Prepared by :- Mr. P.P.Khairnar...Determination of heat transfer coefficient in forced convection 67...

HEAT TRANSFER LABORATORY MANUAL

MGM’S JNEC, AURANGABAD, MECHANICAL ENGINEERING DEPT.

MAHATMA GANDHI MISSION’S

JAWAHARLAL NEHRU ENGINEERING COLLEGE,

AURANGABAD. (M.S.)

DEPARTMENT OF MECHANICAL ENGINEERING

HEAT TRNASFER LABORATORY

MANUAL

Prepared by :- Mr. P.P.Khairnar



MAHATMA GANDHI MISSION’S

JAWAHARLAL NEHRU ENGINEERING COLLEGE,

AURANGABAD. (M.S.)

DEPARTMENT OF MECHANICAL ENGINEERING

HEAT TRNASFER LABORATORY

MANUAL

Prepared By Approved By

Mr. P.P.Khairnar Dr.M.S.Kadam

(Head of Department)



Mission and vision of the Department

VISION OF MECHANICAL DEPARTMENT

To establish the state of the art learning center in Mechanical Engineering which will impart

global competence, enterprising skills, professional attitude and human values in the student.

MISSION OF MECHANICAL DEPARTMENT

1. To impart quality technical education to the students.

2. To develop comprehensive competence in the students through various modes of

learning.

3. To enable students for higher studies and competitive examinations.

4. To facilitate students and industry professionals for continuous improvement and

innovation.

PROGRAM EDUCATIONAL OBJECTIVES:

[1] Use core competence acquired in various areas of Mechanical Engineering to solve

techno-managerial issues for creating innovative products that lead to better livelihoods &

economy of resources.

[2] To establish themselves as effective collaborators and innovators to address technical,

managerial and social challenges.

[3]To equip students for their professional development through lifelong learning and career

advancement along with organizational growth.

[4] Serve as a driving force for proactive change in industry, society and nation.

PROGRAM SPECIFIC OUTCOMES

Student should have

1) An ability to work professionally in mechanical systems including design, analysis,

production, measurement and quality control.

2) An ability to work on diverse disciplinary tasks including manufacturing, materials,

thermal, automobile, robotics, mechatronics, engineering software tools, automation

and computational fluid dynamics.



’HEAT TRANSFER’ EXPERIMENTS

SUBJECT: - Heat Transfer

CLASS: - Third Year Mechanical Engineering

LIST OF EXPERIMENTS

Sr.No Name of Experiment

Page No.

From To

I Determination of thermal conductivity of insulating powder 05 10

II Determination of thermal conductivity of given metal rod 11 15

III Determination of thermal conductivity of composite slab 16 26

IV Determination of heat transfer coefficient in natural

convection 27 32

V Determination of critical heat flux 33 41

VI Study of performance of parallel and counter flow heat

exchanger 42 48

VII Determination of emissivity of given surfaces 49 53

VIII Determination of Stefan Bolzmann constant 54 66

IX Determination of heat transfer coefficient in forced

convection 67 75

X To determine the heat transfer coefficient for natural & forced

convection from pin-fin. 76 78

XI To determine the following for a shell & tube heat exchanger 79 85

XII To study film wise and drop wise condensation and to

determine the overall heat transfer coefficient 86 94

Time Allotted for each Practical Session = 02 Hrs.



EXPERIMENT NO: I

AIM: To determine thermal conductivity of an insulating powder.

APPARATUS: Standard equipment for the determination of thermal conductivity of

insulating powder.

The apparatus consists of two thin walled concentric copper spheres. The inner sphere houses

the heating coil. The insulating powder (Asbestos powder – lagging material) is packed

between the two shells. The power supply to the heating coil is by using a knob and is

measured by an indicator displaying power in watt.

RTD thermocouples are used to measure the temperatures. Thermocouples 1 to 4 are

embedded on inner sphere and 5 to 10 are embedded on the outer shell. Temperature readings

in turn enable to find out the thermal conductivity of the insulating powder packed between

the two shells.

SPECIFICATIONS:-

Sr No. Name Specifications Qty.

1 Material of inner & Outer sphere COPPER 01

2 Radius of the inner sphere (ri) 50 mm 01

3 Radius of the outer sphere (ro) 100 mm 01

4 Thermocouples RTD Type 10

5 Insulating Powder Asbestos Magnesia 01

6 Heater coil Strip heating element 200 watts Sandwiched

Between mica sheets 01

NOTE:

RTD Thermocouples 1 to 4 embedded on inner sphere to measure Ti.

RTD Thermocouples 5 to 10 on outer sphere to measure. Insulating Powder

– Asbestos Magnesia Commercially available powder and packed between

two spheres.



THEORY: It is advised that students should write the theory on the subject in their own

words under the guidance of the teacher.

Consider the transfer of heat by conduction through the wall of a hollow sphere formed by

the insulating powdered layer packed between two thin copper spheres.

Let

ri = Radius of inner sphere in meter.

ro = Radius of the outer sphere in meter.

Ti = Average temperature of the inner sphere in °c

To = Average temperature of the outer sphere in °c

Where,

T1 + T2 + T3 + T4

Ti = ------------------------------

4

T5 + T6 + T7+ T8+ T9+ T10

and To = ------------------------------------------

6

Note that T₁ to T₁₀ denote the temperatures of thermocouples 1 to 10 from the

experimental values of q, Ti and To the known thermal conductivity K can be

determined as

Q x (ro – ri)

K = --------------------------------------------

4 x p x ri x ro x (Ti – To)



PROCEDURE:

1. Keep the power knob to zero position by turning the knobs in anticlockwise

direction.

2. Give supply to Control Unit through the stabilizer unit.

3. Switch on the Control Unit.

4. Check the display it will be lighted up. If not then change the fuse from 1 back side of

control unit.

5. Check for leakage of current in apparatus body.

6. If the leakage of current is detected then switch off the supply and check the electrical

connections and earthing system.

7. Press start button from front side of control panel.



8. With the help of up key see the heater power and all 10 temperatures on control unit

display.

9. Now give supply to heater by rotating the knob H1.

10. After getting the proper steady state, all the observations are recorded.

11. The same procedure is repeated for going to higher temperature by giving more

power.

12. Note down the readings in the observation table as given below.

OBSERVATION:-

1. Radius of inner sphere (ri) = 50 mm

2. Radius of outer sphere (ro) = 100 mm

3. Q=Heat supplied by Heater (Watt)



OBSERVATION TABLE:-

READING SET-1 SET-2 SET-3

Q = Heat supplied by Heater (Watt)

SR.NO THERMOCOUPAL READING IN OC

Temp. for inner sphere in ⁰C

1 T1

2 T2

3 T3

4 T4

Temp. for outer sphere in ⁰C

5 T5

6 T6

7 T7

8 T8

9 T9

10 T10

CALCULATION:- 1. Q =The Rate of heat supplied in Watt.

2. Tmi = Mean temperature of inner surface in 0C

T1 + T2 + T3 + T4

Tmi = ------------------------------

4



3. Tmo = Mean temperature of outer surface in 0C

T5 + T6 + T7+ T8+ T9+ T10

Tmo = ------------------------------------------

6

4. K =thermal conductivity of an insulating powder in W / m K

Q (ro – ri )

K = ---------------------------------------

4 x x ri x ro x (Ti – To)

RESULT: The thermal conductivity of the given insulating powder is =…………

w/mK.

CONCLUSION:

Also expected in this Journal: 1. Concept: Fourier‟s Law of heat conduction

2. Derivation: Fourier‟s Law of heat conduction as applied to a sphere

3. Graph (s): None

4. Diagrams:

a. Set-up

b. Schematic of Temperature Gradient



EXPERIMENT NO: II

AIM: To determine the thermal conductivity of metal rod.

APPARATUS: Standard equipment for the determination of thermal conductivity metal

rod.

The apparatus consists of a metal bar one end of which is heated by n electric heater

while the other projects inside a cooling water jacket. The middle portion of the bar is

insulated with asbestos filled in a cylindrical shell concentric to the rod. The temperature of

the bar is measured at 9 different sections, while radial temperature distribution is measured

with 4 separate thermocouples. Two thermocouples are also provided at the inlet & exit of

the cooling water jacket.

THEORY:

It is advised that students should write the theory on the subject in their own words under the

guidance of the teacher.

INTRODUCTION:

Thermal Conductivity is the physical property of the material denoting the ease with the

particular substance can accomplish the transmission of thermal energy by molecular motion.

Thermal Conductivity of a material is found to depend on the chemical composition

of the substance of which it is composed. The phase (i.e. gas, liquid, solid) in which it exists.

Its crystalline structure if a solid, the temperature and pressure to which it is subjected, and

whether or not it is a homogeneous material.

Table 1. THERMAL CONDUCTIVITY OF SOME MATERIALS.

Thermal Conductivity

Kcal/hr- oc

State

Solid(Metals)

Pure Copper 330 20oC

Brass 95 20oC

Steel 46 20oC

Stainless Steel 14 20oC



MECHANISM OF THERMAL ENERGY CONDUCTION IN METALS

Thermal energy may be conducted in solids by two modes:

• Lattice Vibrations and

• Transport by free electrons.

In good electrical conductors a rather large no. of free electrons moves about in the lattice

structure of the material. Just as these electrons may transport electric charge, they may also

carry thermal energy from a high temperature region to a low temperature region. In fact,

these electrons are frequently referred as the electron gas. Energy may also be transmitted as

vibrational energy in the lattice structure of the material. In general, however this latter mode

of energy transfer is not as large as the electron transport and it is for this reason that good

electrical conductors are almost always good heat conductors viz. Copper, Aluminum and

Silver.

