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NASA NTRS1987

Combustion rate limits of hydrogen plus hydrocarbon fuel: Air diffusion flames from an opposed jet burner technique

Combustion of H2/hydrocarbon (HC) fuel mixtures may be considered in certain volume-limited supersonic airbreathing propulsion applications. Effects of HC addition to H2 were evaluated, using a recent argon-bathed, coaxial, tubular opposed jet burner (OJB) technique to measure the extinction limits of counterflow diffusion flames. The OJB flames were formed by a laminar jet of (N2 and/or HC)-diluted H2 mixture opposed by a similar jet of air at ambient conditions. The OJB data, derived from respective binary mixtures of H2 and methane, ethylene, or propane HCs, were used to characterize BLOWOFF and RESTORE. BLOWOFF is a sudden breaking of the dish-shaped OJB flame to a stable torus or ring shape, and RESTORE marks sudden restoration of the central flame by radial inward flame propagation. BLOWOFF is a measure of kinetically-limited flame reactivity/speed under highly stretched, but relatively ideal impingement flow conditions. RESTORE measures inward radial flame propagation rate, which is sensitive to ignition processes in the cool central core. It is concluded that relatively small molar amounts of added HC greatly reduce the reactivity characteristics of counterflow hydrogen-air diffusion flames, for ambient initial conditions.

Pellett, Gerald L.↗

NASA NTRS1989

Interaction of a vortex and a premixed flame

The interaction of a vortex structure and a premixed flame is studied. The presence of pressure gradients in the vortex and density gradients in the flame result in a complicated interaction. This interaction has been examined when the flame and vortex are fully coupled and in two special cases where they are decoupled: a frozen flame case and a frozen vortex case. In the frozen flame case the main effect of the flame on the vortex is through the barocline torque term. This has been modeled for high Damkoehler numbers. In the frozen vortex case the main effect, at moderate Damkoehler numbers, is to convect the flame around the vortex. At low Damkoehler numbers, depending on the length scales, pockets of unburned gas can form or the flame structure can be significantly changed. The two frozen cases provide a basis for understanding the full interaction.

Ferziger, Joel H.↗

NASA NTRS1989

An Investigation of Flame Spread over Shallow Liquid Pools in Microgravity and Nonair Environments

Experiments of interest to combustion fundamentals and spacecraft fire safety investigated flame spread of alcohol fuels over shallow, 15 cm diameter pools in a 5.2 sec free-fall, microgravity facility. Results showed that, independent O2 concentration, alcohol fuel, and diluent types, microgravity flame spread rates were nearly identical to those corresponding normal-gravity flames for conditions where the normal gravity flames spread uniformly. This similarity indicated buoyancy-related convection in either phase does not affect flame spread, at least for the physical scale of the experiments. However, microgravity extinction coincided with the onset conditions for pulsating spread in normal gravity, implicating gas phase, buoyant flow as a requirement for pulsating spread. When the atmospheric nitrogen was replaced with argon, the conditions for the onset of normal-gravity pulsating flame spread and microgravity flame extinction were changed, in agreement with the expected lowering of the flash point through the thermal properties of the diluent. Helium-diluted flames, however, showed unexpected results with a shift to apparently higher flash-point temperatures and high normal gravity pulsation amplitudes.

Ross, Howard D.↗

NASA NTRS1991

An investigation of flame spread over shallow liquid pools in microgravity and nonair environments

Experiments of interest to combustion fundamentals and spacecraft fire safety investigated flame spread of alcohol fuels over shallow, 15 cm diameter pools in a 5.2 sec free-fall, microgravity facility. Results showed that, independent O2 concentrations, alcohol fuel, and diluent types, microgravity flame spread rates were nearly identical to those corresponding normal-gravity flames for conditions where the normal gravity flames spread uniformly. This similarity indicated buoyancy-related convection in either phase does not affect flame spread, at least for the physical scale of the experiments. However, microgravity extinction coincided with the onset conditions for pulsating spread in normal gravity, implicating gas phase, buoyant flow as a requirement for pulsating spread. When the atmospheric nitrogen was replaced with argon, the conditions for the onset of normal-gravity pulsating flame spread and microgravity flame extinction were changed, in agreement with the expected lowering of the flash point through the thermal properties of the diluent. Helium-diluted flames, however, showed unexpected results with a shift to apparently higher flash-point temperatures and high normal gravity pulsation amplitudes.

