Comparison from modelling and experiment of electrothermal plasma temperature and pressure within granular propellant beds

Previous work conducted in a 155 mm gun chamber simulator investigated the temperature and pressure of electrothermal plasma as it expanded into an inert, high loading density propellant bed. Temperature measurements were made using analysis of spectroscopic recordings of incident radiative flux through a window of translucent energetic material. The propellant geometry used in this previous work was of a hypothetical design using monolithic grains, allowing for a well defined experiment. However, modelling work using the QinetiQ two-dimensional computational fluid dynamics code, EDEN-IB, failed to predict the experimentally determined temperature. The reason for this was believed to be the violation of assumptions within the two-phase flow model. Hence, the work has been repeated using a grain geometry more suited to the assumptions within the model. This paper presents the results of the repeat experimental work. Relatively high plasma temperatures (around 14,500 K) were measured within the central ullage tube of the charge, but immediately upon entering the charge, plasma temperatures fell to around 4000 K. The temperatures and pressures within the bed were comparable to those measured with the initial geometry. Also within this paper, comparisons are made with results from modelling these recent tests using EDEN-IB. The model again predicts hot plasma within the bed. Violation of the two-phase assumptions is unlikely to be the reason for the disparity between experiment and modelling work. Rather, the cause may be the existence of an optically thick boundary layer between the core plasma and the surface of the propellant, which modelling is not predicting. The existence of such boundary layers has been suggested before although this work highlights their importance with regard to energy transfer mechanisms leading to electrothermal ignition.