With increase in the temperature, however the increased lattice vibrations come in the way of

transport by free electrons & for most of the pure metals the thermal conductivity decreases

with increase in the temperature.

PROCEDURE:

Keep the dimmer-stat at minimum voltage position. Switch ON the electric

supply.

Adjust the dimmer-stat to supply a voltage of 80 to 120 volts to the heating

coil. Maintain this constant throughout the experiment.

Start the cooling water supply and adjust it to about 350 cc.

Wait for steady state to be attained.

Note down the readings in the observation table as given below.

Note the mass flow rate of water in kg/s.

OBSERVATIONS:

Length of the metal rod= 425 mm.

Test length of the metal rod= 200 mm

Diameter of the metal rod= Inner diameter of insulation=25 mm.

Outer diameter of the insulation = 55 mm.



OBSERVATION TABLE:

Sr.No 1 2 3 4

Voltage (v)

Current (A)

T1

T2

T3

T4

T5

T7

T8

T9

T10

T11

T12

T13

T14

Mass flow

rate of

water (kg/s)

Plot the temperature gradient with the length of the rod.

CALCULATIONS:



THERMAL CONDUCTIVITY OF A METAL ROD APPARATUS



RESULT: The thermal conductivity of the given metal rod is as shown in the following

table

Sr. No. Local Temperature Thermal conductivity.

1

2

3

4

5

6

7

8

9

CONCLUSION:

Also expected in this Journal:

• Concept: Fourier’s Law of heat conduction

• Derivation: Fourier’s Law of heat conduction as applied to a one-dimensional axial flow through a rod

• Formulas used:

• Graph (s): Temperature Gradient

• Diagrams:

Set-up

Thermocouple positions



EXPERIMENT NO: III

AIM: To determine the thermal resistance &thermal conductivity of composite slab.

APPARATUS:

The apparatus consists of a central heater sandwiched between two sheets. Three types

of slabs are provided on both sides of heater, which forms a composite structure. A small

hand press frame is provided to ensure the perfect contact between the slabs. A heater control

knob is provided for varying the input to the heater and measurement of input to the heater.

Thermocouples are embedded between interfaces of the slabs, to read the temperature at the

surface. The experiments can be conducted at various values of input and calculation can be

more accordingly.

COMPOSITE WALL APPARATUS PHOTO:-

SPECIFICATIONS


01 Slabs

1 Mild Steel 0.3 mtr dia x 0.01 mtr thcickness. 01

2 Backlite 0.3 mtr dia x 0.011 mtr thickness 01

3 Press Wood 0.3 mtr dia x 0.0095 mtr thickness. 01

02 Heater Nichrome heater wound on mice former

and insulator with control unit capacity

300 watt maximum.

01

03 RTD Yog Electrical Solution. 08



THEORY:

Heat transfer is transmission of energy from one region to another as a result of

Temperature difference or gradient.

Since differences in temperature exist all over the universe, the phenomena of

heat flow are as universal as gravitational acceleration.

Mechanical engineers deal with the problem of heat transfer in the design of

A.C engines, refrigeration and air-conditioning plants, steam generation

systems and many others.

The electrical engineers require the knowledge of heat transfer for designing

the Cooling systems of motors, generators and transformers.

The knowledge of heat transfer is essential to the civil engineers in the

construction of dams, tunnels and civil structures.

The heat transfer is equally important to the chemical engineers in freezing,

Condensation, evaporation and boiling point.

MODES OF HEAT TRANSFER:-

Modes of heat transfer are conduction, convection and radiation. In all heat transfer

modes, a temperature difference must exist to cause heat flow and heat always flows

in the direction of lower temperature.

Heat conduction due to the property of matter which causes heat energy to flow

through the matter even if the body is impermeable to any kind of radiation and its

parts are not in motion relative to one another (in the macroscopic sense).

Heat convection is due to the property of moving matter (naturally or under force) to

carry heat energy from higher temperature region to lower temperature region as

internal energy (transporting load from one place to another).

Heat radiation is due to the property of matter to emit and absorb different kinds of

electro-magnetic radiations. The radiation heat transfer between two bodies takes

place without any carrying medium as required in conduction and convection.

It is not the purpose of this chapter to introduce the students with the behavior of the

molecules under each mode of energy transfer but to introduce the laws and ways to

find net transfer of heat energy by each mode or by combined modes.



STEADY AND UNSTEADY STATE HEAT TRANSFER:-

Steady state heat transfer through a body and between the bodies implies that the

temperature of the body varies with the position but not with time. The temperature

at each point of the body remains constant in course of time. The statement can be

represented as,

dT/dt =0 , dT/dx = 0

Where T is temperature and t is time. The heat flow rate (kJ/m2-hr) through the body

or from the body remains constant steady state condition.

In unsteady state heat transfer process, the temperature of the body varies with time

and not with position. This can be stated (dT/dt) = 0. The heat flow rate through the

body or from the body varies with time and location.

The present chapter will deal mostly with steady state heat transfer process.

FOURIER LAW OF CONDUCTION AND THERMAL

CONDUCTIVITY:-

Fourier‟s law of conduction is represented by the equation

Where,

Q = heat flow through a body per unit time.

A = surface area of heat flow (perpendicular to the direction of flow)

dT = temperature difference of the faces of block of thickness dx through which

heat flows.

dx = thickness of body along the direction of heat flow.

The above law can be represented by the equality as.



Where k = constant of proportionality and known as thermal conductivity of a body.

• The negative sign of k in the equation is to take care of the decreasing temperature

along the direction of increasing thickness or the direction of heat flow.

• The temperature gradient is always negative along positive x direction and therefore

the value of Q must be positive.

Units of k are given by

The thermal conductivity of the material is defined as the amount of energy conducted

through a body of unit area and unit thickness in unit time when the difference in

temperature between the faces causing heat flow is 1°C.

Thermal conduction of different bodies is different. The conductivity of the body

depends mainly upon its molecular structure, the specific gravity, moisture content,

temperature and many other factors also affect the thermal conductivity.

Metals are good conductors of heat whereas insula tors are poor conductors of heat.

CONDUCTION OF HEAT THROUGH A SLAB:-

Heat flow through a small elemental thickness dx of a slab (Fig. 1)under steady state

condition is given by Equation. This equation is known as Fourier law of conduction.



Integrating the above equation between the limits of x= 0 to L and T = T1 to T2 as

temperature changes from T1 to T2 through a thickness of slab L,

Fig. 1 Heat flow through a slab Where L/kA is known as thermal resistance of the slab

HEAT TRANSFER - (COMPOSITE WALL):-

a) A hot fluid is separated from a cold fluid by three different solid slabs

Hot fluid at temperature T1 is separated by three layers of solid L1, L2, L3 from a cold

fluid at temperature To as shown in Fig. 2. The steady state heat flow through the system

is given by

Where k1, k2 and k3 are the conductivities of the solid layers 1, 2 and 3 and hi and ho are the

inside and outside convective heat transfer coefficient.



The above equations can be written as given below:

Fig. 2 Heat transfer through composite wall

Adding both sides of the above equations

If there are n layers of solids, then the above equation can be written as



PROCEDURE:-

1. Keep the power knob to zero position by turning the knobs in anticlockwise direction.




control unit.


6. If the leakage of current is detected then switch off the supply and check the

electrical connections and earthing system.




8. With the help of up key see the heater power and all 8 temperatures on control unit

display.


10. After getting the proper steady state, all the observations are recorded. 11. The same

procedure is repeated for going to higher temperature by giving more power.


PRECAUTION:-

Keep knob to zero before start.

Increase the heater power slowly.

Keep all the assembly undisturbed.

Remove air gap between plates by moving hand-press gently.

While removing the plates do not disturb the thermocouples. Operate control panel gently

OBSERVATION:-

COMPOSITE SLABS: 1] Slab thickness : 0.0305 m

A] Mild steel = 0.010 m.

B] Bakelite = 0.011 m.

C] Press wood = 0.0095 m.

2] Slab effective diameter = 0.150 m.



Thermal Conductivity of Slabs :

Mild Steel : 36 to 54 W/m.0C.

Backlite : 0.23 W/m.0C.

Press Wood : 0.13 W/m.0C

OBSERVATION TABLE:-

READING SET - 1 SET - 2 SET - 3

Q=Heat supplied by Heater (Watt)

SR. THERMOCOUPLE READING IN OC

1 T1

2 T2

3 T3

4 T4

5 T5

6 T6

7 T7

8 T8

CALCULATION:-

For calculating the thermal conductivity of composite walls, it is assumed that due to

large diameter of the plates, heat flowing through central portion is flowing at top and bottom

stack of the slab i.e. axial flow. Thus for calculations, central half dia. area, where uni-

directional flow is assumed, is considered. Accordingly thermo-couples are fixed at close to

center of the plates. Also, heat flow rate is to be taken half as same heat is flowing through

top and bottom slab. Here we are considering only one side of slab.

1) Q =The Rate of heat supplied in Watt.

2) A = Area of Plates in m2

A = / 4 d2

Where, d = half diameter of plates in meter.



