Table of Content

______________________________________________________

Summary/Abstract 2

Introduction and Objective 2

Theory 3

Description of Equipment Apparatus 4

Procedure 4

Data, Observations and Results 6

Analysis and Discussions 8

Discussions 13

Conclusions 15

References 15

Appendix 16

1

Abstract____________________________________________________

An experiment was conducted to perform and demonstrate free and force

convection heat transfer using different type extended surface plate. This following

experiment outlines the proper procedure for determining these temperature distribution

along an extended surface and consequently helps the student to demonstrate them

graphically by doing the analysis based on the all the data and readings obtained.

Heat transfer by convection between a surface and the surrounding fluid can be

increased, by attaching thin strips of metal fins to the surface. When heat transfer takes

place by convection from both interior and exterior surfaces of a tube or a plate, generally

fins are used on the surfaces where the heat transfer coefficients are low.

Heat transfer by simultaneous conduction and convection, whether free or forced,

forms the basis of most industrial heat exchangers and related equipment. The

measurement and prediction of heat transfer coefficients for such circumstances is

achieved in the free and forced convection heat transfer apparatus by studying the

temperature profiles and heat flux in an air duct with associated flat and extended transfer

surfaces.

Introduction and Objectives______________________________________________________

In this particular laboratory experiment, students are required to demonstrate the

use of extended surfaces to improve heat transfer from a surface and to demonstrate

convection heat transfer by using different type of extended surface. Besides that, the

temperature distribution along an extended surface is also to be determined at the end of

the experiment.

In addition to that, this experiment is also useful in such a way that it helps to

provide some exposure to the students so that they are able to interpret the obtained test

data and at the same time are able to apply the theory they have learned in class.

2

Theory______________________________________________________

A heated surface dissipates heat to the surrounding fluid primarily through a

process called convection. Heat is also dissipated by conduction and radiation, however

these effects are not considered in this experiment. Air in contact with the hot surface is

heated by the surface and rises due to reduction in density. The heated air is replaced by

cooler air, which is in turn heated by the surface, and rises. This process is called free

convection.

In free convection small movements of air generated by this heat limit the heat

transfer rate from the surface. Therefore more heat is transfer if the velocity is increase

over the heated surface. This process of assisting the movement of air over the heated

surface is called forced convection. A heated surface experiencing forced convection will

have a lower surface temperature than that of the same surface in free convection, for the

same power input.

Convection heat transfer from an object can be improved by increasing the

surface area in contact with the air. In practical it may be difficult to increase the size of

the body to suit. In these circumstances the surface area in contact with the air may be

increased by adding fins or pins normal to the surface. These features are called extended

surfaces. A typical example is the use of fins on the cylinder and head on an air-cooled

petrol engine. The effect of extended surfaces can be demonstrated by comparing finned

and pinned surfaces with a flat under the same conditions of power input and airflow.

3

Description of Experimental Apparatus______________________________________________________

Figure 1: Sketch diagram of Convention Heat Transfer Rig

Procedure

______________________________________________________

Experiment 1

1. The fan assembly is removed from the top of the duct

2. The finned heat exchanger is placed into the test duct.

3. The power cable and temperature sensor cable is connected to the heat

exchanger.

4. The ambient air temperature is recorded.

4

5. To accelerate the heating time, the heater power control is set to 70 watts (90

watts for pinned heat exchanger). Once the plate temperature reaches about

550C (700C for pinned heat exchanger), the heater power is set to 50 watts.

6. After a sufficient time is allowed for the heated plate to achieve steady state

condition, the heated temperature plate (tH) is recorded.

7. The distance of the access holes on the pinned and finned heat exchanger is

measured from the black plate.

8. The temperature probe is inserted into the duct through the hole nearest to the

heated plate, T1, ensuring that the tip of the probe is in contact with the pin.

