Edward G. Groff

The cellular nature of confined spherical propane-air flames

Abstract The transitions of initially smooth spherical laminar flames to polyhedral&cellular flames were observed in propaneair mixtures ignited at the center of a 260-mm-diam constant-volume vessel. Experiments were conducted for initital pressure in the range of 200–500 kPa, equivalence ratio in the range of 0.7-1.0, and initial temperature near 300K. Measurements of flame propagation velocity, burning velocity, and cell size are reported. In contrast to most cellular flames reported in the literature, those in this study had stoichiometric or fuel-lean mixture compositions, with the fuel having a diffusivity less than oxygen for diffusion in air. A diffusional-thermal theory, developed by Sivashinsky to include the relative diffusion of reactants, predicts flame stability. The flame Reynolds number at the onset of the cells was found to correlate in a nearly linear manner with the flame expansion ratio. This behavior is in qualitative agreement with theoretical predictions by Istratov and Librovich based on Marksteins first-order perturbation treatment, which considers the flame to be a hydrodynamic discontinuity in the gas density profile. The cellular flames observed in this study are produced by a hydrodynamic instability mechanism.

An experimental evaluation of an entrainment flame-propagation model

Abstract Experimental burning-velocity and propagation-velocity data are presented for laminar and turbulent flames ignited in a constant-volume vessel. Burning velocities were obtained using a double-kernel technique whereby the expansion component of the propagation velocity is canceled by propagating two flames toward one another. Propagation velocities were obtained from freely propagating flames. The turbulent flow field was the same in both experiments. High-speed schlieren photography was used to determine flame velocities. The burnig-velocity data are used as one input parameter to a two-parameter entrainment flame-propagation model published in the literature. The model is then fit to the flame-radius and propagation-velocity data to determine the other parameter, a characteristic reaction time. It is shown than the model underestimates experimentally observed flame acceleration unless burning velocity is reduced at small flame radii with an empirical term which is a function of flame radius and thickness. With the empirical term the entrainment model does a reasonable job of predicting flame-propagation rates for the flames examined.

Turbulence characteristics of a fan-stirred combustion vessel

The authors previously applied experimental data on spark-ignited flame propagation in a constant-volume vessel to evaluate a phenomenological model of turbulent combustion. In this paper they present detailed information about the devicess turbulent velocity field, aspects of their measurement techniques, and the accuracy and repeatability of their turbulence data. Interest in these issues arises in part from the possible relation of velocity-field fluctuations to cyclic combustion variability in automotive engines.

Microwave effects on premixed flames

It has been proposed in the literature that microwave heating of combustion-generated plasmas in internal-combustion engines can be used to increase the rate of combustion of dilute mixtures. Experiments were conducted on fuel-lean laminar flames held above a porous burner flowing premixed mixtures of fuel (propane, ethylene, or methane) and oxidizer (air or oxygen-argon mixtures). A flame was positioned in a cavity resonated with microwaves at a frequency of about 2.4 GHz, with electric field intensities ranging to over 100,000 V/m. For the lean-mixture air flames (0.6 less than equivalence ratio less than 0.8) examined in this study, burning velocity enhancement increased with electric field intensity to a maximum value of 6 percent. It is concluded that the enhancement can be explained in terms of simple microwave heating of the bulk gases in the flame zone, which yields a greater flame temperature.