|dc.description.abstract||The Loop Heat Pipe (LHP) is a passive, two-phase heat transfer device used, most commonly,
for thermal management of aerospace and aeronautical electronic equipment. A unique feature
is a porous wick which generates the necessary capillary action required to maintain circulation
between the heat source and the heat exchanger. What differentiates LHP devices from
traditional heat pipes, which also work through the use of a wick structure, is the constrained
locality of the wick, placed solely in the evaporator, which leaves the remainder of the piping
throughout the device as hollow. This provides the LHP with a number of advantages, such as
the ability to transport heat over long distances, operate in adverse gravitational positions and to
tolerate numerous bends in the transport lines. It is also self-priming due to the use of a
compensation chamber which passively provides the wick with constant liquid access. These
advantages make LHPs popular in aerospace and aeronautical applications, but there is growing
interest in their deployment for terrestrial thermal management systems.
This research had two aims. Firstly, to create and validate a robust mathematical model of the
steady-state operation of an LHP for terrestrial high heat flux electronics. Secondly, to construct
an experimental LHP, including a sintered porous wick, which could be used to validate the
model and demonstrate the aforementioned heat exchange and gravity resistant characteristics.
The porous wick was sintered with properties of 60% porosity, 6.77x10-13 m2 permeability and
an average pore radius of 1μm. Ammonia was the chosen working fluid and the LHP functioned
as expected during horizontal testing, albeit at higher temperatures than anticipated. For safety
reasons the experimental LHP could not be operated past 18 bar, which translated into a
maximum saturated vapour temperature of 45°C. The heat load range extended to 60 W, 50 W
and 110 W for horizontal, gravity-adverse and gravity-assisted operation respectively.
Because of certain simplifying assumptions in the model, the experimental results deviated
somewhat from predicted values at low heat loads. Model accuracy improved as the heat load
increased. The experimental LHP behaved as expected for 5° and 10° gravity-assisted and
gravity-adverse conditions, as well as for transport line variation, in which performance was
assessed while the total tubing length was increased from 2.5 m to 4 m.
Overall, the construction of the LHP, particularly of the porous wick, its operation and the
modeling of the constant conductance mode of operation proved to be successful. The variable
conductance mode of operation was not accurately modeled, nor was expected behaviour in the
elevation testing encountered, although the reasons for these results are suggested.||en