Showing papers on "Diffusion flame published in 1990"

Flame Stretch and the Balance Equation for the Flame Area

Abstract: When a flame propagates in a nonuniform flow it experiences strain and curvature effects. The fractional rate of change of the flame area constitutes the flame stretch. This quantity is often used to describe the structure and extinction mechanisms of turbulent flames. It also occurs in many recent studies of premixed laminar flames. This article provides a unified view of this concept on the basis of a novel derivation of stretch in terms of strain rate and curvature. The flame stretch, the rate of change of the normal to the flame front and the rate of change of the curvature are deduced from a general transport theorem. As an illustration, the components of flame stretch are evaluated in the case of a direct numerical simulation of the interaction between a pair of vortices and a laminar flame. Another application of flame stretch concerns the determination of the available flame surface density. A balance equation is derived for this quantity and cast in various useful forms thus providing a ...

Planar laser-fluorescence imaging of combustion gases

Abstract: An overview is provided of the planar laser-induced fluorescence (PLIF) method, which currently allows simultaneous combustion measurements at more than 105 flowfield points Important advantages of the method include its relatively high signal strength, ease of interpretation, and applicability for determining several flowfield variables (including concentration, temperature, velocity, pressure and density) Example results are shown for a turbulent non-premixed flame, a spray flame, a rod-stabilized premixed flame, and a diffusion flame from a fuel jet in cross-flow

Morphological Description of Flame-Generated Materials

Abstract: The soot formed in a coannular ethene diffusion flame was extracted by a thermophoretic sampling technique for examination by transmission electron microscopy. A detailed analysis of the particle statistics of the samples is presented. Primary particle diameters range from 10 to 40 nm and their spatial variation within the flame affords quantitative information on the specific soot surface growth and oxidation rates. The moment ratio of the aggregate volume-equivalent diameters (D63D30,) is found to be close to the values predicted for the self-preserving size distribution in either the continuum or free molecular limit. The fractal dimensions (1 62 and 1.74) for Iwo samples examined are in the range reported in other combustion related experiments as well as in computational simulations of particle growth when cluster-cluster aggregation is an important growth mechanism The primary particle diameters that have been reported by various investigators in a wide variety of flame environments are rev...

Measurement of OH rotational temperatures in a flame using degenerate four-wave mixing.

Abstract: Temperatures have been measured in a laminar premixed propane–air diffusion flame using degenerate four-wave mixing (DFWM) of the OH radical. The spectra were recorded simultaneously with laser-induced fluorescence through the (0, 0) band of the OH A2Σ–X2II transition. Individual rotational lines are more clearly resolved in the DFWM spectrum than in the laser-induced fluorescence spectrum, although both are power broadened at laser intensities of 1–2 MW/cm2 at 307 nm. Rotational temperatures have been determined from the DFWM spectra and are in close agreement with temperatures measured with coherent anti-Stokes Raman spectroscopy of nitrogen.

An investigation of diamond film deposition in a premixed oxyacetylene flame

Abstract: Diamond film has been deposited in a single‐nozzle premixed oxyacetylene flame. Results of runs of varying duration suggest that diamond is deposited via the transport of hydrocarbon fragments produced at the secondary flame front. Planar laser‐induced photodissociation fluorescence suggests that this region is rich in C2H species. Emission studies also suggest that the post‐primary flame zone presents a source of C2 radicals that may account for the observed graphite and diamondlike carbon deposited on the substrate exposed to this region of the flame. The results on the premixed flame suggest that it would be possible and more convenient to attempt large area deposition using a multinozzle diffusion flame.

Ignition and behavior of laminar gas-jet diffusion flames in microgravity

Abstract: This paper presents the results of studies on the ignition and behavior of cylindrically symmetric, laminar diffusion flames of methane and propane in quiescent air under microgravity conditions. In prior research, similar flames were ignited in normal gravity and then subjected to the microgravity condition once the flame was established. In this paper, comparisons between the two methods of ignition and observations of the flame behavior and distinct nature of flame color and luminosity are presented. Application of a steady-state, parabolic model has shown satisfactory agreement between the predicted and observed flame heights for those flames that reached a near-steady state in the 2.2s period of microgravity

Blowout limits of turbulent jet diffusion flames for arbitrary source conditions

Abstract: We present a formulation for the blowout limits of turbulent jet diffusion flames issuing from sources with arbitrary geometries and exit conditions into otherwise quiescent environments. It is argued that, while the liftoff characteristics of turbulent diffusion flames appear likely to be controlled by the straining out of flame sheets, the molecular mixing rate at the flame tip controls their blowout characteristics. The concept of a "far-field equivalent source" is introduced, and the local molecular mixing rate in the flow is expressed in terms of the associated far-field scaling laws. Blowout is expected when a resulting algebraic expression reaches a critical value. Results of a "flip" experiment verify the far-field equivalent source formulation. Measurements of the blowout limits over a range of geometries, fuels, and diluents show good agreement with the predictions from this formulation.

