| The condensation heat transfer phenomenon of multi-component non-azeotropic mixture exists in the fields of cascade condensation of natural gas and condensation of mixed refrigerants.During the cascade condensation of natural gas,the varies components(e.g.methane,ethane,propane,nitrogen etc.)condense in order of boiling point.When one component condenses,the others play as non-condensable gases.The existence of non-condensable gas causes severe deterioration of the vapor condensation heat transfer.Therefore,the investigation of the flow and heat transfer mechanism of non-azeotropic mixtures has important guiding significance for strengthening the heat transfer characteristics.In this paper,the condensation flow and heat and mass transfer characteristics of gas mixtures at different stages of natural gas liquefaction was studied through theoretical analysis of heat and mass transfer,numerical calculation via user defined functions and visualization experimental research.And a proper fin form was proposed for strengthening the condensation process of multi-component gas mixture.A visualization experimental system was designed to study the flow pattern and heat transfer characteristics of propane vapor condensation on the vertical plate.The effects of the different wall subcooled temperatures(temperature difference between wall and saturated gas)and liquid film flow patterns on the heat transfer coefficient were analyzed.The research shows that the condensate film on the plate is wave laminar flow on the experimental conditions.When the liquid film Reynolds number is between 59 and 149,there are long isolated interface waves(with a length of 8 to 10 mm)on the condensation wall.And when the liquid film Reynolds number is between 149 and 321,the curvature of the interface waves increase and its length decrease slightly,meanwhile the waves break and merge losing stability and turning to irregular dense and short waves.Compared with the predicted value of Nusselt theory,the average condensation heat transfer coefficient obtained in the experiment is higher,which is because the fluctuation of the liquid film can enhance the heat transfer coefficient.With the increase of the wall subcooled temperature and the liquid film Reynolds number,the intensified interfacial waves enhance the condensation heat transfer coefficient.The average condensation heat transfer coefficient decreases with the increase of the wall subcooled temperature,but the descending rate decreases gradually.The variation of the average Nusselt number with the liquid film Reynolds number obtained by the experiment show good agreement with Kutaladze's correlations.A numerical model was proposed to predict the filmwise condensation characteristics of ultralow temperature vapor with non-condensable gas on an isothermal vertical plate,based on the diffusion layer theory.The Volume of Fluid(VOF)method was adopted to capture the gas-liquid interface,and the mass diffusion in the gas phase was solved using the Species Transport Model.A user defined function(UDF),which was written using a cell iteration method and based on the mass conservation on the gas-liquid interface,was applied to deal with the mass and energy source term of the condensing process.The governing equations are solved numerically in the CFD software using an iterative program.At the last stage of cascade natural gas liquefaction process,the methane condenses with a small amount of nitrogen playing as noncondensable gas.The condensation heat and mass transfer characteristics at the working conditions of wall subcooled temperature of 2 K?20 K and the noncondensable gas mole fraction of 2%?13%were analyzed.The numerical results show that the flow heat and mass transfer in the condensation process mainly occur near the cold wall within a range of 10 mm.A small amount of noncondensable gas can reduce condensation heat transfer coefficients significantly.When the mole fraction of non-condensable gas is 2%,the heat transfer coefficient is reduced by 58.2%,at the subcooled temperature of 15 K.During the condensation process of the gas mixture,there is a gas boundary layer with gas concentration gradient near the condensate liquid film.And the gas boundary layer is about two orders of magnitude thicker than the liquid film.Under the calculation conditions,the thermal resistance of the gas boundary layer reaches a maximum value of about 60 times of the liquid film,revealing that the diffusion resistance in the gas boundary layer plays a major role in the deterioration of heat transfer.In addition,the increase of both the inlet nitrogen mole fraction and the the wall subcooled temperature will decrease the condensation heat transfer coefficient.Although the increase of noncondensable gas mole fraction will reduce the liquid film thickness,its contribution to heat transfer is negligible compared with the increased diffusion resistance caused by the decrease in the vapor concentration gradient.The increase of the wall subcooled temperature will increase the thickness of both the liquid film and gas boundary layer,which could increase the resistance in condensation heat and mass transfer.The condensation