9. The temperature of the next two holes on the fins. (T2 and T3) are measured.

Experiment 2

1. The fan assembly is placed on to the top of duct.

2. Insert the temperature probe is inserted into the duct through the hole nearest

the heated plate, T1, ensuring that the tip of the probe is in contact with the pin

3. The fan speed control is set to low speed.

4. After the temperature has achieve steady state condition (~3 minutes), the

temperature probe reading is noted

5. This procedure is repeated at medium and maximum speed

6. The finned heat exchanger is removed and is then replaced with the pinned

heat exchanger.

5

Data and Observations

______________________________________________________

Input Power = 70 W

Ambient air temperature (tA) = 28.6 0C

TEMPERATURE LOCATION T (0C) FINNED PLATE PINNED PLATE(T- tA) (T-tA)

HEATED PLATE TEMP, tH 50.0 21.4 50.0 21.4T1 53.7 25.1 50.8 22.2T2 52.9 24.3 57.9 29.3T3 52.1 23.5 60.1 31.5

TABLE 2.1: FREE-CONVECTION RESULTS

FAN SPEEDFINNED HEAT EXCHANGER PINNED HEAT EXCHANGER

T1 T2 T3 T1 T2 T3

0 30.9 38.5 43.2 50.8 57.9 50.8LOW 32.0 38.4 45.3 57.2 53.3 41.9

MEDIUM 31.6 33.6 38.6 48.1 42.6 35.5HIGH 31.6 32.6 36.4 43.5 43.6 32.8

TABLE 2.2 EXTENDED SURFACE TEMPERATURE AT DIFFERENCE FAN

SPEED

FAN SPEEDFINNED HEAT EXCHANGER PINNED HEAT EXCHANGERHEATER TEMP,

TH(TH-TA)

HEATER TEMP, TH

(TH-TA)

0 31.8 3.2 50.0 21.4LOW 34.6 6.0 60.6 32.0

MEDIUM 37.0 8.4 58.2 29.6HIGH 38.1 9.5 58.3 29.7

TABLE 2.3 HEATER TEMPERATURE AT DIFFERENCE FAN SPEED

FAN SPEEDRATE OF HEAT TRANSFER, q (W)

FINNED HEAT EXCHANGER PINNED HEAT EXCHANGER0 0.896 2.447

LOW 1.680 3.659MEDIUM 2.352 3.385

HIGH 2.660 3.396

6

TABLE 2.4 RATE OF HEAT TRANSFER AT DIFFERENCE FAN SPEED

Sample of Calculation

q=T H−T ARCond

whereRCond=1hA

hambient=40Wm2/K

For finned heat exchanger:

A=l×w=0.10×0.07=7×10−3m2

RCond=1hA

= 1

40×7×10−3=3.571

q= 6.03.571

=1.680W

For pinned heat exchanger:

A=2πrh=2π ×0. 0065×0.07=2.8588×10−3m2

RCond=1hA

= 1

40×2.8588×10−3=8.7448

q= 32 .08.7448

=3.659W

Analysis and Results______________________________________________________

All the required graphs are shown on the following pages:

7

Graph 1: Graph of Temperature (T-tA), versus distance for each plate

Graph 2: Graph of Temperature (tH-tA) against fan speed for each of the plates

Graph 3: Graph of extended surface temperature against distance from the black

plate for both heat exchangers at various fan speed

Graph 4: Graph of heat transfer rate against fan speed for both exchangers

8

heated plate temp. t1 T2 T30

5

10

15

20

25

30

35

Graph of Temperature against Distance

Finned platePinned plate

Distance

Tem

pera

ture

Graph 1: Graph of Temperature (T-tA), versus distance for each plate

9

0 LOW MEDIUM HIGH0

5

10

15

20

25

30

35

Graph of Temperature against Fan Speed

Finned heat exchangerPinned heat exchanger

Fan Speed

Tem

pera

ture

Graph 2: Graph of Temperature (tH-tA) against fan speed for each of the plates

10

0 LOW MEDIUM HIGH0

10

20

30

40

50

60

70

Graph of Extended Surface against Temperature.

