The Research On Convective Heat Transfer In Supercritical Carbon Dioxide Pipe

As we all know,with the increasing global greenhouse effect,the emission and treatment of carbon dioxide has become a very important topic in the society.The status of CFC and other traditional refrigerants as the initiator of greenhouse effect and ozone depletion is under increasing threat in the refrigeration industry.The development of science and technology make people thermodynamics characteristics and supercritical carbon dioxide is good had a deeper understanding,and as refrigerating agent,carbon dioxide laser is a kind of safety,environmental protection,health,economy and green natural fluid is abundant.In the development of automotive air conditioning,commercial refrigeration,super computer cooling and other industries,carbon dioxide has a very wide application prospect with its unique advantages.At present,the international study on the flow heat transfer in supercritical carbon dioxide microtubules is still at the initial stage,and more research focuses on the experimental study on the flow of carbon dioxide in circular tubules.Therefore,the research on numerical simulation of supercritical carbon dioxide flow heat transfer in micropipelines has gradually become the focus of researchers.In this paper,the influence of thermal buoyancy lift on the heat transfer rate of carbon dioxide micro-tube is discussed.Then,under the premise of not changing the flow state,the change of supercritical carbon dioxide convective heat transfer rate in different pipe diameters(0.50mm,1.00mm,1.75mm and 2.80mm)was calculated.Finally to simplify the complex strata structure by uniform distribution sphere of porous medium model,by discussing CO2 flow characteristics between the solid sphere to reflect or characterization of crustal rock formation in the process of shale gas displacement in the heat transfer characteristics of CO2 flow.The influence of fluid temperature and flow velocity on the flow characteristics and heat transfer performance of supercritical carbon dioxide porous media is discussed.The results show that:1)supercritical fluid has a special thermal physical property,and the physical change is violent near the critical point.In laminar flow state,the thermal physical properties of carbon dioxide in the vicinity of the critical point change dramatically,resulting in a significant buoyancy force,which in turn leads to secondary flow intensification of heat transfer in the tube.As the flow continues,when the fluid reaches steady state development flow,the heat transfer property of carbon dioxide is consistent with the constant physical fluid.2)with the constant enlargement of pipe diameter,pipe same location section maximum eccentric axial velocity,the greater the degree from z = 50 mm to z = 200 mm,the smaller the wall shear stress gradient convergence,the convective heat transfer intensity is weaker,smaller thermal equilibrium rate;In other words,the smaller the pipe diameter,the greater the convective heat transfer intensity and the faster the thermal equilibrium rate.3)by changing the temperature and flow rate at the inlet of the fluid,the changes in the physical quantities of the flow unit and the heat exchange situation were analyzed.With the same inlet velocity,when the inlet temperature rises,the viscosity of the system becomes larger and the fluid velocity attenuates.When the inlet temperature is constant and the inlet velocity increases,the attenuation degree of the temperature decreases,and the convective heat transfer intensity between the fluid and the solid wall increases.The increase of inlet velocity and temperature can increase the rate of convective heat transfer in the system.When import velocity increases to a certain degree,so the heat exchange rate and temperature is no longer a positive(negative)related to the distribution,the simplified structure of the ball diameter and carbon dioxide at the critical point of special physical properties have close correlation.