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Heat Sink Characterization

Experimental Objectives. Determine how air velocity and input power influence heat sink performance.Determine how air velocity and input power influence pressure drop across heat sink.Determine which parameters are most significant to heat sink performance.Clearly show the maximum heat that can be dissipated while keeping the designated heat sink temperature below 65

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Heat Sink Characterization

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    1. Heat Sink Characterization Lab Partners: Evan Kontras Will Linders Date of Presentation: 12/10/09 Date of Experiment: 11/20/09, 12/4/09

    2. Experimental Objectives Determine how air velocity and input power influence heat sink performance. Determine how air velocity and input power influence pressure drop across heat sink. Determine which parameters are most significant to heat sink performance. Clearly show the maximum heat that can be dissipated while keeping the designated heat sink temperature below 65 C using only ambient air.

    3. Experimental Setup

    4. BLOCK DIAGRAM OF SETUP

    5. Experimental Procedure

    6. Effects of Air Velocity and Input Heat Multiple measurements were taken of the temperature on the top and bottom of the heat sink, but because the bottom of the heat sink will have a higher temperature due to being in contact with the heat source (assuming ambient air of 35 C), we were more concerned with this measurement.

    7. Five separate heat inputs were selected over the range from 0 to 80 VAC. For each heat input setting, five blower settings ranging from 20 to 40 VDC (we are interested in relatively low speeds) were cycled through. Therefore, measurements for every combination of heat input and blower speed were taken. The other measured parameters aside from heat sink temperature include air inlet temperature, pressure drop across heat sink, shunt resistance, and pressure drop in flow chamber.

    8. Natural Convection An important consideration for heat sinks with low air velocities is natural convection. As the heat sink base increases in temperature, heat is conducted, convected, and radiated to the nearby air. As this air increases in temperature, it becomes less dense and begins to move upward. As this warm air rises, cool dense air moves in below it due to pressure differences, replacing it. This air now becomes heated, and the cycle continues.

    9. Natural Convection Schematics

    10. Natural convection can be a very effective mode of heat transfer, and in fact disturbing the natural flow and replacement of heated air with the use of forced convection can lead to higher surface temperatures of the heat sink.

    11. Design of Experiment Factorial Analysis Factors Air velocity (min 9.1 ft/s, max 28.6 ft/s) Power input (min 0 volts, max 80 volts) Response Heat sink Temperature

    13. We can see from the previous plot that natural convection is indeed very important for our heat sink at low air velocities, for heat inputs of above around 20-30 VDC. The plot below shows the minimum air velocity at which the temperature of the heat sink is the same as with that obtained through natural convection alone. Increasing the air velocity beyond these values for a given heat input decreases the temperature below that obtained by natural convection alone. In other words, for a given temperature of heat sink, in order for forced convection to be more beneficial that natural convection, this minimum velocity must be used. For temperatures below approximately 40 C there is negligible change in heat dissipation over all velocities.

    14.

    15. The effects of input heat and air velocity on pressure drop were analyzed. The following plots show the linear relationship between both parameters and pressure drop. It is clear how much more sensitive the pressure drop is to blower speed. For a small change in blower speed, just 20 VDC, the pressure drop changes by approximately 600 inches of water. On the contrary, for a relatively large change in input heat, 80 VDC, the pressure drop only changes by around 50 inches of water. Blower speed is by far the more important parameter to consider when designing a heat sink or selecting a fan to operate under specific pressure drop conditions.

    16. Pressure Drop Sensitivity

    17. Recommendations Depending on the amount of heat input to the heat sink, natural convection may be a viable option. However, for any input heat above 60 VDC the natural convection will not be enough to keep the heat sink at or below 65 C and the minimum air velocity must be used. Within the range of our tested velocities, which are relatively low as we were directed to consider, the maximum heat dissipated while keeping the heat sink temperature below 65 C is shown to be approximately 70 W (70 VAC heat input at 1A), using an air velocity of 75 ft/s. If pressure drop is a major concern, it is recommended to use the lowest blower setting possible, with heat input being of little concern.

    18. Uncertainties in Measurement Uncertainty of type-K thermocouple readings: 0.75% Uncertainty of PX-26 pressure transducer : 0.25% Error of heat input setting to measured VAC readings: 6.2% (averaged)

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