Numerical Investigation On Flow And Heat Transfer Of Supercritical Carbon Dioxide

The supercritical CO₂ Brayton cycle system has many advantages,such as less equipment,low thermal inertia,flexible operation and high thermal efficiency.Based on above advantages,the system has been widely considered for application in multiple energy fields,such as the next generation nuclear reactor,high-temperature concentrating solar power system,coal-fired power plant and so on.As thermal power conversion medium,the issue of supercritical CO₂ flow and heat transfer is very significative to heater design in supercritical CO₂ power cycle system.However,the buoyancy effect and flow acceleration effect proposed according to the supercritical single-phase fluid assumption cannot perfectly explain the supercritical heat transfer phenomenon observed in the experiment investigation.There are still many controversies about the heat transfer mechanism of supercritical fluid.In particular,there is still no consensus on the cause of supercritical heat transfer deterioration(HTD).In this dessertation,RANS numerical simulation,experiment research and theoretical analysis are employed to reveal the characteristic of supercritical CO₂ pool natural convection and forced convection heat transfer in a circular tube.The research results can guide theoretically for the heat exchanger design and operation in supercritical CO₂ power cycle.In order to explore the similarities and differences between supercritical poolheat transfer and subcritical pool boiling heat transfer,the law and mechanism of supercritical CO₂ pool heat transfer on the horizontally heated wire are studied by numerical simulation.The qw??T curves corresponding to supercritical CO₂ pool heat transfer are obtained in a wide range of parameters.Under the condition that the fluid temperature Tb in pool is lower than pseudo-critical temperature Tpc,it is observed that supercritical pool heat transfer shows the characteristics of natural convection heat transfer when the heating wall temperature Tw is lower than Tpc,which is similar to the subcritical natural convection heat transfer.The supercritical pool heat transfer shows the characteristics of pseudo-film heat transfer when Tw is higher than Tpc,which is similar to subcritical film heat transfer.However,the supercritical CO₂ pool heat transfer only shows the characteristics of natural convection heat transfer.The influence of parameters involving wall temperature Tw,pool temperature Tb and operating pressure P on supercritical pool heat transfer is discussed in detail.It is considered that the supercritical pseudo-film heat transfer in pool is mainly restricted by the thermal resistance R of gas-like film.In view of the fact that the existing research are mainly focused on convective heat transfer of supercritical CO₂ in vertically upward tube,the forced convective heat transfer of supercritical CO₂ flowing downward in a vertical circular tube with 10 mm diameter is experimentally investigated,covering the ranges of pressures 7.58?15.5 MPa,heat fluxes 145?217 kW/m2,and mass fluxes 580?1000 kg/m2s.And then the wall temperature and convective heat transfer coefficient during supercritical CO₂ heat transfer are obtained.The experimental results indicate that the wall temperature increases with the increase of heat flux qw or the decrease of mass flux G,and the convective heat transfer coefficient decreases gradually.The heat transfer enhancement will occur at low heat flux or high mass flux G.In order to further reveal the influence of flow direction on the supercritical heat transfer,a comparative analysis of the heat transfer between the vertical up-flow and down-flow of supercritical CO₂ was carried out through numerical simulation.The simulation results indicate that,compared with the upward flow,the supercritical CO₂ reflects superior heat transfer performance when flowing downward,while the HTD can be inhibited.The heat transfer difference between up-flow and down-flow increases under high heat flux or low both mass flux and pressure.Then analogous to the subcritical film heat transfer model in a vertical tube,the physical model named pseudo-film heat transfer for supercritical CO₂ is established according to the assumption of supercritical phase transition,including the gas-like film with low density covering the heating tube wall and the liquid-like phase with high density in the core region.The thermal resistance mechanism is proposed,which comprehensively reflects the effects of multiple factors such as the thickness of gas-like film or liquid-like region,thermal physical properties and turbulence flow on supercritical heat transfer.It can be found that,compared with the core liquid-like thermal resistance RL,the thermal resistance RG of gas-like film near the wall has a dominant influence on thermal diffusion.The locally larger RG is a decisive factor leading to HTD.In addition,RG and RL are both smaller in downward flow than that in upward flow,which reasonably explains why the wall temperature is lower under downward flow.It is also very significant to heater design and safe operation to explore the heat transfer characteristics and mechanism of supercritical CO₂ under half-side heating mode.According to the supercritical pseudo-film heat transfer model,the difference of heat transfer performance between circumferential uniform and half-side heating modes,and the reason of HTD during half-side heating are revealed by numerical simulation.The results indicate that HTD mechanism under half-side heating is the same as that under uniform heating.Moreover,the gas-like film thermal resistance RG,h and the core liquid-like thermal resistance RL,h under half-side heating are smaller than those under uniform heating.It indicates that half-side heating mode can inhibit the HTD occurrence,and critical heat flux is higher.Inclined tube is an important layout form in heat exchanger.The behavior of supercritical CO₂ heat transfer is numerically investigated in an inclined tube.It can be found that the supercritical CO₂ heat transfer is similar to that of the subcritical two phase flow in an inclined circular tube.The HTD mechanism is the same as that in the vertical tube,and the HTD is also related to the secondary flow intensity in the cross section of the tube.The greater qw and the inclination angle are,the greater the difference in wall temperature between top generatrix and bottom generatrix is.As a result,when the qw is large and the supercritical CO₂ is across the pseudo-critical point during heat transfer,it is not recommended to employ the inclined channel in the engineering field to prevent the tube wall failure caused by the severe sharp circumferential wall temperature difference.In addition,unlike supercritical CO₂ heat transfer in a vertical tube,the wall temperature Twi at top generatrix is generally higher when supercritical CO₂ flows downward in an inclined tube compared with inclined up-flow.