Analysis Of Convective Heat Transfer Characteristics Of High-efficiency Heat Exchangers In Transcritical ORC Systems

Low grade heat widely exists in engine exhaust gas,industrial waste water,renewable energy,etc.Nowadays,the Supercritical organic Rankine cycle is one of the most effective technology to recover low-grade thermal energy.At the same time,the design and optimization of gas heater in supercritical carbon dioxide Rankine cycles can rise the efficiency of the system effectively.The supercritical fluid changed dramatic physical in the pseudocritical region,which caused the convective heat transfer characteristics much more complicated than the subcritical state.In this paper,supercritical CO₂ is taken as working fluid and explored the convective heat transfer characteristics in high efficiency heat exchanger by numerical simulation of CFD.Heat transfer characteristics are also analyzed under difference kinds of thermal boundary conditions that similar with recovery various of low-grade heat energy.The main research can be listed are as follows:The heat transfer characteristics of supercritical CO₂ flowing upward through a vertical smooth straight tube?ST?with 9 mm diameter were investigated under the condition of constant heat flux?UHF?.In the pseudo-liquid region,the heat transfer deterioration phenomenon were occurred,especially under the conditions of higher heat flux q and lower mass flow rate G.The wall temperature were appeared seriously with joint effect of boundary force and shear stress.When the fluid temperature is above the critical temperature,the specific heat Cp decreased rapidly and wall temperature rise simply.With the effect of wall curvature,heat transfer deterioration disappeared in the helically coiled tube heat exchanger?HCT?,the axial wall temperature reduced greatly,and the heat transfer efficiency is higher compared with ST.With the combined action of the buoyancy force and the centrifugal force,the local heat transfer characteristics are greatly difference.When heat transfer was occurred by buoyancy force,circumferentially maximum(T_iw,max)and minimum wall temperatures(T_iw,min)occur at?=90°and 270°respectively.When only stronger centrifugal force exist,T_iw,max and T_iw,minw,min locate at?=0°and 180°respectively.Heat transfer characteristics are quite difference when supercritical CO₂ was heated under the thermal boundary condition of constant heat flux?UHF?and constant boundary condition?CBC?.The buoyancy force was weakened under CBC condition,so the working fluid was heated more uniformly and the circumferential wall temperature difference(T_iw,dif)gradually decreases along the axial direction,.Since the buoyancy force was weakened,the empirical correlations,which obtained based on UHF conditions cannot predicted Nu effectively.However,the correlation prediction error is within±20%through compared with the correlations of scholar Mao and scholar Li.Because their correlations were obtained under the condition that the buoyancy force was weakened.In anther word,the thermal boundary conditions were affected heat transfer characteristics seriously.