高参数水喷雾闪蒸的理论分析与实验研究

发布时间：2019-01-24 18:32

【摘要】：在能源短缺与环境污染问题日益突出的今天,如何提高能源利用率、减轻生产生活领域的污染成为关系到我国经济发展与人民生活水平的关键课题。在水泥、钢铁等高耗能行业中,工业余热的高效利用方法具有重要的研究价值。将闪蒸引入高耗能行业余热发电系统是进一步提高工业余热利用效率、减轻生产热污染的有效手段。余热利用程度与闪蒸方式、参数水平、闪蒸效率等因素密切相关。因此,对闪蒸机理与特性进行分析与研究具有重要意义。本文围绕在余热回收领域有重要应用价值的高参数水喷雾闪蒸过程展开全面研究,进行了有深度的理论分析与数值模拟,开展了广泛参数的实验研究,掌握了高参数水喷雾闪蒸过程的基本规律与特性。在此基础上,从雾化喷嘴的角度提出了较好的优化方案。首先,本文运用欧拉-拉格朗日法思想,建立喷雾闪蒸数学模型。在欧拉坐标系下求解蒸汽的流动与传热特性,在拉格朗日坐标系下求解喷雾液滴的运动与闪蒸。其中,液滴闪蒸过程因其机理特殊性而无法使用常规蒸发模型预测。本文从闪蒸蒸发机理特殊性出发,建立新的过热液滴闪蒸数学模型,描述闪蒸过程中液滴质量、直径与温度的变化规律。采用数值方法求解喷雾闪蒸数学模型,编写用户自定义函数,对CFD软件进行二次开发,实现了喷雾闪蒸过程全尺寸三维数值模拟。通过计算得出闪蒸罐内部喷雾场与流场的发展过程及稳态特性。结果表明,喷雾在发展初期具有较高过热度,蒸发剧烈,引发喷嘴出口附近蒸汽高速流动,液滴温度和直径随蒸发而迅速减小。随着喷雾的继续发展,蒸发剧烈程度明显减缓,同时液滴初始动量影响力减弱,喷雾径向扩展趋势减小。喷雾场与流场共同发展至稳定状态后,中心轴线附近液滴速度较高,喷雾外缘液滴速度较低。蒸汽流场出现对称的大尺度旋涡。对喷雾闪蒸特性进行参数敏感性分析,探究喷雾锥角、液滴粒径大小、喷射方向、初始速度和闪蒸压力对闪蒸特性的影响。结果表明,喷雾锥角的增大可以提高喷雾的覆盖程度,并使闪蒸进行的充分程度稍有提高。液滴粒径的减小虽使喷雾宽度降低,但显著提高闪蒸速度,喷嘴出口附近蒸汽流速也显著增加。流体向上喷射时无法充分利用闪蒸罐内部空间,且存在蒸发不完全和蒸汽带水的可能性,在实际应用中并不推荐。喷雾初始速度的减小能够延长液滴在闪蒸罐内停留时间,使闪蒸进行的更加充分。闪蒸压力的提高意味着流体过热度的减小,闪蒸强度明显降低,喷雾宽度也因曳力的增大而降低。在均匀喷雾闪蒸过程数值研究基础上,建立了非均匀喷雾闪蒸模型,对液滴尺寸分布符合Rosin-Rammler函数的喷雾闪蒸过程进行求解。发现粒径较小的液滴群集中分布于轴线附近,粒径较大的液滴群径向覆盖范围较广。喷雾温度在轴向上呈现顶部高、底部低的特点,在径向上呈现中心低、外缘高的特点。喷雾液滴速度变化幅度大于均匀喷雾,粒径较小的液滴群速度变化较大。喷嘴出口附近蒸汽流速与湍流强度显著提高。蒸汽流场出现了尺度不同、方向相反的双旋涡。基于以工业余热为热源的大型喷雾闪蒸实验台,开展喷雾闪蒸实验研究。鉴于目前闪蒸实验参数普遍较低的现状,在实验中提高参数水平,将初始水温提高至100℃以上,将过热度范围扩大至30～46℃,将闪蒸罐运行压力保持为正压,拓展了闪蒸的实验研究范围。实验结果表明,闪蒸蒸发量随初始温度的提高而增大,随闪蒸压力的提高而减小。流体喷射方向向下时蒸发量更高,蒸汽带水更少。喷嘴尺寸对闪蒸的影响体现在雾化效果与液体停留时间两个相反的方面,需根据闪蒸空间大小与喷嘴特性选择合适的喷嘴。过热度是闪蒸的驱动力,经拟合得出过热度与闪蒸效率的经验公式。提出以无量纲的Jakob数作为闪蒸过程的特征数,分析不同实验条件下Jakob数与闪蒸效率的关系,对实验数据进行无量纲化多元回归分析,提出Jakob数、无量纲压力与闪蒸效率的经验公式。在对喷雾闪蒸进行全面分析的基础上,考虑到喷嘴内部发生相变的可能性及其对喷雾特性和喷雾闪蒸的影响,进一步将喷嘴内部纳入研究范围。将多相流模型与降压相变模型相结合,对实验用含双S形叶片的喷嘴内部流动与闪蒸进行数值模拟。在此基础上提出一种改进的喷嘴结构,增设一组S形叶片,并将叶片两端与喷嘴内壁的几何关系由相切改为垂直。分析表明,改进的结构在促进闪蒸、减轻积垢与增大通流能力方面具备优势。
[Abstract]:With the increasingly prominent energy shortage and environmental pollution, how to improve the utilization of energy and to reduce the pollution in the field of production life has become a key subject of the economic development and people's living standard in our country. In the high energy consumption industry, such as cement, steel, etc., the efficient utilization method of industrial waste heat has important research value. The high-energy-consumption industry waste heat power generation system is introduced into the high-energy-consumption industry, so that the utilization efficiency of the industrial waste heat can be further improved, and the effective means for reducing the production heat pollution are reduced. The degree of waste heat utilization is closely related to the factors such as flash evaporation method, parameter level and flash efficiency. Therefore, it is of great significance to analyze and study the flash mechanism and characteristics. In this paper, a comprehensive study on the high-parameter water spray flash process with important application value in the field of waste heat recovery is carried out. The theoretical analysis and numerical simulation are carried out, and the experimental research on the wide parameters is carried out. The basic law and characteristics of the high-parameter water spray flash evaporation process are mastered. On this basis, a better optimization scheme is put forward from the angle of the atomizing nozzle. First, this paper uses the Euler-Lagrange method to set up a mathematical model of spray flash evaporation. The flow and heat transfer characteristics of the steam are solved in the Euler coordinate system, and the motion and the flash of the spray droplets are solved in the Lagrange coordinate system. in which the liquid drop flash process cannot be predicted using a conventional evaporation model due to its particularity. In this paper, a new mathematical model of the flash evaporation is set up from the particularity of the evaporation mechanism of the flash evaporation, and the change of the quality, diameter and temperature of the liquid drops in the flash evaporation process is described. The numerical method is used to solve the mathematical model of spray flash evaporation, and the user-defined function is written, and the second development of the CFD software is carried out to realize the full-size three-dimensional numerical simulation of the spray flash process. The development process and the steady-state characteristic of the spray field and the flow field inside the flash tank are obtained by calculation. The results show that the spray has a high degree of superheat in the early stage of development, the evaporation is violent, the high-speed flow of the steam near the outlet of the nozzle is induced, and the temperature and the diameter of the liquid drop rapidly decrease with the evaporation. With the continued development of the spray, the degree of evaporation is obviously reduced, and the initial momentum influence of the droplets is