| The plate-fin heat exchanger(PFHE)has the advantages of compact,strong thermal conductivity and high adaptability,PFHE is widely used in aviation,automobile,air separation,air conditioning refrigeration and other fields.The plate-fin heat exchanger is the key equipment in the air separation plant.Plate-fin heat exchanger is needed for the main heat exchanger,subcooler,condenser and so on,meanwhile the energy consumption of large air separation equipment is especially high,so it is necessary to utilize LNG cold energy into air separation equipment to reduce energy consumption.Therefore,it is necessary to study the flow and heat transfer characteristics of supercritical LNG in the plate-fin heat exchanger.This project is mainly to study the flow and heat transfer characteristics of supercritical LNG in the air separation plate-fin heat exchanger.Firstly,the thermophysical properties of supercritical LNG at operating pressures ranging from 7.1MPa to 9.8Mpa were analyzed.The effects of different inlet velocity,different operating pressure and different wall temperature of supercritical LNG on flow and heat transfer characteristics were analyzed with single channel of flat fin as the research object.The results show that the heat transfer coefficient increases first and then decreases as the temperature increases,and reaches the maximum at the critical temperature of LNG.Increasing the inlet speed or decreasing the wall temperature can improve the heat transfer performance,but also bring about a large pressure drop.Although the pressure drop can be reduced by increasing the operating pressure,the influence on heat transfer is not large.Second,in order to improve the flow and heat transfer characteristics of plate-fin heat exchanger,this topic proposed new trapezoidal fin,a new type of trapezoidal fin was proposed according to the wavy fin,trapezoidal fin multi-channel as the research object,using the liquid-solid-liquid coupling heat transfer numerical simulation research of supercritical LNG in trapezoidal fin channel within the flow and heat transfer characteristics,the research is mainly focused on the groove shape,inlet velocity,different operating pressure and trapezoidal bending Angle's influence on the flow and heat transfer characteristics.The results show that with the increase of LNG inlet speed,the heat transfer performance of trapezoidal corrugated fin and triangular corrugated fin is better and better,but the pressure drop is larger and larger,and the trapezoidal corrugated fin has better heat transfer characteristics than triangular corrugated fin,and the heat transfer performance of trapezoidal corrugated fin is 1.8 times that of triangular corrugated fin.With the increase of trapezoidal bending Angle,the heat exchanger has better heat transfer performance.The heat transfer performance of fins with a bending Angle of 30°is about 1.6 times that of 15°,but the pressure drop is about 1.7 times that of 15°.This also indicates that the heat transfer effect is not better with the larger Angle of trapezoidal fins.Finally,based on the multi-stage response surface method and the Box-Behnken test,this project analyzed the influence of bending Angle,the inlet speed and operating pressure of supercritical LNG on the heat transfer coefficient and pressure drop through software Design Expert.The results show that:The order of the influence of the coefficient is:the supercritical LNG inlet speed is greater than the bending angle and the bending angle is greater than the operating pressure;The order of influence on pressure drop is:the inlet velocity of supercritical LNG is greater than the operating pressure,and the operating pressure is greater than the bending Angle.The optimal values of heat transfer coefficient and pressure drop are:when the bending Angle is 30°,the operating pressure is 9.8MPa and the inlet speed of supercritical LNG is 1.89m/s,the heat transfer coefficient is 6350W/m~2·K and the pressure drop is 293.319Pa. |
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