On Two-Phase Magnesium-Teflon Pyrotechnic Igniter Flow in Closed Cavities

Abstract

One dimensional pyrotechnic igniter flow inside closed cavities is analyzed. To simulate the flow which is two - phase, reacting and unsteady, different flow models are proposed. The governing inviscid two phase conserv ation equations are solved by using accurate gas and solid phase Riemann solvers. The outputs of the models are compared with the experimental data. It is found out that the model which includes ignition delay and the combustion of pyrotechnic powder succe ssfully simulates the flow inside the cavity. It is also detected that the mass flow rate of pyrotechnic granules and particle size of the solid products of Magnesium -Teflon combustion are important parameters affecting the physics of the flow. INTRODU CTION Magnesium -Teflon pyrotechnic powder is widely used in tactical solid rocket motors igniters for reliable, repeatable igniton. Magnesium -Teflon pyrotechnic powder, when ignited, produces solid, liquid and gaseous products. The existence of very hot s olid and liquid particles and condensable and reactive species in Magnesium -Teflon combustion products enables fast heat -transfer by almost all possible modes, conduction from impinging solid particles, forced convection from gaseous species, high thermal radiation, exothermic condensation and solidification. Therefore, in order to model the ignition transient of a solid rocket motor with pyrotechnic igniter, two -phase pyrotechnic igniter flow must be modeled adequately. Numerical one dimensional pyrotech nic igniter flow models are presented here. The initial form of the Magnesium -Teflon pyrotechnic powder varied parametrically in these models. In the first model it is assumed that granules burned in the igniter and the products of combustion are in gas ph ase. In the second model it is assumed that granules burned in the igniter and the products of combustion are in both gas and solid phase. In the third model it is assumed that granules are ignited in the igniter and discharged to the cavity and the combus tion takes place in the cavity and all products are in gas phase. In the fourth model it is assumed that granules are ignited in the igniter and they are discharged to the cavity. The combustion takes place in the cavity and products are in gas and solid phase. Finally in the fifth model, it is assumed that some portion of the granules are ignited in the igniter and discharged to the cavity with the rest of the unignited granules. The ignition of the non -ignited granules takes place in the cavity. The co mbustion takes place in the cavity and all products are in gas and solid phase. METHOD The unsteady one dimensional conservation equations which are in integral form are solved numerically. The governing inviscid two phase conservation equations are solve d by using accurate gas and solid phase Riemann solvers. To analyze the validity of the proposed model for the igniter flow, an experimetal study is conducted to measure the time dependent gas pressure variation inside a long thick -walled cylindrical tube. The Magnesium -Teflon pyrotechnic igniter is ignited at one side of the tube and the gas pressure is measured at the other side. This pressure time data is used for validation of the proposed models. Following section describes the mathematical model and t he numerical method used for the solution of this mathematical model. The unsteady one dimensional conservation equations, namely, conservation of mass, momentum and energy are treated for the solid phase and gas phase seperately. The interaction between t he phases is due to mass, momentum and heat transfer.