Investigation of oscillatory stretch effects on the structure and extinction of counterflow diffusion flames

The effect of oscillatory stretch on laminar opposed jet diffusion flames is investigated both numerically and experimentally. At high excitation frequency, the peak extinction strain rate of the oscillating flame can be extended well beyond the steady-state extinction limits. At moderate frequencies, the OH radical concentrations are measured and compared to the complex chemistry, detailed transport simulations. The OH oscillations in both the experiment and simulation show similar phase delays but the magnitude of the OH variation in the experiments is greater. At moderate frequencies, the time dependent OH variation is quasi-steady where the time dependent flame can be described by a series of steady-state flames. However, at low strain rates, the time-dependent flame does not quite recover to its steady-state value. INTRODUCTION Many experimental investigations have studied the effects of steady stretch on nonpremixed reaction zones. However, in practical combustion devices the aerodynamic stretch imposed on reaction zones is seldom steady. The imposed stretch is constantly changing with time due to fluctuations or turbulence in the flowfield. If the time scales associated with changes in the flow field are on the order of the kinetic and transport times, flames can no longer be considered to be steady. Thus, simulations of these reaction zones calculated with steady-state assumptions may be inadequate to describe the flame structure and emission characteristics. * Graduate Student, Student Member AIAA t Professor, Senior Member AIAA * Research Staff, Member AIAA Copyright © 1997 by R.W. Pitz. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Many analytical and computational efforts have been made to investigate the effects of time dependent stretch on flames (Clarke and Stegen, 1968; Carrier et al., 1975; Saitoh and Otsuka, 1976; Haworth et al., 1988; Maup et al., 1990; Stahl and Warnatz, 1991; Cetegen and Bogue, 1991; Darabiha, 1992; Ghoniem et al., 1992; Barlow and Chen, 1992; Egolfopoulos, 1994a, b; Egolfopoulos and Campbell, 1996; Im et al., 1995; and Petrov and Ghoniem, 1995). Darabiha's calculations show that as the frequency of a sinusoidal strain rate oscillation increases the flame response decreases until the flame no longer responds (Darabiha, 1992). This suggests that over small time intervals the flame can be aerodynamically stretched beyond the steady-state extinction limit, in effect making the flame appear stronger than steadystate ID extinction calculations suggest. Moderate oscillation frequencies (50 Hz) were also investigated and show deviations from the steady-state solutions (peak product concentrations and peak temperature) for the perturbed strain rate extremes. In addition, delays are observed between the imposed stretch and flame structure response. The general flame response time is determined to be on the order of the inverse of the mean strain rate (Darabiha, 1992). Ghoniem et al. (1995) model oscillatory stretched flames utilizing one step chemistry and unity Lewis number (equi-diffusion). Numerical results concerning oscillatory stretch induced extinction are of specific interest for the current investigation. Ghoniem et al. show that, under oscillatory stretch, diffusion flames can exist with peak strain rates beyond the steady-state extinction limit. Increasing the frequency of oscillation allows the peak strain rate to be extended further beyond the steady-state extinction limit. Egolfopoulos and Campbell (1996) have also investigated the effects of oscillatory stretch on diffusion flame structure and extinction using detailed transport and chemistry. Flame structure investigations indicate that at low frequencies the flame response is quasisteady. As frequency is increased, the amplitudes of the induced structural oscillations are reduced and delayed relative to the imposed strain rate. Finally, at high frequencies the flame fails to respond to strain rate oscillations of the external flow field. The phenomena of extinction delay, partial extinction and re-ignition were also investigated. Counterflow flame calculations indicate that at certain strain rates and frequencies, the reaction can be partially extinguished and re-ignited as the strain rate is relieved. Partial extinction can result in reduced H radical concentration, weakening the flame over several cycles and eventually causing global extinction. The first effort to experimentally investigate the effects of time dependent stretch on diffusion flames was conducted by Saitoh and Otsuka (1976) to compliment their analytical study. Photographs were taken of oscillatory stretched counterflow diffusion flames to quantify the amplitude of flame zone oscillation. The results indicate that observed flame zone oscillation decreases as the frequency of the oscillatory stretch increases. Further experimental investigations of time dependent stretch effects on counterflow diffusion flames have been lacking. Recently, Brown and Pitz (1996) and Kistler et al. (1996) have presented oscillatory stretch induced extinction measurements of diluted CHL|-air diffusion flames. Both efforts (in general agreement with previous numerical studies) indicate that as the frequency of strain rate oscillation increases the flame can be extended further beyond the steady-state extinction limit. DeCroix and Roberts (1996) later presented similar measurements of CRi-air and propane-air diffusion flames. While flame extinction measurements provide valuable information on time dependent blow off limits, "measurements of time dependent flame structures are required for code validation and investigation of structural response times. In this work, the effect of oscillatory stretch on the extinction and structure of an opposed jet flame is investigated. The peak strain rate at extinction is measured at various oscillation amplitudes and frequencies. Using laser induced predissociative fluorescence (LIPF), the OH radical concentration is temporally and spatially resolved in the oscillating flame. The OH measurements are compared to complex chemistry, detailed transport calculations of the oscillating opposed jet flame. COMPUTATIONAL METHODOLOGY The mathematical model and the governing equations of the system employed here follow those of Kee et al. (1988). The unsteady behavior of counter-flowing streams is studied using a second order implicit scheme. Calculations are performed in the axisymmetric configuration at one atmosphere pressure, 300 K upstream temperature, and with plug-flow boundary conditions. The reaction mechanism used in the present study is taken from GPJ-Mech 1.2 (Frenklach et al., 1994), which consists of 32 species and 177 elementary reaction steps, and is specifically developed and optimized for natural gas (CH*, C2Hs) combustion. Identical inphase periodic oscillations, with given frequency (f) and amplitude fraction of the mean (A), were imposed at the two nozzle exits (Vf and Va), and are described by: Vj(t) = VCo + V sin(27tft) = Vto [1+A sin(wt)]; V.(t) = V^ + Va' sin(27tft) = V.,0 [1+A sin(wt)], where Vf;0 and ¥„_„ are respectively the mean exit velocities at fuel and oxidizer nozzles, Vf and Va the amplitudes of the imposed velocity fluctuations, and co=27if the angular frequency of oscillation. A steady-state diffusion flame with velocity boundary conditions of Vj,, and Va,0 is utilized as the initial state upon which oscillation is subsequently applied. EXPERIMENTAL METHODOLOGY Oscillatory OpposedJet Burner The central part of the oscillatory counterflow burner (Fig. 1) is the same burner used in a previous study of steady flames (Trees et al., 1995). The stretched laminar flame is stabilized between two counterflowing jets. Both the air and fuel jet have exit diameters of 24.2 mm and are separated by 12.7 mm. Each jet has a series of screens near the exit to produce a flat top velocity profile. The high aspect ratio (1.9) results in a stable flame that has well defined aerodynamic stretch characteristics. However, the large jet diameter limits investigations to relatively low stretched flame conditions (a < 1600/s) because of the high flow rates required for high stretch cases and the desire to maintain laminar flow in the ducts. Both jets are surrounded by larger ducts. These annular regions serve three purposes. The lower duct supplies a curtain flow of N2 to shroud the flame. The upper duct serves as the exhaust for both the post flame gases and the N2 curtain gas. In addition the upper duct houses a water jacket used to cool the exhaust gases eliminating heat transfer to the inner jet flow. During experiments the burner is run with the air jet (with water jacket) on the top. The global aerodynamic stretch or axial strain rate of the counterflow jet flame is given by,