Research On Influence Of Uneven Air Flow On Heat Transfer Characteristics In Air-cooled Heat Exchanger

As the results of the complicated flow field in the air-cooled heat exchanger (air-cooled cell) and the non-uniformity of air flow rate, the heat rejection capability and flow of the fin-tube bundles at different area varies widely. So it is significant to take the research of the influence of uneven flow on the heat transfer and flow characteristics of air-cooled cell.With the business software FLUENT, the thermal-flow performances of six common types of air-cooled finned tube bundles are numerically simulated, three of which are for direct air-cooled system and others for indirect air-cooled system. As the result, the flow resistance, average convection heat transfer coefficient and heat exchange amount of cooling air through the finned tube heat exchanger model with the windward velocity are presented, which is regarded as the reference value of performance parameter of the finned tube in even flow and the thermal-flow simulation foundation of the air-cooled heat rejection cell later. Then the simulation model of the direct and the indirect air-cooled radiating unit are modeled, and the thermal-flow operating condition of the direct and indirect air-cooled unit models with different windward velocity are numerical simulated with CFD method, through which the flow and heat transfer characteristics in corresponding windward velocity condition are obtained, which are regarded as the value of the performance parameter in uneven air-flow condition. By comparing the corresponding value of the performance parameter in even air-flow condition with the value in uneven condition, the variation of the influence of non-uniformity of air flow rate on flow and heat transfer characteristics with different operating condition is presented.The result shows that, under the influence of the non-uniformity of air flow rate, the higher the speed of fan is, the more uneven the air flow is in direct air-cooled unit; For indirect air-cooled unit, the uneven flow is in ladder-like distribution in both horizontal and vertical direction. In all condition, the average flow resistance, convection heat transfer coefficient and heat exchange amount are decrease to some extent, and along with the increase of fan speed for direct air-cooled unit and windward velocity for indirect air-cooled unit, the relative reduced value of average pressure drop and heat exchange amount grow, the reduced value of average convection heat transfer coefficient diminution. By introducing the uneven flow coefficient(the ratio between performance parameter in uneven flow condition and even flow condition), the change rule of corresponding uneven flow coefficient of the average flow resistance, convection heat transfer coefficient and heat exchange amount(Kp, Kh, KQ) with the increase of fan speed and windward velocity are presented.