In this paper, we experimentally investigate the combustion dynamics in lean premixed flames in a laboratory scale backward-facing step combustor in which flame-vortex driven dynamics are observed. A series of tests was conducted using propane/hydrogen/air mixtures for various mixture compositions at the inlet temperature ranging from 300 K to 500 K and at atmospheric pressure. Pressure measurements and high speed particle image velocimetry (PIV) are used to generate pressure response curves and phase-averaged vorticity and streamlines as well as the instantaneous flame front, respectively, which describe unsteady flame and flow dynamics in each operating regime. This work was motivated in part by our earlier study where we showed that the strained flame consumption speed S c can be used to collapse the pressure response curves over a wide range of operating conditions. In previous studies, the stretch rate at which S c was computed was determined by trial and error. In this study, flame stretch is estimated using the instantaneous flame front and velocity field from the PIV measurement. Independently, we also use computed strained flame speed and the experimental data to determine the characteristic values of stretch rate near the mode transition points at which the flame configuration changes. We show that a common value of the characteristic stretch rate exists across all the flame configurations. The consumption * Corresponding author. 77 Massachusetts Avenue, Room 3-344, Cambridge MA 02139, USA. Fax: +1-617-253-5981. E-mail: ghoniem@mit.edu Preprint submitted to Combustion and Flame April 16, 2013 speed computed at the characteristic stretch rate captures the impact of different operating parameters on the combustor dynamics. These results suggest that the unsteady interactions between the turbulent flow and the flame dynamics can be encapsulated in the characteristic stretch rate, which governs the critical flame speed at the mode transitions and thereby plays an important role in determining the stability characteristics of the combustor.

We investigate the dependence of the recirculation zone (RZ) size and structure on the fuel composition using high-speed particle image velocimetry (PIV) and chemiluminescence measurements for C 3 H 8 /H 2 /air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low equivalence ratios, the time-averaged RZ is comprised of two counter rotating eddies: a primary eddy (PE) between the shear layer and the bottom wall; and a secondary eddy (SE) between the vertical step wall and the PE. The flame stabilizes downstream of the saddle point of the dividing streamline between the two eddies. As equivalence ratio is raised, the flame moves upstream, pushing the saddle point with it and reducing the size of the SE. Higher temperature of the products reduces the velocity gradient in the shear layer and thus the reattachment length. As equivalence ratio approaches a critical value, the saddle point reaches the step and the SE collapses while the flame starts to exhibit periodic flapping motions, suggesting a correlation between the RZ structure and flame anchoring. The overall trend in the flow field is the same as we add hydrogen to the fuel at a fixed equivalence ratio, demonstrating the impact of fuel composition on the flow field. We show that the reattachment lengths (L R ), which are shown to encapsulate the mean RZ structure, measured over a range of fuel * Corresponding author. 77 Massachusetts Avenue, Room 3-344, Cambridge MA 02139, USA. Fax: +1-617-253-5981. E-mail: ghoniem@mit.edu

This paper presents an experimental investigation into mode-transitions observed in a 50-kW, atmospheric pressure, backward-facing step combustor burning lean premixed C 3 H 8 /H 2 fuel mixtures over a range of equivalence ratios, fuel compositions and preheat temperatures.The combustor exhibits distinct acoustic response and dynamic flame shape (collectively referred to as "dynamic modes") depending on the operating conditions. We simultaneously measure the dynamic pressure and flame chemiluminescence to examine the phase between pressure (p ) and heat release fluctuations (q ) in the observed dynamic modes. Results show that the heat release is in phase with the pressure oscillations (θ qp ≈ 0) at the onset of a dynamic mode, while as the operating conditions change within the mode, the phase grows until it reaches a critical value θ qp = θ c , at which the combustor switches to another dynamic mode. According to the classical Rayleigh criterion, this critical phase (θ c ) should be π/2, whereas our data show that the transition occurs well below this value. A linear acoustic energy balance shows that this critical phase marks the point where acoustic losses across the system boundaries equal the energy addition from the combustion process to the acoustic field. Based on the extended Rayleigh criterion in which the acoustic energy fluxes through the system boundaries as well as the typical Rayleigh source term (p q ) are included, we derive an extended Rayleigh index defined as R e = θ qp /θ c , which varies between 0 and 1. * Corresponding author. 77 Massachusetts Avenue, Room 3-344, Cambridge MA 02139, USA. Fax: +1-617-253-5981. E-mail: ghoniem@mit.edu Preprint submitted to Combustion and Flame June 27, 2013 This index, plotted against a density-weighted strained consumption speed, indicates that the impact of the operating parameters on the dynamic mode selection of the combustor collapses onto a family of curves, which quantify the state of the combustor within a dynamic mode.At R e = 0, the combustor enters a mode, and switches to another as R e approaches 1. The results provide a metric for quantifying the instability margins of fuel-flexible combustors operating at a wide range of conditions.

In this paper, we investigate the impact of heat transfer between the flame and the flameholder on the dynamic stability characteristics of a 50-kW backward facing step combustor.We conducted a series of tests where two backward step blocks were used, made of ceramic and stainless steel whose thermal conductivities are 1.06 and 12 W/m/K, respectively. Stability characteristics of the two flame-holder materials were examined using measurements of the dynamic pressure and flame chemiluminescence over a range of operating conditions.Results show that with the ceramic flame-holder, the onset of instability is significantly delayed in time and, for certain operating conditions, disappears altogether, whereas with the higher conductivity material, the combustor becomes increasingly unstable over a range of operating conditions. We explain these trends using the heat flux through the flame-holder and the change in the burning velocity near the step wall. Results suggest a potential approach of using low thermal conductivity material near the flame-holder as passive dynamics suppression methods.

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