Turbulent Buoyant Jet Research Articles

Flow and Separation Characteristics of Three-dimensional Negatively Buoyant Wall Jets

This paper reports a study of the behavior of cold air jets, specifically three-dimensional, incompressible, negatively buoyant, turbulent wall jets, and their separation from horizontal surfaces. With a cold-air distribution system, both the supply air temperature and flow rate are lower than in conventional systems for the same cooling load. The cold air is supplied to a zone at temperatures between 39°F and 49°F (3°C and 9°C) instead of the conventional 55°F (13°C). As the building's ventilation and cooling airflow supply temperature and flow rate are reduced below the conventional values, indoor thermal environment considerations become increasingly important. An analysis using integral forms of the momentum and energy equations is presented for determination of the separation point of buoyant turbulent wall jets. The analysis shows that the separation point is a function of the outlet Richardson number in addition to the geometry of the outlet diffuser or nozzle. Also reported in this paper is an experimental investigation of the mixing and separation of three-dimensional buoyant turbulent wall jets of air discharging into a large room. Velocity and temperature difference profiles and half-width growth rates for these jets are presented. The separation distances are also measured, and compared to the predicted results.

General Integral Formulation of Turbulent Buoyant Jets in Cross-Flow

The turbulent buoyant jet in a nonuniform cross-flow is formulated using a general integral method. The integral equations for the turbulent motion through the jet cross section are derived for a fixed control volume and expressed in vector form. In contrast to previous work, the new formulation does not require the a priori assumption of a velocity and concentration distribution. Coupled with a Lagrangian jet spreading hypothesis and the inclusion of added mass effect, the model is shown to predict correctly the properties of the asymptotic flow regimes in both the near field (straight jet and plume) and the far field (advected line puff and thermal). Predictions of the path-averaged jet properties are shown to be consistently supported by experimental data on visual trajectory and jet width, and centerline minimum dilution. The relation between the present formulation and previous Lagrangian/Eulerian models is also discussed.

Mixing in a two-layer stably stratified fluid by a turbulent jet

Mixing in a two-layer stably stratified fluid by a turbulent buoyant jet was studied primarily through a laboratory experiment. A non-swirling jet was discharged vertically downwards in a fluid system consisting initially of a top layer of fresh water and a bottom layer of salt water. In total, six experimental cases were performed where the jet exit velocity and the density difference between the top and bottom layer were varied. The total depth and the initial depth of the two layers were approximately constant in all the cases studied, and the jet exit was located in the middle of the initial freshwater layer. Both vertical density profiles and time series of density in selected points were determined from conductivity measurements using a data sampling system involving up to 16 probes. The mixing efficiency, defined as the percentage of the supplied kinetic jet energy that is used for increasing the potential energy of the fluid system, was related to a densimetric Froude number based on the intermediate layer depth. Overall, the calculated jet mixing efficiencies displayed higher values than corresponding efficiencies for destratification with air-bubble plumes.

Turbulent convection in buoyant sodium jets

Low Prandtl number convection is investigated in vertical axisymmetric turbulent buoyant jets of sodium discharging into a slowly moving ambient. Measurements of mean velocity and mean temperature profiles are performed. By varying the ratio of momentum to buoyancy flux, that is, the densimetric Froude number, different intensities of buoyancy are obtained. The mixed convection flow is compared with forced convection and plume flow. Using a recently developed temperature-compensated miniature permanent magnet flowmeter probe, Gaussian profiles of velocity and temperature are obtained that are independent of the flow regime at all axial measuring positions. The axial decay of the centerline mean velocity for sodium is the same as for fluids of higher Prandtl number. It shows power laws with powers of −1 for forced convection, −2 3 for a transition region, and −1 3 for plumes. The centerline mean temperature for sodium and a plume decreases with power −1, which is in contrast to the −5 3 decay for plumes and fluids of higher Prandtl number. The results of the sodium experiments are compared with data for water and air flows from the literature.

