Burning rate pressure dependence

Figure 15. Burning rate-pressure dependence at subatmospheric pressures for ammonium perchlorate with different binders...

Generally, bum rate increases or decreases as the pressure is raised or lowered. It is also influenced by the temperature of the propellant and increases with an increase in temperature whereas decreases with a decrease in temperature. It is therefore necessary to fix higher as well as lower temperature limits at the time of development of a propellant keeping in view the mission s requirements. The specified temperature limit for Service applications has been stipulated as -40 °C to +60 °C in India. Burn rate also depends upon the calorific value of the propellant and, other things being equal, the higher the calorific value, the higher the burn rate of the propellant. 

How does the linear burn rate generally depend on the pressure ... 

Low explosives are usually a mixture of two or more materials that burn very rapidly. The rate of burning depends on the materials involved, how well they are mixed, the pressure they are under, anu how ihey are ignited. Because the burn rate is dependent upon pressure, low explosives must be tightly contained before they can be made to explode. Low explosives also are usually much less powerful then high explosives. 

Obviously the pressure dependence of the burning rate in Taylor s expts with porous PETN is much greater than linear... 

When the propellant burning rate is espressed by Eq. (5b), the parameter p /e can be considered as the transient sensitivity of the burning rate to pressure. This parameter depends on the transient combustion characteristics, and its evaluation depends on the particular model of the combustion process. Thus, the acoustic admittance provides the link between experimental observation and theoretical prediction. 

It is worth noticing that the "turbulent burning rates" reported in Figure 7.1.2 have been defined similarly but not exactly as the "turbulent flame speed" mentioned in Section 7.1.2. The mixture has been ignited at the center of the bomb and the dependence of the pressure on time has been recorded. This has enabled to determine the derivative of the burned mixture volume. This derivative is ascribed to a spherical surface whose volume is simply equal to the volume of fully burned products, thus leading to an estimate of the turbulent combustion rate. 

As the blackpowder core of a safety fuse bums, it produces gases which must escape. At the same time the heat of the combustion melts the bitumen and plastic and thus produces side venting through the textile layers. This results in the production of an increased but constant gas pressure, determined by the equilibrium between gas generation and gas lost sideways. As the rate of burning of blackpowder depends markedly on the pressure, it is this process of equilibration which determines the speed of burning of the fuse. 

Thus, th in a kinetically controlled regime is described by a dl law. Furthermore, th is found to be inversely proportional to pressure (for a first-order reaction) under kinetically controlled combustion, and in contrast, independent of pressure under diffusionally controlled combustion (since D P-1). In the kinetically controlled regime, the burning rate depends exponentially upon temperature. 

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