Experiments and modelling of liquid nitrogen jet release and dispersion for fire-related applications

Abstract

Cryogenic liquid nitrogen (LN2) is proposed as an effective fire suppressant/extinguisher for challenging fires. Upon reaching a fire source, this inert cryogen vaporizes expediently and separates the fuel from the air. Additionally, it cools the burning fuel and slows down its pyrolysis/gasification reactions. A prior investigation concentrated on the survivability of single LN2 droplets, following breakup of the jet exiting the nozzle, and the predicted the droplet size that reaches a target. This investigation addresses the application of a liquid nitrogen jet for the extinction of fires with a combination of experiments and theoretical analysis. Small scale laboratory experiments were conducted with a sprayer and larger scale experiments were conducted with a cryogenic hose and nozzle. In these experiments the reach of the liquid nitrogen jets was measured, and their fire extinction capabilities were assessed. Numerical simulations were performed with the ADREA-HF CFD code using the mixture approach and explicit modelling of LN2 evaporation using the Lee model. Numerical results on the reach of a liquid nitrogen jet qualitatively matched the experimental observations for release of a liquid nitrogen jet in open atmosphere. The Lee model coefficient was found to depend strongly on the nozzle diameter. An integral model was also constructed to explore the effects of relevant parameters, such as the LN2 storage tank pressure, the initial jet vapor quality, the jet velocity at the nozzle exit and the effects of the ambient air entrainment. Based on experiments and theoretical analysis, it became evident that for effective extinction of a fire with liquid nitrogen a commercial nozzle should have a large diameter and should be brought close to the burning target, i.e., within a few meters. As this action can pose potential safety risks to firefighters, the advancement of the nozzle towards the fire should be operated remotely.