Research On Flow And Heat Transfer Characteristics Of Transcritical U-tube Evaporator Based On Single-phase/two-phase Metho

Trans-critical organic Rankine cycle(TORC)is an advanced medium and low temperature waste heat utilization technology,which is of great significance to alleviate the energy constraint.As a common connection and steering components,U-bend is widely used in transcritical evaporators.In order to guide the heat transfer design of evaporators,the flow and heat transfer characteristics in U-tubes under two typical arrangements were investigated numerically in this work,where bend effect under the coupling of buoyancy force and centrifugal force was analyzing in depth.The following main conclusions are obtained in this paper.(1)In horizontal flow,two typical vortices appeared in downstream region of the bend due to the competition between centrifugal and buoyancy force.As the decrease mass flow rate,the increase of heat flux and the pipe diameter d,the effect of the bend on the downstream straight section becomes more significant.The affecting distance could be more than 70 times the tube diameter in strongly mixed convection with Grq/Grth>200,with the average enhancement rate of heat transfer soaring up to 42%.As the bend was located at positions with different extents of buoyancy,the rate of enhancement changed from 29%to 49%.However,the effect of bend curvature on heat transfer is not significant.Further details of flow and temperature fields reveal that bend-induced enhanced heat transfer was essentially a synergistic effect of inlet temperature and fading secondary flow.(2)In response to poor accuracy of the traditional single-phase model for predicting heat transfer deterioration HTD,a two-phase heat transfer model based on the pseudo-boiling theory is established and optimized for vertical flow.The results show that when the Mixture model is selected and ΔT is set to 1 K,the prediction results are closer to the experimental values.In addition,three different forms of HTD experimental cases were selected for further study.The effects of saturation temperature Tsat and evaporation frequencyfc on the calculated results were significant.The best predicted results were obtained at Tsat=T+ with fe≈ 200.Overall,the twophase numerical method was found to yield more accurate predictions compared to the singlephase numerical method in this work.Finally,there is a positive correlation between the average gas phase volume fraction and the wall temperature peak.HTD at supercritical pressure is also similar to pseudo-film boiling at subcritical pressure,and the accumulation of gaseous-like VL near the wall is the root cause of HTD.(3)In vertical flow,there are two different flow directions(Up-down and Down-up),and twophase numerical methods are used for the flow and heat transfer characteristics investigation.In forced convection,their average heat transfer performance is comparable.However,in mixed convection,HTD appears significantly only in the upstream straight section of Up-down,and its average heat transfer performance was much lower than Down-up,up to 42%.In addition,the effect of curvature is only manifested within the bend and has less effect on the downstream straight section.The secondary flow structure was changed due to the centrifugal effect,resulting in inhomogeneities in heat transfer characteristics.Finally,the positive correlation between VL and temperature peak is still applicable in the U-tube after evaluating the crosssectional gas phase volume fraction.The movement of VL from the outside to the inside in Updown strengthens the heat transfer;in contrast the increase of VL in the inside weakens its heat transfer characteristic in Down-up.