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<div>Pressure-gain combustion in wave rotors offer the opportunity for substantial improvement in gas turbine efficiency and power, while controlling emissions with fuel flexibility, if provided rapid and reliable ignition of lean mixtures. In addition, tightening emission regulations and increasing availability of gas fuels for internal-combustion engines require more reliable ignition for ultra-lean operation to avoid high peak combustion temperature. Turbulent jet ignition (TJI) is able to address the ignition challenges of lean premixed combustion. Especially, the turbulent hot jet results in faster ignition penetration for wave rotor pressure-gain combustors that have high-frequency operation and fast-burn requirements. Controllability of TJI needs better understanding of the chemistry and fluid mechanics in the jet mixing region, particularly the estimation of ignition delay time and identifying the location of the ignition onset. </div><div>In the present work, numerical and analytical methods are employed to develop models capable of estimating the ignition characteristics that the turbulent hot jet exhibits as it is issued to a cold stoichiometric CH4-H2-Air mixture with varied fuel reactivity blends. Numerical models of the starting turbulent jet are developed by Reynolds-averaged and large-eddy simulation of Navier-Stokes and scalar transport equations in a high-resolution computational domain, with major focus on ignition of high-reactivity fuel blends in the jet near-field due to computational resource limitations. The chemical reactions are modeled using detailed chemistry by well-stirred and partially stirred reactor approaches. Numerical models describe the temporal evolution of jet mixture fraction, scalar dissipation rate, flow strain rate, and thermochemical quantities of the flow.</div><div>For faster estimation of ignition characteristics, analytical methods are developed to explicitly solve governing equations for the transient evolution of the near field and the leading vortex of the starting hot jet. First, the transient radial evolution of the turbulent shear-layer of a round transient jet is analytically investigated in the near-field of the nozzle, where the momentum potential core exists. The methods approximate the mixing and chemical processes in the jet shear and mixing layer. The momentum equation is integrated analytically, with a mixing-length turbulence model to represent the variation of effective viscosity due to the velocity gradients. The analytic predictions of the velocity field and mass entrainment rate of the jet are compared with numerical predictions and experimental findings. In addition, the transport equation of conserved scalars in the jet near-field is solved analytically for the history of the jet mixture fraction. This analytic solution for temperature and species is used, together with available models for instantaneous chemical induction time, to create an analytic ignition model that provides the time and radial location of the ignition onset.</div><div>Lastly, the ignition mechanism within the vortex ring, which leads the starting turbulent jet, is modeled using prior understanding about the mixing characteristics of the vortex. This mechanism is more relevant to low-reactivity fuel blends. Due to the presence of strong mixing at the large-scale, the vortex ring is treated as a homogeneous batch-reactor, which contains certain levels of the jet mixture fraction. This assumption provides the initial composition and temperature of the reactor in which ignition ensues. </div><div>This article-dissertation is developed as a collection of 4 articles published in peer-reviewed journals, one submitted article, and additional unpublished work. The study is laid out in 6 chapters with the following contributions:</div><div>Chapter 1: This chapter numerically investigates the three-dimensional behavior of a transient hot jet as modeled using the Reynolds-averaged turbulence flow. The study aims at providing an insight towards the role of mixing in the ignition progress and how the operating conditions such as fuel mixture and pre-chamber pressure ratio can influence the ignition success. An ignition prediction criterion is developed in this chapter, which helps to predict the ignition success under a broad range of operating conditions.</div><div>Chapter 2: In this chapter, the large-eddy simulation (LES) of hot jet ignition is reported in conjunction with detailed kinetics mechanism and adaptive-mesh refinement. The correlation between local values of mixture fraction gradient and ignition is discussed. Furthermore, the role of methane-hydrogen ratio on the heat release pattern is studied for two specific mixtures.