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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Flame stability in combusting turbulent jets

A simplified theoretical model for turbulent methane/air flames from a subscale power plant burner is developed and applied to predict the stability of flames as a function of (1) ambient atmosphere temperature and composition, (2) initial preheat temperature, and (3) percentage of external product gas recirculation. The flame model equates the rate at which mass is supplied per unit volume to the rate it is chemically reacted per unit volume using a one-step reaction and a perfectly stirred reactor (PSR). The model contains a free parameter which is evaluated by comparing the theoretical results to experimental results available in the literature. The stability results show that premixed, turbulent flames are stable over a wide equivalence ratio range if the ambient gas is air; however, if combustion products are entrained, the stability is sharply reduced.

Nelson, H. F.↗
NASA NTRS1976

Flame stability in combusting turbulent jets

The paper develops a model for the stability of turbulent methane/air flames on the assumption that reaction kinetics is much more important than the fluid dynamics near blowout. Flame stability is predicted as a function of ambient atmospheric temperature and composition, initial preheat temperature, and the percentage of external product gas recirculation. A one-step reaction and a well-stirred reactor are used to equate the rate at which the mass is supplied to the flame to the rate at which the mass chemically reacts in the flame.

Nelson, H. F.↗
NASA NTRS1977

Further calculations based upon a theory of flame spread across solid fuels

A theory of flame spread above solid fuel beds of finite or infinite thickness is considered. Heat transfer ahead of the flame is allowed through both the solid and gas phases. The simplifying assumption of a surface reaction is made. Temperature profiles in both phases, concentration profiles in the gas phase, heat transfer at the surface, and flame spreading rates are determined. It is found that chemical kinetic parameters together with transport properties strongly influence the spreading rate. Heat transfer through the gas phase is the dominant mechanism for energy transfer ahead of the spreading flame over a wide range of Peclet numbers for the gas and solid phases.

Feng, C. C.↗
NASA NTRS1978

Flame retardant spandex type polyurethanes

Flame retardant elastomeric compositions were developed, comprised of: (1) spandex type polyurethane having incorporated into the polymer chain, halogen containing polyols; (2) conventional spandex type polyurethanes in physical admixture flame retardant additives; and (3) fluoroelastomeric resins in physical admixture with flame retardant additives. Methods of preparing fibers of the flame retardant elastomeric materials are presented and articles of manufacture comprised of the elastomeric materials are mentioned.

Howarth, J. T.↗
NASA NTRS1978

Process for spinning flame retardant elastomeric compositions

Flame retardant elastomeric compositions comprised of either spandex type polyurethane having halogen containing polyols incorporated into the polymer chain, conventional spandex type polyurethanes in physical admixture with flame retardant additives, or fluoroelastomeric resins in physical admixture with flame retardant additives were developed. Methods are described for preparing fibers of the flame retardant elastomeric materials and manufactured articles as well as nonelastic materials such as polybenzimidazoles, fiberglass, and nylons, for high oxygen environments.

Howarth, J. T.↗
NASA NTRS1977

Model for premixed turbulent flames

Flame speed and mean degree of reaction profiles are determined for an ensemble mean one-dimensional premixed turbulent flame. Elementary closure models are employed to treat turbulence closure problems arising from the nonlinear convective and chemical reaction rate terms in the time-dependent fuel conservation equation; a constant eddy diffusivity is useed to model correlation terms derived from the convective term. An expression developed for fuel consumption is solved to obtain the turbulent flame speed and the variation of mean fuel mass fraction across the flame. Comparison of the numerical analysis with experimental data indicates the importance of the quenching of the combustion reaction in high-wavenumber eddies.

Gouldin, F. C.↗
NASA NTRS1979

Silicon halide-alkali metal flames as a source of solar grade silicon

The feasibility of using alkali metal-silicon halide diffusion flames to produce solar-grade silicon in large quantities and at low cost is demonstrated. Prior work shows that these flames are stable and that relatively high purity silicon can be produced using Na + SiCl4 flames. Silicon of similar purity is obtained from Na + SiF4 flames although yields are lower and product separation and collection are less thermochemically favored. Continuous separation of silicon from the byproduct alkali salt was demonstrated in a heated graphite reactor. The process was scaled up to reduce heat losses and to produce larger samples of silicon. Reagent delivery systems, scaled by a factor of 25, were built and operated at a production rate of 0.5 kg Si/h. Very rapid reactor heating rates are observed with wall temperatures reaching greater than 2000 K. Heat release parameters were measured using a cooled stainless steel reactor tube. A new reactor was designed.

