Experimental Investigation of the Ignitor Plume/Probellant Interaction
The ignition of gun propellants is still a challenging task that requires igniter compositions optimized to each type of propellant. Propellant ignition is mainly dominated by the local heat transfer to the propellant that must be large enough to initiate a self-sustaining reaction with a heat release which overcomes all internal heat loss. In the case of pyrotechnic ignition, heat transfer from the ignitor plume to the propellant's body works in combination of different mechanisms as convection from hot gases, conduction of hot impinging and penetrating particles or condensing metal vapors on the propellant surface as well as heat radiation from the plume. Normally, the performance of an igniter mixture is characterized by ballistic closed-bomb tests under isochoric conditions measuring the ignition delay time by analyzing the pressure time curve. This method validates the applicability of a selected mixture but does not give any hints to understand the individual mechanism of ignition process. This may help to optimize the ignition mixture. This presentation introduces a method to investigate simultaneously some performance data of the igniter mixture and the ignition process using a chimney-type window bomb under isobaric conditions but on different pressure levels (0.1 to 10 MPa N2). A small amount of igniter mixture is filled in a vertically positioned polymer tube and ignited using a melting wire. In a defined distance that may vary between 5 and 30 mm above the upper tube edge, a single propellant grain is fixed in horizontal position. The expanding igniter plume and its interaction with the propellant grain is observed simultaneously using a color high-speed camera and a fast NIR emission spectrometer with 660 spectra/s. This enables to measure expansion speed and temperature of the plume as well as profiles of temperature and species emission during the interaction with the propellant grain and the ignition delay time. Besides these technical data, the visual analysis of the movie results in additional qualitative but highly interesting information e.g. on the effect of penetrating particles on ignition (Figure 1).To measure ignition delay time, both the video signal and the series of species spectra were analyzed. In some cases, e.g. when the combustion of igniter takes longer than the ignition delay time, only the species spectra results in a quantitative value. In this case, the first appearance of a typical propellant combustion product as e.g. water is used to define propellant ignition (Figure 2). When both signals can be analyzed the ignition delay times coincide fairly in an order of the standard deviation of repeated measurements (Figure 3).In summary the ignition behavior is characterized using the ignition delay times as a function of the pressure level usually best depictable in a log-log-diagram (Figure 3). The absolute value of ignition delay time varies with the formulation of ignitor mixture and the type of propellant. This ranking corresponds with ballistic measurements which were performed with selected igniter mixtures and propellants. As expected, the ignition delay times decrease drastically with pressure. Only sometimes the pressure dependence can be described with a linear plot. But depending on temperature, particle and metal vapor content, the ignitor plume of some mixtures shows different effectivity at low pressure (<1 MPa) or medium pressure >4 MPa. Also, such pressure effects are difficult to measure with ballistic tests.