characteristics at different liquification stages of natural gas and the performance of different binary hydrocarbon vapor mixtures were investigated.The condensation heat and mass transfer characteristics of the propane/methane,ethane/methane,and methane/nitrogen mixtures on an isothermal vertical wall were investigated.At the first and second stages of natural gas liquefaction process,the high content methane plays as noncondensable gas.While at the last stage,the low content nitrogen is the noncondensable component.The study shows that the average heat transfer coefficients of propane/methane,ethane/methane and methane/nitrogen mixtures decrease by 53.4%,58.3%and 19.5%as the noncondensable gas mole fraction increase from 80%to 95%,65%to 85%and 2%to 14%,respectively.Furthermore,high noncondensable gas concentration will result in the approximate linear decrease of heat transfer coefficient.There are waves in the condensate film downstream,at the first and second stages(propane/methane,ethane/methane)with high noncondensable gas concentration,due to the disturbing by gas boundary layer.While,at the last stage(methane/nitrogen)with low noncondensable gas concentration,the liquid film increases along the flow direction without fluctuations.At the methane/nitrogen mixtures condensation stage,the thermal resistance ratio of the gas boundary layer and liquid film is from several to dozens,thus the thermal resistance of the liquid film cannot be ignored.However,at the propane/methane and ethane/methane condensation stages,the thermal resistance of the gas phase boundary layer is more than one hundred times that of the liquid film,therefore the thermal resistance of the liquid film layer is negligible.According to the numerical calculation results,the prediction correlations of the heat transfer coefficient at different condensation stages was obtained by the least squares fit,which can be used as an important basis for engineering calculations.A multi-component gas mixtures condensation mass transfer model was established,and the condensation heat transfer characteristics of the three components gas mixture of propane/methane/ethane were investigated,which was compared with the two components gas mixture of propane/methane.The effects of the inlet noncondensable gas mole fraction and wall subcooled temperature on the flow,heat and mass transfer characteristics were analyzed,on the condition of 10%propane mole fraction,0%?40%ethane mole fraction,50%?90%methane mole fraction and wall subcooled temperature of 10 K-40 K.Different from the monotonic gas gradient distribution in the binary mixture,the mole concentration of methane component has a peak near the cold wall and the mole concentration of the ethane component has a valley near the cold wall,in the condensation process of propane/methane/ethane gas mixture.That is because of the carrying effect of large molecular weight components.In addition,compared with methane gas with small molecular weight,ethane gas with large molecular weight is more likely to accumulate near the cold wall,and the increase of ethane mole fraction will weaken the accumulation of methane near the cold wall.Additionally,when the ethane is added to the propane/methane mixture,the gas boundary layer and the temperature boundary layer separate,and the separation amplitude increases gradually with the increase of the ethane mole fraction.At the same time,the addition of the ethane component will reduce the gas and liquid phase thermal resistance at the same time,thereby increasing the condensation heat transfer coefficient.A "micro-pin" shaped fin was proposed in this paper,according to the position of the main thermal resistance of the non-azeotropic gas mixture condensation heat transfer process.The pins are processed on the fin plate by a stamping process to strengthen the vapor condensation in presence of noncondensable gases.A three-dimensional numerical model of "micro-pin" fin channel was established,and the condensation heat transfer enhancement of the "micro-pin" structure on the propane/methane and methane/nitrogen gas mixtures was studied,which was compared with the straight fin,serrated fin and corrugated fin.The study shows that there is a heat transfer coefficient jump at the position of "micro-pin",indicating that the "micro-pin"structure can strengthen the condensation heat transfer.The small holes formed by stamping can realize the mass and momentum exchange of the fluid on both sides of the fin,which will break the stability of the viscous sublayer and the liquid film.The pins can expand the contact area of the condensation wall surface with the gas-liquid interface and gas boundary layer,and enhance the disturbance of the boundary layer.For the condensation process of propane/methane gas mixture,the comprehensive performance factors of "micro-pin" fin have increased respectively by 1.78,1.68 and 1.69 times compared with straight fin,serrated fin and corrugated fin.Compared with the traditional form of strengthening fins,"micro-pin" fin can strengthen the condensation heat transfer process of non-azeotropic gas mixtures more effectively. |
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