Velocity/Mixture Fraction Statistics of Round, Self-Preserving, Buoyant Turbulent Plumes

1995 ◽  
Vol 117 (4) ◽  
pp. 918-926 ◽  
Author(s):  
Z. Dai ◽  
L. K. Tseng ◽  
G. M. Faeth
Keyword(s):  
Sulfur Hexafluoride ◽  
Gaussian Approximation ◽  
Mixture Fraction ◽  
Turbulence Models ◽  
Test Range ◽  
Radial Gradient ◽  
Gradient Diffusion ◽  
Mass Fluxes ◽  
Normal Manner ◽  
Scalar Variance

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions in the self-preserving portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still and unstratified air environment. Velocity/mixture-fraction statistics, and other higher-order turbulence quantities, were measured using laser velocimetry and laser-induced fluorescence. Similar to earlier observations of these plumes, self-preserving behavior of all properties was observed for the present test range, which involved streamwise distances of 87–151 source diameters and 12–43 Morton length scales from the source. Streamwise turbulent fluxes of mass and momentum exhibited countergradient diffusion near the edge of the flow, although the much more significant radial fluxes of these properties satisfied gradient diffusion in the normal manner. The turbulent Prandtl/Schmidt number, the ratio of time scales characterizing velocity and mixture function fluctuations and the coefficient of the radial gradient diffusion approximation for Reynolds stress, all exhibited significant variations across the flow rather than remaining constant as prescribed by simple turbulence models. Fourth moments of velocity and velocity/mixture fraction fluctuations generally satisfied the quasi-Gaussian approximation. Consideration of budgets of turbulence quantities provided information about kinetic energy and scalar variance dissipation rates, and also indicated that the source of large mixture fraction fluctuations near the axis of these flows involves interactions between large streamwise turbulent mass fluxes and the rapid decay of mean mixture fractions in the streamwise direction.

Velocity Statistics of Round, Fully Developed, Buoyant Turbulent Plumes

1995 ◽  
Vol 117 (1) ◽  
pp. 138-145 ◽  
Author(s):  
Z. Dai ◽  
L. K. Tseng ◽  
G. M. Faeth
Keyword(s):  
Carbon Dioxide ◽  
Sulfur Hexafluoride ◽  
Mixture Fraction ◽  
Power Spectra ◽  
Reynolds Stresses ◽  
Test Range ◽  
Velocity Statistics ◽  
Present Test ◽  
Integral Scales ◽  
Temporal And Spatial

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions within the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still air environment. Velocity statistics were measured using laser velocimetry in order to supplement earlier measurements of mixture fraction statistics using laser-induced iodine fluorescence. Similar to the earlier observations of mixture fraction statistics, self-preserving behavior was observed for velocity statistics over the present test range (87–151 source diameters and 12–43 Morton length scales from the source), which was farther from the source than most earlier measurements. Additionally, the new measurements indicated that self-preserving plumes are narrower, with larger mean streamwise velocities near the axis (when appropriately scaled) and with smaller entrainment rates, than previously thought. Velocity statistics reported include mean and fluctuating velocities, temporal power spectra, temporal and spatial integral scales, and Reynolds stresses.

Structure of Round, Fully Developed, Buoyant Turbulent Plumes

1994 ◽  
Vol 116 (2) ◽  
pp. 409-417 ◽  
Author(s):  
Z. Dai ◽  
L.-K. Tseng ◽  
G. M. Faeth
Keyword(s):  
Carbon Dioxide ◽  
Experimental Study ◽  
Sulfur Hexafluoride ◽  
Mixture Fraction ◽  
Power Spectra ◽  
Probability Density Functions ◽  
Density Functions ◽  
Spatial Correlations ◽  
Integral Scales ◽  
Temporal And Spatial

An experimental study of the structure of round buoyant turbulent plumes was carried out, emphasizing conditions in the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon) dioxide and sulfur hexafluoride) in a still air environment. Mean and fluctuating mixture fraction properties were measured using single-and two-point laser-induced iodine fluorescence. The present measurements extended farther from the source (up to 151 source diameters) than most earlier measurements (up to 62 source diameters) and indicated that self-preserving turbulent plumes are narrower, with larger mean and fluctuating mixture fractions (when appropriately scaled) near the axis, than previously thought. Other mixture fraction measurements reported include probability density functions, temporal power spectra, radial spatial correlations and temporal and spatial integral scales.

