2D Numerical Simulation Of Heat And Mass Transfer In Direct Contact Condensation Of Vapor To Subcooled Water Spray

Direct contact condensation of vapor to subcooled liquid spray is a common form of direct contact heat transfer widely used in industries.Compared to heat exchangers used in the regenerative heat transfer and the indirect contact heat transfer,heat exchangers used in direct contact condensation of vapor to subcooled liquid spray with more simple structures not only have larger heat transfer surfaces and higher heat transfer rate,but also require less maintenance since they are more difficult to be corroded and scale.However,due to the complicated heat and mass transfer mechanism in direct contact condensation of vapor to subcooled liquid spray,there is no systematic theory.So researches on this direct contact condensation are very helpful to development of phase change heat transfer theory and design and rebuilding of equipment used in direct contact condensation.Discharged from pressure-swirl nozzles,the spray liquid with a high swirl speed forms a thin conical film and then the film breaks up into large quantity of droplets as it flows downstream.The heat transfer efficiency of the interphase between the thin liquid spray film and vapor is said to be very high because the flow of the thin liquid film is highly turbulent.With the help of commercial computational fluid dynamics software Fluent,the flow of vapor and water liquid in and out pressure-swirl nozzles,and heat and mass transfer in direct contact condensation of vapor to subcooled water spray film are numerically simulated.Since the flow field in and out a pressure-swirl nozzle can be regarded as axisymmetric,a 2D mesh physical model was built to simulate 3D swirl flow through the 2D Axisymmetric Swirl function in Fluent.The laminar model,RNG k-?,RSM Linear Pressure-Strain model and RSM Low-Re Stress-Omega model were used to simulate two phase flow in a large scale pressure-swirl nozzle with the inlet Re in a range of 25000~53000.The results shows that RSM Linear Pressure-Strain model did best among the four models through comparing thickness,velocity and spray cone angles of the liquid flim at the discharge and discharge coefficients.The RNG k-? which did the poorest was unable to capture air core inside the nozzle.The 2D mesh model was also applied to investigate the influence of geometric parameters on the liquid film at the discharge.The results showed that the film thickness increased and the spray cone angle decreased as the Ls/Ds increased;D0 had a significant influence on the flows;the convergence angle of the nozzle has little influence on the flow.Based on the Lee phase change model,the heat and mass transfer in direct contact condensation of vapor to subcooled water spray film was simulated in the 2D model by setting the accommodation coefficient ri in the Lee model to 50,200,2000,10000 respectively.When ri was 200s-1 or 2000s-1,the temperature variation and heat transfer coefficient variation in the flow direction matched well with those of experiments in literature.So ri=200~2000s-1 is recommended when simulating similar phase change processes.Decrease of the liquid turbulence kinetic energy in the flow direction is the main reason for the decrease of heat transfer coefficient.