Effect of tube heat conduction on the single branch pulsating heat pipe start-up
Abstract
The oscillation inception in the single-branch pulsating heat pipe (PHP, called also oscillating heat pipe)
has been studied in the presence of the heat conduction along the PHP tube, with the imposed both evaporator
heat power and condenser temperature. A start-up regime caused rather by the meniscus/film
evaporation than boiling has been considered. The dynamic equilibrium system state has been analyzed,
where the liquid film is absent and the meniscus is located at a position where the tube temperature
corresponds to the saturation temperature. The temporal evolution of the system responding to an initial
fluctuation shows a non-linear response even for small fluctuations. The stability of the equilibrium state
has been analyzed. The stability threshold corresponds to the start-up criterion. The main result of
the above analysis is the independence of the start-up criterion of the liquid film properties (film shape
and thickness). This result applies to the multi-branch PHP too. Due to the large tube thermal inertia,
influence of the temporal variation of the tube temperature on the threshold can be neglected; only
the equilibrium spatial temperature distribution along the tube matters. The start-up threshold value
is determined for the temperature gradient along the tube, more specifically, its equilibrium value at
the equilibrium meniscus location. It depends only weakly on other system parameters like condenser
temperature or adiabatic section length. An analytical expression for the threshold has been obtained.
The start-up power scales like square root of the tube heat conductivity. The liquid viscous dissipation
is found to be much less important than the energy dissipation via the fluid and solid heat transfer.
Origin : Files produced by the author(s)
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