Research On Turbulent Flow And Heat Transfer Characteristics Of Supercritical Fluids Near The Pseudo-Critical Temperature

The world energy consumption is gradually increasing due to the continuous development of industrial civilization and human society.The fossil energy is now difficult to meet the demand of modern sustainable development owing to its nonrenewable and greenhouse gas emission from combustion.As a clean energy source,nuclear power can complement other renewable energy sources to achieve energy diversification and transformation,thus ensuring sustainable energy supply and utilization.Supercritical fluids(SCFs),especially supercritical water and supercritical carbon dioxide,exhibit broad application prospects in the field of nuclear reactors due to their unique characteristics,including no phase change and high heat transfer efficiency.For instance,they have been used as coolants in supercritical water-cooled reactors and for core heat transfer in gas-cooled fast reactors.Therefore,a fundamental understanding of fluid dynamics and heat transfer at supercritical pressure has become increasingly important.The thermophysical properties of supercritical fluids differ significantly from those of sub-critical fluids,as they will undergo a transition from a liquid phase(high density and high viscosity)to a gas phase(low density and low viscosity)when the temperature crosses the pseudo-critical point.Throughout this process,there is no phase transition,but the density,dynamic viscosity,isobaric specific heat capacity and thermal conductivity undergo drastic variations within a narrow temperature range.Therefore,compared with conventional fluids,the turbulent heat transfer characteristics of supercritical fluids are more complex.Such drastic variations in thermophysical properties lead to a highly coupled relationship between momentum and energy,which affects both the mean and turbulent motion and results in complex deterioration phenomena of heat transfer.In this paper,the direct numerical simulation(DNS)method was used to investigate the turbulent heat convection characteristics of supercritical fluids with drastic variations in thermophysical properties.Unlike the RANS method,DNS allows for an accurate solution of the Navier-Stokes equation,making it highly suitable for studying the complex turbulent flow and heat transfer mechanisms of supercritical fluids.As the flow velocity is much less than the speed of sound in the present simulation,the Navier-Stokes equation at low Mach numbers was used for solving.The governing equations are discretized by the conserved space-time discretization scheme,where the staggered mesh is used.The convection and diffusion terms of the momentum equations are discretized by using second-order central differences.Because of the drastic variations in properties,the convection of the scalar equations is discretized by using the QUICK scheme to eliminate oscillations.Finally,the semi-implicit method is used to solve the equation iteratively.In addition,the program is verified from the three aspects of turbulent flows with constant thermophysical properties,turbulent flows with variable thermophysical properties,and turbulent flows under supercritical conditions.These both contain the comparison of DNS data and experimental data,which proves that the current program can meet the calculation accuracy requirements.Firstly,the near-wall turbulence characteristics of supercritical fluid in the vicinity of the pseudo-critical temperature are investigated.It is found that the density and dynamic viscosity fluctuations in near-wall turbulence of supercritical fluid are minor,which can satisfy the Morkovin hypothesis.However,the fluctuation of specific heat capacity is significant and cannot be ignored.The turbulent kinetic energy budget indicates that the drastic variations of thermophysical properties between hot and cold walls will significantly modify the turbulence production and turbulence diffusion,both of which decrease with the decrease of density and dynamic viscosity.By means of stress equilibrium and semi-local methods,the velocity and temperature scaling laws of near-wall turbulence in supercritical fluids and the similarity of momentum and scalar transport are discussed.The velocity transformation developed by Trettel and Larsson with a semi-local coordinate provides a good description of the near-wall turbulence of supercritical fluids.Upon including how large specific heat variations affect temperature transformation,the logarithmic region of the cooled wall becomes consistent,as does the heated wall in a certain temperature range.In addition,in nearwall turbulence with small temperature differences at supercritical pressure,momentum transport is highly analogous to scalar transport.Secondly,a detailed analysis is conducted to examine the influence of the large localspecific heat of supercritical fluids near the pseudo-critical temperature on turbulent flow and heat transfer.Direct numerical simulations of turbulent heat transfer in a heated tube with constant wall heat flux and inlet bulk Reynolds number of Rein=2700 were performed at supercritical pressure.Four cases with the same geometry and different thermophysical properties were simulated and comparatively studied by complete isolation of the effect of the specific heat capacity from the other thermophysical properties.The effects of the large local specific heat capacity on the temperature,velocity,molecular and turbulent heat flux were analyzed in detail using velocity and thermal statistics and wall heat flux decomposition.The results show that the large local specific heat capacity can lead to a higher enthalpy fluctuation and enhance the turbulent heat transfer.Additionally,a quenching effect caused by the large local specific heat capacity was found in the near-wall region due to the fluctuation of thermal diffusivity.The wall heat flux decomposition shows that the large local specific heat capacity can greatly change the turbulent heat flux which is dominant in SCF heat convection,and the effect of the large local specific heat capacity on SCF turbulent heat transfer is complicated because it can lead to an increase in the mean enthalpy gradient and a decrease in turbulence simultaneously.Thirdly,the heat transfer deterioration(HTD)of SCF in heated vertical tubes at high heat flux to mass flow rate ratios is investigated on the basis of clarifying the influence of the drastic variations in thermophysical properties of SCFs near the pseudo-critical temperature.The heated tube has a length of 75 times the tube diameter to capture the complete heat transfer deterioration process.Both forced and mixed convections are simulated.The results show that primary and secondary HTDs occur in all flows considered herein.The causes of the HTD are comprehensively analyzed using the Fukagata-Iwamoto-Kasagi identity,turbulent heat flux,turbulence production,and turbulent kinetic energy.The FIK decomposition shows that the turbulent contribution Nu2 is the dominant part of the total Nusselt number.The turbulence reduction caused by flow acceleration is the main reason for the decrease in Nu2 and the occurrence of the primary HTD.Furthermore,buoyancy first damps the turbulence,exacerbating the HTD,and then forms an M-shaped velocity profile,which enhances the heat transfer.The secondary HTD,which is less pronounced than the primary one,comes from the decrease in the mean enthalpy gradient and enthalpy fluctuation caused by the position variation of the maximum specific heat.Finally,the turbulent flow and heat transfer characteristics of supercritical fluid with roughness near the pseudo-critical temperature are investigated by using a fast direct numerical simulation method for characterizing hydraulic roughness.The results show that roughness can enhance turbulent shear stress and turbulent heat flux,and effectively improve heat transfer.In summary,this paper provides comprehensive research on the turbulent flow and heat transfer characteristics of supercritical fluids near the pseudo-critical temperature.The impacts of drastic thermophysical property variations are investigated.A novel mean temperature scaling applicable to moderate temperature differences for near-wall turbulence at supercritical pressure is developed.The study also reveals the impact of the large local specific heat of supercritical fluids on turbulent heat transfer in circular pipes in both single and multiple heat coupling properties.The bimodal heat transfer deterioration mechanism induced by the drastic thermophysical property variations of supercritical fluids is revealed.Finally,a fast direct numerical simulation method that characterizes hydraulic roughness is introduced into the calculation of supercritical fluid flow and heat transfer,and the influence of roughness on turbulent heat transfer in supercritical fluids based on the drastic thermophysical property variations is revealed via DNS.The research provides a verified DNS dataset for future development and improvement of SCF RANS models.