Doctoral Dissertations

Date of Award

8-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Trevor M. Moeller

Committee Members

John D. Schmisseur, James L. Simonton, Robert S. Reid

Abstract

Heat pipes passively transfer heat in numerous applications. Traditionally one side of the heat pipe is coupled to a heat source (evaporator) while the opposite side is coupled to a heat sink (condenser). This configuration has working fluid stagnation points at each end of the heat pipe. Other configurations may also prove useful, such as heat pipes with multiple evaporators or multiple condensers. In such heat pipes, additional working fluid stagnation points form at locations dependent on the configuration of the thermal boundary conditions. These stagnation points divide the heat pipe into multiple cells that each have an evaporator and condenser. The total thermal power input and output distributes between these cells. This distribution decreases the length that the working fluid must travel as well as the working fluid velocity. This reduces the pressure drop of the vapor and liquid circulating through the heat pipe allowing it to operate at higher power densities with the same spatial footprint.

To characterize the behavior of stagnation points in such heat pipes, a copper water heat pipe was built and tested with arbitrary boundary conditions. In addition, a non-condensable gas was added at the heat pipe ends to allow insight into the vapor flow direction. The amount of non-condensable gas was either constant or controlled with a pressure controller. Throughout testing the heat pipe behaved in a stable and repeatable manner so long as the number of evaporator (hot) stagnation points was less than or equal to those in the previous state. Forming an additional hot stagnation point during a passive gas loaded heat pipe test led to most of the gas remaining on one side of the hot stagnation point. The results of these tests show stagnation points appear to form rapidly when thermal boundary conditions change. Small fluctuations in temperatures near the working fluid vapor and non-condensable gas interface were observed. These changes were caused by changes in the ambient temperature and the feedback system in the pressure controllers and were not observed when the condenser conditions were better controlled such as in the passive gas-loaded calorimeter tests.

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