Numerical study of inverse H2O2 droplet combustion in methane–nitrogen atmospheres under microgravity

Abstract

Inverse droplet combustion, in which the oxidizer evaporates from the liquid phase and the surrounding gas provides the fuel, remains far less understood than classical fuel-droplet burning. In particular, the intrinsic transport–chemistry coupling of $H_2{O}_2$ droplets in hydrocarbon atmospheres is difficult to isolate experimentally because buoyancy alters flame shape, mixing, and heat transfer. In this work, we numerically investigate the inverse combustion of isolated $H_2{O}_2$ droplets in ${\mathrm{CH}}_4/N_2$ atmospheres at 1 atm under microgravity conditions using a transient, one-dimensional, spherically symmetric model with detailed chemical kinetics, multicomponent transport, phase equilibrium, and gas-phase radiation. The simulations are carried out for $X_{{\mathrm{CH}}_4}=0.55-1.00$ and are extended over a range of initial droplet diameters.

The calculations reproduce the main phenomenological features reported by Matsugi et al. (2023) and reveal the intrinsic structure of the inverse flame in the diffusion-controlled limit under microgravity conditions. After a short transient, the droplets evolve toward a quasi-steady regime consistent with the classical $D^2$-law, showing that diffusion-controlled scaling persists despite the reversal of fuel and oxidizer roles.. The flame exhibits a clear two-layer structure, with an inner region dominated by $H_2{O}_2$ decomposition near the droplet surface and an outer methane-oxidation region sustained by radicals transported from the inner zone. Despite variations in methane concentration and initial droplet diameter, the relative flame position remains nearly constant, indicating that the flame location is primarily controlled by the diffusive balance between outward oxidizer transport and inward methane transport. In contrast, the vaporization rate shows a non-monotonic dependence on the initial droplet size, which arises from the competition between conductive and radiative heat transfer at the droplet surface.

These results provide a detailed kinetic and thermal description of inverse $H_2{O}_2$-droplet combustion under microgravity and establish a physically grounded reference for future microgravity experiments and multidimensional simulations.

Novelty and significance statement: This work provides the first detailed kinetic-level description of inverse $H_2{O}_2$-droplet combustion in ${\mathrm{CH}}_4/N_2$ atmospheres under microgravity conditions. Its significance is not only in extending droplet-combustion modeling to an unconventional configuration, but in clarifying how inverse flames are organized when buoyancy is removed and transport acts in a purely diffusive, spherically symmetric environment. The study shows that reversing the roles of fuel and oxidizer does not eliminate diffusion-controlled scaling, but leads to specific non-intuitive features, including a distinct two-zone flame sustained by the coupling between $H_2{O}_2$ decomposition, radical transport, and outer methane oxidation. It also identifies radiation as a key mechanism governing the droplet-size dependence of the vaporization rate through its coupling with flame geometry and heat transfer. For the combustion community, this work establishes a quantitative reference case for inverse droplet flames and a rigorous benchmark for assessing detailed chemistry, transport, and radiation models in multiphase combustion.