Study on the flow characteristics of nitrogen gas foam in horizontal foam generator

Abstract

With the rapid deployment of large oil tank, a quick and efficient fire extinguishing foam generator becomes particularly critical to relieve the safety concerns. In this work, a cryogenic fluid/bulk fluid mixing apparatus, composed of a horizontal channel and a conical spoiler in it, is developed to generate stable and homogeneous foam flow. The flow and temperature characteristics of the mixing flow field, including flow pattern, contour of velocity and phase fraction, temperature and pressure profiles, are studied by a visualization experiment and numerical simulation. From the results, a typical flow pattern of liquid nitrogen jetting into bulk fluid (water or foam liquid) in the mixing channel is proposed. The rapid phase transition (RPT) of nitrogen liquid in bulk fluid is found to effectively enhance the mass and heat transfer between phases. Besides, the introduction of conical spoiler facilitates the flow mixing effectively. The nitrogen liquid jet is broken into small liquid blocks and vaporizes rapidly in the conical spoiler zone. A backflow zone appears at the downstream of the spoiler and further promotes the homogeneous foam flow. Moreover, compared to the liquid nitrogen/water mixing flow, the direct mixing of liquid nitrogen/foam liquid could accomplish higher foaming ability, and more homogenous and stable foam flow due to its lower surface tension, higher dispersivity, faster interphase heat and mass transfer. The study of characteristics of the mixing flow in this work could provide an important guidance for the design and optimization of foam generators in the compressed air foam systems (CAFSs).