Owen I. Smith

Experimental and numerical studies of sulfur chemistry in H2/O2/SO2 flames☆

Abstract The structure of low pressure (100–150 Torr) rich H 2 O 2 flames doped with SO 2 has been investigated both experimentally and numerically in order to clarify the chemistry of sulfur laden flames. Equivalence ratios from 1.35 to 2.4 have been examined, with peak temperatures between 1800 and 1450K. The basic features of the concentration profiles suggest a two stage flame as most of the hydrogenoxygen chemistry is complete early on (5–7 mm). The sulfur chemistry is, by comparison, continually adjusting itself to changes in temperature and radical concentrations. Numerical simulations accurately predict the general features of these flames, allowing for a detailed description of the important chemistry in various regions of the flame. SO 2 is found to be the dominant sulfur species even in the very rich flames, although H 2 S is by far the most thermodynamically favored species. SO is rapidly formed in the main reaction zone; however, it never achieves partial equilibrium with SO 2 , as has previously been surmised. S 2 is found to be the second most dominant sulfur species and is also not in partial equilibrium. The role of HSO 2 as a channel for radical recombination is shown to be extremely important to the distribution of sulfur species. The calculations also suggest that sulfur chemistry is fast enough to perturb the partial equilibrium of O-atoms in the postflame region. Increasing the initial concentration of SO 2 results in preferential formation of S 2 at the expense of H 2 S. Introduction of H 2 S instead of SO 2 results in a flame which by 6 mm is indistinguishable from its SO 2 doped counterpart.

Mixing enhancement in a lobed injector

An experimental investigation of the non-reactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates streamwise vorticity, producing high strain rates which can enhance the mixing of reactants while delaying ignition in a controlled manner. The lobed injectors examined in the present study consist of two corrugated plates between which a fuel surrogate, CO2, is injected into coflowing air. Acetone is seeded in the CO2 supply as a fuel marker. Comparison of two alternative lobed injector geometries is made with a straight fuel injector to determine net differences in mixing and strain fields due to streamwise vorticity generation. Planar laser-induced fluorescence (PLIF) of the seeded acetone yields two-dimensional images of the scalar concentration field at various downstream locations, from which local mixing and scalar dissipation rates are computed. It is found that the lobed injector geometry can enhance mo...

An experimental study of probe distortions to the structure of one-dimensional flames☆

Abstract In this study we have examined the effect of a molecular-beam mass-spectrometer sampling probe on the structure of a flat H 2 O 2 Ar flame doped with a small amount of HCN. Relative CN concentration and CN rotational temperature fields were obtained by laser-induced fluorescence using the R(0) through R(24) lines of the B-X transition around 385 nm. Measurements were made for several probe positions, both upstream and downstream of the location of peak unperturbed concentration. Reduced CN concentrations were observed within five orifice hydraulic diameters of the probe tip; however, a corresponding perturbation of rotational temperature was not. Suction at the probe orifice did not measurably effect either result, indicating that residence time perturbations do not play a major role. Perturbations of the CN concentration field are discussed in terms of catalyzed recombination of the major flame radicals on the probe surface, coupled with diffusive transport and gas phase chemistry applicable to the CN radical.

TRANSVERSE GAS JET INJECTION BEHIND A REARWARD-FACING STEP

An experimental and theoretical study of the behavior of a transverse gas jet injected behind a rearward-facing step in supersonic flow is described. In-flight iodine planar laser-induced fluorescence imaging was performed to study this flowfield at NASA Dryden Flight Research Center using a flight test fixture situated under the fuselage of an F-104G aircraft. Jet structure and penetration data were obtained, in addition to limited characteristics for jet mixing and for supersonic flow over a rearward-facing step in a flow regime and for injection locations not extensively studied in the past. A model for jet behavior is described that places emphasis on the dynamics of the vortical structures observed to dominate jet cross section. The complex flowfield formed downstream of the step is represented by a combination of empirical and analytical correlations. Model predictions for jet trajectories compare reasonably well with results from the present in-flight experiments as well as with prior wind-tunnel experimental data.

Acoustics of a low-speed dump combustor☆

Abstract The acoustic behavior of a dump combustor operating at very low Mach numbers is explored experimentally and analytically. Two dominant acoustic modes are identified that strongly influence the combustion process in the device: (1) a low-frequency “chugging” mode that is associated with relatively low equivalence ratios (

The Sulfur Catalyzed Recombination of Atomic Oxygen in a CO/O2/Ar Flame

Abstract Flame structure has been determined for two fuel-lean, premixed laminar CO/O2 Ar flames, one of which contained 1.2 percent SO2 by volume. These flames were fairly “dry” (∼75ppm hydrogen as H2O and H2 impurities), but could be stabilized at 26.5 kPa with 10 percent diluent. Concentration profiles were measured for atomic oxygen and SO2 as well as all major species (CO, O2, CO2) by molecular beam-mass spectrometry. Some elements of the calibration and data reduction procedures are discussed. Temperature profiles were measured with a coated thermocouple. At distances greater than 0.3cm from the flameholder the structure of the SO2 doped flame was found to be consistent with a state of balance between the reactions. The catalytic recombination of O-atoms via reactions (4) and (5) practically eliminated the ∼30 percent O-atom superequilibrium overshoot for the sulfur-free flame. Sulfur addition resulted in a considerable increase in temperature, ranging from 50K. near the flameholder to ∼200K. 1cm do...

