Study On The Effect Of Buoyancy In Heat Transfer In Horizontal Flow Of Supercritical Fluids

In recent years,heat transfer industrial process involving supercritical pressure fluid flow have drawn more and more attention because of its characteristics of energy conservation and environmental protection.Because supercritical medium has intense physical change in pseudo-critical region,its convective heat transfer characteristics is much more complicated than subcritical condition.However,there are few studies involving horizontal flow in supercritical pressure heat exchange tubes,and there is a lack of deep analysis and understanding of the heat transfer phenomenon in horizontal tubes.In particular,there are many aspects to be improved in the research on the influence of buoyancy in horizontal tubes,so it is necessary to carry out research on it.The main research contents are as follows.The existing experimental platform for supercritical horizontal flow heat transfer is introduced.R125,R404A and R410A are selected as working medium in the existing supercritical horizontal flow heat transfer experimental platform.The experimental pressure range is 1.05?1.2times of critical pressure.No specific experimental range is set for heat flux(referred as q)and mass flow rate(referred as G).The experimental research is carried out in a wide range.Each device of the system and the experimental difficulties of the start-up and control process of the equipment are introduced.In order to ensure the accuracy of the experimental results,whether the heating power required by the system is uniform and whether the system is reliable are checked.The conversion formula between the temperature of the outer tube wall measured by the experiment and the temperature of the inner tube wall is listed.The influence of changing parameters such as heat flux pressure and inlet temperature on the heat transfer in the tube was studied by using the experimental platform.It is pointed out that q/G can be regarded as a whole under experimental conditions.With the increase of q/G,the influence of buoyancy is greater,and the first type heat transfer deterioration the will be more obvious in the range of T_b<T_pc<T_w.With the increase of pressure,the temperature distribution in the inner wall of the test section increases as a whole,and heat transfer deterioration is more likely to occur at lower pressure.Under different inlet temperature,the overall wall temperature increases with the decrease of inlet temperature,and the corresponding dimensionless temperature T_b/T_pc at the peak of wall temperature gradually decreases with the decrease of T_in.The peak wall temperature and temperature difference(?T)increase.A variety of supercritical fluid(such as carbon dioxide,water and everything)horizontal flow heat transfer experimental data are collected,combined with our own experimental data,different buoyancy criterion for the are evaluated.Results show that the criterion with best adaptability Gr_q/Gr_th have good adaptability to changing pipe diameter,needs to choose the top temperature as the qualitative temperature,but still have its limits.Such as bad performance in gas-like region,no numerical consistency between liquid-like region and gas-like region,its peak value has no correlation with the peak value of?T.It is also found that the enthalpy of the bulk fluid of?T is an important factor to measure the development of buoyancy force in the tube.Finally,the principles to be followed in the revision of the criterion of buoyancy lift are put forward.The criterion of buoyancy was optimized following the principles,and the evaluation of it showed that the optimized criterion is more uniform in order of magnitude,and could be applied to working medium such as R134a,for which the previous criterion was almost not applicable.The prediction of the peak position of temperature difference between the top and bottom wall in the pipe was more accurate.Through the research in this paper,more basic experimental data of the supercritical horizontal pipe are obtained,the influence of buoyancy in the supercritical horizontal pipe is more clarified,and a new criterion and its threshold value of the buoyancy are proposed.