Numerical study of the laminar premixed flame stabilization on a slot burner : comparison between detailed and FGM models

It is still a challenging task to numerically solve flames using detailed chemical kinetics in multidimensional geometries of practical applications. To overcome this difculty, many eforts have been done to develop chemical kinetics reducing techniques, such as ILDM, FPV, FPI, and FGM. Although these techniques are widely discussed in the literature, their implementation is not straightforward. In the present work, the FGM technique is implemented to solve a two-dimensional laminar premixed flame of CH4∕air stabilized by heat losses to the burner rim. This confguration is explored to test the FGM prediction capabilities for some stable conditions, one of them close to the blow-of limit. Temperature, mass fraction of selected species, and the burning velocity variation along the flame surface are presented, and limitations of the technique were identifed by comparing FGM results against the direct integration of the full set of conservation equation. In general, the FGM technique has shown a good quantitative agreement when compared with the direct integration

Incorporating unsteady flow-field effects in flamelet-generated manifolds

In general, simulating combustion can be a very costly job. This is caused by the large number of chemical reacting species that are strongly coupled. Moreover, all the (chemical) time-scales that are present, span a multitude of orders, which results in a very large, stiff system of strongly coupled, nonlinear equations and solving such a system is very CPU-intensive. Fortunately, it appears that many combustion systems are dominated by a handful of (slow) processes only. This is due to the fact that the fastest processes rapidly become exhausted and therefore are often neglected. This has subsequently led to a number of reduction techniques that take advantage of the observation that combustion can often be predicted reasonably accurate by taking only a small number of time-scales into account. In this thesis a reduction technique that was introduced by van Oijen [61], i.e., Flamelet- Generated Manifolds (FGM), is expanded upon. The main goal of this thesis is to study whether unsteady flow effects can be captured within the Flamelet-Generated Manifolds concept. The flames that are studied are one-dimensional, non-premixed, stagnation flames and although FGMwas initially developed for and successfully applied to premixed flames but in principle it can also be applied to non-premixed flames. To that end, first a unified one-dimensional flame model is presented, which can be used to describe (partially) premixed and non-premixed flames. Such a one-dimensional flame model is often referred to as flameletmodel. The first step is to decompose the combustion process into three distinct sub-problems, i.e., 1) fluid motion and mixing of enthalpy and elements, 2) the flame front dynamics and 3) the dynamics of the internal flame structure embedded within this flame front. When flames are considered, it is often useful to use a so-called flame adapted coordinate system, where coordinate surfaces correspond to flame surfaces. The flame front dynamics can be described by the evolution of these flame surfaces, which correspond to iso-surfaces of a so-called principal controlling variable Y, for which a conservation equation can be solved. Applying such a coordinate transformation, leads to a set of quasi-one-dimensional combustion equations, which serve as the basis of the FGM method. Generally it is assumed that perturbations from one-dimensional flame behavior are small, and can therefore be neglected. In order to numerically assess these assumptions, the species conservation equation is subdivided into several individual contributions, i.e., an unsteady term, normal transport, flame stretch, curvature, tangential diffusion and the chemical production and consumption terms, respectively. Three different twodimensional flames are simulated, one unsteady premixed flame, one unsteady nonpremixed flame and one steady non-premixed flame. Using the numerical results from these detailed flame simulations, the individual contributions of the species conservation equations are computed and compared to each other. The results show that besides normal transport and chemistry, flame stretch rate and curvature can also be important in both premixed and non-premixed flame simulations. From the two unsteady flame simulations it also follows that the unsteady contribution can be significant. Furthermore, for the steady non-premixed flame, two different principal controlling variables were chosen, resulting in two different coordinate transformations, i.e., a typical non-premixed flame-adapted coordinate system and a typical premixed one. This is possible due to the fact that the flamelet model derived in this thesis is a unified flamelet model, which is able to describe both (partially) premixed flames as well as non-premixed flames. To study whether the effect of transient, local flow fluctuations can be captured by the FGM approach, both steady and unsteady non-premixed flamelet simulations with a detailed chemistry model are studied. Two different situations are studied, 1) a flame which is significantly strained but still far away from the steady extinction limit and 2) a flame where the applied strain-rate is near or even beyond the steady extinction limit. For both situations, two different Flamelet-Generated Manifolds are constructed, i.e., one based on a set steady flamelet simulations and one based on a set of unsteady flamelet simulations. The chemical compositions found during the steady flamelet simulations form a two-dimensional manifold in composition space. On the other hand, a detailed analysis of the chemical compositions found during the unsteady flamelet simulations shows that the unsteady flamelet simulations form a three-dimensional manifold in composition space. Both manifolds are applied to simulate one-dimensional flames that are subjected to sinusoidally varying strain-rate. The results of both FGM simulations are compared to an unsteady simulation with a detailed chemistry model. Both local observables, like species mass fractions and temperature for example, as well as flamesurface area properties like the integral source-term, are represented well with both manifolds. However, for species that are related to the slowest time-scales it is shown that a three-dimensional manifold may result in less accurate predictions, and more controlling variables may be needed

Systematic reduction of complex tropospheric chemical mechanisms, Part I: sensitivity and time-scale analyses