Ross, Howard D.↗

NASA NTRS1993

Structure of confined laminar spray diffusion flames: Numerical investigation

The structure of confined laminar spray diffusion flames is investigated numerically by solving the gas-phase conservation equations for mass species, continuity, momentum, and energy and the liquid-phase equations for droplet position, velocity, size, and temperature. A one-step global reaction scheme along with six equilibrium reactions are employed to model the flame chemistry. Monodisperse as well as polydisperse sprays are considered. The numerical results demonstrate that liquid spray flames substantially differ from gaseous flames in their structure, i.e., temperature, concentration, and velocity fields, shape, and dimensions under the same conditions. Spray flames are predicted to be taller and narrower than their counterpart gaseous ones and their shapes are almost cylindrical. This is in agreement with experimental observations. The numerical computations also show that the use of the equilibrium reactions with the one-step reaction scheme decreases the flame temperature compared to the one-step reaction scheme without the equilibrium reactions and more importantly increases the surface area of the flame zone due to a phenomenon termed 'equilibrium broadening.' The spray flames also possess a finite thickness with minimal overlap of the fuel and oxygen species. A case for which a fuel-mixture consisting of 20 to 80 percent gas-liquid by mass is introduced into the combustor is also investigated and compared with predictions using only gaseous or liquid fuel.

Mawid, M. A.↗

NASA NTRS1993

Measurements of soot formation and hydroxyl concentration in near critical equivalence ratio premixed ethylene flame

The testing and development of existing global and detailed chemical kinetic models for soot formation requires measurements of soot and radical concentrations in flames. A clearer understanding of soot particle inception relies upon the evaluation and refinement of these models in comparison with such measurements. We present measurements of soot formation and hydroxyl (OH) concentration in sequences of flat premixed atmospheric-pressure C2H4/O2/N2 flames and 80-torr C2H4/O2 flames for a unique range of equivalence ratios bracketting the critical equivalence ratio (phi(sub c)) and extending to more heavily sooting conditions. Soot volume fraction and number density profiles are measured using a laser scattering-extinction apparatus capable of resolving a 0.1 percent absorption. Hydroxyl number density profiles are measured using laser-induced fluorescence (LIF) with broadband detection. Temperature profiles are obtained from Rayleigh scattering measurements. The relative volume fraction and number density profiles of the richer sooting flames exhibit the expected trends in soot formation. In near-phi(sub c) visibility sooting flames, particle scattering and extinction are not detected, but an LIF signal due to polycyclic aromatic hydrocarbons (PAH's) can be detected upon excitation with an argon-ion laser. A linear correlation between the argon-ion LIF and the soot volume fraction implies a common mechanistic source for the growth of PAH's and soot particles. The peak OH number density in both the atmospheric and 80-torr flames declines with increasing equivalence ratio, but the profile shape remains unchanged in the transition to sooting, implying that the primary reaction pathways for OH remain unchanged over this transition. Chemical kinetic modeling is demonstrated by comparing predictions using two current reaction mechanisms with the atmospheric flame data. The measured and predicted OH number density profiles show good agreement. The predicted benzene number density profiles correlate with the measured trends in soot formation, although anomalies in the benzene profiles for the richer and cooler sooting flames suggest a need for the inclusion of benzene oxidation reactions.

