The first steps have been taken in a continuing project to study the suitability of dissipating heat from electronic and electric-actuator equipment aboard an aircraft by using a loop heat pipe, containing water as the working fluid, that delivers the heat to the exterior aircraft skin. The first steps include the following:

  • Partial design and partial construction of an apparatus for experimentation on a heat pipe in a pre-existing centrifuge table test bed, wherein the centrifuge table will be used to simulate acceleration of a high-performance aircraft;
  • Upgrading of the test bed; and
  • Mathematical analysis of transfer of heat from a heated plate (a surrogate for the portion of the aircraft skin receiving the heat from the heat pipe) to air flowing along the plate at zero angle of attack, as a function of altitude and speed.

The experimental apparatus is intended to include a high-temperature fluid loop, a low-temperature fluid loop, various sensors, and a data-acquisition system. As part of the overall design, construction, and upgrading effort, the entire wiring system of the centrifuge has been analyzed and rebuilt. Wiring panels have been built to facilitate the wiring of future experiments. New data-acquisition equipment and a control-and-data-acquisition computer running LabVIEW software have been purchased. The centrifuge table is now controlled by specifying centrifugal acceleration directly (instead of specifying a control voltage proportional to the square root of centrifugal acceleration, as was done prior to the upgrade). Work on developing a hightemperature fluid loop for removing heat from the loop heat pipe has been started. A recirculating chiller that is part of the test bed and has not been used in recent years was flushed and prepared to be filled with new coolant liquid (ethylene glycol) for use in the experiments.

The mathematical analysis of the transfer of heat from a heated plate has provided important insights for the contemplated use of an aircraft skin to reject heat. It was found that the altitude and speed of the aircraft would significantly affect the amount of heat that could be rejected from the skin:

  • Aerodynamic heating of the skin would reduce the heat transfer, and if the Mach number was high enough, no heat would be transferred from the skin to the air.
  • The altitude of the aircraft would affect the free-stream temperature and density, which, in turn would affect the overall heat-transfer coefficient.

It was also shown that assumption of a "standard atmosphere" could result in significant errors in the prediction of the heat dissipation, as compared to the predicted heat dissipation for the atmosphere on an unusually hot or unusually cold day. Finally, the analysis showed that the aircraft skin temperature, which would be directly influenced by an actuator thermalmanagement system, would exert a strong effect on the heat-dissipation rate, especially at low altitude.

This work was done by Scott K. Thomas and Andrew J. Fleming of Wright State University for the Air Force Research Laboratory.

This Brief includes a Technical Support Package (TSP).
Use of a Heat Pipe to Cool Aircraft Electronics

(reference AFRL-0073) is currently available for download from the TSP library.

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This article first appeared in the June, 2008 issue of Defense Tech Briefs Magazine.

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