纯工质R245fa水平管内流动沸腾换热特性的实验研究

【作者】 黄晓艳；

【导师】 王华；

【作者基本信息】 昆明理工大学 ， 热能工程， 2010， 硕士

【摘要】 随着经济的发展,能源消耗越来越大,然而在大量消耗的能源中,有一部分能量是没有利用而白白浪费掉,如在动力、化工、石油、冶金、核能、制冷等工业中均存在此问题,有一部分能量以烟气、热水的形式浪费掉。如在昆明钢铁集团有限公司冶炼厂同样存在大量的每天24小时向外排放的低于300-400℃的中低温烟气,废蒸汽、废热水等余热资源,节能潜力巨大。采用低沸点有机工质为循环工质的有机朗肯循环(ORC)发电是对低品位热量回收利用的有效方式,并将环保性能、化学稳定性、安全性、热力学性能、成本、传热和流动性能作为工质选择所考虑的基本条件。准确的工质的传热系数是换热器设计及设备安全运行的基础,本文对有机工质R245fa的传热性能进行了实验测定,目的是为低温余热发电有机朗肯循环相关换热设备的设计与优化,提供传热设计的可靠依据。分析了水平管内汽液两相流的基本参数,包括：截面含汽率及截面含液率、质量流量、质量含汽率(即：干度)、质量流速、体积流量、体积含汽率、汽相真实流速、汽相折算速度、滑动比、滑动速度、汽液两相流的平均密度、汽液两相流的平均流速等。分析了水平管内汽液两相流的基本流型,包括：单相液体、细泡状流型、汽塞状流型、汽弹状流型、波状分层流型、环状流型。总结并分析了管内两相强制对流换热系数计算的关联式,具体模型有两种：一是区分流型的模型,该模型把有机工质在管内蒸发换热粗略地分为两个换热区：沫态沸腾换热区和两相受迫对流换热区；二是不区分流型的模型,此模型主要有三种计算方法：1、叠加法,具有代表性的关联式为有Gungor和Wingterton公式、Chen公式、Jung公式；2、渐进线法,具有代表性的关联式有Liu和Winterton公式、Wattelet公式；3、增强法,具有代表性的关联式有Shah公式、Kandlikar公式。并分析了用神经网络模型计算管内两相强制对流换热系数。基于以上理论研究的基础上,本文研究了R245fa在水平管内的流动沸腾传热特性,分析了工质质量流速、蒸发温度、加热水质量流速及干度对流动沸腾换热系数的影响,结果为：●流动沸腾换热系数同时受蒸发温度、工质质量流速、加热水质量流速及管内工质干度的影响；●在沸腾开始阶段,随着干度的增大,流动沸腾换热系数升高,在流动沸腾换热系数达到最大值后,随着干度的继续增大,流动沸腾换热系数反而下降；●在相同蒸发温度及热流密度下,随着工质质量流速增大,管内流动沸腾换热系数迅速升高；●在相同工质质量流速及蒸发温度下,随着加热水质量流速的增大,管内流动沸腾换热系数升高；●在相同工质质量流速及加热水质量流速下,随着蒸发温度的升高,管内流动沸腾换热系数降低；本文将实验结果与用Chen公式、Liu-Winterton公式、Shan公式的计算结果进行了比较,结果表明：用Chen公式、Liu-Winterton公式、Shan公式预测R245fa工质管内流动沸腾换热系数的平均误差分别为31.6%、6.3%、37.4%；用Liu-Winterton公式计算R245fa工质的饱和流动沸腾换热具有良好的精度,可以满足工程实际对精度的要求。更多还原

【Abstract】 Energy consumption increases with the development of economic, however, a large number of energy is wasted by the form of hot gas and water emission. This kind of phenomenon widely exists in some industries, such as chemical, petroleum, metallurgy, nuclear power, refrigeration and so on. Taking the smelter in Kunming Iron & Steel Group Co., Ltd as an example, a large number of flue gas, waste steam, waste hot water which lower 300～400℃are uninterruptedly discharged every day. Organic Rankine cycle power generation using low-boiling point organic working fluids is an effective way for low-grade heat recovery. To choice suitable working fluids for Organic Rankine cycle, the basic requirements including environmental performance, chemical stability, security, thermodynamic performance, cost, heat transfer and flow performance must be satisfied.Accurate calculation of working fluid heat transfer coefficient is the foundation of heat exchanger design and safe operation of equipment. Heat transfer performances of organic fluids R245fa are studied in this paper. The main purpose is to provide a reliable heat transfer design basis for the design and optimization of heat transfer equipments of Organic Rankine cycle power generation using low-boiling point organic working fluids.Some basic parameters of gas-liquid two-phase flow inside a horizontal tube have been analyzed, including cross-section gas rate and liquid rate, mass flow, vapor content, mass flow rate; volumetric flow; volumetric fraction, real gas velocity, gas superficial velocity, sliding ratio; sliding speed, the average density of gas-liquid two-phase flow, the average velocity of gas-liquid two-phase flow and so on.Basic flow patterns of gas-liquid two-phase flow inside a horizontal tube have been analyzed, including single-phase liquid, fine bubbly flow, gas plug-like flow, gas slug flow, wavy stratified flow, annular flow.The correlations of calculation two-phase forced convection heat transfer coefficients were summarized and analyzed. In general, there are two categories:the first, models distinguished by flow patterns, organic refrigerant evaporation heat transfer in the pipe roughly divided into two heat transfer areas in this model that are Pulverescent-boiling heat transfer area and the two-phase forced convection heat transfer area, respectively. The second, models of homogeneous flow patterns,There are three main calculation methods in this model:1,superposition method, a representative of the correlation with Gungor and Wingterton the formula, Chen formula, Jung formula; 2,progressive line method, a representative of the correlation with Liu and Winterton formula, Wattelet formula; 3,enhancement method, a representative of the correlation with Shah formula, Kandlikar formula. And using neural network model calculations tube two-phase forced convection heat transfer coefficient was analyzed.Based on the above theoretical study, convective boiling heat transfer characteristics of R245fa inside horizontal tube are investigated at various mass flow rates of hot water, working fluids and at various evaporation temperatures. The investigation results show that convective boiling heat transfer coefficients goes up with the increase of hot water and working fluid mass flow rate, but it is contrary to evaporation temperature.By comparisons of the experimental local heat transfer coefficients with that predicted by Chen correlation, Liu-Winterton correlation and Shah correlation, it is concluded that deviations of these three correlations are 31.6%、6.3%、37.4% respectively. Liu-Winterton correlation is more precise to predict convective boiling heat transfer coefficients of R245fa than Chen correlation and Shah correlation.更多还原