Studies of droplet burning and extinction

A project on droplet combustion, pursued jointly with F.L. Dryer of Princeton University, has now been in progress for many years. The project involves experiments on the burning of single droplets in various atmospheres, mainly at normal atmosperic pressure and below, performed in drop towers and designed to be performed aboard space-based platforms such as the Space Shuttle or the Space Station. It also involves numerical computations on droplet burning, performed mainly at Princeton, and asymptotic analyses of droplet burning, performed mainly at UCSD. The focus of the studies rests primarily on time-dependent droplet-burning characteristics and on extinction phenomena. The presentation to be given here concerns the recent research on application of asymptotic methods to investigation of the flame structure and extinction of hydrocarbon droplets. These theoretical studies are investigating the extent to which combustion of higher hydrocarbons - heptane, in particular - can be described by four-step reduced chemistry of the kind that has achieved a good degree of success for methane flames. The studies have progressed to a point at which a number of definite conclusions can now be stated. These conclusions and the reasoning that led to them are outlined here

Scaling considerations for fire whirls

This brief report, based on a presentation made at the Eighth International Symposium on Scale Modeling, held in Portland, Oregon, in September of 2017, summarizes and evaluates different methods for classifying fire whirls and their scaling laws. It is indicated that a number of relevant non-dimensional parameters are known for fire whirls, and future scale-modeling experiments could provide useful additional information and insights

A simplified model for the intermediate structure of strong fire whirls

A model is described for the overall structure of intense fire whirls, based on a spatially evolving vortex, with circulation enhancement driven by the axial acceleration of low-density gas in the core through the axial pressure gradient. The axial acceleration increases the entrainment rate into the core which, through mass conservation, increases the circulation if the angle between the tangential and radial velocity components remains fixed. The two-zone model employs general balance equations for regions inside and outside a cylinder of fixed radius, each inviscid, the inside region being presumed to have a constant density small compared with the (constant) value outside. In the outside region the tangential component of velocity is assumed to be large compared with the radial component, which, in turn, is assumed to be large compared with the axial component. The model predicts an exponential increase of the circulation with axial distance for sufficiently long whirls, which persists until the fuel in the core is completely consumed. Predictions of the model appear possibly to be consistent with the experimentally observed scaling of flame lengths of strong fire whirls

High-pressure droplet combustion studies

This is a joint research program, pursued by investigators at the University of Tokyo, UCSD, and NASA Lewis Research Center. The focus is on high-pressure combustion of miscible binary fuel droplets. It involves construction of an experimental apparatus in Tokyo, mating of the apparatus to a NASA-Lewis 2.2-second drop-tower frame in San Diego, and performing experiments in the 2.2-second tower in Cleveland, with experimental results analyzed jointly by the Tokyo, UCSD, and NASA investigators. The project was initiated in December, 1990 and has now involved three periods of drop-tower testing by Mikami at Lewis. The research accomplished thus far concerns the combustion of individual fiber-supported droplets of mixtures of n-heptane and n-hexadecane, initially about 1 mm diameter, under free-fall microgravity conditions. Ambient pressures ranged up to 3.0 MPa, extending above the critical pressures of both pure fuels, in room-temperature nitrogen-oxygen atmospheres having oxygen mole fractions X of 0.12 and 0.13. The general objective is to study near-critical and super-critical combustion of these droplets and to see whether three-stage burning, observed at normal gravity, persists at high pressures in microgravity. Results of these investigations will be summarized here; a more complete account soon will be published

Theory of the propagation dynamics of spiral edges of diffusion flames in von Kármán swirling flows

This analysis addresses the propagation of spiral edge flames found in von Kármán swirling flows induced in rotating porous-disk burners. In this configuration, a porous disk is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame adjacent to the disk. Among other flame patterns experimentally found, a stable, rotating spiral flame is observed for sufficiently large rotation velocities and small fuel flow rates as a result of partial extinction of the underlying diffusion flame. The tip of the spiral can undergo a steady rotation for sufficiently large rotational velocities or small fuel flow rates, whereas a meandering tip in an epicycloidal trajectory is observed for smaller rotational velocities and larger fuel flow rates. A formulation of this problem is presented in the equidiffusional and thermodiffusive limits within the framework of one-step chemistry with large activation energies. Edge-flame propagation regimes are obtained by scaling analyses of the conservation equations and exemplified by numerical simulations of straight two-dimensional edge flames near a cold porous wall, for which lateral heat losses to the disk and large strains induce extinction of the trailing diffusion flame but are relatively unimportant in the front region, consistent with the existence of the cooling tail found in the experiments. The propagation dynamics of a steadily rotating spiral edge is studied in the large-core limit, for which the characteristic Markstein length is much smaller than the distance from the center at which the spiral tip is anchored. An asymptotic description of the edge tangential structure is obtained, spiral edge shapes are calculated, and an expression is found that relates the spiral rotational velocity to the rest of the parameters. A quasiestatic stability analysis of the edge shows that the edge curvature at extinction in the tip region is responsible for the stable tip anchoring at the core radius. Finally, experimental results are analyzed, and theoretical predictions are tested

