Flame Propagation and Combustion Processes in Solid Propellant Cracks

The effects of pressurizatio n rate, crack-gap width, crack length, and propellant type on the ignition and flame-spreading processes in isolated AP-based solid propellant cracks have been studied experimentally. Ignition front propagation rates were measured using a high-speed (up to 44,000 pictures/s) camera. Cracks up to 200 mm in length with gap widths as low as 450 j*m were studied. It was observed that the hot gases precede the ignition front. The ignition-front propagation speed increases near the crack entrance, reaches a maximum, and then decreases near the crack tip. The results of parametric study indicate that the time required for the ignition front to reach the crack tip decreases, and that the maximum velocity of the ignition front increases as the pressurization rate or burning rate of the propellant is increased. The maximum pressure in the crack increases with an increase in burning rate or crack length, but decreases with an increase in gap width. Nomenclature a = pre-exponential factor in Saint Robert's burning rate relationship aPn, (mm/s)/(atm)/I An = Andr eev number ,rbdh/a dh = hydraulic diameter of crack, mm L = length of crack, mm n = pressure exponent in Saint Robert's burning rate relationship P = pressure, atm Pmax = maximum pressure in the crack cavity, atm rb = burning rate of solid propellant, mm/s T = temperature, K t =time, s Tf = adiabatic flame temperature of solid propellant, K Tpi = initial propellant temperature, 295 K vfp = convective ignition front propagation velocity, m/s x = axial location, measured from entrance of crack, mm a. = thermal dif fusivity d = gap width of crack p = density, kg/m3