Modelling monodisperse water droplets in hydrogen deflagration using OpenFOAM

In this work, standard OpenFOAM solvers within a URANS framework are used to investigate monodisperse droplet-diameter effects on flame propagation, overpressure development and evaporation trends in an obstructed premixed hydrogen–air deflagration. Although a real water spray consists of a distribution of droplet sizes, a theoretical CFD framework enables systematic isolation and comparison of monodisperse droplet-diameter effects. The model employs an Eulerian–Lagrangian approach, where the premixed hydrogen–air mixture is solved in the Eulerian and the water droplets in the Lagrangian framework. The numerical combustion model is first validated by comparing flame front position and overpressure with experimental data for the obstructed configuration. The validated framework is then used to investigate droplet diameters ranging from 5–100 μm, keeping the total added water mass identical for all droplet-size cases. The results show that droplets in the range of 5–20 μm increase flame propagation time and reduce peak overpressure, with the smallest droplets of 5–10 μm giving the most pronounced mitigation behaviour within the present modelling framework. In contrast, droplets sized 40 μm and 100 μm do not show a clear mitigating effect on peak overpressure. Smaller droplets experience significantly larger evaporation fractions than larger droplets, despite the gas flow carrying a substantial fraction of the smaller droplets out of the open-ended channel. Heat transfer and evaporation are identified as important mitigation mechanisms. A Weber analysis indicates that the assumption of no break-up becomes increasingly uncertain for larger droplets, especially droplets larger than 20 μm.