Siva Prasad Reddy Muppala

Numerical modelling of the pressure dependent reaction source term for premixed turbulent methane/air flames

Effects of pressure are studied numerically for premixed turbulent flames under varied turbulence conditions up to 1.0 MPa. In order to model the pressure dependency of the reaction source term, a set of available experimental Bunsen flame data for methane/air mixtures from Kobayashi et al. are used as reference for a numerical optimisation study, where three parameters of a generalized algebraic closure relation for a reaction progress variable approach are determined. This approach is based on the algebraic turbulent flame speed closure model proposed by Zimont, which is subjected to modifications for influence of pressure and for low turbulence conditions. The net pressure dependency of the turbulent flame speed is slightly above zero, similar to the experimental fit of Kobayashi. Dependency of turbulent velocity is found proportional to u0.55, similar to several experimental data fits by Bradley et al. If the resulting flame angles are represented in terms of sT/sL vs. u/sL, then the fitted curve of the calculated results is closely related to that of the experiments. Both are showing a nonlinear bending behaviour especially at low turbulence intensities, which seems to be related to the influence of laminar flame instabilities.

LES of Triangular-stabilized Lean Premixed Turbulent Flames with an algebraic reaction closure: Quality and Error Assessment

In this LES study, an algebraic flame surface wrinkling model based on the progress variable gradient approach is validated for lean premixed turbulent propane/air flames measured on VOLVO test rig. These combustion results are analyzed for uncertainty in the solution using two quality assessment techniques.

A modelling approach for hydrogen doped lean premixed turbulent combustion

In this study, we investigate some preliminary reaction model predictions analytically in comparison with experimental premixed turbulent combustion data from four different flame configurations, which include i) high-jet enveloped, ii) expanding spherical, iii) Bunsen-like, and iv) wide-angled diffuser flames. The special intent of the present work is to evaluate the workability range of the model to hydrogen and hydrogen-doped hydrocarbon mixtures, emphasizing on the significance of preferential diffusion, PD, and Le effects in premixed turbulent flames. This is carried out in two phases: first, involving pure hydrocarbon and pure hydrogen mixtures from two independent measured data, and second, with the blended mixtures from two other data sets. For this purpose, a novel reaction closure embedded with explicit high-pressure and exponential Lewis number terms developed in the context of hydrocarbon mixtures is used. These comparative studies based on the global quantity, turbulent flame speed, indicate that the model predictions are encouraging yielding proper quantification along with reasonable characterization of all the four different flames, over a broad range of turbulence, fuel-types and for varied equivalence ratios. However, with each flame involved the model demands tuning of the (empirical) constant to allow for either or both of these effects, or for the influence of the burner geometry. This provisional stand remains largely insufficient. Therefore, a submodel for chemical time scale from the leading point analysis based on the critically curved laminar flames employed in earlier studies for expanding spherical flames is introduced here. By combining the submodel and the reaction closure, the dependence of turbulent flame speed on physicochemical properties of the burning mixtures including the strong dependence of preferential diffusion and/or Le effects can be determined.

Modelling Issues of Lean High-Pressure Turbulent Premixed Hydrogen-enriched Hydrocarbon Combustion at Gas Turbine Conditions

(Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent premixed conditions offers the advantages of low temperature operation and thus low NOx emissions. At such ultralean conditions, hydrocarbon flames are inherently unstable, with low range of extinction limits, hampering wide range operability especially at high pressures. However, addition of small amounts of hydrogen, characterized by high burning velocities, has the potential to extend this extinction limit. Hydrogenated fuels offers higher turbulent flame speed ST compared to pure hydrocarbons under identical conditions. These issues are addressed using the existing Algebraic Flame Surface-Wrinkling reaction subclosure in which the influence of high-pressure and the Lewis number were explicitly included [1]. The fuel effects were derived from the flame ball concept by Zeldovich [2]. In the first instance, comparative studies between calculations and experiments are based on turbulent flame speed of lean high-pressure premixed turbulent pure methane flames obtained on a typical sudden-expanding dump combustor [3]. The pure methane mixtures were preheated to 673 K with maximum operating pressure of 10 bar, for a range of equivalence ratios. Simulation studies carried out using the AFSW model predicts an increase in ST for pure methane mixtures in line with experiments. In the second step, reaction model correlation results for hydrogen enriched methane flames were compared with the corresponding measured data obtained on the same burner. Griebel et al. [4] observed a nonlinear increase in ST with hydrogen addition, especially for equivalence ratio 0.50. The maximum hydrogen weightage for mixed fuel mixtures is 50 % by volume. In this part of the study, for the hydrogen blended methane mixtures, the differences between and analytical results from the AFSW model in its basic form and measurements are found to be significant. This increase in S T is explained using critical chemical time scale taken from a numerical study of outwardly propagating spherical flames by Lipatnikov and Chomiak [5]. This time scale characteristic of the critically curved laminar flamelets based on leading point concept [5] addresses the combined preferential-thermo-diffusive effects.