Showing papers on "Diffusion flame published in 1978"

The Turbulent Jet Diffusion Flame in a Co-flowing Stream—Some Velocity Measurements

Abstract: Laser-Doppler measurements of mean velocity, axial and radial components of turbulence and Reynolds shear stress have been made on a turbulent diffusion flame formed by a jet of hydrogen burning in a co-flowing stream of air at an initial velocity ratio of 10 to 1. Results are found to be similar to those for isothermal and heated air jets in co-flowing streams except that there appears to be some suppression of turbulence on the centreline near the hottest part of the flame. The technique used involves some small residual effect of seeding bias and this has been thoroughly analysed and estimates made.

Combustion Behavior of Solid-Fuel Ramjets

Abstract: AN experimental investigation was conducted of the combustion behavior in solid fuel ramjets in order to determine the effects of configuration variables and operating conditions on combustion performance. Variables considered were fuel port flow rates, bypass dump momentum and geometry, and bypass ratio. Contents To be used in a tactical situation, the solid-fuel ramjet has to demonstrate combustion stability and high efficiency over the expected operating envelope of altitudes and Mach numbers. It must also show performance comparable to that of liquid-fuel ramjets and ducted rockets. Combustion studies on the solid-fuel ramjet have been underway at United Technologies—Chemical Systems Division since 1971.l The solid-fuel ramjet has two distinct combustion zones within the fuel grain: the recirculation zone behind the sudden expansion inlet, which provides flame stabiliation; and a diffusion flame in the developing boundary layer region after flow reattachment. Unburned gaseous fuel escapes from under the flame at the aft end of the fuel grain and results in decreased combustion efficiency. Aft mixing chambers and bypass air designs are being used to increase the efficiency. A schematic of the solid-fuel ramjet is shown in Fig. 1. The apparatus employed a polymethylmeth acrylate (PMM) fuel with a fuel port to dump inlet area ratio of 9.0 and a fuel port to nozzle throat area ratio of 4.0. The exit area of the grain was held fixed at the initial port area by using a thin orifice plate. The aft mixing chamber had a length to diameter ratio of 2.93. The aft mixing chamber consisted of three interchangeable sections such that the axial location and angular orientation of the bypass dumps could be varied. A summary of the test conditions is presented in Table 1. Figure 2 presents data obtained without bypass air. The regression rate expression indicates a weaker dependence on pressure than the earlier data of Boaz and Netzer.2 Figure 3 presents data obtained with bypass air. A slightly stronger dependence on pressure was shown while regression rate indicated very little or no dependence on air flux for the values tested. In the bypass situation, the mass flux through the grain is low but the pressure is maintained high, due to the total mass flux through the nozzle throat. These conditions minimize the convective heat flux to the fuel surface. For lower mass flux through the grain, the regression rate increases relative to the air flux; thus, more gas with radiative properties (fuel rich) is present. These results indicate that using bypass with PMM fuel changes the principal wall heat flux mechanism from convection to radiation.

A Split-Operator, Finite-Difference Solution for Axisymmetric Laminar-Jet Biffusion Flames

Abstract: A split-operator numerical method is developed to solve the steady laminar diffusion flame problem. Splitting the chemical kinetic terms from the fluid-mechanical terms ameliorates some of the difficulties associated with the disparate time scales and stiffness in the set of equations which describes highly exothermic diffusion flames. Implicit differencing methods enhance the numerical stability of both operators. Special care is taken to maintain the accuracy of the solution, and the coordinate system is varied continuously to follow the flame shape. The present method allows a detailed study of the complex interaction between the fluid mechanics and the finite-rate chemical kinetics in a flame. Cpk = Dk g Nomenclature constant-pressure heat capacity of the gas mixture constant-pressure heat capacity of the kth species trace diffusion coefficient of kth species acceleration due to gravity enthalpy of species A: per unit mass number of radial nodes number of species pressure radial dimension universal gas constant, ergs/mole-K source term in general transport equation temperature axial velocity radial velocity transformed radial velocity formation rate of kth species by chemical reaction molecular weight of kth species axial dimension mass fraction of kth species coordinate spread angle transformed axial dimension origin of coordinate system referenced to nozzle exit transport coefficient in general transport equation conductivity transformed radial coordinate mass density of the gas mixture viscosity chemical symbol for kth species J K p r R S T u v V &k Wk x Yk a £0 e X rj p fjL Xk Subscripts I = axial node index j ' • = radial node index & — species index n — reaction index e = edge condition