Inbody, Michael Andrew↗

NASA NTRS1994

Laser induced fluorescence measurements and modeling of nitric oxide in high-pressure premixed flames

Laser-induced fluorescence (LIF) has been applied to the quantitative measurement of nitric oxide (NO) in premixed, laminar, high-pressure flames. Their chemistry was also studied using three current kinetics schemes to determine the predictive capabilities of each mechanism with respect to NO concentrations. The flames studied were low-temperature (1600 less than T less than 1850K) C2H6/O2/N2 and C2H6/O2/N2 flames, and high temperature (2100 less than T less than 2300K) C2H6/O2/N2 flames. Laser-saturated fluorescence (LSF) was initially used to measure the NO concentrations. However, while the excitation transition was well saturated at atmospheric pressure, the fluorescence behavior was basically linear with respect to laser power at pressures above 6 atm. Measurements and calculations demonstrated that the fluorescence quenching rate variation is negligible for LIF measurements of NO at a given pressure. Therefore, linear LIF was used to perform quantitative measurements of NO concentration in these high-pressure flames. The transportability of a calibration factor from one set of flame conditions to another also was investigated by considering changes in the absorption and quenching environment for different flame conditions. The feasibility of performing LIF measurements of (NO) in turbulent flames was studied; the single-shot detection limit was determined to be 2 ppm.

Reisel, John R.↗

NASA NTRS1994

Observations of Methane and Ethylene Diffusion Flames Stabilized Around a Blowing Porous Sphere Under Microgravity Conditions

This paper presents the experimental and theoretical results for expanding methane and ethylene diffusion flames in microgravity. A small porous sphere made from a low-density and low-heat-capacity insulating material was used to uniformly supply fuel at a constant rate to the expanding diffusion flame. A theoretical model which includes soot and gas radiation is formulated but only the problem pertaining to the transient expansion of the flame is solved by assuming constant pressure infinitely fast one-step ideal gas reaction and unity Lewis number. This is a first step toward quantifying the effect of soot and gas radiation on these flames. The theoretically calculated expansion rate is in good agreement with the experimental results. Both experimental and theoretical results show that as the flame radius increases, the flame expansion process becomes diffusion controlled and the flame radius grows as gamma t. Theoretical calculations also show that for a constant fuel mass injection rate a quasi-steady state is developed in the region surrounded by the flame and the mass flow rate at any location inside this region equals the mass injection rate.

Atreya, Arvind↗

NASA NTRS1994

Simulation of a turbulent flame in a channel

The interaction between turbulent premixed flames and channel walls is studied. Combustion is represented by a simple irreversible reaction with a large activation temperature. Feedback to the flowfield is suppressed by invoking a constant density assumption. The effect of wall distance on local and global flame structure is investigated. Quenching distances and maximum wall heat fluxes computed in laminar cases are compared to DNS results. It is found that quenching distances decrease and maximum heat fluxes increase relative to laminar flame values. It is shown that these effects are due to large coherent structures which push flame elements towards to wall. The effect of wall strain is studied in flame-wall interaction in a stagnation line flow; this is used to explain the DNS results. It is also shown that 'remarkable' flame events are produced by interaction with a horseshoe vortex: burnt gases are pushed towards the wall at high speed and induce quenching and high wall heat fluxes while fresh gases are expelled from the wall region and form finger-like structures. Effects of the wall on flame surface density are investigated, and a simple model for flame-wall interaction is proposed; its predictions compare well with the DNS results.

Bruneaux, G.↗

NASA NTRS1994

Stationary premixed flames in spherical and cylindrical geometries

Stationary source-free spherical flames ('flame balls') in premixed combustible gases were studied by employing low-gravity (micro-g) environments in a drop tower and an aircraft flying parabolic trajectories to diminish the impact of buoyancy-induced convective flow. Flame balls were found in all mixture families tested when: (1) the Lewis number Le of the deficient reactant was sufficiently low; and (2) the compositions were sufficiently close to the flammability limits. Probably as a consequence of the reduction in buoyant convection, the flammability limits at micro-g were significantly more dilute than those at Earth gravity; for example, 3.35% H2 vs 4.0% H2 in lean H2-air mixtures. By comparison with analytical and computational models, it is inferred that the phenomenon is probably related to diffusive-thermal effects in low-Le mixtures in conjunction with flame-front curvature and radiative heat losses from the combustion products. The chemical reaction mechanism appears to play no qualitative role. In the aircraft experiments, the gravity levels (approximately equal 10(exp -2)g(sub 0)) were found to cause noticeable motion of flame balls due to buoyancy, which in turn influenced the behavior of flame balls. At these g levels, a new type of transient, nearly cylindrical flame structure, termed 'flame strings,' was observed.