3) Mean reading of temperatures in 0C

[T1 + T2] [T3 + T4]

TA = ------------------ TB = ------------------

2 2

[T5 + T6] [T7 + T8]

TC = ----------------- TD = -----------------------

2 2

4) R total = Total thermal resistance of composite slab

b

R total = ----------

K x A

5) Kcomp. = Thermal conductivity of composite slab in W / m.0 C

Q x b

K comp. = ---------------- W / m .0 C.

A (TA - TD )

Where, b = Total thickness of composite slab in meter.

Q = Heat flow in W .

A = Area of slab . m2

7) Effective K = 1/K1 + 1/K2 + 1/k3



RESULT:

The total thermal resistance of the composite slab is = K/w.

The thermal conductivity of the composite slab is = w/mK.

CONCLUSION:


Concept: Fourier’s Law of heat conduction

Derivation: Fourier’s Law of heat conduction as applied to a composite slab with axial heat flow only

Formulas used:

Graph (s): Temperature gradient

Diagrama. Setup



EXPERIMENT NO: IV

AIM: To determine the average surface heat transfer coefficient in natural convection and

compare it with the value obtained by using appropriate correlation.

APPARATUS:

The apparatus consists of a brass tube fitted in a rectangular duct in a vertical fashion.

The duct is open at the top and bottom and forms an enclosure and serves the purpose of

undisturbed surrounding. One side of the duct is made up of a transparent material for

visualization. An electric heating element is kept in the vertical tube, which in turn heats the

tube surface. The heat is lost from the tube to the surrounding air by natural convection. The

temperature of the vertical tube is measured by seven thermocouples. The heat input to the

heater is measured by an ammeter and a voltmeter and is varied by a dimmer-stat. The tube

surface is polished to minimize the radiation losses.

THEORY:

Convection is the heat transfer that occurs as a result of the movement of a fluid, and

hence it is not possible in case of solids. It is well known that a hot metal plate will cool faster

when placed in front of a fan than when exposed to still air. We say that heat is convected

away and we call the process convective heat transfer. Heat will transfer by conduction

through the walls of the fluid container. From the container walls, the heat will transfer by

conduction and radiation into the fluid, causing convection currents to be set up in the fluid.

Consider a heated plate. The temperature of the plate is Tp and the temperature of the fluid in

which it is immersed is Tf. The velocity of flow increases from the wall of the plate to the

surface of the fluid. At the wall, the velocity is zero. Therefore heat transfer here is only by

conduction. The change of temperature over the length (temperature gradient) is dependent on

the rate at which the fluid carries the heat away. This process is described by the Newton's

law of cooling.

q= h.As (Tp-Tf)

The heat transfer rate is related to the overall temperature difference between the wall

and the fluid and the surface area As. The quantity h is called the heat transfer coefficient and



it can be calculated for simple systems, but has to be experimentally determined for complex

systems. It includes the effects of conduction, radiation and convection at the considered

surface. For instance the convective heat transfer coefficient for free convection with water

flowing on a hot vertical plate 1 ft high with a temperature difference of 30°C between the

water and the plate is 4.5 W/m2.0C. The following figure shows the heat transfer from the

surface of a plate. Factors affecting the heat transfer co-efficient are:

i. Shape and inclination of the surface over which convection takes place.

ii. Injection of new fluid Roughness of the surface

iii. Whether the flow is in an open or closed space

iv. Mode of convection (free/forced)

v. Speed of flow and Reynolds‟s number (laminar/turbulent flow)

vi. Presence of Electric and magnetic fields.

Heat transfer using movement of fluids is called convection. In natural convection, the

flow is induced by the differences between fluid densities which result due to temperature

changes.

The heat transfer rate for convection is given by the following equation:

q= h.As (Tsurface-T∞)

h is the convection coefficient, As is the surface area and Tsurface and T∞ are the surface and

ambient temperatures, respectively.

Density of fluid changes with temperature. In general, fluids expand as the temperature rises,

and thus the density decreases (density is the mass per unit volume). Warm fluids therefore

are more buoyant than cooler fluids. A hot object will heat the surrounding fluid, which rises

due to buoyancy force. The warm fluid is then replaced by cool fluid. Similarly cool objects

will draw heat away from the surrounding fluid which then falls due to increased density. The

cool fluid is then replaced by warm fluid initiating convection currents.

For a hot horizontal plate surrounded by air, convection currents form when the air

above the plate starts to rise. The air below the plate, however, cannot rise because the plate is

blocking the flow. The heated fluid under the plate will eventually escape through the sides of

the plate; however, the convective flow below the plate is very small compared to the flow on



top. In general, natural convection is more pronounced (has a higher h) on the topside of a hot

plate or the bottom side of a cold plate.

The convection coefficient is a measure of how effective a fluid is at carrying heat to

and away from the surface. It is dependent on factors such as the fluid density, velocity, and

viscosity. Generally, fluids with higher velocity and/or higher density have greater h. The

convection coefficient for natural convection in gas is generally between 1 w/m2K and 20

w/m2K; typical values for liquids fall between 100 w/m

2K and 1000 w/m

2K.

Correlations for Heat Transfer Coefficients:

Each flow geometry requires different correlations be used to obtain heat transfer coefficients.

Initially, we will look at correlations for fluids flowing in conduits. Most correlations will

take the "Nusselt form":

Nu = aReb Pr

c (correction factors)

The correlations that follow are limited to conduit flow without phase change. Unless

otherwise specified, fluid properties should be evaluated at the "bulk average" temperature --

the arithmetic mean of the inlet and outlet temperatures:

Choosing a Correlation

When choosing a correlation, begin by asking:

What is the geometry? (Flow through a pipe, around an object, over a plane, etc.)

Is there a phase change?

If the flow is laminar, is natural convection important? (The Grash of number will be used for

this.)

Laminar flow and free convection:

Heat usually causes the density of a fluid to change. Less dense fluid tends to rise,

while the more dense fluid falls. The result is circulation -- "natural" or "free" convection.



This movement raises h values in slow moving fluids near surfaces, but is rarely significant in

turbulent flow. Thus, it is necessary to check and compensate for free convection only in

laminar flow problems.

The Grashof Number is used to assess the impact of natural convection

The fluid properties used to calculate the Grash of number should be evaluated at the

film temperature, the arithmetic mean between the bulk and wall temperatures. This will

require determining an additional set of property values.

The Grashof Number provides a measure of the significance of natural convection. When the

Grashof Number is greater than 1000, heat transfer coefficients should be corrected to reflect

the increase due to free circulation. Multiplicative correction factors are available to apply to

the Nusselt Number or the heat transfer coefficient (do NOT use both).

Nu = Nulam (0.87(1 + 015 GrI/3

))

PROCEDURE:

1. Switch on the electric supply and adjust the dimmer-stat to obtain the required heat

input (say 40w, 60w, 70w).

2. Wait for steady state to be attained.

3. Note down the reading in the observation table as given below. 4. Repeat the

experiment at different heat inputs. (Do not exceed 80 watts).

OBSERVATIONS:

O.D. of cylinder, d= 38 mm

Length of cylinder, l= 500 mm



OBSERVATION TABLE:

Sr.No 1 2 3 4

Voltage (v)

Current (A)

T1

T2

T3

T4

T5

T6

T7

T8

CALCULATIONS:

a) Actual heat transfer coefficient:

Rate of heat transfer, Q = V I watts

Average temperature of surface, Ts = (T1+ T2+ T3+ T4+ T5+ T6+ T7)/7

Ambient temperature in the duct, Ta = T8

Area of heat transfer,

As = πdI

Q= hactual As (Ts-Ta)

Hence, hactual= Q / [As (Ts-Ta)]

b) Theoretical heat transfer coefficient:

Theoretical is calculated using the appropriate correlation between Nu, Gr and Pr from heat

and mass transfer data book.

Nu = c (Gr Pr)"



RESULT:

The actual value of heat transfer coefficient was found to be…………… w/m2K and

The theoretical value of heat transfer coefficient was found to be………. w/m2K

CONCLUSION:


Diagrams:

Set-up

Schematic of thermal boundary layer with velocity gradient



EXPERIMENT NO: V

AIM: To study the pool boiling phenomenon upto critical heat flux point

APPARATUS:

The apparatus consists of cylindrical glass container housing and the test heater

(Nichrome wire). Test heater is connected also to mains via a dimmer. An ammeter is

connected in series while a voltmeter across it to read the current and voltage. The glass

container is kept on a stand, which is fixed on a metallic platform. There is provision of

illuminating the test heater wire with the help of a lamp projecting light from back and the

heater wire can be viewed through a lens.

This experimental set up is designed to study the pool-boiling phenomenon up to

critical heat flux point. The pool boiling over the heater wire can be visualized in the different

regions up to the critical heat flux point at which the wire melts. The heat flux from the wire

is slowly increased by gradually increasing the applied voltage across the test wire and the

change over from natural convection to nucleate boiling can be seen.

The formation of bubbles and their growth in size and number can be visualized

followed by the vigorous bubble formation and their immediate carrying over to surface and

ending this in the braking of wire indicating the occurrence of critical heat flux point.


1. Glass container Dia. 186.5 mm. &

Height 97mm 01

2. Nichrome wire size 0.120 mm 01

3. Dimmer stat 10 Amp, 230 volts. 01

4. Voltmeter 0 to 300 V 01

5. Ammeter 0 to 10 AMP 01

6. Thermometer 0 to 100 C 01



THEORY:-

The heat flux supplied to the surface is plotted against (Tw - Ts) the difference

between the temperature of the surface and the saturation temperature of the liquid. It is seen

that the boiling curve can be divided into three regions:

Natural Convection Region

Nucleate Boiling Region

Film Boiling Region

The region of natural convection occurs at low temperature differences (of the order of

10 oC or less). Heat transfer from the heated surface to a liquid in its vicinity causes the liquid

to be superheated.

