Flame spreading and violent energy release (VER) process of aluminum tubing in liquid and gaseous oxygen environments

The possible ignition and combustion of brazed aluminum heat exchangers (BAHXs) used in air separation plants all over the world is of critical concern. Recent so-called "violent energy release" (VER) combustion events in two air separation plants in Asia have resulted in nearly instantaneous destruction of entire industrial plants. The purpose of this research is to systematically observe and record the promoted ignition and flame spreading phenomena of aluminum tubing filled with liquid oxygen (tube-side), surrounded by a shell of gaseous oxygen (shell-side). Parameters that were systematically varied include the tube- and shell-side GOX and LOX pressure, tube- and shell-side GOX and LOX flow rate, tube- and shell-side oxygen purity, igniter location, sample thickness, and GOX quality.;Results indicate the transition to VER burning mode occurs nearly instantaneously. An extremely high flame spreading rate, a high luminosity flame-zone and a very rapid rate of heat release characterize the VER burning mode. Film footage of many combustion events were obtained showing for the first time the transition from a normal burning mode to a VER burning mechanism. A detailed reaction regime map showing the threshold boundaries between the three different burning modes (no self-sustained burning, normal burning, and VER burning) was constructed.;It is believed that the VER flame-spreading mode is a result of high convective mass flux of oxygen into the flame zone, resulting from the density variation between liquid and gas phases of oxygen. This high convective flux can greatly enhance reaction rate by lowering diffusion resistance of oxygen to the flame zone and the molten material surface, enhancing heterogeneous and homogeneous reaction rates as well as the molten material removal rate by atomization. An equation describing the growth behavior of the molten layer is derived. Also, an argument is presented supporting the likelihood of heterogeneous reactions at the molten aluminum surface for VER reaction.;High-pressure burn rate data were obtained of aluminum tubes burning upward in high-pressure quiescent oxygen, with densities approaching that of saturated LOX. Results indicate that no VER reaction exists in this environment, supporting the theory that the VER reaction is a result of high oxygen flux.