Effects of active forcing on non-premixed combustion in coflowing jets

Abstract

Direct Numerical Simulations of the flow field in axisymmetric coannular jet diffusion flames are employed to understand the influences of active forcing techniques on non-premixed combustion phenomena. Simple chemistry governed by finite rate Arrhenius kinetics is used. Attention focuses on the early stages of vortex roll up in the region of flow extending to approximately two jet diameters downstream. Development of reacting flow initial conditions based on Burke-Schumann diffusion flame theory is also presented. INTRODUCTION AND OUTLINE The current study is motivated by a desire to obtain a better physical understanding of active and passive control methods. Several researchers including Ho and Gutmark (1987), Husain and Hussain (1983) and Schadow et al. (1993) have investigated the effects of enhanced combustion with the use of actively forced jets. Considerable attention has focused on the Direct Numerical Simulation (DNS) of axisymmetric shear layers. Sanders et al. (1995) have addressed variable density effects and mixing efficiency in axisymmetric isothermal jets. Hosangadi et al. (1989) and Mahalingam et al. (1990) have examined the effects of buoyancy in combusting jets with simplified chemistry. DNS studies of the Kelvin Helmholtz instability mechanism include the work of Grinstein et al. (1986). A comprehensive study of the effects of variable density and transport coefficients on flow instabilities in * Supported by Office of Naval Research. . tMember AIAA * Senior Member AIAA 2-D turbulent diffusion flames is presented by Yamasbita et al. (1991). Many previous studies have addressed the behavior of reacting shear layers within the context of vorticity dynamics including McMurtry et al. (1989). In most of these investigations the fuel and/or oxidizer jet velocities are perturbed at the inlet using information gathered from a stability analysis of the hydrodynamic flow fields to fix the amplitude and frequency of the imposed perturbations. Overall it is found that the additional heat release due to compressibility effects serve to stabilize the shear layer regions in coflowing jets. In the present study, we examine the effects of active forcing on the non-premixed combustion in 2-D coflowing axisymmetric jets where fuel is in the annulus and hence these stabilizing effects as the flame zone region is moved in relation to the shayer layer position. Direct numerical simulations are used to study the interplay of the flame zone and shear layer in viscous, compressible, reacting flows with simple chemistry. Simple chemistry is used to isolate the effects of non-uniform density caused by chemical reaction on the behavior of the hydrodynamic flow field. Different stoichiometric ratios for one-step global chemistry are investigated to study the location and thickness of the reaction zone in proximity to the shear layer. Derivation of initial conditions for 2-D planar jets based on the Burke-Schumann theory for laminar diffusion flames are outlined in detail. Analysis of vorticity dynamics and overall burn efficiency formulations are employed in conjunction with DNS to provide insight into behavior of the flame zone for several flow fields. Shear layer velocity fields and isolevels of vorticity, fuel mass fraction and reaction rate for two representative cases are presented. An overall fuel burnwhere ing efficiency is developed and computed for the same flow fields. In support of recent experiments (Schadow et al. (1993)),-results are expected to help develop efficient active/passive fuel injection schemes that may be used to tailor the combustion processes in compact waste incinerators. pE = + (5)