Numerical Investigation Of Heat Transfer In Direct Contact Condensation Of Steam To Subcooled Water Spray

In a vacuum column of the crude oil distillation unit,if the technique of direct contact condensation heat transfer(DCCHT)by spraying liquid into vapor is applied as opposed to a section of packing,the pressure drop due to the resistance of packing would be significantly reduced.Thus,the resulting lower bottom pressure of the vacuum column greatly reduces the energy requirement.However,most studies focused on examining the DCCHT on solid walls such as tubes,microchannels and trays and there has not been numerical studies on DCCHT achieved by liquid spray.The DCCHT by spraying liquid into vapor is much complex due to its dependence on many factors such as turbulence in the liquid sheet,sheet breakup and steam condensation.Furthermore,the reported numerical models could not be applied in the DCCHT by liquid spray.And the predictions of heat transfer coefficient(HTC)and flow pattern are mostly based on empirical estimations,which limits its applications in modern industry.Therefore,the numerical study of DCCHT is of great importance in the application of DCCHT,the improvement of heat transfer facilities and the development of phase change heat transfer theory.A computational fluid dynamics(CFD)model is proposed to study the DCCHT and the spray hydraulics of steam to subcooled water spray produced by a pressure-swirl nozzle.In the CFD model,the Volume of Fluid(VOF)is used to track the complex liquid-vapor interface,the kinetic theory-based model is utilized to simulate the mass transfer due to condensation and the interfacial area density is introduced to evaluate the area of the vapor-liquid interface.Comparisons between the simulation results and experimental results in literatures under the same operational conditions show that,deviations of dimensionless temperature profiles are not more than 10%,and the simulated HTC are within the experimental uncertainties.The simulation results also show that condensation does not significantly affect the spray cone angle,although it affects the sheet thickness and sheet breakup length.Additionally,when inlet water flow rate is 0.04～0.08 kg/s,inlet water temperature is 65℃and steam temperature is 100℃,HTC in the DCCHT is about 200k W/m~2/K,which is 2 times more than the HTC in traditional film-wise condensation on solid walls.The proposed CFD model is proved to accurately predict the temperature field,flow pattern,and heat transfer coefficient in the DCCHT,which are of major importance in the analysis and design of heat transfer devices and in particular in the application of DCCHT.