International audienceExplicit mechanisms describing the complex degradation pathways of atmospheric volatile organic compounds (VOCs) are important, since they allow the study of the contribution of individual VOCS to secondary pollutant formation. They are computationally expensive to solve however, since they contain large numbers of species and a wide range of time-scales causing stiffness in the resulting equation systems. This paper and the following companion paper describe the application of systematic and automated methods for reducing such complex mechanisms, whilst maintaining the accuracy of the model with respect to important species and features. The methods are demonstrated via application to version 2 of the Leeds Master Chemical Mechanism. The methods of Jacobian analysis and overall rate sensitivity analysis proved to be efficient and capable of removing the majority of redundant reactions and species in the scheme across a wide range of conditions relevant to the polluted troposphere. The application of principal component analysis of the rate sensitivity matrix was computationally expensive due to its use of the decomposition of very large matrices, and did not produce significant reduction over and above the other sensitivity methods. The use of the quasi-steady state approximation (QSSA) proved to be an extremely successful method of removing the fast time-scales within the system, as demonstrated by a local perturbation analysis at each stage of reduction. QSSA species were automatically selected via the calculation of instantaneous QSSA errors based on user-selected tolerances. The application of the QSSA led to the removal of a large number of alkoxy radicals and excited Criegee bi-radicals via reaction lumping. The resulting reduced mechanism was shown to reproduce the concentration profiles of the important species selected from the full mechanism over a wide range of conditions, including those outside of which the reduced mechanism was generated. As a result of a reduction in the number of species in the scheme of a factor of 2, and a reduction in stiffness, the computational time required for simulations was reduced by a factor of 4 when compared to the full scheme

Systematic reduction of complex tropospheric chemical mechanisms using sensitivity and time-scale analyses

International audienceExplicit mechanisms describing the complex degradation pathways of atmospheric volatile organic compounds (VOCs) are important, since they allow the study of the contribution of individual VOCS to secondary pollutant formation. They are computationally expensive to solve however, since they contain large numbers of species and a wide range of time-scales causing stiffness in the resulting equation systems. This paper and the following companion paper describe the application of systematic and automated methods for reducing such complex mechanisms, whilst maintaining the accuracy of the model with respect to important species and features. The methods are demonstrated via application to version 2 of the Leeds Master Chemical Mechanism. The methods of local concentration sensitivity analysis and overall rate sensitivity analysis proved to be efficient and capable of removing the majority of redundant reactions and species in the scheme across a wide range of conditions relevant to the polluted troposphere. The application of principal component analysis of the rate sensitivity matrix was computationally expensive due to its use of the decomposition of very large matrices, and did not produce significant reduction over and above the other sensitivity methods. The use of the quasi-steady state approximation (QSSA) proved to be an extremely successful method of removing the fast time-scales within the system, as demonstrated by a local perturbation analysis at each stage of reduction. QSSA species were automatically selected via the calculation of instantaneous QSSA errors based on user-selected tolerances. The application of the QSSA led to the removal of a large number of alkoxy radicals and excited Criegee bi-radicals via reaction lumping. The resulting reduced mechanism was shown to reproduce the concentration profiles of the important species selected from the full mechanism over a wide range of conditions, including those outside of which the reduced mechanism was generated. As a result of a reduction in the number of species in the scheme of a factor of 2, and a reduction in stiffness, the computational time required for simulations was reduced by a factor of 4 when compared to the full scheme

New 0-D methodology for predicting NO formation under continuously varying temperature and mixture composition conditions

The development of new diesel combustion modes characterized by low combustion temperatures, to minimize the NOx emissions, has caused a noticeable change in the diesel spray s structure and in the NOx chemistry, gaining relevance the N2O and the prompt routes in detriment of the thermal mechanism.Therefore, to accurately predict the NOx emissions, the detailed chemistry and physics must be taken into account, with the consequence of increasing the computational cost. The authors propose in the current study a new predictive methodology associated to low computational cost, where detailed chemistry and simplified physics are considered. To diminish even more the computational cost, the chemistry was tabulated as a function of temperature and oxygen excess mass fraction (parameter which effectively couples the equivalence ratio and the EGR rate). This tool has been developed with the objective of being applicable in continuously varying temperature and mixture fraction conditions (the diffusion diesel spray context) and was validated with the Two-Stage Lagrangian model (TSL-model) and with real engine measurements. The results in both validation scenarios reflect a high degree of accuracy making it applicable, at least, to perform qualitative predictions. By extension, it is expected to perform similarly in continuously varying temperature conditions (i.e.: homogenous charge compression ignition diesel combustion modes) which are less demanding computationally speaking.The authors would like to acknowledge the contribution of the Spanish Ministry of Economic and Competitively for the financial support of the present research study associate to the projects TRA 2008-06448 (VELOSOOT) and to Dr. V. Golovitchev for his valuable comments and suggestions.Benajes Calvo, JV.; López Sánchez, JJ.; Molina Alcaide, SA.; Redón Lurbe, P. (2015). New 0-D methodology for predicting NO formation under continuously varying temperature and mixture composition conditions. Energy Conversion and Management. 91:367-376. https://doi.org/10.1016/j.enconman.2014.12.010S3673769