Vapor Diffusion And Mass Transfer Research With High Partial Pressure Non-Condensable Gas

The study is based on the project "Energy conversion and optimal allocation method in the production process ",which is the sub-project of 973 project "energy-efficient basic research on iron and steel production process".It is mainly studied on vapor condensation heat transfer in the heat pipe heat exchanger used for the low-temperature waste heat recovery equipment.Literature research shows that a small amount of non-condensable gas mixed with the heat pipe will cause a huge inhibition on heat transfer.Therefore,the heat pipe needs to be evacuated before use,but vacuum needs energy and time consumption,especially large-scale heat pipe heat exchanger,and the vacuum tube will remain a small amount of the non-condensable gas to affect heat transfer.In this paper,we design a non-vacuum loop thermosyphon to ef-fectively solve the inhibition of non-condensable gas on heat transfer.Compared with the traditional loop heat pipe,a non-vacuum gravity loop thermosyphon doesn't require wick,the working fluid circulates in the system by gravity,only non-condensable gas is introduced into the gas-liquid separator installed in the end of the condenser,and the inhibition on in the condensation of heat transfer can be greatly reduced.At the same time,the gas-liquid separator limits the circulation of the non-condensable gas in the loop system and avoids the non-condensable gas makes the cycle effect on the evaporator and the condenser.It is found that,a large amount of non-condensable gas is present in the non-vacuum gravity loop thermosyphon,the vapor forms a mixed heat and mass transfer region in the high-pressure non-condensable gas,and this region is defined as the diffusion gas-block zone.The non-condensable gas blocks the vapor flow in the diffusion gas-block zone.The vapor flows in the flow and diffusion way to reach the liquid film surface f'or condensation heat transfer.Meanwhile,the non-condensable gas reverses the vapor flow direction and diffuse to research the local equilibrium.The eff-ect of non-condensable gas in the loop thermosyphon in the start-up process of the system is studied.It is found that the non-condensable gas content,the liquid filling ratio of the system,the evaporator heat load and' the gas-liquid separator installation position all affect the system start-up time,operating pressure and the temperature of the evaporation and the condenser.The results show that:1)in the start-up process of the loop thermosyphon,vapor and non-condensable gas form a diffusion gas-block zone in the vapor pipe,the vapor is pushed through the diffusion gas-block zone and compresses the non-condensablegas into the condenser and the gas-liquid separator,when the diffusion gas-blockzone comes into the condenser tube,the system pressure is rapidly reduced,the"pressure peak" phenomenon in the start-up process is formed;2)the higher theliquid filling ratio,the greater the pressure during start-up,the longer the start-uptime;the greater the evaporator heat load,the shorter the start-up time;the higherthe non-condensable gas content,the longer the start-up time;3)the greater theinitial effective gas volume in the gas-liquid separator,the easier it is to start the loop thermosyphon.After the non-vacuum gravity loop thermosyphon comes into the work stage of the dynamic balance,a large number of non-condensable gas accumulates in the gas-liquid separator and reverses the direction of the vapor flow to diffuse in the condenser tube,thereby the diffusion gas-block zone is formed,to affect the condensation heat transfer.The experimental results show that:1)the non-condensable gas increases the system operating pressure and the evaporating temperature of the evaporator,that is beneficial to increase the local condensation heat transfer,but will affect the heat transfer capacity of the whole heat transfer system;2)The diffusion gas-block zone enlarges the pressure oscillation of the system due to the self-balance of evaporation and condensation,which increases the temperature amplitude on the condenser tube wall,and the turbulence of the diffusion gas-block zone can enhance the local condensation heat transfer efficiency;3)the diffusion gas-block zone greatly changes the condensate distribution of the vapor in the condense tube,the greater the non-condensable gas content,the higher the condensate vapor concentration in the front of the condenser,when the liquid filling ratio is 70%and the evaporator heat load is 3.0kW,the average heat flux in no vacuum conditions in the condenser is 1.67 times vacuum conditions,and the effective length of the condense tube is shorter than vacuum conditions;4)the presence of ethanol in content of 0.5-1 wt%in the system,will promote the local condensation heat transfer,and reduce the effect on condensation heat transfer of non-condensable gas,finally increase the heat transfer efficiency.In order to detect the distribution of non-condensable gas in the diffusion gas-block zone,a non-contact infrared detection platform is designed,by the use of the different absorption of vapor and non-condensable gas in the specific spectrum of the infrared spectrum,the platform measures the distribution of vapor in the quartz glass condense tube,thereby obtains the distribution of the diffusion zone in the condense tube and the concentration distribution of the components in the diffusion gas-block zone.Finally,the flow diffusion model of vapor and non-condensable gas in the diffusion gas-block zone is established based on the component transport equation.Using Maxwell-Stefan diffusion equation instead of Fick's diffusion equation,the approximate solutions on certain operating conditions are obtained by fluid numerical simulation.The results show that the trends of the vapor distribution in the condense tube are in good agreement in the non-vacuum gravity thermosyphon experiment,the non-contact infrared detection and the diffuse flow model.