Numerical Model And Methods Of Frost Loacal Distribution On Plate-tube Heat Exchangers

In this paper, the local frost mass retardition on plat-fin-round-tube exchangers has been simulated numerically with commercial CFD code FLUENT. As it is known, a great difficulty on convergence will be encounerted if the integeral coupling is applied to solve the heat transfer between fins and air, because the fin (about 0.1m thick) is much thinner than the fin spaces. To solve this problem, first, a new kind of numerical method is developed. In this method, the whole model is separated into two 3D individuals:one is air modular where fin temperature is surpposed as known and updated by the data from the second modular---fin modular, where the heat transfer between the ambient and fins are surpposed as known and updated up the data from the first modular. Comparing other existing similar methods, in this model both modular is solved with 3D and have significantly different number of mesh sizes to each othe. And an algorithm is developed to adepte this kind of method.We also compared the numerical results from this method with those experimental data and with those credible numerical ones. We find our method is successful in the cases we tested. And based on this method, we obtained data about the fin local temperature distribution at dry condition and the local heat transfer rate as well as area-wighted average ones at steady condition.The total average heat transfer coefficient of a 2-row heat exchanger is adherened to the one calculated from Webb's j-factor equations. However, the numerical results show that in a 2-row heat exchanger, heat flux conducted through the first row is almost as twice as that through the second one. And by the same means recorded in the EVSIM, the numerical model has calculated the local average heat transfer rate of each row. And the assumption that each row's heat transfer rate in a multiple exchanger contributes equally to the total average heat transfer rate is questioned.In the next, a unsteady model of frost growth on fins is set up by abstracting the model into a serials of wall surface reactions. The numerical results shows that frost first forms in the front of tubes, and then extands from the root of the tube to the around. The mass retardation is obvious on the sides of the tube, but much less in the rear of the first row tube. It also finds that the frost grows most fast at the frontier. On the other, it grows a little slower where is covered by frost.