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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Comparison of Center Nozzle Staging to Outer Nozzle Staging in a Multi-Flame Combustor

Combustion instability in gas turbines is often mitigated using fuel staging, a strategy where the fuel is split unevenly between different nozzles of a multiple-nozzle combustor. This work examines the efficacy of different fuel staging configurations by comparing axisymmetric and non-axisymmetric fuel staging in a four-around-one model gas turbine combustor. Fuel staging is accomplished by increasing the equivalence ratio of the center nozzle (axisymmetric staging) or an outer nozzle (non-axisymmetric staging). When the global equivalence ratio is ϕ = 0.70 and all nozzles are fueled equally, the combustor undergoes longitudinal, self-excited oscillations. These oscillations are suppressed when the center nozzle equivalence ratio is increased above ϕ Staging = 0.79. This bifurcation equivalence ratio varies between ϕ Staging = 0.86 and ϕ Staging = 0.76 for the outer nozzles, and is attributed to minor hardware differences between each nozzle. High speed CH* chemiluminescence images in combination with dynamic pressure measurements are used to determine the instantaneous phase difference between the heat release rate fluctuation and the combustor pressure fluctuation throughout the combustor. This analysis shows that the staged flame has similar phase relationships for all staging configurations. It is found that axisymmetric staging can be as effective as non-axisymmetric staging; however, the aforementioned hardware variations can impact both the bifurcation equivalence ratio and the effectiveness of staging.

Culler, Wyatt↗
DOE OSTI2019

Nozzle tip wetting in gasoline direct injection injector and its link with nozzle internal flow

Fuel film in the gasoline direct injection injector tip, or so-called nozzle tip wetting, has been found to be an important contributor of particle emissions. Attempts have been made to reduce the nozzle tip wetting by optimizing nozzle geometry designs. However, the inherent mechanism of the nozzle tip wetting formation and its link with nozzle internal flow is still unclear yet due to the lack of direct observations. To overcome this insufficiency, here the nozzle internal flow and the formation process of the nozzle tip wetting were visualized in the real-scale aluminum nozzles using the X-ray phase-contrast technique. Results showed that the needle bouncing, injection pressure, and hole configuration affect the formation of the nozzle tip wetting, while the influence of needle bouncing is the most critical. A further study was conducted to examine the effect of nozzle counterbore diameter on the nozzle tip wetting. It was found that with an increase in counterbore diameter, the nozzle tip wetting slightly increased first and then decreased sharply after the counterbore diameter exceeded 0.40 mm. The mechanisms of the aforementioned phenomena were discussed in detail, which can contribute to the better understandings and control strategies of nozzle tip wetting.

42 ENGINEERING↗
DOE OSTI2020

Nozzle assembly and method for forming nozzle assembly

A nozzle assembly is disclosed, including a CMC nozzle shell, a nozzle spar, and an endwall. The CMC nozzle shell includes a CMC composition and an interior cavity. The nozzle spar is partially disposed within the interior cavity and includes a metallic composition, a cross-sectional conformation, a plurality of spacers protruding from the cross-sectional conformation, the plurality of spacers contacting the CMC nozzle shell, and a spar cap. The endwall includes at least one surface in lateral contact with the spar cap and maintains a lateral orientation of the CMC nozzle shell and the nozzle spar relative to the endwall. The lateral orientation maintains a predetermined throat area of the nozzle assembly. A method for forming the nozzle assembly includes inserting the nozzle spar into the interior cavity, rotating the CMC nozzle shell and the nozzle spar laterally relative to the endwall, and maintaining the lateral orientation.

