<div> <p>Combustion will likely continue to be utilized over the next century to meet the world’s energy needs. As increasingly stringent requirements on emissions, particularly of oxides of nitrogen (NO and NO₂) are imposed on power plants due to their harmful effects on the environment, advanced combustor strategies to limit NO_X productions are needed. One such advanced concept involves axially staging the fuel to create a distributed combustion system. The fundamental problem for staged combustion involves the injection of a reacting jet into crossflow. This canonical problem is modified for this dissertation through injection of a reacting premixed natural gas and air jet into a compressible vitiated crossflow with negligible swirl. In addition, the experimental efforts for this work were conducted at elevated inlet air temperature and combustor pressure.</p><p>The development and performance of a perforated plate burner (PPB) to provide vitiated crossflow and operating using premixed natural gas (NG) and air at engine-relevant conditions is discussed. A significant benefit of using burners with simplified flow fields, such as the PPB, for experimental studies in the laboratory is the potential for decoupling the complex fluid dynamics in typical combustors from the chemical kinetics. The stable operation of the PPB within a high-pressure test rig was validated: successful ignition, effective use of redlines for flashback mitigation, and long duration steady-state operation in both piloted and non-piloted modes were all observed. Exhaust gas emissions measured using a Fourier-transform infrared (FTIR) spectrometer showed very good performance of the PPB in terms of the combustion efficiency and low levels of NO_X<i> </i>in non-piloted operation that were generally within 3 ppm.</p><p>Emissions measurements of the premixed reacting jet in vitiated crossflow were obtained for a variety of conditions and a significant NO_X reduction was achieved when the staged combustor exit Mach number was increased and the axial residence time was decreased. Based on this preliminary investigation, a test matrix was developed to independently vary the exit Mach number for a constant axial residence time by using modular rig hardware to change the length of the axial combustor. Up to 70% reduction in NO_X produced by the axial stage was observed when the combustor exit Mach number was increased from about 0.26 to 0.66 at a constant residence time of 1.4 ms. NO_X reduction based on variation in the Mach number and at a constant residence time has not been previously reported in the literature to the best of our knowledge. This decrease in NO_X is hypothesized to be due to the lower static temperature of a compressible flow and potentially better mixing of the jet with the crossflow due to the interaction occurring at high speeds.</p><p>Based on the strong effect of Mach number for NO_X reduction even at a constant residence time, further investigation using laser-based diagnostics is needed to provide insight on physical processes controlling this phenomenon. An optically-accessible secondary combustion zone was developed and fabricated to study the flame position and structure of reacting jets injected into a high-speed vitiated crossflow. The windowed combustor was capable of long-duration, steady-state operation despite a trifecta of: elevated pressures, high combustion gas temperatures, and high-speed reacting flows. High-speed imaging using OH* and CH* chemiluminescence was used to validate operation of the optically-accessible secondary combustion zone.</p><p>High-repetition-rate (1 – 10 kHz) planar laser-induced fluorescence (PLIF) imaging of OH and CH were performed on both premixed NG-air reacting jets and premixed NG-hydrogen-air reacting jets to investigate the flame structure of the reacting jet within a high-speed crossflow. OH-PLIF was performed in the A-X electronic system using excitation at near 283 nm in the (vʹ = 1, v″ = 0) band and near 311 nm in the (vʹ = 0, v″ = 0) band. The crossflow velocity and equivalence ratio were observed to have a strong impact on the stabilization of the reacting jet flame. Additional insight on the stabilization mechanism was obtained using 50 kHz OH* chemiluminescence imaging. CH-PLIF was performed in the C-X electronic system using R-branch excitation near 311 nm in the (vʹ = 0, v″= 0) band. The CH-PLIF images indicated local stoichiometric regions near the leeward side of jet injection and in regions where significant interaction of the fuel rich jet with the vitiated crossflow is expected. In addition, the CH-PLIF images showed evidence of broken, thickened, non-premixed reaction layers.<br></p></div>

