Lattice Boltzmann Simulation Of Condensation And Freezing On Subcooled Hydrophobic Surfaces

Both condensation and freezing occur frequently in natural environment as well as in many engineering systems.It is a subfield of heat transfer that has been the subject of intensive studies during the past century.In recent years,hydrophobic surface shows great potential in condensation enhancement and anti-icing,however,there still exist many problems to be solved.In this thesis,a profound study of phase-change on hydrophobic surfaces under different wall temperature will be done,including condensation transition on downward-facing hydrophobic surfaces and triple phase-change on an inclined hydrophobic surface with a cryogenic spot.The detailed are described as following:1.Lattice Boltzmann simulations for transition from dropwise to filmwise condensation on downward-facing hydrophobic surfaces.Dynamic behaviors including growth,coalescence and departure of droplets from cooler surface are investigated.Effects of wall subcooling and wettability of the surface on droplet departure diameter,average cycle time and nucleation time are presented.Based on the analysis of temperature distribution,the condensation curve in terms of average heat flux versus wall subcooling from droplet nucleation to peak dropwise condensation heat flux and its subsequent transition to filmwise condensation is obtained numerically for the first time.It is found that,during transition from dropwise to filmwise condensation,there is a graduate reduction in wall heat flux under constant wall temperature conditions,and a sudden drop in average wall temperature under constant wall heat flux conditions.Besides,it is verified that droplets fall down from the subcooled surface at locations according to Taylor’s unstable wavelength during filmwise condensation.2.Establish a triple phase-change lattice Boltzmann model.Based on Gong-Cheng liquid-vapor phase-change model and enthalpy-based solid-liquid phase-change model,a novel model which can simulate condensation and freezing is proposed.In this model,the liquid-vapor phase-change is achieved by Gong-Cheng’s model.The enthalpy in the liquid region is calculated,the solid-liquid interface is traced by updated enthalpy and the velocity condition on solid-liquid interface is treated by the immersed moving boundary scheme.3.Using this triple phase-change model,3D transient problem of saturated vapor condensation and subsequent freezing about a circular cryogenic spot on an inclined hydrophobic flat plate are simulated.The results show a droplet is formed and the liquid adjacent to cryogenic spot freezes into ice.In the beginning,both droplet and ice grow rapidly and the heat flux on cold spot is extremely high.Under the effect of gravity,the liquid film covered on ice flows downward.The condensate flow over ice makes ice elongated in downstream region.As the droplet grows larger,a part of liquid film detaches from ice and forms a moving droplet along the flat plate.The dynamic behaviors of liquid affect the growth of ice,and influence the heat transfer as well.Besides,the effects of tilt angle,wall temperature and contact angle to condensation and freezing are discussed.