| With the increasing development of utilization-technologies of low temperature waste heat,organic Rankine cycle has gained much more attention and been studied vastly,which mainly due to the characteristics of low evaporation pressure and the ability to efficiently utilize medium-low temperature heat source.Organic zeotropic fluids are promising for ORC as working fluids because they possess the feature of temperature glide in phase changing process,which can match the temperature variation of practical heat source.Correspondingly,the available work loss reduces while the cycle efficiency improves.The heat-exchange process of organic zeotropic fluid in evaporator is a typical flow boiling process which is complicated.Flow boiling is an important heat-exchange process which widely exists in various heat-exchange equipment.Consequently,analysis of mechanism of flow boiling process is extremely useful for optimization of heat exchangers as well as the control strategy of ORC system.The main objective of this paper is to study the flow boiling process of organic zeotropic working fluid.Firstly,the author started the work with the flow boiling process of pure working fluid.Four source-terms were analyzed,and they were validated by solving Stefan-problem and film boiling case to test the accuracy.The result showed Hardt model was the best while LEE model was the worst.Secondly,the study of flow boiling process of mixture working fluid began.In this part,the thermal physical properties calculation methods and the treatment of source term in phase change process of mixture fluid were studied and analyzed.The result showed that Banerjee R.model had stronger operability and reasonable accuracy,which eventually was selected as the reference model to carry out the simulation research of flow boiling process of organic zeotropic fluid.The study work was organized as follows: 1)The case from the reference paper which proposed the selected model was reproduced,and high match quality proved the feasibility of this model.2)The mixture used in the reference paper was changed to organic zeotropic fluid(pentane and iso-butane)which we used in our experiments,then ran the numerical simulation and got the results.The results were:1.As a result of counter-flow between gas phase and liquid phase,alongside the x positive direction evaporation rate first increased and then decreased,which also showed small sudden change and dispersion at the right outlet.2.Comparatively speaking,the interaction of shearing force is stronger at the top and bottom walls and is weaker near the interface,so the x velocity drops faster near the walls.Y velocity is a sign of the level of evaporative process in a way.3.The flow patterns of liquid and gas became more developed with the flow behavior happening.And the velocity boundary layer is similar to temperature boundary layer.Heat,which comes from the bottom wall,is transferred from liquid to vapor and ends up being released from the top wall.Temperature changes along this trend.4.Evaporation rate of pentane was higher than of isobutene with the unit of kg/s,which was caused by that molecular weight of pentane is larger than of isobutane.Hence,mass fraction of liquid isobutene was bigger than the initial value near the phase-contacting interface.3)Experiment set had been built to study the flow boiling process of zeotropic mixture.Pentane/isobutane(mass fraction,0.25/0.75)was selected as the research mixture.Four experimental conditions in which each had three different qualities were tested.The results showed that the selected model was accurate,while case4(Temperature of water at inlet is 65 ℃,mass flow rate of water and working fluid are 0.186kg/s and 159kg/(㎡ ·s),respectively)had the best performance to match the measured experimental data.The heat transfer coefficient of zeotropic mixture increased along with the increment of mixture quality,and the degree may be different with different working conditions.Through the comparison between the results of experiments and numerical simulation,it was proved that the method described in this paper can effectively predict the heat transfer coefficient in the flow boiling process of zeotropic mixture. |
