Research On Jet Enhancement Of Flow And Heat Transfer Mechanism In Rectangular Helical Channel

Jet is an effective way to enhance heat transfer,which plays an important role in basic researches and industrial applications.In this paper,jet vortex generator(JVG)is used to enhance the heat transfer of helical channel.Based on CFD Fluent software,k-? turbulence model is used to simulate the flow and heat transfer.The accuracy of the numerical method is verified by 3D laser Doppler Velocimeter(LDV),and the unsteady characteristics of vortex structure are studied.Based on the different jet parameters and helical parameters,the steady numerical simulation of the flow and heat transfer in the helical channel is carried out.The composite flow field,resistance characteristic and acting distance in the channel are analyzed under the conditions of different jet velocity ratio(?j),incidence angle(?),channel curvature(?)and section height width ratio(?).Based on the principle of field synergy,the mechanism of heat transfer enhancement of JVG is revealed.The comprehensive enhanced heat transfer factor JF is used to evaluate the comprehensive enhanced heat transfer effect of JVG.The results show that the formation of a secondary vortex(CFD)rushing toward the wall at the initial stage of the jet enhances the turbulence of the fluid near the inner wall and undergoes a process of gradual decomposition and dissipation in the helical channel.In the research range,with the increase of the jet velocity ratio ?j,the enhanced heat transfer effect of the jet on the helical channel is enhanced,but the flow resistance is also significantly increased;With the increase of the incident angle ?,the enhanced heat transfer effect of the jet is weakened,and the enhanced heat transfer effect of the jet is not significant when ?=75°;For the helical channel with ?=0.1341 and ?=1.11,the maximum acting distance of jet in the helical channel can reach 74 dh(dh is the equivalent diameter of the helical channel).At a certain value of ?j and ?,reducing the curvature ? of the helical channel can improve the heat transfer enhancement effect;With the increase of the ? value of the aspect ratio,the impact of the jet on the inner wall of the channel is enhanced,and the heat transfer enhancement effect is improved.The range of the comprehensive enhanced heat transfer factor JF based on the original flow rate of the helical channel and the increase of the flow rate due to the jet are1.45?2.14 and 1.12?1.28 respectively.The large eddy simulation is used to analyze the development characteristics of the vortices.The shear layer in the core area of the jet is unstable under the action of the main flow,forming two-side entrainment vortices in the helical channel.With the development of the flow,the entrainment vortices are mixed and broken into many small vortices.The ?2criterion is used to judge the vortex coherent structure,and the simulation of the formation,mixing and breaking of the longitudinal entrainment vortices of the jet with larger strength is obtained transformation process.