Anthony P. Hamins

Comparisons of the Soot Volume Fraction Using Gravimetric and Light Extinction Techniques

Abstract Simultaneous optical and gravimetric measurements were performed in the postflame region of an acetylene/ air premixed flame where the temperature of the soot/gas mixture was reduced to 500 K through nitrogen dilution. By combining gravimetric measurements of the collected soot with soot density measurements using helium pyenometry, an accurate value of the soot volume fraction was obtained. The temperature and soot concentration profiles were measured to compare the line of sight light extinction measurement with the point sampling gravimetric measurements. The soot volume fraction obtained by light extinction measurements overestimated the actual soot volume fraction by about a factor of two. By calibrating the optical measurements with the gravimetric soot volume fractions, a dimensionless extinction coefficient, K e , of 8.6 was measured. This value is conjectured to be applicable for soot generated for a variety of fuels and to be valid for extinction wavelengths in the visible to the near-infrared. It was also found that the mass specific light extinction coefficient was found to be 8.0 m 2 /g which is consistent with measurements reported in the literature for a variety of fuels.

An experimental investigation of the pulsation frequency of flames

Measurements of the pulsation frequency in non-premixed flames were conducted for gaseous and liquid fuels. Measurements were performed over a wide range of Froude number (≈10−4 to ≈103), Reynolds number (≈10 to ≈103) and burner diameter (0.0074 m to 0.30 m). The fuel velocity at the burner exit had a weak influence on the pulsation frequency for some diameters. The Strouhal number plotted as a function of the inverse Froude number was shown to correlate the measurements determined here as well as measurements reported in the literature for pulsations in flames burning gaseous, liquid and solid fuels over 14 orders of magnitude in Froude number. Previous measurements of the effect of enhanced gravitational level on the pulsation frequency were also interpreted in terms of the Strouhal number—Froude number relationship. Stability limits in flames were investigated. The critical fuel velocity needed to initiate pulsations was measured as a function of burner diameter for methane and propane flames. Flow visualization was used to measure the pulsation frequency of an isothermal buoyant gas stream over a range of Froude number (10−3 to 1). A plot of the Strouhal number as a function of the inverse Froude number also correlated the measurements, but yielded a different power law exponent than the reacting flow case.

Influence of CF3I, CF3Br, and CF3H on the high-temperature combustion of methane☆

Abstract The effects of a number of flame retardants (CF 3 I, CF 3 Br, and CF 3 H) on the high-temperature reactions of methane with air in a plug flow reactor are studied by numerical simulations using the Sandia Chemkin Code. 1 The dependence of (a) the ignition delay and (b) time for substantially complete reaction as a function of temperature and additive concentrations are calculated. In agreement with experiments, the ignition delay can be increased or decreased by the addition of retardants. The reaction time is always increased by additives. The mechanism for these effects has been examined. It is concluded that the ignition delay is controlled by the initial retardant decomposition kinetics, which releases active species into the system. These species can either terminate or initiate chains. The reaction time is largely a function of the concentrations of the active radicals H, OH, and O that are formed during the combustion process. It is shown that their concentrations, particularly those of H atoms, are lowered in the presence of the retardants. We find that the chemical mechanism governing reaction time is very similar to that which controls the flame velocity and a correlation between decreases in flame velocity and H-atom concentration is demonstrated. The calculations suggest that relative reaction time and H-atom concentrations should be effective measures for the estimation of retardant effectiveness.

Inhibition effectiveness of halogenated compounds

Abstract A numerical study of the inhibition efficiency of halogenated compounds was carried out for C 1 - 2 hydrocarbon-air laminar premixed flames. The inhibition efficiency of CF 3 Br, CF 3 I, CF 3 H, C 2 HF 5 , C 2 F 6 , and CF 4 additives was interpreted using an additive group method. In agreement with measurements, the calculated burning velocity decreased exponentially with increasing additive concentration over a wide concentration range. The inhibition parameter Φ proposed by Fristrom and Sawyer indicating inhibition efficiency was modified to take into account the exponential dependence of burning velocity on inhibitor concentration. The inhibition indices for halogen atoms and groups important in the inhibition process were determined for stoichiometric conditions. The physical and chemical effects of the additives were studied. With increasing additive concentration, the chemical influence of an inhibitor saturates and the physical influence increases. Therefore, use of a composite inhibitor composed of a mixture of an effective chemical inhibitor with a high heat capacity diluent may be beneficial. The contribution of physical and chemical components on inhibitor influence are estimated near entinction. A procedure for determination of a regeneration coefficient, which indicates an effective number of catalytic cycles involving inhibitor during the combustion process, is suggested. The regenation coefficient of HBr in stoichiometric methane-air flame with 1% CF 3 Br added is approximately 7.

Heat Feedback to the Fuel Surface in Pool Fires

Abstract A series of measurements designed to investigate the heat feedback in pool fires burning liquid fuels are reported. Such measurements are essential for the development and validation of detailed models which predict the burning rate of liquid hydrocarbons and solid polymers. The radial variation of the local radiative and local net heat flux incident on the surface of 0.30 m diameter pool fires were measured. A water-cooled, nitrogen purged, narrow view-angle gauge was developed to measure the radiative flux incident on the fuel surface. Measurements of the mass burning rate in a burner composed of annular rings was used to estimate the local heat feedback. A number of different fuels were studied, yielding flames with a wide range of heat release rates and luminosities. Consideration of the heat balance for a control volume enclosing the liquid PPOI indicated that radiation was an important component of the heat feedback for non-luminous fires and a dominant component in luminous fires.

