Vincent McDonell

Lean blowout model for a spray-fired swirl-stabilized combustor

In aero-engine applications, the lean blowoff (LBO) limit plays a critical role in the operational envelope of the engine. The geometry of the combustion chamber primary zone plays a critical role in establishing LBO limits. This is especially true for advanced lean burn concepts which introduce the majority of the combustion air in a manner designed to enhance the rate of mixing with the fuel. In the present study, specific mixer hardware has been designed to develop a systematic, statistically sound test matrix to study the effect of mixer components (primary swirl vane, secondary swirl vane, Venturi, and co-and counterswirl) on LBO. A strategy is employed to develop, based on an existing model, a new predictive model for LBO which accounts for a heterogeneous swirl-stabilized reaction and explicitly relates the geometry of the hardware to the LBO limit. The model predicts the LBO fuel/air ratio at three operating temperatures to within 14% of the measured value. The multivariate experiments used to relate LBO to geometry were also further analyzed to establish the main hardware parameters affecting LBO. Specifically, the Venturi and swirl sense (co- versus counter-swirl) were found to impact LBO at lower air inlet temperatures (294 and 366 K). The Venturi and counter-swirl enhance the atomization and mixing processes which are more rate limiting at lower temperatures, and, as a result, improve stability. At a higher inlet air temperature (477 K), the secondary swirl vane angle also plays a role in determining LBO, with larger angles (75°) generating better stability, which is associated with a stronger recirculation zone. The hardware configuration with the best LBO performance over the different conditions was identified (45° primary swirler, 55° secondary swirler, counter-swirl with Venturi).

Dynamics of a Longitudinally Forced, Bluff Body Stabilized Flame

This paper describes an investigation of the response of bluff body stabilized flames to harmonic oscillations. This problem involves two key elements – the excitation of hydrodynamic flow instabilities by acoustic waves, and the response of the flame to these harmonic flow instabilities. In the present work, data were obtained with inlet temperatures from 311 K to 866 K and flow velocities from 38 m/s to 170 m/s. These data show that the flame front response at the acoustic forcing frequency first increases linearly with downstream distance, then peaks and decays. The corresponding phase decreases linearly with axial distance, showing that wrinkles on the flame propagate with a nearly constant convection velocity. These results are compared to those obtained from a theoretical solution of the G-equation excited by a harmonically oscillating, convecting disturbance. This kinematic model shows that the key processes controlling the response are 1) the anchoring of the flame at the bluff body, 2) the excitation of flame-front wrinkles by the oscillating velocity, and 3) flame propagation normal to itself at the local flame speed. The first two process control the growth of the flame response and the last process controls the decay. These predictions are shown to describe the key features of the measured flame response characteristics.

Effect of ambient pressure on an airblast spray injected into a crossflow

The injection of a fuel spray into a cross stream was studied for its application in rapid fuel ‐air mixing for lean combustion processes. The fuel is injected as either a discrete stream or as a partially to fully atomized jet of droplets. Of particular interest was the penetration of the outer and inner edges of the spray of liquid fuel into the gaseous airstream. The experiment focused on exploring the effect of e ow conditions on the spray surface trajectories from the point of injection to a downstream distance of z/Dfuel = 35. Tests were conducted under ambient pressures of 1, 3, and 5 atm at atomizing air pressure drops varying from 0 to 4.8% for a jet-A fuel e ow of 0.18 g/s and a baseline crosse ow airvelocity of 38 m/s. A modie ed dee nition of the jet-to-crosse owmomentum-e ux ratioq2 wasdeveloped to accommodatea two-phasejetandwassubsequentlyused to obtaina relationshipbetween the e ow conditions and thespray surface trajectories. The effect of the degree of atomization in thespray resulting from the change in operating conditions was incorporated by implementing a pressure ratio correction factor into the correlating equation. Nomenclature Aairbl = area associated with the airblast air, assumed as the difference between Aspray and Aliquid Aliquid = area associated with the fuel injection orie ce Aspray = area associated with the spray injection orie ce Cd = orie ce discharge coefe cient cn = correlation constants, n D0;1;2;3 Dfuel = fuel injection orie ce diameter Dspray = spray injection orie ce diameter q1 = single-phase jet-to-crosse ow momentum-e ux ratio q2 = two-phase jet-to-crosse ow momentum-e ux ratio V = velocity Wecross = crosse ow-associated Weber number ΩairV 2 crossDfuel=ae x = penetration distance z = downstream distance Ω = density

