Molecular Dynamics Study Of The Boiling Heat Transfer Characteristics Of Metfane,Ethane And Their Mixed Nano-Liquid Films

The Printed Circuit Heat Exchanger(PCHE)is a compact、 efficient、 reliable heat exchanger that meets the needs of small size,lightweight,low-temperature and high-pressure resistance of the main cryogenic heat exchanger of Floating Liquefied Natural Gas(FLNG)at sea.Understanding the flow boiling heat exchange mechanism of LNG in PCHE is a necessary prerequisite for the localization of PCHE.LNG is a typical non-azeotropic mixed mass with complex heat transfer characteristics.Due to the existence of temperature slip and concentration gradient,the phase change heat transfer of mixed masses will generate mass transfer resistance compared with that of single masses,resulting in heat transfer deterioration.The heat transfer in the evaporating thin film region is widely present in the evaporation and boiling processes of microchannels,and 80% of the heat transfer in the channel occurs in this region,so the evaporating thin film heat transfer is a common problem in the phase heat transfer of microchannels.It is very important to research the boiling heat transfer characteristics of methane,ethane and their mixed nano-liquid films to promote the localization of PCHE.Therefore,this paper investigates the boiling heat transfer characteristics of methane,ethane and their nano-liquid films by molecular dynamics simulation.In this paper,the molecular dynamics simulations of the boiling heat exchange process of methane,ethane and their mixed nano-liquid films with different ratios at different heat source temperatures are carried out,and the main contents include three parts:(1)The boiling behavior of nano-liquid film with single masses at different heat source temperatures and the variation of related thermodynamic parameters were studied.The results show that,there are only two forms of nano-liquid film boiling:evaporation and explosion boiling;the variation curve of heat flux with superheat is consistent with that of pool boiling;the higher the heat source temperature is,the faster the energy transfer rate from the substrate to the liquid film will be,resulting in a large amount of energy gathering near the substrate and easier to reach the film boiling state.(2)The relevant thermodynamic parameters and the changes of each component in the liquid film were investigated for different ratios of methane/ethane mixed nano-liquid films at different heat source temperatures.The results show that,in both nuclear boiling and film boiling,a small number of molecules that do not vaporize are adsorbed on the solid substrate,which is called the fixed adsorption layer,and the percentage of methane in the fixed adsorption layer is slightly higher than that of methane in the initial nano-liquid film.In nuclear boiling,the proportion of each component vaporized substances varies consistently during the vaporization of different ratios of mixed nano-liquid film.Firstly,the molecular vaporization is dominated by methane,and when the methane ratio in the liquid film except the fixed adsorption layer decreases to about 0.1,the molecular vaporization changes to be dominated by ethane;in film boiling,the phenomenon of nano-liquid film detaching from the solid substrate and the ratio of mixed components have an effect on the change of each component ratio in the vaporization process.When the methane ratio is less than or equal to ethane,the molecular vaporization is dominated by methane before the nano-liquid film is separated from the solid substrate,and the vaporization rates of methane and ethane are gradually the same after the nano-liquid film is separated from the solid substrate.When the ratio of methane is greater than ethane,the molecular vaporization is dominated by methane before the nanofilm detaches from the solid substrate.The vaporization rate of methane and ethane is gradually the same when the vaporization rate of each component in the mixed nano-liquid film is about 0.5.(3)The mass transfer characteristics of mixed and single nano-liquid films were compared and analyzed under different boiling modes.The results showed that,when the nuclear boiling occurred,the vaporization rate of ethane in the mixtures with the increase of methane fraction in the mixtures compared with that of the pure ethane,and the overall vaporization rate and mass transfer rate of the mixtures showed the characteristic of increasing with the increase of methane fraction;when the film boiling occurred,the mass transfer strength was influenced by the vaporization rate of the nano-liquid film before it separated from the solid substrate and the time required for the nano-liquid film to separate from the substrate.When the methane fraction is greater than or equal to ethane,the vaporization rate gradually reaches the maximum,the vaporization rate of methane.while the time required for the nano-liquid film to separate from the substrate decreases with the increase of methane fraction.In summary,in nuclear boiling,the overall vaporization rate and mass transfer are optimal with a larger initial methane fraction in the mixtures;in film boiling,the overall vaporization rate and mass transfer are optimal with a 1:1 initial methane/ethane fraction in the mixtures.Through the research of this paper,the mechanism of different boiling phenomena of nano-liquid films at different heat source temperatures is uncovered,and the optimal mixing group distribution ratio of mass transfer under nuclear boiling and film boiling is obtained,which will provide theoretical significance for the study of flow boiling heat transfer of LNG in PCHE.