reduced, and the spray radial expansion tendency is reduced. When the spray field and the flow field are developed to a stable state, the velocity of the liquid drops near the central axis is higher, and the droplet velocity of the outer edge of the spray is lower. The steam flow field has a symmetric large-scale vortex. The effects of spray cone angle, droplet size, spray direction, initial velocity and flash pressure on the flash characteristics were investigated. The results show that the increase of the spray cone angle can improve the coverage of the spray, and the full extent of the flash evaporation can be improved slightly. Although the size of the droplets is reduced, the spray width is reduced, but the flash speed is significantly increased and the steam flow rate in the vicinity of the nozzle outlet is also significantly increased. when the fluid is injected upward, the inner space of the flash tank cannot be fully utilized, and the possibility of incomplete evaporation and water of the steam is present, and is not recommended in practical application. the reduction in the initial velocity of the spray can extend the residence time of the droplets in the flash tank to make the flash more sufficient. the increase of the flash pressure means a reduction in the degree of superheat of the fluid, a significant reduction in the flash strength, and a reduction in the spray width as a result of the increase in the drag force. On the basis of the numerical study of the uniform spray flash process, a non-uniform spray flash model was established to solve the spray flash process of the Roin-Rammler function. It is found that the droplet group with small particle size is distributed in the vicinity of the axis, and the larger diameter of the droplet group has a wide radial coverage. the spray temperature has the characteristics of high top and low bottom in the axial direction, and has the characteristics of low center and high outer edge in the radial direction. and the change of the velocity of the spray liquid drop is larger than that of the uniform spray, and the change of the droplet group speed with small particle diameter is relatively large. the velocity of the steam near the outlet of the nozzle and the turbulence intensity are obviously improved. The steam flow field has a two-vortex with different dimensions and opposite directions. The experiment of spray flash evaporation is carried out on the basis of a large-scale spray flash experiment table based on industrial waste heat as a heat source. In view of the fact that the current flashing experiment parameters are generally low, the parameter level is raised in the experiment, the initial water temperature is raised to above 100 DEG C, the degree of superheat is increased to 30 to 46 DEG C, the operating pressure of the flash tank is kept to a positive pressure, and the experimental research range of the flash evaporation is expanded. The experimental results show that the flash evaporation increases with the increase of the initial temperature and decreases with the increase of the flash pressure. the evaporation capacity is higher when the direction of the fluid injection is downward, and the steam is less in water. The effect of the size of the nozzle on the flash is reflected in the two opposite aspects of the atomization effect and the liquid residence time, and a suitable nozzle is selected according to the flash space size and the nozzle characteristics. The superheat degree is the driving force of the flash evaporation, and the empirical formula of the degree of superheat and the flash efficiency is obtained by fitting. The relationship between the number of Jakob and the flash efficiency under different experimental conditions is analyzed, and the empirical formula of the number of Jakob, the dimensionless pressure and the flash efficiency is presented. On the basis of a comprehensive analysis of the spray flash, the possibility of the phase change inside the nozzle and its influence on the spray characteristics and the spray flash evaporation are taken into account, and the inside of the nozzle is further included in the scope of the study. The multi-phase flow model is combined with the step-down phase-change model, and the internal flow and the flash of the nozzle with the double S-shaped blade are numerically simulated. On this basis, an improved nozzle structure is proposed, a set of S-shaped blades is added, and the geometric relation between the two ends of the blade and the inner wall of the nozzle is changed to be vertical. The analysis shows that the improved structure has the advantages in promoting the flash evaporation, reducing the scale and increasing the flow capacity.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK124;TK115

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