Study On The Coupling Mechanism Between Evaporation And Evaporation-induced Convection In Annular Pool At Low Pressures

Evaporation process is a non-equilibrium thermodynamic process,which plays a key role in water cycle in nature and widely exists in people’s production and life.However,most of the current studies are performed in environment.In this case,the evaporation rate depends on the diffusion rate of the working medium in the gas phase and the process is obviously different from that when the liquid evaporates in its own vapor.In general,the evaporation rate is relatively large when the liquid evaporates under low pressure environment,the macro flow induced by evaporation exists below the evaporation interface and the non-equilibrium effects at the evaporation interface such as the liquid-vapor interface temperature discontinuity significantly affect the evaporation process.Therefore,the evaporation process under low-pressure vapor environment is complex and need to be further studied.In this dissertation,the evaporation-convection process in annular pool at low pressures is studied by means of experiment and numerical simulation,the energy transfer process of liquid and vapor phases near the evaporation interface under different working fluids,liquid layer depths,vapor pressures and cylinder temperature conditions are systematically studied.The unsteady flow induced by evaporation at low pressures is observed,the physical mechanism and critical conditions of flow destabilization are determined,the magnitude and direction of interface temperature discontinuity is obtained,and the major influencing factors of temperature discontinuity are discussed.The main research results and contents are as follows:（1）The experimental system at low pressures is designed and built,and the numerical simulation model based on kinetic theory of gases（KTG）is established.Through a series of comparison between experiment and simulation,it is determined that the evaporation coefficient is 0.8 when the working fluid is water and ethanol.（2）When the working fluid is water and the depth of liquid layer is 10 mm or when the working fluid is ethanol and the depth of liquid layer is 3 mm,the energy transfer mode of liquid and vapor phases is revealed,the radial distribution of the interface temperature discontinuity is obtained.The results show that there is obvious radial temperature gradient at the evaporation interface near the inner and outer cylinders due to the evaporation cooling effect,therefore,two thermocapillary cells appears below the interface.When the working fluid is water and the cylinder temperature is low,the directions of thermocapillary convection and buoyancy convection are opposite,so the buoyancy cells can be seen below the thermocapillary cells,the uniform temperature layer appears below the interface.When the working liquid is water and the cylinder temperature is high or the working liquid is ethanol,the directions of thermocapillary convection and buoyancy convection are the same,there are only thermocapillary cells in liquid phase which is enhanced by buoyancy.The evaporation rate near the cylinders is much greater than that in other regions,and the energy transferred from liquid phase to the interface is much greater than that transferred from vapor phase to the interface.The interface temperature of vapor side is always higher than that of liquid side.The evaporation rate is largest near the cylinder,so the temperature discontinuity is relatively large near the cylinder.With the decrease of vapor pressure,the evaporation rate increases,the magnitude of temperature discontinuity increases at all points.There is a linear relationship between the interface temperature discontinuity and the heat flux on vapor side,so the empirical expression of temperature discontinuity can be obtained.（3）When the working fluid is water and the liquid depth is 10 mm or when the working fluid is ethanol and the liquid depth is 3 mm,the energy transfer process is analyzed in detail in the liquid pool with external radial temperature gradient,the coupling relationship between evaporation and thermocapillary convection is revealed.Whenβ<1,the energy required for evaporation is mainly transferred from liquid phase to the interface near the inner and outer cylinders.When the working liquid is water and the temperature of cold cylinder is low,one thermocapillary cell appears above the other.The uniform temperature layer exists below the interface,the necessary condition for the formation of the uniform temperature layer is that the two cells are distributed upper and lower sides.When the working liquid is water and the cold wall temperature is high or the working liquid is ethanol,the two cells are distributed left and right sides.Whenβ=1,there is only one thermocapillary cell in liquid phase,the energy is mainly transferred from liquid phase to the evaporation interface near the hot cylinder.The interface temperature of vapor side is higher than that of liquid side at different conditions,due to the largest evaporation rate near the hot cylinder,the temperature discontinuity near the hot cylinder is larger than that in other regions.The temperature discontinuity and heat flux on vapor side also satisfy a fixed linear relationship,the empirical expression is basically the same as that obtained when T_in=T_out=T_w.（4）With the increase of the liquid layer depth,the flow is enhanced by buoyancy and the average evaporation rate increases.When there is no uniform temperature layer in liquid layer,the heat near the cylinders is directly brought to the evaporation interface,the average evaporation rate of the liquid layer increases obviously.When there is a uniform temperature layer below the interface,part of the heat cannot be directly transferred to the evaporation interface,the average evaporation rate increases slightly.When the working liquid is water,the flow in annular pool is axisymmetric steady-state flow at different liquid layer depths.While the working fluid is ethanol,the 3D unsteady flow appears when the liquid layer depth is larger than 7 mm and the vapor pressure is below the critical value.When the liquid depth increases,the heat flux on vapor side tends to decrease,so the magnitude of temperature discontinuity decreases.The empirical correlation of temperature discontinuity is still applicable at different liquid layer depths.（5）In the deep pool filled with ethanol,when the liquid layer depth is larger than 7mm and the pressure ratio is less than the critical value.Due to the inertia of fluid,the lag exists between the variations of the flow resistance and the velocity.The two cells interact continuously,the flow paths of two cells and the location of interaction region vary constantly,the 3D unsteady flow is formed.With the decrease of vapor pressure,the simulated result shows a transition from periodic flow to chaotic flow.