Mixing in a two-layer stably stratified fluid by a turbulent jet

AbstractMixing in a two-layer stably stratified fluid by a turbulent buoyant jet was studied primarily through a laboratory experiment. A non-swirling jet was discharged vertically downwards in a fluid system consisting initially of a top layer of fresh water and a bottom layer of salt water. In total, six experimental cases were performed where the jet exit velocity and the density difference between the top and bottom layer were varied. The total depth and the initial depth of the two layers were approximately constant in all the cases studied, and the jet exit was located in the middle of the initial freshwater layer. Both vertical density profiles and time series of density in selected points were determined from conductivity measurements using a data sampling system involving up to 16 probes. The mixing efficiency, defined as the percentage of the supplied kinetic jet energy that is used for increasing the potential energy of the fluid system, was related to a densimetric Froude number based on the i...

Flame Radiation Distribution From Fires

A methodology and a model are presented for the calculation of radiation distribution from fires. Extensive application of the model is discussed with regard to turbulent buoyant jet fires wherein radiation is controlled by soot and the flame size is such that it produces optically thin conditions. The local soot concentration is modeled, according to previous results, to be 1) proportional to the inverse smoke-point heat release rate, Y,- ll& and 2) a function of the local mixture fraction and temperature. The local volumetric emissive power is averaged over the turbulent fluctuations to obtain the radiation losses for optically thin flame situations. Combustion is modeled, in a standard way, by an ensemble of laminar flamelets whose chemical composition of species is known from state relationships. The combustion model is an integral model that has been validated by extensive data from turbulent buoyant jet flames whereas the radiation model is validated here by comparison with propane turbulent jet flames; this comparison allows the determination of a proportionality coefficient for the soot concentration and radiation term. The generality and soundness of the model has been determined by applying it to predict the jet flame radiation from another fuel, propylene (while maintaining the same proportionality constant for the soot as for the propane). Preliminary results and favorable comparisons with experiments are also shown for pool fires of diameter up to 1 m. The proposed methodology and the model can be applied to an arbitrary burning material whose laminar smoke-point heat release rate has been measured in laboratory experiments.

Analytical solution to the flame trajectory based on the analysis of “Scaling of buoyant turbulent jet diffusion flames” by N. Peters and J. Göttgens

Horizontal jet flames are formed in oil and gas well diverter fires, some accidental leak fires, and furnaces that utilize horizontal fuel injectors. In spite of these applications, only a few studies of flames in the horizontal configuration have been reported in the literature [1–3].

Smoke yields from turbulent buoyant jet flames

Simple correlations for smoke yield in turbulent buoyant jet flames with smoke-point laminar flame heights are proposed and validated by using experimental data for gaseous, liquid and solid fuels. The smoke yield (kg smoke/kg fuel) in turbulent flames was measured for conditions representing turbulent buoyant jet flames (i.e. high effective B-number for solid and liquid fuels) while the laminar smoke-point properties were measured in a standard apparatus for gaseous and liquid fuels and in a new apparatus for solid fuels. The experimental results show, in agreement with the theoretical considerations, that the smoke yield is inversely proportional to the smoke-point heat release rate and proportional to the stoichiometric ratio. The present correlation is not expected to be valid for halogenated components (such as PVC) because the presence of halogens would affect both the gaseous combustion and the soot formation path chemistry. Smoke yields can be predicted for geometries and turbulent conditions other than turbulent jet buoyant flames by means of turbulent combustion models carefully compared and validated with the present and/or other results.

