Experimental Study On The Effect Of Buoyancy And Centrifugal Force On Heat Transfer Of Supercritical Fluid In A Bend And Its Upstream And Downstream Straight Pipe

The transcritical organic Rankine cycle is currently the most efficient system for medium and low temperature waste heat recovery.However,the thermal inhomogeneity caused by the drastic change of the physical properties of the working fluid under supercritical pressure state is the main problem that must be solved.At the same time,as a common connecting pipe in heat exchanger,U-bend not only changes the flow direction in the pipe,but also makes the heat transfer process more complicated due to the introduction of centrifugation and its attenuation property.Based on this,the experimental study on the heat transfer characteristics of supercritical organic working fluid in U-tube under strong buoyancy and centrifugal force was carried out.The main research contents and results are as follows:（1）The U-tubes with two diameters(d_in=8 mm)were experimentally studied under different operating conditions,involving parameters such as heat flux,mass flow rate,inlet temperature,inlet pressure.The distribution of wall temperature and heat transfer coefficient at various axial and circumferential direction was explored.The results show that U-bend can weaken the difference of heat transfer capacity between the top and bottom caused by buoyancy.At the same time,the centrifugal effect will cause the difference of heat transfer capacity between the inside and outside,and the heat transfer capacity at outside is significantly enhanced.（2）When the pipe diameter is large(d_in=8 mm),the buoyancy has a large space for development in the circumferential direction.However,due to the low flow rate,the centrifugal effect is not obvious.At this time,the buoyancy plays a dominant role in the influence of heat transfer,which is named the strong buoyancy condition.Under this condition,the U-tube and straight tube comparative experiments were carried out.Through the comparison of the average heat transfer and local heat transfer between them,it was found that the U-bend can greatly enhance the heat transfer.At high heat flux,the heat transfer enhancement is more obvious,and the heat transfer coefficient is up to 4.68 times of the straight tube.The enhancement of U-bend on flow heat transfer is mainly reflected at the bottom and outside.Based on the quantitative analysis of the development of buoyancy in the two types of tubes,the buoyancy discriminant Gr_q/Gr_th=120 is proposed as the threshold value for the obvious heat transfer enhancement of U-bend.When Gr_q/Gr_th>120,the introduction of U-bend can effectively enhance heat transfer.When Gr_q/Gr_th<120,the heat transfer enhancement of U-bend is not obvious,and the average wall temperature difference between the U-tube and straight tube is within 10 K.（3）When the pipe diameter is small(d_in=8 mm),the centrifugal effect plays a dominant role in the influence of flow and heat transfer in bend and contiguous straight sections.Under the condition of strong centrifugal effect,a new dimensionless parameterψ*is proposed to evaluate the competition between buoyancy and centrifugal effects.Whenψ*>1,the centrifugal effect dominates,and the circumferentially maximum wall temperature appears on the inside.Whenψ*<1,buoyancy effect becomes the dominant factor,and the maximum wall temperature appears at the top.（4）Based on the experimental data,the classical heat transfer correlations are evaluated.The results show that the prediction effect of the Jackson correlation is significantly better than that of the Gnielinski formula in the upstream and downstream straight sections that are not affected by centrifugal effect.In the U-bend,the Jackson correlation still has a good predictive ability at inside.For the outside,the Jackson correlation predicted values are generally lower than the experimental values.The research results of this paper supplement the experimental data gap of supercritical fluid heat transfer characteristics in U-tubes,and provide a scientific basis for the design and optimization of U-tube heat exchangers in advanced transcritical systems.