This Research explores hydrogen combustion in a Jet-in-Crossflow configuration through computational fluid dynamics using ANSYS Fluent commercial CFD software. Three fuel-only hydrogen jets with a momentum flux ratio J of 10, 50, and 115 were introduced axially, using a large eddy simulation with a WALE sub grid model. Detailed chemistry was computed directly with a 9 species hydrogen/air kinetic mechanism. The 4mm jet and crossflow domain utilized an automatic mesh adaptation method centered around the flame shear layer. The study models the second stage of a lab-scale gas turbine test facility at a pressure level of 5atm,a crossflow temperature of 1620K, and crossflow velocity of 75m/s. The models were compared to physical experiments conducted and analyzed with line-of-sight CH* chemiluminescence to create more insight into the phenomena of the combustion process. Flame position along the windward and leeside stabilization points were overlaid, and the validated CFD model utilized to characterize reaction progress as a function of jet entrainment with hot oxidizer. At elevated momentum flux ratio, increased reaction rates along the shear layer of the diffusion flame were attributed to the enhanced contact area between the fuel jet and crossflow oxidizer. The results outline the potential of carbon-free combustion technology and highlight the importance of tuning the operating condition for application in gas turbines.

Transverse jets in crossflow are widely used to enhance mixing between two flow streams. Such jets exhibit complex flow features, and are highly sen- sitive to a wide variety of operating conditions. The focus of this work is the mixing of relatively low Reynolds number jets that are often encountered in the chemical processing industry. The main objective is to determine if the the jet mixing characteristics can be sufficiently altered by changing the nature of the jet inflow. In particular, we study the effect of jet shape and inflow veloc- ity profile on the mixing properties. Four different jet shapes including circle, square, upstream triangle, and downstream triangle are considered. It is found that the jet shape has tremendous impact on the near field dynamics, gener- ating unique vortical structures for each shape. However, the overall mixing rate is unaffected and is controlled by the evolution of the coherent vortex pair (CVP) in the far-field of the jet. Analyses of turbulence modeling constraints and structure of reaction zones for consecutive-competitive reactions are also presented. / text

Jet-in-crossflow is an interaction between a fuel jet and air crossflow commonly found in jet engines. The crossflow is used to break up or atomize the fuel jet for downstream combustion. This interaction between fluids while at low speeds, is predictable, varies greatly at higher speeds. This investigation seeks to (1) create a mechanism for jet-in-crossflow, using mechanical pintles, that is independent of velocity to help increase the predictability and reliability of jet engines and (2) identify key design parameters that will lead to flow independence. Parameters investigated in this experiment include pintle height, angle, and percent of pintle coverage into the jet orifice. Pintles that covered 100 percent of the jet showed a strong deviation from the traditional interaction with no pintle. Relationships were also found between the angle, height, and penetration depth although none as ubiquitous as the jet coverage.

No description available.

Jet in crossflow (JICF) configurations are often used for secondary fuel injection in staged-fuel combustion systems. The high temperature, vitiated crossflow in these systems is inherently unsteady and characterized by random, turbulent fluctuations and coherent, acoustic oscillations. This thesis presents the results of an experimental investigation into the dynamics of non-reacting and reacting jets injected into unsteady, vitiated crossflow. The flow structure and flame stabilization of jets with different momentum flux and density ratios relative to the crossflow are characterized using simultaneous time-resolved stereoscopic particle image velocimetry (SPIV) synchronized with OH planar laser induced fluorescence (PLIF). A modified trajectory scaling law is developed to account for the influence of near-field heat release on the jet trajectory. The second part of this work focuses on the response of a JICF to crossflow forcing. Acoustic drivers are used to excite natural resonances of the facility, which are characterized using the two-microphone method. Spectral analysis of SPIV results shows that, while the jet response to crossflow velocity fluctuations is often negligible, the fluctuating crossflow pressure induces a significant fluctuating jet exit velocity, which leads to periodic jet flapping. The flame response to crossflow forcing is studied using flame edge tracking. An analytical model is developed that predicts the dependence of the jet injector impedance upon important JICF parameters. In the final part of this work, vortex tracking and Mie scattering flow visualization are used to investigate the effect of near-field heat release on the shear layer dynamics. A phenomenological model is developed to explain the effect of combustion on the shear layer stability of density stratified, reacting JICF. The results of this study demonstrate the important effects of near-field heat release and crossflow acoustics on the dynamics of reacting JICF.

The jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration. This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow. Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field. In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.

