MEHB 471

 

HEAT TRANSFER LAB

Author’s Name : ****

Student Number: ****

Lab No. : 2

Date experiment performed :  2000

Due Date:2000

Group Number : ****

Lecturer : ****

 

 

Statement of Purpose/ Objective:

The objective of the experiment is to :-

·        Demonstrate the relationship between power input and surface temperature on free convection

·        Demonstrate the relationship between power input and surface temperature in forced convection

·        Determine the temperature distribution along an extended surface

·        Demonstrate different type of extended surface heat transfer

·        See the effect of different flow velocity on the convection heat transfer

 

Data, Observation and Results:

Experiment 1: Free Convection

Input power : 50.0 Watt

Ambient Air Temperature (t­A) = 24.1oC

 

Distance of probe from the back plate;

 

T1 = 0.01m

T2 = 0.036m

T3 = 0.062m

 

Table 2.1 Free-convection results

 

Temperature Location T

Finned Plate

Pinned Plate

 

 

( T – tA)

 

( T – tA)

Heated Plate temp. (tH)oC

60

35.9

79.5

55.4

T1

57.1

33

70.1

46

T2

54.7

30.6

67.8

43.7

T3

53.4

29.3

66.7

42.6

Air velocity m/s

0.21

 

0.27

 

 

Experiment 2: Force Convection

Table 2.2 Extended surfaces temperature at different velocity

 

Air Velocity m/s

Finned Heat Exchanger

Pinned Heat Exchanger

T1

T2

T3

T1

T2

T3

0

60.5

59.9

58

77.6

76.1

75.2

1.0

58.1

55

54.8

58

52

49.1

1.5

49.8

45.9

42.7

50.7

50.2

44.4

2.0

43.4

39.7

37.1

45.4

41.5

40.4

 

Table 2.3 Heater temperature at different velocity

 

Air Velocity m/s

Finned Heat Exchanger

Pinned Heat Exchanger

Heater temp.(tH­)

(tH – tA)

Heater temp. (tH­)

(tH – tA)

0

64

39.9

85.5

61.4

1.0

62.4

38.3

67.7

43.6

1.5

50.2

26.1

62.0

37.9

2.0

49.6

25.5

53.4

29.3

Pinned Heat Exchanger Total Surface Area, APinned = 173p3[(1.3310-2)2/4]

                                                                                   = 2.26310-3 m3

 

Finned Heat Exchanger Total Surface Area, AFinned = 936.8310-2310310-2

                                                                                  = 0.0612 m3

Sample calculation:

Analysis and Discussion:

From the graph plotted of the temperature of fins versus distance from the back plate for the free convection, it is found that the Pinned plate has a relatively high temperature compared to the Finned plate. In general, the two plates experienced a decreased in temperature along the extended surfaces.

 

The second graph plotted is of the velocity versus temperature (tH-tA) for force convection for each plate. The graph shows that the Pinned heat exchanger still has a relatively high temperature over the Finned heat exchanger. It also reveals that the temperature decrease along with the increase in velocity of air in the rig.

 

For the  graph of extended surfaces temperature versus distance from the back plate for both heat exchangers at various air velocity, it is found that the Pinned heat exchanger has a relatively high temperature compared to the Finned heat exchanger and the temperature is decreasing along the extended surface with the minimum temperature achieved at the farthest point taken on the extended surfaces, for both the Pinned and Finned heat exchangers.

 

The Finned heat exchanger has a total surface area of , AFinned = 0.0612 m3 while the Pinned heat exchanger has a total surface area of, APinned = 2.26310-3 m3. The difference in surface area exposed to the convection process explains the variation of temperature behavior on the both extended surfaces. The Pinned heat exchanger has a total of surface are about 0.00226 m2 compared to 0.0612 m2 of Finned heat exchanger surface area. This is why the Pinned heat exchanger has a relatively high temperature compared to the Finned heat exchanger. A greater surface area exposed to the convection process enabled a much more heat transferred to the air.

 

The three graphs also explain on the differences of heat transfer process for the free and force convection. The free convection depends on the natural process of the air circulation to transfer heat which is not as efficient as the force convection. In force convection, the hot air is quickly removed by the air to give way for the subsequent heat transfer process. The velocity of the air circulation provides a heat transfer rate which is greater than the free convection.

                                                                              

Conclusion:

As a conclusion, the same power input to the both heat exchangers, Finned and Pinned, has resulted in a temperature distribution along the extended surface by means of the conduction process through the material for the free and force convection. The temperature decrease along the extended surfaces right to the farthest position from the heater plate.

 

The determination of the different type of extended surfaces heat transfer is a succes and it can be concluded that the Finned type of extended surface achieved a higher heat transfer than the Pinned type of extended surface. This is due to the total surface area exposed to the air is greater for the Finned than the Pinned extended surface.

 

The different flow velocity certainly improved the heat transfer rate for both Pinned and Finned extended surfaces. This type of process is called a force convection. In a free convection, the air circulation happened naturally as the hot air, which is lighter is replaced by the cool air. This process can be improved by introducing a fan that will increase the air flow rate thus will speed up the heat transfer process.