Low pressure burning rates

Since NC is a fuel-rich nitrate ester, a nitropolymer propellant with a high NC content generates black smoke as a combustion product. In addition, the combustion of nitropolymer propellants becomes incomplete at low pressures below about 3 MPa and black smoke composed of solid carbon particles is formed. This incomplete combustion is caused by the slow rates of the reactions of NO with aldehydes and CO in the combustion wave. Thus, the nitropolymer propellants are no longer smokeless propellants under low-pressure burning conditions. 

Low pressure burning behavior gives information concerning the detailed structure of the flame zone. It is known that the fuel-oxidant reaction zone becomes very weak at very low pressures. Thus, the nature of any remaining exothermic reactions occurring at or near the propellant surface is more obvious in the over-all propellant burning behavior. Burning rates and extinction behavior have been measured for a number of propellant systems and are reported below. These results are then interpreted in terms of the theoretical predictions made previously. 

ADN and compositions of ADN have been shown to be able to undergo self-sustained combustion with higher burn rates than the commonly used oxidizer AP. A basic study showed that the bum rate for compressed ADN was 7.4 mm/s at the sub-atmospheric pressure of 0.2 MPa and 54 mm/s at 6 MPa. Since ADN has a surplus of oxygen, its burn rate increases if a carbon source is added. The low-pressure burning is then expanded down to 0.02 MPa. The burn rate for a mixture of ADN and paraffin in the ratio 90 10 has been determined to be 50 mm/s at 7 MPa with a burn rate coefficient of 0.8 and a flame temperature of 2960 [17]. 

Experimental Determination of the Burning Rate. Experimental determinations of the burning rate are made with the closed tomb for gun propellants and the strand burner for rocket propellants. The closed bomb is essentially a heavy-wahed cylinder capable of withstanding pressures to 689 MPa (100,000 psi). It is equipped with a piezoelectric pressure gauge and the associated apparatus requited to measure the total chamber pressure, which is directly related to the force of the propellant. It also measures the rate of pressure rise as a function of pressure which can then be related to the linear burning rate of the propellant via its geometry. Other devices, such as the Dynagun and the Hi—Low bomb, have also been developed for the measurement of gun propellant performance. 

Shock waves from primers, quite often referred to as brisance or the brisant effect , may cause rupture of propint grains (Refs 19 20). This effect is accentuated at low temps, at which condition the NC grains become more brittle and subject to fracture- This effectively results in the burning of much smaller proplnt particles, radically increasing the burning rate. This then produces very high gun chamber pressures, which have been known to cause guns to expld... 

N. E. Cohen, 13th Symp (Int) Combust (Proc) (1970), 1019—29 CA 76, 61471 (1972) To analyze and explain the mechanism of combustion of powdered metals in contact with a solid oxidizer (AP) with the powdered metal dispersed in solid AP (I), the combustion of various compressed I-Al and I-Mg mixts in N2 under various conditions in a high-pressure window bomb was studied. The regression-rate laws of the mixts at high and low pressures, the pressure limits for deflagration, and the structures of the combustion zone and of the surface were detd. The burning rate of various I-Al mixts, as a function of pressure, I particle size, and mixt ratio was determined by cinematography. The combustion was difficult to achieve... 

The solutions obtained by Rosen show that at low pressures, the burning rate becomes linear in pressure and the surface pyrolysis characteristics are not important. At high pressures, however, the burning rate becomes independent of pressure and is determined almost entirely by the decomposition reactions at the solid surface. Rosen points out that this simple model can... 

The results show that at 2 torr, ku = 2.5 X 10 8 and at 760 torr ku = 1.0 X 10 8 cm.3 molecule-1 sec.-1 This is reasonably good agreement in view of the possible errors. Furthermore, the values of ku obtained are consistent with earlier estimates based on comparisons with similar reactions (10, 19). Our purpose in presenting it here is to illustrate the potential use of flames in estimating more accurate rate constants for reactions like Reaction 14. Of course, the influence of diffusion must always be accounted for in such estimations diffusion is particularly important at low pressures and for small ion concentrations. (It is often advantageous to work at low pressures because the spatial resolution is much better than at 1 atm. At low pressures most measurements are made in or close to the reaction zone itself. At high pressures, where the reaction zone is thinner, measurements are made both in the reaction zone and in the burned gases.)... 

It can be shown that if the pressure index of the propellant exceeds 1 the rate of gas increase by factor 2 exceeds the rate of gas loss by factor 1, so that the pressure builds up in the motor, which finally explodes. Quite apart from such an extreme case, a low pressure index in the propellant is desirable so that irregularities in burning are quickly smoothed out with the least effect on rocket performance. It is for this reason that platonising agents mentioned on p. 181 are important, because they enable a very low pressure index to be achieved at ordinary operating pressures of the order of 14 MPa. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

If Da = 1 is defined as the transition between diffusionally controlled and kinetically controlled regimes, an inverse relationship is observed between the particle diameter and the system pressure and temperature for a fixed Da. Thus, for a system to be kinetically controlled, combustion temperatures need to be low (or the particle size has to be very small, so that the diffusive time scales are short relative to the kinetic time scale). Often for small particle diameters, the particle loses so much heat, so rapidly, that extinction occurs. Thus, the particle temperature is nearly the same as the gas temperature and to maintain a steady-state burning rate in the kinetically controlled regime, the ambient temperatures need to be high enough to sustain reaction. The above equation also shows that large particles at high pressure likely experience diffusion-controlled combustion, and small particles at low pressures often lead to kinetically controlled combustion. 

The temperature profile in the combustion wave of a double-base propellant is altered when the initial propellant temperature Tq is increased to Tq -i- ATq, as shown in Fig. 6.15. The burning surface temperature is increased to -i- AT, and the temperatures of the succeeding gas-phase zones are likewise increased, that of the dark zone from Tgto Tg-t- ATg, and the final flame temperature from 7 to Tf-t- ATf If the burning pressure is low, below about 1 MPa, no luminous flame is formed above the dark zone. The final flame temperature is Tg at low pressures. The burning rate is determined by the heat flux transferred back from the fizz zone to the burning surface and the heat flux produced at the burning surface. The analysis of the temperature sensihvity of double-base propellants described in Section 3.5.4 applies here. 

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