Study On Frosting Characteristics Of Moist Air Near Cold Wall

The water vapor in moist air near the cold wall is frozen to some tiny ice particles and then these tiny ice particles are attached to the cold wall,which is known as the frosting phenomenon of moist air near the cold wall.The frosting phenomenon,widely exists in nature and many engineering fields,is a complicated physical process which involves fluid flow,heat and mass transfer,and phase change.Frosting will affect the operating characteristics of the equipment in most cases.In especial,in some aeronautic and aerospace engineering,the heat exchanger sometimes works under relatively large temperature difference condition(larger than 100K)and occurs the fast frosting phenomenon.For instance,the heat transfer coefficient and the pressure drop on moist air side in the precooler are seriously affected by the frosting phenomenon.During this period(about several tens or hundreds of seconds),the operation of the engine cannot be stopped by the defrosting cycles as applied in the heat exchanger.The frosting phenomenon will cause the heat transfer failure of the equipment,or even cause the catastrophic accident.Therefore,it has significance for the equipment design to study the frosting mechanism and forecast the frosting process.The frosting way of moist air under large temperature difference condition mainly includes two parts:the frosting process of moist air on the wall and the frosting process of moist air near the cold wall.It is different from the ordinary frosting process under small temperature difference condition.The work about the frosting process of moist air under large temperature difference condition is relatively scarce.Therefore,the frosting process of moist air near the cold wall was analyzed from the macroscopic and the microcosmic perspective.It is mainly related to the ice particle formation,the ice particle growth,and the ice particle deposition on the cold wall.As for the first factor,the modified enthalpy-based lattice Boltzmann model was established on the basis of the phase change theory and the formation process of the ice particle.Comparing with the enthalpy-based lattice Boltzmann model,the modified model was verified that it can be used to describe the droplet freezing process involving fluid flow.Then,the freezing process of a suspended droplet and a falling droplet in a cold space were simulated by the modified model,respectively.The temperature distribution in the droplet,the freezing condition of the droplet,the freezing parameters,and its effect factors were obtained.With regard to the second factor,based on the growth process of the ice particle,an improved multi-component enthalpy-based lattice Boltzmann model was proposed to simulate the growth process of a suspended ice particle in moist air flow.The growth shape,the average growth velocity of the ice particle,and its effect factors were investigated,respectively.For the last factor,the deposition process of the ice particle has been analyzed based on the gas-solid two phase flow theory and the snow accumulation phenomenon.The deposition process of the ice particle on the single and the staggered tube-bundle were investigated by the LBM-LGA ice particle deposition model,respectively.The effects of the initial position of the ice particle,the ice particle diameter,and the tube-bundle arrangement on the moving trajectory and deposition location of the ice particle were obtained.Based on the frosting mechanism of moist air near the cold wall and the frosting way of moist air under large temperature difference condition,the improved multi-component enthalpy-based lattice Boltzmann model coupled with the LBM-LGA ice particle deposition model was used to establish the frosting computing platform of moist air under large temperature difference condition.The correctness of the frosting computing platform was verified by comparing with the experimental results.Then,the frosting process of moist air in extremely low temperature staggered tube-bundle channels was investigated.The simulation results were compared with the simulation results of the deposition process of ice particles on the staggered tube-bundle by the LBM-LGA ice particle deposition model in the same condition.The comparison results showed the main frosting way of moist air under large temperature difference condition.Finally,the frosting process of moist air in extremely low temperature tube-bundle channels was investigated by this frosting computing platform.The frosting condition and the heat transfer characteristics of tube-bundle were obtained.The effects of the tube spacing,the tube diameter,the moist air flow velocity,the tube surface temperature,and the moist air relative humidity on the average frost layer thickness of tube-bundle,the average heat exchange coefficient on moist air side,and the pressure drop cofficient on moist air side were discussed,respectively.