Extinction of nonpremixed flames with halogenated fire suppressants

Abstract An experimental, analytical, and numerical study was performed to elucidate the influence of eleven gaseous agents, considered to be substitutes for CF3Br, on the structure and critical conditions of extinction of diffusion flames burning liquid hydrocarbon fuels. The effectiveness of these agents in quenching flames was compared to those of CF3Br and an inert diluent such as nitrogen. Experiments were performed on diffusion flames stabilized in the counterflowing as well as in the coflowing configuration. The fuels tested were heptane in the counterflowing configuration, and heptane, the jet fuels JP-8, and JP-5, and hydraulic fluids (military specifications 5606 and 83282) in the coflowing configuration. The oxidizing gas was a mixture of air and the agent. On a mass and mole basis CF3Br was found to be most effective in quenching the flames and the mass-based effectiveness of the other eleven agents was found to be nearly the same as that of nitrogen. Experimental results were interpreted using one-step, activation-energy asymptotic theories and the results were used to provide a rough indication of the thermal and chemical influence of these agents on the flame structure. To understand in some detail the influence of CF3Br on the structure and mechanisms of extinction of the flame, numerical calculations using detailed chemistry were performed. The calculated structure of counterflow heptane-air diffusion flames inhibited with CF3Br was found to consist of three distinct zones including a CF3Br consumption zone which appears to act as a sink for radicals. The calculated values of the critical conditions of extinction of counterflow heptane-air diffusion flames inhibited with CF3Br were found to agree fairly well with measurements. The study suggests the need for refinement of the inhibition chemistry.

Simultaneous optical measurement of soot volume fraction and temperature in premixed flames

The performance of a three-wavelength optical probe technique for measuring soot volume fraction and temperature was assessed by conducting experiments in the homogeneous environment of a premixed flame. Using a premixed ethylene/air flame, the temperatures and soot volume fractions (fva, based on absorption measurements at 633 nm and fve, based on emission measurements at 900 nm and 1000 nm) were compared with previously reported results. Although the temperatures and mean soot volume fractions compared favorably, the discrepancy between fva and fve prompted new measurements to evaluate the importance of source wavelength on the fva measurements, scattering by soot particles, light absorption by “large” molecules and the use of different indices of refraction reported in the literature. The experiments on the degree of soot scattering and light absorption by “large” molecules indicated that these effects cannot reconcile the observed discrepancy in the soot volume fractions. The measured soot volume fractions were, however, sensitive to the absorption constant and therefore varied significantly when different sets of refractive indices were used. Furthermore, the agreement between fva and fve was improved when extinction measurements were performed with longer wavelength light sources. Isokinetic soot sampling experiments were also performed to compare with the optically-measured soot volume fractions. This technique does not rely on the refractive indices of soot and therefore provides an independent measure of the soot volume fraction. The soot volume fractions measured using this technique compared favorably with the optically measured values (calculated using various indices of refraction).

The Effect Of Sample Size On The Heat Release Rate Of Charring Materials

The burning of a horizontal wood slab situated atop an insulating substrate was modeled using three coupled submodels for the gas-phase, wood, and substrate processes. A global analytical model was used to determine the radiative and convective heat feedback from the gas-phase combustion to the wood surface. The char-fonning wood model was a one-dimensional numerical computation of the density change as a function of position and time. The backside boundary condition of the wood was treated as conductive heat loss into a substrate material modeled by the heat conduction equation. The condensed-phase model results were tested by exposing Douglas Fir samples to an external flux in a nitrogen environment (no combustion). Heat release rate calculations are compared to experimental results for Douglas Fir samples of 0.1 m and 0.6 m diameter. Both theory and experiments show that, for the conditions studied, the heat release rate is nearly independent of the specimen diameter except for the initial peak and the affect of this peak on the first portion of the quasi-steady settling period. Model predictions also indicate that the second peak, which follows the settling period, is very sensitive to the thickness of the insulating substrate.

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s 1 . The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N2 ,C O 2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N 2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 0.01 for N2, 0.773 0.009 for CO2, and 0.437 0.005 for CF3Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 0.004 for N2, 0.187 0.002 for CO2, and 0.043 0.001 for CF3Br.

Numerical Modeling of Pool Fires Using LES and Finite Volume Method for Radiation

The thermal environment in small and moderate-scale pool flames is studied by Large Eddy Simulation and the Finite Volume Method for radiative transport. The spectral dependence of the local absorption coefficient is represented using a simple wide band model. The predicted radiative heat fluxes from methane/natural gas flames as well as methanol pool burning rates and flame temperatures are compared with measurements. The model can qualitatively predict the pool size dependence of the burning rate, but the accuracy of the radiation predictions is strongly affected by even small errors in prediction of the gas phase temperature.