</div><div>Chapter 3: The LES of CH4-H2-Air ignition is extended in this chapter to account for multivariable evaluation of ignition. Joint probability assessment of ignition explains the role of important scalars on the formation and growth of ignition. Also, the effect of CH4-H2 ratio on the spatial distribution of ignition is assessed and discussed.</div><div>Chapter 4: In this chapter, the rate of mass entrainment into the jet in the near-field region is studied. Characterization of the mass entrainment illuminates the understanding of mixing behavior of the starting turbulent jets. Through an exact solution of the momentum equation, this chapter includes a model of the diffusive transport in a round transient jet at high Reynolds numbers.</div><div>Chapter 5: This chapter proposes a method to evaluate the mass/heat exchange between a transient-turbulent jet and a quiescent environment. To analyze the transport phenomena in the jet near-field, the transient diffusion equation in cylindrical coordinates is explicitly solved and its solution is compared with the empirical findings. The transport solution then enables an ignition model to describe the spatiotemporal characteristics of ignition in the near-field.</div><div>Chapter 6: The development of ignition within the vortex ring of the transient jet is investigated in this chapter. The initiation, growth, and departure of the vortex ring are studied using the available empirical correlations and the LES. Using a perfectly-stirred, zero-dimensional representation of the vortex, chemical kinetic calculations provide estimates of ignition delay for various fuel mixtures.</div><div><br></div>

Indiana University-Purdue University Indianapolis (IUPUI) / A chemically reactive turbulent traversing hot-jet issued from a pre-chamber to a relatively long combustion chamber is experimentally investigated. The long combustion chamber represents a single channel of a wave rotor constant-volume combustor. The issued jet ignites the fuel-air mixture in the combustion chamber. Fuel-air mixtures are prepared with different hydrocarbon fuels of different reactivity, namely, methane, propane, methane-hydrogen blend, methane-propane blend and methane-argon blend. The jet acts as a rapid, distributed and moving source of ignition, traversing across one end of the long combustion chamber entrance, induces complex flow structures such as a train of counter rotating vortices that enhance turbulent mixing. In general, a stationary hot-jet ignition process lack these structures due to absence of the traversing motion. The ignition delay of the fuels and fuel blends are measured in order to obtain insights about constant-volume pressure-gain combustion process initiated by a moving source of ignition and also to glean useful data about design and operation of a wave rotor combustor. Reactive hot-jets are useful to ignite fuel-air mixtures in internal combustion engines and novel wave rotor combustors. A reactive hot-jet or puff of gas issued from a suitably designed pre-chamber can act as rapid, distributed and less polluting ignition source in internal combustion engines. Each cylinder of the engine is provided with its own pre-chamber. A wave rotor combustor has an array of circumferentially arranged channels on a rotating drum. Each channel acts as a constant-volume combustor and produces high pressure combustion products. Implementation of hot-jet igniter in a wave rotor combustor offers utilization of available high temperature and high pressure reactive combustion products residing in each of the wave rotor channels as a distributed source of ignition for other channels, thus requiring no special pre-chamber in ultimate implementation. Such reactive products or partially combusted and radical-laden gases can be issued from one or more channels to ignite the fuel-air mixture residing in another channel. Due to the rotation of the rotor channels, the issued hot-jet would have relative motion with respect to one end of the channels and traverse across it. This thesis aims to investigate the effects of jet traverse time experimentally on ignition delay along with other important factors that affect the hot-jet ignition process such as fuel reactivity, fuel-air mixture preparation quality and stratification and equivalence ratio. In this study, the traversing motion of the hot-jet at one end of the main combustion chamber is implemented by keeping the main combustion chamber stationary and rotating a pre-chamber at speeds of 400 RPM, 800 RPM and 1200 RPM. The rotational speeds correspond to jet traverse times of 16.9 ms, 8.4 ms and 5.6 ms respectively. The fuel-air mixture inside the channel is at room temperature and pressure initially and its equivalence ratio is varied from 0.4 to 1.3. The cylindrical pre-chamber is initially filled with a 50%-50% methane-hydrogen blend fuel and air mixture at room pressure and temperature