Introduction: Convection can be determined as the transfer of heat energy by the movement movement of matter. There is two type of convection that is natural and forced f orced convection. convection. Natural convection is the mechanism mechanism that used to transport t ransport the heat by density differences in the fluid occurring due to the temperature temperature gradients. Forced convection convection is the t he mechanisms mechanisms also transport the heat in fluid motion that t hat generated by an external external source such such as pump, fan fan and suction device. device. Forced convection is more efficient compare to natural convection.
Objective: Thermal resistance for flat plate, finned heatsink are measured under natural and forced convection conditions.
Methodology/ Procedures: Experiment 1 (Forced Convection)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
The fan assembly assembly is placed on the t op of the duct. The finned heatsink is placed into the duct. The heater switch is turned on. The heater power control is set to t o 15 Watts (the control knob is turned clockwise) The air flow velocity is set to 0.5 m/s (the control knob is t urned clockwise) The heatsink temperature is recorded in every 1 minute. When temperature does not change in 1 minute, it can be considered that the system reached steady state. While waiting, The finned and pinned heatsink geometry is measured and their approximate surface areas is calculated. Steady state and ambient temperatures is recorded. The fan speed control knob is set to give a reading of 1.0, 1.5, and 0.0 m/s air speed. Steps 6 and 8 is repeated.
Results: Power : 15W Surface areas : Finned Heatsink : 0.125m²
Pinned Heatsink : 0.055m²
Flat Plate : 0.011m²
Surface
Air flow velocity, v (m/s)
Heat sink Temperature, T p (°C)
Ambient Temperature, Ta (°C)
Temperature rise T p-Ta (°C)
Finned heat sink
0 0.5 1.5 0.5
68.5 60.7 57.5 70.2
25.0 25.4 25.1 24.9
43.5 35.3 32.4 45.3
Flat plat
Thermal resistance (T p-Ta)/Q, (°C/W) 2.90 2.35 2.16 3.02
Graph of Air velocity against Thermal resistance 4
e 2.9 c n3 a t s i s 2 e R l a 1 m r e 0 h T 0
3.02 2.35
0.2
0.4
2.16
0.6
0.8
1
1.2
1.4
1.6
Air velocity
Finned heatsink
Flat plate
Figure 1: Graph of Air velocity against Thermal resistance
Based on the Figure 1, the graph showed the air flow velocity against thermal resistance for finned heatsink and flat plate. By giving an air flow velocity 0.5 m/s for finned heatsink and flat plate its showed there is different value of both thermal resistance. The higher value of thermal resistance in 0.5 m/s air flow velocity is flat plate which give the value of 3.02°C/W which is much higher than finned heatsink by 2.35°C/W.By comparing the surface area, flat surface have smaller surface area(0.011m²) compare finned heatsink (0.125m²). In this experiment, when temperature reached to the steady state is when temperature do not change in 2 minute where there is no cooling take place in this experiment.So there is relationship between thermal resistance and surface area.The larger surface are a so the thermal resistance becomes lower. So this experiment showed that finned heatsink have larger surface area and lower thermal resistance compare to the flat plate.
Graph of Time (min) against Finned Heatsink Temperature at 0.5m/s(°C) 70
) C ° ( s 60 / m50 5 . 0 t 40 a e r 30 u t a r 20 e p m10 e T k 0 n i 0 s t a e H d
Series1
5
10
15
20
25
30
35
40
45
50
Time (min)
Figure 2: Graph of Time against Finned Heatsink Temperature The graph in Figure 2 showed that the time against temperature rise for 0.5 m/s air flow velocity for finned heatsink. Based on the graph, finned heatsink does not take a longer ti me to reach heatsink temperature because of finned heatsink have larger surface area and lower thermal resistance.Based on the graph also there is a liner graph showed by the finned heatsink.
Discussion: 1. I think that the causes of the different thermal performance (temperature) for finned/pinned heatsinks and flat plate is because when compared the thermal resistance of t hese three heatsinks and plate in this experiment, it shows that for finned heatsink, the thermal resistance is 2.35°C/W which is the lowest compared to pinned heatsink(0.82°C/W) and flat plate(3.02°C/W). This happened due to the heatsinks have different the number of total surface area. We know that, when the number of surface area is increased, the number of thermal resistance will decrease. By comparing these 3 heatsinks and plate we can conclude that finned heatsink have larger surface area compare to other.
2. During the experiment, the heatsink temperature is changes as air velocity changes due to no cooling take place when air temperature reached heatsink temperature. In the experiment showed that the finned heatsink temperature changes where there is air flow velocity changes. If the air flow velocity increased it will take a shorter time to achieve heatsink temperature. 3. By conducting this experiment, the possible causes of error in temperature measurement is the apparatus of the experiment. This is because while conducting the experiment, the value of the apparatus is quite changes in certain time. The sensitivity of the value of air flow velocity changes about ±0.1 m/s while conducting the experiment. Thus, we can conclude that the value of air flow velocity changes due to the value of temperature changes. Besides that, another possible error is human error which is t hat might be taking the value of the temperatures inconsistently. Between both errors, the most significant error is from the apparatus of the experiment which give extremely effects to the reading value of the temperature.
Conclusion: As conclusion, surface area of heatsink and the number of air flow velocity can affected the number of thermal resistance. From the experiment we can conclude that if increased the number of air flow velocity and the surface area of heatsink become larger, the lower the thermal resistance. When there is no air flow velocity, it much take longer time to reached heatsink temperature because natural convection take place by cooled it down. Thus f orced convection take shorter time to reached heatsink temperature.
References: 1. http://www.sfu.ca/~mbahrami/ENSC%20388/Notes/Natural%20Convection.pdf 2. http://people.csail.mit.edu/jaffer/SimRoof/Convection/
