Ignition of solid propellants by forced convection

Abstract

Experimental data are reported for the ignition of single grains of solid propellant in a stream of gas at high temperature. The investigation encompassed gas temperatures from 578" to 1,O7O0K., gas velocities corresponding to free-stream Reynolds numbers from 156 to 624, a complete range of oxygen-nitrogen m e s , and a few oxygen-carbon dioxide mixtures. Pyrocellulose and double-base propellants were tested. The grains were approximately 1/8 in. in diameter and extended through the gas stream, so that ignition was forced to take place on the cylindrical surface rather than on the end of the grain. The exposure before ignition was measured for a large number of grains. The data can be represented by an equation that is consistent with the known effect of flow rate on conveckve heat Gansfer -and the known rates, an indication that both processes are EXPERIMENTAL APPARATUSThe experimental apparatus consisted principally of a furnace and auxiliaries to provide a uniform stream of hot gas of known dimensions, temperature, velocity, composition, and turbulent intensity; a device to inject a grain of powder into the gas stream transversely; and a device to detect ignition and to measure the time of exposure prior to ignition. The apparatus is shown in Figure 1.Gas from high-pressure tanks was h t passed through rotameters a t regulated effect of temperature on chemical reaction important in ignition.pressure and then through a heating unit consisting of a l%-in. I.D. Zircotube (aluminum oxide) packed with %-in. cylinders rind locrited inside an electric resistance furnace with a cylindrical Globar (silic-on carbide) element. The heated gas then passed through a Gin.-long by I-in. I.D. adiabatic tube packed with %-in. spheres for temperature measurement and finally through two 52-mesh, 0.004-in. wire screens spaced 1% in. apart to control the velocity profile and turbulence. The gas thus emerged as a cylindrical stream 1 in. in diameter with a velocity profile which deviated less than 5% from iiniformity and an intensity of turbulence of less than 2%.Radiation from the top of the adiabatic tube was minimized by the use of platinum screens and by wrapping the pellets in the top rows with platinum foil. The adiabatic section was well insulated, and the temperatures of the gas stream and packing were assumed to be equal near the outlet, so that the gas temperature might be measured with a single-wire, buttwelded Chromel-Alumel thermocouple in a porcelain tube extending across the adiabatic tube. Since the temperature of a gas stream issuing from a tube is prcservcd in a' region beyond the exit, the measured temperature was further assumed to be the gas temperature in the test zone. The assumptions involved in the temperature mensurement were discussed in detail in a previous paper (a). The thermocouple actuated a continuous-line temperature recorder indicator, which was checked periodically with a precision potentiometer. The furnace temperature waa controllcd by an off-on switch on the primary side of the step transform...

Heat Transfer and Chemical Kinetics in the Ignition of Solid Propellants

“…Linear surface ignition temperatures have also been calculated for the data of Brier and coworkers (6) on the ignition by forced convection of M2 in 20% oxygen-80% nitrogen mixtures. For these calculations, the maximum local heat transfer coefficient was used.…”

“…Cook and Olson (7) arrived at a similar conclusion. Brier and coworkers (6) reported that increasing oxygen content results in a decrease in ignition time. One may conclude that under conditions of forced convection, gas phase kinetics contribute significantly to the over-all kinetics of ignition to flame burning.…”

“…Other investigators have examined the ignition process using a variety of igniting media for the transfer of thermal energy to the propellant (7,3,6,7). In this study, small cylindrical grains were investigated in an apparatus employing radiation and natural convection as the mechanisms of heat transfer.…”

Guidelines for Modeling Based on Experiences with Radiantly Stabilized Combustion

“…An investigation, in connection with the onset of the U.S. space program, of the ignition of solid propellants by a stream of very hot gas, revealed that the process depended critically on chemical reactions in the gas-phase (Churchill et al). 1 This unexpected finding prompted a subsidiary investigation of the contribution of the energetic chemical reactions of combustion to convective heat transfer. It was decided to pursue this diversionary effort in the context of combustion inside a tube because of the extensive database for forced convection without reaction in tubular flow.…”

Projectile Impact Ignition Characteristics of Propellants II. Single, Double and Triple Base Propellants

“…This may have been due to a partial decomposition of the propellant without the formation of flame. 11 is known that the gentle heating by a flameless ignition source can cause nitrocellulose and some nitrocellulose-containing propellants to decompose without the formation of flame in a fizz-type of burning (Churchill et al, 1956;Roth and Wachtell, 1962). The decomposition reaction proceeds in a smoldering-like manner.…”