Stability of submerged diffusion flames in subsonic and underexpanded supersonic gas-fuel streams

Abstract: 1. The standoff distance of the stabilization ring of the base of a lifted flame increases along the surface of maximum velocity fluctuations, causing it to stand off from the surface of stoichiometric composition in the direction of lean fuel mixtures and finally resulting in extinction. 2. There is a limiting value of the injection nozzle diameter (determined by the fuel composition, ambient pressure, fuel temperature, and ambient temperature), above which absolute stability of the lifted flame is ensured for any velocities. 3. A stability model has been proposed, along with analytical relations, making it possible to determine the domain of flame instability for subsonic and supersonic fuel streams discharging into a stationary surrounding air space.

An Experimental Study of Flame Spread and Burning in Arrays of Monosize Hydrocarbon Droplets

Abstract: The combustion of two-dimensional arrays of falling n-heptane droplets was studied by black-and-white and colour schlieren photography which could clearly distinguish between evaporating and burning droplets. No individual droplet flames were seen in this work. The first ignited droplet trailed a wake flame and acted as a moving ignition source, producing downward flame spread rates which were the same as the fall velocity of this droplet and much higher than the upward flame spread rate through the vapour. All drops above the first ignited one burned in a collective cylindrical flame. Simple models of flame spread through the array were consistent with experimental trends.

An Asymptotic Analysis of Nitric Oxide Formation in Turbulent Diffusion Flames

Abstract: Exploiting the fact that the NO formation in diffusion flames lakes place in a narrow region around the maximum temperature, an asymptotic expansion in terms of a small parameter ∊ is performed. This parameter contains the maximum temperature, the activation energy of the NO formation rate and the second derivative of the temperature profile at maximum temperature. Closed form solutions are obtained for the NO profiles within the flame as well as for the total NO mass formation rate. The results agree favourably with experimental results of a hydrogen-air jet diffusion flame.

Sooting Characteristics of Liquid Pool Diffusion Flames.

Abstract: : This investigation deals with a liquid fuel diffusion flame and examines the use of the smoke point test as a means of qualitatively measuring the ability of a fuel to produce soot relative to other fuels. Results indicate the necessity of controlling the initial conditions in order to obtain meaningful measurements. This thesis reports a new technique for the smoke point determination that has proved to be more accurate and reproduceable than previous methods. Recent studies indicate water addition in a premixed flame chemically suppresses soot formation. As a result, addition of water inside a diffusion flame is a likely direction to pursue. Both water in fuel emulsions and direct steam injection were used in the present investigation. The results indicate a dominant thermal effect and a possible secondary chemical effect of water on soot formation. Blending of various fuel types reveals the domination of an aromatic fuel over an aliphatic when determining a combined smoke point of the mixture. Applying this information to alternative hydrocarbon fuels, the oil shale and coal derived fuels, having a higher percentage of aromatics than conventional fuels, produce soot more readily than their petroleum derived counterparts. Testing of oil shale and conventional fuels supplied by the Air Force verifies this result.

Rayleigh temperature profiles in a hydrogen diffusion flame

Abstract: Rayleigh scattering from a hydrogen-air laminar jet diffusion flame in combination with a numerical model of the flame has been used to determine temperature profiles. The model predictions of species concentration are used to calculate a mean Rayleigh cross-section which is used to relate the Rayleigh scattered intensity to temperature. Using an argon ion laser producing 7.5 watts at 488 nm and an optical multichannel analyzer (OMA), the scattered light was imaged into a spectrometer. The OMA was rotated 90 degrees to its normal orientation, allowing scans to be taken along the spectrometer exit slit. This resulted in a spatially resolved Rayleigh signal along the laser beam through the entire flame. Spatial resolution of 0.18 mm on each of the 500 detector elements with good signal-to-noise ratios was achieved even with integration times of only 0.03 second. Since the entire profile is made simultaneously, particulate perturbed profiles are easily recognized and discarded. Transverse profiles are presented to show flame structure. Axial profiles are compared to radiation corrected thermocouple measurements.

