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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Summer Aerosol and Trace Gas Observations in Houston, Texas Using an Adaptable Mobile Facility

An aerosol container featuring a shared inlet system was deployed to Houston, Texas in July 2022, enabling direct, high-time-resolution in situ measurements of aerosols and trace gases. The internal rack system and floorplan was designed for adaptable modularity to elucidate aerosol physicochemical processes at fine scales. The design allowed for the deployment of a core instrument suite and additional customized research grade instruments. A heterogeneous mixture of aerosols was observed during three regimes: (1) intermittent black carbon (BC) and diurnal variations in aerosol chemical composition, (2) observed particle growth associated with SO 2 , (3) transported supermicron dust. The high variability of observed particles and gases in high time resolution indicated a complex urban area with multiple local and regional sources and processes. Particle growth rates of 7–16 nm/hr were observed for submicron particles during periods when SO 2 was >0.5 ppbv. Two periods of multi-day long-range transport events of dust from the African Sahara were observed in the supermicron and submicron particle modes with total mass concentrations up to 30 μg m −3 . Aerosol scattering angstrom exponents and extinction coefficients (B ext ) increased with humidity as a function of particle composition. The measurements demonstrate collaborative capabilities that can be used to increase observations of aerosol processing, microphysical and optical properties, internal mixing state, and supermicron aerosol that are not parameterized or missing in global Earth energy system models.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Assessing the Effects of Temperature and Oxygen Vacancy on Band Gap Renormalization in LaCrO 3-δ : First-Principles and Experimental Corroboration [plus supplemental information]

Understanding the temperature dependence of functional properties in high-temperature gas sensors is vital for applications in combustion environments. Temperature effect on the electronic structure due to electron-phonon coupling is a key property of interest as this influences other responses of sensors. In this work, we assess the impact of temperature on band gap renormalization of pristine and oxygen-vacant LaCrO 3–δ perovskite employing Allen–Heine–Cardona theory with first-principles simulations and corroborate with experimental observation. Antiferromagnetic cubic LaCrO 3 shows a direct ground-state band gap of 2.62 eV that is reduced by over 1 eV due to the presence of oxygen vacancies, which can form endothermically. We find excellent agreement in temperature-dependent band gap shift in LaCrO 3 between theory and an in-house experiment, proving that the theory can adequately predict renormalization on the band gap in a magnetic system. Band gaps in cubic LaCrO 3–δ are found to monotonically narrow by 1.13 eV in pristine and by around 0.62 eV in oxygen-vacant structures as temperature increases from 0 to 1500 K. The predicted band gap variations are rationalized using an analytical model. Furthermore, the experimental zero-temperature band gaps are extracted from the model fits that can provide useful insights on the simulated band gaps.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2012

Arctic Tundra Vegetation Functional Types Based on Photosynthetic Physiology and Optical Properties

Climate change in tundra regions may alter vegetation species composition and ecosystem carbon balance. Remote sensing provides critical tools for monitoring these changes as optical signals provide a way to scale from plot measurements to regional patterns. Gas exchange measurements of pure patches of key vegetation functional types (lichens, mosses, and vascular plants) in sedge tundra at Barrow AK, show three significantly different values of light use efficiency (LUE) with values of 0.013+/-0.001, 0.0018+/-0.0002, and 0.0012 0.0001 mol C/mol absorbed quanta for vascular plants, mosses and lichens, respectively. Further, discriminant analysis of patch reflectance identifies five spectral bands that can separate each vegetation functional type as well as nongreen material (bare soil, standing water, and dead leaves). These results were tested along a 100 m transect where midsummer spectral reflectance and vegetation coverage were measured at one meter intervals. Area-averaged canopy LUE estimated from coverage fractions of the three functional types varied widely, even over short distances. Patch-level statistical discriminant functions applied to in situ hyperspectral reflectance successfully unmixed cover fractions of the vegetation functional types. These functions, developed from the tram data, were applied to 30 m spatial resolution Earth Observing-1 Hyperion imaging spectrometer data to examine regional variability in distribution of the vegetation functional types and from those distributions, the variability of LUE. Across the landscape, there was a fivefold variation in tundra LUE that was correlated to a spectral vegetation index developed to detect vegetation chlorophyll content.

