Flame Spreading and Violent Energy Release (VER) Processes of Aluminum Tubing in Liquid and Gaseous Oxygen Environments

Abstract

This work is motivated by the need for a fundamental understanding of the unintentional combustion of aluminum heat exchange equipment used in liquid oxygen (LOX) service; in which combustion can be quite energetic and cause severe damage. The promoted ignition, flame spreading, and combustion phenomena of aluminum tubing filled with LOX, surrounded by a shell of gaseous oxygen (GOX), were systematically observed and recorded. Parameters studied include the tube- and shell-side pressures, GOX and LOX flow rates, and GOX quality (the mass fraction of GOX in the two-phase flow inside the tube sample). Results from high-speed movie films indicate that under certain operating conditions, a transition from a “normal” (much slower) burning mode to an extremely rapid and violent burning mode occurs. This transition was shown to occur in some cases after a period of delay and in other cases immediately after the onset of ignition. The “Violent Energy Release,” or VER burning mode, is characterized by an extremely high flame spreading rate (1-2 orders of magnitude higher than the flame propagation rate in a purely GOX environment), a high luminosity flame-zone, and a very rapid rate of heat release. Within the range of initial conditions tested, the major parameters that tend to enhance the degree of violence of aluminum combustion in a LOX/GOX environment include increasing initial pressure and decreasing GOX quality (higher LOX fraction). A burning regime map showing the threshold boundaries between conditions of no self-sustained flame propagation, “normal” burning, and VER burning mode is presented. It was found that the threshold boundaries between the different regimes of burning are essentially determined by the tube-side conditions. In addition a detailed insight into the controlling mechanisms of the VER reaction has been obtained, and a physical description of the transition mechanism is presented.