An Experimental Study Of Boiling And Condensation Co-existing Heat Transfer In A Phase Change Chamber

The inner space of the flat plate heat pipe is closed and small,therefore there is a strong interaction between its boiling and condensation processes that are taking places simultanuously.In order to strengthen the heat transfer performance of the flat plate heat pipe,and to deepen its understanding of the internal boiling and condensation coexisting phase change heat transfer,a boiling and condensation coexisting phase change thermal test system was constructed and used to conduct experimental studies of boiling and condensation coexisting phase change heat transfer in the phase change chamber.Combined with the experimental data and visualization observations,the effects of filling ratio,heat flux,heating surface structure and physical properties of the working fluid on the heat transfer characteristics were analyzed.Deionized water was used as the working fluid,experimental data and visualization images of boiling and condensation coexisting phase change heat transfer in a phase change chamber with smooth,equal-width grooved,variable cross-sectional-area grooved and array finned heating surface were analyzed.The effects of the heating surface structure,liquid filling ratio and heat flux on the boiling and condensation coexisting phase change heat transfer characteristics were analyzed systematically.The experimental results confirm there is an optimum liquid filling ratio at which the phase change chamber aquires its best heat transfer performance.The optimum filling ratio are 36.0%,11.4% and 36.0% for phase change chambers with smooth,equal-width grooved and array finned heating surface,respectively.It should be stressed here that in the phase change chamber with equalwidth grooved heating surface,the heat transfer effect of the boiling and condensation surface are the best when the liquid filling ratio is 36.0% and 11.4%,respectively.Since the thermal resistance of condensation is the dominant thermal resistance in the heat transfer process of the phase change chamber,the optimum liquid filling ratio of the phase change chamber is that of the condensation heat transfer.As the heat flux increases,the boiling becomes intense,the frequency and speed of the boiling working liquid scouring the condensation surface increases,hence the heat transfer on the condensation surface is improved.The boiling working liquid and the condensate fall back into the boiling liquid to disturb the boiling liquid,the growth of the boiling bubbles is accelerated,and the interaction between the bubbles is enhanced,so that the boiling and condensation heat transfer are strengthened at the same time.The grooved and array finned structure increase both the solid-liquid contact area and the active nucleation sites of the boiling surface at the same time.The bubbles on the boiling surface increase.The growth and detachment speed of the bubbles accelerates.The visualization observationss show that the grooved and array finned structure not only strengthen the boiling heat transfer,but also enhance the interaction between boiling and condensation in the phase change chamber.The boiling liquid frequently flushes the condensation surface,thins the condensate film,and thus strengthens the heat transfer of the condensation surface.Ethanol was used as the working liquid,experimental data and visualization images of boiling and condensation coexisting phase change heat transfer in a phase change chamber with equal-width grooved and array finned heating surface were analyzed,and the influences of the physical properties of the working fluid on the boiling and condensation coexisting phase change heat transfer characteristics and start-up characteristics in the phase change chamber were analyzed.Same results as that of deionized water,the interaction between boiling and condensation in the phase change chamber using ethanol as the working fluid also increases with the increase of heat flux,and the heat transfer coefficient increases with the heat flux increase.When the filling ratio is 36.0%,the heat transfer performance of the phase change chamber with the equal-width grooved and the array finned heating surface is the best.Due to the surface tension,thermal conductivity,latent heat,and specific heat capacity of the deionized water is larger than that of ethanol,and the viscosity of deionized water is small,therefore,when using the deionized water as the working fluid,the detachment diameter of the boiling bubbles is larger and the detachment speed is faster,the interaction between boiling and condensation in the phase change chamber is stronger,so the heat transfer performance is better.When using ethanol as the working fluid,the phase change chamber starts faster,and the stable operation temperature is higher,ethanol is more suitable for those with higher response speed requirements,and deionized water is more suitable for high heat flux applications.