Experiment And CFD Simulation Of Steam Condensation Heat Transfer Process Outside The Tube

With the increasingly serious energy shortage caused by the rapid development of modern industry,energy conservation and emission reduction have become a key issue to be solved urgently.Condensation is an efficient means of enhancing heat transfer.Its mechanism is complex and there are many influencing factors.The study of momentum,energy and mass transfer in the condensation process is of great significance to improving the heat transfer efficiency of the condenser and energy utilization.With the development of computer level and fluid dynamics theory,computational fluid dynamics(CFD)has become an effective method for studying the condensation process.CFD simulation has significant advantages in analyzing the mass and heat transfer characteristics of the liquid film.This paper explored the numerical simulation method of steam condensation outside the tube.Based on the experimental and theoretical analysis,the two-dimensional CFD models of the steam condensation outside the vertical tube and the horizontal tube were construct.The liquid film flow and heat transfer characteristics of steam condensation outside the tube were studied.The main research results are as follows:(1)The geometric models of the casing annulus were constructed,and different multiphase flow models and interphase models were studied.The results showed that using VOF model,with the liquid phase as the main phase,the simulation results were more reliable.For capturing the phase interface,the geometric reconstruction method and the CICSAM method were more accurate,but the CICSAM solution saved more computing resources.The phase change mass transfer intensity factor r was determined by the heat transfer and temperature difference between phase interface and steam.(2)A two-dimensional CFD model of steam condensation outside the vertical tube was built.The effects of surface tension,wall contact angle,wall material,and tube type on the condensation heat transfer process were investigated.The results showed that when the r was 60,000,the temperature difference between phase interface and steam was less than 0.1K,and the error between the simulated and the experimental values of the average condensation heat transfer coefficient was within ±25%,indicating that the CFD model was reliable within this operating range.The steam velocity magnitude gradually decreased along the axial direction.There was a radial velocity directed to the condensing wall.A continuous liquid film formed on the cold wall along the flow direction,and the ripples formed on the surface of the liquid film under gas disturbance and surface tension.The thermal resistance was concentrated inside the liquid film with a linear temperature gradient.Additionally,the effect of surface tension was negligible.With the increase of the contact angle,the film condensation gradually turned into droplet condensation,and the condensation heat transfer coefficient ascended.Moreover,the copper tube with high thermal conductivity had a better heat transfer performance.(3)Experimental and CFD methods were used to study the steam-air heat transfer process in the horizontal tube-in-tube condenser.The experimental results showed that the total heat transfer coefficient was mainly influenced by the air flow.The larger the air flow,the higher the total heat transfer coefficient.The higher the steam saturation temperature,the higher the heat transfer rate due to the higher heat transfer driving force.Coupling the model of convective heat transfer process in the tube and the model of condensation process in the casing annulus,the error between the simulation results and the theoretical formula or the experimental results was within 士 10%,validating the feasibility and accuracy of the coupling CFD model.In the small range of heat flux(103～104 W/m2),the heat transfer coefficient of steam outside the horizontal tube was two orders of magnitude smaller than the value calculated by the theoretical formula,which might be due to the poor ductility of the liquid film generated under the low condensation.(4)According to the range of heat flux,the classical Nusselt formula was modified.The correlations of the heat transfer coefficient for the film condensation outside the vertical tube and the horizontal tube were obtained:αNu=0.733[gρl(ρl-ρv)λl3hlv/μlLq]1/3(104<q<105 W/m2);αNu=0.0083[gρi(ρl-ρv)λl3hlv/μld1q1]1/3(103<q<104 W/m2),the average deviation between the values calculated by these correlations and the CFD simulation results or the experimental values was within±20%.