Heat Transfer Characteristics Of Evaporator And Influence Of Superheat On Stability In ORC System

Evaporator,as a traditional component,its importance is usually easy to be ignored.However,evaporator can affect the ORC system in various aspects,such as heat transfer performance,volume size,working fluid filling,and system cost.Since the heat transfer performance is the most important factor for ORC system,it is important to understand the mechanism of two-phase heat transfer in evaporator for the design and optimization of ORC system,the construction of high efficiency ORC power plant and the promotion of ORC technology.Two main contens are included in this papar: the evaporator heat transfer characteristics and the influence of superheat on the stability of the system.In terms of heat transfer characteristics,the heat transfer criterion equation is the basis of the evaporator design,reflecting the flowing heat transfer essence of fluid in the evaporator.In this papar,based on the analysis results of the existing heat transfer criterion equation,the heat transfer criterion equation which can reflect the essence of the flow boiling heat transfer is obtained by using the dimensionless analysis method.In terms of the influence of superheat on the system,the concept of the entrainment at evaporator is proposed.The influence of superheat on the stabililty of the system with entrainment is analyzed using thermodynamics,and the relationship between the entrainment and the superheat in stable operation is obtained.In addition,according to the heat transfer equation obtained,the existing ORC system was modified,and using visualization method,the analysis results about entrainment at the outlet of evaporator under low superheat was verified.A critical liquid level in the evaporator was obtained.Under this level the system can run stably.Firstly,the main factors affecting the flow boiling heat transfer are analyzed,according to the various heat transfer equations.With the same exchanger geometry,fluid operating conditions,the different heat transfer criterions are analyzed with various working fluids.The results show that the heat flux has strong influence on the heat transfer coefficient and the influence of the mass rate is weak.In different equations,the same factors play different roles,which indicats that the correlation obtain with water and other working fluid can not be used for ORC directly,and study on the flow boling heat transfer of working fluid in ORC has to be carried out.An experiment rig used to measure the heat transfer coefficient of working fluid outside tube was built,and a smooth tube is selected as reference,the heat transfer of R245 fa and R601 a outside the corrugated and Turbo-EHP tubes were investigated.The main research contents are the influence of the average dryness,heat flux and saturation temperature on heat transfer coefficient.The results show that the boiling heat transfer coefficient increases first and then decreases with the increase of heat flux.In addition,Turbo-EHP tube has the best heat transfer performance,followed by the corrugated tube and the smooth tube.For Turbo-EHP and smooth tubes,based on the experimental data,some dimensionless variables associated with the heat transfer process are analyzed firstly.8 dimensionless parameters are analyzed by the regression analysis method.In order to fully consider the influence of vapor on the heat transfer process,equivalent Reynolds number Reeq and the equivalent boiling number Boeq were used to replace Re and Bo.In addition,heat transfer criterion for R245 fa and R601 a is proposed.The saturated flow boiling heat transfer experiment of R245 fa in plate evaporator was carried out,and the impact of dryness and superheat at evaporator outlet and heat flux was studied.In order to reduce the influence of the subcooled liquid on the calculation of the boiling heat transfer coefficient,the single-phase heat transfer performance in the preheater was studied,firstly.Based on the single-phase heat transfer result,the available heat transfer area in the plate evaporator was calculated.Results indicate that the heat transfer coefficient increases with the heat flux,and decreases with the superheat increase at evaporator outlet.Based on experiment data,a heat transfer coefficient about R245 fa saturated flow boiling in plate evaporator was proposed.Investigation on the effect of superheat on the stability of ORC system was carried out.With thermodynamic analysis method,the effect of superheat with entrainment on the system was studied,and the relationship between entrainment and superheat for stable operation was obtained.For plate evaporator,the relationship between entrainment and superheat was verified by visualization method.