Conversion of fuels containing nitrogen to oxides of nitrogen in hydrogen and methane flames

Abstract: Nitrogen-containing compounds such as hydrogen cyanide, acetonitrile, acrylonitrile, pyridine, benzonitrile, ammonia and methylamine, which are typical of the products likely to be encountered during the decomposition of nitrogen-containing polymers in fires, have been introduced into hydrogen and methane flames burning in oxygen-argon atmospheres. There is a complete conversion of fuel nitrogen in all cases to oxides of nitrogen and molecular nitrogen. The relative conversion to oxides of nitrogen (as NOx/N2) increases as the injection rate of nitrogen-containing fuels is decreased. The relative yields of oxides of nitrogen tend to be similar with methane and hydrogen premixed flames and markedly greater than observed with hydrogen diffusion flame. In all cases the yield of oxides of nitrogen-containing products such as hydrogen cyanide can also present a toxic risk during the burning of nitrogen-containing polymers, particularly when high temperature are involved. The combustion of these products in flame zones cannot be assumed to alleviate the additional toxic risk because of their conversion to oxides of nitrogen.

Diffusion flame in free convection

Abstract: This US Bureau of Mines report on experimental and theoretical studies of diffusion flames in free convection (1) explores the cross-coupling between buoyancy-induced flows and the combustion process as it relates to the size, structure, and radiative balance of flames, (2) presents new data for the size-dependent radiance oscillations of diffusion flames and for the flame, and (3) compares both amplitude and freqency of the measured oscillations with the theoretical predictions that are based on a new view of the structure of diffusion flames. This new view is at variance with the traditional one, which assumes uniform reaction in the diffusive mixing zone. It recognizes the significance of the role played by convective buoyancy and its duality in both aiding and impeding flame propagation. Buoyancy quenches propagation at low burning velocities, introducing a real discontinuity in the combustion rate. A flammable volume is thus defined as that bound between lean- and rich-limit contours. However, in the flow disturbances generated by combustion waves that consume that volume, buoyancy aids propagation by promoting the convective mixing of a new flammable volume. The measured radiance oscillations are manifestations of these propagation and remixing cycles.

Wärme‐ und Stofftransport in Tankflammen

Abstract: Heat and mass transfer in pool flames. Theoretical and experimental concept of heat and mass transport in liquid pool flames are reviewed. The heat and mass transfer processes in the interface between flame and liquid fuel are outlined, and the concept of the burning rate as a parameter for the quantitative description of a pool fire is discussed. The methods of calculation and experimental results concerning flame length and flame shape as well as buoyancy forces are described. In pool flames, a lower region (flame neck) and an upper region (flame plume) are distinguishable. Both the flame neck and the flame plume show a flow pattern obtained from short or long periods of observation. The flow patterns of the flames have been measured by the techniques of equidensitometry and Laser-synchronous-interferometry. The flow pattern observed over a short period of time gives an insight into the eddy field of the pool flame whose turbulence is measured. The flow pattern observed over a long time is obtained from time averaged values of temperature, heat radiation, concentration etc.

The effect of diluent gases on ion formation in hydrocarbon flames

Abstract: It has been demonstrated that the addition of N2, CO, Ne, Ar, Kr, Xe, CO2, and CF4 to a H2-air diffusion flame containing He and traces of CH4 enhances the level of ion formation. This enhancement is proportional to the concentration of both CH4 and the diluent gas, consistent with the proposition that the diluent gas participates in the ion formation process.

Direct determination of ammonium-nitrogen by flame emission spectrometry in a hydrogen-nitrogen diffusion flame

Abstract: A method for the determination of trace amounts of ammonium ion in aqueous solution has been developed that utilises the chemiluminescent emission at 336.0 nm of the NH species produced when ammonia gas generated from alkaline sample solutions is introduced into a hydrogen-nitrogen diffusion flame.The method has been applied successfully to the determination of the exchangeable ammonium-nitrogen content of soils. A practical detection limit of 0.2 µg ml–1 has been obtained for a 5-ml sample.

