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基于最速降线型液体分布器的竖管降膜蒸发特性研究

作者:完美论文网  来源:www.wmlunwen.com  发布时间:2019/10/8 9:25:20  

摘要:机械蒸汽再压缩(MVR)蒸发系统因具有占地小、能耗低、传热效果好等特点,广泛应用于石油化工、生物制药、废水处理等领域。本文以MVR蒸发系统中的降膜蒸发器为研究对象,针对其核心部件之一的液体分布器展开研究,设计了一种最速降线型液体分布器并对竖管降膜进行了实验研究和数值模拟。本文旨在优化液体分布器的布膜效果,提高降膜蒸发器的换热性能,进一步推广MVR蒸发系统的应用。主要工作如下:

(1)竖管降膜冷态实验:设计了一种最速降线型液体分布器,在此基础上设计并搭建了竖管降膜冷态实验系统,完成了四种工质的竖管降膜冷态实验。以最小降液流率为指标,通过与三种传统插件型液体分布器对比,评价了最速降线型液体分布器的性能,分析了进料流量、缓冲槽液位高度、料液浓度以及工质类型对平均液膜厚度的影响。结果表明:相比传统插件型液体分布器,最速降线型液体分布器下的管内流动和管口成膜情况较好,具有较小的最小成膜流量和最小降液流率,其最小降液流率实验值与Hartley理论值的最大误差为9.54%;进料流量越大,缓冲槽液位越高,平均液膜厚度越大;料液浓度越高,平均液膜厚度越大;工质的黏度和表面张力越大,平均液膜厚度越大。

(2)竖管降膜流动数值模拟:基于最速降线型液体分布器,建立了竖管降膜流动的三维模型,对竖管降膜流动过程进行了数值模拟,对计算域进行了网格划分及网格无关性验证,将模拟值与冷态实验值进行了对比。分析了气相流速、液相流速以及工质类型对平均液膜厚度的影响。结果表明:模拟与实验所得的平均液膜厚度变化趋势一致,差别在9.2%~13.7%;对于纯水,当最大液相进口流速0.8m/s时,气相流速超过1m/s会使平均液膜厚度急剧下降,因此气相流速不能超过1m/s;沿管长方向,平均液膜厚度随气相流速的增大而减小,且减小幅度随气相流速的增大而增大;液相流速越大,液膜分布越均匀,液膜厚度越大,流动液膜的湍流度和波动特性越强;工质黏度越大,表面张力越大,平均液膜厚度越大。

(3)竖管降膜热态实验:基于竖管降膜冷态实验结果,设计并搭建了竖管降膜热态实验系统,完成了氯化钠溶液的竖管降膜热态实验研究。分析了进料流量、进料温度、传热温差、热流密度、真空度和料液浓度对平均对流传热系数的影响。结果表明:层流和湍流状态下,平均对流传热系数随进料流量的增大呈现先增大后减小的趋势;平均对流传热系数随着进料温度和真空度的增大而增大,随着氯化钠溶液浓度的增大而减小;传热温差小于8℃时,平均对流传热系数随着传热温差的增大而增大,传热温差大于8℃时则相反;层流状态下,平均对流传热系数随着热流密度的增大而减小,湍流状态下,平均对流传热系数随着热流密度的增大呈现先增大后减小的趋势。

Mechanical vapor recompression (MVR)evaporation system is widely used in petrochemical, bio-pharmaceutical,wastewater treatment and other fields due to its small footprint, low energyconsumption and good heat transfer effect. In this paper, the falling filmevaporator in MVR evaporation system was taken as the research object, and theliquid distributor of one of its core components was researched. Thebrachistochrone liquid distributor was designed and the falling-film ofvertical tube was studied by experiment and numerical simulation. This paperaims to optimize the filming effect of the liquid distributor, improve the heattransfer performance of the falling film evaporator, and further promote theapplication of the MVR evaporation system. Main tasks as follows:

(1) The experimental study on the cold filmof the vertical tube falling film: The brachistochrone liquid distributor wasdesigned. Based on this, a vertical tube falling film cold state experimentalsystem was designed and built, and four kinds of working medium vertical tubefalling film cooling were completed. The performance of the brachistochroneliquid distributor was evaluated by comparison with three conventional plug-inliquid distributors with the minimum downflow rate as an indicator. The effectsof feed flow rate, buffer tank liquid level height, feed liquid concentrationand working fluid type on the average liquid film thickness were analyzed. Theresults show that compared with the traditional plug-in type liquiddistributor, the flow in the tube and the nozzle film formation under thebrachistochrone liquid distributor are better, and it has the smaller minimumfilm forming flow rate and minimum falling liquid flow rate,and the maximum error between the experimental value of the minimumflow rate and the theoretical value of Hartley is 9.54%; The larger the feedflow rate, the higher the liquid level of the buffer tank that results thelarger the average liquid film thickness; The higher the liquid concentrationthat results the larger the average liquid film thickness;The larger the viscosity and surface tension of the working fluid,the greater the average thickness of the liquid film.

(2) Numerical simulation of falling filmflow in vertical pipe: Based on the brachistochrone liquid distributor, athree-dimensional model of the falling film flow of the vertical pipe wasestablished. The flow process of the falling pipe of the vertical pipe wasnumerically simulated. The calculation domain was meshed and verified by gridindependence. The values were compared to the cold experimental values. Theeffects of gas phase flow rate, liquid phase flow rate and working fluid typeon the average liquid film thickness were analyzed. The results show that theaverage liquid film thickness of the simulated and experimental results isconsistent with the difference of 9.2%~13.7%. For pure water, when the maximumliquids inlet flow rate is 0.8m/s, the gas flow rate exceeds 1m/s, which willmake the average liquid film. The thickness decreases sharply, so the gas flowrate cannot exceed 1 m/s; Along the length of the tube, the average liquid filmthickness decreases with the increase of the gas phase flow rate, and thedecrease amplitude increases with the increase of the gas phase flow rate; Thelarger the liquid phase flow rate, the more uniform the liquid filmdistribution, and the larger the liquid film thickness. The stronger theturbulence and fluctuation characteristics of the flowing liquid membrane; Thelarger the viscosity of the working fluid, the larger the surface tension andthe larger the average liquid film thickness.

(3) Experimental study on the falling filmthermal state of vertical tube: Based on the experimental results of thevertical tube falling film cold state, the experimental system of the verticaltube falling film thermal state was designed and built.

关键词:MVR;降膜蒸发器;液体分布器;实验研究;数值模拟

MVR; Falling-film evaporator; Liquiddistributor; Experimental research; Numerical simulation

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