Importance of phase change timing in a self-sustained oscillatory flow

Abstract

This paper experimentally shows the effect of phase change timing on the oscillation amplitude for a vapor bubble-liquid plug exposed to a thermal gradient in a capillary tube. The case studied is the basic unit of pulsating heat pipes (PHP) and self-oscillating fluidic heat engines (SOFHE), implemented here by heating a tube at its closed end (evaporator) and cooling it at the opposite open end (condenser). A vapor bubble is formed at the closed end, trapped by a liquid plug. Under certain conditions, the liquid plug starts to oscillate due to periodic evaporation–condensation in the vapor bubble. The temperature, pressure, and volume of the vapor bubble are measured to calculate the phase change rate, for two different working fluids (water and ethanol). We first show a regime where the liquid plug's oscillations exhibit a beating phenomenon, where the amplitude periodically increases and dies down due to the slow dryout of the thin film left behind the meniscus. We then use a simple approach to manipulate the phase change using a wicking fiber inserted in the tube. This modification yields stable oscillations with constant amplitude due to a significant increase in the phase change rate. For ethanol, no oscillations are observed without the fiber. Adding the wicking fiber leads to oscillations with a different phase change profile than with water. To maximize work done by phase change on the liquid plug, the phase change force needs to be optimally in phase with velocity. To evaluate this phase angle, a new tool is proposed that is based on integrating the product of the phase change force and velocity over a cycle at different phase angles. By doing so, the maximum theoretical work at ideal phase angle is calculated. It is used to define Phase Change Effectiveness as a dimensionless number (φ) that equals the ratio of the phase change work to the maximum theoretical work. The phase change profile for water has a φ number of 0.56, showing room for further improvement, while ethanol shows an effective phase change profile with a φ number of 0.99. For a lower maximum work, a more effective phase change profile yields a higher oscillation amplitude, which is at 10.2 mm for ethanol compared to 7.2 mm for water. This study opens a new path towards engineering the phase change to optimize the oscillations in pulsating heat pipes and self-oscillating fluidic heat engines.