Potassium kinetics in heavily seeded atmospheric pressure laminar methane flames

Abstract Hydroxyl radical decay rates were measured in laminar atmospheric pressure CH 4 O 2 N 2 Ar flames (Φ = 0.85-1.1) with and without the addition of potassium (mole fractions up to 3.6 × 10−4). Flames were stabilized on a flat-flame burner shrouded by nitrogen. OH number density profiles were determined from laser absorption at 309.28 nm [A-X, OO Q2(6)]. Potassium profiles were obtained from laser absorption on the 404.53-nm transition. Addition of potassium was observed to accelerate the OH decay rate, with the additive influence being most pronounced at higher equivalence ratios. The influence of K was nonlinear, and increasing seeding levels produced progressively less acceleration of the OH decay rate. The measured potassium atom number density decayed slowly with distance above the burner for fuel-rich conditions but decayed rapidly in lean flames. Potassium reaction mechanisms were tested against the experimental data in a series of numerical simulations, and the flollowing mechnism produced the most satisfactory agreement: K + O 2 + M = KO 2 + M KO 2 + H = KO + OH KO + H 2 O = KOH + OH KOH + H = K + H 2 O K + OH + M = KOH + M Based on a best fit to the experimental data, a rate coefficient for K + OH + M → KOH + M was estimated as 5 × 10−32 cm6 molec−2 s−1 at 2000 K. A two-reaction model suggested by Jensen appears to be a global approximation of the above mechanism. Also, addition of sodium to a Φ = 1.1 flame produced an OH decay profile indistinguishable from that measured with potassium seeding, suggesting similar chemistry for both alkali metals.

Probe-induced distortions in the sampling of one-dimensional flames

A procedure is developed for estimating the influence of a probe on the temperature and residence time distribution of gases sampled from a one-dimensional flame. Estimates are made for stoichiometric methane-oxygen-argon flames at4 and 105 Pa, with particular attention devoted to the influence of the included angle and orifice diameter of the probe. Significant temperature distortions are predicted in the mean temperature of the sampled gas, with the largest distortions corresponding to large cone angles and small orifice diameters. Calculated residence time distributions are found to broaden as the orifice diameter is increased, whereas the mean residence time is essentially unchanged. The residence time distribution is compared with characteristic times for chemical reaction in an effort to characterize composition distributions resulting from homogeneous processes.

Destruction of liquid and gaseous waste surrogates in an acoustically excited dump combustor

Abstract Destruction of gaseous and liquid waste surrogates is studied in a two-dimensional dump combustor configuration. Two different waste surrogates are examined: sulfur hexafluoride, which is injected in the gaseous phase and pyrolyzed at high temperatures, and acetonitrile, which is injected in the liquid phase and can be burned in the presence of high concentrations of oxidizing species. Waste surrogates are injected through movable ceramic plugs into the recirculation zones within the dump combustor cavity. The movable plugs allow the combustor cavity length to be altered, in turn exciting or damping various acoustic modes of the device. Strong coupling among the fluid mechanics, acoustics, and combustion/incineration processes are observed in this device; these processes are representative of dump combustors in general. Among the important observations in this study is that waste destruction is strongly affected by the flame and recirculation zone stability. When the flame is perturbed by large vortical structures corresponding to low-frequency “chugging” oscillations, or when it is chaotically destabilized (while acoustically quiet), the recirculation zones into which waste is injected can be destabilized, and DREs for the surrogate are reduced. When the flame is stabilized under conditions which are acoustically quiet, or when the flame is only slightly wrinkled or disturbed, as under high-frequency mode conditions, the recirculation zones are stable and waste is usually destroyed well. Waste destruction itself, however, is also observed to affect recirculation zone and flame stability in addition to affecting the devices acoustic signature.

Enhancement of Fuel-Nitrogen Oxidation by Fuel-Sulfur in Fuel Rich Flames

Abstract C2H2/O2/Ar and H2/O2/Ar premixed, laminar, one-dimensional flames, seeded with small amounts of fuel-N (C2N2) and fuel-S(SO2) and burning at 1 atm were investigated in order to gather information concerning fuel-S/fuel-N interactions in combustion systems. Temperature and nitric oxide profiles were obtained from the burner surface to 5 cm down-stream (—∼56 ms residence time). Large increases in fuel-NO emissions were observed due to SO2 addition to the premixed gases (up to 60 percent for acetylene and 140 percent for hydrogen flames at equivalence ratios of 1.2). The enhancement of NO emissions is analysed in terms of the possible interactions of sulfurous species with the fuel-nitrogen oxidation mechanism. Results are compared with those obtained previously in methane flames, and found to show reasonable qualitative agreement. The effect of SO2 addition on flame temperature is also discussed in terms of sulfur species equilibrium in the flame and the effect of flame inhibition (via sulfur catal...