Turbulent Buoyant Jet Research Articles

The Study of Flame Length for the Horizontal Jet Flame of Carbon Dioxide and Propane Mixed Gas

Inert gases, such as carbon dioxide (CO₂), are often presented in fuel gases and influence their combustion characteristics significantly. This paper investigates the horizontal length of buoyant turbulent jet flame under different boundary conditions, including different nozzle diameters, heat release rates, and CO₂ volume fractions. According to the experiments, it is found that the horizontal length of jet flame increases with the heat release rate under the same nozzle diameter. Additionally, the flame length initially increases with CO₂ concentration but then decreases. Based on an analysis of the buoyancy and momentum flux of the turbulent jet flame, a new correlation has been developed, which relates the horizontal flame length to the heat release rate and CO₂ volume fraction. This correlation can be served as a valuable reference for fire prevention and control in gas leakage fire scenarios.

High-Fidelity Velocity and Concentration Measurements of Turbulent Buoyant Jets

Accurate models of turbulent buoyant flows are essential for the design of the cooling circuit of nuclear reactors and passive safety systems. However, available models fail to fully capture the physics of turbulent mixing when buoyancy becomes predominant with respect to momentum. Therefore, high-fidelity experiments of well-controlled fundamental flows are needed to develop and validate more accurate models. We analyze experiments of positive and negative turbulent buoyant jets, both in uniform and stratified environments, with the aim of understanding the thermal hydraulics of turbulent mixing with variable density and providing high-fidelity data for the development and validation of turbulence models. Non-intrusive, simultaneous particle image velocimetry and laser-induced fluorescence measurements were carried out to acquire instantaneous velocity and concentration fields on a vertical section parallel to the axis of a jet in the self-similar region. The refractive index matching method was applied to measure high-resolution buoyant jets with up to 8.6% density difference. These data are free of the typical errors that characterize optical measurements of buoyancy-driven flows (e.g. natural and mixed convection) where the refractive index of the fluid is inhomogeneous throughout the measurement domain. Turbulent statistics and entrainment of buoyant jets in uniform and stratified environments are presented. These data are compared with non-buoyant jets in a uniform environment, as a reference to investigate the effects of buoyancy and stratification on turbulent mixing. The results will be used for the assessment of current turbulence models and as a basis for the development of a new model that captures turbulent mixing.

Scalar transport and nucleation in quasi-two-dimensional starting jets and puffs

We experimentally investigate the early-stage scalar mixing and transport with solvent exchange in a quasi-two-dimensional (quasi-2D) jet. We inject an ethanol/oil mixture upward into quiescent water, forming quasi-2D turbulent buoyant jets and triggering the ouzo effect with initial Reynolds numbers, Re0=420,840, and 1680. We study two different modes of fluid supply: continuous injection to study a starting jet and finite volume injection to study a puff. While both modes start with the jet stage, the puff exhibits different characteristics in transport, entrainment, mixing, and nucleation, due to the lack of continuous fluid supply. We also inject a dyed ethanol solution as a passive scalar reference case, such that the effect of nucleation for the ethanol/oil mixture can be disentangled.For the starting jets, the total nucleated mass from the ouzo mixture seems very similar to that of the passive scalar total mass, indicating a primary nucleation site slightly above the virtual origin above the injection needle, supplying the mass flux like the passive scalar injection. With continuous mixing above the primary nucleation site, the mildly increasing nucleation rate suggests the occurrence of secondary nucleation throughout the entire ouzo jet.For the puffs, we show that the puff with the smallest Re0 propagates the fastest and its entrainment lasts the longest. We attribute the superior performance to the buoyancy effect, which transforms a turbulent puff into a turbulent thermal, and has been proven to have stronger entrainment. Although the entrainment and nucleation reduce drastically when the injection stops, the mild mixing still leads to non-zero nucleation rates and the reduced decay of the mean puff concentrations for the ouzo mixture.Adapting the theoretical framework established in Landel et al. (2012b) for quasi-2D turbulent jets and puffs, we successfully model the transport of the horizontally-integrated concentrations for the passive scalar. The fitted advection and dispersion coefficients are then used to model the transport of the ouzo mixture, from which the spatial–temporal evolution of the nucleation rate can be extracted. The spatial distribution of the nucleation rate sheds new light on the solvent exchange process in transient turbulent jet flows.

