Views
5 years ago

Premixed Flames as Gasdynamic Discontinuities - Combustion ...

Premixed Flames as Gasdynamic Discontinuities - Combustion ...

Premixed Flames as Gasdynamic Discontinuities - Combustion

Premixed Flames as Gasdynamic Discontinuities: Integral Momentum Analysis C. Bruzzese ∗ , H. Dirks, A.G. Class Institute for Nuclear and Energy Technologies, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany Abstract Viewed on hydrodynamic length scales, a typical premixed laminar flame is thin and can be described as a gasdynamic discontinuity separating the fresh mixture from the burned products. We perform an integral momentum analysis of cylindrical premixed flames viewed as gasdynamic discontinuities and quantitatively investigate the jump of momentum at the surface of discontinuity. We interpret the jump of momentum as the result of a localised force and compute the surface tension of the surface of discontinuity, which is negative. For numerical simulations, we suggest an implementation that implicitly satisfies the momentum balance and that is based on distributions. Introduction A premixed laminar flame is typically thin compared to the characteristic length scales of the fluid flow and can be viewed as a gasdynamic discontinuity separating the fresh mixture from the burned products. The first model of a flame as a gasdynamic discontinuity was proposed by Darrieus [1] and by Landau [2]. In the Darrieus-Landau model, the surface of discontinuity representing a flame propagates normal to itself at the constant adiabatic flame speed of a planar flame. The model predicts unconditionally unstable planar flames, which is contradictory to laboratory observations. In first attempts to overcome this contradiction (e.g. [3]), the flame speed relation was modified by introducing phenomenological dependencies of the flame speed on curvature and strain of the discontinuity surface. In a next generation of investigations (e.g. [4-6]), the flame speed relation was derived by asymptotic methods from first principles, considering a simplified chemical one-step reaction, thermal expansion and diffusive processes. Recent investigations [7-9] have generalised the representation of flames as gasdynamic discontinuities, e.g. in inhomogeneous mixtures, for arbitrary reaction orders or variable transport coefficients. In the unified model of flames as gasdynamic discontinuities of Class et al. [8], theoretical investigations have revealed that a negative surface tension has to be attributed to the surface of discontinuity representing a general premixed flame. Surface tension has consequences on the stability of an interface. Positive surface tension tends to stabilise a fluid interface since it requires energy to increase the area of the interface. Therefore, a negative value of the surface tension of a flame surface is expected to destabilise it. The flame speed relation must overcome both the Darrieus-Landau instability and the destabilising effect of the negative surface tension. Specific Objectives Our objective is to perform quantitative studies of the momentum balance at a surface of discontinuity representing a realistic premixed flame with complex ∗ Corresponding author: christian.bruzzese@iket.fzk.de Proceedings of the European Combustion Meeting 2009 chemistry and to compute quantitative values of the forces and the surface tension in the surface of discontinuity. In this paper, we recall the unified model of flames as gasdynamic discontinuities of Class et al. [8] and formulate a general integral momentum balance of premixed flames. In order to analyse the effect of curvature of the discontinuity surface exclusively, we apply the formulation to the case of stationary cylindrical flames that result from detailed simulations. Furthermore, we propose an implementation method that takes into account the forces in the surface of discontinuity. The unified model of flames as gasdynamic discontinuities Our analysis is based on the unified model of flames as gasdynamic discontinuities [8]. We will introduce the model only to the extent needed to perform an integral momentum analysis. The model describes the propagation of a premixed flame in unconfined space in the low Mach number regime. The main concept exploited in the derivation of the model is the introduction of two sets of governing equations. One set contains all the equations governing the propagation of a premixed flame within the range of validity of the model. It includes chemistry and is able to describe the reactive structure of the flame. This set is therefore referred to as reactive model. The governing equations can be found in standard books about combustion, e.g. [10]. The second set contains the equations of the hydrodynamic model. It does not include terms accounting for chemical reactions and can only describe the inert flow field ahead of and behind the flame. In the hydrodynamic model, the internal reactive structure of the flame is not resolved but is replaced by a surface of discontinuity where the variables jump from their values in the unburned state to their corresponding values in the burned state. Far ahead of and behind the flame, the solutions of the reactive model and the hydrodynamic model are by definition identical. Differences between the models exist only within the flame structure. This fact is

Laminar Premixed Flames - Propulsion and Combustion Laboratory
here - Combustion Centre - International Flame Research Foundation
Turbulent Non-Premixed Combustion Analysis - TU Delft
Experimental and LES Study of Low Swirl Flame
Turbulent combustion (Lecture 3) Non-premixed flames
Combustion Control of Premixed Swirl Flame Using Additional ...
Cyclic Flame Propagation in Premixed Combustion - GALCIT ...
Self-Induced Instabilities of Laminar Premixed Flames Anchored on ...
Measurement of turbulent transport in premixed turbulent flames by ...
Experimental Study of the Response Behaviour of Premixed Flames ...
A priori analysis of turbulent flamelet combustion in a premixed ...
Premixed and Stratified Flame Diagnostics Based on 3D Flame ...
Temperature effect on soot size distribution of sooting premixed flames
Electric Field Control of a Premixed Turbulent Flame at High Pressure
Numerical Simulation of Turbulent Premixed Flames with ...
On the Modeling of Premixed Combustion under Varying ...
Measurement of flame surface density for turbulent premixed flames ...
The response of swirling premixed flames to velocity perturbations P ...
Diagnostics of Reactive Zone in Premixed Flames via Acetone-OH ...
Numerical Study of Premixed Laminar Spherical Flames
Direct Simulation of Hydro-dynamically Unstable Premixed Flames
Premixed Combustion for Acetylene-Hydrogen Fuel Mixtures used ...
Dynamic motion of lean swirling premixed flame generated by ...
Scalar Characteristics of a Premixed Turbulent Stagnating Flame ...
large eddy simulation of turbulent premixed flames propagation in a ...
Dynamically thickened flame LES model for premixed and non ...
large eddy simulation of turbulent premixed flames propagation in a ...
Soot Diagnostics for Non-Premixed and Partial Premixed Flames
Low Mach Number Simulation of Turbulent Premixed Combustion
towards an extension of tfc model of premixed turbulent combustion