Hafner, Matthew Troy↗
DOE OSTI2017

The Effect of Transient Fuel Staging on Self-Excited Instabilities in a Multi-Nozzle Model Gas Turbine Combustor

Combustion instability in gas turbines can be mitigated using active techniques or passive techniques, but passive techniques are almost exclusively used in industrial settings. While fuel staging, a common passive technique, is effective in reducing the amplitude of self-excited instabilities in gas turbine combustors at steady-state conditions, the effect of transients in fuel staging on self-excited instabilities is not well understood. This paper examines the effect of fuel staging transients on a laboratory-scale five-nozzle can combustor undergoing self-excited instabilities. The five nozzles are arranged in a four-around-one configuration and fuel staging is accomplished by increasing the center nozzle equivalence ratio. When the global equivalence ratio is φ = 0.70 and all nozzles are fueled equally, the combustor undergoes self-excited oscillations. These oscillations are suppressed when the center nozzle equivalence ratio is increased to φ = 0.80 or φ = 0.85. Two transient staging schedules are used, resulting in transitions from unstable to stable operation, and vice-versa. It is found that the characteristic instability decay times are dependent on the amount of fuel staging in the center nozzle. It is also found that the decay time constants differ from the growth time constants, indicating hysteresis in stability transition points. High speed CH* chemiluminescence images in combination with dynamic pressure measurements are used to determine the instantaneous phase difference between the heat release rate fluctuation and the combustor pressure fluctuation throughout the combustor. This analysis shows that the instability onset process is different from the instability decay process.

Culler, Wyatt↗
DOE OSTI2018

Hole number effect on spray dynamics of multi-hole diesel nozzles: An observation from three- to nine-hole nozzles

For the simplicity of optical diagnostics, studies on fuel spray and combustion characteristics of modern diesel engines have been conducted using the investigation nozzles having the reduced number of holes (two- or three-hole nozzles) compared to the production nozzles. However, the similarity of spray dynamics between the investigation nozzles and production nozzles has not been confirmed yet. Also, the hole number effect on spray dynamics has been investigated only in a limited range of hole number making it difficult to derive the clear trend and understandings of the hole number effect. In this study, the hole number effect on spray dynamics of multi-hole diesel nozzles has been investigated in a wide range of hole number with sufficient data points (three, five, six, eight, nine-hole) by using the X-ray Phase-Contrast Imaging technique. The non-linear trend in the spray structure, as well as the dynamics results, was observed as increasing the hole number regardless of injection pressure. Discussions were made accordingly to understand the observed result trend. In conclusion, the study suggests that great attention is needed when deciding the number of nozzle holes for the optimized engine combustion.

42 ENGINEERING↗
DOE OSTI2018

Comparison of Equivalence Ratio Transients on Combustion Instability in Single-Nozzle and Multi-Nozzle Combustors

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinement from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

Chen, Xiaoling↗
DOE OSTI2018

Comparison of equivalence ratio transients on combustion instability in single-nozzle and multi-nozzle combustors

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinements from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2018

Evaluation of Shot-to-Shot In-Nozzle Flow Variations in a Heavy-Duty Diesel Injector Using Real Nozzle Geometry

Cyclic variability in internal combustion engines (ICEs) arises from multiple concurrent sources, many of which remain to be fully understood and controlled. This variability can, in turn, affect the behavior of the engine resulting in undesirable deviations from the expected operating conditions and performance. Shot-to-shot variation during the fuel injection process is strongly suspected of being a source of cyclic variability. This study focuses on the shot-to-shot variability of injector needle motion and its influence on the internal nozzle flow behavior using diesel fuel. High-speed x-ray imaging techniques have been used to extract high-resolution injector geometry images of the sac, orifices, and needle tip that allowed the true dynamics of the needle motion to emerge. These measurements showed high repeatability in the needle lift profile across multiple injection events, while the needle radial displacement was characterized by a much higher degree of randomness. A robust and previously validated computational setup from the authors’ research group using a commercial computational fluid dynamics (CFD) code has been adapted for the eight-hole heavy-duty common-rail diesel injector used for the measurements. The simulation results obtained using the x-ray-scanned geometry have been validated against available experimental data of mass flow rate at the nozzle exit. The fuel mass flow rate has then been analyzed at three different injection pressures that cover the conditions at which the injector typically operates during normal engine operation. Finally, the average off-axis motion of the needle has been perturbed (based on the variability found in the experimental measurements) to generate three new cases that present different amplitude and phasing of the radial displacement with respect to the baseline average motion. Furthermore, this revealed the effects of off-axis motion on shot-to-shot and orifice-to-orifice variations.