Olsen, D. B.↗
NASA NTRS1978

A theoretical study of a laminar diffusion flame

Theoretical models of an axisymmetric laminar diffusion flame are discussed, with an emphasis on the behavior of such flames at increasing pressures. The flame-sheet or Burke-Schumann model (in terms of Bessel functions) and various boundary layer numerical solutions are presented and their results compared with experimental data. The most promising theoretical model combines the numerical flow field solution of the Patankar-Spalding computer code with the Pratt-Wormeck chemical reaction subroutine. The flame shapes for pressures of 1, 5, 10, 20, and 50 atmospheres were computed and agree remarkably well with experimental data. There is a noticeable shape change with pressure, believed to be a result of buoyancy effects. The chemical concentration profiles do not exhibit much dependence on pressure, a reflection of the fact that only one chemical mechanism was utilized at all pressures.

Frair, K. L.↗
NASA NTRS1979

Application of multispectral color photography to flame flow visualization

For flames of short duration and low intensity of radiation a spectroscopical flame diagnostics is difficult. In order to find some other means of extracting information about the flame structure from its radiation, the feasibility of using multispectral color photography was successfully evaluated. Since the flame photographs are close-ups, there is a considerable parallax between the single images, when several cameras are used, and additive color viewing is not possible. Each image must be analyzed individually, it is advisable to use color film in all cameras. One can either use color films of different spectral sensitivities or color films of the same type with different color filters. Sharp cutting filters are recommended.

Stoffers, G.↗
NASA NTRS1980

Pollutant emissions from flat-flame burners at high pressures

Maximum flame temperatures and pollutant emission measurements for NOx, CO, and UHC (unburned hydrocarbons) are reported for premixed methane air flat flames at constant total mass flow rate over the pressure range from 1.9 to 30 atm and for equivalence ratios from 0.84 to 1.12. For any given pressure, maxima typically occur in both the temperature and NOx emissions curves slightly to the lean side of stoichiometric conditions. The UHC emissions show minima at roughly the same equivalence ratios. The CO emissions, however, increase continually with increasing equivalence ratio. Flame temperature and NOx emissions decrease with increasing pressure, while the opposite is true for the CO and UHC emissions. The NOx data correlate reasonably well as a function of flame temperature only. Four flameholders, differing only slightly, were used. In general, the temperature and emissions data from these four flameholders are similar, but some differences also exist. These differences appear to be related to minor variations in the condition of the flameholder surfaces.

Maahs, H. G.↗
NASA NTRS1980

A comparison of transport algorithms for premixed, laminar steady state flames

The effects of different methods of approximating multispecies transport phenomena in models of premixed, laminar, steady state flames were studied. Five approximation methods that span a wide range of computational complexity were developed. Identical data for individual species properties were used for each method. Each approximation method is employed in the numerical solution of a set of five H2-02-N2 flames. For each flame the computed species and temperature profiles, as well as the computed flame speeds, are found to be very nearly independent of the approximation method used. This does not indicate that transport phenomena are unimportant, but rather that the selection of the input values for the individual species transport properties is more important than the selection of the method used to approximate the multispecies transport. Based on these results, a sixth approximation method was developed that is computationally efficient and provides results extremely close to the most sophisticated and precise method used.

Coffee, T. P.↗
NASA NTRS1982

Time resolved density measurements in premixed turbulent flames

Premixed, turbulent flames are important in connection with investigations of fundamental, turbulent-reacting-flow processes and the study of practical combustion devices, such as spark ignition engines and premixed, prevaporized gas turbine combustors which burn premixed reactants. The considered investigation is concerned with the application of laser induced Rayleigh scattering to measure the gas density in premixed, methane-air flames. A description is provided of the results of density and velocity measurements in an open, lean, premixed methane-air flame stabilized in grid turbulence of low Reynolds number. It is found that where applicable, Rayleigh scattering can be used to good advantage to measure molecular number density. Mean and rms density results show that the mean flame thickens with axial distance but that the maximum in rms does not change appreciably.

