Research on the Mixing Mechanism and Parameter Optimization of Liquid Nitrogen Foam Generator

Abstract

Nitrogen foam is expected to be an effective method to extinguish large-scale fires and suppress explosions. The key to its foaming process is that gas nitrogen (N2) and a foam solution are mixed uniformly in a foam generator. Moreover, liquid nitrogen (LN2) has been proposed as a source of gas nitrogen to generate nitrogen foam in previous experimental works. In this paper, the flow and heat transfer characteristics between liquid nitrogen and the foam solution are explored by numerical methods, which are then utilized to optimize the parameters of the foam generator. It is found that the flow pattern of the foam generator with a cone spoiler is a stratified flow by establishing the mixture and Lee’s evaporation-condensation model in ANSYS Fluent. Moreover, the spoiler in the foam generator plays a crucial role in breaking LN2 into droplets and increasing the contact area between phases, and a distance of 10 mm from the inlets to the spoiler is recommended. From previous results, an unreasonable flow rate ratio of LN2 to a foam solution may lead to icing or a low volume fraction of nitrogen, thus a flow rate ratio of 1:50 is determined in the foam generator. As for the shape of the spoiler, the spiral spoiler shows the best foaming performance compared to cone and semisphere spoilers, due to its fluid rotation instead of translation, which effectively increases the N2 volume fraction of foam from 0.616 to 0.717. Therefore, the mixing characteristics of the foam generator in this work lays a foundation for devising a practical nitrogen foam generator.