Numerical Analysis Of Buoyancy Force Affects On Heat Transfer Of Supercritical CO₂ In Heated Helically Coiled Tube

With the widespread use of high-power electronic equipment in the aerospace and military fields,people have made higher demands on environmental control systems.However,temperature control was one of the most important technologies for aircraft,aviation equipment and aviation workers.Due to the fact that the aircraft was subjected to high gravity and microgravity in practice,the researchers payed more and more attention to the study of heat transfer under high gravity or microgravity.At present,the heat transfer experiment and simulation study under the condition of high gravity or microgravity were concentrated in the two-phase heat transfer process.Under the condition of high gravity and microgravity,the heat transfer characteristics of supercritical fluids such as aviation kerosene,CO₂,H₂O,N₂ and so on were rarely reported.In the case of high gravity or microgravity,the drastic change of the buoyancy force was the main factor influencing the change of the heat transfer characteristic due to the different gravity environment.Because of the experimental conditions on the surface of the earth,it could not achieve the experimental study of abnormal gravity conditions.Therefore,the numerical simulation method was used to explore the effect and mechanism of the buoyancy force on the heat transfer of supercritical CO₂ in the helically coiled tube under abnormal gravity conditions.Under the condition of high gravity,it was equivalent to suppressing the interference of centrifugal force,which highlighted the essential characteristics of the buoyant force.Under the condition of microgravity,the buoyancy force was very small,which highlighted the centrifugal force.In addition,the flight direction of the aircraft was not fixed.Therefore,this paper studied the heat transfer of supercritical CO₂ in helically coiled tube by using different flow directions and high gravity or microgravity.In this paper,we applied two kinds of different specifications of helically coiled tube under the heated condition of the research status of fluid flow,through the turbulence model of comparative,finally this paper chose renormalization group RNG k-? model.And the application of this model was compared with the existing experimental data,the average deviation of the simulation results and experimental results was 0.27%.Firstly,the numerical simulation of the heat transfer in helically coiled tube at horizontal and vertical flow direction were carried out by changing the flow direction,to detect the influence of buoyancy force on the heat transfer and the interrelation between the direction of flow.The influence of mass flow rate,inlet pressure,heat flux and the different flow directions of supercritical CO₂ on heat transfer and pressure drop was studied in this paper,and further analyzed variable physical properties,under different flow directions coupling effect of.buoyancy force and centrifugal force in helically coiled tube.Secondly,the effects of the different gravities on the supercritical CO₂ heat transfer and pressure drop in the vertical flow direction and the horizontal flow direction were studied.The mechanism of heat transfer was analyzed by studying the coupling effect of buoyancy and centrifugal force on the section of the shape of the secondary flow.It was found that the heat transfer coefficient of 0.5g,g and 2g enhances with the increase of gravity before the quasi-critical temperature,and it had obvious peak at the quasicritical temperature,but after the quasi-critical temperature,the heat transfer coefficient was basically the same.Whether the horizontal flow direction or the vertical flow direction,microgravity and high gravity before the quasi-critical point had a weakening effect and strengthen influence on the heat transfer respectively.But the effect of weakening and strengthening after quasi-critical points was not significant.Finally,according to a large number of the numerical data,this paper presented in the case of high gravity or microgravity heat transfer relationship.