Study On Multi Longitudinal Vortex Structure In View Of Fluid-based Heat Transfer Enhancement In A Tube

Flow and heat transfer of the work fluid are common process in the power, nuclear power, refrigeration, chemical,petroleum, aerospace and other industries. A variety of heat transfer problems widely exist in the heat exchangers of these industries. In the process of vigorously conserving energy, reducing emissions and developing new resources of energy, there are also large mounts of heat transfer problems. Therefore, theoretical analysis, numerical calculation and experimental verification of the convective heat transfer process in a tube are carried out in this thesis. Under the support of fluid-based heat transfer enhancement techniques, the multi longitudinal vortex structure elements which can reduce the flow resistance on the premise of enhancing heat transfer have been found.On the basis of existing entropy generation minimum principle and entransy dissipation minimum principle, this thesis summarizes the main methods and techniques in the convective heat transfer enhancement. Based on the first and second law of thermodynamics, the irreversibility of convection heat transfer process has been discussed, and the available energy analysis has been carried out through the convection heat transfer process. The advantage of the available energy analysis is that the available energy has the same unit with the energy, so it is easy to understand and explain the irreversibility of convection heat transfer process.For laminar and turbulent convective heat transfer, the synergy among vectorial physical quantities of a fluid particle is analyzed to reveal the relation between the multi-field synergy mechanism and heat transfer enhancement. Additionally, the efficiency evaluation criterion (EEC) is proposed to evaluate the overall performance of heat transfer enhancement. Meanwhile, using synergy angles α、β、γ、θ、η, a unified evaluation system and corresponding evaluation indexes for heat transfer enhancement are suggested. Neural network model has been used to establish the relations between the physical quantities of the fluid particle and the efficiency evaluation criterion (EEC). And the genetic algorithm has also been used to get the optimal group of physical quantities synergy angles.According to the fluid-based heat transfer enhancement techniques which are different from the traditional surface-based heat transfer enhancement techniques, and combined with multi longitudinal vortex structure obtained by the irreversibility optimization of convection heat transfer process, numerical studies on a series of tube inserts have been carried out, and the overall heat transfer performance, entropy generation analysis, physical quantity synergy analysis and available energy analysis have also been carried out based on the numerical results. Helical screw-tape inserts with four different widths (w=7.5mm,12mm,15mm and20mm) have been investigated for different Reynolds number ranging from4000to12000. A three-dimensional turbulence analysis of heat transfer and fluid flow is performed by numerical simulation. The simulation results show that the helical screw-tape can induce a longitudinal swirl in the tube and the PEC value of the helical screw-tape inserts of different width varies between1.58and2.35. The numerical investigation of multi-longitudinal vortices in a tube induced by triple and quadruple twisted tapes insertion show that the maximal PEC value can be reached to2.5under laminar flow condition. Multi helical screw-tape can also induce multi longitudinal vortex structure in a tube, and the numerical results further validate that multi longitudinal vortex structure can efficiently enhance the heat transfer in the tube, the maximal PEC value can be reached to2.90-5.10under laminar flow condition, the synergy angles β and θ, are calculated, and the numerical results verify the synergy regulation among physical quantities of fluid particle in the flow field of convective heat transfer, which can guide us to get the optimum design. In conjunction with a certain size, four delta wing longitudinal vortex generator can induce a interesting flow structure with four vortexes inside and four vortexes outside the fluid, the maximal overall heat transfer performance PEC value can be reached to2.9. Through the numerical simulation of heat transfer and flow characteristics of five star rod inserts in a tube, it can be found that ten vortexes flow structure are formed in a tube successfully, this flow structure can also make good performance in disturbing fluid in the tube and the maximal overall heat transfer performance PEC value can be reached to3.5. All these tube inserts structures mentioned above can provide good guidance to the design, modification and optimization of the heat exchanger.According to the theoretical and numerical analysis, this thesis proves that the four longitudinal vortex generator insert is an effective heat transfer enhancement. In the Reynolds number range of3000-19000, the Nusselt number is increased by1.20～2.93 times, the flow resistance increases by3.5～6.5times, compared with the plain tube. With the increase of the longitudinal vortex generators spacing and the decrease of the attack angle, the increased ratio of heat transfer coefficient has decreased; and the the increased ratio of flow resistance coefficient first decreases and then increases with the increase of the Reynolds number, and has also decreased with the increase of the longitudinal vortex generators spacing and the decrease of the attack angle. Under turbulent flow condition, the heat transfer performance, the PEC value can reach up to1.65, which is an effective overall heat transfer enhancement. The experimental investigation has proved that the multi longitudinal vortex structure can increase heat transfer and resistance increases slightly.