Study On Condensation Heat Transfer Of Binary Mixture Refrigerant In Helical Tube

As the main equipment of liquefied natural gas(LNG),coil-wound heat exchanger is widely used in large natural gas liquefaction equipment because of its compact structure and high heat exchange rate,which can realize heat transfer of various working fluids and high operating pressure.At present,the large coil-wound heat exchanger in China rely on import and lack of technology for designing and manufacturing.The mixture refrigerant in the coil-wound heat exchanger belongs to the hydrocarbon zeotropic refrigerant,and the heat transfer process of two-phase phase change is extremely complex.Because of the lack of high precision heat transfer correlation to calculate the two phase heat transfer coefficient in the coil-wound heat exchanger,it is very difficult to optimize the design of the coil-wound heat exchanger.Therefore,the condensation heat transfer characteristics of hydrocarbons mixtures in helical tube are studied by experiment and simulation in this paper.First of all,an experimental facility is designed and built to measure the condensation flow and heat transfer characteristics of hydrocarbons mixtures in a single helical tube of coil-wound heat exchanger under real working conditions,and the test method for measuring the inner wall temperature of the inner tube of the double casing for test section is studied,the experimental system and experimental data processing methods are introduced.On the basis of a well built experimental facility,the condensation heat transfer characteristics of propane are studied,and the influence of different operating parameters on the condensation heat transfer coefficients of propane is analyzed.The result show that the experimental facility is stable and the test accuracy is up to the experimental requirements.Then,the flow patterns of mixture refrigerants methane/propane condensate flow in the tube are observed by the flow pattern visualization system,and five main flow patterns of slug flow,stratified flow,wavy flow,semi annular flow and annular flow are obtained.Secondly,a three-stage physical model is proposed by rationally simplifying the structure of the test section,and a reasonable calculation grid is given,then a numerical model of forced flow condensation heat transfer for mixture refrigerant in tube is established.And the condensing flow patterns of the mixture refrigerant are simulated by the established heat transfer model,the flow pattern simulated consistent with the experimental observations is obtained,the validity of the model is verified.In addition,considering the influence of mixing effect on condensation heat transfer of mixture refrigerant,a new method of condensation heat transfer calculation for mixture refrigerant is proposed,which combines heat transfer numerical model with Silver method.By comparing the simulation results with the experimental data,it is found that the maximum deviation is less than ±25%.It provides a calculation model for the study on condensation heat transfer characteristics and condensation flow pattern of mixture refrigerants in helical tubes.Then,the condensation heat transfer characteristics of methane/ethane and ethane/propane in a helical tube are simulated by the established condensation calculation model,and the effect of mass flux,vapor quality,saturation pressure and heat flux on condensation heat transfer coefficient is analyzed.The results show that the condensation heat transfer coefficient increases with the increase of vapor quality and mass flux,while decreases with the increase of saturation pressure.The heat flux has no effect on heat transfer coefficient.In addition,the heat transfer law of different mixture ratio of mixture refrigerant is also studied,and the influences of different structural parameters(diameter,helical angle,curvature diameter)on the condensation heat transfer coefficient are analyzed.The result show that the heat transfer coefficient increases with the decrease of tube diameter.The helical angle has little effect on the heat transfer coefficient.With the increase of helical angle,the heat transfer coefficient increases very little.The heat transfer coefficient decreases with the increase of curvature diameter.Therefore,in the actual process of production and processing for coil-wound heat exchanger,reducing the curvature diameter properly can enhance the heat transfer coefficient.Finally,the flow pattern data are compared with the existing flow patterns,and the boundary lines of the Kim flow pattern are used to divide the flow pattern for the condensing flow in the helical tube.According to the new flow pattern boundary line,the flow pattern is divided into non annular flow and annular flow.Comparing the heat transfer coefficient data of different flow patterns with the existing condensing correlations,it is found that the Shah correlation is better for non annular flow data,and the deviation is 15.4%.The Chen correlation has a good prediction for the annular flow,with a deviation of 13.9%.On this basis,a new condensation heat transfer correlation for different flow patterns in a helical tube is proposed.The overall deviation is 13.8%,and all the data fall within the range of the deviation of ±30%.This study can provide theoretical support for the design of large coil-wound heat exchanger.