Static and Dynamic Liquid-Vapor Phase Distribution in the Capillary Evaporator of a Loop Heat Pipe

Abstract

The liquid–vapor phase distribution and displacement in the capillary evaporator of a loop heat pipe (LHP) are key phenomena affecting the steady state and transient operating characteristics. This study intends to analyze the liquid-vapor interface behavior in the capillary evaporator that causes operational instability and enhances the heat-transfer while performing optical observation in the transparent cylindrical evaporator during the LHP operation. A quartz wick-acetone LHP system was designed and fabricated, which operated successfully with a maximum heat flux of 5.9 W/cm2. Phase displacement in various operations, such as the start-up involving nucleate boiling, capillary limit, hysteresis, and step-up of the heat load were observed. Binarized image quantitatively processed revealed dynamic characteristics on the contact surface between the wick and case. Comparison of the phase displacements during the start-up involving nucleate boiling and nucleate boiling after normal start-up showed that the equilibrium vapor phase on the contact surface between the evaporator case and wick is formed by both imbibition and drainage. On the step-up-down test of the heat load, a visual evidence of hysteresis of the evaporator heat-transfer coefficient due to the phase distribution in the wick was noticed. The simulation results showed that the residual liquid phase along the three phase contact line within the case, wick and grooves, observed by the visualization experiment, is of low temperature. Therefore, the distribution of the residual liquid can enhance the evaporator heat-transfer coefficient. This characteristic is the key aspect of optimizing the porous structure of the wick.