42 ENGINEERING↗
DOE OSTI2021

Effect of Nozzle Curvature on Supersonic Gas Jets Used in Laser-Plasma Acceleration

Supersonic gas jets produced by converging-diverging (C-D) nozzles are commonly used as targets for laser-plasma acceleration (LPA) experiments. A major point of interest for these targets is the gas density at the region of interaction where the laser ionizes the gas plume to create a plasma, providing the acceleration structure. Tuning the density profiles at this interaction region is crucial to LPA optimization. A "flat-top" density profile is desired at this line of interaction to control laser propagation and high energy electron acceleration, while a short high-density profile is often preferred for acceleration of lower-energy tightly-focused laser-plasma interactions. A particular design parameter of interest is the curvature of the nozzle's diverging section. We examine three nozzle designs with different curvatures: the concave "bell", straight conical and convex "trumpet" nozzles. We demonstrate that, at mm-scale distances from the nozzle exit, the trumpet and straight nozzles, if optimized, produce "flat-top" density profiles whereas the bell nozzle creates focused regions of gas with higher densities. An optimization procedure for the trumpet nozzle is derived and compared to the straight nozzle optimization process. We find that the trumpet nozzle, by providing an extra parameter of control through its curvature, is more versatile for creating flat-top profiles and its optimization procedure is more refined compared to the straight nozzle and the straight nozzle optimization process. Furthermore, we present results for different nozzle designs from computational fluid dynamics (CFD) simulations performed with the program ANSYS Fluent and verify them experimentally using neutral density interferometry.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Ceramic matrix composite turbine nozzle shell and method of assembly

A ceramic matrix composite turbine nozzle includes a primary outer nozzle platform; a primary inner nozzle platform; and an airfoil-shaped body extending between the primary inner and primary outer nozzle platforms. The body includes core plies defining a cavity; composite wrap plies circumscribing the core plies and defining an airfoil shape; a secondary outer nozzle platform in contact with the primary outer nozzle platform; and a secondary inner nozzle platform in contact with the primary inner nozzle platform. Each composite wrap ply has two layers of unidirectional fibers oriented transverse to each other and has first and second longitudinal edges. The first and second longitudinal edges are cut into fingers, which are folded in a transverse direction away from a turbine nozzle longitudinal axis and are interleaved between platform plies to define the secondary inner nozzle platform and the secondary outer nozzle platform.

de Diego, Peter↗
DOE OSTI2019

Effects of Nozzle Radial Position, Separation Angle, and Gas Flow Partitioning on the Mixing, Eye Area, and Wall Shear Stress in Ladles Fitted with Dual Plugs

Mixing time, slag eye area, and wall shear stress in a ladle fitted with dual plugs have been studied as a function of operating variables, namely, gas flow rate, radial nozzle position, separation angle between nozzles and flow rate partitioning. While the mixing time and slag eye area have been experimentally investigated in a scaled water model (with and without a top slag layer), the shear stress on the vessel wall has been studied computationally via a RANS-based turbulent, three-dimensional coupled Eulerian–Lagrangian (VOF-DPM), multiphase flow model. Experimental and computational results have indicated that the mixing, eye area, and wall shear stress depend on the gas flow rate, i.e., while mixing efficiency increases with the increasing gas flow rate, the slag eye area, and wall shear stress, in contrast, become more pronounced with the increasing gas flow, leading to an undesirable operating regime. The radial nozzle position, separation angle between the nozzles and flow rate partitioning at any given flow rate also influence the ladle process performance, influencing the mixing, eye area, and wall shear stress, albeit to a varied degree. Within the range of operating conditions studied, an expected inverse correspondence between mixing time and shear behavior was observed. Experimental and computational results in conjunction have indicated that the best arrangement of porous plugs or gas injection nozzles for superior ladle process performance is dependent on the gas flow rate and is specific to the desired objective (i.e., decreasing the mixing time, or slag eye area and wall shear stress). Hence, a unique gas injection practice cannot and should not be suggested as the optimum for ladle metallurgy in steelmaking. Nevertheless, if the mixing time is the parameter of primary interest, a nozzle configuration with equal flow partitioning (1:1) between the nozzles and identical nozzle radial positions (0.7R/0.7R, 45 deg) should suffice for both low and high gas flow rates. In contrast, a nonidentical nozzle radial position (0.7R/0.5R, 90 deg) and an unequal gas flow rate per nozzle (1:3) appear preferable if both ladle eye and shear stresses, rather than mixing, are the issues of concern.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Multi-hole gasoline direct injection:In-nozzle flow and primary breakup investigated in transparent nozzlesand with X-ray

