Research Of Convection Enhanced Heat Transfer Characteristics In A Tube

Taking coal as the main consumption structure to increase the energy shortage problem in China,to accelerate the development of energy security strategy,reduce over-dependence on various types of primary energy,optimize energy structure,promote the rational and efficient use of energy,and achieve energy conservation and emission reduction are imperative.Enhanced heat transfer technology,can achieve the oil,metallurgy,chemical,electronic devices and many other areas of heat transfer equipment efficient heat transfer,energy saving purposes.Therefore,the study of heat transfer enhancement,that is,the study of improving the heat transfer performance of the heat exchanger has always been the focus of scholars.The study of convective enhancement heat transfer technology in pipes aims at increasing the disturbance of the fluid to the boundary layer,strengthening the heat transfer between the fluid in the core area and the wall surface fluid,thereby reducing the energy consumption and improving the technology of comprehensive heat exchange capacity.In this paper,the study of convective enhancement thermal characteristics in the tube is performed.The main work includes the following aspects:First of all,based on the theory of enhanced heat transfer,this paper proposes a new type of tube-in-core region to be inserted into an asymmetric height rectangular winglet vortex generator.Use the numerical simulation software Fluent to study its inclination angle and wing height for its effect of characteristics heat transfer performance and resistance under turbulent flow conditions.The results show that the longitudinal vortex generated by the rectangular wing can indeed strengthen the mixture of the cold fluid in the core region and the hot fluid in the wall,and the larger the inclination of the rectangular winglets is,the greater the Nusselt number（Nu）is.Compared with the light pipe,Nu and（friction factor）f increase by 1.16².49 and 2.09¹2.32 times,respectively,Nu and f increase with the increase of non-dimensional winglet height H₂/D,through the optimization of the engineering structure,the optimal structural parameters were obtained,ie,the inclination of the wingletβ=30°and H₁/D=H₂/D=0.5.At this time,the optimal comprehensive evaluation factor PEC was 1.18.By using the field synergy principle and the entransy dissipation extremum principle are used to analyze the calculation results.It is concluded that the synergistic angle of field decreases with the increase of rectangular winglet’s inclination angle and wing height.The larger the inclination angle is,the higher the wing height is and the smaller the thermal dissipation value,the smaller the heat transfer resistance.Based on this,the reliable empirical correlations of Nu and f are given based on the simulation data,which provides a design reference for this type of design.In order to verify the enhanced heat transfer efficiency of rectangular winglets,the Nusselt number,friction factor and comprehensive heat transfer evaluation factor of the inserted winglet in the tube were compared with those of other scholars.The results showed that the winglet enhanced heat transfer performance is better,and the next study can improve comprehensive evaluation performance by drilling holes in rectangular winglets.Secondly,in order to further improve the comprehensive evaluation factor,a new type of triangular winglet with inserted spiral arrangement in the tube is proposed,and its flow and heat transfer characteristics are numerically studied.From the velocity field,temperature field,pressure field and convection heat transfer eigenvalues,the influence of the angle of the triangulation winglet and the number of triangular winglet on the flow and heat transfer is revealed.In addition,compared with the results of the rectangular winglet,the results show that the inclinationβof the winglet is positively correlated with Nu and f and negatively correlated with the PEC.Compared with the rectangular winglets,the Nu maximum increases by 17.0%and the f maximum decreases by 33.33%.The influence of the number of triangular winglet and the convective heat transfer characteristics was studied.When n=16 compared with n=4,the Nu only increased by 9.18%,while the f increased by 52.94%,indicating that with the triangle winglet increasing number,the increase in Nusselt number is far less than the increase in friction factor.Therefore,the number of triangular winglets cannot be increased without limit.When Re=5000 and the number of triangular winglets n=4,the maximum PEC=1.39 is obtained,which is 17.8%higher than that of the rectangular winglet.Finally,the water is used as working medium,the convective heat transfer experimental platform with asymmetrically arranged rectangular winglets in a circular tube was set up,and experimental data were measured.Comparing the experimental results with the simulation results,the maximum errors of Nusselt number and friction factor are 8.76%and 10.61%respectively,indicating that the experimental results are in good agreement with the simulation results.In addition,the uncertainty of the experiment was analyzed,the maximum uncertainties of the Reynolds number（Re）,Nu,f measured in the experiment were 8.32%,10.11%,and 8.37%,respectively.In order to verify the reproducibility of the experimental measurements,the repeatability test was performed approximately 5 months after the start of the experiment to verify the stability of the measuring instrument.The results showed that the maximum deviations of Nu and f were 5.82%and 4.19%,respectively,and the experimental repeatability was good.