Carolyn R. Kaplan

Computations of enhanced soot production in time-varying CH4/air diffusion flames☆

Abstract Recent experimental measurements of soot volume fraction in a flickering CH4/air diffusion flame show that for conditions in which the tip of the flame is clipped, soot production is ≈ 5 times greater than that measured for a steady flame burning with the same mean fuel flow velocity (Shaddix et al., Ref. 9). This paper presents time-dependent numerical simulations of both steady and time-varying CH4/air diffusion flames to examine the differences in combustion conditions which lead to the observed enhancement in soot production in the flickering flames. The numerical model solves the two-dimensional, time-dependent, reactive-flow Navier-Stokes equations coupled with submodels for soot formation and radiation transport. Qualitative comparisons between the experimental and computed steady flame show good agreement for the soot burnout height and overall flame shape except near the burner lip. Quantitative comparisons between experimental and computed radial profiles of temperature and soot volume fraction for the steady flame show good to excellent agreement at mid-flame heights, but some discrepancies near the burner lip and at high flame heights. For the time-varying CH4/air flame, the simulations successfully predict that the maximum soot concentration increases by over four times compared to the steady flame with the same mean fuel and air velocities. By numerically tracking fluid parcels in the flowfield, the temperature and stoichiometry history were followed along their convective pathlines. Results for the pathline which passes through the maximum sooting region show that flickering flames exhibit much longer residence times during which the local temperatures and stoichiometries are favorable for soot production. The simulations also suggest that soot inception occurs later in flickering flames, and at slightly higher temperatures and under somewhat leaner conditions compared to the steady flame. The integrated soot model of Syed et al. (Ref. 12), which was developed from a steady CH4/air flame, successfully predicts soot production in the time-varying CH4/air flames.

Flow-field effects on soot formation in normal and inverse methane-air diffusion flames

Abstract We investigate the effects of the flow-field configuration on the sooting characteristics of normal and inverse coflowing diffusion flames. The numerical model solves the time-dependent, compressible, reactive-flow, Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. A benchmark calculation is conducted and compared with experimental data, and shows that computed peak temperatures and species concentrations differ from the experimental values by less than 10%, while the computed peak soot volume fraction differs from the experimental values by 10–40%, depending on height. Simulations are conducted for three normal diffusion flames in which the fuel/air velocities (cm/s) are 5/10, 10/10, and 10/5, and for an inverse diffusion flame (where the fuel and air ports have been reversed) with a fuel/air velocity of 10/10. The results show significant differences in the sooting characteristics of normal and inverse diffusion flames. This work supports previous conclusions from the experimental work of others. However, in addition, we use the ability of the simulations to numerically track soot parcels along pathlines to further explain the experimentally observed phenomena. In normal diffusion flames, both the peak soot volume fraction and the total mass of soot generated is several orders of magnitude greater than for inverse diffusion flames with the same fuel and air velocities. In normal diffusion flames, soot forms in the annular region on the fuel-rich side of the flame sheet, while in inverse flames, the soot forms in a fuel-rich region on top of the flame sheet. Surface growth is the dominant soot formation mechanism (compared to nucleation) for both types of flames; however, surface growth rates are much faster for normal diffusion flames compared to inverse flames. Soot oxidation rates are also much faster in normal flames, where the dominant soot-oxidizing species is OH, compared to inverse flames, where the dominant soot-oxidizing species is O 2 . In the inverse flames, surface growth continues after oxidation has ceased, causing the peak soot volume fraction to be sustained for a long period of time, and causing the emission of soot, even though the quantity of soot is small. Comparison of soot formation among the three normal diffusion flames shows that the peak soot volume fraction and total mass of soot generated increases as the fuel-to-air velocity ratio increases. A larger fuel–air velocity ratio results in a longer residence time from the nucleation to the oxidation stage, allowing for more soot particle growth. When the fuel-to-oxidizer ratio decreases, there is less time for surface growth, and the particles cross the flame sheet (where they are oxidized) earlier, resulting in decreased soot volume fraction.

Dynamics of a strongly radiating unsteady ethylene jet diffusion flame

This work was sponsored by the Naval Research Laboratory through the Office of Naval Research. Computing time was provided by Numerical Aerodynamic Simulator (NAS) and the Naval Research Laboratory.