The superheated liquid rises to the free liquid surface by natural convection, where

Vapour is produced by evaporation. As the temperature difference (Tw – Ts) is increased,

nucleate boiling starts. In this region, it is observed that bubbles start to form at certain

locations on the heated surface.

Region II consists of two parts. In the first part, II – a, the bubbles formed are very few

in number. They condense in the liquid and do not reach the free surface. In the second part,

II – b, the rate of bu bbles formation and the number of locations where they are formed

increase. Some of the bubbles now rise all the way to the free surface. With increasing

temperature difference, a stage is finally reached when the rate of formation of bubbles is so

high, that they start to coalesce and blanket the surface with a vapour film. This is the

beginning of the region III viz film boiling.

In the first part of this region III-a, the vapour film is unstable, so that the film

boiling may be occurring on a portion of the heated surface area, while nucleate boiling

may be occurring on the remaining area. In the second part, III-b, a stable film covers the

entire surface. The temperature difference in this region is of the order of 1000 C and

consequently radiative heat transfer across the vapour film is also significant.

It will be observed that the heat flux does not increase in a regular manner with the

temperature difference. In region I, the heat flux is proportional to (Tw – Ts) ⁿ, where „n‟ is

slightly greater than unity. When th e transition from natural convection to nucleate boiling

occurs the heat flux starts to increase.



more rapidly with temperature difference, the value of n increasing to about 3.at the end of

region II, the boiling curve reaches a peak. Beyond this, in the region II-A, in spite of

increasing temperature difference, the heat flow increases with the formation of a vapour

film. The heat flux passes through a minimum at the end of region III-a. it starts to increase

again with (Tw – Ts) only when stable film boiling begins and radiation becomes

increasingly important.

It is of interest to note how the temperature of the heating surface changes as the heat

flux is steadily increased from zero. Up to point A, natural convection boiling and nucleate

boiling occur and the temperature of the heating surface is obtained by reading off the value

of (Tw – Ts) from the boiling curve and adding to it the value of Ts. If the heat flux is

increased even a little beyond the value of A, the temperature of the surface will shoot up to

the value corresponding to the point C. it is apparent from figure 1 that the surface

temperature corresponding to point C is high.

For most surfaces, it is high enough to cause the material to melt. Thus in most

practical situations, it is undesirable to exceed the value of heat flux corresponding to point A.

This value is therefore of considerable engineering significance and is called the critical or

peak heat flux. The pool-boiling curve as described above is known as Nukiyam pool Boiling

Curve. The discussions so far has been concerned with the various type of boiling which

occur in saturated pool boiling. If the liquid is below the saturation temperature we say that

sub-cooled pool boiling is taking place. Also in many practical situations, e.g. steam

generators; one is interested in boiling in a liquid flowing through tubes. This is called forced

convection boiling, may also be saturated or sub-cooled and of the nucleate or film type.

Thus in order to completely specify boiling occurring in any process, one must state

Whether it is forced convection boiling or pool boiling,

Whether the liquid is saturated or sub cooled, and

Whether it is in the natural convection nucleate or film boiling region.



PROCEDURE

1. Fill the tank with water.

2. Fix the Nichrome wire to two studs. Check no loose connection is present.

3. Dip the Nichrome wire into the water and make the electrical connections

4. Check all the MCB are at OFF position and also the heater connection is removed from

Three pin provided at right side of control panel.

5. Now on only two MCB not the heater. And immersed thermometer at top of the glass

to measure water temperature.

6. Note the current reading in steps of 1 amp till a maximum current of 10 ampere.

7. Between each reading the time interval of two min is allowed for steady state to

establish.

8. Water temperature is noted with a thermometer at the beginning and at the end of the

experiment. The average of these two is taken as the bulk liquid average temperature.

OBSERVATIONS:

Length test heater= l= mm.

Diameter of the test wire= d= mm.

Surface area= A= _dl= mm2.

OBSERVATION TABLE:-

Sr.No Water/Bulk Temp

T in oC

Voltage

(V)

Ampere

(I)

1

2

3

4

CALCULATION:-

Q = heater power in Watts

Q = V X I Watts



q = critical heat flux in w / m2

q = Q/A in w / m2



RESULT:

The thermal conductivity of the given metal rod is as shown in the following table.

Sr.No Water/Bulk Temp

T in oC

Critical Heat Flux

1

2

3

4

5

6

7

8

CONCLUSION:

Also expected in this Journal

Concept: Boiling Phenomenon, pool boiling curve & its various regions

Formulas used:

Graph (s): Variation of critical flux with pool temperature

Diagrams:

A) Set-up

B) Pool Boiling Curves



EXPERIMENT NO: VI

AIM: To determine the following for parallel flow and counter flow double pipe heat

exchangers:

1. LMTD

2. Effectiveness

3. Overall heat transfer coefficients

APPARATUS:

It consists of a tube in tube type of heat exchanger. A heater is provided for heating

the water. The hot water flows through the inner tube and the cold water flows through the

annular space between the inner and outer tubes. A pipe and valve arrangement is provided

for reversing the direction of the cold water. Thermo wells are fitted at the inlets and outlets

of both hot and cold water. The temperatures of the hot and cold water at these points are

measured with mercury in glass thermometers. The volumetric flow rates of the hot and cold

water are measured using a measuring jar and a stopwatch.

THEORY:

A device used to transfer heat between fluids that are at different temperatures and

separated by a solid wall is termed a heat exchanger. Common uses for heat exchangers are

found in waste heat removal, air-conditioning, power production and chemical processing.

The general representation of a heat exchanger is as shown in the figure above.

Types

Heat exchangers are normally classified according to type of construction and flow

arrangement. Heat exchangers can have several different flow arrangements. One

arrangement is where the fluids both enter the heat exchanger from the same side and flow

parallel to each other until the exit point. This is called Parallel Flow heat exchanger. Another

common type is the Counter Flow in which the opposite happens. Here, one fluid enters the

heat exchanger at the exit point of the other fluid, and exits at that fluids entrance point. A

third type of heat exchanger is the Cross Flow heat exchanger, in which one fluid flows

perpendicular to the other. The main difference between the flow arrangements lies in the



temperature distribution along the length of the heat exchanger, and the relative amounts of

heat transfer under given temperature specifications for specified heat exchanger surfaces. A

counter flow heat exchanger requires a minimum area; a parallel flow heat exchanger requires

a maximum area while a cross-flow heat exchanger requires an area in between.

Parallel Flow

In a parallel flow heat exchanger, both the hot and cold fluid flow in the same

direction. They enter together at one end, flow through in the same direction and leave

together at the other end. The temperature difference between the two fluids decreases

asymptotically along the increasing length of the exchanger. The outlet temperature of the

cold fluid never exceeds that of the hot fluid.

The temperature profiles for parallel flow and counter flow heat exchangers is shown in the

figures below: Here the subscripts '1' and '2' stand for inlet and outlet respectively and the

subscripts 'h' and' c' stand for hot and the cold fluids respectively.

Counter Flow

Counter flow heat exchangers are the opposite of parallel flow heat exchangers. In a counter

flow design, the entrance of the hot fluid of the heat exchanger is the exit of the cold fluid,

and at the other end of the exchanger, the hot fluid exit is the cold fluids entrance. In contrast

to the parallel flow, exchanger, this configuration provides for heat transfer between the hotter

portions of the two fluids at one end, as well as between the colder portions at the other. The

difference in temperature between the two fluids along the length of the exchanger is nowhere

near as larger as it is for the inlet of the parallel flow design. The outlet temperature of the

cold fluid may exceed the outlet temperature of the hot fluid.



1. The heat exchanger is insulated from its surroundings, in which case the only heat exchange

is between the hot and cold fluids.

2. Axial conduction along the tubes is negligible

3. Potential and kinetic energy changes are negligible.

4. The fluid specific heats are constant.

5. The overall heat transfer coefficient is constant

Common designs incorporate concentric tubes, also known as double-pipe arrangement. This

consists of one pipe placed concentrically inside another of larger diameter. This design can

have parallel or counter flow configurations. This is largely used for sensible heating or

cooling of process fluids where small heat transfer areas are required.

For a hollow cylinder exposed to a convection environment on its inner and outer surfaces,

the electrical resistance analogy would appear as in the following figure where, again TA and

TB are the two fluid temperatures. Note that the area for convection is not the same in both

fluids in this case, these areas depending on the inside tube diameter and the wall thickness.

In this case the overall heat transfer would be expressed by

In accordance with the thermal network shown in the figure, the terms Ai and Ao represent

the inside and cut side surface areas of the inner tube. The overall heat transfer coefficient

may be based on either the inside or the outside area of the tube. Accordingly



Fouling Factors

After a period of operation the heat transfer surfaces for a heat exchanger may become coated

with various deposits present in the flow systems, or the surfaces may become corroded as a

result of the interaction between the fluids and the material used for construction of the heat

exchanger. In either event, this coating represents an additional resistance to the heat flow,

and thus results in decreased performance. The overall effect is usually represented by a

fouling factor or fouling resistance Rf, which must be included with the other thermal

resistances making up the overall heat transfer coefficient.

