Combustion Wave Speed Research Articles

Combustion wave speeds of nanocomposite Al/Fe 2O 3: the effects of Fe 2O 3 particle synthesis technique

Combustion wave speeds of nanoscale aluminum (Al) powders mixed with iron oxide (Fe 2O 3) were measured as a function of Fe 2O 3 synthesis technique and fuel/oxidizer composition. Three reactant synthesis techniques were examined; two focus on sol–gel processing of nanoscale Fe 2O 3 particles and the third utilizes commercially available nanoscale Fe 2O 3 powder. Nanoscale aluminum particles (52 nm in diameter) were combined with each oxidizer in various proportions. Flame propagation was studied by igniting low-density mixtures and taking data photographically with a high-speed camera. Both open and confined burning were examined. Results indicate that the combustion wave speed is a strong function of the stoichiometry of the mixture and a slightly fuel-rich mixture provides an optimum combustion wave speed regardless of oxidizer synthesis technique. Oxidizers processed using sol–gel chemistry originally contained impurities which retarded the combustion wave speeds. When the same oxidizers are annealed at moderate temperatures, the new heat-treated oxidizer shows a dramatic improvement, with combustion wave speeds on the order of 900 m/s.

Non-adiabatic combustion waves for general Lewis numbers: wave speed and extinction conditions

AbstractThis paper addresses the effect of general Lewis number and heat losses on the calculation of combustion wave speeds using an asymptotic technique based on the ratio of activation energy to heat release being considered large. As heat loss is increased twin flame speeds emerge (as in the classical large activation energy analysis) with an extinction heat loss. Formulae for the non-adiabatic wave speed and extinction heat loss are found which apply over a wider range of activation energies (because of the nature of the asymptotics) and these are explored for moderate and large Lewis number cases—the latter representing the combustion wave progress in a solid. Some of the oscillatory instabilities are investigated numerically for the case of a reactive solid.

Ignition and Combustion Behaviors of Nanocomposite Al/MoO3

ABSTRACTFlame propagation in Al/MoO3 thermites is measured as a function of bulk density and initial sample temperature. The composites are composed of nano-scale reactant particles mixed and pressure molded to between 49 and 73% of the theoretical maximum density. A relationship between cylindrical die pressure and final pellet density is derived. Experiments are also performed by initially pre-heating samples to a uniform temperature ranging from 20 to 200 °C. Ignition sensitivity was determined by measuring the ignition delay time and temperature using a 50-W CO2 laser and thermocouples, respectively. Combustion wave speeds were measured using high-speed imaging diagnostics. Results allow comparison of combustion behaviors associated with nano- vs. micron-scale particle composites. The nano-scale particle composites are extremely sensitive to ignition, especially when initially preheated. Combustion wave speeds for the compressed nano-composites were found to double when compared to the micron-scale composites.

Structure of a pressure driven flame in porous media

The model of a combustion wave driven by local elevation of pressure in inert porous media filled with flammable gas is investigated. The combustion wave structure is studied using the Zel'dovich approach. Analytical formulae for the combustion wave speed and for values of temperature and pressure behind the flame front are derived.

Radiation enhanced combustion wave speeds

Reactiondiffusion systems are known to admit travelling wave solutions which can be interpreted as combustion waves if the reaction term is exponential in temperature following the Arrhenius law. T...

Superadiabatic combustion of methane air mixtures under filtration in a packed bed

Filtration combustion in porous media differs substantially from combustion in a homogeneous media. The difference is the heat transfer between the filtrated gas and the porous medium under conditions of active heat transfer over a highly developed internal solid surface. The reactive transfer is characterized by a thermal wave velocity that determines the velocity of heat accumulation in a porous medium due to the filtrated flow and by a reaction wave velocity. A resonance condition can occur when both velocities nearly coincide and the heat of reaction becomes localized in a moving thermal wave. A temperature can be reached much in excess of the adiabatic reaction temperature and thus self-propagating reactions are possible even in a weakly exothermic medium. The propagation of the localized resonance wave and the superadiabatic reaction effect is confirmed by experiments and the results of a simple mathematical analysis. The model is compared with predictions for the thermal and combustion wave speeds and demonstrates good agreement.

Characteristics of water-base foam combustion

Systematic experimental studies have been carried out to characterize the combustion of water-base foams with various hydrogen- and methane-air mixtures. The dependencies of the combustion wave speed (S) on the expansion ratio of foam (K), concentration of surface-active agent (SAA) in detergent solution, and bubble size of foam have been investigated. The combustion of foams with hydrogen mixtures has been established to be steady state. The effects of unsteadiness arising from combustion of methane foams are due to sound generation in the system and its interaction with the flame. The flame speed in foam with rapidly burning gas mixture has been demonstrated to decrease with decreasing expansion ratio and the foams with slowly burning mixtures have demonstrated to be a non-monotonic function of K, including an anomalous region of increasing flame speed with decreasing K. It has been demonstrated that the increase in both the SAA concentration in a detergent solution and the mean size of foam bubbles have no effect on the form of S(K) dependence but leads, however, to the increase in the flame speed values. The characteristics of foam combustion have been shown to be due to the effects of heat transfer from the gas to foammore » films and aerosol droplets forming upon destruction of foams by a flame front. Taking into account the interphase interaction provided the basis for derivation of a simple model of foam combustion and allowed to predict the flame speed dependences on the controlling parameters. The characteristics of combustion predicted by the model are in fair agreement with experimental data.« less