L.-D. Chen

Buoyant diffusion flames

Planar visualization was employed to study flame structure and low frequency flame oscillation. Two distinct vortices were visualized in the flames studies: large toroidal vortices outside the luminous flame and small roll-up vortices inside the luminous flame. The flame oscillation frequency and the convective velocity of the toroidal vortices were measured for ethylene, methane, and propane diffusion flames over a wide range of test conditions. The frequency was typically in the range 10 to 20 Hz and the convective velocity was approximately, 0.8 m/s. The frequency of the toroidal vortices was found to correlate with the flame oscillation frequency. The potential effects of the toroidal vortices on the flame dynamics at low fuel flow rates are discussed, for example, low frequency flame oscillation, non-linear flame bulge motion, and quenching of the luminous flame surface.

Preliminary results of a numerical-experimental study of the dynamic structure of a buoyant jet diffusion flame

Abstract Preliminary results of a joint numerical-experimental investigation of the dynamic structure of a buoyant jet diffusion flame are presented. The purpose of this effort is to determine the nature of the unsteady interactions between flames and their associated vortex motions. A direct numerical simulation of an unsteady low-speed propane-air jet diffusion flame is carried out utilizing the flame sheet and conserved variable approximations. Counterrotating vortex structures internal and external to the flame surface appear and move upward along with flame sheet bulges. Tip-cutting (flickering) occurs with a frequency of 11–15 Hz. These dynamic features bear close resemblance to those observed experimentally by means of the reactive Mie scattering (RMS) technique. Comparisons between computational and experimental (determined using thin filament pyrometry) near-instantaneous radial temperature profiles at four heights in the flame also show good agreement. The assumption of zero gravity in the computation results in the complete cessation of dynamical activity, thus demonstrating the important role that buoyancy plays in the behavior of this flame.

Visualization of jet flames

INTRODUCTION was also used recently to examine CH and C2 Flow visualization has long been employed in distributions in ethylene and acetylene gaseous experimental studies of fluid mechanics and heat flames and in heptane-air spray flames [7]. transfer phenomena. Two widely used methods The method described in [2] and utilized in the utilize (1) Mie scattering of light from small present study differs from previous MSM techparticles and (2) the variation in refractive index, niques in how particles are introduced into the commonly known as Schlieren (density gradient) flow. Particles are formed in the flow, at a and shadowgraph (second derivative of density reaction interface where TiCl4<g) and H20(g) react gradient). The MSM (Mie scattering method) to form TiO2(~), as opposed to preformed particles generally requires foreign materials to be added to in previous MSM studies, e.g., [3, 6, 8]. Reactive the flow. The RIM (refractive index method) is systems have been used in flow visualization, e.g., applicable only when noticeable variations in [9, 10, l l ] ; however, the reactive TIC14 scheme density are observed. Detailed reviews of these used in this paper appears to be the more easily methods can be found in Merzkirch [1]. applied to gaseous combusting systems and proThere have been several 2-D flow visualization vides visualization of surfaces that have clear techniques used for qualitative and quantitative physical significance to the combustion process. measurements. For example, large vortex strucAlso, it should be noted that the reaction of TiC14tg) tures were observed in diffusion flames stabilized and H20~g) in the air occurs under isothermal by a bluff body [2], Lorenz/Mie scattering [3] and conditions and yields submicron particles. This is Rayleigh scattering [4] were used for concentraparticularly important since energy is not added to tion measurements of axisymmetric jets. Simultaor taken from the combustion system under study, neous mapping of species concentration and ternand the submicron particles respond to flow perature was recently conducted using Rayleigh variations almost instantaneously. and Raman spectroscopy [5], and streak photograThe objectives of this paper are to describe the phy was employed to obtain velocity vectors of a reactive TiCl4, 2-D MSM for the visualization of water-tunnel flow [6]. Laser induced fluorescence jet diffusion flames, and to compare images

The Structure of Jet Diffusion Flames

This paper presents the structural characteristics of free, round, jet diffusion flames as obtained using a new 2D laser sheet lighting visualization technique referred to as the RMS (Reactive Mie Scattering) method. The results of analyzing photographs and high speed movies of flames using the RMS method are discussed in terms of the visible flame structure. The fuel veloCity is varied from 0.16 to 17 m/s. The presence of large toroidal vortices formed outside the visible flame zone have been known for many Years but their importance in determining the dynamic structure of free jet diffusion flames has not been fully appreciated. The influence of the outer vortices on flame structure is prevalent for near laminar and transitional flames and diminishes for near turbulent flames. They are believed to result from a Kelvin-Helmholtz type instability formed by a buoyantly driven shear layer. They appear to be responsible for flame flicker defined by the separation of the flame tip or the oscillations of the flame surface and for determining the shape of the mean, rms, and pdf radial profiles of temperature. Vortex structures have also been observed inside the visible flame zone. In transitional flames established by a contoured nozzle, these structures are shown to be on the scale of the 10 mm diameter nozzle, toroidal, and coherent for a long distance downstream. However, they may have only a minimal impact on the mean temperature characteristics of transitional flames. Their impact on the visible flame structure of near turbulent flames is large. At high fuel velocities, coalescence of the large vortices appear to be correlated with the formation of small 3D vortices which are randomly distributed in size and space. Collisions of the small vortices with the visible flame front produce small localized flamelets which are responsible for the wrinkled appearance of the visible flame surface. The localized stretching of the flame surface is believed to invoke finite rate chemistry effects. Indeed, collisions are observed where the flame stretch is large enough to cause localized holes to form in the flame surface. This appears to occur when the radial velocities of the inner vortices are large. Holes formed near the lip of the jet are postulated to be one mechanism that induces flame lift-off.

