Numerical Investigation On Natural Convection Heat Transfer Of The Annular Structure In Heat Exchangers And Solenoids

Qualitative and quantitative research on the flow and heat transfer characteristics of structures related to natural convection phenomena in Engineering practice and normal life are of great value in academic research and industrial applications.Based on the new model proposed by Bej an et al.,the numerical research conducted in the range of Rayleigh number 10 ?Ra ?5×105 for the annular monomer and multibody structures widely involved in coil heat exchangers and solenoids frequently used in industry.It can not only ensure the accuracy of numerical calculation,but also greatly reduce the required calculation area size by the symmetric boundary conditions adopted in this model.The problems of the huge calculation amount and high cost caused by the large space calculation area have been solved by the technically methods.Based on this model,the natural convection of a fixed small size(L/D=0.5)annulus monomer structure in air(Pr = 0.72)in the range of 10 ?Ra ?5×105 is first numerically calculated.By analyzing the temperature nephogram,pressure nephogram,flow field,dimensionless temperature and velocity distribution,local Nusselt number and local resistance coefficient of the model under different Rayleigh numbers,it can be seen from the analysis results that due to the influence of stack effect and heating effect in the middle of the annulus,the influence changes with the change of the Rayleigh numbers.When the horizontal thermal boundary layer is tangent(in the condition of Ra=104),the ratio Nu/Nu0 of the average Nusselt number of the annulus to the average Nusselt number of the isothermal cylinder under the same condition reaches the maximum of 1.019.When the Rayleigh number continues to increase or decrease(the thermal boundary layer is crossing or separating),Nu/Nu0 are all less than 1.In addition,the universal and accurate heat transfer correlations for annulus monomer structures at different stages within the above Rayleigh number research range are obtained respectively.Secondly,based on the research of fixed-size monomer structure,the natural convection heat transfer process of horizontally scaled(0.1?L/D?5)annulus monomer structure in 15 cases with Ra=10 is continuously numerically studied to explore the influence of horizontal spacing change on heat transfer capability.Similarly,the temperature nephogram,flow field diagram,dimensionless temperature and velocity distribution of the above representative structures are analyzed,and it is found that when L/D is small,the thermal boundary layers of the two sections cross and overlap each other,and the thermal boundary layers have a large deviation to the central axis.In addition,it is found that when L/D increases from 0.1,the average Nusselt number shows a trend of decreasing first and then increasing,and the minimum point is located near L/D=0.4.When L/D continues to increase,the average Nusselt number is close to the heat transfer of a horizontal isothermal cylinder.In addition,the results of thermophysical parameters of the 15 examples are given for reference in industrial applications.Finally,in order to explore the influence of the upstream and downstream preheating effect on the heat transfer between the vertical annular arrays,it has been carried out a numerical simulation on the vertical two-body annular structure and the temperature nephogram,flow field diagram,local Nusselt number,local resistance coefficient,average Nusselt number and other data of the above structure has also been analyzed.The results show that when the thermal boundary layer is tangent at the lower annulus,it still intersects at the upper annulus,and the dimensionless temperature at the center line is about 0.43.The movement speed of the fluid inside the annulus is generally higher than that of the fluid outside.And the speed difference gradually increases in the vertical direction.The dimensionless velocity at the lower tangent of the upper annulus surged from 0.6 units when L'/D=0.5 to 0.84 units when L'/D=2.0.Finally,the fitting correlations between the average Nusselt number and L'/D in the range Ra=104,L/D=0.5,0? L'/D ?2 are obtained.The numerical research on natural convection heat transfer of the above three representative annular structures is carried out in this paper.The obtained results provide certain basis for structural design and heat transfer optimization of coil heat exchangers and solenoids commonly used in engineering heat transfer,and have certain academic significance and engineering reference value.