Logarithmic Mean Temperature Difference

F-or a double pipe heat exchanger with a hot and a cold fluid flowing through it, we calculate

the heat transfer in the arrangement with a simple formula q= UA Tm

Where U= Overall heat transfer coefficient

A = surface area for heat transfer consistent with definition of U Tm = suitable mean

temperature difference across the heat exchanger

LMTD is determined by the following equation

Where the subscripts i and 0 stand for the inlet and outlet positions; and h and c stand for the

hot and cold fluid. The above equation is valid for parallel flow heat exchangers. For counter

flow heat exchangers, the inlet and outlet temperature values for the cold fluid get reversed in

the equation.

Heat Exchanger Effectiveness

One way of measuring a heat exchanger's performance is to calculate its effectiveness. The

heat exchanger effectiveness is defined as the ratio of the actual heat transfer to the heat

transfer attainable in an infinitely long counter flow exchanger. So, by defining the

effectiveness in this way, we have a much simpler expression for our effectiveness:



If, for any given exchanger, the effectiveness is known we can write the heat transfer equation

as:

PROCEDURE:

1. With the help of the valves provided both the hot and cold water are made to the

flow in same direction (Parallel flow arrangement).

2. Adjust the flow rate of hot and cold water with the help of the valves

3. Measure the flow rate of the hot and cold water with the help of a measuring jar and a

stopwatch. Note the flow rates.

4. Switch ON the heater and wait for steady state to be attained.

5. Note the inlet and outlet temperatures of the hot and cold water.

6. Also note the voltage and current supplied to the heating element.

7. Repeat the experiment after reversing the direction of the hot water. (Counter flow

arrangement)

OBSERVATIONS:

Diameter of inner pipe, di = 8.1 mm

Diameter of outer pipe, do = 13.14 mm

Length of pipes, I = 2m

OBSERVATION TABLE:

Type of Cold Water Hot water

flow Flow rate Tci Tco Flow rate Thi Tho

(lit/s) (0C) (

0C) (lit/s) (

0C) (

0C)

Parallel

flow

Counter

flow



CALCULATIONS:

RESULTS:

For parallel flow arrangement

1. LMTD= °C

2. Ui actual= W/m2K

3. Ui theoretical= W/m2

K

4. Uo actual= W/m2K

5. Uo theoretical= W/m2K

6. ε =

For Counter flow arrangement:

1 LMTD = oC

2. Ui actual= W/m2K

3. Ui theoretical= W/m2K

4. Uo actual= W/m2K

5. Uo theoretical= W/m2K

6. ε =



CONCLUSION:


1) Diagrams:

Set-up

Different flow arrangements

Set-up ckt



EXPERIMENT NO: VII

AIM: To determine the value of emissivity for a gray body.

APPARATUS:

The experimental set up consists of two circular Aluminum plates identical in size and

is provided with heating coils sandwiched. The plates are mounted on brackets and are kept in

an enclosure so as to provide undisturbed natural convection surroundings. The heat input to

the heater is varied by separate knob and it‟s power is measured by using an indicator.

The temperatures of the plates are measured by RTD separate wires are connected to

points to get the average surface temperatures of the plates.

Another thermocouple is kept in the enclosure to read the ambient temperature of enclosure.

Plate 1 is blackened by a thick layer of the lamp black to form the idealized black

surface whereas plate 2 is the test plate whose emissivity is to be determined. The heater

inputs to the two plates are dissipated from the plates by conduction, convection and

radiation.

The experimental set up is designed in such a way that under steady state conditions the heat

dissipation by conduction and convection is same for both the plates when the surface

temperatures are same and the difference in the heater input readings is because of the

difference in the radiation characteristics due to their different emissivities. The schematic

arrangement of the set up is shown in the figure.

PHOTO OF EMMISSIVITY MEASUREMENT APPARATUS:-



SPECIFICATIONS:-


1. Fuse 2 amp 01

2. Thermocouple RTD Type; Yog Electrical Solution. 03

3. Black plate Aluminium material 150 mm diameter 01

4. Test plate Aluminium material 150 mm diameter 01

THEORY:-

Emissivity being a property of the surface depends on the nature of the surface and

temperature.

It is obvious from the Stefan Boltzmann‟s law that the prediction of emissive power of

the surface requires knowledge about the values of its emissivity and therefore much

experimental research in radiation has been concentrated on measuring the values of

emissivity as function of surface temperature.

The present experimental set up is designed and fabricated to measure the property of

emissivity of the test plate surface at various temperatures.

It is denoted by „ϵ‟

E

ϵ = ---------

Eb

Absorptivity of a black body is 1 and by the knowledge of Kirchoff‟s law,

emissivity of the black body becomes unity.



PROCEDURE





control unit.





8. With the help of up key see the all 2 heater powers and all 3 temperatures on control

unit display.

9. Gradually increase the input to the heater to black plate and adjust it to some value

viz.12, 15, 20 watts. Adjust the heater input to test plate slightly less than the black

plate 8, 10, 12 watts etc.

10. Check the temperatures of the two plates with small time intervals and adjust the input

of test plate only, by the dimmerstat so that the two plates will be maintained at the

same temperature.

11. This will require some trial and error and one has to wait sufficiently (more than one

hour or so) to obtain the steady state condition.

12. After attaining the steady state condition, record the temperatures, power readings for

both the plates.

13. The same procedure is repeated for various surface temperatures in increasing order.

OBSERVATION S:

d = Diameter of plates =

A = Area of plates = (p / 4) x d2 =

Eb = Emissivity of black plate = 1

s = Stefan Boltzman constant. = 5.669 x10-8 w/m²/ K4



OBSERVATION TABLE:-

Sr.No

Black Plate Test Plate Ambient

Temperature

Power Qb

(P1) in

watt

Tb

T1

Power Qs

(P2) in

watt

Ts

T2

Ta

T3

1

2

3

CALCULATION:-

Qb = Heater input to black plate in Watt

Qb = P1 in Watt

· QS = Heater input to test plates in Watt

QS = P1 in Watt

· A = Area of plates in m 2

A= (π/4)x d2

= 0.0201m2

· E = Emissivity of test plate

Qb – Q S = (Eb – E) sA (TS4 – Ta

4)

PRACAUTION:-

1. Use stabilized AC single-phase supply (preferably).

2. Always keep the knobs at zero position before start.

3. Gradually increase the heater inputs.

4. See that the black plate is having the layer of lamp black uniformly.



NOTE: - There is a possibility of getting absurd results if the supply voltage is

fluctuating or if the input voltage is not adjusted till the satisfactory steady state

condition is reached.

PROPERTY TABLE:-

MATERIAL TEMPERATURE EMISSIVITY

METALS

Polished copper

Steel, Stainless Steel

Nickel

20 c 0.15, Increases with

temperature

Aluminum (Oxidized) 90 - 540 c 0.15 to 0.35

NON

METALS

Brick, Wood, Marble,

Water 20 - 100 c 0.80 to 1

RESULT: The emissivity of the test plate is…………………. .

CONCLUSION:


1) Concept: Radiation, Emissivity, Shape factor

2) Formulas used:

3) Graph (s): None

4) Diagrams:

a. Set-up



EXPERIMENT NO: VIII

AIM: To determine the value of Stefan-Boltzmann constant experimentally.

APPARATUS:

The apparatus is centered on a flanged copper hemisphere B fixed on non-conducting

plate A. The outer surface of B is enclosed in a metal water jacket used to heat B to some

suitable constant temperature. The hemispherical shape of B is chosen solely on the grounds

that it simplifies the task of draining the water between Test Piece & hemisphere. Four RTD

thermocouples are attached to various points on surface of Hemisphere to measure its mean

temperature.

The test piece, which is mounted in an insulating Bakelite sleeve S. is fitted in a hole,

drilled in the center of base plate „A‟. A RTD thermocouple is used to measure the

temperature of Test Piece (T5).

The thermocouple is mounted on the disc to study the rise of its temperature. When the

disc is inserted at the temperature T5 (T5 T) i.e. the temperature of the enclosure, the

response of the temperature change of the disc with time is used to calculate the Stefan

Boltzmann constant.

SPECIFICATIONS


1. Hemispherical enclosure diameter 200 mm 01

2. Base plate, Bakelite diameter 310 mm 01

3. Test disc diameter 20 mm 07

4. Mass of test disc 0.004 Kg 01

5. Specific heat of the copper test disc 0.41868 KJ / Kg°C

6. No of RTD on Hemisphere 04

7. No of RTD on Test piece 01



8. No of RTD control heater 01

9. Water heater capacity (2KW) 01

10. Hot water Bath 5.75 liters Capacity. 01

NOTE: Using lampblack to make their absorptivities to be approximately

unity blackens the inner surface of hemisphere. The test piece is also blackened.

THEORY

CONCEPT OF BLACK BODY

It is defined as ideal body, which observes all incident radiant energy without reflecting or

transmitting any energy. This applies for radiation of all wavelengths & for all angle of

incidence.

Thermal Radiation is the important mode of heat transfer observed in several engineering and

other applications. All bodies at temp above ⁰ K emit energy in the form of radiation.

Different theories are developed to study the Radiation Heat transfer like Maxwell theory

explains the radiation phenomenon as propagation of energy in the form of electromagnetic

wave, while Max Plank‟s hypothesis treats it as energy being carried through photons or

quanta‟s Whichever of these theories are used, it is convenient to classify all electromagnetic

radiant energy emission in terms of wavelength.

