Investigation On The Departure Of Droplet/bubble From The Substrate In Condensation/boiling Heat Transfer

The gas-liquid phase change includes condensation and boiling.Due to thestrong coupling among the flow,heat and mass transfer across the gas-liquid interface,the phase change heat transfer is quite a complex process.In order to enhance phase-change heat transfer,the droplets during condensation or bubbles during boiling should depart from the substrate quickly,this is the focus in our study.On the superhydrophobic surface,the droplet can bounce off from the condensing surface spontaneously,which can effectively improve the efficiency of droplet condensation and results in the efficient energy utilization.With the Diffuse Interface Method the coalescence-induced droplet jumping is simulated numerically.The influence of droplet size difference and contact angle hysteresis on the jumping of droplet,and the energy conversion analysis are studied.This study is of great significance for the design of efficient dropwise condensation surface.It is found that from nanometer size to millimeter size,with the increase of droplet size,the jumping velocity of dimensionless droplets increases gradually as the droplet size increases,after reaching the maximum,the velocity of droplet jumping decreases because of the increase of gravity.When the contact angle is constant,the larger the radius difference of droplet is,the smaller the jumping speed is,and the time when droplet leaving the substrate is later than the maximum vertical velocity.The larger the contact angle hysteresis,the smaller the drop velocity of droplets,and the moment that the jump time will appear earlier than the maximum vertical velocity.In addition,we study the change and movement of bubbles,the growth and detachment of bubbles on different heating surfaces from superhydrophobic surface to superhydrophilic surface.This study is of great significance for studying the mechanism of nucleate boiling and the phase change heat transfer.We use the Level Set method for numerical simulation,assuming that the bubble is under the action of buoyancy,viscosity and interfacial tension.The effects of different Re number and Bo number on the growth and disengagement of bubbles are analyzed and the critical parameters for bubble departure is found.The smaller the contact angle of the substrate,the larger the contact area with the substrate,the greaterthe viscous force at the bottom,the greater the horizontal component of the pressure and the smaller vertical component,so it is difficult to departure.