Impacts of oxidant characteristics on the ignition of n-propanol-air hydrothermal flames in supercritical water

Abstract

Hydrothermal flame is produced in an aqueous environment beyond the thermodynamic critical properties of water. It is an interface developed at the contact of oxidant and fuel in supercritical water and depends on the operating parameters. In-situ diffusion-limited hydrothermal flames were generated in a novel supercritical flame reactor designed by the National Aeronautics and Space Administration (NASA) at Glenn Research Center to investigate the impacts of oxidant flow rate and temperature on flame ignition and stabilization. The reactor system comprises of n-propanol as fuel and air as the oxidant. Two-dimensional simulation studies were performed to interpret different thermal events occurring during the process. Temperatures inside the reactor were recorded at different times to determine the onset and propagation of hydrothermal flames. Temperature profiles obtained via simulations were compared with the experimental data at near-critical temperatures (380 °C and 20.5 MPa). The study of oxidant flow rate on ignition and temperature profile at near-critical and supercritical conditions (400 °C and 22.5 MPa) was conducted by varying the air flow rate ranging from 0.5 to 3 mL/s. A flow rate of 1.5 mL/s was found to be optimal with the spontaneous ignition of hydrothermal flames. The effect of inertial and buoyant forces on hydrothermal flames was qualitatively explained using the non-dimensional Reynolds and Froude numbers. The ignition delay times of hydrothermal flames for near-critical and supercritical reactor conditions for different flow rates are reported. Ignition mechanism and impact of the oxidant characteristics during supercritical water oxidation were inferred using a two-dimensional simulation model for n-propanol-air.