Research On The Behavior Of Obstructed Buoyant Jets

The characteristics of rounded flowing in near region of obstructed buoyant jet in static ambient were mainly discussed in the paper. As one type of diffuser improving dilution of contamination, the knowledge about it will be advanced by researching and probing into it deeply, and the design and choice in environmental engineering firstly can be made depending on them. Secondly, there are great application value and practical significance existing by researching on the problem of rounded buoyant flows because it occurs widely in our lives and production in industry and agriculture, for example, the rounding and diffusion of powders discharged by chimneys and cooling towers after it encountered buildings, and resisting force received by ships in ocean and the air dynamics problems for aero crafts. Researching on it will enrich our jet theories and provide grounds for investigation on more complicated flows.Three parts can be identified in the paper as followings: researching states in the first part (the first chapter); in the second part, establishing of mathematical model applied in the paper (the second chapter) and researching on important parameters in the model (the third chapter); researching on the characteristics of buoyant obstructed jet in the third part (the fourth, fifth, sixth chapter). Main contents in each chapter are introduced as followings:Previous researching methods and fruits about diffusers have been introduced in the first chapter and summarization and evaluation have been made. Jets correlated closely with diffusers have been classified. Comparison of physical characteristics of jet fluid and ambient is often the criterion of jet classifications and three types of jet can be identified based on it: momentum jet, plume jet and buoyant jet. The previous two types of jets are the specializations of the third. There are three researching methods on jet: experiments, theory analysis and numerical simulation. Comparison and summarization of the three methods have been made also.Four types of diffusers can be sorted based on their application in different environmental conditions: coflowing, alternating, staged and Tee and their sketch maps are presented also. Four typical methods improving dilution by other researchers are introducedhere: obstructed buoyant jet, buoyant wall jet, single-port suction manifolds and pre-dilution manifolds. The previous two methods can be ranked as trying to change flowing states around jet orifice to improve dilution. The third method has some concepts of pre-dilution, however, contaminants may overflow from the suction ports and improved dilution is rather limited. The fourth method is rather effective but cost more than others and some comparison must be made with other methods in designs.In the chapter 2, un-compressed viscous basic momentum equation for liquids, Navier-Stokes equation, was educed based on prime physical laws: mass conservation law and momentum conservation law. The time-averaged turbulent equations were deduced from Navier-Stokes equation by Boussinesq hypothesis aiming at obstructed buoyant jet researched in the paper. The turbulent energy k and dissipation ratio e were solved by their convection-diffusion equations. All these equations were translated into dimensionless. Hybrid finite analysis method was also introduced in detail here.In the chapter 3, jets with buoyancy or not from single circular orifice in static ambient were simulated using two-dimensional k-e model under cylinder coordinate. The results of axial velocity and temperature from numerical simulation match well with those supposed by previous researchers and the mathematical model was confirmed. The effect of Prandtl number on results by numerical simulation was investigated qualitatively on the base of the values commended by other scholars. It was found that it had little effect on velocity even varied under a large range but had rather great effect on temperature. The results of axial temperature simulated by the model are smaller than those in experiments when the value of Prandtl number tends to be little.In the chapter 4, many numerical simulations, were done on obstructed buoyant jet. The results of axial velocity and concentration followed well with those gained in experiments and it was confirmed that the mathematical model could simulate the obstructed buoyant jet effectively. Three areas behind the disc can be divided on the base of numerical results: circumfluence area, transitional area and self-similarity area. The variation law of the length of circumfluence area was found with Froude number, the diameter of disc and the distance of disc from the orifice. At the same time it was found that axial dilution couldn't be improvedevidently when the ratio of the distance of disc from the orifice to the diameter of the disc was less than 1. And there was abnormal rounded flowing type existed. Two measures were supposed in the chapter on how to control the flowing types of sewage discharging under complex varying ambient conditions when the diffuser was used.The ranges of normal rounded flowing for different diameters of the disc were researched aiming at the emergence of normal and abnormal rounded flowing. The critical Froude numbers for rather small diameters of the disc increase with increasing of the distance of the disc from the orifice. When the diameter of the disc equals to that of the orifice, there is not abnormal rounded flowing existed but the axial dilution is less than that of previous diameters of the disc. And it shows that the effect of the disc on improving dilution was not exerted sufficiently. The law for occurring of reverberated and bifurcated flowing was discovered also in numerical simulations.In chapter 5, the three-dimensional turbulent model considering buoyancy was used to simulate the buoyant flow from square orifice in static ambient. Experiments were made on the buoyant jet from square orifice in half infinite static ambient to confirm the validity of the model. Concentration on the axis under different conditions was obtained. The results from experiments accord with those from the model. The concentration and longitudinal velocities on axis from square orifice were compared with those from circular and it found that the two graphs of concentration and velocities respectively accorded well. The shapes of temperature and longitudinal velocity distribution on different height cross-sections followed well with those from circular orifice.In chapter 6, some experiments were made for the buoyant jet from a square orifice with a square disc placed on it in static ambient and concentration along the axis in self-similar area behind disc was measured. A three-dimensional mathematical model was established to simulate the whole flowing under different conditions. All the results predicted by the numerical calculation were substantiated by the experiments. The results were compared with experiential formula for obstructed round buoyant vertical jets in static ambient and it was found that the two concentration distributions had good accordance. Star shape of temperature contours on cross sections in the near areas from the disc was found. The shape will transformto concentric circles alike to the round buoyant vertical jet in self-similar area with increasing of the distance from the disc.Based on a series of numerical calculations, the behavior of flow field in obstructed square buoyant vertical jet was summarized and analyzed. Based on the axial line velocity distribution, the flow after the disc can be divided into three regions: i.e. circumfluence region, transitional region and self-similar region. The three regions can also be distinguished based on the axial velocity. The axial velocity in self-similar region was found to obey the same law. The contours of pressure on cross sections of different heights were displayed and formation, expansion, rupture and disappearing of negative pressure regions were found.Chapter 7 is the conclusion and prospect. It sums up the research methods and all the results and points oul the prospects in the future.