A Hypothetical Burning-Velocity Formula for Very Lean Hydrogen-Air Mixtures

Very lean hydrogen-air mixtures experience strong diffusive-thermal types of cellular instabilities that tend to increase the laminar burning velocity above the value that applies to steady, planar laminar flames that are homogeneous in transverse directions. Flame balls constitute an extreme limit of evolution of cellular flames. To account qualitatively for the ultimate effect of diffusive-thermal instability, a model is proposed in which the flame is a steadily propagating, planar, hexagonal, close-packed array of flame balls, each burning as if it were an isolated, stationary, ideal flame ball in an infinite, quiescent atmosphere. An expression for the laminar burning velocity is derived from this model, which theoretically may provide an upper limit for the experimental burning velocity

Four-step and three-step systematically reduced chemistry for wide-range H₂–air combustion problems

The feasibility of developing multipurpose reduced chemistry that is able to describe, with sufficient accuracy, premixed and non-premixed flames, one-dimensional detonations, high-temperature autoignition, and also low-temperature autoignition is explored. A four-step mechanism with O and OH in steady state is thoroughly tested and is shown to give satisfactory results under all conditions. The possibility of reducing this to a three-step mechanism, to decrease computation times without compromising the range of applicability is then investigated. The originality of this work resides in introducing a single species X, representing either HO₂ for high-temperature ignition or H₂O₂ for low-temperature ignition. An algorithm is defined that covers the entire range without significant degradation of accuracy. Integrations show promising results for different laminar test cases, and applicability to turbulent flows is indicated.This work was supported by the UE Marie Curie ITN MYPLANET, by the Spanish MCINN through Project # CSD2010-00010, by the Comunidad de Madrid through Project # S2009/ENE-1597, and by the US AFOSR Grant # FA9550-12-1-0138

Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover

This paper addresses homogeneous ignition of hydrogen-oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit. A three-step reduced mechanism involving H2, O2, H2O, H2O2 and HO2, derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO2 consumption through HO2 + HO2 → H2O2 + O2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H2 + O2 → 2H2O and 2H2O → H2O2 + H2. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure.This work was supported by the Comunidad de Madrid through project # P2009/ENE-1597. The first two authors also acknowledge support from the EU through the Marie Curie ITN MYPLANET and from the Spanish MCINN through projects # ENE2008-06515 and CSD2010-00011.European Community's Seventh Framework ProgramPublicad

The chemistry involved in the third explosion limit of H₂–O₂ mixtures

The third explosion limit of hydrogen oxidation in closed vessels has always been thought to be the result of the competition between homogeneous gas-phase reactions and diffusion of hydroperoxyl radicals to the walls, where they are destroyed. It has recently been observed that this species actually follows a chemical-kinetic steady state in this regime, with the consequence that its diffusive rate toward the catalytic walls becomes irrelevant. Here we show that the critical explosion conditions are determined instead by the fate of hydrogen peroxide, which emerges as the controlling reactant for the resulting gas-phase chemistry. A simple, accurate analytic expression for the third explosion limit follows from identification of the critical conditions for existence of weakly reactive, diffusion&-reaction solutions, thereby providing the answer to a long-standing problem that in early work was characterized as being hopelessly difficult.This work was supported by the US AFOSR Grant # FA9550-12-1-0138, by the Comunidad de Madrid through Project #P2009/ENE-1597, and by the Spanish MCINN through Project #CSD2010-00011

Synthesis of Multi-Wall Carbon Nanotubes Using Unseeded Hydrocarbon Diffusion Flames

A method is provided for synthesizing carbon nanotubes from unseeded methane-air diffusion flames. A novel stainless steel and Ni—Cr wire probe is also provided for collecting carbon nanotubes from those diffusion flames