Theory

Convection means the transfer of heat where the warmer air or liquid that have higher speed and less density moving upward while the cooler air that have less speed and high density moving downward. For example, if we look at a lighted up candle. If we place our hand just above the candle, for sure your hand will get hot but if you place your hand just side to the flame, you just feel less hot compared to the above. We can refer to the picture 1.

Figure 1

That’s all about convection. However, in this experiment, we are study about forced convection. Forced convection occurs when the flow of gas or liquid is circulated by pumps or fans.In this experiment, we are studying about adding pins or fins to the surface that will increase the rate of convection of heat. Heat flow by convection(Qͦ) is calculated by using formula 1.

Q̽�=hA (Tsurface−Tsurruonding)The effect of surfaces can be demonstrated by comparing pinned and finned surfaces with a flat plate under the same condition and flow.

Figure 2: pinned surfaces

Figure 3: finned surfaces

This experiment conduct by placing the heated surface in a test duct. The surfaces was connected to a heater power control that supply heat to heat the surface. In the test duct, it was a fan that control the speed of air in test duct. Figure 4 shows how this experiment will be conducted.

Figure 4

Apparatus

Procedure

a. The heat exchanger was placed into the test duct and ambient temperature was recorded(T͚)

Figure 4: heat exchanger

b. The heater power control was set up to 270W. The temperature was allowed to rise to 80°c and then adjusted back the heater power control to 20W.

Figure 5: heater power control

c. Stopwatch started since the temperature was 80°c and wait for 7 minutes before recording the heated plate surface temperature(Ts)

d. Step (b) repeated. The fan speed control was set up to give a reading of 1.5m/s on the thermal anemometer. The condition was allowed for 7 minutes to stabilize the reading before recording again the Ts.

e. Step (b) and (d) was repeated but the fan speed is adjusted to get 2.5m/s. the Ts was recorded for that speed.

RESULT

Ambient air temperature (T∞ ) = 23.5 C ̊�

Power input ( Q ) = 270 W

TABLE A FINNED PLATE

Air velocity(m/s)

Heater Temperature (Ts)( C ) ̊�

Ts- T∞( C ) ̊�

h( W/(m. C )) ̊�

0 62.4 38.9 96.131.5 49.8 26.3 142.192.5 44.0 20.5 182.42

TABLE B PINNED PLATE

Air velocity(m/s)

Heater Temperature (Ts)( C ) ̊�

Ts- T∞( C ) ̊�

h( W/(m. C )) ̊�

0 66.5 43.0 181.481.5 47.3 23.8 327.882.5 42.4 18.9 412.88

surface temperature (Ts - T∞) air velocity38.9 026.3 1.520.5 2.5

surface temperature (Ts - T∞) air velocity43 0

23.8 1.518.9 2.5

9 × Lw + Abase

A graph of Air velocity against Surface temperature (Ts - T∞)

15 20 25 30 35 40 450

0.5

1

1.5

2

2.5

3

Graph of Air velocity against Surface temperature (Ts - T∞)

Finned ̊PlatePinned ̊Plate

Surface temperature (Ts - T∞)

Air

velo

cit

y (

m/s

)

SAMPLE CALCULATION

Area of base = 0.11 m × 0.1 m

=0.011 m

Area finned plate =

= 9 × (0.068 × 0.1) + 0.011

=0.0722 m2

Area pinned plate = 17 ×πDL2

+ Abase

= 17 × π (0.013)(0.068)

2 +0.011

= 0.0346 m2

Convection heat transfer coefficient (h)

h = Q̽

A s(T s−T∞)

= 270W

0.0722(62.4−23.5)

= 96.13 W / m2 . C ̊