Mridul Kumar

Effect of deformation on flame spreading and combustion in propellant cracks

A comprehensive theoretical model was formulated to study the development of convective burning in a solid propellant crack which continually deforms due to burning and pressure loading. The effect of interrelated structural deformation and combustion processes was included in the theoretical model. The set of coupled, nonlinear, governing partial-differential equations was solved numerically. Several regions of partial crack closures were observed experimentally in narrow cracks (gap width -450 /on). Predicted results indicate that the partial closures may generate substantial local pressure peaks along the crack, implying a strong coupling between chamber pressurization, crack combustion, and propellant deformation, especially when cracks are narrow and chamber pressurization rates are high. Predicted results for ignition-front propagation and pressure distribution are in good agreement with experimental data. Both theoretical and experimental results indicate that the maximum pressure in the crack cavity is generally higher than that in the chamber. Under the conditions studied, it was found that the initial flame-spreading process is not affected substantially by propellant deformation.

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 Roberts 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 Roberts 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

A comprehensive erosive-burning model for double-base propellants in strong turbulent shear flow

Abstract A comprehensive aerothermochemical model of erosive burning of double-base propellants has been developed. The present analysis includes detailed modeling of the combustion process, taking into account both chemical kinetics and diffusion effects in the gas phase. Different reactions simulating the fizz zone and the final flame zone (including dark and luminous zones) are considered in the gas phase. The instantaneous governing partial differential equations are Favre averaged to take into account variable density effects. The turbulence modeling consists of a two-equation ( k -ϵ) turbulence closure model for the final Favre-averaged conservation equations. The set of governing equations is solved numerically. Predicted results compare well with experimental data of Burick and Osborn for a noncatalytic double-base propellant. Predicted temperature profiles in the boundary layer correctly depict the characteristics of double-base propellant combustion. The main mechanisms for augmentation of the burning rate is the increase in heat feedback caused by turbulence-enhanced reactions and transport properties in the fizz zone.

Ignition of Solid Propellant Crack Tip under Rapid Pressurization

Under rapid chamber pressurization rates (~ 105 atm/s or higher), the closed end of a solid propellant crack was observed to ignite prior to the arrival of the convective ignition front. Tests were conducted in an inert crack with propellant only at the tip to eliminate some of the possible mechanisms for crack-tip ignition. Ignition delay time, defined as the time lag between the arrival of the pressure wave at the tip and the subsequent onset of emission of luminous light from the propellant surface, has been measured as a function of chamber pressurization rate. A theoretical model has been developed to explain the tip ignition phenomena. The model considers: a one-dimensional transient heat conduction equation for the solid phase; one-dimensional, unsteady mass and energy conservation equations for the gas phase near the crack tip; and an experimentall y observed pressure-time trace near the crack-tip region in place of the gas phase momentum equation. Both experimental and theoretical results indicate that the ignition delay time decreases as the pressurization rate is increased. Theoretically calculated ignition delay times are in good agreement with the experimental data. Based upon this agreement, ignition near the crack-tip region is considered to be caused by the enhanced turbulent transport of energy in the gas phase, which is driven by strong compression waves.

A comprehensive model for AP-based composite propellant ignition

A comprehensive model and numerical solutions for ignition of AP-based composite solid propellants are presented. The analysis simulates the ignition process of a propellant sample, located in a stagnation region, under rapid pressure loading conditions. Specific features considered in the model include: 1) detailed chemical kinetics information for the ignition of AP-based composite propellants, 2) two-dimensional (axisymmetric) geometry for the composite propellant, and 3) rapid pressurization of the gas phase. An implicit finite difference scheme is used to solve the set of transient, second-order, coupled, inhomogeneous, nonlinear, governing partial differential equations. Numerical solutions reveal a number of important events occurring during the ignition sequence, including: igniter gas penetration to the region near the sample surface, combustion of unburned species upon arrival of compression waves, heat transfer to the propellant, pyrolysis of the oxidizer and fuel, and gas-phase reactions leading to ignition. The model correctly predicts the experimental observation that the ignition delay time decreases as the pressurization rate is increased. The various ignition criteria considered show the same trend as that measured experimentally.

Ignition of composite propellants in a stagnation region under rapid pressure loading

Ignition of AP-based composite solid propellants located at the tip of an inert crack wasinvestigated both experimentally and theoretically. The ignition process was observed by simultaneously using a high-speed (≈40,000 pictures/s) camera and a fast-response photodiode system. Heat flux to the propellant surface was measured with a thin-film heat-flux gage. Effects of pressurization rate, crack-gap width, and igniter flame temperature on the ignition process were studied experimentally. Experimental results indicate that the ignition-delay time decreases and the heat flux to the propellant surface increases as the pressurization rate is increased. Results of the theoretical analysis which employed separate solid-phase energy equations for fuel and oxidizer and the measured heat flux to the propellant surface are in good agreement with experimental data. The decrease in ignition delay with increasing pressurization rate is caused by enhanced heat feedback to the propellant surface at higher pressurization rates. This augmentation in heat feedback to the propellant at higher pressurization is believed to be a result of a combination of the following mechanisms: heating due to compression-wave reflection at the closed end; heat release due to burning of unreacted igniter species near the tip, behind the compression wave; and enhanced heat transfer due to recirculating hot gases near the tip.