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Show Paper No. 17 A NEW TURBULENT COMBUSTION MODEL BASED ON FLAME SURFACE DENSITY CONCEPT: Application to a Ramjet Combustor M.H. SENNOUN* and A. CHARETTE Universite du Quebec a Chicoutimi 555, bould de VUniversite, Chicoutimi, G7H 2B1, Quebec, Canada ABSTRACT This paper presents a computational study of a two-dimensional steady-state turbulent premixed propane-air flame in a ramjet combustor. The results are obtained with a turbulent combustion model which is based on the flamelet concept and a new description of the source and sink terms (production and destruction) of the flame surface density (FSD) balance equation. Predictions of the flow field structure, temperature, flame surface density, and heat release rate are compared with experimental measurements and other numerical calculations in the case of a ramjet combustor. The qualitative and quantitative agreement between numerical predictions and experimental results indicates that the model proposed in mis paper provides a an adequate description of turbulent premixed reactive flows. INTRODUCTION All practical combustion systems are based on turbulent combustion (aircraft engines, rockets, piston engines, industrial burners and furnaces, etc). Turbulent combustion modelling is of central importance in several applications and is recognized as one of the hardest and most challenging problems known in the physical sciences. Initial models of turbulent combustion have relied on the idea that chemistry is fast with respect to mixing. On mis basis, Spalding [21] devised the Eddy-Break-Up model in which the mean reaction rate is given in terms of a typical turbulence time and of the local mean square fluctuation of the fuel. This basic model has been used extensively because it is simplicity. It has, however, a limited range of validity and cannot reproduce many of the uends observed in the experiments. Another class of models for the chemistry/turbulence interaction is based upon probabilistic representations, by using pdf s (probability density function) or by solving pdf evolution equations wich themselves require special assumptions in order to effect closure. This approach is fraught with major uncertainties and computational burdens, and is presendy impractical for applications when complex chemistry is to be considered. This approach has been most notably developed by Bilger [1], Bray and Moss [4], Borghi [3], Pope [191 and Kollman [10]. Our primary emphasis, however, will be on the third class of models which are those based upon the flamelet hypothesis, the notion that the turbulent combustion field can be described in terms of an array of laminar elements or flamelets which collectively constitute a turbulent flame. These elements are transported, suained and wrinkled by the turbulent flow but retain a basic structure that is, according to the hypothesis, uniquely related to that of an appropriate laminar reference flame. There are several approaches to the flamelet description. However they all share the following ingredients : 1) A laminar flamelet submodel providing the structure and properties of the suained reactive elements; 2) A description of the turbulent flow comprising mass average equations describing the mean flow variables and the main species mass fractions and relevant closure equations; 3) A set of rules which couple the flamelet submodel to the turbulent flow description. In the Coherent Flame Model (CFM) (Marble and Broadwell [12] and Candel et al. [6]), a balance equation for the flame surface density (FSD), accounting for transport, diffusion, production and destruction of flame area, plays the role of an interface between the local flamelet model and the global mean turbulent flow model. In certain cases a simplified model for the reactive laminar sheets is utilized while in others the structure of the reference flame is accordingly studied by mathematical modelling with a reasonably full treatment of chemical corresponding author. 1 |