The most commonly used law of thermal radiation is the Stefan Boltzman law which states

that heat flux or emissive power of a black body is proportional to the forth power of absolute

temperature of the surface and is given by:

Eb = σ T4 W / m

2

Where,

Eb = Emissive power (W / m2)

σ = Stefan Boltzman constant = 5.699x 10-8 W /m2 K4

T= Temperature of Black body (0 K)



The Stefan Boltzman law can be derived by integrating the Plank‟s law over the entire

spectrum of wavelengths from 0 to ∞, though historically it is worth noting that the Stefan

Boltzmann law was independently developed before Plank‟s law.

The radiation energy falling on Test Piece from the enclosure is given by:

E = AD T⁴σ ----------- (1)

Where,

AD = Area of the disc D in m²

T = Average temperature of the enclosure Recorded by the

Thermocouple K

The emissivity of the disc D is assumed to be unity (Black disc) the radiant energy,

emitted by disc D into enclosure will be

Et = AD T54 σ----------- (2)

The net heat input to disc D per unit time is given by (1) - (2)

E - Et = σ AD (T4- T5

4)--------- (3)

If the test piece has mass m and specific heat S then in a short time after test piece is

inserted in A,

m.s. (dT/dt) = σ AD (T4 -T5

4)

OR,

m.s. (dT/dt)T= 0

σ = -----------------------------

AD (T4 -T5

4)



In this equation, m.s. (dT/dt) t = 0 denotes the rate of rise of temperature of the test

piece at the instant when its temperature is T5 and will vary with T5. it is clearly best

measured at time t = 0 before heat conducted from A to test piece begins to have any

significant effect.

This is obtained from plot of temperature rise of test piece with respect to time and

obtaining its slope at t = 0 when temperature is T5. This will be the required value of dT/dt at

t=0. The thermocouple mounted on disc is to be used for this purpose.

Note that the test piece with its insulating sleeve S is placed quickly in position and

start the timer and record the temperature at fixed time intervals. The whole process is

completed in about 30 seconds of time.

Longer test piece is left in position; the greater is the probability of errors due to heat

conduction from A to test piece .the experiment is repeated for obtaining better results.

PROCEDURE

1. Check the ball valve fitted at bottom of water bath is at closed position.



2. Make tap water connection by using a flexible pipe up to the hot water bath till

it with water.




control unit.




8. Press START button from front side of control panel.



9. With the help of UP key see the all 6 temperatures on control unit display.

10. Now start the heater by following procedure.

a) After giving power supply and press START button we see the below screen.

b) Now press ENTER button.

c) Now press UP key to ON the heater.

d) Now press ENTER button.



11. The heater is started to heat the water.

12. Also we have to set the heater cut off for one time only. We are using one RTD (T6) as

thermostat. While temperature reached at set points it automatic cut off the heater power

supply. Foe setting see following process:-

a) While power on the control panel it shows.

b) Now press ENTER button.

c) Now press UP button till SYSTEM SETTING screen comes.

d) Now again press ENTER button and feed password 3000 with the help of UP button.

) After that press ENTER.



f) Set the OUTPUT SETTING with the help of UP & SHIFT key.

TEMP CHANNEL: 06 (for heater cut off) CUT OFF TEMP: 090.0 (for heater

cut off)

OFF TIME: 05 (in sec) g) Now press ENTER button.

h) Now press again ENTER button to exit.

i) Now we set all the parameters.

j) Now press START button.



13. The immersion heater up to a temperature of about 90°C heats the water in the tank.

14. The test piece is removed before pouring the hot water into the jacket from bottom end of the

enclosure.

15. When water crossed to 90°C (indicated as T6) then o pen the ball valve fitted at bottom of

water bath is at closed position.

16. The hot water is poured in the water jacket.

17. The hemispherical enclosure and will come to some uniform temperature T in a short time

after filling the hot water in the jacket the thermal inertia of hot water is quite adequate to

present significant cooling in the time required to conduct the experiment.

18. Note down all the 4 temperature readings after getting steady state. 19. The enclosure will

soon come to the thermal equilibrium conditions. 20. The test piece is now inserted in A at a

time when its temperature is saying T5 (to be sensed by a separate thermocouple).

21. No. Of reading (Say 10 No.) Can be taken at the interval of 5 Sec. For 5 seconds

notifications we see a star (*) mark indicating after every 5 seconds near to temperature

T5. Note down every time the star (*) come to temperature T5.

.



22. After getting all the readings drain all water by opening ball valve provided at right side of

enclosure.

OBSERVATION:-

Type of material = Copper

d = Diameter of Test Disc = 20 mm

t = Disc Thickness = 1.6 mm

W = Weight of Disc =0.004 kg

OBSERVATION TABLE:-

Sr.no. Thermocouple No Temperature °c

1 T1

2 T2

3 T3

4 T4



Observation table for different temp. of test disc at every 5 sec

Time in sec 5 10 15 20 25 30 35 40 45

Temp in 0c

CALCULATION

Ta = Average temperature in 0C

T1 + T2 + T3 + T4

Ta = -----------------------------

4

T5 = Temperature of disc in 0C

T5 = at the instant when it is inserted in 0C

T5 = …….. 0C

dT / dt = Slope from graph of dT against dt.

σ = Stefan boltzman constant in W / m2 0

K4



σ = ………………………w/m2 o

K

Plot the graph Time interval and temperature (T5)

GRAPH:

dT / dt graph for slope



RESULT: The experimental value of Stefan-Boltzmann constant was found to be

……….w/m2K

4

CONCLUSION:


1) Concept: Radiation, Stefan-Boltzmann constant

2) Formulas used:

3) Graph (s): Temperature against time with tangent drawn at time=0, slope

of the tangent shown

4) Diagrams:

a. Set-up



EXPERIMENT NO: IX

AIM: To determine the heat transfer coefficient in forced convection

INTRODUCTION:-

In many practical situations and equipment‟s, we invariably deal with flow of fluids in tubes

e.g. boiler, super heaters and condensers of a power plant, automobile radiators, water and air

heaters or coolers etc. the knowledge and evolution of forced convection heat transfer

coefficient for fluid flow in tubes is essentially a prerequisite for an optional design of all

thermal system

Convection is the transfer of heat within a fluid by mixing of one portion of fluid with the

other. Convection is possible only in a fluid medium and is directly linked with the transport

of medium itself.

In forced convection, fluid motion is principally produced by some superimposed velocity

field like a fan, blower or a pump, the energy transport is said due to forced convection.

APPARATUS:

The apparatus consists of a blower unit fitted with the test pipe. The test section is

surrounded by a Nichrome band heater. Four thermocouples are embedded on the test section

and two thermocouples are placed in the air stream at the entrance and exit of the test section

to measure the air temperature. Test pipe is connected to the delivery side of the blower along

with the orifice to measure flow of air through the pipe. Input to the heater is given through a

dimmer stat and measured by meters.

It is to be noted that only a part of the total heat supplied is utilized in heating the air.

A display indicator is provided to measure temperatures of pipe wall at various points in the

test section. Airflow is measured with the help of orifice meter and the water manometer

fitted on the board.



ASSEMBLY DIAGRAM OF FORCED CONVECTION APPARATUS:-

SPECIFICATIONS:-


1. Pipe diameter (Do) 33 mm. 01

2. Pipe diameter (Di) 28 mm 01

3. Length of test section (L) 570 mm. 07

4. Air Blower 1 35 No 01

5. Orifice Diameter (d) 14 mm 01

6. Band Heater

400mm with 100mm Equi- distant

4holes of 6mm ID-33mm, 400

watt,

01

7. Manometer U-tube, acrylic body, +/- 100mm, 01

8. Thermocouples RTD Type 06



PROCEDURE :-





control unit.







8. With the help of up key see the all 6 temperatures on control unit display.


10. Also start the blower by rotating knob H2. Set the desired blower speed.

11. After getting the proper steady state, all the observations are recorded. 12. The same

procedure is repeated for going to higher temperature by giving more power.




PRECAUTION:-

1. Keep the dimmer stat at zero position before switching ON the power supply.

2. Increase the Power gradually.

3. Do not stop the blower in between the testing period.

4. Do not disturb thermocouples while testing. Don‟t exceed 200 watts

5. Operate push buttons of control panel gently.

OBSERVATION:-

Outer diameter of the pipe (Do) = 33 mm

Inner diameter of the test pipe (D i) = 28 mm

Length of the test section (L) = 570 mm

Diameter of the orifice (d) = 14 mm

OBSERVATION TABLE:-

Sr Heater Temperature in c Manometer

No Power

reading of

(H1) T1oC T2

oC T3

oC T4

oC T5

oC T6

0C water

In watt h in meter



CALCULATION:-

Ao = Area of Cross Section Orifice in m2

π

Ao = -------------- x d2

4

· Q = Volume flow rate in m3 / sec

Q = Cd x Ao x √2 x g x h x (ρw / ρa)

Where,

Cd = Coefficient of discharge of orifice = 0.68

Ao = area of cross section of orifice in m2

ρw = Density of water = 1000 Kg/m3

ρa = density of air at ambient temp. = 1.03 Kg/m3

h= manometer reading in meter

ma = mass flow rate of air in Kg / sec

ma = Q x ρa

Where,

ρa = Density of air at Ambt. temp. = 1.03 Kg/m3

· ∆T = Temperature rise in air in 0C or

0K

∆T = (T6 – T 1)

