Heat Transfer Deterioration And Instability Of Supercritical CO₂

Due to its excellent thermal properties and environmental protection,CO₂ is widely used in trans-critical cycle thermal systems.Among them,the trans-critical CO₂Rankine cycle is a low-grade waste heat recovery technology with good application prospects,which can not only improve energy efficiency but also reduce the cost of power generation.When the trans-critical CO₂ Rankine cycle system runs near the pseudo-critical point,the complex phenomena such as heat transfer deterioration and unstable flow caused by the severe nonlinear variation of thermo-physical properties of fluid are the key factors to affect the safety and energy conversion efficiency of the system.To date,researches about supercritical CO₂ mainly focus on heat transfer deterioration,and there are only few studies on the flow instability of supercritical CO₂in the vertical tube,especially under the condition of low heat flux in the waste heat recovery system.In this paper,the flow instability of supercritical CO₂ in vertical tube is investigated by experiments,and the transient response characteristics of supercritical CO₂ heated in vertical tube and horizontal tube are investigated by numerical simulation.Firstly,the heat transfer deterioration and flow instability in the vertical upward flow of supercritical CO₂ are studied experimentally,and the effects of heat flux,mass flow rate,pressure and inlet temperature on heat transfer deterioration and flow instability are analyzed.The experimental conditions are as follows:the mass flow rate is 315～477 kg/m²·s,the pressure is 7.5～8.5 MPa,the inlet temperature is 10～20°C,and the heat flux is 53～92 k W/m².The results show that when the heat flux exceeds the critical deteriorating heat flux,the heat transfer state of supercritical CO₂ flow in the tube will change from a stable state to an oscillating state,and the system pressure,mass flow rate,wall temperature and outlet fluid temperature oscillate periodically with time.It is also found that the oscillation of supercritical CO₂ occurs in the process of heat transfer deterioration to heat transfer recovery.In the oscillation stage,with the increase of heat flux,pressure,inlet temperature and mass flow rate,the flow of supercritical CO₂ gradually becomes stable.However,under the same experimental conditions,there is no oscillation phenomenon in the downward flow,which indicates that the variation of heat transfer mode under the action of buoyancy force is the main reason for the oscillation of supercritical CO₂ in the tube.Up to now,the researches on the instability of supercritical fluids are mainly carried out through experiments,and the transient response mechanism of supercritical fluids after heating and the transient distribution of temperature field and density field inside can not be obtained.In this paper,in order to obtain the dynamic response characteristics of a certain transient state after supercritical CO₂ is heated,the transient response of flow and heat transfer of supercritical CO₂ in vertical and horizontal flow is studied by numerical simulation.The effects of pressure,mass flow rate,heat flux and inlet temperature on the transient flow and heat transfer process of supercritical CO₂ are studied.The numerical simulation covers a pressure range of 8～10 MPa,a mass flow rate range of 3.5～10 g/s,and a heat flux range of 5～60 k W/m².The results show that the oscillation of supercritical CO₂ occurs in both vertical and horizontal tubes at the initial heating stage（the first stage）.At the end of the oscillation,it reaches a steady state after transient convection（the second stage）.The thermos-acoustic oscillation at the early stage of heating is due to the thermal expansion caused by the decrease of the density of supercritical CO₂ with the increase of temperature,while the unheated cold fluid has inertia,which forms the pressure wave under the pressure disturbance in the tube,and then causes the change of fluid velocity.The pressure waves appear periodically,causing periodic oscillation of pressure and velocity.In addition,the pressure,velocity and temperature in the tube all oscillate periodically,and the closer to the critical pressure is,the greater the amplitude is.The greater the heat flux,mass flow rate and inlet temperature,the greater the oscillation amplitude of the pressure,velocity and temperature in the tube,while the frequency is not affected.