Howard R. Baum

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.

A model for combustion of firebrands of various shapes

The lifetime of a firebrand before burning out controls the maximum distance a firebrand can travel to cause spotting. Thus, combustion of firebrands of various shapes and sizes and their burnout time during transport is studied. The analysis assumes “quasi-steady” burning. In the present context, “quasi-steady” means that the rate processes controlling the gas phase fuel consumption and energy release are much faster than the particle fuel depletion time or the gas phase transport times. The Reynolds number based on the overall particle dimension and velocity relative to the ambient flow is assumed to be small. The gas phase combustion processes are represented by the evolution of a mixture fraction variable. It is shown that the velocity field near the particle can be described by a potential flow whose functional form is determined by the mass conservation equation and that this flow satisfies the particle surface boundary conditions. Gas phase solutions are obtained for two-parameter family of firebrand shapes composed of oblate and prolate ellipsoids of revolution. Prolate ellipsoids range from a thin needle to a sphere and oblate ellipsoids range from a sphere to a thin disc. Thus, they cover all possible firebrand shapes. The ambient velocity field does not need to be aligned with the firebrand axis of symmetry, so that the composite velocity and mixture fraction fields are three-dimensional. While a variety of steady-state condensed phase models are compatible with this picture, results are first presented for an ablating solid describable by the Spalding B number. B-numbers representative of flaming combustion of wood firebrands and glowing combustion of remaining char are used. All quantities are calculated as a function of ellipsoidal aspect ratio, B number, and the Reynolds number. Surprisingly, it is found that the firebrand burnout time is shape independent. All possible shapes were considered by using oblate and prolate ellipsoids of different sizes and aspect ratios. The burnout time depends only on the firebrand mass under the assumptions used.

Local Extinction of Diffusion Flames in Fires

The objective of the present study is to use large activation energy asymptotic (AEA) theory to bring basic information on the extinction limits of non-premixed flames. The AEA analysis leads to an explicit expression that predicts the occurrence of flame extinction in the form of a critical Damkohler number criterion; the criterion provides a unified framework to explain the different extinction limits that are observed in non-premixed combustion (i.e., aerodynamic quenching, thermal quenching, and dilution quenching). The critical Damkohler number criterion is then formulated in terms of six input variables; these variables characterize the magnitude of flame stretch, the magnitude of the flame heat losses, and the composition and heat content of the fuel and oxidizer supply streams; these input variables thereby contain information on (laminar or turbulent) flow-induced perturbations, deviations from adiabatic combustion, and air and fuel vitiation. Different two-dimensional flammability maps are then presented using different assumptions aimed at reducing the dimension of the parameter space from six to two. While providing a limited view point, these flammability maps provide valuable insights; it is found for instance that diffusion flames are more sensitive to air vitiation than fuel vitiation.