Joseph Emmett Shepherd

We report an experimental study on the processes of fuel injection, vaporization, mixing, and ignition under conditions simulating standardized autoignition tests with uniform gas and surface temperatures in a closed volume. An optically-accessible, instrumented, 1-liter cubic test cell was fabricated from passivated metal with fused-silica windows. The cell is electrically heated from the top and bottom with feedback control to create a uniform temperature air atmosphere. Liquid fuel (n-hexane) is injected by a syringe pump to control the injected volume and rate. Vaporization and mixing of the fuel were characterized by direct imaging, infrared absorption, and background-oriented schlieren (BOS). Combustion events were captured using BOS, fine thermocouples, and direct imaging.Results show that in a nearly uniform temperature initial atmosphere, fuel dispersion after vaporization is dominated by diffusion, but localized fuel concentrations can be created by convection from small temperature variations. Depending on temperature, dispersion, and ignition can occur on similar timescales, leading to ignition in a partially mixed environment. Measurements show a two-stage ignition behavior in stratified atmospheres: slowly propagating, combustion of the cool-flame variety followed by formation of a rapidly propagating hot flame. High-speed imaging highlights the early formation and growth of the ignition kernel and transition between combustion modes. Two distinct ignition modes were observed, which exhibit similar temperature profiles measured by our thermocouple but differ in light emission and peak pressure. These observations suggest further refinements of autoignition test methodology.

The reactivity of transverse waves in detonations of methane, oxygen and nitrogen are experimentally assessed using MHz rate schlieren and chemiluminescence imaging. In these highly unstable mixtures, the mode of wave propagation is more complex than what is described by the cellular instability model that is conventionally used for weakly unstable mixtures. Behind the low-speed leading shock in unstable waves, the processed gas remains essentially unreacted until transverse waves reach this region. In highly unstable waves, the transverse waves have a range of reactivity, that is rates of reaction in the flow immediately behind the wave. In this study, we present examples of transverse waves for near-limit detonations and analyse four cases in detail. In some cases, these waves appear to be essentially non-reactive or cause very slow reaction. In other cases, the transverse waves can be highly reactive. In the most extreme example, the transverse wave is propagating at the Chapman–Jouguet speed with a small reaction zone, i.e. a transverse detonation. A reactive oblique shock model is used to approximate the triple-point configuration of this case as a double-Mach reflection, which shows good agreement with the images. The reaction evolution along path lines is analysed using detailed reaction mechanisms and considerations about flow-field unsteadiness. Length scales of the energy release and expansion processes within the reaction zone region are used to explain the observed modes of wave propagation and interaction.