Validation of a Flamelet Approach to Modelling 3-D Turbulent Combustion Within an Airspray Combustor

2002 ◽  
Author(s):  
A. G. Kyne ◽  
M. Pourkashanian ◽  
C. W. Wilson ◽  
A. Williams
Keyword(s):  
Mixture Fraction ◽  
Turbulence Models ◽  
Predictive Performance ◽  
Design Tool ◽  
Aviation Fuel ◽  
Lookup Table ◽  
Product Cycle ◽  
Discrete Phase Model ◽  
Flamelet Model ◽  
Wide Range

Over the past two decades Computational Fluid Dynamics (CFD) has become increasingly popular with the gas turbine industry as a design tool. By applying CFD techniques during the early stages of designing a product, engineers can establish the key parameters and dimensions of a system before any experimental trial and error tests are made, thus reducing the product cycle time and costs. This study compares CFD predictions with a comprehensive set of experimental measurements made at QinetiQ on the combustion of aviation fuel within a modem airspray combustor. The performances of two separate models describing the chemical interactions are compared. First, an equilibrium model was employed and linked to the 3D commercial solver, FLUENT 5.5, through a mixture fraction/PDF lookup table approach. Similarly a flamelet model was implemented using a recently developed detailed chemical reaction mechanism describing aviation fuel combustion which has previously received rigorous testing with regard to its predictive performance over a wide range of combustion conditions (Patterson et al., 2001). Both cases predicted heat transfer through a new non-adiabatic PDF lookup table generator developed within the department. This allowed the implementation of a discrete phase model that treats the fuel entering the combustor as a fine liquid spray before evaporating and arriving in the gaseous phase. Two turbulence models (k-ε and Reynolds Stress models) were also used and the results of each compared.

scholarly journals Reynolds-Averaged Navier-Stokes Modeling of Turbulent Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz Mixing Using a Higher-Order Shock-Capturing Method

2019 ◽  
Author(s):  
Oleg Schilling
Keyword(s):  
Sulfur Hexafluoride ◽  
Mixing Layer ◽  
Turbulence Models ◽  
Shock Capturing ◽  
Navier Stokes ◽  
Weno Reconstruction ◽  
Third Order ◽  
Central Difference ◽  
Reynolds Averaged Navier Stokes ◽  
Good Agreement

Abstract A numerical implementation of a large number of Reynolds-averaged Navier–Stokes (RANS) models based on two-, three-, four-equation, and Reynolds stress turbulence models (using either the turbulent kinetic energy dissipation rate or the turbulent lengthscale) in an Eulerian, finite-difference shock-capturing code is described. The code uses third-order weighted essentially nonoscillatory (WENO) reconstruction of the advective fluxes, and second- or fourth-order central difference derivatives for the computation of spatial gradients. A third-order TVD Runge–Kutta time-evolution scheme is used to evolve the fields in time. Improved closures for the turbulence production terms, compressibility corrections, mixture transport coefficients, and a consistent initialization methodology for the turbulent fields are briefly summarized. The code framework allows for systematic comparisons of detailed predictions from a variety of turbulence models of increasing complexity. Applications of the code with selected K–ε based models are illustrated for each of the three instabilities. Simulations of Rayleigh–Taylor unstable flows for Atwood numbers 0.1–0.9 are shown to be consistent with previous implicit LES (ILES) results and with the expectation of increased asymmetry in the mixing layer characteristics with increasing stratification. Simulations of reshocked Richtmyer–Meshkov turbulent mixing corresponding to experiments with light-to-heavy transition in air/sulfur hexafluoride and incident shock Mach number Mas = 1.50, and heavy-to-light transition in sulfur hexafluoride/air with Mas = 1.45 are shown to be in generally good agreement with both pre- and post-reshock mixing layer widths. Finally, simulations of the seven Brown–Roshko Kelvin–Helmholtz experiments with various velocity and density ratios using nitrogen, helium, and air are shown to give mixing layer predictions in good agreement with data. The results indicate that the numerical algorithms and turbulence models are suitable for simulating these classes of inhomogeneous turbulent flows.

scholarly journals Effects of the turbulence model and the spray model on predictions of the n-heptane jet fuel–air mixing and the ignition characteristics with a reduced chemistry mechanism

2017 ◽  
Vol 231 (14) ◽  
pp. 1877-1888 ◽  
Author(s):  
Haiqiao Wei ◽  
Xi Chen ◽  
Wanhui Zhao ◽  
Lei Zhou ◽  
Rui Chen
Keyword(s):  
Mixture Fraction ◽  
Combustion Process ◽  
Computational Cost ◽  
Turbulence Models ◽  
Fuel Mixture ◽  
Theory Model ◽  
Influential Factors ◽  
Gas Jet ◽  
Reduced Chemistry ◽  
Jet Theory

