| Heat exchangers were key process equipment widely used in many industrial fields,such as petroleum,chemical,and electric power.With the development of the industrialization and the depletion of energy and resources,the requirements for high efficiency,compactness,low-pressure drop and specialty of heat exchangers were increased.In this paper,the outward helically corrugated tube was proposed which can satisfied high-pressure working condition.A numerical method was employed to explore the complex flow and enhanced heat transfer mechanism under the coupling secondary flow and rotational flow.The convection entropy generation model was used to analyze the global irreversibility of heat transfer and viscous dissipation.Combined with the flow features,the effects of complex flow patterns on heat transfer and resistance were obtained.Based on the multi-objective optimization theory,the triple-indexes evaluation system was built with considering the heat transfer,flow resistance,and overall heat transfer performance,simultaneously.Finally,the helically corrugated tube was used in a double-pipe heat exchanger.The enhanced heat transfer mechanism of the shell side was investigated,and the tube-shell sides matching flow rate was obtained by the multi-objective optimation method.The Reynolds stress model(RSM)and large eddy simulation(LES)methods were employed to predict the turbulent flow features and heat transfer performance of smooth tube,transverse corrugated tube,and helically corrugated tube.The results were compared with the experimental data,empirical results and results by the published articles.The results show that the large eddy simulation method was more precise on local features,both the Reynolds stress model and large eddy simulation accurately predicted the separation and reattachment location of the detached vortex,also the magnitude of the back-flow.When predicted the average heat transfer and resistance of smooth tube and corrugated tube by the RSM and LES,the errors among the numerical results,experimental data and empirical correlations are within ±10% indicated that,both two numerical methods possessed high precision.To save the calculation efficiency,the RSM was chosen to numerically calculate the turbulent flow and heat transfer performance o f helically corrugated tubes.Through the comparison of the flow fields,local and average heat transfer for transverse corrugated tube and different structures helically corrugated tubes,the effect of corrugation structures on the production of secondary flow,spiral flow,and strong turbulent pulsation were studied.Also,the effect of the complex flow features on heat transfer and friction performance were discussed.The results show that the secondary flow and turbulent pulsation were inhibited by the rotational flow.With the increase in corrugation height,the secondary flow and spiral flow were both increased,while the turbulent pulsation increased first and then decreased.With the increase in corrugation pitch,the secondary flow nearly the same,the spiral flow increased and turbulent kinetic energy decreased.The local heat transfer and friction factor along the corrugation surface indicated that the local heat transfer reached a maximum at the windward of corrugation,while it reached a minimum at secondary flow region and even lower than it at the smooth surface.The local friction factor reached a maximum at the initial and the end of corrugation.In addition,the heat transfer performance was slightly weakened by the rotational flow,but the flow resistance was decreased significantly.According to the convection heat transfer entropy model,the irreversibilities of heat transfer and viscous dissipation in corrugated tubes were numerically investigated.The local heat transfer entropy generation distribution of fluid in the boundary layer verified the conclusions by the local heat transfer performance obtained in chapter three.The heat transfer entropy generation distribution of the main flow region shown that,though the secondary flow was disadvantages to the heat transfer between the boundary layer and surface wall,the heat transfer of main flow region was improved.In the strong turbulence pulsation flow region,the heat transfer entropy was lowest;it indicated that the turbulence pulsatio n was meaningless to heat transfer.The local friction entropy generation indicated that the viscous dissipation irreversibilities were mainly produced at the boundary layers of smooth surface,secondary flow,and the downstream region.Based on the surface center composite design,the three-factors of Reynolds number,corrugation height,and corrugation pitch were designed with five-levels.The second-order response models of the three targets of heat transfer,flow resistance and overall heat transfer performance were established based on the response surface method,with a goodness of fit above 96%.The sensitivities of the three response models were also studied.The Pareto optimal boundary of the three-objectives(heat transfer,flow friction,and overall heat transfer performance)was obtained by the multi-objective genetic algorithm.The optimized helically corrugated tube was applied in the double-pipe heat exchanger.The mechanism of enhanced heat transfer for the shell side of the double-pipe heat exchanger,the design of shell diameter and the matching flow rate of shell-and-tube sides were all studied.The results show that the distribution of local heat transfer and friction coefficients for shell-side was same as the tube-side.The average performances have shown that,with the shell diameter increased,the total heat transfer and total pressure drop both decreased.When the shell diameter reached to 38 mm,the total heat transfer performance was declined slowly and the total pressure drop decreased significantly and chosen as the optimal shell diameter.The multi-objective optimization of the tube-shell flow matching was carried out,and the Pareto optimal solutions were obtained with the total heat transfer,total pressure drop and overall heat transfer performance as three targets. |
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