Abstract
This paper explores the mechanism of convective heat transfer enhancement in a new perspective. In this paper, a new parameter called heat convection velocity is proposed based on the field synergy principle. It is defined as the velocity projection on the temperature gradient vector and reflects the magnitude of the velocity component that contributes to heat convection. Three typical cases are taken into consideration to investigate the influence factors of Nusselt number theoretically. The results indicate that the Nusselt number can be enhanced by increasing the mean heat convection velocity and the dimensionless mean temperature difference. Through theoretical analysis, three suggestions are found for designing heat transfer enhancement components: (a) the overall synergetic effect should be improved; (b) the fluid with lower temperature gradient should be guided to the region where the temperature gradient is higher; (c) temperature distribution should be an interphase distribution of hot and cold fluid. Besides, the heat convection velocity is used to investigate the mechanism of convective heat transfer in the smooth tube. It is found that the increase of Nusselt number is due to the increase of heat convection velocity. In addition, according to design suggestions, a new insert is invented and inserted in the circular tube. With heat convection velocity analysis, it is found that there is much potential of increasing heat convection velocity for enhancing the convective heat transfer in the circular tube.
Highlights
Convective heat transfer process exists widely in industrial applications ranging from thermal energy conversion to waste heat recovery, including steam generation, chemical industry, metallurgy industry, agricultural products drying, and etc. [1]
Theoretical principles in terms of convective heat transfer enhancement techniques can be classified into four major categories: increasing the heat transfer area, destroying and thinning boundary layers, generating swirling and secondary flow, and raising the turbulence intensity [4,5,6]
Similar to the analysis in the duct flow, it is known from Equation (38) that the increase of heat convection velocity is conducive to the heat transfer enhancement
Summary
Convective heat transfer process exists widely in industrial applications ranging from thermal energy conversion to waste heat recovery, including steam generation, chemical industry, metallurgy industry, agricultural products drying, and etc. [1]. Liu et al [23,24] proposed the physical quantity synergy between the velocity and pressure gradient in the consideration of reducing power consumption and extended field synergy principle to turbulent flow Afterwards, He and Tao [5] carried out a comprehensive review of field synergy principle in convective heat transfer, involving mechanisms, techniques, and performance evaluation. Where the local synergy angle β of velocity and temperature gradient is defined as β = arccos It is shown in Equation (4) that convective heat transfer performance could be greatly enhanced with the increase of cosine value of synergy angle when the velocity magnitude and temperature gradient magnitude are specified. It is necessary to carry out an in-depth study on the mechanism of convective heat transfer enhancement and extend field synergy principle
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