Suppression of hydrogen/oxygen/nitrogen explosions by fine water mist containing sodium hydroxide additive

•We measure LBV for H2/air/N2 mixtures with the introduction of 0.5 M NaOH mist.•Vented deflagration tests are also carried out to find inerting limits with NaOH mist.•With NaOH mist a transition occurs at a critical density corresponding to inerting.•We discuss the inhibition of ignition by NaOH mists and importance of droplet size.•We present a diagram with approximate flammability limits for 0.5 M NaOH mists.

Complementary sets of experiments, consisting of burning velocity measurements and vented explosion tests, have been undertaken for a wide range of hydrogen–oxygen–air test compositions using fine water mist with NaOH additive (SMD ∼ 4 μm). In contrast to pure water mists, burning velocity measurements identified a critical mist concentration (for a given gas composition) above which a sudden large decrease in burning velocity is observed. The critical concentration was also found to correspond to an inerting concentration during vented explosion testing. Prior to reaching the critical concentration, the NaOH additive had a negligible effect on both the burning velocity measurements and explosion tests. This clearly indicates that the NaOH additive is acting as a chemical inhibitor. The inhibiting effect is generally considered to occur due to homogeneous gas phase mechanisms and it is thought likely that only the fraction of the entire mist (with droplet diameter < 2.5 μm) would evaporate sufficiently quickly to allow vaporised NaOH to take part in the inhibition. The experimental data obtained have enabled the construction of an inerting map to facilitate the design of a practical mist inerting system.