Blast Mitigation by Water Mist, (3) Mitigation of Confined and Unconfined Blasts

Abstract

Abstract : This is the third in a series of reports focusing on numerical simulations of blasts and blast mitigation. This report uses the models developed in the first two to specifically examine the effect of water mists consisting of sub-50-micron droplets on blast shock-fronts and the development of quasi-static overpressure in enclosed spaces. In unconfined blasts, results showed that the water mist does not directly suppress the secondary reactions. Mitigation of the shock-front is accomplished mainly through momentum extraction and not vaporization. Quantitative results determined the exact effect of 5 to 50 micron sized droplets on the shock-front. Droplet size was found to play a secondary role compared to mass loading. Smaller droplets were less effective close to the explosive, while further downstream, the optimum droplet size depended on mass loading. The total amount of water mass between the observer and explosive was the most important factor in determining the amount of mitigation seen by the observer. Simulations were also conducted to determine the effectiveness of water mist to mitigate quasi-static pressure build-up in enclosures. Comparisons with experiments conducted at NSWC were used to validate the models and showed that the models could predict the overall mitigation efficiency to within a few percent. Absolute values of the simulation quasi-static overpressure tended to be slightly higher than experimental values, most likely because various loss mechanisms (incomplete combustion, absorption of energy by walls, and venting) were not incorporated into the model. Results also suggested that multi-dimensional simulations were required compared with simple thermodynamic and one-dimensional computations. Finally, additional needed computational work is summarized in the report. Many of these improvements are implemented and are currently being tested, and will help substantially in simulating blast mitigation scenarios of interest to the Navy and DoD.