T1 (finned)T2 (finned)T3 (finned)T1 (pinned)T2(pinned)T3(pinned)

Extendard Surface

Tem

pera

ture

Graph 3: Graph of extended surface temperature against distance from the black plate for both heat exchangers at various fan speed

11

0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.5

1

1.5

2

2.5

3

3.5

4

Graph of Heat Transfer Rate against Fan Speed

FINNED HEAT EXCHANGERPINNED HEAT EXCHANGER

Fan Speed

Rate

of H

eat T

rans

fer

0 LOW MEDIUM HIGH

Graph 4: Graph of heat transfer rate against fan speed for both exchangers

12

Discussions______________________________________________________

Data and Graph Analysis

From Data, Observation and Results section of this report, we had plotted the

graph where we can see the relationship between the temperature and distance for each

plate. From Graph 1, for a finned plate, initially when we heated the plate the temperature

is increase. After a few moment, the temperature drop linearly until T3 distance. On the

other hand, for the pinned plate the temperature increases constantly until T3.

As for Graph 2, it can clearly be shown we can see that pinned heat exchanger

have high temperature difference compared to finned heat exchanger. This indicates that,

pinned plate release less heat compare to finned plate.

Besides that, we can come up with a few relationships by observing the pattern of

the curves illustrates by Graph 3. For both pinned and finned heat exchanger, at position

T2 and T3, the temperature seems to be decreasing as the speed of the fan increases. In

contrary, at position T1, as the speed of the fan went up from 0 to the highest speed, the

temperature for finned heat exchanger increases while for pinned heat exchanger, the

temperature decreases.

Comments on the correlation between total surface area of the heat exchanger and the

temperature achieved and which of the extended surfaces has greater surface area.

For finned plate, it is square in shape. The heat release from the plate is high, and

thus the temperature stored is low. On the other hand, for pinned plate, it is cylinder in

shape. The heat release from the plate is low compared to finned plate. Thus, the

temperature stored in the pinned plate is high.

13

For a heat exchanger with 100 % efficiency, the whole of the extended surface should be

at the same temperature as the backplane, why this is not achievable in the experiment?

According to thermodynamic law of conduction of heat transfer, total amount of

energy transferred will not be 100 % efficiency because they will be have some minor

loses during the heat conduction. Technically, when the heat is transferred from one

medium to another, there will be some friction that will cause some significant heat

losses.

Which extended surface have higher heat transfer rate? Why?

Pinned plate have high heat transfer rate. It is because it is cylinder in shape. The

cylinder shape basically made the pinned plate released heat in slow rate.

14

Conclusions

______________________________________________________

From this experiment, we can see that the use of extended surfaces enhance heat

transfer from a surface. It is found that finned extended surface releases heat faster than

pinned extended surface due to higher surface area.

In theory, higher flow velocity would encourage convection heat transfer. This

theory is clearly illustrated in Graph 4 where rate of heat transfer increases as the fan

speed increases.

Generally, the temperature decreases as we measure from position 1 to 3. This

temperature distribution pattern is depicted in Graph 3. However, discrepancy may occur

due to some errors. For instance, the inconsistency of the fan speed that caused by worn

out equipment may greatly affect the entire experiment.

In conclusion, this experiment can be consider as successful since all the

objectives have been covered.

References

______________________________________________________

Instruction manual from the Heat Transfer & Applied Thermodynamics Lab

2012, Convection Heat Transfer,

http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

2012, Wikipedia, Convection

http://en.wikipedia.org/wiki/Convection

2012, Wikipedia, Heat Exchanger

http://en.wikipedia.org/wiki/Heat_exchanger

15

2012, The theory behind heat transfer

http://www.distributionchalinox.com/produits/alfa-laval/echangeurs/heat-transfer-

brochure.pdf

Incropera, DeWitt, Bergmann, Lavine, Fundamentals of Heat and Mass Transfer, 7 th

Edition, Wiley Asia Student Edition

Yunus A. Cengel, Michael A. Boles, Thermodynamics An Engineering Approach, 7th

Edition, Mc Graw Hill

Appendix______________________________________________________

Finned Extended Surface

16