The Effects of Fuel Sooting Tendency and the Flow on Flame Radiation in Luminous Turbulent Jet Flames

Abstract In this paper two observations are emphasized with regard to flame radiation in luminous turbulent jet flames: (a) the radiant fraction in turbulent buoyant jet flames is a weak function of the fuel sooting tendency as expressed by the laminar smoke-point heat release rate; and (b) moreover, for very sooty fuels, the radiant fraction saturates to a constant value which is independent of the laminar smoke-point heat release rate, although it depends on the adiabatic flame temperature. These observations can be explained by considering that cooling of the flames is enhanced and the effective radiation temperature drops as soot increases and thus radiant losses increase, until for very sooty fuels flame extinction in the upper part of the flames occurs. The physics of flame radiation phenomena are discussed in the light of a global similarity analysis for flame radiation together with the use of extensive experimental data. In the analysis, complementary to the usual hydrodynamic length scale for combustion, a new radiation length scale is introduced which characterizes the effects of radiant losses on the flow. Scaling relationships of the turbulent radiant fraction in terms of the laminar smoke-point heat release rate are developed first for turbulent buoyant jet flames, wherein flow effects are small, and then extended to momentum-dominated turbulent jet flames. The present results have direct practical significance (a) Tor predicting radiant fractions in fires, and (b) for the design of enhanced flame radiation burners. The physics presented here for soot concentration and radiant cooling can be incorporated in detailed k-ε-g or other turbulence models.

Calculation of vertical mixing in plane turbulent buoyant jets on the basis of the algebraic model of turbulence

An algebraic model of turbulence, involving buoyancy forces, is used for calculating velocity and temperature fields in plane turbulent vertical jets in a non-homogeneous stagnant medium. A new approach to the solution of the governing system of partial differential equations (continuity, conservation of momentum, heat (buoyancy), turbulent kinetic energy, dissipation rate and mean quadratic temperature fluctuation) is suggested which is based on the introduction of mathematical variables. Comparison is made between the results of the present calculations with experimental and numerical data of other authors.

Characterization of gaseous helium jet dispersion to atmosphere

A major ground based experiment to be performed for the Superfluid Helium On Orbit Transfer (SHOOT) programme is the accidental loss of the vacuum guard of the superinsulated dewar. The design of the dewar vent path requires adequate mass removal after a preset pressure is reached due to external heat transfer. The exiting helium creates a turbulent buoyant jet, expanding in air with entrainment of the jet interface to the surrounding. Transient analysis is performed for axial and radial jet temperature prediction using the self-similarity assumption applied to mass, momentum and the energy balance equations of helium. The predicted jet temperature profiles with vertical and radial expansion up to 1.6 and 1.0 m, respectively, demonstrate the low temperature core established by gaseous helium. For all time steps, the axial and radial temperature predictions are observed to be within 8 and 20%, respectively.

Interfacial Mixing Caused by Turbulent Buoyant Jets

Laboratory experiments were carried out to investigate mixing and entrainment induced by a planar turbulent buoyant jet impinging on a stable density interface. The jet was released horizontally into the homogeneous bottom layer of a fluid system in which the top linearly stratified layer was separated from the homogeneous layer by a density discontinuity. This flow configuration has similarities to those that occur in solar ponds and ocean thermal energy conversion plants. The results reveal markedly different entrainment mechanisms, and thus entrainment laws, for different jet regimes that depend on the inlet buoyancy of the jet. Measurements included the entrainment velocity, interfacial‐layer thickness, and time evolution of vertical density profiles. Based on observations of entrainment mechanisms, simple theoretical models are developed to predict entrainment laws for slightly buoyant and neutrally buoyant jet regimes, and the model predictions are compared with the experimental results.

The plane vertical turbulent buoyant jet

The plane vertical turbulent buoyant jet

Closure to “ Analysis of Turbulent Buoyant Jet in Density‐Stratified Water ” by Ruochuan Gu and Heinz G. Stefan (August, 1988, Vol. 114, No. 4)

Closure to “ <i>Analysis of Turbulent Buoyant Jet in Density‐Stratified Water</i> ” by Ruochuan Gu and Heinz G. Stefan (August, 1988, Vol. 114, No. 4)

Discussion of “ Analysis of Turbulent Buoyant Jet in Density‐Stratified Water ” by Ruochuan Gu and Heinz G. Stefan (August, 1988, Vol. 114, No. 4)

Discussion a propos de l'article de Ruochuan Gu et Heinz G. Stefan paru dans la meme revue en Aout 1988, vol. 114, no 4

