Stability And Mixing Character For Buoyant Jets In Quiscent Shallow Water

The near field stability and mixing characteristics of buoyant jets produced by thermal diffuse in quiescent shallow water are analyzed. A simple way to dispose of the large quantities of waste heat, resulting from thermal power plants and in the operation of pumped storage hydroelectric plants, is to discharge the heated condenser water through a submerged outfall located at the bottom of estuaries and costal waters, the so-called once-through condenser cooling system. The quite restrictive thermal standards that have been adopted recent years by the governments severely limit the permissible temperature rises and mixing zones around sites of heated water discharge, and have led to the design and construction of large sophisticated structures that enhance the near-field dilution of the effluent. More and more scientific workers are majored in the prediction for the characters of buoyant jets both in stagnant and flowing, confined and unconfined environment. An understanding of the induced excess temperature distribution, in relationship to the hydrodynamic characteristics of the buoyant discharge, and the hydrodynamic reaction between the heat buoyant discharge and the receiving water, is essential for a sound design which utilizes to its fullest the cooling capacity of the ambient water-body to meet the appropriate standards. The information can also provide a basis for an assessment of the aquatic impact of the thermal effluent. So the research on the stability and mixing character of heated buoyant jets has academic and practical meaning. It can predict the diffuse and transportation characteristic of thermal heat, optimize the designing of diffuser, and present theoretic sopport for the environmental evalution.Heated buoyant jets are strongly influenced by the interaction of its inertia forces and the buoyancy. For the sewage jets, however, the role of the momentum flux is minor and its mixing is governed by its buoyancy flux, and the interaction dynamics are largely nonexistent in sewage discharges. So analysis techniques employed to determine the mixing capablity of thermal diffusers (at least in the stagnant range) must be different from the simple buoyant jet theories for sewage diffusers. In such a case, four flow regions of distinct hydrodynamic properties can be discerned: (I) Initial Buoyant Jet Region, inwhich the discharge mixes with the ambient fluid by turbulent entrainment as it rising to the surface; (IT) Surface Impingement Region, within which the buoyant mixed flow is turned into a outward moving supercritical flow; (TTT) Internal Hydraulic Jump Region, transforming the flow into a sub-critical flow region downstream with an abrupt change of upper layer thickness and loss of energy; (IV) Stratified Counter Flow Region, with negligible entrainment across the interface. Anterior three zones are defined as near field and the forth zone is defined as far field. A stable near field is defined as one in which a buoyant surface layer is formed which does not communicate with the initial buoyant jet zone. The near field is defined as unstable whenever the layerd flow structure breaks down in the discharge vicinity, resulting in recirculating zones or mixing over the entire water depth.Based on their achievements of the former researchers, the flow behavior of different types of thermal buoyant jets is studied in this paper, to predict under what combinations of discharge and ambient characteristics the near field will be stable or unstable, and to find stability criterion and mixing characters for different discharging types. Using numerical method, the plane vertical thermal buoyant jet, vertical round buoyant jet and horizontal buoyant jets are numerically analyzed.The buoyancy extended k-e turbulence model and the Hybrid Finite Analytic Method (HFAM) are used to simulate the 2D and 3D thermal 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.The characters for the three dimensional thermal buoyant jets in the flowing environment are analyzed experimentally. Using the Micro ADV and temperature measuring system, the velocity field and temperature field of the heated buoyant jets at different cross-to-jet velocity ratios are measured. The test data processed by the professional soft WinADV presented by Sontek Company revealed the characteristics of those mean flow parameters, such as the temperature contours in the symmetry plane, the velocity vector in the symmetry plane, and the trajectory based on the maximal velocityand maximal temperature. The turbulent flow parameters such as the relative kinetic energy distribution and the distribution of the relative turbulence intensity are also gave out. Furthermore, the experimental runs are numerically simulated using the buoyancy extended k-s turbulence model, and the calculated distribution of velocity and temperature field, the bifurcation phenomenon, the horse-shoe configuration and the trajectories are all in good agreement with the experiments. Thus the reliability of the methematical model is testified, and it can be used to the simulation of other types of thermal buoyant jets.The discharge stability is purely dependent on the near-field behavior of the jets, or the dynamic interaction of the buoyant jet region, the surface impingement region and the internal hydraulic jump region, and is independent of the far-field geometry of the receiving water. The stability criterion is a function of the relative submerged depth, and source densimetric Froude number. For a given relative submerged depth, the stable domain can be approached by decreasing the densimetric Froude number; All these are well demonstrated by the numerical analysis for the plane buoyant jet, the round vertical buoyant jet and the horizontal buoyant jet with different combinations of discharge and ambient characteristics.The far-field geometry of the receiving medium has a significant influence on the bulk mixing characteristics of the discharge. This is especially important for unstable discharges in which the continuous build-up of species concentrations in the recirculation process is limited by the buoyancy-driven convection in the far-field. Stable discharge configuration is more propitious to the diffusion of waste heat, for the re-entrainment of warm fluid in unstable conditions will decrease the near field dilution.The flowing characteristics and discharge stability for horizontal buoyant jets are numerically analyzed. Its flowing configuration is symmetrical, and no bifurcation phenomena are found. Under the buoyancy effect, the thermal jets rise to the surface and diffused downstream. The water temperature in the surface is significant higher than that in the bottom. At the same relative depth of submergence, the decay rate of temperature in the symmetry plane of horizontal buoyant jets will decrease with the increase of densimetric Froude number; for a given densimetric Froude number, the decay rate of temperature willincrease with the increase of relative depth of submergence. It's indicted that the diffusion of thermal contamination can be quicken, under the condition of increase relative submergence, decrease discharge velocity, increase the port diameter or increase the temperature difference between the jet and the ambient water.Under stable discharge, the thermal buoyant jets can be diluted rapidly, and the temperature in the near field is high; under unstable discharge, the temperature in the near field is lower, but the influence field download is extended very far. What kind of discharge pattern should be chosen in practical application is decided by the different requirements of ecology and environment. The formula for the minimal surface dilution for horizontal buoyant jet under stable and unstable discharge is given out, and the effects of the densimetric Froude number to the minimal surface dilution is inconspicuousness under unstable discharge. The formula for the location of maximal surface temperature and the maximal surface velocity for horizontal buoyant jet under stable and unstable discharge are given out, which present quantitative criteria for the subdivision of the flow field, and conduce to the realization of the flowing configuration.The comparision of stability criterion with that for vertical round buoyant jet indicates that for the horizontal buoyant jet the surfacr layer becomes unstable at lower densimetric Froude numbers, which result in a horizontal acceleration of the ambient flow. Farther analysis believes that the asymmetrical discharge condition with a horizontal velocity component is more unstable than the symmetric discharge.