Pulsating propagation and extinction of hydrogen detonations in ultrafine water sprays

TL;DR

This study uses advanced simulation methods to analyze how hydrogen detonations propagate, pulsate, and extinguish in ultrafine water sprays, revealing complex interactions affecting detonation stability and speed.

Contribution

It introduces a detailed simulation of hydrogen detonation behaviors in ultrafine water sprays, identifying multiple propagation modes and the effects of droplet size and water loading.

Findings

Three detonation modes identified: pulsating, propagating then extinguishing, and immediate extinction.

Pulsating detonation involves periodic changes in speeds and distances between reaction and shock fronts.

Critical droplet size significantly reduces the likelihood of detonation extinction.

Abstract

The Eulerian-Lagrangian method is applied to simulate pulsating propagation and extinction of stoichiometric hydrogen/oxygen/argon detonations in ultrafine water sprays. Three detonation propagation modes are found: (1) pulsating propagation, (2) propagation followed by extinction, and (3) immediate extinction. For pulsating detonation, within one cycle, the propagation speeds and the distance between reaction front (RF) and shock front (SF) change periodically. The pulsating phenomenon originates from the interactions between gas dynamics, chemical kinetics, and droplet dynamics inside the induction zone. Multiple pressure waves are emanated from the RF within one cycle, which overtake and intensify the lead SF. An autoigniting spot arises in the shocked gas after the contact surface. The relative locations of SF, RF, shock-frame sonic point, and two-phase contact surface remain unchanged in a pulsating cycle, but their distances have periodic variations. Moreover, the unsteady behaviors of detonation extinction include continuously increased distance between the RF and SF and quickly reduced pressure peaks, temperature, and combustion heat release. The decoupling of RF and SF leads to significantly increasing chemical timescale of the shocked mixture. The hydrodynamic structure also changes considerably when detonation extinction occurs. Moreover, the predicted map of detonation propagation and extinction illustrates that the critical mass loading for detonation extinction reduces significantly when the droplet size becomes smaller. It is also found that for the same droplet size, the average detonation speed monotonically decreases with water mass loading. However, detonation speed and pulsating frequency have a non-monotonic dependence on droplet size under a constant water mass loading.