Influence of Buoyancy on Turbulent Hydrogen/Air Diffusion Flames

Abstract: A model which treats the axisymmetric turbulent mixing of co-flowing streams, including non-equilibrium chemistry, is applied to the prediction of buoyant H2/air diffusion flame properties. The model, which employs the Donaldson/Gray eddy viscosity formulation, is first tested against the nonbuoyant laboratory H2/air flame data of Kent and Bilger. It is shown that centerline temperatures and species mole fractions can be predicted to good accuracy. However, the calculated rate of radial transport of energy and mass is greater than that measured in the laboratory experiments. The results of a series of parametric calculations on buoyant flames and comparisons with buoyant flame length data show that: (1) flame properties scale with nondimensional distance for Froude numbers (Fr) greater than about 106, (2) buoyancy significantly affects temperature decay rates downstream of the location of maximum temperature (after all the H2 has burned), and (3) the predicted influence of Fr on buoyant flame len...

Transient Extinction History of a Solid Fuel Diffusion Flame

Abstract: The transient extinction history of a diffusion flame in the stagnation-point boundary layer of a solid fuel is presented. The numerical analysis shows that dynamic extinction is possible by a rapid reduction of an environmental factor, such as Damkohler number or oxygen concentration. The extinction transition can be divided into two stages separated by a very short period of active onset of flame extinguishment. During the first stage, a quasi-steady gas flame is maintained. Then the flame diminishes swiftly in a fraction of the gas phase characteristic time, followed by the second stage, i.e., the slow cooling-down of the solid.

Simultaneous Measurement of Fluctuating Temperature and Velocity in a Combustion Field

Abstract: A velocimeter constructed is composed of detectors and analog computing circuits through which the signals from detectors are directly converted to necessary values. The detectors have a thermocouple, static-and total-pressure probes respectively connected to condenser microphones and a differential pressure meter. With this constitution this velocimeter revealed its performance to be in the same level as a hot-wire velocimeter for a flameless field and provided more accurate turbulence degree in a combustion field than the earlier methods based on the measurement of only pressure variation. Through the application for an open diffusion flame, it was shown that the time constant of thermocouple can be estimated from Nusselt's number equation by Ahmad in combination with the signals obtained, and further that the correlation between temperature and velocity fluctuations becomes negative at the inside of maximum temperature location and positive at its outside.

A two zone model of flame propagation applied to H2 + air flames and HCL inhibited flames

Abstract: This communication presents a two zone flame model which allows computation of flame propagation velocity and average properties in the radical generation region and fuel attack region of the flame The approach is computationally simpler than formal flame theory calculations; however, it provides only limited information since detailed structural information is sacrificed for computational simplicity The zonal approach to flame structure is described and justification is provided for the various approximations which characterize the model The model is largely a chemical one; however, the balance maintained between the radical molecule attack reaction and diffusional transport of radicals into the reaction region controls the propagation rate The model was tested on five H2 + air flames, and the computed propagation velocities compared favorably with experimental values HCL inhibition of the stoichiometric H2 + air flame was investigated, and the zonal approach was found suitable for studying flame inhibition

Studies of Opposed Flow Diffusion Flames with Solid-gas Interface— I. Experimental Measurements of Ammonium Perchlorate-Hydrocarbon Systems

Abstract: To examine the gas-kinetics parameters pertinent to the flame reactions of ammonium perchlorate (AP) based propellants we have employed the heterogeneous opposed flow diffusion flame (HOFD). In this combustion system a diffusion flame is formed in the stagnation region between a gaseous stream of fuel and an opposing stream of oxidizer originating at the solid surface of A P. With increasing fuel flux the steady-state mass flux of AP reaches a value at which the kinetics of reaction limit the consumption of reactants. This limiting condition provides information on the gas phase kinetic parameters of the combustion system. Both the composition of the solid oxidizer and the gaseous fuels were varied in ( The experimental studies. The effect of different catalysts and solid fuels on the burning rate characteristics of the solid phase were examined. A number of gaseous hydrocarbon fuels were used . Based on the results kinetic expressions are derived for the overall combustion progress in the diffus...

Coherent Antistokes Raman Spectroscopy In Sooty Diffusion Flames

Abstract: The main reason for doing spectroscopy in a sooty diffusion flame, rather than in a well characterized and controlled environment, is that such a flame is thought to be a good model for the early stages of many jet combustors where particle concentrations, temperature, and possibly turbulence are likely to be similar. However, there are important differences. For example, the total pressure in a practical combustor is several atmospheres and it is pressure rather than an excess of fuel which causes soot formation in that case.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.