Nonlinear filtering for low-velocity gaseous microflows

Gaseous flows in microfluidic devices are often characterized by relatively high Knudsen numbers. For such flows, the continuum approximation is not valid, and direct simulation Monte Carlo (DSMC) may be used to find an appropriate solution. For low-velocity flows, where the fluid velocity is much smaller than the mean molecular velocity, large statistical fluctuations in the solution mean that the features of the flow may be obscured by noise in the solution. The use of a high-order, nonlinear monotone convection algorithm, flux-corrected transport (FCT), as a filter to extract the solution from the noisy DSMC calculation is described. The diffusion, antidiffusion, and flux-limiting properties of FCT are discussed in terms of their filtering properties. The effects of filtering with FCT are demonstrated for a series of test problems, including a square wave with superimposed random noise, and low-and high-velocity and low- and high-Knudsen-number microchannel flows

Gravitational effects on sooting diffusion flames

Simulations of a laminar ethylene-air diffusion flame burning in quiescent air are conducted to gain a better understanding of the effects of buoyancy on the dynamics and behavior of heavily sooting flames under normal-, partial-, micro-, zero-, and negative-gravity conditions and under conditions of gravitational jitter. The simulations solve the time-dependent reactive-flow Navier-Stokes equations coupled with submodels for soot formation and multidimensional radiation transport. Results from the computations follow many of the trends that have been experimentally observed in nonbuoyant diffusion flames. Due to the significant reduction in buoyancy-induced convection, diffusion becomes the dominant mechanism of transport. Microgravity flames are much longer and wider than their earth gravity counterparts due to the reduction in axial velocity and the thicker diffusion layers. In microgravity, flame flicker disappears due to the lack of a buoyancy-induced instability and the entire sooting region is much larger. The reduction in the axial velocity results in significantly longer residence times, allowing more time for soot particle surface growth, and resulting in greatly enhanced soot volume fraction. The enhanced soot production results in increased radiative heat losses, resulting in reduced flame temperatures. By tracing the path lines along which a soot parcel travels, the simulations show significant differences in the local environments through which soot passes between earth-gravity and microgravity flames.

The effect of coflow velocity on a lifted methane-air jet diffusion flame

The effect of coflow velocity on flame liftoff is studied using numerical simulations of methane-air diffusion flames. The numerical model solves the time-dependent, axisymmetric (2-D), Navier-Stokes equations coupled to submodels for chemical reaction and heat release, soot formation, and radiation transport. The computations predict a flame structure similar to that observed experimentally. The flame stabilization point is located on the stoichiometric surface in the inner shear layer. Animations of the simulations show that the radial and axial location of the stabilization point varies in time by 1–2 cm, as the flow field is distorted by passing vortices in the inner shear layer. Flame liftoff heights compare well with those observed experimentally. The computations show that liftoff height increases with jet exit velocity and with the air coflow velocity. For higher coflow velocities, the inner-shear-layer vortices begin to form farther downstream, and the jet spreads more slowly. The scalar dissipation rate along the stoichiometric contour decreases sharply at the liftoff height. All of the liftoff data for a wide range of jet (20–50 m/s) and coflow (10–1500 cm/s) velocities collapse onto one curve when liftoff height is plotted against an effective velocity. These data indicate that the momentum of the coflowing stream acts in combination with the jet momentum to push the base of the flame farther away. This suggests that the momentum at the flame base is a strong factor in determining flame liftoff height.

A combinatorial approach to microfluidic mixing

A new computational approach to the modeling and design of efficient microfluidic mixers is demonstrated. The mixers created provide far more rapid mixing than previous designs. A set of mixer components is created and mapped using a traditional Navier–Stokes fluid solver. The maps are used to quickly model the effect each component has on the lateral distribution of species in the channel. For a mixer of a given length, all the possible combinations of components can be evaluated, and the best mixer for a given metric can be found. Although the mixers presented in this study are short (length-to-width ratios below 8), they show degrees of mixing comparable to much longer mixers found in the literature.

