【作者】 杨中华；

【导师】 槐文信；

【作者基本信息】 武汉大学 ， 港口、海岸及近海工程， 2004， 博士

【摘要】 本文研究了静止环境和流动环境中的负浮力射流的特性。负浮力射流在环境领域有着广泛地应用,如工业和生活污水排海排江,电厂冷却水排入江河,从烟囱、冷却塔等建筑物排出的粉尘、废热气体流入大气、船舶废水排入港口水域。当废水或废气排放到环境流体中时,废流体的密度往往与环境流体的密度不同,这种由于密度差而产生的浮力不仅影响平均流本身,而且更主要的影响湍流及受湍流影响的动量、热量和质量的交换。因此,对浮力作用下的射流研究不仅具有重要的学术意义,而且对工程实践也具有重要的实际意义,能够为正确分析、准确预报污染物的扩散、输移规律及其影响途径,制定合理、有效的控制和保护措施,减少扩散器各种繁复设计参数(如形状、尺寸等)的率定等问题提供理论基础。本文的主要工作就是在前人试验研究的基础上,结合试验和数值模拟,对多种形式的负浮力射流的水流流动特性进行分析和研究,探讨污染物的输移扩散规律,给出射流影响区域具体划分方法和计算公式,对设计排海工程扩散器,评价扩散器的稀释效果及污染范围提供理论基础。文章首先对负浮力射流的试验研究成果进行了总结和分析,介绍了有关的主要控制参数和研究热点,然后对一种目前研究较少的热水负浮力射流进行了试验研究,最后结合试验研究的成果,对多种形式的负浮力射流进行了数值模拟和预报。数值研究分静止和流动两部分进行,分别研究了圆形和平面、垂直和倾斜、盐水负浮力和热水负浮力等不同的射流形式。负浮力射流是一种变密度流,目前的研究多停留在试验阶段。对几种较为显著的特征量进行测量分析,主要包括射流最大上升高度、分离点的位置、底部扩展层厚度、各特征位置的稀释度等等,对于流场内部结构的研究则很少。理论分析认为,对于静止环境中的盐水负浮力射流,射流的最大上升高度等特征量的主要影响因素主要是动量通量、浮力通量和流量通量,通过量纲分析进一步得到了特征量与出口密度弗劳德数的线性关系,由于试验工况和测量方法的差别,不同的试验研究得出的不同的经验系数。对现有的试验资料进行整理,并进行曲线拟合,得到适用于不同出口密度弗劳德数的经验公式。静止环境中的倾斜射流可以扩大喷射的影响区域,增加稀释度,因而在实际上具有更多的应用。射流角度不同,特征量也不一样。对多种倾角下的射流进行了试验研究表明,当倾角为60 o时,圆形负浮力射流的轨迹最长,其稀释度最大,对60 o倾角的试验研究得到了相应的最大上升高度、分离点的位置和相应的稀释度计算式,它们均与出口密度弗劳德数有关。流动环境中的负浮力射流特性不仅和出口密度弗劳德数有关,还要受到射流比和排放角度的影响。通过量纲分析可以得到各特征量的计算公式,它是射流比和出口密度弗劳德数的函数。在流动环境中,垂直射流的稀释效果比倾斜射流(向下游倾斜)的稀释效果要好。盐水负浮力射流的试验研究较多,而对于另一种负浮力射流,即由温度变化引起的负浮力射流则研究的较少。本文对流动环境中的平面热水负浮力射流进行了试验研究。采用Micro ADV测量系统和温度测量仪器对流场和温度场进行了测量,得到了热水负浮力射流的时均和紊动特性。不同的试验工况采用改变环境流速和排放角度来实现,为了对比分析浮力作用对射流的影响,我们也作了纯射流的试验。对比表明,不同的排放角度下,负浮力射流和纯射流的影响区域大小关系不同,说明热水负浮力射流中,温度变化对射流的影响不仅表现在负浮力作用一个方面,还表现在紊动强度的增强,而两者的强弱对比决定了温度负浮力射流的流动规律。对射流的紊动分析表明,横流来流及出流断面的紊动较弱,但出流断面较之来流断面的紊动有所增强;涡心及分离点断面的紊动较强。通过本文试验可以看到,Micro ADV能够精确的采集流场的速度样本,并且它的采集频率很高,从而为分析流场的紊动特性提供了很大的方便,但是在整理试验成果中同时发现,所采集的流场紊动数据沿探头发射方向偏小,这一问题在以后的流场测量中应引起注意。在数值模拟方面,建立考虑浮力作用的k -ε双方程湍流模型,采用混合有限分析方法对二维和三维负浮力射流进行数值模拟。为了避免相同结点上求解所有的未知量时出现的不合理的锯齿状压力分布,而应用交错网格法,边壁条件的处理采用壁函数方法。对静止环境的圆形和平面负浮力射流进行了数值模拟和预报,采用试验资料对数学模型和方法进行了验证。分析了最大入侵高度、断面和轴线上的流速、密度(或温度)和湍动能变化以及典型断面上的湍动能和动量平衡。对于倾斜负浮力射流,根据流动的特性可以将射流影响区域划分成为三个区,即:射流区、回流区和水平扩展区,射流区和水平扩展区及射流区内部分区的界限通过计算公式给出,计算还给出了收缩断面的位置及该断面物理量的分布。通过对不同排放角度下平面负浮力射流的计算,分析了排放角度对负浮力射流的影响,给出了考虑排放角度影响的倾斜射流最大上升高度计算公式。通过对负浮力射流的远区计算表明,高密度流体以异重流的形式沿底部向下游推进,分析了异重流的头部形状和远区断面流速分布规律。流动环境中的负浮力射流相对于静止环境中的负浮力射流增加了环境来流的作用,因此在分析流动规律的时候必须考虑流速比的影响。对流动环境中的负浮力射流,分别模拟了平面垂直负浮力射流、平面倾斜负浮力射流、平面热水负浮力射流和圆形垂直负浮力射流。对于平面负浮力射流,分别分析了最大入侵高度、断面上的流速、密度(或温度)和湍动能变化以及典型断面上的湍动能和动量平衡。类似静止环境中倾斜负浮力射流的方法,同样可以将射流影响区域分区,给出了分区的界限和相应的计算公式。结合本文的试验,对平面热水负浮力射流同样进行了数值模拟和预报,计算得到了不同排放角度和射流比情况下的涡心、分离点位置和最大入侵高度。对比分析表明,对于流动环境中的热水负浮力射流,倾斜射流比垂直射流影响区域要小的多,射流角度越小,各特征量的值越小。射流比越大,各特征量的值也越大。对湍动能及其耗散率分布的分析可以得到,在喷口下游有两个相对较高的区域,一个在喷口附近,一个在回流区内,回流区内湍动能最大值存在滞后的现象。湍动能耗散率在主流中心线上较大,向两侧逐渐减小。在靠近喷口的下游水面附近,有一个相对较小的区域。在水面上,喷口下游的湍动能及其耗散率均衰减很快,然后逐渐增大,在分离点位置附近达到一个最大值,然后逐渐减小。在喷口下游,温度的最大值并不在分离点处,而是在距离喷口相对较近的一个位置。流动环境的圆形负浮力射流属于三维问题,采用三维紊流模型对其进行了数值模拟,分析了这类射流的流动状态和密度扩散规律,给出了不同射流比下的流线图和断面密度等值线图。发现了喷口附近存在一个相对的低密度区以及射流撞击底部后向侧壁分流、密度分布呈现中间低两侧高的现象。更多还原

【Abstract】 The characteristics of negatively buoyant jets in static and flowing environment have been investigated in this paper. Negatively buoyant jets are studied in connection to many environmental and geophysical application in which the buoyancy force is a direction opposite to the flow. Examples are industrial discharge such as brine, living sewage disposal, cooling water of power plant which is discharged to the river, dust or waste hot gas which is let to the atmosphere from chimney and cooling tower, and bilge water which is released to the port water area. The released water and gas are often different from the environmental liquid in density. The buoyancy which is caused by the difference of density has effect not only on the flow itself, but also on the exchange of mass, momentum, and