Experimental Study of a Model Gas Turbine Combustor Swirl Cup, Part 11: Droplet Dynamics

As the second part of a study to characterize the performance of a 3 x scale gas turbine combustor swirl cup, the focus of the present article addresses the droplet dynamics. Droplet axial, radial, and tangential velocities as well as size were measured using phase Doppler interferometry. Bimodal droplet axial and tangential velocities distributions were observed in the shear layer close to the exit plane of the swirl cup. Bimodal droplet radial velocity distributions around the closing point of the on-axis recirculation zone and at the periphery of the spray were observed as well. Droplet velocity histograms, based on droplet size classes, reveal how these velocity bimodal distributions are formed and explain why velocity fluctuations for larger droplets are greater than those of the smaller droplets in some regions. Correlations between size and velocity, individual velocity components, and size and flow angle provide evidence of intermittent flowfield structures superimposed on the local and global turbulence. Specific flowfield structures and flowfield intermittencies are found to give rise to these bimodal distributions. Overall, these data add measurably to the understanding of the two-phase transport that can occur in practical systems. 2 refs.

Gas and drop behavior in reacting and non-reacting air-blast atomizer sprays

A detailed study of the two-phase flow produced by a gas-turbine air-blast atomizer is performed with the goal of identifying the interaction between the two phases for both nonreacting and reacting conditions. A two-component phase Doppler interferometry is utilized to characterize three flowfields produced by the atomizer: (1) the single-phase flow, (2) the two-phase nonreacting spray, and (3) the two-phase reacting spray. Measurements of the mean and fluctuating axial and azimuthal velocities for each phase are obtained. In addition, the droplet size distribution, volume flux, and concentration are measured. The results reveal the strong influence of the dispersed phase on the gas, and the influence of reaction on both the gas and the droplet field. The presence of the spray significantly alters the inlet condition of the atomizer. With this alteration quantified, it is possible to deduce that the inertia associated with the dispersed phase damps the fluctuating velocities of the gas. Reaction reduces the volume flux of the droplets, broadens the local volume distribution of the droplets in the region of the reaction zone, increases the axial velocities and radial spread of the gas, and increases the anisotropy in the region of the reaction zone. 20 refs.

Experimental Study of a Model Gas Turbine Combustor Swirl Cup, Part I: Two-Phase Characterization

The behavior of droplets and the continuous phase (i.e., gas in the presence of the droplets) downstream of a 3 x model gas turbine combustor dome swirl cup is characterized via phase Doppler interferometry in the absence of reaction. The goal is to improve the understanding of droplet-gas interaction in a complex flow typical of that produced by engine hardware. Three components of continuous phase and droplet velocities were measured along with droplet size. The measurements reveal that (1) at the exit plane of the swirl cup, more uniform and finer droplets are produced relative to the atomizer alone, (2) both the continuous phase and droplets recirculate, (3) the region downstream of the swirl cup into which droplets join the recirculation is correlated with droplet size, and (4) significant slip velocities exist between the continuous phase and the droplets which are also correlated with droplet size and reflect a strong momentum exchange between the phases. 11 refs.

