Optimization Numerical Research On Sub-cooled Boiling Heat-transfer Performance Of Plate Heat Exchanger With Dimples And/or Protrusions

FXH can avoid low-temperature corrosion effectively while reducing exhaust gas temperature of boiler through the introduction of sub-cooled boiling process and condensation process. In this paper, with water as working medium and the background of FXH’s application in recycling of waste heat in power station boiler, the goal driven optimization study of plate heat exchanger with dimples and protrusions in the process of sub-cooled boiling heat-transfer is carried. In this paper, the double loop parameters optimization model is proposed innovatively. Against the disadvantages of dimples and protrusions structure in the process of sub-cooled boiling heat-transfer, the fusiform structure is presented. The following is the main work and results in this paper:(1) Through the analysis of literatures and characteristics of the this study, the method of using PEC = (Nu/Nuo)l(f/f0)1/3 as the main evaluation index and Mixture model, RNG k-ε model and Enhanced Wall Functions to simulation, is identified. Also, the UDF of the mass and heat transfer between liquid and vapor in process of sub-cooled boiling heat-transfer is written. Based, on this method, the process of sub-cooled boiling heat-transfer of flat flow is simulated. Results show that, the void fraction increases along the flow direction, and decreases with the increase of inlet velocity. With the increase of mass flow rate, the total heat exchange amount increase, but the heat exchange amount of unit mass flow rate decreases. When Re is small, with the increase of Re,f decreases sharply, with Re continuing to increase, and f still decreases, but the change is leveling off, until tending to a constant value.(2) The double loop parameters optimization model, in which the structure parameter is used as the inner loop and Re is used as the outer loop, is proposed innovatively. The goal driven optimization study of dimple channel in the process of sub-cooled boiling heat-transfer is carried. Results show that the volume fraction of water vapor in the dimples is significantly higher than that on the plate near the dimples, which proves that the plate with dimples has the very good strengthening effect on phase-change heat transfer. For the same dimple, the enhancement on heat-transfer of dimple’s front is better than that of dimple’s back With different flow parameters, for dimple channel, the common optimal structure parameters is that R equals 7 mm, Hd equals 2 mm and P equals 20 mm. The comparison results between optimal structure parameters flow and flat flow show that, under the condition that the increase of f is not obvious, Nu of the optimal structure parameters flow is 1.97～2.13 times of flat flow’s, therefore, the optimal structure parameters flow has high value of PEC, with the range of PEC as 1.86～1.86.(3) In view of the weaknesses of existing dimple flow channel, a fusiform structure of flow channel is put forward. The simulation results of the fusiform structure flow channel show that the void fraction of fusiform structure flow channel has the same change rule with the existing optimal structure parameters flow channel and the flat flow channel, but the value is higher than that of the existing optimal structure parameters flow channel and the flat flow channel. Compared with the existing optimal structure parameters flow channel, the f of fusiform structure flow channel changes little, even the value of f is slightly smaller, but the Nu of the new structure flow channel has obvious increase, so the PEC of the fusiform structure flow channel is improved from 1.86～2.11 to 1.98～2.27.