Study Of Single-phase Heat Transfer Enhancement In Tubes Based On Longitudinal Swirl Flows

The application of heat transfer enhancement techniques for energy saving is significant for relieving the increasingly serious energy,environment and atmosphere situation that China is faced with.When a technique is used,heat transfer performance of heat exchangers will improve,but the pressure drop also increases rapidly.Too much pressure drop will dent the energy-saving performance gained by the heat transfer enhancement technique.Therefore,it is necessary to investigate highly efficient techniques that can keep a better balance between heat transfer enhancement and pressure drop.According to the optimum flow fields obtained from heat transfer optimization,this dissertation proposes the concept of single-phase heat transfer enhancement techniques in tubes based on longitudinal swirl flows.The main principle of this technique is to generate the flow fields characterized by longitudinal swirl flows using specific structures like tube inserts or artificial roughness.A novel heat transfer enhancement device named vortex rod is firstly proposed in this dissertation.Numerical simulations are carried out to investigate the heat transfer and flow characteristics in the tube with vortex rod inserts.It is found that three pairs or six longitudinal swirl flows are induced in the tube.To optimize the geometric parameters of vortex rods for the maximum heat transfer enhancement with the minimum pressure drop,multi-objective optimization is conducted.The optimum heat transfer enhancement factor is found to be 1.27.In addition,sensitive analysis on heat transfer enhancement in a tube with conical strip inserts is performed using RSM method.The objectives are the Nusselt number ratio and the friction factor ratio,and design parameters include Reynolds number,the conical strip filling ratio and the pith ratio.Results indicate that the sensitive of Nusselt number ratio to the conical strip filling ratio and Reynolds number is positive,while the sensitive of Nusselt number ratio to the pitch ratio is negative.The sensitive of friction factor ratio to Reynolds number is positive,but the sensitive of friction factor ratio to the conical strip filling ratio and the pitch ratio is a little complicated that the interaction between the two design parameters should be taken into consideration.Artificial roughness in the form of discrete inclined grooves is also put forward for heat transfer enhancement in this work.It is found that longitudinal swirl flows are generated in the tube.Increasing the groove inclination angle and the number of circumferential grooves,and decreasing the groove pitch ratio can not only increase the heat transfer performance,but also decrease the irreversibility of the process.Considering that fact that the geometry of ribs is similar to that of grooves,discrete inclined ribs are also proposed for heat transfer enhancement.In this work,the effect of rib arrangements on the flow pattern and heat transfer performance in the tube is studied.It is found that single longitudinal swirl flow is generated in the P-type ribbed tube,while multiple longitudinal swirl flows are induced in the V-type ribbed tube.Compared to the single longitudinal swirl flow,the flow pattern of multiple longitudinal swirl flows is more effective for heat transfer enhancement.In addition,numerical investigations on the thermal-hydraulic performance in the tube with the combined use of discrete inclined grooves and ribs are performed.The results show that multiple longitudinal swirl flows generated in the rib-grooved tube improve the heat transfer performance significantly.The heat transfer performance and pressure drop for the rib-grooved tube are 1.58-2.46 times and 1.82-5.03 times as those of a plain tube,respectively.The performance evaluation criterion(PEC)is in the range of 1.19-1.68.To verify the longitudinal swirl flows generated in the enhanced tubes,Particle Image Velocemetry(PIV)experiments are conducted in the enhanced tubes such as the tube with vortex rod inserts,the tube with conical strip inserts and the tube with discrete inclined ribs.The flow fields measured by PIV are compared with those obtained by numerical simulations.Results indicate that longitudinal swirl flows are the main characteristics of the flow fields in the enhanced tubes.Therefore,PIV experiments confirm the reliability of numerical results.In summary,this dissertation proposes the concept of single-phase heat transfer enhancement techniques in tubes based on longitudinal swirl flows.Based on this concept,several tube turbulators are proposed to generate longitudinal swirl flows for heat transfer enhancement.In addition,PIV experiments verify that longitudinal swirl flows are the main flow characteristics in the enhanced tubes.