Effect of hardware geometry on gas and drop behavior in a radial mixer spray

The demands on current and future aero gas turbine combustors are requiring a greater insight into the role of the injector/done design and manufacturing tolerances. This paper systematically isolates manufacturing tolerances and focuses on hardware design. The target is the structure of the two-phase flow and combustion performance associated with practical injector/dome hardware. A spray injector with two radial inflow swirlers was custom designed to (1) maintain tight tolerances and strict assembly protocol and (2) thereby isolate the sensitivity of performance to hardware design. Although it represents practical hardware, the custom set is a unique modular design that (1) accommodates parametric variation in geometry, (2) retains symmetry, and (3) maintains effective area. Swirl sense and the presence of a venturi were found to be the most influential. The venturi acts as a fuel prefilming surface and constrains the highest fuel mass concentration to an annular ring near the centerline. Coswirl enhances the radial dispersion of the continuous phase, and counterswirl increases the level of mixing that occurs in the downstream region of the mixer. The combined effect of the two parameters (swirl sense and venturi) revealed that the largest drop sizes, which penetrate the continuous phase flow, are formed with coswirl and without venturi. The smallest drop size distributions were found to occur for the counterswirl configuration with venturi. In the case of counterswirl without venturi, the high concentration of fluid mass is found in the center region of the flow. The lean blowout (LBO) equivalence ratio was lower for counterswirl configurations for reasons that involved the coupling of the centerline recirculation zone with the location of high fuel concentration emanating from smaller droplets. In the coswirl configuration, a lack of fuel drops exists in the reaction anchoring region, thereby leading to poor stability characteristics.

Scaling of the Two-Phase Flow Downstream of a Gas Turbine Combustor Swirl Cup: Part I—Mean Quantities

A production gas turbine combustor swirl cup and a 3×- scale model (both featuring co-axial, counter-swirling air streams) are characterized at atmospheric pressure. Such a study provides an opportunity to assess the effect of scale on the behaviour of the continuous phase (gas in the presence of spray) and droplets by comparing the continuous phase velocity, droplet size, and droplet velocity at geometrically analogous positions. Spatially resolved velocity measurements of the continuous phase, droplet size, and droplet velocity were acquired downstream of the production and 3×-scale swirl cups by using two-component phase-Doppler interferometry in the absence of reaction. While the continuous phase flow fields scale well at the exit of the swirl cup, the similarity deviates at downstream locations due to (1) differences in entrainment, and (2) a flow asymmetry in the case of the production hardware. The droplet velocities scale reasonably well with notable exceptions. More significant The droplet velocities scale reasonably well with notable exceptions. More significant differences are noted in droplet size, although the presence of the swirl cup assemblies substantially reduces the differences in size that are otherwise produced by the two atomizers when operated independent of the swirl cup.

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part I: Experiments and Data Analysis

This article describes measurements of the response of bluff-body stabilized flames subjected to transverse acoustic waves. It is the first of a two-article series. The objective of this work was to extend prior studies of this nature to much higher Reynolds numbers and more severe environments that more closely mimic conditions encountered in applications. To this end, experiments were performed at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The data show that the disturbance field and the flame front response amplitude exhibit a nonmonotonic spatial distribution with interference patterns. The phase of the flame front response at the forcing frequency varies nearly linearly with downstream distance, and corresponds to a phase speed that is slightly less than the mean flow velocity. Significantly, these results show that the key features of the flames magnitude and phase characteristics are quite similar to those observed in much lower flow velocities.

Impact of Biodiesel on Fuel Preparation and Emissions for a Liquid Fired Gas Turbine Engine

This paper reports the fuel injection, vaporization, and emissions characteristics when running a 30 kW gas turbine engine on biodiesel (B99), and diesel fuel distillate # 2. Compositional analysis is used to assess the distillation of these fuels and a comparison is made with ethanol. The role of the liquid properties on fuel preparation and the subsequent engine performance is also assessed. The results show that while compositionally simple, biodiesel features some properties that result in inferior atomization and longer evaporation times compared to DF2. In addition, the overall spray behavior differs substantially in terms of density and width. The measured NO and CO emissions levels produced by the engine reveal significant increases with the use of biodiesel which is expected given the inferior fuel preparation characteristics. It appears reasonable to modify the fuel injector in order to overcome the deleterious effects observed for biodiesel. The benefits of blending biodiesel and ethanol in order to eliminate duel fuel operation during cold startup are discussed.Copyright