Research On Frosting Conditions And Applicability Of Liquefied Natural Gas Ambient Air Vaporizer

Natural gas（NG）plays an important role in environmental protection and energy structure adjustment.Liquefied natural gas（LNG）has the high energy density and small storage space,which has developed rapidly in China and abroad in recent years.LNG needs to be regasified into gaseous natural gas at temperatures not less than 5 ℃ before being sent to urban gas networks.With the proposal of the national goal of ‘carbon peak,carbon neutralization’,reducing LNG gasification energy consumption has also received more and more attention.LNG ambient air vaporizer（AAV）directly uses the heat energy in the natural environment,which is an effective means to reduce the energy consumption of LNG gasification,and has a broad development prospect.However,because the coupled heat and mass transfer characteristics of LNG AAV is still not clear,AAVs lack relevant theoretical guidance in design,application and operation switching,resulting in poor operation effect of LNG AAV.Therefore,it’s needed to study the coupled heat and mass transfer performance of LNG AAV.In this paper,the dynamic coupled heat and mass transfer characteristics of LNG AAV,the complex frosting characteristics of the surface of the fin during the cryogenic liquid gasification process,and the correlation between the climate and the outlet temperature of the natural gas and the growth rate of frost layer on the surface of the AAV are studied by means of experiment,field measurement and numerical simulation.On this basis,the switching operation judgment index and judgment standard of AAV is analyzed,and the climate zoning for the design and application of LNG AAV is studied based on auxiliary heater degree days.This study would lay a theoretical foundation for the design,application and optimal operation of LNG AAV.Firstly,a coupled heat and mass transfer model of LNG AAV considering the bubble point change of LNG is established,which can describe the temperature change of LNG during the gas-liquid phase change pipe section.The frost growth model is verified by the experimental data under cryogenic temperature conditions from previous literatures;and the validation results show that the correlation for frost layer density under normal cold condition can not accurately grasp the influence of wall temperature on the frost layer density,especially when the wall temperature is lower than-78 ℃.Considering the cryogenic characteristics of LNG,these correlations for frost layer density are not applicable to the frost growth process on the surface of LNG AAV.In order to get the accurate frost growth model on the surface of the AAV,the characteristics of frosting on the surface of a finned tube in the regasification process of cryogenic liquid are investigated by a series of experiments.The frost morphology is observed to reveal the evolution of frost morphology on the surface of a finned tube in the regasification process of liquid nitrogen.It was found that a new “wisteria-type” frost crystal is formed in the area of the fins with an initial temperature drop rate of higher than2.32℃·min^-1.Image analysis method is used to measure the frost thickness,and the spatial distribution characteristics of frost thickness and its influencing factors are analyzed.The results show that the influence of LNG flow rate and ambient temperature on frost growth is greater than that of relative humidity.Furthermore,based on the experimental data,the correlation for frost layer density suitable for the frosting on the surface of a finned tube in the regasification process of cryogenic liquid is obtained,and verified by the experimental data from other scholars.The results show that the new correlation for frost layer density can accurately predict the frost layer growth process on the surface of a finned tube in the regasification process of cryogenic liquid under different conditions.Taking a typical LNG AAV in a LNG gasification station in north China as the research object,the dynamic coupled heat and mass transfer characteristics of LNG AAV in reality is studied through field measurement,and the coupled heat and mass transfer model of LNG AAV is verified with both the measured data and foreign experimental data.The results of the field measurement show that the outlet temperature of natural gas decreases with the operation time due to frosting,but the average heat transfer coefficient of LNG AAV does not decrease significantly with the operation time due to the fluctuation of LNG gasification flow.Furthermore,because LNG absorbs a lot of heat from the surrounding environment,the ambient temperature near the ground around LNG AAV decreases significantly.The model validation results show that the coupled heat and mass transfer model established in this paper can well predict the variation of outlet natural gas temperature under different conditions.Based on the coupled heat and mass transfer model of LNG AAV,the coupled heat and mass transfer characteristics of LNG AAV are further studied.The distribution characteristics of frost thickness and fluid temperature along the tube length under different operation time are discussed,and the effect of air temperature distribution near ground,air temperature and humidity,LNG gasification flow rate and fin structure on the average frost growth rate and the outlet natural gas temperature are investigated.According to the growth rate difference of frost layer on the surface of AAV under different air temperatures and humidity,the frosting condition is divided into the heavy frosting conditions,general frosting conditions and low temperature frosting conditions.Finally,based on the coupled heat and mass transfer characteristics of the LNG AAV,the energy demand,frosting heat loss coefficient,frosting area loss coefficient,frost coverage rate and other index parameters are defined to judge the switching operation of LNG AAV.According to the variation of these indicators under different conditions,the determination method of the judgment standard of the AAV switching operation is proposed,and the calculation example is analyzed.At the same time,using the mathematical methods of correlation analysis,dispersion analysis and so on,the regionalization indexes for the application of climate zoning in the design of AAV are discussed.The spatial difference of the regionalization indexes is carried out by using the collaborative Kriging spatial difference method.Combined with K-means clustering analysis and the idea of hierarchical regionalization,the boundaries of the regionalization indexes are determined,The climate of LNG AAV design and application in China is divided into eight climate zones by using Arc Map software.Through analyzing the changes of meteorological parameters in each climate zone,the optimization strategy of LNG AAV design and operation in each climate zone is proposed.This study can provide a theoretical basis for the switching operation of LNG AAV,and has important theoretical and practical significance for improving the operation performance of LNG AAV.