energy. The research on negatively buoyant jets has academic and practical meaning. It can predict the diffuse and transportation of pollutant, give relevant effective protect plan, and reduce the conformation of design parameter of outfall (figure and dimension etc.).Based on the former test research, physical examination and numerical simulation are made to study the flow behavior of negatively buoyant jets, to find the rule of pollutant transportation and diffusion, and to give the demarcation of jet region. In this paper, the test work of negatively buoyant jets is summarized to analyze the main control parameter and research point of which. Then a kind of negatively buoyant jets that is named by hot water negatively buoyant jets in flowing environment is studied by physical examination. Numerical simulation is made to study each kind of negatively buoyant jets. The jets form varied in spout shape, jet angel, environment and buoyancy.Density is changeable in negatively buoyant jets, so it causes many difficult in study. The main study on negatively buoyant jets is experiment study by measuring some significant character parameters such as the maximum vertical penetration, position of separate point, thickness of bottom horizontal spreading layer, and dilution of these points. Few studies on inner conformation of jet flow are made by now. Theoretical analysis indicates that the maximum penetration of brine negatively buoyant jets in static environment is mainly dominated by the mass flux, the momentum flux and the buoyancy flux. Farther dimension analysis shows that the relation between the character parameters and jet Froude number is linear. Different coefficients are got by different examination. By summarizing these experiment data, a formula that can be used widely is found then. Inclined jets can extend the jets influencing region and increase the dilution. It is found by examination that a 60oangel gave the longest trajectory and therefore the highest dilution. Since then, examination on 60o angel jets is made to get the formula of maximum penetration, position of separate point and their dilution. For negatively buoyant jets in flowing environment, the character parameters varied with the jet Froude number and jet ratio. Better dilution effect is made in flowing environment than in static environment.More attention is paid on brine negatively buoyant jets, and the experiments on hot water negatively buoyant jets are very little. In this paper, hot water negatively buoyant plane jets in flowing environment are studied by experiment. Micro ADV system is used to measure the velocity and turbulent parameter. The whole field temperature is measured also. The parameters of experiment varied with the change of flowing velocity and jet angle. Pure jets experiments are made also to find the influence of buoyancy in negatively buoyant jets. It is found that the contrast of influenced area size between hot jets and pure jets is different in different jet angles. This means that the change of temperature has effect not only on negatively buoyancy, but also on the intensity of turbulence. The contrast of these two influences dominates the trend of jet flow. The turbulent kinetic energy of upriver current is week and the turbulent kinetic energy on vortex center and separate point is much stranger. In this study, Micro ADV proved to be a good tool in velocity measuring. The collection frequency of which is very high so it has the advantage to study the turbulence of the flow. It is also found that the turbulence parameter is smaller on sending direction of the