This contribution describes the flow field inside modern gasoline direct-injection nozzles and sprays. Starting from the internal nozzle flow, results from transparent real-size nozzles are shown, where a significant vapor fraction even for cold fuel conditions is proven. Based on vapor fraction inside the nozzle, evidence for (super-)sonic flow conditions inside the nozzle is shown. The nozzle outlet velocity is determined by means of X-ray structure tracking velocimetry, which is a very powerful measurement technique to gain access to the very dense spray at the nozzle outlet. The X-ray velocities are compared to values that are determined by means of optical—phase Doppler anemometry/laser Doppler anemometry and Schlieren imaging—measurement techniques. By extrapolating the maximum droplet velocities found by laser Doppler anemometry in the more downstream regions of the spray to the nozzle outlet region, very similar velocities to the one derived from the X-ray measurements close to Bernoulli velocity are evaluated for typical gasoline direct-injection engine conditions. A third access to the nozzle outlet velocity is given by the derivation of penetration curves. The combination of vapor fractions and outlet velocities provides a measure for the initial spray momentum.

42 ENGINEERING↗
DOE OSTI2022

Investigation of the effects of passive pre-chamber nozzle pattern and ignition system on engine performance and emissions

The impact of passive pre-chamber (PC) internal volume, nozzle hole pattern (i.e. with and without a central axial nozzle), and PC igniter plug type on performance and emissions was investigated in an optically accessible, single-cylinder, gasoline research engine. The two PC igniter plugs investigated were a conventional inductive coil spark plug and a nanosecond repetitively pulsed (NRP) plasma discharge system previously demonstrated to accelerate early flame propagation. The baseline PC design featured a funnel shaped internal volume with a PC tip that contained six radial nozzles and a larger central axial nozzle. Two additional PC tip geometries were evaluated where either the baseline internal volume was increased or the axial nozzle was removed and the radial nozzle diameters were increased. A sweep of charge equivalence ratios ( ϕ) from nearly stoichiometric to the lean limit was performed for a fixed engine speed (1300 revolutions per minute), and engine load (3.5 bar gross indicated mean effective pressure). Time-resolved PC and main chamber (MC) pressure data as well as MC emissions data were collected to analyze engine performance and emissions characteristics. Combustion in the MC was further investigated using high-speed excited methylidyne radical (CH*) chemiluminescence imaging. Collected results highlighted that while all PC tips and ignition systems exhibited similar performance and emissions down to ϕ = 0.8, relevant differences in thermal efficiency and emissions for leaner charge mixtures were observed, with the results highly dependent on the nozzle pattern and ignition system. Major deviations were correlated to preferential de-pressurization of the PC through the axial nozzle for lean conditions that was not observed for mixture conditions closer to stoichiometric. Results show that a combination of radial and axial nozzle patterns in the PC extended lean-stability limits at the low-load condition evaluated. Further benefits were observed with the use of NRP ignition systems due to faster combustion within the PC volume provided that the volume was sufficiently large.

