Preliminary experimental study of liquid droplets explosively induced

The explosive release of liquids is a subject studied in the field of defense, in particular for fuel air charges [1,2], but also for civilian applications such as fighting forest fires [3].

During the detonation of a charge in contact with a liquid, a cloud of droplets expands very quickly. It comes from the growth of instabilities at the interface with the detonation products and the liquid : this phenomenon, which is quite difficult to map, still remains a challenge today.

A way to investigate it is to access to physical quantities of interest which are the size and the concentration of the droplets in the cloud. There is no off-the-shelf device for accessing these measurements. This is why a study has been carried out for several years at CEA GRAMAT to test different measurement methods

The preliminary approach consisted in studying low droplet density cloud where classical optical method was relevant.

For that a campaign during which an explosive charge (1.3 g eq TNT) was immersed in a cylindrical container filled with 1 kg of water. A high-speed camera fitted with a Cassegrain-type telescope was implemented and associated with a Fresnel lens powered by a pulsed coherent light source.

The aim was to film by ombroscopy in a control volume of 3 cm³ on the outskirts of the cloud in order to record the shadows of the droplets. An image processing algorithm specially designed for this purpose was applied and then made it possible to discriminate between droplets outside the area of interest and then to measure their size.

The results of this campaign provided very interesting data for the continuation of the study, attesting feasibility of a measurement by transmission in the near infrared for denser clouds.

[1] D. R. GARDNER, «Near-field dispersal modeling for liquid fuel-air explosives,» Sandia National Laboratory, Sandia, 1990.

[2] S. SINGH et V. SINGH, «Extended Near-Field Modelling and Droplet Size Distribution for Fuel-Air Explosive Warhead,» Defence Science Journal, vol. 51, n°13, pp. 303-314, 2001.

[3] K. STEFANSKI, «Explosive Formation and Spreading of Water-Spray Cloud – Experimental Development and Model Analyses,» Central European Journal of Energetic Materials,, vol. 6, n°13-4, pp. 291-302, 2009.