降膜式蒸发器出气口液滴夹带风险预测及优化设计研究

Studies on Risk Prediction and Optimization Design of Droplet Entrainment at Outlet of Falling Film Evaporator

为解决降膜式蒸发器出气口液滴夹带和压缩机吸气带液问题,本文采用计算流体动力学(CFD)进行流动-传热-相变过程中蒸发器液滴夹带空间分布和流体动力学参数研究,并结合K1、K2典型样机实际测试结果验证数值方法的准确性,构建了以蒸发器出气口液滴夹带率(φ≤0.37%)、水平气流通道平均速度(v_(H)≤0.24 m/s)、出气挡板出口平均气流速度(v_(B)≤2.18 m/s)等目标控制参数来评估蒸发器出气口带液的风险,基于建立的目标控制参数对设计的K3~K6降膜蒸发器系列进行带液风险评估,结果表明K3和K4满足控制参数的要求,具有较低的带液风险,但K5和K6的目标控制参数具有较高的带液风险。基于预测的高风险出气口带液模型,通过降低分配器出口流速、增大筒体直径、调整管层间距对目标控制参数群进行了优化设计并进行了换热管外流态分布分析,结果表明,优化后蒸发器出气口的液滴夹带率比优化前降低75%以上,管外流态主要以滴状流的形式存在,且换热管外表面液相分布较为均匀。对优化后的K5和K6实验样机实测发现蒸发器出气口不存在带液现象,该带液风险控制目标参数可为降膜式蒸发器的优化设计提供一定的关键依据。

To solve the problem of droplet entrainment at the outlet of a falling film evaporator and the liquid carryover of a compressor,the spatial distribution of droplet entrainment and the fluid hydrodynamic parameters of the evaporator were investigated in this study by utilizing computational fluid dynamics(CFD)during the phase change process.The accuracy of the numerical method was verified by the test results of prototypes K1 and K2,and the objective control parameters,including the amounts of droplet entrainment(φ≤0.37%),average velocity of horizontal flow(v_(H)≤0.24 m/s),and average flow velocity at the outlet of baffle(v_(B)≤2.18 m/s)were established to evaluate the risk of the droplets entrainment at the evaporator outlet.Based on the established objective control parameters,the risk assessment of the falling film evaporator models(K3~K6)was carried out.The results showed that K3 and K4 met the requirements of the control parameters and had a lower risk of carrying liquid.In comparison,K5 and K6 had a higher risk of carrying liquid.Based on the predicted high-risk model with droplet entrainment,control parameter groups,such as reducing the outlet velocity of the distributor,increasing the diameter of the cylinder,and adjusting the gaps between the tube bundles,were adopted in the optimization design,and the distribution of the flow pattern outside the heat exchange tube was analyzed.The results indicated that the entrainment rate of droplets with the optimized evaporator is reduced by more than 75%compared to that before optimization,the state of the outside tube mainly exists in the form of droplet flow,and the distribution of the liquid phase outside the tube is relatively uniform.It was found that there is no liquid at the outlet of the falling-film evaporator through an optimized test(K5,K6),which is expected to provide a fundamental basis for the optimization design of the subsequent falling-film evaporator.