Reynolds-averaged Navier–Stokes simulations with an improved spray model and a realistic chemistry mechanism are performed for turbulent spray flames under diesel-like conditions in a constant-volume chamber. Comprehensive numerical analyses including two turbulence models (the renormalisation group k– ε model and the standard two-equation k– ε model) with different model coefficients are made. The distribution of the fuel mixture fractions is a very important factor affecting the combustion process. In this study, we also use the entrainment gas-jet model, modifications of the the spray model coefficient and two turbulence models to investigate extensively the influence of the gas-jet theory model on the fuel–air mixture process. First, a non-reacting case is validated by comparing the liquid-phase penetration and the vapour-phase penetration and also the mixture fractions at different axis positions. Second, approriate methods are confirmed according to accurate mixture fraction distributions to validate the combustion process. Because of the large number of species and reactions, the calculation of chemically reacting flows is unaffordable, particularly for three-dimensional simulations. Hence, the dynamic adaptive chemistry method for efficient chemistry calculations is extended in this work to reduce the computational cost of the spray combustion process when a reduced chemistry mechanism is used. The results show that, in the evaporation case, the gas-jet theory model can be used to obtain a relatively accurate fuel vapour penetration length with different influential factors and that improved numerical methods can effectively reduce the mesh dependence for the spray evaporation process. It is demonstrated that the Schmidt number Sc and the turbulence models significantly influence the mixture fraction distribution. Very good agreement with available experimental data is found concerning the ignition delay time and the flame lift-off length for different oxygen concentrations owing to the accurate fuel mixture fraction.

Numerical investigation of methanol bluff-body flame using variants of RANS turbulence models with conditional moment closure model

2019 ◽  
pp. 21-30
Author(s):  
R. N. Roy ◽  
S. Sreedhara
Keyword(s):  
Reynolds Stress ◽  
Bluff Body ◽  
Mixture Fraction ◽  
Turbulence Models ◽  
Moment Closure ◽  
Closure Model ◽  
Conditional Moment ◽  
Conditional Moment Closure ◽  
Rans Turbulence Models ◽  
The Mean

In this article, conditional moment closure model (CMC) along with four variants of RANS turbulence models is used for investigating a methanol bluff-body flame. This work attempts to establish the accuracy of turbulence models in predicting the mixing fields, which results in improved predictions of the mean and variance of mixture fraction. This ensures an accurate probability density function (pdf) of the mixture fraction field which is used to obtain unconditional quantities from the conditional quantities calculated from CMC closure. The flow and mixing field are calculated using ANSYS Fluent software by incorporating four different turbulence models viz. standard k-ε (SKE), modified k-ε (MKE), RNG k-ε and Reynolds stress turbulence models. Flow field simulations have been coupled with an in-house CMC solver to obtain the mean flame structure. Profiles of mixture fraction showed an excellent agreement with the experimental data when Reynolds stress turbulence model was used. The unconditional mean temperature and species mass fraction obtained from the CMC model shows improved predictions when coupled with the Reynolds stress turbulence models. Because of inaccurate mixing field and hence the pdf predicted from SKE, MKE and RNG k-ε models, the unconditional quantities showed significant deviations from the experimental results.

Numerical Investigation of Combustion Characteristics in a Liquid Fuelled Can Combustor

2012 ◽  
Author(s):  
Vivek Pandey ◽  
Ashoke De ◽  
Abhijit Kushari
Keyword(s):  
Mixture Fraction ◽  
Critical Role ◽  
Turbulence Models ◽  
Reacting Flow ◽  
Swirl Number ◽  
Recirculation Bubble ◽  
Air Mass ◽  
Primary Zone ◽  
Sst Model ◽  
Exit Temperature

In the present study, reacting flow in a can combustor is numerically simulated and the results are compared with the data available in open literature. Different grids are considered in order to compare air mass-flow distributions through air entry ports. The fuel used for reacting flow calculations is a kerosene surrogate. In combination with continuity and momentum equations, the transport equations for mean mixture fraction (f) and mixture fraction variance (f″) are solved, linking the instantaneous thermo-chemical state of the fluid through this conserved scalar and its variance. Three sets of atomizer air swirls with swirl number (SN) 0.8, 1, and 1.2 have been tested along with different RANS based turbulence models. It is observed that k-ω-sst model appears to exhibit most realistic solutions in terms of the pattern factor and peak exit temperature. This is due to better prediction of turbulence kinetic energy generation at the primary zone of the combustor. However, the swirl in atomizing air plays a critical role in anchoring the flame in the primary zone of the combustor due to formation of recirculation bubble. Moreover, it is observed that the spray stream is convected downstream for a swirl number slightly below 1.0, thereby affecting the pattern factor of the combustor.