Linear stability analysis of gas-liquid interface

This paper extends the Rayleigh theory to study the interface instability of axisymmetric gas-liquid flows. The dispersion equations of cylindrical liquid sheets were derived and numerical solutions were obtained. Two families of instability curves were found; one for symmetric disturbances and the other for antisymmetric disturbances. Two limiting cases can be recovered from derived dispersion equations. The equation recovers the Rayleigh instability of round jets for symmetric disturbances and recovers the hollow jet or submerged jet instability for antisymmetric disturbances. Effects due to ambient fluid density and ambient fluid velocity on liquid-sheet instabilities were examined.

Effects of radiative and conductive transfer on thermal ignition

Abstract The purpose of this study is to examine the thermal ignition phenomenon when the interaction of conductive and radiative heat transfer for a plane layer medium with an Arrhenius heat generation is considered. The medium contains absorbing and emitting gases, soot, and particles where scattering is included for the particles. The medium is enclosed between isothermal and reflecting walls. Numerical results are reported for the heat release that causes a sudden increase in the medium temperature. The influence of medium composition, particle scattering, and wall emittance on the heat release for thermal ignition is reported. For the values of the parameters eamined, the results indicate that when absorbing gases, soot, and particle radiation effects are taken into account, the heat release may be two orders of magnitude higher than that when only conduction is considered.

A numerical/experimental study of the dynamic structure of a buoyant jet diffusion flame

An overview of a joint numerical/experimental investigation of the dynamic structure of a low-speed buoyant jet diffusion flame is presented. The dynamic interactions between the flame surface and the surrounding fluid mechanical structures are studied by means of a direct numerical simulation closely coordinated with experiments. The numerical simulation employs the full compressible axisymmetric Navier-Stokes equations coupled with a flame sheet model. Counterrotating vortex structures both internal and external to the flame surface are seen to move upward along with flame sheet bulges. These buoyancy-driven dynamic features compare well with those observed experimentally by means of phase-locked flow visualizations over entire flame-flickering cycles. The flicker frequencies measured both computationally and experimentally also compare well. Other aspects of this investigation which are discussed include sudden jumps in flicker frequency with increasing coflow velocity and the utilization of background pressure changes to simulate gravitational force variations experimentally.

Time evolution of a buoyant jet diffusion flame

Predictions of the time and spatial evolutions of a buoyant jet diffusion flame by a direct numerical simulation are experimentally evaluated. The simulation involves the solution of the full time-dependent Navier-Stokes equations in conjunction with a flame sheet model. The experiments involve a vertical, buoyant diffusion flame in a coflowing air environment. The fuel is a propane and nitrogen mixture (50% by mass) and the burner is a long tube (22.94 mm inside diameter) with a sharpo lip at the exit. Planar visualizations and thin-filament-pyrometry temperature measurements are phase-locked to the naturally periodic oscillation of the flame. A comparison of experimental and theoretical results for seven phase angles shows that the model gives an adequate representation of the time evolution of the flame. Temperature profiles obtained at ten axial locations show reasonable agreement when a 20% reduction in heat release is used to approximate the radiative heat loss from the flame. Time-averaged velocity, measurements in the near-nozzle region also compare favorably with the prediction.

Analytical Solution for Two-Phase Flow in PEMFC Gas Diffusion Layer

Thermal and water management is critical to fuel cell performance. It has been shown that gas diffusion layer (GDL) can impose the mass transport limit; for example, it can block the reactant transport to active layer when flooding occurs at high current density conditions. Micro porous layer (MPL) in conjunction with backing layer (BL) has been used as a GDL material and was shown to be effective for water management. To study the transport processes in GDL and MPL modified GDL, an analytical solution is derived current study for calculation of two-phase, multicomponent transport in GDL. Two models were considered, the unsaturated flow model (UFM) and the separate flow model (SFM). Comparison of the calculated saturation level and oxygen mass fraction shows that UFM calculation can underestimate, as well as overestimate the saturation and oxygen concentration. The SFM was used to study the effects due to GDL property variations. The calculation shows that increase in liquid water transport in an MPL modified GDL is due to the abrupt change of liquid water flow rate when a step change in porosity or permeability is imposed. The calculation further shows that particle size of around 1 μm would be a good choice for MPL as it results in higher oxygen concentration at active layer and lower saturation in GDL.Copyright

Numerical modeling of buoyant ethanol-air wick diffusion flames

Abstract A numerical model based on a conserved-scalar approach is presented for buoyant ethanol-air wick diffusion flames at atmospheric and subatmospheric conditions. The model incorporates an equation that describes the interface condition of wick combustion. The prediction yields similarity solutions for flat-plate ethanol-air wick diffusion flames, but not for cylindrical wick diffusion flames. The flat-plate solution yields a mass burning rate per unit surface area following the x −1 4 dependence of the classical similarity solution, where x is the streamwise distance. A pressure dependence of P0.644 is predicted for the flat-plate overall mass burning rate, in agreement with the P 2 3 dependence reported in the literature. The cylindrical wicks have a mass burning rate per unit surface area that deviates from the x −1 4 dependence. The predicted mass burning rate, however, does not substantially deviate from the flat-plate solution for cylinders with a moderate aspect ratio (of the order one). The deviation in mass burning rate is most pronounced when needle-like cylinders are considered. The variable-property effects are also examined. The results show that the Chapman gas and constant-Prandtl-number assumptions are not adequate for wick diffusion flames, even at the subatmospheric-pressure condition studied.