A Study On Mechanism And Components Of Disturbing Fluid For Convective Heat Transfer Enhancement

Heat transfer enhancement is the key to improving energy conversion efficiency,which helps to relieve energy supply pressure and reduce carbon emissions.Heat transfer enhancement is also a safety requirement for high heat flux components such as chips.As for the widespread single-phase duct flow,it is not only the frontier of current research but also a difficult problem for worldwide researchers to design efficient heat transfer enhancement components with low flow resistance.To this end,focusing on the theme of enhancing heat transfer efficiently with low resistance,this dissertation carried out following works:This dissertation revealed the effect of fluid disturbance on the convective heat transfer.Based on the field synergy principle,the effective heat transfer velocity(V_h)was proposed and defined as the velocity projection on temperature gradient.By theoretical analyses,it was obtained that the increase in Nusselt number was equal to the product of the increase in effective heat transfer velocity and the increase in integration of dimensionless temperature gradient magnitude.Thus,four methods for heat transfer enhancement were obtained.In addition,the local heat convection number(Lu)was proposed to describe the local heat convection performance.The heat convection intensity factor(H_f)was proposed to evaluate the enhancement degree of overall heat convection performance.This dissertation explored the low-resistance flow pattern in convective heat transfer for a variety of operating conditions.The governing equations of the optimized flow pattern were derived theoretically by using the variational principle with the objective of minimizing exergy destruction in heat transfer process for a given power consumption.The variational optimization method based on minimizing exergy destruction was extended from laminar flow to turbulent flow and fluid-solid coupled heat transfer conditions.It was found that the optimized flow pattern under various operating conditions(uniform thermal boundary,non-uniform thermal boundary,and fluid-solid coupled heat transfer)was multiple longitudinal swirls flow combined with alternating warm and cold temperature distribution.This dissertation proposed low-resistance components to enhance the laminar heat transfer in the tube with uniform thermal boundaries inside the shell-and-tube heat exchanger.Inclined two-stage bifurcated sheets were proposed in the tube by following the constructal law of nature.In the condition of constant wall heat flux with water as working fluid,the maximum values of R3 and EEC were 6.66 and 1.32,respectively.In order to simplify the bifurcation structure,inclined single-stage quadruple bifurcated paddle sheets were proposed.The maximum values of R3 and EEC were 6.05 and 1.38,respectively,for a constant wall heat flux with water as working fluid.This dissertation proposed low-resistance components to enhance the turbulent heat transfer in solar collectors with non-uniform thermal boundaries.In the solar air heater,inclined trapezoidal vortex generators were proposed.For the upwind aligned arrangement at Re=6000-18000,the maximum values of R3 and EEC reached 1.89 and 0.98,respectively,with a maximum increase in the thermal efficiency of 24%and a maximum increase in the exergy efficiency of 31%.In the absorber tube of concentrating solar collector,the longitudinal swirls impinging cooling strategy was proposed and inclined curved-twisted baffles were subsequently proposed.As the inlet temperature increased from 400K to 600K,the temperature difference between the upper and lower halves of the tube wall could be reduced by 55.1%on average,and the overall efficiency and the exergy efficiency were increased by 0.52%and 0.22%on average,respectively.This dissertation proposed low-resistance components to enhance convective heat transfer in minichannel heat sinks where it is thermal and hydraulic developing.Inclined rectangular sheets were proposed to enhance the laminar heat transfer.At Re=100-1100,the maximum values of R3 and EEC were 4.22 and 1.12,respectively.This dissertation proposed a heat transfer enhancement strategy of enhancing the original flow pattern by forming longitudinal swirls flow.V-shaped ribs were subsequently proposed to enhance the turbulent heat transfer.At Re=3600-6800,the maximum values of R3 and EEC were 2.06 and 0.89,respectively.