and at an equivalence ratio of 1.1. These conditions were chosen based on prior evidence of ignition rapidity with the jet properties. The hot-jet is issued by rupturing a thin diaphragm isolating the chambers. Using high frequency dynamic pressure transducer pressure histories, the diaphragm rupture moment and onset of ignition is measured. Pressure traces from two transducers are employed to measure the initial rupture shock speed and ignition delay. Schlieren images recorded by a high speed camera are used to identify ignition moment and validate the measured ignition delay times. Ignition delay is defined as time interval from the rupture moment to onset of ignition of fuel-air mixture in the main combustion chamber. The ignition system is designed to produce diaphragm rupture at almost exactly the moment when jet traverse begins. Ignition delay times are measured for methane, propane, methane-hydrogen blends, methane-propane blend and methane-argon blend. The equivalence ratio of the fuel-air mixtures varied from 0.4 to 1.3 in steps of 0.1 for stationary-hot jet ignition experiments and in steps of 0.3 for traversing hot-jet ignition experiments. Hot-jet ignition delay of fuel-air mixtures, for both stationary hot-jet ignition process and traversing hot-jet ignition process, generally increased with increasing equivalence ratio. For stationary hot-jet ignition delay, the minimum ignition delay occurs between Ф = 0.4 to Ф = 0.6 for the tested fuel-air mixtures. Both stationary and traversing hot-jet ignition delay depended on fuel reactivity. In particular, the shortest ignition delay times were observed for a fuel blend containing hydrogen. Among pure fuels propane exhibited slightly shorter ignition delay times, on average, compared to pure methane fuel. The addition of argon to pure methane, intended to control fuel density and buoyancy, increased the ignition delay. The traversing hot-jet ignition delay generally increased with increasing jet traverse times. To explain the variations in the measured hot-jet ignition delay and investigate qualitatively the effect of buoyancy on flame propagation and mixture stratification, the fuel-air mixture inside the main combustion chamber was ignited using a spark plug to generate a propagating laminar flame. The laminar flame propagated within the flammable regions of the channel in ways that sensitively reveal variations in local fuel-air mixture equivalence ratio. Flame luminosity images from a high speed camera and schlieren images revealed the fuel-air mixture being highly stratified depending on the density difference between the fuel and air and provided mixing time (0 s, 10s ,30s for most fuels). The lack of buoyancy-driven spreading caused the fuel to remain in the vicinity of the fuel injector resulting in significant longitudinal stratification of the fuel-air mixture. Lighter fuels stratified to the top of the chambers and heavier fuel stratified to the bottom of the chamber. Increasing the mixing time, which is defined as the time interval from end of fuel injection into the chamber to the triggering of the spark plug, improved the buoyancy-driven spreading and extended the flammable region as evidenced by the schlieren and flame luminosity images. The maximum pressure developed in the combustor for the three ignition processes, namely, stationary hot-jet ignition, traversing hot-jet ignition and spark ignition process in laminar flame propagation experiments were compared. Stationary hot-jet ignition process generally exhibited the highest pressure being developed in the chamber. Variations in heat loss, fuel-air mixture leakage and mass addition mechanisms reduced the maximum pressure for spark ignition and traversing hot-jet ignition process.

Indiana University-Purdue University Indianapolis (IUPUI) / 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 jet composition and temperature are determined by its source in a pre-chamber with a hydrogen-methane mixture with an equivalent ratio of 1.1, and a fuel mixture ratio of 50:50 (CH4:H2 by volume). The rupture pressure of a diaphragm in the pre-chamber, which is related to the mass flow rate and temperature of the hot jet, can be controlled by varying the number of indentations in the diaphragm. The main chamber composition is varied, with the use of four equivalence ratios (1.0, 0.8, 0.6, and 0.4) and two fuel mixture ratios (50:50, and 30:70 of CH4:H2 by volume). 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. The trend of the maximum pressure in the main chamber with respect to the equivalence ratio and the fuel mixture ratio describes that the equivalence ratio 0.8 shows the highest maximum pressure, and the fuel ratio 50:50 condition reveals lower maximum pressure in the main chamber than the 30:70 condition. 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.