Heat Transfer Characteristics Of Supercritical Pressure Fluids Used In Organic Rankine Cycles In Horizontal Tubes

Organic Rankine cycle?ORC?is one of the most promising technology to for utilization of renewable energy and waste heat such as solar energy,biomass energy,geothermal energy and industrial waste heat.Supercritical ORC?SORC?has higher thermal efficiency and lower exergy loss,so it has been attracting more and more attention.The heat transfer in the vapor generator is one of the most important issue in SORC systems.This thesis investigated the heat transfer characteristics of ORC working fluids at supercritical pressures in horizontal tubes,aiming to provide guidance for vapor generator design and system optimization of SORC.To investigate the heat transfer characteristics of supercritical fluids,an experimental system was built,with which supercritical heat transfer experiments with R134a were conducted in wide parameter ranges,extending the existing experimental parameter ranges.The parameter influences on heat transfer were analyzed.The definitions of heat transfer enhancement?HTE?and deterioration?HTD?were clarified,based on which the buoyancy effect was analyzed and results showed that the buoyancy effect led to HTE and HTD in axial direction and temperature non-uniformity in circumferential direction.Existing buoyancy criteria were evaluated and the threshold values these criteria were determined with experimental data of R134a.Moreover,a new buoyancy criterion of qd_in^0.7/G^1.2,which is of simple form and easy to be used in engineering applications,was developed to predict the maximum temperature difference between the top and bottom surfaces of the horizontal tube.To further investigate the heat transfer mechanisms,numerical simulations were conducted with typical heat transfer cases.Results show that the difference in heat transfer coefficients of the top and bottom surfaces is caused by the difference in turbulence intensity and the integrated effect of specific heat,with the former one being the dominate factor.Compared with forced convection,the turbulence is intensified on the bottom surface and restrained on the top surface,which results in the HTE on the bottom and HTD on the top surface.The secondary flow due to the buoyancy force intensified the turbulence for the bottom surface,while secondary flow vortex centers occurred near the top region of the tube,impeding heat transfer from the top fluid to the main flow and thus lead to the HTD.In addition,the mechanisms of diameter and pressure effects on heat transfer were analyzed through numerical simulations.Fluid-to-fluid scaling for supercritical heat transfer can effectively reduce the number of experiments by converting experimental data of the model fluid to the prototype fluids.Fluid-to-fluid scaling method for vertical flow was investigate based on abundant experimental data.Based on numerical results,the axial velocity gradient was used with a thermal resistance analogy to de rive a new dimensionless number,Re^-0.9?_A,to scale the mass flux.Then,a set of fluid-to-fluid scaling laws were developed and validated,extending the capability of scaling methods to mixed convection heat transfer.