S.S. Shy

Simulation of turbulent burning velocities using aqueous autocatalytic reactions in a near-homogeneous turbulence

Abstract In 1992, Ronney, Shy and coworkers introduced experimentally an aqueous autocatalytic reaction system to simulate premixed turbulent combustion in a well-know Taylor-Couette (TC) flow field. This chemical system can produce self-propagating fronts with characteristics which more closely match those assumed by some current theories of turbulent combustion than do gaseous flames, so that these fronts may be used to assess the viability of models of turbulent combustion. In this work, values of the simulated turbulent burning velocities (ST) were measured in a newly designed, fully three-dimensional and nearly homogeneous turbulent flow field that was generated by a pair of concurrently vertically vibrated grids in a chemical tank. Effects of flow velocity spectra were obtained from laser doppler velocimetry. Visualizations of the internal structure of propagating fronts were detected by laser-induced fluorescence of a pH indicator. It was found that there are three transitions of the front propagation rates U T (≡ S T S L ) in terms of a normalized turbulent intensity U (≡ u′ S L ) and a turbulent Karlovitz number (Ka) and Reynolds number (Re). Markstein numbers are probably close to unity. (1) There is a nearly linear increase of UT with U only when Ka

DIRECT AND INDIRECT MEASUREMENTS OF FLAME SURFACE DENSITY, ORIENTATION, AND CURVATURE FOR PREMIXED TURBULENT COMBUSTION MODELING IN A CRUCIFORM BURNER

This paper analyzes experimentally the Bray–Moss–Libby (BML) model and the flame surface density (R) transport equation using premixed flames propagating through isotropic turbulence in a new cruciform burner and, thus, makes the analogy in both cases for the first time. The burner consists of a long vertical vessel that provides a downward propagating, lean premixed C3H8/air flame and a horizontal vessel. The latter is equipped with a pair of counter-rotating fans and perforated plates at each end to generate nearisotropic turbulence between two perforated plates. Visualization of turbulent flame fronts is obtained from high-speed laser sheet imaging. Several hundred runs at the same experimental conditions are carried out to obtain sufficient images in the central uniform region that are then processed to extract flame wrinkling

Experimental analysis of flame surface density modeling for premixed turbulent combustion using aqueous autocatalytic reactions

In the flamelet framework for premixed turbulent combustion, a transport equation for the flame surface density, commonly known as the Σ-equation, can be formulated but requires closure assumptions. We have applied an aqueous autocatalytic reaction, which produces a self-propagating chemical surface (liquid flame) with characteristics closely matching many of those assumed by flamelet models to extract full spatial statistics relating to the Bray-Moss-Libby model. The present work reports, for the first time, measurements of some unclosed terms in the Σ-equation using liquid flames in a nearly isotropic turbulent flow field. The three-dimensional form of nearly stationary isotropic turbulence was generated near the core region between a pair of vibrating grids in a chemical tank, as verified by laser-Doppler velocimetry. Visualization of propagating surfaces is via a high-speed, successive planar chemically reacting, laser-induced fluorescence technique to extract flame surface density, vector normal to the front, and curvature. Unlike gaseous flames, liquid flames with essentially constant laminar propagating (burning) velocity are approximately free of thermal expansion and heat loss effects and thus may be useful for developing a very basic understanding of the interrelationship between production by hydrodynamic straining and destruction by propagating effects in the Σ-equation, relevant to flamelet models. It is found that the propagation term is negligible and the curvature term has three different modes across the turbulent front brush, respectively (1) mainly negative, (2) positive/negative: production at the reactant side/destruction at the product side, and (3) mainly positive. The first two modes constitute nearly 90% of all possible modes found for a typical aqueous propagating front, indicating that the curvature behavior is more complicated than that generally assumed by flamelet models (mode 1 only) and that of direct numerical simulation and gaseous V flame results (mode 2 only). Two simple schemes are included to explain these results. Finally, measurements of the total propagating surface area production (flame stretching) suggest that the collisions or reconnections of flamelets may be important for the coexistence of these three different curvature modes.

Spatially resolved flamelet statistics for reaction rate modeling using premixed methane-air flames in a near-homogeneous turbulence