Application of the k-ε Turbulence Model to Buoyant Adiabatic Wall Plumes

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Michael A. Delichatsios ◽  
C. P. Brescianini ◽  
D. Paterson ◽  
H. Y. Wang ◽  
J. M. Most
Keyword(s):  
Stokes Equations ◽  
Reynolds Shear Stress ◽  
Mixture Fraction ◽  
Turbulence Models ◽  
Lateral Spread ◽  
Adiabatic Wall ◽  
Mean Velocity ◽  
Wall Boundary Conditions ◽  
The Mean ◽  
Wall Plume

Computational fluid dynamics based on Reynolds averaged Navier–Stokes equations is used to model a turbulent planar buoyant adiabatic wall plume. The plume is generated by directing a helium/air source upwards at the base of the wall. Far from the source, the resulting plume becomes self-similar to a good approximation. Several turbulence models based predominantly on the k-ε modeling technique, including algebraic stress modeling, are examined and evaluated against experimental data for the mean mixture fraction, the mixture fraction fluctuations, the mean velocity, and the Reynolds shear stress. Several versions of the k-ε model are identified that can predict important flow quantities with reasonable accuracy. Some new results are presented for the variation in a mixing function for the mixture normal to the wall. Finally, the predicted (velocity) lateral spread is as expected smaller for wall flows in comparison to the free flows, but quite importantly, it depends on the wall boundary conditions in agreement with experiments, i.e., it is larger for adiabatic than for hot wall plumes.

Mass and internal-energy transports in strongly compressible magnetohydrodynamic turbulence

2018 ◽  
Vol 84 (6) ◽  
Author(s):  
N. Yokoi
Keyword(s):  
Magnetic Field ◽  
Internal Energy ◽  
Transport Coefficients ◽  
Turbulence Models ◽  
Direct Interaction ◽  
Multiple Scale ◽  
Mhd Turbulence ◽  
Cross Diffusion ◽  
Gradient Diffusion ◽  
High Reynolds

Turbulent mass and internal-energy transports in strongly compressible magnetohydrodynamic (MHD) turbulence are investigated in the framework of the multiple-scale direct-interaction approximation, an analytical closure scheme for inhomogeneous turbulence at very high Reynolds numbers. Utilising the analytical representations for the turbulent mass and internal-energy fluxes and their transport coefficients, which are expressed in terms of the correlation and response functions, turbulence models for these fluxes are proposed. In addition to the usual gradient-diffusion transports, cross-diffusion transports mediated by the density variance and the transports along the mean magnetic field mediated by the compressional or dilatational turbulent cross-helicity (velocity–magnetic-field correlation coupled with compressive motions) are shown to arise. These compressibility effects are of fundamental importance since they provide deviations from the usual gradient-diffusion transports. Analogies of the dilatational cross-helicity effects to the magnetoacoustic waves are also argued.

Proposal of United Combustion Model for Premixed and Diffusion Flames and Its Evaluation

2008 ◽  
Author(s):  
Shin-Ichi Inage
Keyword(s):  
Diffusion Flame ◽  
Diffusion Flames ◽  
Laminar Flame ◽  
Mixture Fraction ◽  
Turbulence Models ◽  
Premixed Combustion ◽  
Combustion Model ◽  
Laminar Flame Speed ◽  
Turbulent Diffusion Flame ◽  
And Diffusion

A premixed flame assisted by the burning of a diffusion flame is used in gas-turbine combustors to reduce NOx emissions. A united model that can be applied to the premixed and diffusion flames is therefore required to simulate the combustion phenomena. This paper proposes such a united model based on the author’s premixed combustion model for reactive progress variable equation. The proposed model has the following features. 1) It includes the laminar flame speed and the gradient of the mixture fraction as parameters. When the gradient of the mixture fraction is close to zero, the model is also close to the previous premixed combustion model as an asymptotic form. 2) It considers the effects of pressure in the combustor, unburned gas temperature, and flame stretch on combustion based on the laminar flame speed. 3) It can be applied to all types of turbulence models like the k-ε model, large eddy simulations, and direct simulations in the case of wrinkled laminar flames. The effect of turbulence is considered through the turbulent eddy viscosity of all turbulence models. To verify the accuracy of the model, the opposed diffusion flame presented by Tsuji and Yamaoka was numerically simulated, as an example of a laminar diffusion flame. Further, a turbulent diffusion flame, which was assisted by the burning of a pilot jet, was demonstrated using the united combustion model as an example of the turbulent diffusion flame discussed by Barlow and Frank. The flame was known as Sandia Flame D. Model results were in good agreement with the experimental data and this agreement confirmed the proposed united model was able to accurately simulate both diffusion flames.

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