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UNIVERSITI TEKNOLOG I MARA

FINAL EXAMINATION

COURSE

COURSE CODE

EXAMINATION

TIME

THERMAL ENGINEERING

MEC551

JANUARY 2012

3 HOURS

INSTRUCTIONS TO CAN DIDATES

1

2.

This question paper consists of two (2) parts : PART A (5 Questions)

PART B (3 Questions)

Answer ALL questions from PART A and two (2) questions from PART B in the Answer

Booklet. Start each answer on a new page.

3. Do not bring any material into the exam ination room unless permission is given by the

invigilator.

4.

Please check to make sure that this examination pack consists of :

i) the Que stion Paper

) an Ans wer Booklet - provided by the Faculty

i) a Property Tables Booklet - p rovided by the Faculty

DO NOT TURNT ISP GEUNTILYOU RE TOLD TO DO SO

This examination paper consists of

7

printed pages

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PART A

QUESTION 1

w

A flat plate of thickness 0.8 cm , area o f

A

- ISO cm

2

a n c

j thermal conductivity m is

subjected to constant heating at a rate of Q=

8 0 0

W

a

t its bottom surface. When steady

state operation is reached the upper surface temperatures of the plate is measured to be

80 C.

x

A = 1 5 0 cm

2

T = 80 C

T

1 A ' A

T

0.8 cm

For steady one dime nsional conduction heat transfe rthroug h the plate,

a) calculate the heat flux q

Q

(W/m

2

) at the lower surface,

b) express the differential equation and the boundary conditions for steady one

dimensional heat conduction through the plate, and

c) obtain the variation of temperature on the plate and calculate the temperature at the

bottom of the surface.

(10 marks)

QUESTION 2

a) W hat is the physical significance of the Nusselt Num ber? How it is defined?

(3 marks)

b) An engine oil at 80 C flows over a 10-m long flat plate who se temp erature is 30 C

with a velocity of 2.5m/s. Determine the rate of heat transfer over the entire plate per

unit width. (kW /m)

Given : Prop erties of the eng ine oil at T

f

= 55C :

density., p = 867kg/m

3

kinem atic viscosity, = 7.045X1Q

-5

m

2

s

conductivity,k = 0.1414W/mC

Prandlt number, P r= 1SS1

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(7 marks)

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QUESTION

3

a) Sketch and define the tem perature profiles for parallel flow and counter flow in a

double-pipe heat exchanger.

(4 m arks)

b) 0.8 kg/s of wa ter is heated from 35 to 90 C with oil flow of 0.9 kg/s in a counter-flow

heat exchanger. The specific heat of oil is 2.0 kJ/kg.K. The oil enters the heat

exchang er at a temp erature of 180 C and leaves at 110C. The O verall heat transfer

coefficient is 450 W/m

2

.K. Calculate the area of heat exchanger using LMTD method.

Given: Specific heat of water,

Cw

= 4180 J/kg.K.

(6 marks)

QUESTION 4

An ideal vapor compression cycle uses R134a as a refrigerant and has a flow rate of

120 kg/min. The pressure in the evaporator of the refrigerator is 0.14 Mpa and the pressure

in the condenser is 0.8 Mpa. Draw a T-s diagram and if the isentropic efficiency of the

compressor is 100 percent, find:

a) the refrigerating effect (kW ),

b) the work input (kW), and

c) the COPret of the system .

(10 marks)

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QUESTION 5

Liquid propane (C

3

H

8

) at 25 C is burned in a space he ater at a rate of 0.05 kg /min with the

stoichiometric amount of air. The air enters this space heater steadily at 1 atm from the

comp ressor at 22 7 C. The c omb ustion products leave at 1 atm and 72 7 C but only 98

percent of the carbon burns to C0

2

and the remaining 2 % burns to form CO but all the

hydrogen in the fuels bum to H

2

0 .

a) W rite a balanced com bustion equation.

b) Determine:

i) the air fue l ratio (kg air/kg fue l), and

ii) the ma ss flow rate of air (kg/min).

What are the main factors contribute to the incomplete combustion process? Which

substance is the m ain product of incomplete com bustion, CO or OH ? , and explain.