Ronney, P. D.↗

NASA NTRS1994

Temperature Field During Flame Spread over Alcohol Pools: Measurements and Modelling

A principal difference between flame spread over solid fuels and over liquid fuels is, in the latter case, the presence of liquid-phase convection ahead of the leading edge of the flame. The details of the fluid dynamics and heat transfer mechanisms in both the pulsating and uniform flame spread regimes were heavily debated, without resolution, in the 1960s and 1970s; recently, research on flame spread over pools was reinvigorated by the advent of enhanced diagnostic techniques and computational power. Temperature fields in the liquid, which enable determination of the extent of preheating ahead of the flame, were determined previously by the use of thermocouples and repetitive tests, and suggested that the surface temperature does not decrease monotonically ahead of the pulsating flame front, but that there exists a surface temperature valley. Recent predictions support this suggestion. However, others' thermocouple measurements and the recent field measurements using Holographic Interferometry (HI) did not find a similar valley. In this work we examine the temperature field using Rainbow Schlieren Deflectometry (RSD), with a measurement threshold exceeding that of conventional interferometry by a factor of 20:1, for uniform and pulsating flame spread using propanol and butanol as fuels. This technique was not applied before to flame spread over liquid pools, except in some preliminary measurements reported earlier. Noting that HI is sensitive to the refractive index while RSD responds to refractive index gradients, and that these two techniques might therefore be difficult to compare, we utilized a numerical simulation, described below, to predict and compare both types of field for the uniform and pulsating spread regimes. The experimental data also allows a validation of the model at a level of detail greater than has been attempted before.

Miller, Fletcher J.↗

NASA NTRS1996

The effects of complex chemistry on triple flames

The structure, ignition, and stabilization mechanisms for a methanol (CH3OH)-air triple flame are studied using Direct Numerical Simulations (DNS). The methanol (CH3OH)-air triple flame is found to burn with an asymmetric shape due to the different chemical and transport processes characterizing the mixture. The excess fuel, methanol (CH3OH), on the rich premixed flame branch is replaced by more stable fuels CO and H2, which burn at the diffusion flame. On the lean premixed flame side, a higher concentration of O2 leaks through to the diffusion flame. The general structure of the triple point features the contribution of both differential diffusion of radicals and heat. A mixture fraction-temperature phase plane description of the triple flame structure is proposed to highlight some interesting features in partially premixed combustion. The effects of differential diffusion at the triple point add to the contribution of hydrodynamic effects in the stabilization of the triple flame. Differential diffusion effects are measured using two methods: a direct computation using diffusion velocities and an indirect computation based on the difference between the normalized mixture fractions of C and H. The mixture fraction approach does not clearly identify the effects of differential diffusion, in particular at the curved triple point, because of ambiguities in the contribution of carbon and hydrogen atoms' carrying species.

Echekki, T.↗

NASA NTRS1997

Strain-Rate-Free Diffusion Flames: Initiation, Properties, and Quenching

For about a half century, the stabilization of a steady planar deflagration on a heat-sink-type flat-flame burner has been of extraordinary service for the theoretical modeling and diagnostic probing of combusting gaseous mixtures. However, most engineering devices and most unwanted fire involve the burning of initially unmixed reactants. The most vigorous burning of initially separated gaseous fuel and oxidizer is the diffusion flame. In this useful idealization (limiting case), the reactants are converted to product at a mathematically thin interface, so no interpenetration of fuel and oxidizer occurs. This limit is of practical importance because it often characterizes the condition of optimal performance (and sometimes environmentally objectionable operation) of a combustor. A steady planar diffusion flame is most closely approached in the laboratory in the counterflow apparatus. The utility of this simple-strain-rate flow for the modeling and probing of diffusion flames was noted by Pandya and Weinberg 35 years ago, though only in the last decade or so has its use become internationally common place. However, typically, as the strain rate a is reduced below about 20 cm(exp -1), and the diffusion-flame limit (reaction rate much faster than the flow rate) is approached, the burning is observed to become unstable in earth gravity. The advantageous steady planar flow is not available in the diffusion-flame limit in earth gravity. This is unfortunate because the typical spatial scale in a counterflow is (k/a)(sup 1/2), where k denotes a characteristic diffusion coefficient; thus, the length scale becomes large, and the reacting flow is particularly amenable to diagnostic probing, as the diffusion-flame limit is approached. The disruption of planar symmetry is owing the fact that, as the strain rate a decreases, the residence time (l/a) of the throughput in the counterflow burner increases. Observationally, when the residence time exceeds about 50 msec, the inevitably present convective (Rayleigh-Benard) instabilities, associated with hot-under-cold (flame-under-fresh-reactant) stratification of fluid in a gravitational field, have time to grow to finite amplitude during transit of the burner.

