Modeling of turbulent premixed flames using flamelet-generated manifolds

Efficient and reliable numerical models have become important tools in the design and optimization process of modern combustion equipment. For accurate predictions of flame stability and pollutant emissions, the use of detailed comprehensive chemical models is required. This accuracy, unfortunately, comes at a very high computational cost. The flamelet-generated manifold (FGM) method is a chemical reduction technique which lowers this burden drastically, but retains most of the accuracy of the comprehensive model. In this chapter, the theoretical background of FGM is briefly reviewed. Its application in simulations of premixed and partially premixed flames is explained. Extra attention is given to the modeling of preferential diffusion effects that arise in lean premixed methane–hydrogen–air flames. The effect of preferential diffusion on the burning velocity of stretched flames is investigated and it is shown how these effects can be included in the FGM method. The impact of preferential diffusion on flame structure and turbulent flame speed is analyzed in direct numerical simulations of premixed turbulent flames. Finally, the application of FGM in large-eddy simulations is briefly reviewed

Flamelet-generated manifolds : development and application to premixed laminar flames

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Improving FGM: multiple chemical time scales:Applied to predict the fate of NO and CO in Nozzle Guide Vanes

In this work, we propose an innovative approach intended to improve the general accuracy of the Flamelet Generated Manifolds chemistry reduction method. It is based on increasing the dimensionality of the FGM by allowing for an additional degree of freedom. This extra dimension of the manifold accounts for chemical kinetics, describing conversion of one species into others. We follow the ideas of ILDM to perform a time scale analysis of chemical source term. It is done locally in each grid point of the FGM, yielding the local chemical time scales and the directions in composition space of their corresponding reaction groups. The assumption is that the chemistry evolution quickly vanishes in the directions of the fast reaction groups. This means that the reaction process develops only along the few “slowest” directions. Then the FGM is extended locally by the directions of the slow chemistry. Still, the movement in the direction of the extension is bound to the narrow vicinity of the original manifold. An additional transport equation needs to be solved for the secondary reactive control variable used to parametrize the movement in the direction of the extension. The movement on the manifold along the 1D FGM is still parametrized by the reaction progress variable, which is the main reactive control variable. This approach is not restricted to just one additional dimension. On the contrary, an arbitrary number of chemically reactive dimensions can be included.\u3cbr/\u3eThe performance of the new developed model is examined in one-dimensional test configurations, which simulate the process of expansion of a mixture of burnt gases. The purpose of this theoretical exercise is to obtain conditions severe enough for the thermochemistry to go off the FGM with one chemical degree of freedom, in the direction of the secondary reactive dimension. In the utilized test cases, this is achieved by the high rate at which the expansion happens. These fast time scales of the change of the thermodynamic variables can interact with the post flame chemistry, for example altering the concentration of the pollutants. The idea of expansion or compression of burnt gases can be related to several applications. Often, expansion of burnt gases is used to convert the released heat into work. Here, we adopt a somewhat idealized test case setup that resembles the idea of the gas turbine stators, also called the nozzle guide vanes (NGV). There, the burnt gases, formed in the combustor, are led through a decreasing area duct. Accompanied by the decrease of temperature and pressure, the velocity increases almost up to the speed of sound. Due to the high velocity, the residence time inside the NGV is small, resulting in a high rate of cooling and expansion.\u3cbr/\u3eThe results of the test problems show that the FGM method with one additional reactive dimension yielded a better agreement with the detailed chemistry simulations. Improvements are observed for the source terms of the reactive control variables and for the species composition. Also, the accuracy of the CO and NOx predictions increased by an order of magnitude

Predicting NO Formation with Flamelet Generated Manifolds

With the increasing tightness of emission limits, models for pollutant formation become more and more important. Steady flamelet models have proven to be very efficient methods to model combustion. However, formation of nitrogen oxide and soot are relatively slow chemical processes, which cannot be assumed in quasi-steady state and therefore need additional treatment. In this study, two methods to model NO formation using steady flamelet tables are investigated. The results are compared with simulations using the full chemistry model. It appears that accurate results can be obtained from the table when alpha is large enough (alpha > 10) to minimize interpolation errors. Solving an additional transport equation for NO is less sensitive to interpolation errors of the look-up procedure and results in slightly more accurate results than direct look-up

Modelling of premixed counterflow flames using the flamelet-generated manifold method

In the recently introduced flamelet-generated manifold (FGM) method the ideas of the manifold and the flamelet approach are combined: a manifold is constructed using one-dimensional flamelets. In this paper the effect of flame stretch on theaccuracy of the FGM method is investigated. In order to isolate the effect of flame stretch, premixed methane/air counterflow flames are simulated. In the case of unit Lewis numbers a one-dimensional manifold is sufficient to model the main effects of flame stretch. A manifold with two progress variables reproduces the results computed using detailed kinetics almost exactly. When non-unit Lewis numbersare used, the enthalpy and element composition of the burnt mixture change, which may influence the mass burning rate significantly. If these composition changes are included in the manifold using one additional controlling variable, the results agreewell with detailed computations