Arvind Atreya

A Simplified Model for the Pyrolysis of Charring Materials

A simplified model of the pyrolysis of charring materials is analyzed. The effects of moisture are neglected, and the heat of pyrolysis is assumed equal to zero. Four stages of pyrolysis are obtained: (i) inert heating, (ii) initial pyrolysis, (iii) thin char, and (iv) thick char. Formulas for the volatile mass efflux, m, are obtained in stages (ii), (iii), and (iv); m = 0 in the first stage. The calculations indicate that the surface temperature controls the volatile production rate in the initial pyrolysis stages (the kinetically controlled regime), while the temperature gradient controls the volatile production rate in the thick char stage (the diffusion-controlled regime). Comparisons of the calculated results with numerical computations are made for the volatile mass efflux, the surface temperature, and the density.

A one-dimensional model of piloted ignition

Abstract In this article a model of piloted ignition is analyzed. The two-dimensional coupled solid and gas phase problem is simplified by assuming that the mass evolution rate from the combustible solid is a known function of time and by employing a plane rather than a point ignition source. Thus only a transient one-dimensional analysis of the gas phase is necessary. The equations are solved numerically using a fast scheme especially suitable for combustion problems. The pilot flame is modeled as a thin slab of gas that is periodically raised to the adiabatic flame temperature of the stoichiometric mixture. The effects of (1) ignition source location, (2) fuel mass evolution rate from the surface, and (3) surface temperature of the solid are investigated. An explanation is offered for the preignition flashes often observed experimentally. A rational criterion for positioning of the pilot flame is proposed. The minimum fuel flow rate, by itself, is found insufficient for predicting the onset of piloted ignition; heat losses to the surface play an important role. Also, the conditions at extinction of a steady diffusion flame are found to be very similar to those for piloted ignition.

Wind-aided flame spread over charring and non-charrring solids: An experimental investigation

This paper presents the results of a detailed experimental investigation on laminar forced flow wind-aided flame spread over wood (with external radiation) and PMMA in the ceiling configuration. The speed of propagation of the pyrolysis front and flame front and the production rates of major chemical species are measured as a function of time. The objectives are to study the dependence of the flame and pyrolysis front speeds on the free stream velocity and the oxygen mass fraction and to infer the local fuel pyrolysis rates from the measured production rates of major chemical species. Several important conclusions are derived from this study: (i) In the configuration studied, the pyrolysis front and the flame front, for both wood and PMMA, were found to be much closer to each other than predicted by the theoretical models. This is also true for the pyrolysis front and the flame front speeds regardless of the free stream velocity and/or the oxygen mass fraction. (ii) The pyrolysis front and the flame front speeds for both wood and PMMA vary nearly linearly with the free stream velocity as predicted by the theoretical models. With oxygen mass fraction they vary as Y o ∞ 1.1 for wood and Y o ∞ 1.4 for PMMA. (iii) Species measurements show that the pyrolysis mass flux becomes almost constant with the downstream distance from the leading edge for both wood and PMMA. This is in disagreement with Emmons steady boundary layer burning theory, where the mass flux decays as x −0.5 . Clearly, a steady state is not achieved in the entire burning zone. Nevertheless, the flame spread rate is predicted well by models that use Emmons solution in the burning zone. This indicates that the flame spread rate depends primarily on local heating of the solid by the flame tip in the adjacent preheat zone.

Observations of methane and ethylene diffusion flames stabilized around a blowing porous sphere under microgravity conditions

This paper presents the experimental and theoretical results for expanding methane and ethylene diffusion flames in microgravity. A small porous sphere made from a low-density and low-heat-capacity insulating material was used to uniformly supply fuel at a constant rate to the expanding diffusion flame. A theoretical model which includes soot and gas radiation is formulated but only the problem pertaining to the transient expansion of the flame is solved by assuming constant pressure infinitely fast one-step ideal gas reaction and unity Lewis number. This is a first step toward quantifying the effect of soot and gas radiation on these flames. The theoretically calculated expansion rate is in good agreement with the experimental results. Both experimental and theoretical results show that as the flame radius increases, the flame expansion process becomes diffusion controlled and the flame radius grows as gamma t. Theoretical calculations also show that for a constant fuel mass injection rate a quasi-steady state is developed in the region surrounded by the flame and the mass flow rate at any location inside this region equals the mass injection rate.

Wind-aided flame spread over an unsteadily vaporizing solid

A simple but realistic model for the flame length and the flame front speed during windaided flame spread on a thick vaporizing solid like PMMA is presented. The model predictions compare favorably with the experiments conducted in the ceiling configuration. This model is based on the following experimental observations: (i) During wind-aided flame spread, the solid-phase undergoes transient pyrolysis while the gas-phase remains quasi-steady, (ii) In the ceiling configuration, the flame stand-off distance is much smaller than the thermal boundary layer thickness. To incorporate the first observation into the model, an expression was derived for the transient mass flux as a function of the net incident heat flux. This expression was verified by transient pyrolysis experiments conducted on PMMA for heat fluxes ranging from 1.6 to 5 W/cm 2 in N 2 atmosphere. The second observation was exploited to obtain considerable simplification in the gas-phase enabling an explicit expression for the convective heat flux from the flame to the solid. This heat flux was corrected for shielding due to blowing of fuel mass from the solid surface. A comparison of the model with the experimental results show that shielding of heat transfer due to blowing and the radiative heat loss from the sample surface have a large effect on the flame length and the flame front speed. It was also found that for cases where the flow is laminar and flame radiation is small, surface heat loss causes the pyrolysis front speed to eventually become zero for a thick sample. This may occur even before the pyrolyzing zone achieves steady state as assumed by previous theoretical models. Once the pyrolysis front speed becomes zero, the flame spread rate is controlled by the speed of the burnout front which propagates behind the pyrolysis front for thin or charring materials.

