Heat sinks are one of the primary mechanisms today for thermal management of electronics. In the high altitudes reached by modern military aircraft, the capacity for air cooling is reduced due to the rarefied atmosphere. With an increase in altitude there is a subsequent decrease in the density of air. A review of the literature shows a lack of research done on pin-fin heat sinks with impingement flows at low Reynolds number conditions. Experimental testing will determine the thermal resistance of a pin-fin heat sink with impingement flow at low absolute pressures. A test apparatus will be constructed, and experiments will be conducted within a hypobaric chamber. In a hypobaric chamber, it is possible to simulate altitudes up to 30 000 meters by reducing the absolute pressure using a vacuum pump. Temperature is regulated and air is circulated within the chamber. The test apparatus, which is to be completely enclosed within the hypobaric chamber, consists of a centrifugal blower forcing air through a duct. Air is impinged upon a pin-fin heat sink heated with uniform flux on the base. Incident air flow is along the axis of each circular pin-fin, and exhaust from the heat sink will be transverse to the pins. Feedthroughs are available in the chamber wall for supplying electrical power to the blower, for taking temperature measurements with embedded thermocouples, and for measuring blower shaft speed. Temperature measurements are made in the base of the heat sink, in the air, and at other points to characterize other heat losses from the apparatus. Blower speed is monitored with an optical tachometer, and by similarity laws for turbomachinery it will be possible to determine the air flow impinging upon the heat sink. Pressure in the chamber will be varied in several steps up to the equivalent of a 30 000-meter altitude, and at each step a correlation will be made between heat sink thermal resistance and Reynolds number of the impinging air.

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