probe. This should be noticed in future experiment.The k-εturbulence model and the Hybrid Finite Analytic Method (HFAM) is used to simulate the 2D and 3D negatively buoyant jets. The staggered grid technique is adopted to obtain the correct pressure and velocity at each grid point. At solid surfaces, the wall function is used to relate the values at the first grid points outside the viscous sub layers to the boundary conditions.Round and plane negatively buoyant jets in static environment are studied by numerical simulation. The model and method are modified by experiment data. The maximum penetration, velocity and density (or temperature) and turbulent kinetic energy on the section, balance of kinetic and momentum are analyzed. For incline plane negatively buoyant jet in static environment, the region of this flow can be divided into three parts, they are jets region, recirculation region and horizontal spreading region. The demarcation between jets and horizontal spreading region and the inner demarcation of jets region are given also. The position of the contracted section and the variations of velocity, density and turbulent kinetic energy along the contracted section are given too. A new equation is given that predict the maximum penetration of planar dense jets with different injection angles and initial densimetric Froude number by simulating plane negatively buoyant jets with different jets angle. Calculation of far-field behavior indicated that the denseness flow is the main form of far-field flow. The shape of denseness flow head and velocity distribution on section are analyzed.For negatively buoyant jet in flowing environment, several jet forms are concerned such as plane and round, hot water and brine, vertical and incline. The maximum penetration, velocity and density (or temperature) and turbulent kinetic energy on the section, balance of turbulent kinetic energy and momentum are analyzed. Like jets in static environment, the region can be divided into different part also, limit of each part are analyzed.Based on the experiment of this paper, numerical simulation is made to study the behavior of hot water plane negatively buoyant jets in flowing environment. The position of vortex center and separate point and the maximum penetration in different angles are given. It is found that the influence field of incline jets is much smaller than which of vertical jets. Small jet angles get small character parameters. Large velocity ratios get large character parameter. There are two relatively high areas of turbulent kinetic energy and dissipation for the downstream region. One area is near the spout and the other is in the vortex center. The position of maximum turbulent kinetic energy lags behind the vortex center in recirculation region. The dissipation is high at the mainstream line and decreases from the mainstream line to both sides along the cross section. At the water surface, the turbulent kinetic energy and dissipation attenuate fast and then increase and reach to a maximum at the separate point. The maximum of temperature is located near the spout and is not at the separate point.The round negatively buoyant jets in flowing environment belong to 3D problem. 3D numerical simulation is made to study the behavior and the flow state and diffusion of density are analyzed. Streamlines and concentration contours in different section are given. It is also found that there is a low concentration region behind the inlet and the jets branch off after impinging the boundary and the concentration near the side is higher than which in the middle of the channel.更多还原