Conditions for Oscillation Onset and Flow Stagnation in Oscillating Heat Pipes

Phase changes are believed to be the predominant mode of heat transfer in oscillating heat pipes (OHPs) which are partially filled with liquid and loop through the hot and cold zones of a heat exchanger. Evaporation takes place in the hot zone and condensation in the cold zone. The net effect of phase changes alone is accumulation of liquid slugs in the cold zone. In our previous work, we have demonstrated the mechanism for the onset of oscillations when there is a sufficient temperature gradient between the two zones. An explicit criterion was found through the examination of stability of an equilibrium corresponding to liquid slugs filling up the parts of OHP in the cold zone. However, due to the assumed symmetry of simultaneous heating in the evaporation zone and cooling in the condensing zone, that model cannot explain the experimental observations that certain OHPs cannot be driven into oscillations with large imposed input power into the evaporation zone. In this paper, we modify the evaporation model to reflect the fact that condensation in the cold zone is zero at thermodynamic equilibrium, and it starts only as a result of a pressure rise above the thermal dynamic equilibrium following the imposition of heat flux into the hot zone. With this modification, it is shown that with a higher heat flux to the hot zone, the mean pressure inside the heat pipes rises. Because of the pressure dependence of the stability criterion, the condition for the onset of oscillations is no longer met and oscillations cease. This is consistent with our previous result since the stability criterion is dependent on the mean pressure and higher pressure requires a larger temperature gradient for the onset of oscillations. Through numerical simulations that are based on the modified evaporation model, we show that motions in the OHP may become stagnant due to the build-up of the mean pressure in the OHP.