蒸汽在高分压不凝气体中扩散流动的传质研究

发布时间：2018-01-05 09:04

本文关键词：蒸汽在高分压不凝气体中扩散流动的传质研究　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：本研究基于973项目"钢铁生产过程高效节能基础研究"子课题"生产过程的能量转化与优化配置方法"。主要针对中低温余热回收设备中的热管换热器进行蒸汽凝结换热的研究。文献研究表明,热管中混入少量不凝气体就会对装置换热效果造成巨大抑制。在使用前需要进行抽真空操作,而抽真空耗能耗时,特别是大型热管换热装置,且抽真空后管内仍会残留少量不凝气体影响换热。本文设计开发一种不抽真空的重力回路热虹吸管,有效解决不凝气体对换热设备带来凝结换热的抑制。相比传统回路热管,不抽真空的重力回路热虹吸管无需管芯材料,工作液依靠重力在系统内自循环,仅在冷凝管末端增加气液分离器,将不凝气体引入气液分离器,可大幅降低不凝气体在冷凝管中对蒸汽凝结传热的抑制。同时,气液分离器限制了不凝气体在回路系统内循环流动,避免不凝气体对蒸发器和冷凝器换热造成循环影响。研究发现,不抽真空重力回路热虹吸管内存在大量不凝气体时,蒸汽在高分压不凝气体中形成混合传热传质区域,我们将此区域定义为扩散栓塞区。在扩散栓塞区不凝气体阻碍蒸汽流动,蒸汽通过流动+扩散的方式到达液膜表面凝结换热,同时不凝气体逆蒸汽流动方向发生逆向扩散并达到局部区域的动态平衡。实验研究了重力回路热虹吸管内不凝气体对系统启动过程的影响发现,不凝气体含量、系统充液率、蒸发器热负荷以及气液分离器安装位置均会对系统启动时间、运行压力及蒸发器和冷凝器温度造成影响。研究表明:1)重力回路热虹吸管启动过程蒸汽与不凝气体在蒸汽管中形成扩散栓塞区,蒸汽推动扩散栓塞区,并压缩不凝气体进入冷凝管及末端的气液分离器,当扩散栓塞区进入冷凝管后,系统压力迅速降低,形成启动过程"压力峰"现象;2)系统充液率越高,启动过程压力越大,启动时间越长;蒸发器热负荷越大启动时间越短;不凝气体含量越高,启动时间越长;3)气液分离器内有效初始气相容积越大,回路热虹吸管启动越容易。不抽真空重力回路热虹吸管进入动态平衡工作阶段后,大量不凝气体聚集在气液分离器内并逆蒸汽流动方向,向冷凝管逆向扩散,与蒸汽形成扩散栓塞区,影响冷凝管内的凝结换热。实验表明:1)不凝气体提升系统运行压力及蒸发器的蒸发温度,有利于提高局部凝结换热量,但会影响整个换热系统的换热能力;2)扩散栓塞区放大了由于蒸汽蒸发凝结自平衡引起的系统压力震荡,使冷凝管管壁温度振幅增大,而扩散栓塞区的震荡可以提升局部的凝结换热效率;3)扩散栓塞区的存在大幅改变了冷凝管内蒸汽的凝结分布,不凝气体含量越大,蒸汽凝结越集中在冷凝管前段,70%充液率蒸发器热负荷为3.0kW时,不抽真空工况在冷凝管前段的平均热流密度是抽真空工况的1.67倍,有效冷凝管长度比抽真空工况短;4)系统内添加含量为0.5-1wt%乙醇后,会促进蒸汽的局部凝结换热,降低不凝气体对凝结换热的影响,提升换热器的换热效率。为了检测扩散栓塞区内不凝气体的分布,我们设计了一套非接触式红外检测平台,利用水蒸气和不凝气体(空气)对红外光谱中特定光谱吸收率的不同,测量水蒸气在石英玻璃冷凝管中的分布,得到了冷凝管内扩散栓塞区的分布位置及扩散栓塞区内组分浓度分布。最后基于组分输运方程建立了扩散栓塞区内蒸汽与不凝气体的流动扩散模型,使用Maxwell-Stefan组分扩散代替组分输运方程中的Fick定律扩散项,并数值计算得到模型在某些工况下的近似解。将扩散栓塞区模型计算得到的数值解与实验数据相比较,发现模型能准确描述出冷凝管内蒸汽及NCG的扩散流动规律。
[Abstract]:This study is based on the 973 project "steel production of" energy efficient process based sub project "of the production process of energy conversion and optimal allocation method. Mainly for low temperature waste heat recovery equipment in heat pipe steam condensation heat transfer heat exchanger. Literature research shows that heat pipe mixed with a small amount of non condensable gas will change the effect of heat on the device caused great inhibition. Before use to operate the vacuum, vacuum pumping and energy consumption and time-consuming especially for large heat pipe heat exchanger, and the vacuum tube will remain a small amount of non condensable gases affect heat transfer. This paper designs a vacuum gravity loop thermosyphon Straw and effectively solve the non condensable gas on heat exchangers bring condensation heat transfer is inhibited by. Compared with the traditional gravity loop heat pipe, loop thermosyphon Straw without vacuum without tube core material, working fluid by gravity self circulation within the system, only in the cold The condensate tube end to increase gas-liquid separator, the non condensable gas into a gas-liquid separator, which can greatly reduce the non condensable gas in the pipe to inhibit condensation of steam condensation heat transfer. At the same time, the gas-liquid separator limits the gas circulation in the loop system, avoid non condensable gas on the evaporator and condenser heat transfer caused by circulation influence. Found that without vacuum gravity loop thermosyphon Straw in the existence of a large number of non condensable gas, steam to form mixed heat and mass transfer area in the high pressure gas in this area, we will define the diffusion zone. The steam flow in the embolism hinder the diffusion of non condensable gas embolism zone, steam flow through the way to the film surface diffusion + condensation heat transfer, dynamic balance and non condensable gas steam flow direction inverse reverse diffusion and reach the local area. Experimental study of the gravity loop thermosyphon Straw in non condensable gas on system startup That