A Study Of Heat Transfer Enhancement In Tubes With Fluid Mixing

With the rapid development of the economy,energy shortages and environmental problems are becoming more and more serious.Hence,improving energy efficiency is of great significance for balancing the contradiction between energy consumption growth and energy conservation and emission reduction targets brought about by economic development.As one of the heat transfer methods,convective heat transfer is widely used in heat exchange equipment in various industrial fields.Improving the convective heat transfer performance of heat exchange units or equipment is of great significance for improving energy efficiency.In this paper,the single-flow convective heat transfer process in the tube is taken as the research object,and the convective heat transfer performance evaluation,heat transfer mechanism and a series of heat transfer enhancement techniques research are carried out.Based on the available potential equilibrium equation,the dissertation derives the expression of the local exergy destruction rate of the convective heat transfer process.Exergy efficiency of the heat transfer process is defined to evaluate the performance of the enhanced heat transfer technology from the perspective of the second law of thermodynamics.In addition,based on the conservative equation of fluid mechanical energy expressed by the pressure form,the corresponding synergy equation is derived to reveal the relationship between the flow pressure drop and the synergistic performance of the fluid velocity field and pressure gradient,and a representation of the experimental dimensionless correlation of Eu number versus Re and Li numbers is obtained.Moreover,the dimensionless correlation of a plain heat transfer tube is achieved by hydraulic experimental measurement.Guided by the optimized flow field and temperature field obtained by the heat transfer optimization,the dissertation obtains three new types of elements for heat transfer enhancemtnt through design and improvement with numerical simulation: central slant rod inserts,multiple conical strips inserts and bidirectional conical strip inserts.The results indicate that: 1.The central slant rod inserts realizes a multi-longitudinal vortex flow in the tube which is similar to the optimized flow field,so that the heat transfer performance in the tube is effectively enhanced with no much increase in flow resistance,the heat transfer and flow resistance are increased to 1.81–5.05 and 2.49–6.93 times that of the plain tube,respectively,the overall heat transfer performance PEC value and R3 value range in 1.34–2.71 and 1.74–4.60,respectively;2.The multiple conical strips inserts allows the vortex intensity of the multi-longitudinal vortex flow in the tube to be significantly enhanced compared to the central slant rod inserts,so the heat transfer performance is also significantly improved,but when the Reynolds number and the cone center angle are large,backflow eddies and flow dead zone are generated on the back of the conical strips and result in a significant increase in flow resistance.In general,the heat transfer and flow resistance are 2.54–7.63 and 2.40–28.74 times that of the plain tube,respectively,and the overall heat transfer performance PEC value and R3 value are ranged in 1.79–3.31 and 1.23– 6.05,respectively;3.The bidirectional conical strip inserts allows the vortex intensity of the multi-longitudinal vortex flow to be further enhanced on the basis of the multi-taper insert,and allows the formation of the backflow eddyand the flow dead zone to be inhibited at the same time.Therefore,the heat transfer performance has been further improved.The heat transfer and flow resistance are 2.35–9.85 and 2.37–21.18 times that of the plain tube,respectively,and the overall heat transfer performance PEC value is ranged in 1.75–3.93.In addition,PIV experiments are carried out to measure the flow field in the three enhanced heat transfer tubes and make comparison with the numerical results.The experimental results confirm that the multi-longitudinal vortex flows are formed in the enhanced tubes and agree well with the simulation results with the relative deviations limited in 10%,which indicate that the numerical method used in the dissertation has a high reliability for calculating the flow and heat transfer performance in the heat exchanger tube.For the purpose of completely destroying the thermal boundary layer,a new type of tube inserts,fluid exchanger inserts,is proposed and numerical studies on its flow and heat transfer performance are conducted in this dissertation.The fluid exchanger inserts can completely interchange the cold fluid in the center of the tube and the hot fluid in the boundary and consequently completely destroy the thermal boundary layer periodically and effectively enhance the heat transfer.The overall heat transfer performance(PEC)of the fluid exchanger inserts can reach to 1.24?1.62.Furthermore,the results of exergy destruction analysis indicate that the exergy efficiency of the fluid exchanger inserts is increased by 0.3% to 1.9% compared to the plain tube.In this dissertation,the conical strip inserts is used to improve the heat transfer,thermal efficiency and thermo-mechanical performance of a parabolic trough receiver(PTR),and numerical studies have been carried out.The conical strip inserts can significantly improve the local heat transfer coefficient in the high heat flux region,thereby effectively reducing the peak temperature and circumferential temperature difference with the reductions in peak temperature and the circumferential temperature difference ranging in 9–230K and 7–219K,respectively.The heat loss is consequently decreased by up to 82.1%,and the thermal efficiency is increased by 0.02%–5.04%.Its average heat transfer is increased by 45%–203%,while the flow resistance is increased to 6.17–17.44 times that of plain PTR,the PEC value ranges in 0.70–1.33.The results of entropy production and exergy efficiency analysis show that the entropy production is reduced by up to 74.2% and the enthalpy efficiency is increased by up to 5.7% compared with plain PTR.In addition,to investigate the optimal flow field for heat transfer in PTR,the effects of different rib arrangement on the flow field and thermal performance of PTR are numerically studied in this work.The results show that the ribbed tube can effectively improve the performance of PTR.Moreover,the optimized flow field in the revelant optimization study,two reverse longitudinal vortex flow that are flushed downward,are obtained in enhanced PTRs with SCSIR and SADIR.Therefore,they achieve the best performance,as the peak temperature and heat loss are reduced by up to 177 K and 80.1%,respectively.The PEC values and modified thermal efficiencies range in 0.62 –1.56 and 0.70–0.73,respectively,with the modified thermal efficiency being improved by up to 1.9%.The results of entropy analysis show that the SCSIR and SADIR obtain the lowest entropy genetation rate,and the maximum reduction of entropy generation rate are 73.9% and 72.7%,respectively.