Density ratio effects on reacting bluff-body flow field characteristics

The wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio, ρ_uρ_b, over the Reynolds number range of 1000-3300. We show that two fundamentally different flame/flow behaviours are observed at high and low ρ_uρ_b values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given ρ_uρ_b value. Rather, the flow field is highly intermittent in a transitional ρ_uρ_b range, with the relative fraction of the two different flow/flame behaviours monotonically varying with ρ_uρ_b . This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.