A Theoretical Description of Flame Spreading over Solid Combustibles in a Quiescent Environment at Zero Gravity

Abstract: A theoretical model is presented that can be used to predict the structure and rate of spread of an attached diffusion flame moving over a thermally thin, pyrolyzing combustible placed in a gravity-free, quiescent, oxidizing environment. The gas-phase model includes steady-state, two-dimensional momentum, energy, and species equations while the solid-phase model consists of continuity and energy equations, the solution to which provide boundary conditions for the gas-phase problem. The spread rate appears as an eigenvalue in both the gas- and solid-phase equations. The numerical procedure developed to solve the system of equations is stable even for spread rates comparable to normal velocities present at the fuel-gas interface. Solid fuel pyrolysis is modelled using a first-order Arrhenius decomposition while both finite-rate and infinite rate chemistry in the gas phase are considered. Computed spread rates increase with increasing oxygen concentration in the ambient and are generally a factor of...

Experiments on axisymmetrically pulsed turbulent jet flames

Abstract: An experimental study of the effects of strong axisymmetric pulsing on a free, vertical turbulent jet diffusion flame is presented. The jet flame was pulsed over the frequency range of 2-1300 Hz with amplitudes ranging from 0.13-0.89 of the centerline jet velocity. Conditionally averaged centerline velocity measurements and flame photographs were obtained to characterize the pulsed jet flames. The centerline evolution of the pulse waveforms was examined and found to be dependent on the pulse frequency; the centerline decay of the pulse amplitude increased with increasing pulse frequency. The visual dimensions of the pulsed flame were also frequency dependent. Significant changes in the local flame structure were observed, which exhibited a constant local nondimensional frequency of about 0.2, consistent with a preferred-mode coupling between the pulsing and the jet flame structure. Several features of the results also suggest a coupling between the forcing and low frequency structure in the outer preheat layer surrounding the jet flame.

Synthesis of diamond powder in acetylene oxygen plasma

Abstract: Diamond particles 10–500 nm in diameter were produced by microwave‐assisted combustion of acetylene in oxygen. Both premixed and diffusion flame configurations were investigated. A mixture of cubic and hexagonal polytypes of diamond were identified. Larger particle sizes were observed at lower reactor pressure and higher C to O atomic ratios. C to O atomic ratios between 0.83 and 1.0 produced crystalline diamond powder while other ratios produced graphite, soot, and amorphous carbon phases. Diamond formation was not observed when reaction pressures were above 150 Torr.

A simple model of flames

Abstract: This paper presents a simple model for modeling and rendering of flames. The technique is based on physical model of flame processes. This flame model accounts for both laminar and turbulent flames, with premixed and diffusion combustion. The basic geometry of flames relies on the laminar flame model. An anisotropic density field function is used to define the geometry of laminar flames. A stochastic function is added to purturb the basic geometry of flames to simulate the turbulent flames. Luminous flames from carbon particle scattering is added to the model by approximating the Mie scattering function. The color of flames is determined by the combustible fuel and it is also dependent on the flame region inside the flames. A volume rendering technique based on ray tracing is used to render the flames, which we call the volume tracing.

Full numerical simulation of coflowing, axisymmetric jet diffusion flames

Abstract: The near field of a non‐premixed flame in a low speed, coflowing axisymmetric jet is investigated numerically using full simulation. The time‐dependent governing equations are solved by a second‐order, explicit finite difference scheme and a single‐step, finite rate model is used to represent the chemistry. Steady laminar flame results show the correct dependence of flame height on Peclet number and reaction zone thickness on Damkohler number. Forced simulations reveal a large difference in the instantaneous structure of scalar dissipation fields between nonbuoyant and buoyant cases. In the former, the scalar dissipation marks intense reaction zones, supporting the flamelet concept; however, results suggest that flamelet modeling assumptions need to be reexamined. In the latter, this correspondence breaks down, suggesting that modifications to the flamelet modeling approach are needed in buoyant turbulent diffusion flames.

Mechanistic studies of toluene destruction in diffusion flames

Abstract: The concentrations of stable species in an atmospheric-pressure, laminar diffusion flame burning a fuel mixture of methane and toluene (1 mole percent) were measured using a microprobe gas sampling/mass spectrometer system. These measurements are compared to results from a flame burning pure methane under identical temperature and flow conditions. In each of the flames studied little or no change is observed in the peak concentrations of a number of major chemical species such as water, carbon monoxide, and carbon dioxide. Small increases occurred in the peak concentrations of hydrogen, acetylene, methylacetylene, vinylacetylene, diacetylene, and butadiene. The presence of increased amounts of these products suggests limited decomposition of the benzyl or phenyl radicals which are likely initial intermediates of aromatic pyrolysis. Relatively larger increases occurred in the peak concentrations of a number or aromatic compounds including phenylacetylene, styrene, and naphthalene. For the toluene ...