Fendell, Francis↗

NASA NTRS1997

Structure and Stability of Burke-Schumann Diffusion Flames

The general goal of this NASA Grant is twofold: to improve our understanding of (1) the influence of buoyancy on the stability and structure of Burke-Schumann type diffusion flames, and (2) the effects of buoyancy on vortex-flame interactions in co-flow diffusion flames. A numerical code with a higher order accuracy for spatial discretization is developed in this project for simulation of time-dependent diffusion flames by Sheu and Sheu and Chen, and an extended reduced mechanism is incorporated for prediction of methane oxidation and NO(x)(NO, NO2, and N2O) formation and emission from methane Burke-Schumann diffusion flame (BSDF) as reported in Sheu, and Sheu and Chen. Initial investigation of vortex and flame interaction within the context of fast chemistry is reported. Experiments are conducted in reduced pressure to study the lift-off and stabilization of methane-fueled BSDF in reduced buoyancy environments due to reduced pressure. Measurements of temperature and species concentrations are made in normal and reduced pressure environments to study the effects of buoyancy on the structure of BSDF, and will be reported in this paper. To study the buoyancy effects on the lift-off and stabilization of methane-fueled jet diffusion flames in coflowing air, a glovebox investigation, Enclosed Laminar Flames (ELF), has been proposed and approved for space-based testing on the fourth United States Microgravity Payload (USMP-4) mission, scheduled for October 1997. A brief description of the ELF investigation is also presented.

Lee, Yong G.↗

NASA NTRS1997

Structure of Microgravity Transitional and Pulsed Jet Diffusion Flames

This paper describes results obtained in a study of pulsed gas jet diffusion flames to better characterize the recently observed vortex/flame interactions in microgravity transitional and turbulent diffusion flames, and to improve the understanding of large-scale structures in corresponding normal-gravity flames. In preparation for a space experiment, tests were conducted in the 5.18-Second Zero-Gravity Facility of the NASA Lewis Research Center. Both unpulsed and pulsed laminar flames were studied and numerical modeling of these flames was carried out for data comparison and model validation. In addition, complementary tests for a series of unpulsed flames were conducted on-board the NASA KC-135 research aircraft. The microgravity transitional and turbulent gas-jet diffusion flames have been observed to be dominated by large-scale disturbances, or structures. These structures first appear intermittently in the flame at Reynolds numbers (based on the cold jet injection properties) of about 2100. With increase in injection Reynolds number, the rate of intermittent disturbances increases until the generation becomes continuous at Reynolds numbers of 3000 and higher. The behavior of these structures depends upon the velocity and temperature characteristics of the jet/flame shear layer. These characteristics are different in normal gravity and microgravity.