Experimental Study of the Behavior of Horizontally Buoyant Turbulent Jet Flames in Acoustic Waves

Natural gas is most commonly transported by pipeline. However, leaks sometimes occur during transport, often leading to jet fires that cause substantial economic damage and property damage. It is therefore crucial to improve the efficiency of jet fire suppression. This paper focuses on the change of flame morphology of horizontal jet fires under different boundary conditions (acoustic pressure, frequency and jet exit velocity) and experimentally analyses the horizontal, vertical and trajectory length variation laws of the flame. Then a prediction model relating the horizontal length, vertical length and trajectory length of the horizontal jet flame under the action of acoustic waves is established.

Experimental study, dimensional analysis and an integral model for horizontal buoyant turbulent jet fires under opposing wind

This work demonstrates a comprehensive model for horizontal turbulent jet flame geometries under opposing wind, for which few data or models can be found in the literature. This situation can be a practical scenario of offshore drilling platform with a horizontal flare under opposing wind. The opposing wind pushes the jet flame back, which eventually turns around and upwards, pointing finally downstream in the windward direction. The flame geometries are characterized by four parameters including horizontal length and vertical height from nozzle exit to the location of farthest point that flame envelope could reach before the flame turns around as well as horizontal length and vertical height from nozzle exit to the location of flame tip. Experiments are carried out for horizontal jets discharged from circular nozzles having diameters of 3, 5, 7 and 9 mm and employing propane as fuel with opposing wind speeds from 0.35 to 1.08 m/s. The locations of flame tip and the flame farthest point decrease with increasing opposing wind for a constant heat release rate. Dimensional analysis is derived from a physical model considering the momentum of fuel jet, flame buoyancy due to the flame temperature rise, opposing wind momentum and normalized flow rate at stoichiometric conditions at flame tip. The normalized flame geometry parameters based on the proposed model are well represented by two characteristic length scales derived by accounting for the interaction between excess jet-wind momentum and flame buoyancy as well as that between opposing wind momentum and flame buoyancy. In addition, the normalized mass flow rate at stoichiometric conditions determines the total length of flame trajectory. Moreover, an integral model is deduced by an air entrainment model and top hat profiles for the mass and momentum conservations to predict well the flame geometries of horizontal jet fires under opposing wind.

Micro-droplet nucleation through solvent exchange in a turbulent buoyant jet

Solvent exchange is a process involving mixing between a good solvent with dissolved solute and a poor solvent. The process creates local oversaturation which causes the nucleation of minute solute droplets. Such ternary systems on a macro-scale have remained unexplored in the turbulent regime. We experimentally study the solvent exchange process by injecting mixtures of ethanol and trans-anethole into water, forming a turbulent buoyant jet in the upward direction. Locally, turbulent mixing causes oversaturation of the trans-anethole following turbulent entrainment. We optically measure the concentration of the nucleated droplets using a light attenuation technique and find that the radial concentration profile has a sub-Gaussian kurtosis. In contrast to the entrainment-based models, the spatial evolution of the oversaturation reveals continuous droplet nucleation downstream and radially across the jet, which we attribute to the limited mixing capacity of the jet. Although we are far from a full quantitative understanding, this work extends the knowledge on solvent exchange into the turbulent regime, and brings in a novel type of flow, broadening the scope of multicomponent, multiphase turbulent jets with phase transition.