Abstract Flamelet models have been widely applied to predict premixed turbulent combustion, such as for instance, the Bray-Moss-Libby (BML) model in which spatial flamelet statistics and, thus, mean reaction rate were deduced from a mean reaction progress variable ( c ) and a mean crossing frequency. Recently, Shy et al. introduced a methodology based upon a downward propagating premixed flame through a near-isotropic turbulent flow field in a cruciform burner with a pair of specially designed ion probes for quantitative measurements of turbulent burning velocities. In this work, we report detailed measurements of important spatial statistical properties of these propagating turbulent methane-air flames for experimental analysis of the BML model using high-speed laser sheet tomography technique. Four cases are studied, including both lean and rich conditions, with equivalence ratio φ = 0.9 and 1.2, and two different turbulent intensities u ′/ S L ≈ 1.4 and 4.1 where S L is the laminar burning velocity. Each case contains up to five hundred runs at the same experimental conditions, so that sufficient images in the central near-isotropic region can be obtained to extract contours of reaction progress variable ( c ), flamelet crossing lengths, crossing frequencies, flame wrinkling lengths ( L y ), flamelet crossing angles (θ), coefficient g in the BML model, and flame surface density (Σ). The symmetric profile of flamelet crossing frequency ν y as a function of c is found for diffusionally stable flames, where the maximum value of ν y occurs at c = 0.5. For diffusionally unstable flames, the profile of ν y tends to be asymmetric (skewed to the burned side), revealing the effect of Lewis number on ν y . It is found that L y , evaluated along contours of c , is almost constant for all values of c . Its magnitude decreases with increasing turbulent intensities and is much smaller than the integral length scale in the unreacted turbulent flow. As Lewis number is varied, values of L y for diffusionally unstable flames are larger than that for diffusionally stable flame. These results differ from those obtained with Bunsen flames and liquid flames, indicating that the BML model needs a precise closure for L y . The overall mean cosine value of θ (= σ y ) is measured to be 0.61 for u ′/ S L ≈ 1.4 and 0.67 for u ′/ S L ≈ 4.1, in contrast to 0.5 found for Bunsen flames but very close to 0.65 measured in liquid flames, suggesting that σ y is probably not a universal constant as assumed by the BML model. The coefficient g is found to be better described by an exponential relationship ( g = 2) than a gamma-two relationship ( g = 1), a result consistent with previous Bunsen flame measurements. Other quantities of interest, such as crossing frequencies, auto-correlations of c , and distributions of actual crossing angle along c contours, are also examined. These results may be used to improve the BML model.

Laboratory Simulation of Flamelet and Distributed Models for Premixed Turbulent Combustion Using Aqueous Autocatalytic Reactions

Abstract Flamelet models have been widely applied to predict premixed turbulent combustion because of their simplicity for the description of chemical features in a turbulent flow field. We had used an aqueous autocatalytic reaction which produced an irreversibly propagating front with characteristics closely matching many of those assumed by flamelet models, to simulate premixed turbulent combustion. We then studied experimentally the influence of turbulence on this reaction-diffusion propagating front. The turbulence was generated by a pair of vertically vibrating grids in a chemical tank and was found to be nearly stationary and isotropic in the core region between the two grids, as verified by laser Doppler velocimetry. Visualization of these turbulent propagating fronts in the nearly isotropic region was obtained using the chemically reacting, laser-induced fluorescence (LIF) technique. In this paper, these planar LIF images were then processed to extract the mean reaction progress variable (c¯), the...

More on Minimum Ignition Energy Transition for Lean Premixed Turbulent Methane Combustion in Flamelet and Distributed Regimes

At the 31st Combustion Symposium, Shy et al. found a transition on minimum ignition energy (MIE) of methane-air mixtures at the equivalence ratio φ = 0.6 in intense isotropic turbulence, where ignition energies of a spark-electrode was quantitatively measured by an energy-adjustable high-power pulse ignition system. Using the same methodology, this paper presents for the first time two new MIE data sets at φ = 0.7 and 0.8 over a wide range of turbulent intensities. It is found that MIE transition due to different modes of turbulent combustion depends on a turbulent Karlovitz number (Ka) indicating the time ratio between chemical reaction and turbulence, for which MIE first increases gradually with Ka and then increases drastically when Ka > K a c ≈ 4 ∼ 9 depending also on φ. The effect of the electrode gap on ignition energies and turbulence influence to centrally-ignited, outwardly propagating flames are also discussed.

Numerical Simulations of Premixed Flame Ignition in Turbulent Flow

The ignition process in a prescribed flow field was investigated in the frame of a 2D thermal-diffusion model. It is assumed that the heat release in the course of chemical reaction has no influence on flow. The latter is obtained by a hydrodynamic model without taking into account chemical reactions. Initial conditions were represented by a fixed size square domain δ filled with hot combustion products with constant temperature. If the flame is ignited at a chosen minimum value of initial temperature this value is referred to as the ignition temperature Tign for a given size of the hot domain. The minimum ignition energy is determined as a product of ignition temperature and square domain size Eign = Tign δ2. The dependencies of minimum ignition energy on characteristics of time-independent, space-periodic flow field are obtained. The ignition process in the flow field, which is pre-calculated in the frame of the two-dimensional Euler equation for freely decaying turbulence, is also studied. It is found that theoretical results obtained within thermal-diffusion models allow to explain qualitatively some experimental results on the ignition in the turbulent flow.

Does Density Ratio Significantly Affect Turbulent Flame Speed

In order to experimentally study whether or not the density ratio σ substantially affects flame displacement speed at low and moderate turbulent intensities, two stoichiometric methane/oxygen/nitrogen mixtures characterized by the same laminar flame speed SL = 0.36 m/s, but substantially different σ were designed using (i) preheating from Tu = 298 to 423 K in order to increase SL, but to decrease σ, and (ii) dilution with nitrogen in order to further decrease σ and to reduce SL back to the initial value. As a result, the density ratio was reduced from 7.52 to 4.95. In both reference and preheated/diluted cases, direct images of statistically spherical laminar and turbulent flames that expanded after spark ignition in the center of a large 3D cruciform burner were recorded and processed in order to evaluate the mean flame radius R̄ft

More on Global Quenching of Premixed CH4/Diluent/Air Flames by Intense Near-Isotropic Turbulence

\bar {R}_{f}\left (t \right )

Four-Dimensional Measurements of the Structure of Dissipative Scales in an Aqueous Near-Isotropic Turbulence

and flame displacement speed St=σ−1dR̄fdt