Di Sabatino, Francesco↗
DOE OSTI2018

The effect of non-axisymmetric fuel staging on flame structure in a multiple-nozzle model gas turbine combustor

Fuel staging is often used to suppress combustion instabilities in a gas turbine engine. This strategy is accomplished by applying differing flow rates of fuel to different nozzles of a multinozzle combustor. As such, the mixture composition and critical flame properties like flame speed and heat release may vary from nozzle to nozzle. In this experiment, five identical nozzles are arranged with a central nozzle surrounded symmetrically by four others; this is commonly referred to as a “four-around-one” configuration. The fuel is staged by adding more fuel through the middle and one outer nozzle, where staging through the middle nozzle will be referred to as axisymmetric staging, and staging through an outer nozzle is non-axisymmetric staging. This staging is analyzed through the use of tomographic image reconstruction. Images are taken in 5 degree increments over a 180 degree span around the flame for each staging condition. We compare three cases: one without staging, one with axisymmetric staging, and another with non-axisymmetric staging. Fuel staging can cause deviations in the flame structure where neighboring flames meet each other. Nonaxisymmetric versus axisymmetric staging are found to result in different flame shapes and stabilization methods due to the difference in the number of flames that are interacting with the staged nozzle.

Sekulich, Olivia↗
DOE OSTI2022

Investigation of Needle Motion Profile Effect on Diesel Spray in Near-Nozzle Field

A variety of needle-motion profiles are used in diesel injectors. However, it is unclear what the underlying mechanism is to determine the needle-motion profiles and how they affect the spray dynamics. It has been of significant interest to examine how the spray dynamics will change if only altering the needle valve opening speed or closing speed while all other parameters are kept the same. The different needle-motion profiles were obtained using a piezo nozzle (Nozzle #P) and a solenoid nozzle (Nozzle #S), which have identical nozzle geometry. By utilizing the X-ray imaging technique, it was observed that the average needle valve speed of Nozzle #P was 51% higher at the opening stage but 17% lower at the closing stage than Nozzle #S. When the needle valve lift is low (approximately 200 μm), the needle valve opening speed has a crucial effect on spray dynamics. The faster needle valve opening of Nozzle #P results in a 42% larger spray spreading angle and 34% lower spray velocity at the downstream field. The spray dynamics may be controllable by properly designing the needle-motion profiles in the scenarios of the low needle lifts. However, when the needle valve is sufficiently open (approximately over 200 μm), almost identical spray characteristics were observed regardless of the needle-motion profiles.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Flame Edge Dynamics and Interaction in a Multi-Nozzle Can Combustor with Fuel Staging

The characterization and mitigation of thermoacoustic combustion instabilities in gas turbine engines is necessary to reduce pollutant emissions, premature wear, and component failure associated with unstable flames. Fuel staging, a technique in which the fuel flow to a multi-nozzle combustor is unevenly distributed between the nozzles, has been shown to mitigate the intensity of self-excited combustion instabilities in multiple nozzle combustors. In our previous work, we hypothesized that staging suppresses instability through a phase-cancellation effect in which the heat release rate from the staged nozzle oscillates out of phase with that of the other nozzles, leading to destructive interference that suppresses the instability. This previous theory, however, was based on chemiluminescence imaging, which is a line-of-sight integrated technique. In this work, we use high-speed laser-induced fluorescence to further investigate instability suppression in two staging configurations: center-nozzle and outer-nozzle staging. An edge-tracking algorithm is used to compute local flame edge displacement as a function of time, allowing instability-driven edge oscillation phase coherence and other instantaneous flame dynamics to be spectrally and spatially resolved. Analysis of flame edge oscillations shows the presence of convecting coherent fluctuations of the flame edge caused by periodic vortex shedding. When the system is unstable, these two flame edges oscillate together as a result of high-intensity longitudinal-mode acoustic oscillations in the combustor that drive periodic vortex shedding at each of the nozzle exits. In the stable cases, however, the phase between the oscillations of the center and outer flame edges is greater than 90 degrees (~114 degrees), suggesting that the phase-cancellation hypothesis may be valid. This analysis allows a better understanding of the instantaneous flame dynamics behind flame edge oscillation phase offset and fuel staging-based instability suppression.

Doleiden, Daniel↗