Mean Flow and Mixing Properties of a Vertical Round Turbulent Buoyant Jet in a Weak Crosscurrent

Mean Flow and Mixing Properties of a Vertical Round Turbulent Buoyant Jet in a Weak Crosscurrent

Analysis of Flow and Thermal Characteristics in a Rectangular Chamber with a Hollow Cylinder Under a Horizontal Turbulent Buoyant Jet

Analysis of Flow and Thermal Characteristics in a Rectangular Chamber with a Hollow Cylinder Under a Horizontal Turbulent Buoyant Jet

Analysis of Flow and Thermal Characteristics in a Rectangular Chamber with a Hollow Cylinder Under a Horizontal Turbulent Buoyant Jet

Analysis of Flow and Thermal Characteristics in a Rectangular Chamber with a Hollow Cylinder Under a Horizontal Turbulent Buoyant Jet

Multi-spatio-temporal scales PIV in a turbulent buoyant jet discharging in a linearly stratified environment

Time-resolved particle image velocimetry is implemented with a camera array and several laser sheets; this results in a multi-spatio-temporal scale system that is modular and flexible. The setup is optimized to capture global flow features, while locally resolving in space and time near the Kolmogorov scale. The apparatus relies extensively on machine vision CMOS cameras; they are inexpensive and stream to computer hard drive with virtually continuous time-resolved records (up to one hour for the current system). This leads to statistically converged data and also helps in minimizing down time between experimental runs. Details of the implementation and design of experiment are reported.The system instruments a vertical buoyant jet discharging in a linearly stratified environment. Refractive index matched aqueous solutions enable precise optical deployment. The density difference is 3% and the fluids have similar dynamic viscosity. The jet Reynolds number is 2.00×104 and is above the mixing transition. Such flows are typically challenging to instrument and few velocity field data are available to date. Velocity statistics are reported as well as first insights gained from the campaign.

A Global-Scale Investigation of Stochastic Similarities in Marginal Distribution and Dependence Structure of Key Hydrological-Cycle Processes

To seek stochastic analogies in key processes related to the hydrological cycle, an extended collection of several billions of data values from hundred thousands of worldwide stations is used in this work. The examined processes are the near-surface hourly temperature, dew point, relative humidity, sea level pressure, and atmospheric wind speed, as well as the hourly/daily streamflow and precipitation. Through the use of robust stochastic metrics such as the K-moments and a second-order climacogram (i.e., variance of the averaged process vs. scale), it is found that several stochastic similarities exist in both the marginal structure, in terms of the first four moments, and in the second-order dependence structure. Stochastic similarities are also detected among the examined processes, forming a specific hierarchy among their marginal and dependence structures, similar to the one in the hydrological cycle. Finally, similarities are also traced to the isotropic and nearly Gaussian turbulence, as analyzed through extensive lab recordings of grid turbulence and of turbulent buoyant jet along the axis, which resembles the turbulent shear and buoyant regime that dominates and drives the hydrological-cycle processes in the boundary layer. The results are found to be consistent with other studies in literature such as solar radiation, ocean waves, and evaporation, and they can be also justified by the principle of maximum entropy. Therefore, they allow for the development of a universal stochastic view of the hydrological-cycle under the Hurst–Kolmogorov dynamics, with marginal structures extending from nearly Gaussian to Pareto-type tail behavior, and with dependence structures exhibiting roughness (fractal) behavior at small scales, long-term persistence at large scales, and a transient behavior at intermediate scales.

Vertical Round Buoyant Jets and Fountains in a Linearly, Density-Stratified Fluid

Turbulent round buoyant jets and fountains issuing vertically into a linearly density-stratified calm ambient have been investigated in a series of laboratory experiments. The terminal (steady-state) height of rise and the mean elevation of subsequent horizontal spreading have been measured in positively buoyant jets (at source level), including pure momentum jets and plumes, as well in momentum-driven negatively buoyant jets (fountains). The results from experiments confirmed the asymptotic analysis that was based on dimensional arguments. The normalized terminal height and spreading elevation with respect to the elevation of injection of momentum-driven (positively) buoyant jets and fountains attained the same asymptotic values. The numerical results from the solution of entrainment equations, using an improved entrainment coefficient function, confirmed the results related to buoyancy dominant flows (plumes), while their predictions in momentum-driven flows were quite low if compared to measurements.

