Ignition and combustion of lunar propellants

The ignition and combustion of Al, Mg, and Al/Mg alloy particles in 99 percent O2/1 percent N2 mixtures is investigated at high temperatures and pressures for rocket engine applications. The 20 micron particles contain 0, 5, 10, 20, 40, 60, 80, and 100 weight percent Mg alloyed with Al, and are ignited in oxygen using the reflected shock in a shock tube near the endwall. Using this technique, the ignition delay and combustion times of the particles are measured at temperatures up to 3250 K as a function of Mg content for oxygen pressures of 8.5, 17, and 34 atm. An ignition model is developed which employs a simple lumped capacitance energy equation and temperature and pressure dependent particle and gas properties. Good agreement is achieved between the measured and predicted trends in the ignition delay times. For the particles investigated, the contribution of heterogeneous reaction to the heating of the particle is found to be significant at lower temperatures, but may be neglected as gas temperatures above 3000 K. As little as 10 percent Mg reduces the ignition delay time substantially at all pressures tested. The particle ignition delay times decrease with increasing Mg content, and this reduction becomes less pronounced as oxidizer temperature and pressure are increased

A model of flame propagation in rich mixtures of coal dust in air

A two-phase combustion model describing fundamental coal dust flame propagation phenomena is developed to treat general fuel rich mixtures. The model includes heterogeneous combustion, pyrolysis of the coal, and homogeneous combustion of volatile matter and the optically thick limit for radiative heat transfer. Calculations for coal (fuel) rich mixtures in air were done for equivalence ratios of 3-8. Predicted burning velocities for 50 [mu]m particles of coal with 36% volatile matter indicated a broad maximum of 37 cm/s at an equivalence ratio of 4 (0.367 kg/m3). The minimum computed velocity was 9 cm/s at [phi] = 8 (0.733 kg/m3). The burning velocity was found to increase as the particle size decreased. The chemical kinetics model was highly simplified, but based on experimental information. The predicted flame temperatures and structures compare well with recent experimental data published by the authors. The structure of the flames was found to be strongly influenced by radiative heat transfer. Flame thicknesses were predicted to exceed 10 cm for most conditions studied.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25749/1/0000309.pd

Experimental Study of Constant Volume Sulfur Dust Explosions

Dust flames have been studied for decades because of their importance in industrial safety and accident prevention. Recently, dust flames have become a promising candidate to counter biological warfare. Sulfur in particular is one of the elements that is of interest, but sulfur dust flames are not well understood. Flame temperature and flame speed were measured for sulfur flames with particle concentrations of 280 and 560 g/m3 and oxygen concentration between 10% and 42% by volume. The flame temperature increased with oxygen concentration from approximately 900 K for the 10% oxygen cases to temperatures exceeding 2000 K under oxygen enriched conditions. The temperature was also observed to increase slightly with particle concentration. The flame speed was observed to increase from approximately 10 cm/s with 10% oxygen to 57 and 81 cm/s with 42% oxygen for the 280 and 560 g/m3 cases, respectively. A scaling analysis determined that flames burning in 21% and 42% oxygen are diffusion limited. Finally, it was determined that pressure-time data may likely be used to measure flame speed in constant volume dust explosions

The Effects of Natural Gas Cofiring on the Ignition Delay of Pulverized Coal and Coke Particles

This paper presents the results of a study designed to determine the effects of natural gas cofiring on particle ignition delay for variously sized pulverized coal and coke particles exposed to realistic combustor conditions. A fluidized bed feeder injects small numbers of particles (typically three to five) into a drop tube furnace at temperatures from 1300K to 1500K with heating rates up to 105 K/sec. Individual particle ignition times are recorded using an optical sensor at the furnace entrance and a photomultiplier tube at the furnace exit. Ignition delay measurements were performed for various inlet gas velocities, particle volatilities and gas compositions (including variations in oxygen, methane, natural gas, nitrogen and carbon dioxide concentrations). Ignition measurements with particles of different volatile contents, ranging from 7.5% to 36.1%, show that addition of 1% methane by volume reduces the ignition delay of low volatile particles to a level similar to the ignition delays for high volatile coal of the same particle size. Experimental results are compared with ignition delays predicted by using a thermal model of particle behavior coupled with two ignition models–one model based on energy absorption and the other based on devolatilization. The thermal model includes the effects of gas phase combustion, particle size and swelling, gas and particle velocity and temperature. The energy-ignition model requires an experimentally determined ignition energy for each tested coal. The devolatilization-ignition model predicts ignition delay using a single value for the minimum volatile concentration required for ignition for all tested coals. Both ignition models accurately predict the measured ignition delay for various volatile contents and sizes in cofiring experiments