scholarly journals Conditional Moment Closure Modelling of a Lifted H2/N2Turbulent Jet Flame Using the Presumed Mapping Function Approach

2015 ◽  
Vol 2015 ◽  
pp. 1-24
Author(s):  
Ahmad El Sayed ◽  
Roydon A. Fraser
Keyword(s):  
Mixture Fraction ◽  
Mapping Function ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Jet Flame ◽  
Velocity Statistics ◽  
Conditional Moment Closure ◽  
Consistent Manner ◽  
Radical History

A lifted hydrogen/nitrogen turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure (CMC) supplemented by the presumed mapping function (PMF) approach for the modelling of conditional mixing and velocity statistics. Using a prescribed reference field, the PMF approach yields a presumed probability density function (PDF) for the mixture fraction, which is then used in closing the conditional scalar dissipation rate (CSDR) and conditional velocity in a fully consistent manner. These closures are applied to a lifted flame and the findings are compared to previous results obtained usingβ-PDF-based closures over a range of coflow temperatures (Tc). The PMF results are in line with those of theβ-PDF and compare well to measurements. The transport budgets in mixture fraction and physical spaces and the radical history ahead of the stabilisation height indicate that the stabilisation mechanism is susceptible toTc. As in the previousβ-PDF calculations, autoignition around the “most reactive” mixture fraction remains the controlling mechanism for sufficiently highTc. Departure from theβ-PDF predictions is observed whenTcis decreased as PMF predicts stabilisation by means of premixed flame propagation. This conclusion is based on the observation that lean mixtures are heated by downstream burning mixtures in a preheat zone developing ahead of the stabilization height. The spurious sources, which stem from inconsistent CSDR modelling, are further investigated. The findings reveal that their effect is small but nonnegligible, most notably within the flame zone.

scholarly journals Consistent Conditional Moment Closure Modelling of a Lifted Turbulent Jet Flame Using the Presumedβ-PDF Approach

2014 ◽  
Vol 2014 ◽  
pp. 1-25 ◽  
Author(s):  
Ahmad El Sayed ◽  
Roydon A. Fraser
Keyword(s):  
Mixture Fraction ◽  
Turbulent Jet ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Jet Flame ◽  
Gradient Diffusion ◽  
Conditional Moment Closure ◽  
History Of ◽  
Closure Modelling

A liftedH2/N2turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure. The conditional velocity (CV) and the conditional scalar dissipation rate (CSDR) submodels are chosen such that they are fully consistent with the moments of the presumedβprobability density function (PDF). The CV is modelled using the PDF-gradient diffusion model. Two CSDR submodels based on the double integration of the homogeneous and inhomogeneous mixture fraction PDF transport equations are implemented. The effect of CSDR modelling is investigated over a range of coflow temperatures (Tc) and the stabilisation mechanism is determined from the analysis of the transport budgets and the history of radical build-up ahead of the stabilisation height. For allTc, the balance between chemistry, axial convection, and micromixing, and the absence of axial diffusion upstream of the stabilisation height indicate that the flame is stabilized by autoignition. This conclusion is confirmed from the rapid build-up ofHO2ahead ofH,O, andOH. The inhomogeneous CSDR modelling yields higher dissipation levels at the most reactive mixture fraction, which results in longer ignition delays and larger liftoff heights. The effect of the spurious sources arising from homogeneous modelling is found to be small but nonnegligible, mostly notably within the flame zone.

Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method

2013 ◽  
Author(s):  
Scott Martin ◽  
Aleksandar Jemcov ◽  
Björn de Ruijter
Keyword(s):  
Turbulent Combustion ◽  
Large Scale ◽  
Reaction Rates ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Small Scale ◽  
Conditional Moment ◽  
Progress Variable ◽  
Conditional Moment Closure ◽  
Scale Turbulence

Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided. The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled. The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.

Study on Conditional Statistics in Two-Dimensional Liquid Jet With the Second-Order Chemical Reaction

2011 ◽  
Author(s):  
Tomoaki Watanabe ◽  
Hiroki Yasuhara ◽  
Yasuhiko Sakai ◽  
Takashi Kubo ◽  
Kouji Nagata ◽  
...  
Keyword(s):  
Turbulent Flow ◽  
Chemical Reaction ◽  
Mixture Fraction ◽  
Spectroscopic Method ◽  
Second Order ◽  
Moment Closure ◽  
Liquid Jet ◽  
Two Dimensional ◽  
Conditional Moment ◽  
Conditional Moment Closure