(10 marks)

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PA R T B

QUESTION 1

Figure Q1 il lustrates a combustion system using propane as fuel and supplied with 40 % of

excess air to generate energy for a feed water heating. Both reactants are assumed to be at

the standard reference state. Due to non-uniform mixing, 7 % of the carbon content of the

fuel reacts poorly with oxygen and forms carbon m onoxide.

The Feed water heating is achieved by using a single tube, 2-shell and 4-pass heat

exchanger. T he feed w ater supply is at 50 C and 2 8 kg/min feed rate, and exits in a heated

state of 250C. The hot gas from the combustion chamber reaches the heat exchanger at

700 C, slightly less than the combustion product temperature due to internal cooling

mechanisms, and exits at 300 C. The tube is thin-walled, 12 cm in diameter, with a heat

transfer coefficient of 300 W/m

2

.K. At the shell side, the heat transfer coefficient is 60

W /m

2

.K.

For this particular system:

a) Calculate the fue l feed rate in (L/min) if the fuel density is 0.5 kg/liter,

b) Determine the thermal power of the combustion process (MW ), and

c) If the specific heat at constant pressure of the feed wa ter is 49 00 J/kg.K, determine the

minimum length of the heat exchanger based on the length of the tube (m).

Air

25C & atm

5 kg/min

O

Combustion system

Flue Gas

300C

Burner

Combustion

products

827C

Heat

Exchanger

700C

6

Propane

25C &

atm

FW exit 250C FW inlet 50C

Figure Q1

(25 marks)

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QUESTION 2

In a manufacturing process, a transparent tinted is being bonded to a composite substrate

as shown in Figure Q2. To cure the bond at temperature T

u

the respective substrate has

been heated, which give the uniform temperature of

T

=

79 C w hile the free surface of the

tinted (T

s

= 4 0 C) is exp osed to air at

7,

and a co nvection heat transfer coefficient /?.For the

calculated

T

fl

|

m

,

the properties of the air at atm are as follows;

DENSITY

p[kg/m

3

]

1.092

THERMAL

CONDUCTIVITY

K[W/(m.C)]

0.02735

DYNAMIC

VICOSITY

u[kg/(m.s)]

1.963X10

5

KINEMATIC

VISCOSITY u

[m

2

/s]

1.798X10

5

PRANDTL

NUMBER

Pr

0.7228

Under condition of steady state and one dimensional heat transfer, calculate:

a) the rate of heat transfer, Q (W),

b) the interface tem pera ture,

T

(C),

c) the air tem perature , T . (C),

d) if the flow is assum ed to be comp letely turbulent over the entire plate, calculate the

approximate total heat transfer (W), and

e) give your prediction for the rate of heat transfer if the velocity of the air is increased

about 45 %.

V.

=

m/s

^ :

0.5m

1.2m

y

p r-

t

jr.

___

jc%TOtfreVrr

B

/>

2 1m 7 1 1m

To

Figure Q2

> -

Given:

Nu = 0.664Re

5

Pr

1/3

(for laminar flow)

Nu = 0.037Re

08

Pr

1/3

(for turbulent flow)

Nu = [0.037Re

08

-871] Pr

1/3

(for mixed flow)

K

A

=27 1W/m.c

K

B

=

11 5

W/m.c

K

ti

nted=0 4W/m.c

(25 m arks)

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QUESTION 3

An autom otive air-cond itioner is used to treat air cabin at 1 atm , 30 C, and 80 percent

relative humidity at a rate of 10 m

3

/min, by passing the air through the cooling system which

consist of cooling co ils. The cooling process produces water in the condensate at 14 C. The

air leaves the cooling coils and re-enters the cabin as saturated condition. The cooling

system uses R-134a as the working fluid and applies an ideal vapour-compression

refrigeration cycle, operates between saturated pressure of 1.4 bar and 8 bar. The

refrigerant enters the compressor in saturated phase at a rate of 16.9 kg/s and it is

isentropically comp ressed.

For the automotive air-conditioner system described above, sketch the schematic of the

complete system and calculate:

a) the cooling load of the cooling coil (kW ),

b) the exit temp erature of the air cabin (C),

c) the rate of condensate removal (kg /s), and

d) the required work input to the system (kW).

(25 marks)

END OF QUESTION PAPER

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