Nanoscale Vaporization Heat Transfer Characteristics Of R32/R1234yf Mixtures

Mixture working fluids have the advantages of complementary components and active design,therefore it has a promising future in the application of thermal system.The key to improve the comprehensive performance of the thermal system is to understand the mechanism of vaporization heat transfer of mixture working fluids.At present,there are few studies on the nanoscale vaporization heat transfer of mixture working fluids,the detailed and in-depth understanding of the mechanism is missing,and the relevant conclusions on the vaporization of pure working fluids cannot be directly generalized to mixture working fluids.Researches on nanoscale and molecular level can effectively reveal the mechanism of vaporization heat transfer of mixture working fluids.In this work,R32/R1234 yf was taken as the main research object,by combining molecular dynamics simulation and theoretical research,the evaporation of sessile droplet on the heating surface,suspended droplet evaporation,liquid film vaporization and bubble growth were studied respectively,the mechanism of vaporization heat transfer of mixture working fluids was studied at the molecular level.For the evaporation of sessile droplet on a smooth heated surface,it was found that the concentration gradient inside the droplet generated by the non-uniform evaporation of the mixture drove the internal flow and made the contact line oscillate and unstable.The oscillation amplitude of the contact line is positively correlated with the mole fraction of volatile components.Considering the effect of mass transfer resistance caused by interfacial concentration gradient,a semi-theoretical model was developed,and the average prediction deviation of droplet evaporation of mixture working fluids is reduced from 59% of the traditional model to 26%.For the evaporation process of nanoscale suspended droplets with Kn close to 1,the simulation results were compared with those predicted by the kinetic theory model and the macroscopic diffusion-based model,respectively.The modified kinetic theory model has higher prediction accuracy.Considering the scale effect,the thermal conductivity in the macroscopic diffusion-based model is modified,and a theoretical model for predicting the effective thermal conductivity is proposed.The average prediction deviation of the modified model for pure working fluids is reduced by 80%.Combined with the dynamic change of components at the interface during the evaporation,a theoretical model suitable for the evaporation of mixture nanodroplets was proposed,and the average prediction deviation was further reduced by 30% after considering the concentration gradient on the basis of the correction of thermal conductivity.The vaporization process of nanoscale liquid film on heating surface was studied.The applicability of Hertz-Knudsen-Schrage equation for predicting the vaporization rate of pure and mixture nanoscale liquid film was verified.Since R32 is more easily adsorbed by the solid surface than R1234 yf,increasing the concentration of volatile component R32 will significantly advance the boiling point,resulting in easier boiling of the mixture liquid film.The initial boiling stage is inertial-controlled stage,and the heat transfer deterioration mechanism is mainly due to the difference of diffusion rate of each component,which leads to the interface saturation pressure lower than the saturation pressure under nominal component.The nanobubble growth on heated surface was studied.Increasing the concentration of R1234 yf can delay the bubble inception time,inhibit the nucleation and growth of bubbles,and the dynamic process of bubble growth of the mixture is closer to that of pure R1234 yf due to the difference of diffusion rate for different components.Restricted by local heating conditions,the bottom edge of the bubble is pinned,and the bubble grows in a constant footprint radius mode.Compared with the bubble growth model in bulk liquid phase,a bubble growth model in the inertial-controlled stage on the heated surface is proposed,and the average prediction deviation is less than 10%.The bubble growth rate increases first and then decreases with the increase of wall lyophilic characteristics.As the mole fraction of R1234 yf increases,the sensitivity of bubble growth rate to wetting characteristics becomes stronger.