A well-resolved numerical study of a turbulent buoyant helium jet in a highly-confined two-vented enclosure

A numerical study of a turbulent buoyant helium jet developing in a two-vented cavity is conducted based on a well-resolved numerical simulation. A sufficiently large exterior region is modelled in the computational domain to approach the natural inlet/outlet conditions of the vents, leading to a good agreement between the numerical results and available experimental particle image velocimetry (PIV) measurements. For a release of helium with a flow rate of 5 Nl.min-1 in an air-filled cavity about 15 cm high, the flow structure and the helium dispersion are analysed illustrating the strong confinement effect enhanced by a cross-flow from the lower vent. By tracking the jet axis deviation and comparing the classic plume description in terms of global fluxes estimated from DNS results to the Morton et al.’ theory [1], we quantify the effect of the confinement and the cross-flow on the flow structure. Finally, we highlight a blocking zone at mid-cavity height originated from confinement, where helium accumulates.

Flame geometry of downward buoyant turbulent jet fires under cross flows: Experiments, dimensional analysis and an integral model

This study investigates the flame geometry of vertically downward facing buoyant turbulent jet fires issuing from a nozzle subject to cross flows, which deflect the jet flow by interacting with downward momentum issuing from the source and the upward buoyant forces owing to the flames. The flames turn eventually upwards as the buoyant forces dominate. The objective of this work is to understand the physics and develop non-dimensional correlations and modeling for the flame geometry of this jet fire configuration never investigated before. For this purpose, experiments are conducted having 4 circular nozzles (diameter at 3, 4, 5 and 7 mm) with propane employed as fuel under various heat release rates. The cross flows are generated by a wind tunnel having a uniform cross flow air speed varying from 0.31 m/s to 2.08 m/s. The horizontal and downward distance from the nozzle to the lowest point of the jet flame, the vertical thickness of the flame at the lowest point, the flame vertical height from lowest point to the flame tip, the flame tip horizontal projection distance are measured. Three characteristic length scales developed by dimensional analysis considering initial downward momentum M0, flame buoyancy g′, inertial cross flow force ua2 together with the normalized air required for complete combustion Sm˙f/ρaua×(ua2/g′)2, are found to represent well the aforementioned properties of the flame geometry. An integral model, considering the mass and momentum conservations, is developed to predict the trajectory and geometric properties of the downward jet flame under cross flow showing good agreement with the experimental results.

Improvements of turbulence model for analysis of hydrogen stratification erosion by turbulent buoyant jet

Light gas stratification erosion by steam turbulent buoyant jet is the typical phenomenon about containment thermal hydraulics and hydrogen safety. Overprediction of stratification erosion rate has been observed in computational fluid dynamics (CFD) analysis by using current turbulence model based on simple gradient diffusion hypothesis (SGDH). To solve this problem, the effects of buoyancy term, coefficient and turbulent Schmidt number are discussed and a modified k-ε model with buoyancy term based on generalized gradient diffusion hypothesis (GGDH) which contains vertical and horizontal density gradient is developed. In modified k-ε model, besides the value of coefficient, the buoyancy term based on SGDH is modified into GGDH. The strain tensor is modified into rotating tensor to suppress the overestimated local turbulent kinetic energy generated in flow stagnation zone. Comparison between analysis results and experimental data shows that improvement effect of modified model on prediction of light gas stratification erosion rate is significant.

Physical models of flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with different heat release rates

This paper investigated the flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with the same base dimension whilst having different heat release rates (HRRs). Propane was used as fuel. The HRR of one of the burners was fixed at 8, 16, 24, 32 and 40 kW, respectively. By increasing the HRR of the other burner, the interaction behaviors of two jet flames were studied under various burner spacings. A total of 15 categories of HRRs combinations of two jet flames were conducted in this work. Two flames heights are different to lead the heat pressure on both sides of the flame to be non-axisymmetric. And the interaction law, air entrainment mechanism etc. become more complicated than two same HRRs flames. Results showed that with increasing burner spacing, the flame merging behaviors can be divided into three stages. The flame height firstly deceases to a minimum value, then slightly rises again and finally maintains stable due to the weak interaction of non-merging flames. Two effective weighting factors are supposed to quantize the maximum and minimum flame heights to derive a flame height model which then is validated using the experimental data in literatures. The merging point height increases with increasing HRR and burner spacing. Based on the air entrainment mechanism of two interaction flames, a general method for calculating the global volume air entrainment is presented to build a physical entrainment model. Above flame height and physical entrainment models are suitable for the calculation with two flames combinations with the same and different HRRs, which greatly extends the application scope of interaction fires. In addition, two creative methods of calculating flame height and global air entrainment volume have been provided a reference for the research on the combustion behaviors of multiple flames with different HRRs combination.

