Experimental And Numerical Research On Turbulent Convection Heat Transfer Of Supercritical Pressure Fluid

With the rapid development of science and technology,the research on the supercritical pressure fluid flow and heat transfer in vertical pipe has drawn wide attention in the field of industrial technology such as thermal power,utilization of nuclear energy,aerospace,chemical and chemical engineering.In the present paper,the supercritical pressure fluid flow and heat transfer in the tube were studied by experimental method and numerical simulation.The influence of fluid thermal properties,buoyance effect,flow acceleration,insert helical structure and circumferential direction non-uniform heat conditions on the supercritical fluid convection heat transfer were studied.Under higher heat flux conditons,the buoyancy and flow accelaeration have a coupling effect on the heat transfer of supercritical pressure CO₂ in the vertical tube.For downward flow,the flow acceleration effect is stronger than the buoyance effect in the range of 7?10^-8<r<8?10^-7.Heat transfer deterioration occurs due to the coupling effect,and the wall temperature changes nonlinearly.For upward flow,when the local Bo*in the range of 6?10^-7<Bo*<8?10^-6,the coupling effect lead to flow disturbance,and the upward flow convection heat transfer intensity is slightly higher than the downward flow.A method that considers the turbulent heat flux has been proposed to improve the prediction accuracy of numerical simulation.A buoyance effect model that accounts for the production of turbulent kinetic energy and a turbulent Prandtl number model accounting for turbulence thermal diffusion,which are both based on the turbulent heat flux model,was adopted in the original AKN k-?model.Experimental results and direct numerical simulations?DNS?data were used to validate the performance of the modified model.The value of the turbulent Prandtl number in the buffer layer,with 5<y⁺<60,is essential for accurate reproductions of experimental data,particularly for cases where shear production is much larger than buoyancy production.Within a certain range of heat flux,non-steady heat transfer occurs when the supercritical pressure CO₂ flows upward in the tube.The characteristics of non-steady heat transfer are that the wall temperature,outlet temperature,and inlet flow rate exhibit periodic oscillations,the frequency is about 0.05 Hz.The main reasons for the non-steady heat transfer of supercritical pressure CO₂ is the increased pressure drop and coupling effect of buoyance and acceleration caused flow disturbance.The field synergy principle could be used to analysis the convection heat transfer of supercritical pressure fluid.The increase of field synergy angle between the velocity and temperature gradient lead to heat transfer deterioration.By means of the field synergy principle,the insert helical structure is arranged at 20%radius close to the wall,which can strengthen the convective heat transfer of supercritical pressure CO₂ in vertical pipe and eliminate the heat transfer deterioration and non-steady heat trnasfer phenomenon.The buoyancy effect on circumferential direction non-uniform heat conditions is different from that of uniform heat conditons.The fluid density is highest on the adiabatic side and lowest on the heating side.The buoyancy effect makes the shear production of turbulent kinetic energy significantly larger than that of the uniform heat flow near the heating side.In the present paper,Gr_b/Re_b^2.7 is used to measure the effect of buoyancy on the heat transfer in the form of circumferential direction non-uniform heat conditions.When Gr_b/Re_b^2.7>5?10^-5,the buoyancy effect deteriorate heat transfer to the supercritical pressure fluid at the heating side in the sinusoidal heating form.