On gravity-driven liquid nitrogen jets reach and horizontal spread for extinction of ground fires by aerial means

Abstract

This paper describes experimental and numerical results on the reach and the spread of gravity-driven jets of liquid nitrogen (LN₂) on the ground for applications to fire extinction by aerial means. A series of experiments released LN₂ jets from different elevations in ambient air to measure their reach and spread distances upon the impingement. A numerical model was developed to simulate the behavior of such jets. Upon validation, the numerical model was used to further predict the LN₂ pool mass and spreading distances under various release configurations. Results showed that the LN₂ survivability is greatly affected by the release height of the cryogen, since the LN₂ quantity reaching the ground decreases as the release height increases. Moreover, releasing larger initial LN₂ quantities and, most importantly using larger nozzle diameters, both the LN₂ pool mass and spreading diameter can be extended. Additional experiments were conducted where cryogen jets were released onto small (∼300 cm²) alcohol pool fires; results showed that only limited quantities of the LN₂ evaporated in transit to the fire, and small amounts of the cryogen expediently snuffed the fires. A simplified model also suggested that in fire supression/extinction by LN₂ the fuel cooling mechanism is of secondary importance compared to the mechanism of separating the fuel from oxygen.