Prechamber Jet Ignition and Combustion Computational Analysis of Ethylene-Air Mixture in a Wave Rotor Combustor

For a wave-rotor combustor, prechamber ignition systems offer several advantages over conventional localized spark ignition. For other internal combustion applications, prechamber ignition penetrates the fresh charged mixture, engenders non-localized ignition sites, produces reliable and spontaneous ignition at cold start operating conditions thereby leading to reduction of emission formation. This research applies detailed computational chemical and fluid dynamic simulation to investigate the ignition and combustion processes in a previously tested experimental wave rotor combustor using a prechamber ignition system. The combustion system built and tested at Purdue University comprised a rotating wave-rotor drum with multiple main combustion chambers and a stationary pre-combustion chamber to produce a traversing hot reactive jet for rapid ignition in the wave rotor. The traversing jet delivers hot gas with some active free radical species into a premixed ethylene-air mixture in each constant-volume combustion chamber from a continuous-flow prechamber torch igniter positioned at a fixed location adjacent to one end of the wave rotor with a clearance gap that allows relative motion between the rotor and its end wall.

In this work, the computational domain consists of three channels of the wave rotor combustor, with combustion simulated fully in one channel, to optimize computational cost while closely mimicking the traversing jet motion and subsequent ignition and combustion processes. The finite-volume fluid dynamics simulation alternately uses transient Reynolds Averaged Navier-Stokes (RANS) two-equation eddy-viscosity model, and in another simulation case, a Large Eddy Simulation (LES) to model turbulence. The simulation was set up to begin at a point in the wave rotor cycle when the channel has been filled and assumes a uniform mixture with substantial but variable residual turbulence.

In-rotor pressure trace, flame propagation, and heat release rates predicted from the simulation are compared with experimental data previously collected from an instrumented wave rotor constant volume combustor rig. There are significant aspects of the experimental wave rotor that could not be measured, but which may substantially affect its combustion processes: turbulent kinetic energy, turbulence scales of the mixture prior to ignition, scalar dissipation rates, scalar mixture fraction, in-rotor vortex structures, and the level of active radicals in the traversing hot jet. These ignition and combustion characteristics are not available from the wave rotor experiments. Thus, the present research estimates these quantities using computation simulation method to gain enhanced insight into the ignition mechanism of the wave rotor combustor.

The pre-chamber combustion model employed in this study is manipulated to understand how dosing of free-radical species enhances localized ignition initiation and accelerates overall fuel burn rate in the wave rotor channel. The turbulent kinetic energy in the channel is also varied to understand the sensitivity of the combustion process to turbulence intensity and scales as the flame propagates along wave rotor channel away from the initial ignition jet zone. The findings from this study significantly advance the understanding of a demonstrated wave rotor combustor with potential to revolutionize wide range of pressure-gain combustor applications and particularly gas turbine engine technology.

The RANS simulation result validates the influence of acoustic instability in the wave rotor through pressure wave-flame collision impact on flame area seen in previous studies, and how the resulting flame wrinkling structure enhances heat release rate (HRR) in the wave rotor combustor. However, other key results observed from this simulation also reveal the critical role of baroclinic torque generated from Rayleigh-Taylor hydrodynamic instability on the flame front wrinkling. Additionally, the combustion regime was observed to be established in the thin reaction zone in the wave rotor, suggesting that combustion process in the wave rotor is controlled by turbulent mixing.

Variability studies of the prechamber nozzle jet performed using Large Eddy Simulation model shows how the jet nozzle geometry significantly impact ignition and combustion phenomena including ignition delay time, mixing and jet entrainment process, as well as overall heat release rate magnitude in the wave rotor combustor. Results from the current study show that a jet issued from a converging-diverging nozzle produces rapid mixing, ignition and less HRR, while the converging nozzle jet produces enhanced jet entrainment and higher overall heat release rate. To understand the scalar mixing process in the jet, the localized turbulence frequency, scalar dissipation rate and mixture fraction scalar quantities are analyzed to estimate mixing efficiency of the jet within the scalar field. Rapid ignition in the head vortex is observed in the wave rotor through analysis of temporal and spatial evolution of the reacting radical species as a function of the chemical progress variable and mixture fraction. Further investigation of the high-intense mixing process reveals the presence of localized flame annihilation within the reaction zone.