Velocity Field and Scalar Field Measurements of Turbulent Buoyant Round Jets in a Two-Layer Stratified Environment

Results reported in the literature have shown that the turbulence models currently implemented in computational fluid dynamics (CFD) commercial codes (e.g., ANSYS-CFX, STAR-CCM+, and FLUENT) have a tendency to overestimate thermal stratification and underestimate turbulent mixing when buoyancy effects become dominant with respect to momentum effects. Also, standard large eddy simulation models cannot fully capture the behavior of jets interacting with stratified environments because the assumption of turbulence isotropy of the smaller scales breaks down. Because of light diffraction and image distortion, it is challenging to apply nonintrusive optical flow measurements, like particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), to get experimental data for CFD validations when there are density variances involved in the flow. However, a refractive index matching (RIM) technique that has been recently developed in our Experimental and Computational Multiphase Flow Laboratory allows us to perform high-resolution measurements of velocity fields and scalar fields for turbulent buoyant jet flow in the presence of density differences as high as 8.6%. To form a fully turbulent round free jet, an experimental facility was designed with a jet nozzle diameter of 2 mm, located at the bottom of a cubic tank with 30-cm side length. The jet flow is established by a servo-engine-driven piston to eliminate possible fluctuations introduced by the motor. A high-fidelity synchronized PIV/PLIF system was utilized in conjunction with RIM to measure the velocity and concentration fields in the self-similar regions of a jet flow with a density difference of 3.16% for aqueous solutions. With Reynolds numbers of 4000 and 10 000, the jet impinging with a two-layer stably stratified environment is compared to the positively buoyant jet with lighter fluid injected into denser surroundings. Detailed quantifications of the measurement uncertainties are also carried out. The experimental results are presented in terms of turbulent statistics and the analysis of jet penetration depths.

Formation of compound droplets during fragmentation of turbulent buoyant oil jet in water – ADDENDUM

Formation of compound droplets during fragmentation of turbulent buoyant oil jet in water – ADDENDUM

EVALUATION OF K-EPSILON MODEL FOR TURBULENT BUOYANT JET

The modelling of a turbulent buoyant jet is challenging due to the complex nature of such flow, which consists of two fluids with different densities, as well as the multi-scale flow phenomena associated in both space and time. In this paper, the k-epsilon turbulence model is applied to model a turbulent buoyant jet at different flow regimes including laminar and turbulent. The velocity field and centerline velocity are in good agreement with the experiments, as well as the expected results based on jet theory. Moreover, the distribution of the radial velocity matches with Gaussian distribution. The k-epsilon model is an appropriate turbulent model that can be applied for larger Reynolds number flow simulation.
 Keywords: k-epsilon, turbulence model, CFD, turbulent buoyant jet.

Estimating the trajectory length of buoyant turbulent jet flames issuing from a downward sloping nozzle

Downward sloping buoyant turbulent jet fires occur frequently during flammable gas leakage, while its flame trajectory, especially the quantitative length, is not well described. The flame trajectory lengths of downward jet fires with five sloping angles (0°, 15°, 30°, 45°, 60° relative to the horizontal) and two nozzle diameters (8 and 10mm) were experimentally investigated. A novel method to quantify the flame trajectory length is proposed based on the flame outlines, and the results obtained show good agreement with the existing data of horizontal jet fires. For cases with differently inclined angles, the normalized flame trajectory length as a function of nozzle diameter can be correlated with the Froude number Fr1/5 and dimensionless heat release rate Q*2/5, but not as function of the tilting angle as this is not considered in the existing correlations. In order to establish a global model for predicting the flame trajectory length of downward sloping jet fires as function of the tilting angle of the nozzle, a remodified Froude number (Frf*) is proposed, and the correlated result can describe all the current experimental results together with that of horizontal and vertically downward-oriented jet fires in the literature.