Dandekar, K. V.↗
NASA NTRS1982

On stability of premixed flames in stagnation - Point flow

A quantitative description of flame stabilization in stagnation-point flow is proposed. Asymptotic and stability analyses are made for a flame model where the density of the gas is assumed to be constant and the reaction zone is assumed to be narrow and concentrated over the flame front. It is shown that, if blowing is sufficiently strong, the corrugations disappear and a plane flame results. The phenomena cannot be fully described by means of classical linear stability analysis.

Sivashinsky, G. I.↗
NASA NTRS1983

Turbulent flame propagation and combustion in spark ignition engines

Pressure measurements synchronized with high-speed motion-picture records of flame propagation have been made in a transparent-piston engine. The data show that the initial expansion speed of the flame front is close to that of a laminar flame. As the flame expands, its speed rapidly accelerates to a quasi-steady value comparable with that of the turbulent velocity fluctuations in the unburned gas. During the quasi-steady propagation phase, a significant fraction of the gas behind the visible front is unburned. Final burnout of the charge may be approximated by an exponential decay in time. The data have been analyzed in a model-independent way to obtain a set of empirical equations for calculating mass burning rates in spark-ignition engines. The burning equations contain three parameters: the laminar burning speed, a characteristic speed (uT), and a characteristic length (lT). The laminar burning speed is known from laboratory measurements. Tentative correlations relating uT and lT to engine geometry and operating variables have been derived from the engine data.

Beretta, G. P.↗
NASA NTRS1984

Analytic modeling of a spray diffusion flame

A detailed model for a spray diffusion flame is described. The model is based on the boundary layer form of the equations of motion, with droplet transport accounted for using a discretized droplet size distribution function. Interphase transport of mass and energy are accounted for, with a flame-sheet model used to describe the combustion process on a droplet scale. Near dynamic equilibrium is assumed for the description of droplet transport; droplets can diffuse relative to the gas phase. Gas-phase mixing is accounted for using a two-equation turbulence model; buoyancy effects are included, with a temperature fluctuation equation used to account for buoyancy effects on turbulence structure. Thermal radiation from gas-phase CO2 and H2O is included. Gas-phase chemical kinetics are modeled using a 20-reaction, 10-species version of the advanced quasi-global chemical kinetics formulation. Results are compared with data for a vaporizing Freon spray and a pentane spray flame. It is shown that the computational approach provides a reasonably valid picture of the overall development of a spray diffusion flame, and, furthermore, provides a useful tool for the parametric examination of the spray combustion process.

Harsha, P. T.↗
NASA NTRS1985

Behavior of the lean methane-air flame at zero-gravity

A special rig was designed and constructed to be compatible with the NASA Lewis Research Center Airborne Research Laboratory to allow the study of the effect of gravity on the behavior of lean limit in a standard 50.4 mm (2 in.) internal diameter tube when the mixtures are ignited at the open end and propagate towards the closed end of the tube. The lean limit at zero gravity was found to be 5.10% methane and the flame was found to extenguish in a manner previously observed for downward propagating flames at one g. It was observed that g-jitter could be maintained at less than + or 0.04 g on most zero g trajectories. All of propagating lean limit flames were found to be sporadically cellularly unstable at zero g. There was no observable correlation between the occurrence of g-jitter and the lean limit, average propagation speed of the flame through the tube or the occurrence of cellular instability.

Noe, K. A.↗
NASA NTRS1985

Growth of a diffusion flame in the field of a vortex

In the present study of the growth of a diffusion flame in the field of a vortex, the motion in the core is converted into a solid body rotation. The flame extension and distortion kinematics are presented, and the effect of the local flow field on local flame structure is analyzed in detail. The combustion field is found to consist of a totally reacted core region whose radius is time-dependent, and an external flame region which consists of a pair of spiral arms that extend at large radii toward their original positions on the horizontal axis. Two similarity rules are formulated which are independent of kinematic viscosity.

Marble, F. E.↗
NASA NTRS1985

Effects of buoyancy on gas jet diffusion flames - Experiment and theory

Theoretical and experimental research on the effects of buoyancy on gas-jet diffusion flames is described. Part of this research involves an assessment of existing data obtained under reduced-gravity conditions. The results show that uncertainties in the current understanding of flame structure exist and further research is required before reliable predictions of ignition, stabilization, and propagation of flames under microgravity conditions can be made. Steady-state and transient theories have been developed and used in the analysis of existing drop-tower data and new data obtained from a stationary experiment involving inverted flames. The result of this research has led to the definition of a microgravity experiment to be performed in space.

Edelman, R. B.↗