Effect of an Altitude-Dependent Background Atmosphere on Shuttle Plumes

T HE shuttle ionospheric modification with pulsed localized exhaust (SIMPLEX) experiments conducted by the Naval Research Laboratory (NRL) were designed to 1) assess the effects of rocket exhaust interactions in the ionosphere and 2) mimic large ionospheric disturbances that occur naturally [1–5]. These experiments use space shuttle orbital maneuver subsystem (OMS) engines to inject exhaust over ground radar sites. The shuttle exhaust provides a high-speed neutral gas that streams through the ambient plasma of the ionosphere. The neutral exhaust molecules exchange chargewith ambient O ions in the ionosphere. This interaction between the plasma and neutrals results in the formation of ion-ring and beam velocity distributions of plasma particles in the ionosphere. During the SIMPLEX experiments, these distributions are studied with ground radars using incoherent scatter of radio waves from electrons in the ionosphere. To date, six SIMPLEX burns [1–5] have been conducted over incoherent scatter radar sites at various locations, as shown in Table 1. In these experiments, the relative velocity between the ambient ions and injected neutrals is much faster than that which occurs naturally in any region of the ionosphere. Auroral electric fields can accelerate ions to velocities between 0.5 and 2:5 m=s [6]. The neutral injections from the space shuttle in orbit yield exhaust velocities of between 4.7 and 10:7 km=s relative to the background neutrals and ions. During the shuttle burn experiments, the highspeed neutrals chemically react with the stationary plasma. During auroral plasma convection, however, the high-speed ambient O ions and electrons in the ionosphere (accelerated to a high velocity by auroral electric fields) chemically react with stationary neutral molecules or atoms [5,6]. Consequently, the primary difference between high-speed plasma convection and space shuttle OMS burns is the reference frame for the relative ion and neutral motion [5]; both phenomena result in the formation of ring-beam ion velocity distributions. The SIMPLEX experiments were designed to reproduce the naturally occurring ion-ring distributions, which can create ionospheric instabilities leading to regions of plasma turbulence. In addition, the experiments also provide a unique way to examine gas-phase physical and chemical phenomena in the hypersonic and hyperthermal energy regime, which is relatively unstudied and difficult to reproduce in a laboratory [7]. The first step to modeling the ionospheric interactions of space shuttle exhaust is to describe the neutral flow from the exit plane of the shuttle OMSengines into the expanse of the upper atmosphere. In this paper, we use the direct simulation Monte–Carlo (DSMC) method [8] to simulate the shuttle burn and to study the interaction between the shuttle exhaust and the neutral species of the background atmosphere. In the following sections, we present simulation results showing the time evolution of the shuttle plume and background, and we discuss the effect of the altitude-dependence of the background atmosphere.

Ignition phenomenon of solid fuel in a confined rectangular enclosure

Abstract In this study the ignition model of a PMMA (polymethyl-methacrylate) wall has been proposed in a two-dimensional rectangular enclosure when it is initially exposed to a high-temperature source. The effect of surface radiation has been taken into account. It was found that the transient development of thermo-fluid fields was strongly governed by wall heating due to surface radiation. In particular, the rapid heating of the adiabatic floor made the flow very unstable, creating complex secondary recirculating flows. Depending on the hot source temperature, the ignition process was controlled either by mixing and transport of fuel vapor and oxidizer in the vicinity of the hot wall, or by infiltration of hot air into the region near the PMMA wall. Copyright

Towards the development of a multiscale, multiphysics method for the simulation of rarefied gas flows

We introduce a coupled multiscale, multiphysics method (CM 3 ) for solving for the behaviour of rarefied gas flows. The approach is to solve the kinetic equation for rarefied gases (the Boltzmann equation) over a very short interval of time in order to obtain accurate estimates of the components of the stress tensor and heat-flux vector. These estimates are used to close the conservation laws for mass, momentum and energy, which are subsequently used to advance continuum-level flow variables forward in time. After a finite time interval, the Boltzmann equation is solved again for the new continuum field, and the cycle is repeated. The target applications for this type of method are transition-regime gas flows for which standard continuum models (e.g. Navier-Stokes equations) cannot be used, but solution of Boltzmanns equation is prohibitively expensive. The use of molecular-level data to close the conservation laws significantly extends the range of applicability of the continuum conservation laws. In this study, the CM 3 is used to perform two proof-of-principle calculations: a low-speed Rayleigh flow and a thermal Fourier flow. Velocity, temperature, shear-stress and heat-flux profiles compare well with direct-simulation Monte Carlo solutions for various Knudsen numbers ranging from the near-continuum regime to the transition regime. We discuss algorithmic problems and the solutions necessary to implement the CM 3 , building upon the conceptual framework of the heterogeneous multiscale methods.