Bahadori, M. Yousef↗

NASA NTRS1997

Soot Formation in Freely-Propagating Laminar Premixed Flames

Soot formation within hydrocarbon-fueled flames is an important unresolved problem of combustion science. Thus, the present study is considering soot formation in freely-propagating laminar premixed flames, exploiting the microgravity environment to simplify measurements at the high-pressure conditions of interest for many practical applications. The findings of the investigation are relevant to reducing emissions of soot and continuum radiation from combustion processes, to improving terrestrial and spacecraft fire safety, and to developing methods of computational combustion, among others. Laminar premixed flames are attractive for studying soot formation because they are simple one-dimensional flows that are computationally tractable for detailed numerical simulations. Nevertheless, studying soot-containing burner-stabilized laminar premixed flames is problematical: spatial resolution and residence times are limited at the pressures of interest for practical applications, flame structure is sensitive to minor burner construction details so that experimental reproducibility is not very good, consistent burner behavior over the lengthy test programs needed to measure soot formation properties is hard to achieve, and burners have poor durability. Fortunately, many of these problems are mitigated for soot-containing, freely-propagating laminar premixed flames. The present investigation seeks to extend work in this laboratory for various soot processes in flames by observing soot formation in freely-propagating laminar premixed flames. Measurements are being made at both Normal Gravity (NG) and MicroGravity (MG), using a short-drop free-fall facility to provide MG conditions.

Lin, K.-C.↗

NASA NTRS1997

Premixed Atmosphere and Convection Influences on Flame Inhibition and Combustion (Pacific)

Flame spread over flat solid fuel beds is a useful paradigm for studying the behavior of more complex two-phase nonpremixed flames. For practical applications, two of the most important elements of flame spreading are the effects of (1) the ambient atmosphere (e.g. pressure and composition) and (2) the flow environment on the spread rate and extinction conditions. Concerning (1), studies of flame spread in vitiated air and non-standard atmospheres such as those found in undersea vessels and spacecraft are particularly important for the assessment of fire hazards in these environments as well as determination of the effectiveness of fire suppressants. Concerning (2), the flow environment may vary widely even when no forced flow is present because of buoyancy effects. Consequently, the goal of this work is to employ microgravity (micro g) experiments to extend previous studies of the effects of ambient atmosphere and the flow environment on flame spread through the use of microgravity (micro g) experiments. Because of the considerable differences between upward (concurrent-flow) and downward (opposed-flow) flame spread at 1g (Williams, 1976, Fernandez-Pello, 1984), in this work both upward and downward 1g spread are tested. Two types of changes to the oxidizing atmosphere are considered in this work. One is the addition of sub-flammability-limit concentrations of a gaseous fuel ('partially premixed' atmospheres). This is of interest because in fires in enclosures, combustion may occur under poorly ventilated conditions, so that oxygen is partially depleted from the air and is replaced by combustible gases such as fuel vapors, H2 or CO. Subsequent fire spread over the solid fuel could occur under conditions of varying oxygen and gaseous fuel content. The potential significance of flame spread under vitiated or partially premixed conditions has been noted previously (Beyler, 1984). The second change is the diluent type, which affects the radiative properties of the mixture as well as the Lewis number (Le) of the reactants in the atmosphere, which for oxygen is defined as the thermal diffusivity of the bulk mixture divided by the mass diffusivity of oxygen into the bulk mixture. Understanding the effect of diluent type is desirable because in some undersea and spaceborne habitations, it is desirable to use diluent gases other than nitrogen. Prior experiments have shown that both radiation (Bhattacharjee and Altenkirch, 1993) and Lewis number (Zhang et al, 1992) effects are important in flame spreading problems.

Honda, Linton K.↗

NASA NTRS1997

Interactions Between Flames on Parallel Solid Surfaces

The interactions between flames spreading over parallel solid sheets of paper are being studied in normal gravity and in microgravity. This geometry provides interesting opportunities to study the interaction of radiative and diffusive transport mechanisms on the spread process. Radiative losses from flames are recognized as an important component of flame spread. These loss mechanisms are changed when the flame interacts with other flames. Most practical heterogeneous combustion processes involve interacting discrete burning fuel elements, consequently, the study of these interactions is of practical significance. Owing largely to this practical importance, flame interactions have been an area of active research, however microgravity research has been largely limited to droplets. Consideration of parallel solid surfaces has been limited to 1-g studies. The parallel spread geometry offers several distinct differences from single sheet spread, the appearance of the internal flame, instabilities of the internal flame, pressure dependence of spread rate and spread rate variation with separation distance. These issues are being studies as part of this program and discussed in this paper.

Urban, David L.↗