| Heat exchangers are widely used in various fields of energy systems.With the development of the economy,the demand for high-efficiency and energy-saving heat exchangers is more obvious.The update of heat transfer enhancement will promote the revolution of energy technology and realize the high-quality development of the energy industry.In this paper,tube-side external expansion corrugated tubes are developed and taken as the research objects.Through the combination of experiment and numerical simulation,heat transfer,resistance,and comprehensive performance of corrugated tubes with different structures are revealed,and the experimental correlation is fitted.Through the study of velocity,temperature,turbulent kinetic energy,and turbulent eddy dissipation distributions,the enhanced heat transfer mechanisms of corrugated tube heat exchangers are analyzed.Using the evaluation methods of heat transfer efficiency conversion rate and efficiency evaluation coefficient,the optimal flow velocity matching on both sides and the best efficiency evaluation coefficient of corrugated tube heat exchangers are obtained.Based on the multi-objective optimization method,the structural parameters and working parameters of corrugated tube heat exchangers are optimized,and the optimal solution is solved.Through studies in this thesis,the performance of corrugated tubes and mechanisms are comprehensively expounded,the optimal flow direction setting and flow rate matching on both sides of corrugated tubes are identified,and structural and working parameters of corrugated tube heat exchangers are optimized,and guided the design of heat exchangers.According to the test standard,the enhanced heat transfer element test platform is designed and built.Three kinds of corrugated tubes with different structures are made.Working condition parameters and data processing methods in the experiment are determined,and the sensors of the platform are calibrated and analyzed for uncertainty.At the same time,the numerical model is established,the Reynolds stress transport model is selected,and the meshing method of near wall hexahedral dense mesh coupled polyhedral mesh in the main flow area is used.The number of meshes is determined through the analysis of mesh independence.Then,heat transfer and resistance performance of smooth tube and corrugated tube are tested experimentally and simulated.Compared with the empirical correlations and numerical simulation results,experimental results are verified.The performance of symmetrical corrugated tubes with different structural parameters is studied by experiments.Compared with the smooth tube,the heat transfer performance and resistance performance of symmetrical corrugated tube on tube side are increased by 26.70%-39.09% and 69.42%-176.93%,and on shell side increased by 41.45%-60.08% and 129.30%-269.98%.The experimental results show that the pitch of corrugations changes the number of corrugations and affects heat transfer and resistance performance on both sides.With the increase in height of corrugations,the heat transfer and resistance performance increase gradually,and the increased range of resistance performance is larger.With the increase of fillet radius,heat transfer and resistance performance decrease slightly,and the reduction of resistance performance is larger relatively,which improves the comprehensive performance by a small margin.The analysis of the mechanism shows that heat transfer and resistance performance of symmetrical corrugated tubes are related to secondary flow,which is generated on tube-side and shell-side corrugations.Secondary circulations are formed on the tube-side upstream and shell-side downstream,and small eddies are formed at the intersection between secondary circulations and core flow.the pitch of corrugations changes the interaction between corrugations,the height of corrugations alters the scale of secondary circulations and small eddies,and fillet radius affects the intensity and distribution of secondary circulations and small eddies.An asymmetric tube heat exchanger with large fillet radius has four different flow direction settings.Heat transfer and resistance performance of asymmetric corrugated tubes are studied,and the optimal flow direction setting is obtained.Under the optimal flow direction setting,performance of asymmetrical corrugated tubes with different structural parameters is also investigated.Compared with the smooth tube,heat transfer performance and resistance performance of asymmetrical corrugated tube on tube side are increased by 20.86%-33.86% and 39.79%-119.24%,and on shell side increased by 36.74%-54.83% and 54.62%-195.88%.The results show that,in reverse flow,the large fillet radius of the asymmetric corrugated tube is located on the tube-side upstream,and the comprehensive performance is the best;in parallel flow,the comprehensive performance is better when the large fillet radius is located on tube-side downstream.The large fillet radius of asymmetric corrugated tube brings down the secondary circulations and small eddies,and reduces the heat transfer and resistance performance.On the pitch of corrugations,the large fillet radius reduces the length of straight section and improves the interaction between corrugations;Large fillet radius forms different fillet angles with the height of corrugations,which affects heat transfer performance,resistance performance,and the intensity of secondary flow;With the increase of large fillet radius,the improvement of comprehensive performance is restricted.The performance of helical corrugated tubes with different structural parameters is studied by experiments.Compared with the smooth tube,the heat transfer performance and resistance performance of helical corrugated tube on tube side are increased by 25.82%-41.63% and 24.78%-79.61%,and on shell side increased by 39.28%-58.37% and 53.23%-125.95%.The experimental results show that,compared with the symmetrical corrugated tube,the resistance performance of the helical corrugated tube is reduced significantly,while the heat transfer performance is not depressed greatly,and the comprehensive performance is improved remarkably.Helical flow produced by helical structure weakens the intensity of secondary circulation,but it increases the volume of the secondary circulation and the absolute flow path of the fluid.The angle of the helical structure is determined by the pitch of corrugations,which changed the intensity of the helical flow and the axial secondary flow;The increase of height still improves the heat transfer and resistance performance;The increase of fillet radius partly reduces the heat transfer and resistance performance.Heat transfer efficiency conversion rate of the smooth tube and corrugated tube heat exchangers are studied,and the optimal flow velocity matching on both sides are obtained.The comprehensive efficiency evaluation coefficient of corrugated tube heat exchangers is studied.The average comprehensive efficiency evaluation coefficient of the symmetrical corrugated tube heat exchanger is 1.20-2.22 times better than that of smooth tube,the asymmetric corrugated tube heat exchanger is1.21-2.95 times,and the helical corrugated tube heat exchanger is 1.35-3.19 times.Based on the multi-objective optimization theory,a second-order linear regression model is established for the three objective functions of heat transfer,resistance,and comprehensive efficiency on helical corrugated tube heat exchanger.The pitch,height,fillet radius,Reynolds number on tube side and shell side are taken as the response factors.Through the variance and response surface analysis of the regression model,the optimal solutions of geometric and operating parameters are obtained,which provides a reference for the optimal design of helical corrugated tube heat exchanger. |
PDF Full Text Request