Study On The Behavior Of Negatively Buoyant Jets

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.