Study On The Mechanisms Of Heat Transfer Enhancement In Wavy Condensate Films

Condensation heat transfer enjoys a wide range of applications in desalination,refrigeration,power generation industrial fields in that it offers a very effective means of transporting energy.As the interfacial waves on the condensate film play an important role in enhancing condensate rate,it is of great significance to develop a reasonable and accurate mathematical model and investigate the microscopic mechanism of heat transfer enhancement.A numerical model has been developed to simulate the evolution of wavy condensate film on a vertical condenser surface in which the wavy interface is tracked by a non-orthogonal coordinate transformation.A sinusoidal disturbance was introduced by a fictitious body force with mean zero in one period,which avoids the effect of disturbance on wavy film evolution.The profiles of local film thickness,pressure,velocity,temperature and heat transfer coefficients were predicted by a programming work with Fortran language based on an improved SIMPLER algorithm.This research is directed at developing a deeper understanding of the heat transfer enhancement mechanism by analyzing the microscale phenomenon of flow and heat transfer in wavy condensate film.The simulation results show that a good agreement between the numerical predicted timeaveraged Nusselt numbers and those of the empirical correlations verifies the accuracy of the numerical model.The evolution of wavy liquid film and velocity profiles are subjected to the pressure gradients existed in the non-sinusoidal solitary waves.The non-linear temperature profiles prove that both convection and conduction contribute to the mechanism to heat transfer in the liquid film.The normal velocity close to the wall,which significantly contributes to change the temperature gradient at the wall,plays an important role in the heat transfer of the wavy condensate film.A disturbance frequency of 16 Hz associated with the critical film Reynolds number was determined by analyzing the numerically generated neutral stability curve.The regular-spaced waves,whose wave amplitudes increase continuously,couldn't be generated on the interface when the disturbance frequency is lower than 16 Hz.The effects of the disturbance frequencies on wave characteristics show that the resulting waves inherit the frequency of disturbance.The increase in heat transfer area caused by film waves contributes negligibly to the heat transfer enhancement.Compared with other disturbance frequencies,the wave effect on heat transfer enhancement is the most significant at 16 Hz because of the minimum time-averaged thickness and the most notable variation of the normal velocity close to the wall in solitary waves.The minimum time-averaged thickness and the maximum proportion of substrate film length in the wave length proves that the film thinning effect is dominated by the length of the film substrate rather than the substrate film thickness.The variation of the normal velocity accounts for the convection effect of heat transfer enhancement due to the fact that the positive normal velocity,which flows away from the wall,contributes to increasing the temperature gradient and heat transfer coefficient.The predicted heat transfer coefficient for the wavy film at 16 Hz is found to be 30 percent greater than that of the Nusselt solution.Film thinning effect and convection effect contribute 6 percent and 24 percent to the heat transfer enhancement,respectively.