Natural and Forced Convection

Natural and Forced Convection INTRODUCTION: Forced convection is a mechanism, or type of transport in which fluid motion is generated by an external source (like a pump, fan, suction device) the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat. Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source (like a pump, fan, suction device) but only by density differences in the fluid occurring due to temperature gradients.

OBJECTIVE: To measured thermal resistance for flat plate, finned heatsink, and pinned heatsink under natural and force convections condition.

METHODOLOGY: 1. 2. 3. 4.

The fan were assembled on top of duct and the flat plate were placed inside the duct. The heater were switched to on. The power control and air flow velocity then were set up to 15 Watt and 0.5 m/s. Heat sink temperature then were recorded every 1 minute and were stops when the temperature does not change in 2 minutes. 5. Then, the experiment was repeated by using finned heat sink and pinned heat sink.

RESULTS: Input power = 15 W

Surfaces

finned heatsink

pinned heatsink

flate plat

Ambient air flow Heatsink velocity, v temperature, Tp temperature, (m/s) (⁰C) Ta (⁰C) 0 57.6 24.9 0.5 36.2 25.6 1 32.5 24.7 1.5 31.5 24.2 0 48.5 24.8 0.5 37.4 24.5 1 31.7 24.2 1.5 31.2 25.1 0 78.7 25.1 0.5 71.6 24.9 1 68.6 24.5 1.5 63.1 24.4

Thermal Temperature resistance rise (⁰C) (⁰C/W) 32.7 2.18 10.6 0.71 7.8 0.52 7.3 0.48 23.7 1.58 12.9 0.86 7.5 0.50 6.1 0.41 53.6 3.57 46.7 3.11 44.1 2.94 38.7 2.58

TABLE1:data of difference surface and difference air flow velocity

Thermal resistance (⁰C/W)

THERMAL RESISTANCE AGAINST AIR FLOW VELOCITY

4 3.5 3 2.5 2 1.5 1 0.5 0

PINNED FLATE PLATE FINNED 0

0.5

1

1.5

2

Air velocity (m/s)

FIGURE 1: relationship between air flow velocity and thermal resistance in steady state.

From the figure 1 above, we can see the results of relationship between air flow velocity and thermal resistance in steady state for pinned heatsink, finned heatsink and flat plate. Based on the figure, we can conclude that the thermal resistance for the flat plate are the highest. For air flow velocity 0.5m/s thermal resistance reading for flat plate are 3.11 ⁰c/w. Thermal resistance for pinned heatsink at 0.5m/s air velocity is 0.86⁰c/w. Reading of thermal resistance for finned heatsink at 0.5m/s air velocity is 0.71⁰c/w. air flow velocity and thermal resistance for finned heatsink are linear. As the air flow velocity increase we can see that thermal resistance decrease respectively. For all type heatsink(pinned heatsink, finned heatsink and flat plate) the trend of the graph are same, where the air velocity flow increase, the thermal resistance decreased. For 0.5m/s air velocity, thermal resistance for finned heatsink is 0.71m/s, pinned heatsink is 0.86m/s and flat plate is 3.11m/s. For 1.0 m/s air velocity, thermal resistance for finned heatsink is 0.52m/s, pinned heatsink is 0.5m/s and flat plate is 2.94m/s. For 1.5m/s air velocity, thermal resistance for finned heatsink is 0.48m/s, pinned heatsink is 0.41m/s and flat plate is 2.58m/s. Increasing of air velocity from 0.5m/s to 1.5m/s were called force convection. The natural convection happened when the air velocity flow is 0m/s, that’s mean the heatsink were cooled. Thermal resistance reading for finned heatsink in natural convection is 2.78⁰c/w, for pinned heatsink is 1.58⁰c/w, and flat plate is 3.52⁰c/w. Natural convection take more time for the heatsink temperature to stable/constant compared to force convection.

TEMPERATURE RISE (⁰C)

RELATIONSHIP OF TIME AND TEMPERATURE RISE FOR 0.5m/s CASE

40 30 20 FINNED

10 0 0

5

10

15

20

25

TIME (MIN)

FIGURE 2: relationship of time and temperature rise

30

35

40

Figure 2 shows that the temperature rise as the time increase. So that, the temperature rise for finned haetsink are linearly with the time. From the graph, we can see it reached the steady state at 37min and its temperature is 36.2⁰c/w.

DISCUSSION; 1. Based on the experiment that had been conducted, thermal performance (temperature) for finned heat sink, pinned heat sink and flat plate are different due to the surface area. Greater the surface area will cause lower the thermal performance. Finned heat sink surface has greater surface area compared to the others. The value of its thermal resistance for 0.5m/s air flow velocity is 0.71⁰c/w. This value is the smallest value of thermal resistance compared to the others(pinned heatsink and flat plate). 2. As predicted in the pre-lab the lowest thermal performance are finned heatsink, followed by pinned heatsink and the hottest thermal performance are flat plate. The results of the experiment that being conducted were same as predicted in pre-lab. 3. The heatsink temperature changes as air velocity changes because the results of heat transfer. As the air velocity increase the heatsink temperature decrease. The movement of air velocity in the duct affect the transfer of heat at the surface of heatsink. Faster the air flow velocity cause slower the heat transfer to the surface of heatsink and cause the heatsink temperature lower. 4. There are some errors occurs during the experiment were conducted.  Machine error; the value of velocity and power are supposed to be the same (as being set up) throughout the experiment. However the value of air velocity changes ( ±0.1m/s) while the experiment were conducted , same problem happened for heater power control.  Human error; this happened while record the results of heatsink temperature for every one minute. Not focusing on the experiment and sometimes missed a few seconds while taking the reading. Cause the value of temperature are not exactly being recorded in 1minutes.  Surrounding; the machine sometimes are sensitive to the surrounding (temperature, heat, air flow). This cause the reading of air flow velocity and power control were changing during the experiment conducted. 5. Among all the errors that occur, the most significant error are machine error that cause the air velocity and power control changing while the experiment were conducted.

CONCLUSION; What can I conclude, the higher the surface area, thermal performance (temperature) of the heatsink became lower. As the air velocity flow changes, the heatsink temperature also changes. I can said that, increasing of air velocity cause the heatsink temperature decrease. Natural convection of the heatsink take more time than force convection.

REFERENCES: http://www.dummies.com/how-to/content/transferring-heat-through-convection-naturalversu.html ( 4 Oktober 2014, 2.15 pm) www.qats.com/../Qpedia_Jun08_Heat_Sink_Thermal_Resistance.html (6 Oktober 2014, 4.03 pm) http://en.wikipedia.org/wiki/Heat_sink http://amrita.vlab.co.in/?sub=1&brch=194&sim=791&cnt=7