Experimental and Numerical investigation of hot-jet ignition with shock effects in a constant-volume combustor

poster abstractA wave rotor, an array of channels arranged around the axis of a cylindrical drum, can be used as a combustor in gas turbine engines in order to reduce the consumption of the fuel by increasing the fuel efficiency. Since the wave rotor combustor consumes fuel in constant volume channels, the engine system derives benefit from not only high temperature of the combusted gas, but also high pressure by containing the hot gas in the channels. Combustion of gas mixture in one of channels ignited by hot jet penetration under the necessity of rapid ignition accompanies complex non-steady phenomena, such as shock wave propagation, shock-flame interaction, and vortex generation in the channel. Especially, when a shock wave passes through the flame surface, the heat release rate and fuel consumption rate can be suddenly increased by a deformation of the flame surface, which are closely related with the combustion time of the fuel mixture. This research aims to investigate the ignition process, and the shock-flame interaction in a constant volume combustor experimentally and numerically to extract useful information for future wave rotor combustor design. Varıous mixtures of CH4 and H2 with equivalence ratio 1.0 were set as fuel for the main chamber, providing variation in chemical kinetic timescale. The hot gas jet consists of combusted gas mixture of a fuel composed of 50% CH4+ 50% H2 (by volume), burned in the pre-chamber with air at equivalence ratio 1.1. For experimental research, three dynamic pressure transducers were installed on the main chamber to measure the pressure changes caused by shock waves and flame propagation in the main chamber. Time-dependent flame and shock wave images up to 20,000 fps were obtained by a high speed camera, and a Z-type schlieren system. The schlieren technique, an optimum system to capture shock waves in the channel, utilizes light deviation due to flow density gradient, visualizing flows which are invisible to the human eye. In numerical research, adaptive mesh refinement for velocity and temperature, and multi-zone reaction modeling to speed up the kinetics were used to analyze turbulent combustion with minimum computational cost. Advanced post-processing techniques were used to calculate flame surface area, heat release rate, and vorticity deposited on flame surface to understand the flame wrinkling and surface increase. Finally, pressure data in main chamber, flame propagation speed, and the large scale of vortices under different initial conditions obtained from the experimental study were compared to the numerical results under the same conditions in order to suggest reference data for designing future wave rotors

Three-Dimensional Simulation of Turbulent Hot-Jet Ignition for Air-CH4-H2 Deflagration in a Confined Volume

This work describes essential aspects of the ignition and deflagration process initiated by the injection of a hot transient gas jet into a narrowly confined volume containing air-CH4-H2 mixture. Driven by the pressure difference between a prechamber and a long narrow constant-volume-combustion (CVC) chamber, the developing jet or puff involves complex processes of turbulent jet penetration and evolution of multi-scale vortices in the shear layer, jet tip, and adjacent confined spaces. The CVC chamber contains stoichiometric mixtures of air with gaseous fuel initially at atmospheric conditions. Fuel reactivity is varied using two different CH4/H2 blends. Jet momentum is varied using different pre-chamber pressures at jet initiation. The jet initiation and the subsequent ignition events generate pressure waves that interact with the mixing region and the propagating flame, depositing baroclinic vorticity. Transient three-dimensional flow simulations with detailed chemical kinetics are used to model CVC mixture ignition. Pre-ignition gas properties are then examined to develop and verify criteria to predict ignition delay time using lower-cost non-reacting flow simulations for this particular case of study

Experimental investigation of hot-jet ignition of methane-hydrogen mixtures in a constant-volume combustor

Investigations of a constant-volume combustor ignited by a penetrating transient jet (a puff) of hot reactive gas have been conducted in order to provide vital data for designing wave rotor combustors. In a wave rotor combustor, a cylindrical drum with an array of channels arranged around the axis spins at a high rpm to generate high-temperature and high-pressure product gas. The hot-gas jet ignition method has been employed to initiate combustion in the channels. This study aims at experimentally investigating the ignition delay time of a premixed combustible mixture in a rectangular, constant-volume chamber, representing one channel of the wave rotor drum. The ignition process may be influenced by the multiple factors: the equivalence ratio, temperature, and the composition of the fuel mixture, the temperature and composition of the jet gas, and the peak mass flow rate of the jet (which depends on diaphragm rupture pressure). In this study, the main mixture is at room temperature. The sudden start of the jet upon rupture of the diaphragm causes a shock wave that precedes the jet and travels along the channel and back after reflection. The shock strength has an important role in fast ignition since the pressure and the temperature are increased after the shock. The reflected shock pressure was examined in order to check the variation of the shock strength. However, it is revealed that the shock strength becomes attenuated compared with the theoretical pressure of the reflected shock. The gap between theoretical and measured pressures increases with the increase of the Mach number of the initial shock. Ignition delay times are obtained using pressure records from two dynamic pressure transducers installed on the main chamber, as well as high-speed videography using flame incandescence and Schileren imaging. The ignition delay time is defined in this research as the time interval from the diaphragm rupture moment to the ignition moment of the air/fuel mixture in the main chamber. Previous researchers used the averaged ignition delay time because the diaphragm rupture moment is elusive considering the structure of the chamber. In this research, the diaphragm rupture moment is estimated based on the initial shock speed and the longitudinal length of the main chamber, and validated with the high-speed video images such that the error between the estimation time and the measured time is within 0.5%. Ignition delay times decrease with an increase in the amount of hydrogen in the fuel mixture, the amount of mass of the hot-jet gases from the pre-chamber, and with a decrease in the equivalence ratio. A Schlieren system has been established to visualize the characteristics of the shock wave, and the flame front. Schlieren photography shows the density gradient of a subject with sharp contrast, including steep density gradients, such as the flame edge and the shock wave. The flame propagation, gas oscillation, and the shock wave speed are measured using the Schlieren system. An image processing code using MATLAB has been developed for measuring the flame front movement from Schlieren images. After the combustion occurs, the frequency of the pressure oscillation by the traversing pressure wave increases compared to the frequency before ignition, showing a similar trend with the maximum pressure in the chamber. The frequency is the fastest at the equivalence ratio of 0.8, and the slowest at a ratio of 0.4. The fuel ratio 30:70 cases show slightly faster frequencies than 50:50 cases. Two different combustion behaviors, fast and slow combustion, are observed, and respective characteristics are discussed. The frequency of the flame front oscillation well matches with that of the pressure oscillation, and it seems that the pressure waves drive the flame fronts considering the pressure oscillation frequency is somewhat faster. Lastly, a feedback mechanism between the shock and the flame is suggested to explain the fast combustion in a constant volume chamber with the shock-flame interactions. (Abstract shortened by ProQuest.