Effects of Fine Water Mist on a Confined Blast

Abstract

A computational, multi-phase, model has been developed to study the interactions between water droplets and radial expansion of a gas cloud in a spherical chamber. Initial conditions for the gas cloud are specified based on chemical equilibrium calculations for the detonation of a high explosive (RDX). Mono-dispersed water droplets are injected at uniform concentration into the chamber prior to the expansion. A Lagrangian model is used to track the breakup of the parent drops near the shock front to form child drops, which are extremely small. The NavierStokes solutions show that the child droplets accumulate near the shock front and evaporate at 100 times higher rate than the parent droplets. Latent heat absorption is the dominant mechanism followed by the sensible heat absorption by the water vapor (and droplets), and momentum absorption from the high velocity gases by the child droplets. The simulations also show that the water vapor formed by the evaporation increases the gas density at the shock front. The increased density and reduced gas temperature (cooling) have opposite effects on the pressure at the shock front. This leads to only a modest suppression in the pressure. At realistic droplet concentrations (0.08 kg droplets/m 3 of air), the water mist is shown to evaporate completely in a short time (2.42 ms) prior to shock reflection at the chamber wall mainly due to the breakup at the shock front. High concentrations of mist may be desirable, but are difficult to achieve in practice at the total flooding conditions.

Blast Mitigation Using Water Mist: Test Series II

“…The suppression results obtained for the aluminized explosives, Destex and PBXN-109 confirm thermodynamic and Navier-Stokes calculations that predicted the overpressure would be reduced in the presence of water mist [22]. This is significant because it was suggested that water mist in the chamber would lead to reactions between aluminum and water to form hydrogen.…”

“…Recent simulations of a 50 lb detonation of high explosive in a 3.5 m radius spherical chamber performed by Ananth et al indeed highlight the significance droplet breakup has on blast suppression [22]. The simulations show that a shock front propagates ahead of the thermal front immediately following the detonation.…”

“…Once the shock front has passed, these values will be subjected to heat and shear forces generated behind the shock front [21,22]. Since the total evaporation rate of the mist could play a significant role within the short time scales of an explosion, a key objective in this test series was to produce and maintain steady state water mist conditions inside the chamber.…”

“…where L is the latent heat of water vaporization, T  is 673 K and T s is the water surface temperature corresponding to water vaporization, 373 K. T  was chosen based on the simulations performed by Ananth that suggest the water droplets encounter temperatures between 600 and 700 K behind the shock front at 2 milliseconds [22]. Figure 8 shows the time in which it takes for droplets having diameters between 5 and 400 m to evaporate without breaking up.…”

“…Two different mechanisms of blast suppression are proposed by Schwer et al [24] and Ananth et al [22], however they both suggest suppression is achievable with mist and that greater suppression is achieved with higher mist concentrations. Detonation experiments in Figures 13 and 14 along with the overpressure results in Table 7 once again emphasize that the mist conditions that produced the highest measured concentration of water in the chamber had the greatest effect on suppressing the overpressures produced by the detonation of TNT and PBXN-109.…”