Burning Rate Enhancement Research Articles

The relationship between plateau burning behavior and ammonium perchlorate particle size in HTPB-AP composite propellants

The burning rates of series of bi- and trimodal AP-HTPB propellants have been incorporated into an empirical statistical model which describes a hypothetical pentamodal polydisperse AP pseudopropellant. This pseudopropellant is the linear summation of five unimodal polydisperse AP pseudopropellants. The five AP particle size distributions cover the range from 400 to 20 μm weight median diameter). The experimental propellants have been further subdivided into plateau burning and nonplateau burning propellants. The statistical model has been used to generate a plateau burning and a nonplateau burning pentamodal AP pseudopropellant. Plateau burning behavior in this propellant system (when compared to nonplateau burning behavior) appears to result from an enhanced burning rate at low pressures and a reduced burning rate at high pressures for the unimodal 20 μm AP pseudopropellant. It is speculated that such behavior is consistent with the presence of a binder melt on the burning surface. The larger AP unimodal pseudopropellants also show low burning rates and plateau burning behavior. Such behavior appears to be the result of variations in the importance of the AP monopropellant flame as a function of pressure and AP particle size. A melt layer on the AP particle surface during combustion may be the cause of the plateau burning behavior in the larger AP unimodal pseudopropellants. The temperature sensitivity of the unimodal pseudopropellants increases as a function of the AP particle size. Plateau burning behavior is maintained over the temperature range from 0° to 55°C.

An experimental investigation of burning rate and mass transfer in a turbulent fluidized bed

Burning rates of carbon spheres are measured in a 102 mm diameter bed fluidized in the turbulent regime. Results suggest carbon burns much faster than in a bubbling fluidized bed operating under otherwise similar conditions—excepting velocity. The enhanced burning rate is attributed to high mass transfer rates in a turbulent fluidized bed. This indirect observation of high mass transfer rate in turbulent fluidized beds is confirmed by a series of mass transfer (sublimation of naphthalene) experiments in the same bed. For the prediction of mass transfer in turbulent fluidized beds, a correlation of the Sherwood number is proposed. It shows a fair agreement with experimental data.

Thin-flame theory for the combustion of a moving liquid drop: effects due to variable density

The combustion of a moving liquid fuel drop has been investigated. The drop experiences a strong evaporation-induced radial velocity while undergoing slow translation. In view of the high evaporation velocity, the flow field is not in the Stokes regime. The combustion process is modelled by an indefinitely fast chemical reaction rate.While the flow and the transport in the continuous phase and the drop internal circulation are treated as quasisteady, the drop heat-up is regarded as a transient process. The transport equations of the continuous phase require analysis by a singular perturbation technique. The transient heat-up of the drop interior is solved by a series-truncation numerical method. The solution for the total problem is obtained by coupling the results for the continuous and dispersed phases.The enhancement in the mass burning rate and the deformation of the flame shape due to drop translation have been predicted. The initial temperature of the drop and the subsequent heating influence the temporal variations of the flamefront standoff ratio and the flame distance. The friction drag, the pressure drag and the drag due to interfacial momentum flux are individually predicted, and the total drag behaviour is discussed. The circulation inside the drop decreases with evaporation rate. A sufficiently large non-uniform evaporation velocity causes the circulation to reverse.

The mechanism of burning rate catalysis in composite propellants by transition metal complexes

The order of catalytic burning rate enhancement by metal phthalocyanine complexes in an HTPB/AP propellant is: Fe > Cr(F) > Cu > V(O) > Mn > Ni > Zn, Na, Mg. Evidence is presented that the metal phthalocyanine complexes catalyze binder thermal decomposition, so consequently condensed phase chemical reactions can be rate limiting at lower pressures during combustion. From analogies drawn from the known redox potentials of the closely related and isoelectronic metal porphrin complexes, it is inferred that reversible oxidation potentials of the metal phthalocyanine complexes can provide a mechanistic rationale for the observed order of catalytic burning rate efficiencies.

