1993-03-01

Magnesium Heat Sink Evaluations 930415

A system has been constructed to estimate heat dissipated from geometrically identical heat sinks and pinfins extruded from magnesium (M1A) and aluminum (6063-T5)alloys. Two longitudinal fins of circular cross sectional area, machined from aluminum and magnesium, are used to calibrate the equipment. Thermocouples were mounted on an aluminum connecting cylinder between a heater and the pin fins or heat sinks. Insulation around this same cylinder and the heater keeps most of the heat flowing axially to be dissipated by the pin fins and heat sinks. Thermocouples were also mounted on the longitudinal fins to measure temperature distribution. Measured temperature distributions on the aluminum and magnesium pin fins were in good agreement with theoretical models. After calibration of the measuring equipment, the heat dissipated through actual aluminum and magnesium heat sinks was measured as a function of air velocity over the heat sinks.
Aluminum and magnesium heat sinks were then tested at two different heater settings with air velocities of 0, 0.4, 1, 3, and 7 m/s (0, 79, 197, 590, and 1378 fpm). The results of these tests showed that for low air velocities (below 1 m/s or 197 fpm) the performance of magnesium and aluminum heat sinks is very similar. The estimated thermal transmission ratio (the ratio of the heat dissipated from the heat sink to the excess base temperature) of the magnesium heat sink was approximately only 5 to 10 percent lower than the aluminum heat sink. At very high velocities, however, the magnesium heat sink thermal transmission ratios were about 15-25% lower than the aluminum heat sink because the base temperatures of the aluminum and magnesium heat sinks are much smaller than those at a moderate and lower velocity cases. Thus, a small difference in the low values of base temperatures (a few degrees) can result in a large difference in the thermal transmission ratios. The estimated heat dissipated from magnesium heat sinks for all of the velocities were approximately within 5 percent of the estimated heat dissipated from the aluminum heat sink. This indicates that the heat dissipated from a magnesium heat sink is comparable to the aluminum heat sink.
In addition to the experimental work, a theoretical study of two transient heat flow conditions is included to show how magnesium compares to aluminum.

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