Experimental Investigations On Two-phase Flow And Heat Transfer Characteristics In Shell Side Of LNG Spiral-wound Heat Exchanger

LNG spiral wound heat exchangers have the advantages of high effectiveness,high operation pressure and large scale unit,and have been used as main cryogenic heat exchangers in large-scale LNG plants.The LNG spiral-wound heat exchanger is the key equipment in liquefaction process,and its performance can directly determine the LNG production of the whole liquefaction process.Due to 70%of the thermal resistance is in the shell side,the heat transfer and pressure drop characteristics in the shell side of LNG spiral-wound heat exchanger is the key consideration for the design and optimization of such heat exchangers,and should be clarified.In the present study,the test method for the heat transfer and pressure drop characteristics of the cryogenic non-azeotropic two-phase flow in the shell side of LNG spiral wound heat exchanger is proposed.Based on the test method,the test rig of heat transfer and pressure drop characteristics of two-phase flow in the shell side of LNG spiral wound heat exchanger is established,which is the first visualization test rig for such experimental research.The test rig can reflect the structure and parameter of the practical large-scale LNG spiral wound heat exchanger,realize the cryogenic test conditions,realize the regulation of the multiple working conditions in a wide range,and satisfy the explosion-proof requirements of the hydrocarbon working fluids.According to the actual working fluids in the shell side of LNG spiral wound heat exchanger,the pure propane and ethane/propane binary mixtures with molar fractions of 10%/90%to60%/40%are chosen as the test fluids.The working conditions and structural parameters in the analysis can reflect the operation conditions and structure features of the practical LNG spiral wound heat exchanger,covering the radial tube pitch and tube diameter ratio of 1.08¹.33,the longitude tube pitch and tube diameter ratio of 1.17¹.50,the mass flux of 40⁸0 kg m^-2 s^-1,the vapor quality of 0.2⁰.9,the heat flux of 4¹0 kW m^-2and the operation pressure of 0.25 MPa.The flow patterns,heat transfer and pressure drop data of the cryogenic non-azeotropic two-phase flow in the shell side of LNG spiral wound heat exchanger are acquired through the experiment.The influence of fluid compositions,working conditions and structure parameters on the heat transfer and pressure drop characteristics are analyzed,and the following achievements are acquired.（1）The shell-side flow patterns for LNG spiral-wound heat exchangers are clarified and the flow pattern map is developed.The shell-side flow patterns for LNG spiral-wound heat exchangers,including the column falling film flow,the droplet falling film flow,shear flow and mist flow,are observed,and can be influenced by the working conditions,the geometry parameters and the fluid compositions.The effects of working conditions on the flow patterns include:as the vapor quality increases,the flow pattern transfers from the column falling film flow to the droplet falling film flow,the shear flow and the mist flow;as the mass flux increases,the vapor qualities for the flow pattern transition from the column falling film flow to the droplet falling film flow and from the droplet falling film flow to the shear flow decrease,but the vapor quality for the flow pattern transition from the shear flow to the mist flow increases.The effects of geometry parameters on the flow patterns include:as the row pitch increases,the vapor qualities for the flow pattern transition from the column falling film flow to the droplet falling film flow,from the droplet falling film flow to the shear flow,and from the shear flow to the mist flow increase;as the tube pitch increases,the vapor qualities for the flow pattern transition decrease.The effects of fluid compositions on the flow patterns include:for the mixed fluid with the components of ethane and propane,as the mole fraction of ethane increases,the vapor qualities for the flow pattern transition from the column falling film flow to the droplet falling film flow and from the droplet falling film flow to the shear flow decrease,but the vapor quality for the flow pattern transition from the shear flow to the mist flow increases.Based on the experimental results,the flow pattern map and transition principle are developed.（2）The heat transfer characteristics of two-phase flow in the shell side of LNG spiral-wound heat exchanger are clarified.The experimental results show that the heat transfer characteristics of two-phase flow in the shell side of LNG spiral-wound heat exchanger are influenced by the the working conditions,the geometry parameters and the fluid compositions.The effects of working conditions on the heat transfer characteristics of two-phase flow include:as the vapor quality increases,the heat transfer coefficient increases at first and decreases sharply at the vapor quality of 0.7⁰.9;as the heat flux increases,the heat transfer coefficients of the column falling film flow,the droplet falling film flow,and the shear flow keep constant at first and then incrases,but the heat transfer coefficients of the mist flow decreases;as the mass flux increases,the heat transfer coefficient with the low heat flux increases gradually,but the heat transfer coefficient with the high heat flux varies non-monotonically.The effects of geometry parameters on the heat transfer characteristics of two-phase flow include:as the tube pitch increases,the heat transfer coefficient gradually decreases,and the decrease incrases with the increasing vapor quality and heat flux;as the row pitch decreases,the heat transfer coefficient at the mass flux of 40 kg m^-2 s^-1 and the vapor quality larger than0.4 increases at first and then decreases,but for other conditions,it increases monotonically.The effects of fluid compositions on the heat transfer characteristics of two-phase flow include:as the vapor quality is less than 0.7,the heat transfer coefficient decreases with the increasing mole fraction of ethane,and the maximum decrease is 21%;as the vapor quality is larger than 0.7,the heat transfer coefficient increases with the increasing mole fraction of ethane,and the maxmum increase is 27%.（3）The heat transfer correlation of two-phase flow in the shell side of LNG spiral-wound heat exchanger is developed.The developed correlation is capable of reflecting the effect of working conditions,geometry parameters and fluid compositions on the heat transfer characteristics of two-phase flow in the shell side,and the deviation for 95% of the experimental data is within±20%.（4）The pressure drop characteristics of two-phase flow in the shell side of LNG spiral-wound heat exchanger are clarified.The experimental results show that the two-phase pressure drop characteristics in the shell side of LNG spiral-wound heat exchanger are influenced by the the working conditions,the geometry parameters and the fluid compositions.The effects of working conditions on the two-phase pressure drop characteristics include:the frictional pressure drop increases with the increasing mass flux and vapor quality,and the increase gradient increases with the increasing vapor quality.The effects of geometry parameters on the two-phase pressure drop characteristics include:the frictional pressure drop decreases with the increasing tube pitch and the increasing row pitch.The effects of fluid compositions on the two-phase pressure drop characteristics include:as the vapor quality is less than 0.4,the frictional pressure drop decrases with the increasing the mole fraction of ethane,and the maximum decrease is 17%;as the vapor quality is larger than 0.6,the frictional pressure drop increases with the increasing mole fraction of ethane,and the maximum increase is 41%.（5）The two-phase pressure drop correlation in the shell side of LNG spiral-wound heat exchanger is developed.The developed correlation is capable of reflecting the effect of working conditions,geometry parameters and fluid compositions on the two-phase pressure drop characteristics in the shell side,and the deviation for 95%of the experimental data is within±25%.