Modelling the mitigation of hydrogen deflagrations in a vented cylindrical rig with water fog and nitrogen dilution

Abstract

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of mitigating the overpressure rise, should a hydrogen deflagration in a vented enclosure occur. A numerical CFD gas explosion code (FLACS) has been used to simulate the pressure-time curves and the rate of pressure rise generated following the ignition of different hydrogen–oxygen–nitrogen mixtures in a small scale vented cylindrical explosion rig. This has allowed the potential mitigating effect of nitrogen dilution (reduced oxygen) and very fine water fog, used both alone and in combination, to be explored and permitted their direct comparison with corresponding experimental test data.The numerical simulation results obtained reproduce general trends in behaviour similar to those found in the experimental tests. The initial rate of pressure rise is slowed and the time taken to reach 0.1 barg increased, as the oxygen index is reduced and the number of fogger units operating is raised. However, the combination of nitrogen dilution and water fog is also more effective than predicted in some cases. Thus, the numerical simulation under-predicts the level of mitigation produced at higher fog densities for nitrogen diluted H2–O2–N2 mixtures, where the experimental results suggests that total explosion suppression can be achieved.