Numerical Study On Characteristics Of Heat And Mass Transfer Of Zeotropic Working Fluid During Flow Boiling

In order to cope with the increasingly prominent problems of energy shortage and environmental pollution,renewable energy and waste heat power generation technologies have attracted widespread attention,among which the Organic Rankine Cycle technology is an important part.The zeotropic mixture has the characteristic of temperature glide during phase-change processes,and can significantly improve the system performance when applied to the Organic Rankine Cycle.Evaporator is one of the key components of the Organic Rankine Cycle system,and it accounts for the largest exergy lose in the system.Therefore,study on the flow boiling characteristics of zeotropic mixture in evaporator is of great significance for the optimization of the heat exchanger and the improvement of the system performance.The heat and mass transfer characteristics of zeotropic mixture during flow boiling were numerically simulated.Firstly,the existing source models were reviewed,and the applicability,pros and cons of each model were clarified.Among the source models of pure working fluids,the Lee model is simple and convenient to apply,but there is an artificial coefficient in it.It is difficult to directly determine the value without the comparison with the experimental data.The two-fluid model has a solid theoretical foundation and could give better results in the study of flow boiling in comparison.Among the source models of mixed working fluids,Banerjee's work is worthy of recognition,but the model lacks consideration of convective mass transfer,with a potential space to improved.Then,based on the thermal phase change model of two-fluid model,the flow boiling heat transfer characteristics of the mixed working fluid were numerically simulated,and the influence of the change of thermo-physical properties was mainly investigated.The results show that,among the parameters of the number of active nucleation sites,the change of vapor density has the greatest influence on the flow boiling heat transfer coefficient and the influence of surface tension is the smallest.There is no effect on the heat transfer coefficient by only changing the saturation temperature.The prediction accuracy of heat transfer correlation can be significantly improved by introducing the Weber number.After that,the steady state model of the evaporator was constructed to numerically study the mass transfer characteristics of the zeotropic working fluid composed of pentane/isobutane during the flow boiling.And the effects of inlet composition and evaporation pressure were investigated.The results show that,the mass fraction of the more volatile component isobutane during the whole flow boiling process is always reduced first and then increased,showing the form of a quadratic curve.As the inlet isobutane component increases,the position of the minimum isobutane mass fraction in the evaporator shifts backwards.As the evaporation pressure increases,the influence of flow boiling on the mass fraction of isobutane is gradually reduced.Finally,based on the Banerjee model,the influence of convective mass transfer was introduced to develop a new source model.And the heat and mass transfer characteristics of zeotropic working fluid composed of pentane/isobutane were simulated during the flow boiling.The results show that,under the simulated condition,the flow boiling of working fluid is mainly affected by convective evaporation,and dryness begins to appear when the dryness reaches 0.7.