The Characteristics And Mechanisms Of Heat Transfer Deterioration Of Supercritical CO₂ Heated In The Helically Coiled Tubes

The Trans-critical Rankine Cycle?TRC?is an advanced technology in heat-work conversion with low grade energy.Due to the good thermos-physical properties,thermal stability and eco-friendliness CO₂ has good application prospect and research value.The improvement of the efficiency of the TRC system closely related to the in-depth study on the heat transfer characteristics of supercritical CO₂?S-CO₂?in the tubes.But,the significant variation of the thermal-physical properties near the pseudo-critical temperature may induce the abnormal heat transfer during the heat transfer process.The heat transfer deterioration which usually accompanied with the abrupt increase of the wall temperature and decrease of the heat transfer coefficient will not only harmful to equipment,but also threaten the stability of the power system.Many scholars have conducted the experiment measurement,numerical simulation and theoretical analysis on the heat transfer deterioration in straight tube,and some achievements has been obtained.As a kind of high-efficiency heat transfer tube,the helically coiled tube have the advantages of compact structure,high heat exchange efficiency and large utilization ratio of per unit space which can enhance the heat transfer and suppress heat transfer deterioration.However,the studies on the heat transfer characteristics of S-CO₂ in helically coiled tube are relatively fewer.To provide valuable basic database and physical model for the practical process of TRC systems,this paper combines experimental and numerical investigation on the heat transfer characteristics and heat transfer deterioration mechanisms of S-CO₂ flowing in heated helically coiled tube.The heat transfer differences between straight tube?d=8mm,L=2000 mm?and helically coiled tube?L=2000mm,d=8mm,R=180mm,2?b=32mm?were compared.The effects of heat flux,mass flux and pressure on the heat transfer performance were analyzed.The results showed that the heat transfer deterioration occurred in the straight tube at G=150kg/?m²s?,p=8MPa and q=20.2kW/m².But,the heat transfer deterioration did not occur in the helically coiled tube under the same boundary conditions which indicates the helically coiled can effectively suppress the heat transfer deterioration.In addition,by comparing to the forced convective heat transfer empirical correlation it can be found that the heat transfer was enhanced in the helically coiled tube due to the combined effects of the centrifugal force and the buoyancy force.The validation by the experimental data and various numerical models showed that the SST k-?can exactly predict the heat transfer in the helically coiled tube.Therefore,the SST k-?model was used to simulate the heat transfer of S-CO₂ heated in the helically coiled tube at various inclination angles with heat flux q=20.5kW/m²,mass flow G=262kg/?m²s?and pressure p=8MPa.The inclination angle effects?the coupling relationship between the buoyancy force and flow direction?on the heat transfer performance in both axial and circumferential were analyzed.The results showed that in vertical helically coiled tube,the buoyancy force was always perpendicular to the flow direction.The heat transfer coefficient along the flow direction is determined by the magnitude of the buoyancy force.When the helically coiled tube was placed at an inclined position,the coupled angle between buoyancy and flow direction changes periodically and the amplitude increases with the decreasing of the inclination angle.Hence,the heat transfer coefficient in the inclined helically coiled tube was not only affected by the buoyancy force,but also affect by the coupling angle of the buoyancy force and the flow direction.What's more,the oscillation of the heat transfer coefficient indicates the heat transfer stability in the inclined helically coiled tube is poor than the vertical one.To further analyze the mechanism of heat transfer deterioration in the helically coiled tube,it is necessary to investigate the heat transfer characteristics of the S-CO₂heated in the straight tube?d=4mm,L=2000mm?.Through the decoupling method?compared with?=constant,g=0 and heat transfer characteristics under normal conditions?,the effects of buoyancy and flow acceleration on heat transfer deterioration was analyzed.The results showed that the buoyancy force plays a leading role in heat transfer deterioration compared with the flow acceleration.According to wall temperature,the heat transfer deterioration was divided into three stages:initial stage,stable stage and recovery stage.The analysis on the distribution of velocity and turbulent energy on the corresponding section indicated that the hear transfer deterioration will occurred when M-type velocity was formed on the section and the peak of the velocity was located at buffer layer?5<y⁺<30?.Based on the relationship between the thermal resistance of the boundary layer a new heat transfer correlation was proposed.The numerical simulation was used to compare the heat transfer characteristics between the straight tube and the helically coiled tube.Consistent with the experimental results,the helically coiled tube has a certain inhibitory effect on the heat transfer deterioration,but the heat transfer in the helically coiled tube also deteriorated near the critical point at the p=8MPa,G=400kg/?m²s?and q=70kW/m².Based on the theoretical analysis of the buoyancy force in the straight pipe,the buoyancy criteria for determining vertical and horizontal helically coiled pipes are proposed,respectively.In order to analyze the mechanism of heat transfer deterioration in the helically coiled tube,the velocity and the turbulent energy distribution of the characteristic line on different sections of the helically coiled were captured.The results showed that the turbulent flow energy at the outside is significantly higher than that at the inner side and the inclination angle has little effect on the turbulent kinetic energy at the outside.The coupling effects of centrifugal force and buoyancy force mainly affect the velocity and the turbulent kinetic energy distribution at the inside of the cross section.Hence the local heat transfers deterioration occurred at the inner side at a lower heat flux.Different from the heat transfer deterioration in straight tube,the M-type velocity also appears in the helically coiled tube,but the peak point always located at the outside of the buffer layer and the heat transfer deterioration did not occur.The local heat transfer deterioration mainly occurred at the inner side of the cross section where the velocity was suppressed.