Qa = Heat carried away by Air in kJ / sec or Watts

Qa = ma x Cp x ∆T

Where,



Cp = specific heat of air= 1.005 KJ / oK Kg

Ta = Average Temperature of Air in 0C

(T1 + T6)

Ta = ----------------

2

Ts = Average Surface Temperature in 0C

T2 + T3+ T4 +T5

Ts = -----------------------

4

As = Test Section Surface Area in m2

As = π x Di x L

Where,

Di = Inner diameter of the test pipe in meter

L = Length of the test section in meter

h = Heat Transfer Coefficient in W / m2k

Q

h = -------------------

A (Ts – Ta)

Ac = Cross Test Section Area in m2

π

Ac = ------- x Di2

4

V = Mean Velocity of Flow through tube in m / sec

Q

V = -----------

Ac



Re = Reynold’s Number

V Di

Re = ------------

Ʋ

Where,

Ʋ= Kinematic Viscosity at bulk mean Temp. i.e. (T1 + T6) in

m2/ s

Pr = Prandtle Number

Pr = 0.7 at Avg. Temperature

Nu = Nusselt Number

Nu = 0.023 x Re0.8

x Pr0.3

h = heat transfer coefficient calculated by using the correlations

Nu = 0.023 Re0.8

Pr0.3 ………… For Re >10000

Nu = 0.036 Re0.8

Pr0.3 ………… For Re >2300

h x D Nu

= ------------

K

Where,

K = thermal conductivity of air at avg. temp. in …………..w / m k



RESULT: The heat transfer coefficient in forced convection was found to be

……….w/m2K

CONCLUSION:


a. Concept: Heat Transfer in forced convection, Important correlations

b. Formulas used:

c. Diagrams:

i. Set-up

ii. Location of the thermocouples



EXPERIMENT NO: X

AIM: To determine the heat transfer coefficient for natural & forced convection from pin-

fin.

APPARATUS:

The apparatus consists of a pin-fin provided with an electric heater at one end. This assembly

is kept in a duct and provided with thermocouples along the length. A blower is fitted to this

duct to provide a constant supply of high velocity air. This blower is fitted in such a way that

its inlet is at the end of the duct. The heater coil is fitted with voltmeter and ammeter. An

orifice meter with a U-tube differential type manometer is fitted to the delivery pipe to

measure the velocity of the air.

THEORY:

In order to increase the rate of cooling or heating, the heat flow area is increased by

attachment of fins or solid materials to the wall of heat exchangers on that side which the

coefficient of heat transfer is small. These fins are widely used in engineering heat transfer

applications like transformer motors, IC engines, etc.. External surfaces in the form of rods

are known as pin fins or spines and one in the form of continuous plates are known as fins.

They may be located longitudinally or circumferentially. The cross sectional shape of

extended surface in a plane normal to the wall is known as the profile of spine or fin.

PROCEDURE:

1) Keep the dimmer-stat to the heater at minimum voltage position. Switch ON the electric

supply.

2) Adjust the dimmer-stat to supply a particular voltage to the heater. Maintain this constant

throughout the experiment.

3) Wait for steady state to be attained in natural convection mode.

4) Note down the readings in the observation table as given below for natural convection

mode.

5) Now turn the blower on.

6) Again, note down the readings in the observation table as given below for forced

convection mode.



OBSERVATIONS:

Density of manometric fluid= 1000 kg/m3.

Diameter of orifice= 15mm.

Diameter of pin= 12.7 mm.

Duct size= 10x15 cm2.

Coefficient of discharge= 0.64.

Diameter of delivery pipe= 28 mm.

Length of pipe= 40 cm.

OBSERVATION TABLE:

Plot the temperature gradient with the length of the pin.

CALCULATIONS:



RESULT:

Heat transfer coefficient by-

1. Natural convection:

a. Experimentally w/m2K.

b. Theoretically w/m2K.

2. Forced convection:

a. Experimentally w/m2K.

b. Theoretically w/m2K.

CONCLUSION:


a. Concept: Heat transfer through pin-fin

b. Derivation: Heat transfer through pin-fin (All 3 conditions)

c. Graph (s): Temperature gradient along the length of fin

d. Diagrams:

i. Set-up

ii. Location of thermocouples



EXPERIMENT NO: XI

AIM: To determine the following for a shell & tube heat exchangers:

I) LMTD

II) Effectiveness

III) Overall heat transfer coefficients

APPARATUS:

The apparatus consists of a shell & tube type heat exchanger (two pass-tube side). The tube

side is supplied with hot water while the cold side with cold water. Thermocouples are

provided at the entry & exit of both the water circuits. There is

provision of a measuring flask & a stop-watch to measure flow rate on either side. The water

is heated with the help of two geysers. Water is continuously supplied to

both the circuits.

THEORY:

A device used to transfer heat between fluids that are at different temperatures and separated

by a solid wall is termed a heat exchanger. Common uses for heat exchangers are found in

waste heat removal, air-conditioning, power production and chemical processing. Heat

exchangers may be constructed in different fashions.

Common designs incorporate concentric tubes, also known as double-pipe arrangement. This

consists of one pipe placed concentrically inside another of larger diameter. This design can

have parallel or counter flow configurations. This is largely used for sensible heating or

cooling process fluids where small heat transfer rates are required. Other designs incorporate

a shell-in-tube construction.

Shell and Tube Heat Exchangers

They are built of round tubes mounted in large cylindrical shells with the tube axis parallel to

the shell. They are widely used as oil coolers, power condensers, preheaters in power plants

and steam generators in nuclear power plants. Specific shell and tube designs differ by the

number of shell and tube passes. Baffles are commonly used in heat exchangers to increase

the convection coefficient.



Resistance to Heat Transfer

The ability to transfer heat from one fluid to another through some form of solid wall is

dependent on a variety of factors:

1. Surface area between the two fluids.

2. Rate of heat transfer through a dividing wall which separates the two fluids.

3. Rate of heat transfer from one layer to the next (if more than one layer is present) in

the dividing wall.

4. Rate of heat transfer from the hot fluid to the dividing wall.

5. Rate of heat transfer from the dividing wall to the cold fluid.

Surface Area

Increasing a common surface area between two fluids will increase overall heat transfer rate

between the fluids. This could be accomplished by incorporating more tubes in a shell and

tube heat exchanger.

Typical Values for Overall Heat Transfer „U‟ are-

1. Plate Heat Exchanger, liquid to liquid U range 1000›4000Wm-2K-1

2. Shell and tube, liquid inside and outside tubes U range 150›1200 Wm-2K-1

3. Spiral Heat Exchanger, liquid to liquid U range 700›2500Wm-2K-1

The general representation of a heat exchanger is as shown in the figure below.

U= Overall heat transfer coefficient Wm-2K-1

A= Heat Exchanger Surface Area m2

h1=Heat transfer coefficient-hot side (Wm-2K-1)

h2= Heat transfer coefficient-cold side (Wm-2K-1)

t=Temperature (oC)

θ=Temperature difference (oC)

Overall Heat Transfer Coefficient

For a hollow cylinder exposed to a convection environment on its inner and outer surfaces,

the electrical resistance analogy would appear as in the following figure, where, again TA and

TB are the two fluid temperatures. Note that the area for convection is not the same in both

fluids in this case, these areas depending on the inside tube diameter and the wall thickness.

In this case, the overall heat transfer would be expressed by



In accordance with the thermal network shown in the figure, the terms Ai and AO represent

the inside and outside surface areas of the inner tube. The overall heat transfer coefficient

may be based on either the inside or the outside area of the tube. Accordingly,

Fouling Factors

After a period of operation, the heat transfer surfaces for a heat exchanger may become

coated with various deposits present in the flow systems or the surfaces may become

corroded as a result of the interaction between the fluids and the material used for

construction of heat exchanger. In either event, this coating represents a additional resistance

to the heat flow, and thus results in decreased performance. The overall effect is usually

represented by a fouling factor or fouling resistance Rf, which must be included with other

resistances making up the overall heat transfer coefficient.

Logarithmic Mean Temperature Difference

For a double pipe heat exchanger with a hot and cold fluid flowing through it, we calculate

the heat transfer in the arrangement with a simple formula

Q=UAΔTm

Where, U=overall heat transfer coefficient

A=surface area for heat transfer consistent with definition of U

Tm=suitable mean temperature difference across the heat exchanger

LMTD is determined by following equation



Where the subscripts I and o stand for the inlet and outlet positions and h and c stand for the

hot and cold fluids.

The above equation is valid for parallel flow heat exchangers. For counter flow heat

exchangers, the inlet and outlet temperature values for the cold fluid get reversed in the

equation.

LMTD Correction Factor For Countercurrent Shell and Tube Heat Exchanger Log Mean

Temperature difference (LMTD) is defined for countercurrent heat transfer. A double pipe

heat exchanger provides true countercurrent flow. Other equipment geometries will depart

from countercurrent flow. When this happens, the effective MTD is lower than the LMTD. A

LMTD correction factor has been defined to account for this circumstance. The LMTD

correction factor (F) is defined and used as follows :

F= (Effective MTD) / (LMTD)

Heat Exchange Duty Q = (Overall Coefficient U)*(Area A)*(F)*(LMTD)

F is related to heat transfer geometry and temperatures. This section contains formulas

for countercurrent multi-tube pass shell and tube heat exchangers.

Temperatures And LMTD Definition

T1= Shell IN t1= Tube IN

T2= Shell OUT t2= Tube OUT

The definition of LMTD is the same for all heat exchangers. Shell and tube designations are

immaterial in this definition.

