| Natural gas is generally stored and transported after being liquefied at an ultra-low temperature.It is necessary to vaporize LNG first before it is used.LNG is accompanied by the generation of cold energy during the vaporization process,and this part of the cold energy can be recycled through heat exchangers and other devices.The spiral tube heat exchanger has the characteristics of compact structure and high heat transfer coefficient.Compared with the straight tube heat exchanger,the spiral tube heat exchanger can generate secondary flow through centrifugal force to achieve the purpose of improving heat exchange efficiency,and can meet the basic requirements of cold energy recovery in the LNG vaporization process.In this article,the numerical simulation method was used to study the characteristics of the inlet Reynolds number,the spiral diameter of the spiral tube,the spiral tube pitch,and the inner diameter of the spiral tube on the heat transfer efficiency and pressure drop of the spiral tube heat exchanger.The numerical calculation results were compared with existing boiling experiment results.The comparison of thermal correlation calculation results provides a basis for the structural optimization of spiral-tube heat exchangers.The main research content and conclusions of this article are as follows:(1)The geometric model of the spiral coaxial casing was established.Based on the basic flow control equations,the heat transfer of the gas-liquid two-phase flow in the spiral coaxial casing and the interaction force between the two phases are calculated.Nucleate boiling model of Kurual,Podowski and Lavieville were used to simulate the flow and heat transfer process of LNG in a spiral tube heat exchanger.(2)In order to verify the reliability of the selected calculation model,the established numerical calculation model was used to perform numerical calculations on the existing experimental process.The average deviation is less than 10%,which indicates that the numerical calculation results have a high degree of fit with the experimental results,and the experimental results are in good agreement with the numerical calculation results,which verifies the feasibility of the numerical calculation model.(3)Under the condition that the inlet Reynolds numbers are 13500?53500 respectively,the effects of different spiral tube diameters,spiral tube pitches and spiral tube inner diameters on the vapor volume fraction distribution,pressure change and heat transfer efficiency of the spiral tube heat exchanger are studied.It is determined that the above-mentioned single size parameter such as the spiral pitch diameter affects the pressure change and heat transfer efficiency under the condition of other spiral tube dimensions unchanged,the higher the inlet Re number,the greater the overall pressure loss;the greater the spiral tube pitch Smaller,the greater the overall pressure loss;the spiral diameter of the spiral tube and the diameter of the spiral tube have little effect on the overall pressure change.When the vapor volume fraction is below 0.2,the heat transfer coefficient increases the fastest.As the vapor volume fraction increases,the heat transfer coefficient increases slower.Under the condition that the inlet Re is less than 100,000,the larger the Re number and the larger the Nu number,the better the heat transfer efficiency;the pitch has almost no effect on Nu;the larger the spiral diameter of the spiral tube,the smaller the number of Nu;the larger the diameter of the spiral tube,the larger the Nu number.(4)The boiling heat transfer mechanism in the tube and the calculation formula of the boiling heat transfer coefficient in the straight tube and the spiral tube are described,and the difference between the boiling heat transfer coefficient computer system of the straight tube and the spiral tube is compared and analyzed.Comparing the calculation results of the boiling heat transfer coefficient of the spiral tube in the existing experimental research with the numerical calculation results,comparing the numerical calculation results of Nu number with the Cui formula,the calculation deviation is 25.16%,the standard deviation is 5.4%,and the obtained straight line is fitted The R-square number is 0.881,and the calculated value is close to 74.84% of the calculated value of the Cui formula.Therefore,the coefficient of the Cui formula is corrected,and the deviation between the corrected formula and the numerical calculation result is within15%.The revised Cui formula fits well with the numerical calculation results,and can be used as an empirical formula for calculating the boiling heat transfer of LNG in the spiral tube.In this thesis,numerical simulation methods are mainly used to calculate the LNG boiling flow process in spiral tube heat exchangers of different structures,and the law of fluid flow and heat transfer characteristics in the heat exchanger is obtained.The structural optimization of the double-pipe heat exchanger provides reference and reference. |
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