Flow Boiling Of Organic Working Fluid In Horizontal Tube Under Organic Rankine Cycle Conditions

Organic Rankine Cycle?ORC?can efficiently utilize the widely spread low-to mid-temperature heat sources.Decided by the level of temperature,ORC has higher evaporation pressures than refrigeration.This paper selects R134a as the working fluid.Existing R134a flow boiling studies are mostly targeted at low-pressure range suitable for refrigeration,so ORC is lacking in fundamental experimental data and heat transfer rules.Through experiments and numerical analysis,this paper studies R134a flow boiling under high pressures to provide guidance for design and operation of ORC evaporators.After validating the experimental rig and numerical model,research is conducted from two aspects of heat transfer phenomena and correlation.Phenomena study includes effect of such parameters as pressure,mass flux and heat flux.Correlation study adapts existing correlations to obtain predictions with higher accuracy.Characteristics of heat transfer under normal conditions are studied based on experiments with pressures of 2.5-3.3 MPa(0.62-0.81_?8?.Increasing pressure leads to earlier dryout at top wall,which expands along the tube,so major flow pattern in tube is stratified flow with partial dryout.Change in flow pattern affects heat transfer fundamentally.Dryout and its expansion leads to decreasing trend of heat transfer coefficient with quality.Pressure and heat flux slightly increase heat transfer at low quality region?<around 0.3?while mass flux enhances heat transfer over the whole range of quality.To obtain heat transfer of stratified flow under high pressures,this paper proposes a new correlation combining Gungor and Winterton correlation and Wojtan correlation with prediction accuracy of 12.5%for various pressures,mass fluxes and heat fluxes.The new correlation can act as effective reference for predicting heat transfer performance of ORC evaporators.Characteristics of sharp increase of wall temperature under abnormal conditions are studied based on experiments with pressures of 2.5-3.92 MPa(0.62-0.97_?8?.Wetted wall in stratified flow can effectively limit the amplitude and rate of temperature rise of unwetted wall,avoiding the sharp increase.So CHF?Critical Heat Flux?phenomena at bottom wall is the triggering mechanism of sharp increase of wall temperature in the tube.Increasing pressure may change the type of CHF,which is negative to prevent sharp increase of wall temperature.Increasing mass flux can increase the CHF value and partially compensate for negative effects brought by increasing pressure.In near-critical region with higher sensitivity,this paper adapts existing CHF correlation to horizontal orientation and achieves accuracy of 7.3%,which provides guidance for setting proper heat flux values of ORC evaporators.