Critical phenomenon in compartment fires with liquid fuels

The objective of this paper was to analyze in detail the behaviour of liquid-fuel compartment fires and to present precise information on the whole aspect of the fire phenomenon for use in modelling compartment fires. Two hundred experiments were conducted to research the behaviour of liquid-fuel compartment fires as a function of the ventilation parameter AH and compartment size, where A is the area of the ventilation opening and H is its height. Especially the influence of the compartment on the burning behaviour was investigated, using various size compartments, and the total aspect of this influence was presented. A critical phenomenon was observed for a pertinent condition of the ratio of compartment size to fuel tray size. This phenomenon, which is closely associated with the enhancement of burning rate and oscillatory combustion, may be substantially controlled by the dynamic equilibrium between the rate of fuel gas supply and the rate of air supplied by ventilation. A maximum burning rate may be obtained at an extreme value on this equilibrium. For methanol pool fires, the maximum burning rate was up to 7.2 times the open burning rate. The temperature within the compartment near the maximum burning condition was unexpectedly uniform and isothermal, due to fluctuation of turbulent flames and recirculation of gas. These experimental results were compared with previous research in detailed discussion.

Observations on the burning of droplets in the absence of buoyancy

Experimental results are reported on the burning of free droplets of heptane and decane in oxygennitrogen atmospheres. The burning occurred in a freely falling chamber providing 2.2 sec of negligible gravity. Enhanced burning rates and a phenomenon of flash extinction were observed and attributed to thermophoretic buildup and ignition of carbon near the droplet surface.

Combustion of ammonium perchlorate-based composite propellants in presence of methylammonium perchlorates

The effect of tri- and tetramethylammonium perchlorates (MAP-3 and MAP-4) on the burning rate of ammonium perchlorate (AP) based propellants has been determined at various pressures. Both additives increase the burning rate; however, MAP-3 has a moderate effect, whereas MAP-4 has a rather large effect. To explain the results, the thermal decomposition and calorimetric values of the propellants having these additives have been examined. Compound MAP-3 affects the thermal decomposition rate considerably, whereas MAP-4 has virtually no effect on the decomposition rate. The contrasting effects of MAP-4 on decomposition and burning rate suggest that the enhancement of burning rate may be due to the catalysis of gas-phase reactions. Further, detailed differences between behaviour of MAP-3, and MAP-4 appear to be attributable to the melting and low-temperature exotherm of MAP-3 and nonmelting and high-temperature exotherm of MAP-4.

Compartment fires with non-cellulosic fuels

In this paper the burning behaviour of fully-developed fires of non-cellulosic materials is considered. It is shown that many of the features of a fire involving a liquid or thermoplastic material can be described by a model in which thermal radiation is the dominant form of heat transfer. Results from such a model are compared with experiments performed in a 1/3-scale fire compartment with a variety of fuels and configurations, and with published data for non-cellulosic materials. The radiative enhancement of burning rate was up to six times the ‘open’ burning rate for ethanol pool fires. The influence of the fuel and burning behaviour on the flame external to the compartment is also considered. It is shown that it is not sufficient to base heat transfer calculations on window geometry and compartment temperature alone; allowance must also be made for external combustion and flame emissivity.

The mechanism of super-rate burning of catalyzed double base propellants

Previous investigators have offered qualitative explanations for the large, pressure-dependent, burning behavior changes seen in nitrate ester propellants when lead or copper salts are added. The most developed qualitative models are those of Camp and of Powling and co-workers but both exhibit certain discrepancies with experiment. New evidence reported here derives from radiation-assisted burning tests in which the spectral content (particularly ultraviolet) of the impinging radiation was varied; contrary to the original hypothesis of Camp, the UV component of the radiation yielded no special burning rate enhancing effect. Experimental evidence recently presented by the authors shows that the burning rate enhancement by lead or copper compounds is a result of acceleration of the fizz zone reactions; this is accompanied by an increased production of carbonaceous material at the burning surface. Therefore it is hypothesized that the acceleration is due to a shift in equivalence ratio toward the stoichiometric when potentially burnable fuel molecules are instead carried through the fizz zone as solid carbon. A simplified mathematical model of this hypothesis is developed based on representing the equivalence ratio change as a shift from a single normal reaction pathway toward a second, more reactive pathway. This hypothesis successfully explains the appearance of a region of enhanced burning rate and its disappearance at higher pressures but it is shown that sudden disappearance (mesa burning), sometimes seen experimentally, requires a further mechanism whose nature is not yet clear.