The evolution of the temperature field during cavity collapse in liquid nitromethane. Part I: Inert case

TL;DR

This study investigates the physical mechanisms behind hot spot formation during cavity collapse in liquid nitromethane, using detailed 2D and 3D simulations to understand temperature evolution without chemical reactions, highlighting complex wave interactions and hot spot origins.

Contribution

The paper provides a detailed analysis of hot spot generation mechanisms during cavity collapse in nitromethane, emphasizing the importance of 3D simulations for accurate temperature prediction.

Findings

Hot spots can reach temperatures more than twice the shock temperature, potentially causing ignition.

Complex wave interactions, including Mach stems, contribute to hot spot formation.

3D simulations are essential for accurate modeling of temperature evolution and ignition timing.

Abstract

We study the effect of cavity collapse in non-ideal explosives as a means of controlling their sensitivity. The aim is to understand the origin of localised temperature peaks (hot spots) which play a key role at the early stages of ignition. Thus we perform 2D and 3D numerical simulations of shock induced gas-cavity collapse in nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. To understand the relative contribution between these two processes we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which accounts for the large density difference across the gas-liquid interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic and numerical data. Previous studies identified the impact of the upwind side of the cavity wall to the downwind one as the main reason for the generation of a hot-spot outside of the cavity; this is also observed in this work. However, it is apparent that the topology of the temperature field is more complex than previously thought and additional hot spots locations exist, arising from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots we follow the complex wave patterns generated and identify the temperature elevation or reduction generated by each wave. This allows to track each hot spot back to its origins. We show that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing the maximum temperature observed in the domain in 2D and 3D simulations we show that 3D calculations are necessary to avoid belated ignition times in reactive scenarios.