Strain Effects in Partially Premixed Methane-air Jet Flames

Abstract

A large-eddy simulation (LES) subgrid combustion model has been developed based on an innovative application of the linear-eddy model to the counter flow configuration (LEM-CF). This subgrid model is developed by parameterizing flame statistics from the LEM-CF sub-model in terms of a reduced set of variables. A database of closure statistics for LES transport equations for these variables is then generated. The run-time application of the model within LES retrieves the closure statistics from the database resulting in a computationally efficient formulation. One key feature of this formulation is that it includes the LEM-CF prediction of turbulent flame extinction limits to develop a localized extinction criterion for the LES. This extinction criterion is a function of the LES resolved scale strain rate which may be directly computed from the LES resolved scale velocity field. The model's capabilities for capturing strain rate and local extinction effects were assessed through its application to the Sandia piloted partially premixed CH 4 /air-air jet flame series. A comparison of model predictions for scalar means and RMS fluctuations for Flame D and F of this series showed good overall agreement with the experimental data. This included good predictions of the temperature and product species in the neck region of Flame F which is dominated by local extinction effects. The model was also applied to determine the blow-out jet bulk velocity for this configuration which was found to be in reasonable agreement with what is available from the experiment. Through this blow-out assessment, three flame modes for this configuration are also identified beyond the Flame F condition. The flame dynamics of a highly oscillating flame mode just prior to blow-out were also investigated. I. IntroductionFluid dynamic strain effects in nonpremixed, partially premixed, and premixed flows play an important role in the evolutionary development and stabilization of flame structure in both commercial and military gas turbine systems. For commercial combustors, the operation at lean conditions for the sake of efficiency and pollutant suppression results in an enhanced susceptibility of the flame to strain rate effects that may lead to local extinction, instability and blow-off. For military systems, the high strain rate environment within high performance gas turbine augmentors results in difficultly in establishing flame stability over the entire flight envelop. For these extreme conditions, experimental correlations are inadequate at establishing the operational limits of these systems. As a result, many gas turbine manufacturers are turning to computational techniques such as large eddy simulation (LES) to assess sub-system performance and to aid in hardware design and modification. However, this approach is limited by the accuracy of sub-models for turbulence-chemistry interactions that are employed. These sub-models play a critical role in establishing the accuracy of the prediction of turbulent extinction limits that affect flame s...

A framework for data-based turbulent combustion closure: A priori validation

“…We have observed that bandwidths ranging between 0.005 and 0.05 for a Gaussian kernel function are enough to smoothly capture the important characteristics present in the experimental data. The joint KDE is 14 implemented in Python using the in-built functions available in their scikit-learn library [31]. These joint PDFs are constructed at each axial and radial location in the flame using the instantaneous pointwise measurements.…”

“…The two limits of a presumed PDF shape with a limited set of parameters and the solution for a PDF transport equation leave ample room in between for intermediate approaches, which may be based on the construction of PDFs based on simulation [11][12][13][14] or experimental data. For example, Goldin and Menon [11,12], Sankaran et al [13] and Calhoon et al [14] built joint scalar PDFs, which are parameterized by an appropriate set of lower moments using simulations based on the linear-eddy model (LEM) [15].…”

“…by adopting a flamelet assumption) or via the multi-environment PDF method [10].The two limits of a presumed PDF shape with a limited set of parameters and the solution for a PDF transport equation leave ample room in between for intermediate approaches, which may be based on the construction of PDFs based on simulation [11][12][13][14] or experimental data. For example, Goldin and Menon [11,12], Sankaran et al [13] and Calhoon et al [14] built joint scalar PDFs, which are parameterized by an appropriate set of lower moments using simulations based on the linear-eddy model (LEM) [15].The PDFs are generally trained on simpler canonical flows, such as scalar decay in homogeneous turbulence or co-flowing jet configurations. The LEM is a 1-D model that can accommodate any degree of chemical complexity based on the coupling of reaction and diffusion processes in a deterministic fashion and a stochastic implementation for turbulent stirring.…”

A framework for data-based turbulent combustion closure: A posteriori validation

“…Nonetheless, these moment-based models tend to be more computationally efficient than more advanced models that seek to directly evaluate unclosed terms in the governing equations, such as the PDF transport approach [3], the LEM-LES [4] and the ODT-LES [5] approaches. The use of lowdimensional stochastic models, such as the linear-eddy model (LEM) [6] and the one-dimensional turbulence (ODT) model [7] to construct PDFs and conditional means also provide an a priori tabulation approach for both conditional statistics and PDFs [8][9][10][11][12].…”