AN ANALYSIS OF THE TWO-DIMENSIONAL TURBULENT BUOYANT JET

Abstract The two-dimensional turbulent buoyant jet is analyzed using a vertical length scale defined by L = M0F0 F0−23 where Ma is the rate at which momentum is added at the source and F0 is the rate at which buoyancy is added. Introduction of the length scales into the equations of motion shows the ratio x/L directly controls the bouyancy term. The pure momentum jet corresponds to the limit x/L→;0 and the pure buoyant plume to x/L→∞. Utilizing an eddy viscosity model, a similarity solution is presented for the buoyant plume, a series solution is used to develop this eddy viscosity model, and a numerical solution is presented for the buoyant jet. Comparisons are made with available experimental data.

Axial Dilution in Obstructed Round Bouyant Jet

Experimental results concerning the axial concentration (or dilution) distribution of a round vertical turbulent buoyant jet of fresh water, discharged in a calm ambient saline solution and obstructed by a thin concentric disc of diameter D, placed horizontally at a distance H from the jet exit section of diameter d are presented and discussed. The functional relationship of axial concentration in the region beyond the disc is well established quantitatively and the important parameters affecting the concentration distribution are determined. A comparison of the present results with those of the unobstructed jet prove the effectiveness of the arrangement studied in increasing dilutions after the disc.

Investigations of round vertical turbulent buoyant jets

The axial and radial velocity components w and u, and the concentration c of a Rhodamine 6G dye were measured simultaneously in a turbulent buoyant jet, using laser-Doppler anemometry combined with a recently developed laser-induced-fluorescence concentration measurement technique. These non-intrusive techniques enable measurements in a region of plume motion where conventional probe-based techniques have had difficulties. The results of the study show that the asymptotic decay laws for velocity and concentration of a tracer transported by the flow are verified experimentally in both jets and plumes. The momentum and volume fluxes and the mean dilution factor are determined in dimensionless form as a function of the normalized distance from the flow source. Contradictory results from earlier experimental plume investigations concerning the decay laws of w and c and the plume width ratio bc/bw are discussed. The turbulence properties and the transition from momentum-driven jets to buoyancy-driven plumes are presented. The turbulence is found to scale with the mean flow as predicted by dimensional analysis and self-similarity. Buoyancy-produced turbulence is found to transport twice as much tracer as jet turbulence. Although velocity statistics in jets and plumes are found to be highly self-similar there is a strong disparity in the distribution of tracer concentration in the two flows. This occurs in the time-average mean flows as well as the r.m.s. turbulent quantities. Instantaneous concentration fluctuations are found to exceed time averages by as much as a factor of 3. The experimental results should provide a reasonable basis for validation of computer models of axisymmetric plumes.

Analysis of Turbulent Buoyant Jet in Density‐Stratified Water

A turbulent buoyant jet in a stratified ambient fluid is investigated. The integral analysis approach is used and extended. By applying the similarity hypothesis for velocity and temperature profiles and deriving an entrainment function from the integral equations, a general model is developed. The model predicts jet trajectories, center‐line temperature or density and velocity decay, and the dilution on the axis of a turbulent buoyant jet discharged at an arbitrary angle into a density‐stratified or uniform ambient. The pressure distribution is not hydrostatic as in most other models. The effects of density change and jet curvature on pressure are explicitly evaluated. Predictions obtained with the model are compared with experimental data.

The round vertical turbulent buoyant jet

Experiments on round vertical turbulent buoyant jet have been performed using tap water as the jet liquid and saltwater solution as the ambient liquid. Measurements were made of the centerline axial velocity and the centerline and radial concentrations. The integral forms of the momentum and tracer equations were integrated on the basis of an assumption concerning a function of the spreading coefficients. The validity of this assumption has been verified by the experimental results and the analysis led to unified analytical expressions concerning the axial velocity and concentration distributions along the jet axis. The present findings are discussed and compared with previously reported works.