Dependence of the Bluff Body Wake Structure on Flame Temperature Ratio

Abstract

This paper describes an experimental investigation of the wake and flame characteristics of a bluff body stabilized flame. Prior investigations have clearly shown that the wake structure is markedly different at "high" and "low" flame density ratios. This paper describes a systematic analysis of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio, ρ u /ρ b , over the range 1.7< ρ u /ρ b <3.4. This paper shows that two fundamentally different flame/flow behaviors are observedcharacterized here as Kelvin-Helmholtz and Von-Karman vortex street dominated -at high and low ρ u /ρ b values, respectively. These are interpreted here as a transition from a convectively to absolutely unstable flow. This transition manifests itself in several ways with decreasing ρ u /ρ b values, including (1) the spectrum of the flame motion and flow field transitions from a distributed to a narrowband peak at St~0.24, and (2) the correlation between the two flame front branches monotonically increases. Finally, the intermittent nature of the flow field is emphasized, with the relative fraction of the two different flow/flame behaviors monotonically varying with ρ u /ρ b .

Convective and Absolute Instabilities in Reacting Bluff Body Wakes

“…Note that the backflow ratio used in this study is related to the Λ parameter in Yu and Monkewitz [21] by 1 1 (4) This leads to the following dispersion relation: 1 For the fuel/air mixtures tested in this study, the density ratio is quite close to the temperature ratio, because the pressure and molecular weight of reactants and products is almost the same. For example, the average molecular weight changes from 27.90 to 27.88 kg/kmol from reactants to products.…”

“…In non-reacting flows, the bluff body wake is absolutely unstable, and characterized by large scale, asymmetric vortex shedding [13] known as the Von Karman vortex street. This instability has a characteristic frequency of [12] D U St f D BVK (1) where St D is the Strouhal number. For circular cylinders, St D is independent of Reynolds number (St D = 0.21) in the turbulent shear layer, laminar boundary layer regime, ~1000<Re D <~200,000 [14].…”

Frequency Locking and Vortex Dynamics of an Acoustically Excited Bluff Body Stabilized Flame

“…Indeed, a variety of prior studies have noted fundamental differences in the dynamic character of the flame and/or flow field at different velocity and fuel/air ratio conditions [27][28] , particularly under near blowoff conditions [29][30][31][32][33][34][35] or in flames utilizing highly preheated reactants 23,29,36 . Erickson et al 23 and Emerson et al [37][38] presented calculated results of flames with various density ratios, showing that a large sinuous flow feature gradually grows in prominence as density ratio across the flame is decreased below values of approximately 2-3. A significant additional observation from the latter study was that the transition to absolute instability did not appear abruptly as the density ratio crossed some threshold value.…”

“…The experimental rig, shown in Figure 4, consists of two premixed, methane-air combustors in series. This facility, the bluff body geometries, and the diagnostics are detailed in previous work [37][38] and so are only briefly described here. The main components are a vitiator, secondary air and fuel inlets, a flow settling section, acoustic drivers, and the test section.…”

“…Conditional sampling was based on sine fits between short duration, time-local chunks of the unsteady centerline vorticity signal at x/D = 3 and a sine wave at the natural frequency; free parameters for the sine fits were amplitude and phase. Such a procedure is outlined in more detail in previous work, where short duration sine fits were used to characterize the behavior of flow intermittency [37][38] . Sampling for ensemble averaging was phase-locked to the phase of these sine fits.…”