process, gas content, system filling rate, the evaporator heat load and a gas-liquid separator installation position will affect the system startup time, operating pressure and temperature of the evaporator and the condenser. The results show that: 1) gravity loop thermosyphon Straw starting process steam and non condensable gas in the steam pipe in the form of diffusion embolism steam diffusion zone, the embolization area, a gas-liquid separator and compressed non condensable gases into condensing tube and at the end, when the spread of embolism area into the condenser tube, quickly reduce the system pressure, the formation of starting process of "pressure peak" phenomenon; 2) system liquid filling rate is high, start-up pressure is, the longer the start the greater the heat load of evaporator; start time is short; the non condensable gas content is high, the start time is longer; 3) in the gas-liquid separator effective initial gas phase volume bigger, loop thermosyphon Straw start more easily without vacuum gravity. The loop thermosyphon Straw into dynamic equilibrium stage, a large number of non condensable gas accumulation in the gas-liquid separator and inverse steam flow direction, the reverse diffusion to the condenser tube, forming the diffusion area and the effect of embolization steam condensing tube condensation heat transfer. The experimental results show that: 1) the non condensable gas extraction temperature and evaporator operating pressure evaporation system, is conducive to the improvement of local condensing heat transfer, but will influence the heat transfer capacity of heat exchanger system; 2) diffusion area enlarged embolism due to steam condensate system since the pressure balance caused by the shock, the condensing tube wall temperature amplitude increases and the diffusion area of the shock can enhance the embolization of the condensation heat transfer efficiency of the local 3); diffusion embolism zone exists significantly changed the condensing tube steam condensate distribution, gas content is bigger, more concentrated in front of steam condensation condensation tube, filling rate 70% evaporator heat load of 3.0kW, The average heat flux without vacuum conditions in front of the condenser pipe is 1.67 times of the vacuum conditions, effective condensation tube length than the vacuum condition is short; 4) within the system to add content of 0.5-1wt% ethanol, will promote the local heat transfer of steam condensation, reduce the impact of non condensible gas, improve thermal efficiency heat exchanger. In order to detect distribution of diffusion gas embolism in the District, we designed a non-contact infrared detection platform, the use of water vapor and non condensable gas (air) to specific infrared spectra, absorption rate, water vapor distribution measurement in quartz glass condensing tube, get the distribution and diffusion zone in condensation pipe embolism embolism diffusion area within the component concentration distribution. Finally based on component transport equations of the diffusion zone embolism steam and incondensable gas flow diffusion model, using the Maxwell-Stefan component expansion Instead of scattered component lose Fick transport equation in law of diffusion, and the numerical calculation of the approximate solution is obtained under some conditions. The model solutions are compared with experimental data of numerical diffusion model to calculate the embolization area, found that the model can accurately describe the diffusion of steam and NCG tube condensation flow patterns.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK124

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