Auxiliary Variables

…………………….(2)



……………………(3)

The LMTD correction factor F is obtained from the relevant charts for the values of P & R.

Heat Exchanger Effectiveness

One way of measuring a heat exchanger‟s performance is to calculate its effectiveness. The

heat exchanger effectiveness is defined as the ratio of the actual heat transfer to the heat

transfer attainable in an infinitely long counter-flow exchanger. So, by defining the

effectiveness in this way, we have a much simpler expression for our effectiveness:

If, for any given exchanger, the effectiveness is known, we can write the heat transfer

equation as:

q= εCmin(Thin-Tcin)

PROCEDURE:

1) Start the tube-side water-flow. After ensuring that the water is running, start one of the

geysers.

2) Start the cold water supply after a while. Keep the flow-rate at the cold water side higher

than the hot water side. Maintain both the flows at a moderate level.

3) Wait for steady state to be attained.

4) Note down the reading in the observation table as given below.



OBSERVATIONS:

1. Shell

a. Inner Diameter = 208 mm

b. Thickness = 6 mm

c. Material = M. S.

2. Tubes

a. O.D. = 19 mm

b. I.D. = 17 mm

c. Material = Copper

d. Pitch = 30 mm

e. No. of tubes = 32

f. Length = 500 mm

OBSERVATION TABLE:

CALCULATIONS:



RESULT:

1) LMTD= °C

2) Ui actual= W/m2K

3) Ui theoretical= W/m2K

4) Uo actual= W/m2K

5) Uo theoretical= W/m2K

6) ε =

CONCLUSION:


a. Concept: Shell & Tube Heat Exchanger, Correlations

b. Graph (s): None

c. Diagrams:

d. Set-up



EXPERIMENT NO: XII

AIM: To study film wise and drop wise condensation and to determine the overall heat

transfer coefficient.

APPARATUS:

The equipment consists of a glass cylinder in which steam generation takes place. The lower

portion of glass cylinder houses suitable electric heater for steam generation. A cover is

provided for the container for filling the water. The glass cylinder houses to water cooled

copper condensers, one of which is chromium plated to promote drop wise condensation and

the other is in its natural state to give film wise condensation. Separate connections of two

condensers for passing water are provided. One rotameter with two valves and appropriate

piping can be used for measurement of water flow rate in one of the condensers under test. A

digital temperature indicator provided has multipoint connections, which measures

temperatures of steam, two condensers, water inlet and outlet temperature of condenser water

flow.

SPECIFICATIONS:-

1. CONDENSERS:

A) One chromium plated for drop wise condensation and one having natural Finish

for film wise condensation. Both are identical in construction.

B) Dimensions:

1. Outer Diameter (Do) = 19 mm

2. Inner Diameter (Di) = 18 mm

3. Length (L) = 150mm

C) Fabricated from copper with reverse flow in concentric tubes. Fitted with

Thermo couples for surface temperature measurement.



2. MAIN UNIT:

M.S. fabricated construction comprising of test section and steam generation section.

Test section is provided with glass cylinder for visualization of the process.

3. HEATING ELEMENTS:

Suitable water heater operated through 10 Amps. Switch. Heating element

(2KW) is protected from over-heating by thermostat and power can be controlled by

solid-state thyristor drive.

4. INSTRUMENTATION:

a) TEMPERATURE INDICATOR: Digital, 8-channel temperature indicator with

cold junction compensation using RTD thermocouples 0-200 C.

b) ROTAMETER: Eureka Make 10 –100 LPH capacity for mea suring water

flow rate.

c) Thermostat control for heater

THEORY

In all applications, the steam must be condensed as it transfers heat to a cooling medium, e.g.

cold water in the condensers of a generating station, hot water in a heating calorifier, sugar

solution in a sugar refinery etc. During condensation very high heat fluxes are possible and

provided the heat can be quickly transferred from the condensing surface to the cooling

medium, heat exchangers using steam can be compact and effective.

Steam may condense on to a surface in two distinct modes, known as “Film wise” and “Drop

wise”. For the same temperatu re difference between the steam and the surface, drop wise

condensation is much more effective than film wise. In practical plants it rarely occurs for

prolonged periods.

FILM WISE CONDENSATION:-

Unless specially treated, most materials are wettable and as condensation occur a film

condensate spreads over the surface. The thickness of film depends upon a number of factors,

e.g. the rate of condensation, the viscosity of the condensate and whether the surface is

vertical or horizontal, etc.



Fresh vapour condenses on to the outside of the film and heat is transferred by conduction

through the film to the metal surface beneath. As the film thickens, it flows downwards and

drips from the low points leaving the film intact and the film of liquid is a barrier to the

transfer of heat and its resistance accounts for most of the difference between the

effectiveness of film wise and drop wise condensation.

DROP WISE COMDENSATION

By specially treating the condensing surface, the contact angle can be changed and the surface

becomes „non-wettable‟. As the stea m condenses, a large number of generally spherical

beads cover the surface. As condensation proceeds, the beads become larger, coalesce, and

then stricken downwards over the surface. The moving bead gathers on the static beads along

its downward in its trail. The „bare‟ surface offers very little re sistance to the transfer of heat

and very high heat fluxes are therefore possible.

Unfortunately, due to the nature of the materials used in the construction of condensing heat

exchangers, film-wise condensation is normal. (Although many bare metal surfaces are „non-

wettable‟ this is not true of the oxide film, which quickly covers the bare material).

PROCEDURE:

1) Fill up water in main unit by hand valve, 1 cm above the heater.

FIGURE NO. – 1



2) After filling the water close the hand valve. Start water flow through one of the

condensers, which is to be tested and note down water flow rate in rotameter.

Ensure that during measurement, water is flowing only through the condenser

under test and second valve is closed.

3) Switch ON the heater and set the thermostat temperature.

4) Steam generation will start slowly in the steam generator of the unit and as the

steam rises to test section, gets condensed and falls down in the cylinder.

5) Depending upon type of condenser under test drop wise or film wise

condensation can be visualized.

6) If water flow rate is low then steam in the chamber will condensate less. If the

water flow rate is changed then condensation will occur at more of less

atmospheric pressure.

7) Observations like temperature, water flow rate, are noted down in the observation

table at the end of each set.



PRECAUTIONS :-

• Do not start heater supply unless water is filled in the test unit.

• Operate gently the selector switch of temperature indicator to read various

temperatures.

PLATED CONDENSER (DROP WISE CONDENSATION)

OBSERVATION TABLE:

Sr. Water inlet Water outlet to Surface temp. of Water flow Steam

no. to plated condenser plated rate temp.

condenser T2 in (o C) condenser Mw in T6 in (

o C)

T1 in ( o C) T3 in (

o C) (LPH)



PLAIN CONDENSER (FILM WISE CONDENSATION):-

OBSERVATION TABLE:

Sr. Water inlet Water outlet to Surface temp. of Water flow Steam

no. to plain condenser plain condenser rate temp.

condenser T4 in (o C) T5 in (

o C) Mw in T6 in (

o C)

T1 in ( o C) (LPH)

CALCULATION:-

We will first calculate the valves of the inside and outside heat transfer coefficients

and then the valves of the overall heat transfer coefficient.

A) Reynold‟s number for water flowing inside tube condenser.

The properties of water at average bulk mean temperature at

(Tsat + Ts)

Tf = --------------- from table

2

Density p = 1 = Kg / m

3

vf

v = m2 / sec.

Thermal conductivity K = W /m k

(Prandtl Number) Pr =

Tsat = Steam temperature . (T6 or T7)



Ts = Surface temperature of condenser (T3 or T5)

4 x m

Reynolds Number Re = ……………………………….

Π x D x ρ x v

Here

m = Mass flow rate of water flowing inside condenser

D = inside diameter of condenser tube = 0.018 mtr

• Now Calculate Nussult Number Nu

= 0.023 (R) 0.8

(Pr)0.4

• Heat transfer Rate inside the condenser tube

hi = Nu x v/Di

where, Di = Tube inside diameter in meter =0.018 mtr.

• The valve of ho (tube outside heat transfer rate)will be calculated from

following equation

Where,

g = 9.81 m /s2

Properties At Saturated Liquid At Tf =0 K



Tf = (Tsat + Ts) / 2

where,

Tsat. = steam temperature . (T6 or T7)

Ts = Condenser surface temperature (T3 or T5)

ρl = Kg/m3 (from chart)

µl = N s/ m2 (from chart)

Kl = W/ m k (From Chart)

Properties of saturated vapour at TS = steam temperature 0C

ρv = Kg/m3 (From chart)

hfg = kJ/kg ( From Chart)

E) The correctness of assumption of laminar flow of condensate can be verified

by calculating Reynolds number,

R =

4 ho x L (Tsat - Ts )

µ l x h fg

where,

L= Length of condenser tube = 0.150 mtr.

m. =

Ө kg / sec

hfg



H) Overall Heat transfer coefficient

RESULT:

Type Ui (w/m2K) Uo (w/m2K)

Filmwise Condensation

Dropwise Condensation

CONCLUSION:


a. Concept: Phenomena governing dropwise & filmwise condensation.

b. Graph (s): None

c. Diagrams:

i. Set-up

ii. Condensation on Drop & Film type tubes

Prepared by :- Mr. P.P.Khairnar...Determination of heat transfer coefficient in forced convection 67...

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Transcript of Prepared by :- Mr. P.P.Khairnar...Determination of heat transfer coefficient in forced convection 67...