During the past decade, new advanced combustion systems that share the same basic concept of using a substantially diluted and high-temperature oxidizer in the reaction volume have gained a great deal of interest regarding their application in industrial and power systems. These novel combustion technologies have proved to offer significant benefits compared to traditional combustion techniques. These benefits include reductions in pollutant emissions and energy consumption, as well as a higher and more uniformly distributed heat flux. This entails the potential to, for example, reduce the size of equipment in industrial units or increase production rates while fuel consumption and the subsequent CO2 emissions are decreased or maintained at the same level. Although the development of these new combustion technologies has occurred fairly recently, it has gained worldwide recognition. During the past few years the technique has been used commercially with several different types of burners. Despite its widespread use, the basic understanding of the chemical-physical phenomena involved is limited, and a better understanding of the combustion phenomena is required for more effective utilization of the technology. The objectives of this work have been to obtain fuel-jet characteristics in combustion under high-temperature, low-oxygen conditions and to develop some theoretical considerations of the phenomena. The effect of the preheat temperature of the combustion air, combustion stoichiometry and the fuel-jet calorific value on flame behavior was investigated. Temperature and heat-flux distribution were also studied using a semi-industrial test furnace to see if similar flame features would be found for the small- and large-scale experiments. Particle Image Velocimetry (PIV) was used for the first time to obtain information on the flow dynamics of a fuel jet injected into a crossflow of oxidizer at either a normal temperature or a very high temperature. Light emission spectroscopy was used to collect information on time-averaged radical distributions in the combustion jet. Jet turbulence, time-averaged velocity distribution, fuel-jet mixing, the distribution of radicals such as CH, OH and C2, and flame photographs were investigated. The results showed delayed mixing and combustion under high-temperature low-oxygen-concentration conditions. The combustion air preheat temperature and oxygen concentration were found to have a significant effect on the burning fuel-jet behavior. The results of the semi-industrial-scale tests also showed the features of even flame temperature and heat flux. / QC 20100610

abstract: The atomization of a liquid jet by a high speed cross-flowing gas has many applications such as gas turbines and augmentors. The mechanisms by which the liquid jet initially breaks up, however, are not well understood. Experimental studies suggest the dependence of spray properties on operating conditions and nozzle geom- etry. Detailed numerical simulations can offer better understanding of the underlying physical mechanisms that lead to the breakup of the injected liquid jet. In this work, detailed numerical simulation results of turbulent liquid jets injected into turbulent gaseous cross flows for different density ratios is presented. A finite volume, balanced force fractional step flow solver to solve the Navier-Stokes equations is employed and coupled to a Refined Level Set Grid method to follow the phase interface. To enable the simulation of atomization of high density ratio fluids, we ensure discrete consistency between the solution of the conservative momentum equation and the level set based continuity equation by employing the Consistent Rescaled Momentum Transport (CRMT) method. The impact of different inflow jet boundary conditions on different jet properties including jet penetration is analyzed and results are compared to those obtained experimentally by Brown & McDonell(2006). In addition, instability analysis is performed to find the most dominant insta- bility mechanism that causes the liquid jet to breakup. Linear instability analysis is achieved using linear theories for Rayleigh-Taylor and Kelvin- Helmholtz instabilities and non-linear analysis is performed using our flow solver with different inflow jet boundary conditions. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

<div>The atomization of jets and droplets undergoing breakup in high-speed flows has been experimentally measured and theoretically modeled. Systems for producing individual droplet breakup and full jet breakup were designed, and a wide range of diagnostics were developed and adapted to measure the results with reduced uncertainty.</div><div><br></div><div>A detailed methodology for investigating high-speed sprays in the Purdue Experimental Turbine Aerothermal Lab is presented. Optical diagnostic techniques were carefully selected and optimized for the test section geometries and flow features, such that images could be collected at high frequencies of 20 kHz with high resolutions. Developed image processing routines are outlined to demonstrate how backlit imaging with specialized lenses allowed for more accurate spray depth measurements in supersonic conditions, which were then used in regression modeling routines to derive empirical correlations that factored in test section geometry, flow conditions, and injector design. A Mie scattering imaging technique was used for quantitative analysis of the supersonic spray plume profile and measurement of the spray width. 20 kHz shadowgraphy provided sufficient gradients for analysis of the unsteadiness of the spray and surrounding supersonic flow at the point of injection. Droplet sizes and velocities were measured in subsonic conditions using digital in-line holography, in which recent advancements to the reconstruction algorithm were implemented to reduce out-of-plane measurement uncertainty, and phase Doppler particle analysis.</div><div><br></div><div>The breakup of a single drop undergoing multi-mode breakup was analytically characterized, with the proposal of a new breakup criterion in the Taylor analogy breakup model. Hill vortices within the drop were proposed as a new flow mechanism promoting multi-mode breakup. Product drop sizes from the ring breakup were predicted and compared with experimental results.</div>