| Shell and tube heat exchangers occupy 30% to 40% share of the heat exchanger market and play an extremely important role in industries such as power plant thermal systems,metallurgy,steelmaking,refrigeration,chemistry,and especially new energy fields.How to improve the heat exchange performance of shell and tube heat exchangers is of great significance to the energy conservation of high energy-consuming industries.This work studies performance augments on compound heat transfer processes in ladder type helical schemes of countercurrent U-tube feedwater heaters,including heat transfer enhancement and flow performance studies in helical channel in the half-cylindrical space of countercurrent U-tube feedwater heaters,and two-phase flow performance of the horizontal tube inserting the inclined wire mesh tube for enhancing the condensation heat transfer.The purpose of this work is to solve the problem of flow dead zones in traditional segmental baffle U-tube heat exchanger and the difficulty of removing the condensate from the outer tube wall,thus achieving the goal of improving heat transfer coefficient and the comprehensive performance.The main research contents and results of the work are as follows:(1)Aiming at the characterized facts that the current helical baffle heat exchangers are set for the cylindrical space while the countercurrent U-tube feedwater heaters require work in the half-cylindrical space,the numerical model of ladder type helical baffle heat exchanger was established and simulated for half cylindrical space,under the background of supercooling of the condensate section and superheated steam cooling section in a feedwater heater.The characteristics of flow,heat transfer and temperature uniformity in the ladder type helical channels are investigated.Visualization results prove that the ladder type helical baffle heat exchanger has higher heat transfer coefficient and better temperature and flow uniformity compared with the lateral dominant flow in traditional double or single segment baffle heat exchanger as well as the axial dominant flow in the egg crate baffle heat exchanger.The superheated steam or supercooling water forms helical flow along the channel in the halfcylindrical space with secondary vortexes.It reveals that,with the structure of bending baffles and two sets of the baffles overlapping in the middle conjunction line,the ladder type helical baffles can suppress the shortcut leakage and guide the fluid to mix or exchange from the upstream helical channel to the downstream channel,thereby improving the heat transfer performance.(2)Unilateral ladder folded schemes,unilateral ladder plane schemes and bilateral plane schemes were numerically studied in half cylindrical space,and simulation results have been established and compared in terms of velocity fields,pressure fields and temperature fields.Numerical results show that in the research range where the shell side temperature is 510-683 K,the pressure is 6.45-6.5MPa,and the axial Reynolds number Rez,o is 16949-90948,the unilateral ladder plane scheme with a smaller incline angle has a larger flow cross-sectional area,lower flow velocity and higher comprehensive index.The different geometric structures such as different incline angles and the project length of the bending section of unilateral ladder folded schemes have less influence on the heat transfer coefficient,but a greater influence on the pressure drop and comprehensive index.For a given baffle spacing,the unilateral ladder folded scheme with a larger incline angle or with shorter projection length of inclined section has a higher heat transfer coefficient,of which the pressure drop is also higher;while the unilateral ladder plane scheme has the smallest incline angle or the longest projection length of inclined section and a lower pressure drop,which is recommended from the perspective of the more convenient manufacture and highest comprehensive index.(3)In the research range where shell side inlet flow rate of the superheated steam cooling section is 0.361-0.722 kg/s,temperature is 550-683 K,pressure is 6.45-6.5MPa and axial Reynolds number Rez,o is 43280-90948,the results show that the shell-side heat transfer coefficient of ladder type helical baffle heat exchangers is averagely 2.99% higher than that of the corresponding double-segment scheme,and the average shell-side pressure drop of the ladder type helical baffle heat exchangers is 52.95% lower than that of the corresponding double-segment schemes,the comprehensive indexes hoΔpo-1 and hoΔpo-1/3 are respectively140.5% and 36.3% higher than those of the corresponding double-segment schemes with identical baffle pitches in the discussed range.The bilateral plane type helical baffle scheme shows the lowest shell-side pressure drop and Euler number among all schemes.The ratios of comprehensive indexes hoΔpo-1 and hoΔpo-1/3 of the bilateral plane type helical baffle scheme to the unilateral ladder type with folded baffle H*-72 are relatively 1.867 and 1.133 on average,of which the ratios are 1.207 and 1.034 of the unilateral ladder type with plane baffle scheme H*-\,respectively.(4)The experimental test of countercurrent U-tube feedwater heaters was built,and the experimental results were compared and analyzed with the results of the numerical models,which verified the reliability of the numerical models and methods.The experimental results show that under the same working conditions,the shell-side heat transfer coefficients of the unilateral ladder folded baffle heat exchanger and the unilateral ladder plane baffle heat exchanger are respectively 5.45% and 1.13% higher than those of the corresponding segmental baffle heat exchanger on average,and the pressure drop is reduced by 7.01% and 14.53% on average compared to the corresponding segmental baffle heat exchanger under the same operating conditions,respectively,which shows the superiority of the ladder type helical baffle shemes of countercurrent U-tube feedwater heaters in enhanced heat transfer performance.(5)For the condensation heat transfer outside the horizental tube bundle in the condensation section of the countercurrent U-tube feedwater heater,the horizontal tube inserting the inclined wire mesh tube was proposed in aims of reducing the thickness of the condensate film and accelerating the condensate draining,and the liquid film flow and local heat and mass transfer in the complex structure channel on the surface of a single horizontal circular tube were compared with those on the inclined wire mesh tube inserted outside the horizontal circular tube.Two-dimensional and three-dimensional models of flow and condensation on the wire mesh tube were established,using SST k-ω turbulence model,VOF two-phase flow model and the user-defined mass transfer process.The CFD simulation results show good agreement with the correction correlation of Nusselt,and the deviations of the twodimensional model and the three-dimensional model with Nusselt correction are less than 11.6%and 5.1%,respectively.The accuracy and precision of numerical methods and models are verified by showing the distribution of the liquid film thickness along the circumferential angle of the horizontal circular tube and by comparing the similar trends of the distributions of volume fraction and temperature contours.Two-dimensional unsteady state simulation results show that the temperature and thickness of the liquid film on the wire mesh tube fluctuate,and the volume fraction of vapor decreases gradually.The three-dimensional steady-state simulation results show that the liquid film thickness and liquid volume fraction on the heat transfer tube reduce when inserting wire mesh tube.The results show that the wire mesh tube structure leads to forming a uniform liquid film on the surface of the wire mesh because of capillary force,and the condensate flows quickly from the horizontal tube to the surface of the wire mesh tube under the effects of surface tension and gravity.Numerical simulation results show that the local heat transfer coefficient of the tube surface with the metal wire mesh tube is increased by an average of 12% compared with the single horizontal tube.The way of inserting metal wire mesh tube outside the horizontal tube can improve the heat transfer coefficient outside the tube,and provide a theoretical reference for the condensation enhancement heat transfer outside the horizontal tube bundle and the condensation section. |
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