Heat transfer during wind-aided flame spread on a ceiling mounted sample

In this paper the effect of wind speed and ambient oxygen mass fraction on heat transfer to the surface underneath and ahead of the flame during wind-aided laminar flame spread has been investigated experimentally and analyzed according to a simple model. These experiments were performed in the ceiling configuration. High temperature ceramic solids in-strumented with surface and in-depth thermocouples were used downstream of the PMMA sample experiencing flame spread. Simultaneous transient measurements of the PMMA surface temperature, ceramic solid temperatures, gas-phase temperatures, and the flame tip location were made. Since there was little excess pyrolyzate in all the experiments, the flame length was only slightly larger than the pyrolysis length. Thus, heat transfer measurements underneath the flame were conducted with a steady state methane diffusion flame in the boundary layer over the ceramic detectors mounted in the ceiling configuration. Results of heat transfer measurements downstream of the flame tip for the steady state flame agree well with those of the transient flame spread experiments. This shows quasi-steady gas-phase behavior during wind-aided flame spread and implies that any transient behavior is primarily due to the solid-phase. It was also found that for these laminar flames, convection is the dominant mode of heat transfer to the sample surface both ahead and underneath the flame for ambient oxygen concentration conditions. However, flame radiation becomes more significant as Y ox increases. Models for surface temperature downstream of the pyrolysis front and the pyrolysis front spread rate were also developed from these heat transfer correlations. The model predictions agree well with the measurements.

Fire Growth on Horizontal Surfaces of Wood

Abstract The results of an extensive experimental work conducted on transient three-dimensional fire spread on horizontal surfaces of various woods up to two feet in diameter indicate that flame spread on wood is sensitive to factors both internal and external to the sample. The transport of fuel gases from the burning to the unburned zone presents a new mechanism of flame spread unique to wood. These experiments show that (i) forward radiative heat transfer from the flame is the primary accelerating mechanism, (ii) forward gas phase heat transfer is a local phenomenon independent of fire size, (iii) conduction of heat parallel to the spread surface does not contribute significantly to the flame spread process, (iv) reradiation from the surfaces of wood and char is the primary heat loss mechanism, and (v) energetics due to the desorption of adsorbed moisture has a very noticeable effect on the flame spread process. Using these experin~ental observations and treating flame spread as a continuous ignition p...

Effect of fuel dilution by CO2 on spherical diffusion flames in microgravity

This paper presents the experimental results for expanding spherical diffusion flames in microgravity. The flames are fueled with ethylene diluted with various concentrations of CO2. A comparison is made between flames with pure ethylene and the diluted flames to test the effect of increased radiative heat loss predicted by the CO2 addition. A small porous spherical burner was used to produce the aerodynamically stabilized gaseous spherical diffusion flames. Measurements taken from these flames include radius, flame radiation and flame temperature. The results indicate that as the ethylene flow rate decreases corresponding to increased CO2 dilution, the flame temperature and radiation decreases and the flame radius is decreased as well. The radiation data shows that soot formation in the diluted flames is inhibited, but that gas radiation from CO2 remains significant despite the reduced burning rates of these flames and may be enhanced by the CO2 added to the fuel.

Transient Measurements of Temperature and Radiation Intensity in Spherical Microgravity Diffusion Flames

This work reports transient measurements of flame growth, temperature field, and hemispherical spectral radiation emission in expanding spherical diffusion flames. These measurements were made to study the effect of various diluents (N2, CO2, He) on properties of microgravity diffusion flames. Diluents were chosen to replace nitrogen on the oxidizer side of the flame, maintaining a constant oxygen composition. Experiments were conducted at atmospheric pressure using a porous spherical burner aerodynamically supporting an ethylene (C2H4) diffusion flame in an oxidizing atmosphere. A color CCD camera was used for visual observations and to obtain the flame growth rate. Temperature field was measured by an array of thermocouples that were corrected for radiation and conduction heat transfer and time constant. Radiation intensity was measured by an array of photo detectors sensitive to selected spectral regions. Experimental results show that Lewis number of the oxidizer does not have a dominant effect on flame growth. Influence of a CO2/He mixture despite controlling ρCP, the energy storage component of the reactant mixture in comparison to N2 diluent, reduced soot formation and flame temperature due to increased energy transfer. The CO2/He mixture increased the volume averaged temperature and lowered the flame emission indicating both an increase in heat transfer and reabsorption of the energy. Helium diluent lowered the flame temperature, increased the volume averaged temperature, and maintained flame emission. CO2 diluent lowered the flame temperature, decreased the volume averaged temperature, and also lowered flame emission.