It is important in engineering to elucidate the mechanism of a chemical reaction in turbulent flow. But there are still few studies on reacting turbulent flow in a liquid phase. In this study, the two-dimensional liquid jet with the second-order reaction (A+B←R) is investigated. The concentrations of the species R and the conserved scalar (which is the concentration of other species independent of the above chemical reaction) are measured simultaneously by the optical fiber probe based on light absorbtion spectroscopic method. The concentrations of species A and B are obtained from the conserved scalar theory. Regarding the velocity field, the streamwise velocity is measured by the hot-film anemometer. The moment closure methods are often used for the prediction of turbulent flow. But it is difficult to apply it to the reacting turbulent flow because of the high non-linearity of the reaction rate terms. It is commonly known that the values of concentrations depend strongly on the mixture fraction (which is a conserved scalar) defined as the normalized concentration of the species which is independent of reaction. Hence, Conditional moment closure (CMC) methods are useful for the prediction of the turbulent flow with chemical reactions. In this study, conditional scalar statistics are investigated by using the conditional moment closure methods and experimental data. It is shown that the conditional averages of concentration of reactant and product species approach the equilibrium limit (which correspond to the limiting case of the fast chemical reaction) in the downstream direction and the value of the conditional scalar (mixture fraction) dissipation decreases and its distribution varies in the downstream direction and comes to show the local minimum value near the point η = ξS (which is the stoichiometric value of the mixture fraction).

Large Eddy Simulation of an Enclosed Turbulent Reacting Methane Jet With the Tabulated Premixed Conditional Moment Closure Method

2016 ◽  
Vol 138 (10) ◽  
Author(s):  
Carlos Velez ◽  
Scott Martin ◽  
Aleksander Jemcov ◽  
Subith Vasu
Keyword(s):  
Turbulent Combustion ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Combustion Model ◽  
Conditional Moment ◽  
Eddy Simulation ◽  
Conditional Moment Closure ◽  
Major Species ◽  
Large Eddy ◽  
Closure Method

The tabulated premixed conditional moment closure (T-PCMC) method has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes in Reynolds-averaged Navier–Stokes (RANS) environment by Martin et al. (2013, “Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method,” ASME Paper No. GT2013-95092). Here, the premixed conditional moment closure (PCMC) method is extended to large eddy simulation (LES). The new model is validated with the turbulent, enclosed reacting methane backward facing step data from El Banhawy et al. (1983, “Premixed, Turbulent Combustion of a Sudden-Expansion Flow,” Combust. Flame, 50, pp. 153–165). The experimental data have a rectangular test section at atmospheric pressure and temperature with an inlet velocity of 10.5 m/s and an equivalence ratio of 0.9 for two different step heights. Contours of major species, velocity, and temperature are provided. The T-PCMC model falls into the class of table lookup turbulent combustion models in which the combustion model is solved offline over a range of conditions and stored in a table that is accessed by the computational fluid dynamic (CFD) code using three controlling variables: the reaction progress variable (RPV), variance, and local scalar dissipation rate. The local scalar dissipation rate is used to account for the affects of the small-scale mixing on the reaction rates. A presumed shape beta function probability density function (PDF) is used to account for the effects of subgrid scale (SGS) turbulence on the reactions. SGS models are incorporated for the scalar dissipation and variance. The open source CFD code OpenFOAM is used with the compressible Smagorinsky LES model. Velocity, temperature, and major species are compared to the experimental data. Once validated, this low “runtime” CFD turbulent combustion model will have great utility for designing the next generation of lean premixed (LPM) gas turbine combustors.

IMPLEMENTATION OF THE CONDITIONAL MOMENT CLOSURE MODEL TO A TURBULENT NONPREMIXED H2/CO-AIR FLAME STABILIZED ON A BLUFF BODY

1999 ◽  
Vol 23 (3-4) ◽  
pp. 425-433 ◽  
Author(s):  
S.- H. Kim ◽  
T. Liu ◽  
K.Y. Huh
Keyword(s):  
Bluff Body ◽  
Reaction Rates ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Turbulent Diffusion Flame ◽  
Recirculating Flow ◽  
Conditional Moment Closure ◽  
Mass Fractions ◽  
Turbulent Nonpremixed

A turbulent nonpremixed flame of H2/CO-air stabilized on a bluff-body is simulated by the conditional moment closure (CMC) model. Full spatial variation of the conditional quantities is taken into account for an elliptic recirculating flow field. Comparison has shown reasonable agreement for the conditional and Favre mean temperature and mass fractions of CO and H20 between calculation and experiment. Overprediction of the peak OH mass fraction is attributed to inaccurate modelling of the conditional scalar dissipation rate. The CMC model is capable of predicting major features of a turbulent diffusion flame characterized by finite chemical reaction rates.

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.

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