Effect of tube heat conduction on the single branch pulsating heat pipe start-up

International audienceThe 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 evaporatorheat power and condenser temperature. A start-up regime caused rather by the meniscus/filmevaporation 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 temperaturecorresponds to the saturation temperature. The temporal evolution of the system responding to an initialfluctuation shows a non-linear response even for small fluctuations. The stability of the equilibrium statehas been analyzed. The stability threshold corresponds to the start-up criterion. The main result ofthe above analysis is the independence of the start-up criterion of the liquid film properties (film shapeand 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; onlythe equilibrium spatial temperature distribution along the tube matters. The start-up threshold valueis determined for the temperature gradient along the tube, more specifically, its equilibrium value atthe equilibrium meniscus location. It depends only weakly on other system parameters like condensertemperature 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 dissipationis found to be much less important than the energy dissipation via the fluid and solid heat transfer