Huemmrich, Karl F.↗
DOE OSTI2020

On the pressure and temperature dependence of adsorption densities and other thermodynamic properties in gas shales

Black shale source rocks have become a key natural gas resource in the US and China. Unlike in conventional hydrocarbon reservoirs, a significant fraction of the gas in shales is adsorbed onto the inordinate specific surface areas (SSA) of micropores inside organic matter with densities exceeding those of bulk gas by an order of magnitude. Yet, the distribution of adsorbed versus bulk gas remains poorly understood. Experiments measure excess adsorption isotherms, which are corrected to absolute adsorption amounts by a factor that involves the density, ρ abs , of the adsorption layer(s). Constant values of ρ abs are widely used in the literature but, we argue, are inconsistent with the pressure- and temperature-dependence of adsorption behavior. In this work, we propose a more reliable approach that assumes a constant volume of each adsorption layer that can be determined from the SSA of the substrate. We measure nine high-pressure methane excess adsorption isotherms up to 15 MPa for three shale samples from China and Germany at temperatures of 65, 75, and 95 °C, and low-pressure carbon dioxide and nitrogen isotherms to estimate the SSA. We also investigate another 24 isotherms at 35, 50, and 60° C or 8 Chinese shale samples and 3 isotherms at 60, 90, and 120° C for an Argentinian shale sample from the literature. A new variation of the multi-layer Ono-Kondo lattice model is introduced to derive the temperature and pressure dependent adsorption layer densities, which are subsequently used to obtain the absolute adsorption isotherms of interest. Finally, this practical methodology has the potential to significantly improve our estimates of shale gas-in-place resources, and adsorption modeling more broadly.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1983

Evaluation of ceramics for stator applications: Gas turbine engines interim report. Stator fabrication and evaluation

The objective was to assess current ceramic materials, fabrication processes, reliability prediction, and stator durability when subjected to simulated automotive gas turbine engine operating conditions. Ceramic one-piece stators were fabricated of two materials, silicon nitride and silicon carbide, using two near-net-shape processes, slip casting and injection molding. Non-destructive evaluation tests were conducted on all stators identifying irregularities which could contribute to failures under durability testing. Development of the test rig and automatic control system for repeatably controlling air flow rate and temperature over a highly transient durability duty cycle is discussed. Durability results are presented for repeated thermal cycle testing of the ceramic one-piece stators. Two duty cycles were used, encompassing the temperature ranges of 704 to 1204 C (1300 to 2200 F) and 871 to 1371 C (1600 to 2500 F). Tests were conducted on 28 stators, accumulating 135,551 cycles in 2441 hours of hot testing. Cyclic durability for the ceramic one-piece stator was demonstrated to be in excess of 500 hours, accumulating over 28,850 thermal cycles. Ceramic interface forces were found to be the significant factor in limiting stator life rather than the scatter in material strength properties or the variation in component defects encountered.

Arnon, N.↗
DOE OSTI2022

Impact of Variable Gas Mixtures on Bubble Size Distribution and Mass Transfer in Gas Fermentation Reactors

Gas fermentation has emerged as a promising new technology for the generation of fuels and chemicals from mixtures of greenhouse and energy rich gas streams (CO2/CH4/H2/CO) via microbial bioreaction. Example pathways include biomethanation (CO2/H2 to CH4), biogas upgrading, CO fermentation and wet-waste conversion. Effective Gas-liquid mass-transfer is an important physical phenomenon that determines the design and scale-up of these systems. There is currently a knowledge-gap regarding bubble-size distributions when using a mixture of gases with vastly different properties, which can have a significant impact on overall mass-transfer. For example, hydrogen bubbles are more buoyant compared to other relatively heavier gases (CO2/CH4/CO), resulting in a large distribution of residence times and bubble sizes. This work therefore develops a deeper understanding of bubble dynamics and interphase mass transfer in such heterogenous gas mixtures through well-resolved computational models. We use a detailed multiphase computational-fluid-dynamics (CFD) model to study the impact of gas-mixtures on overall mass-transfer in bubble column and air-lift reactors. The CFD tool previously developed by the authors (1) for simulating aerobic fermentation reactors at scale is used in this study. The Reynolds-averaged mass, momentum, energy, and species transport equations are solved for interpenetrating gas and liquid phase in this model. We use a population balance-based bubble-size-distribution model that is validated against small-scale experiments in our solver. Results pertaining to multiple simulations of gas-fermentation reactors are presented where gas mixtures with varying compositions of CO2/CH4/CO/H2 are imposed at the sparger boundaries. The spatio-temporal variations in bubble-size distribution and mass transfer coefficient is analyzed for varying superficial velocities and gas-compositions for varying sizes of bubble-column and airlift reactors.

BIOMASS FUELS,MATHEMATICS AND COMPUTING↗
NASA NTRS1997

Flow Effects on the Flammability Diagrams of Solid Fuels

A research program is currently underway with the final objective of developing a fundamental understanding of the controlling mechanisms underlying the flammability diagrams of solid combustible materials and their derived fire properties. Given that there is a high possibility of an accidental fire occurring in a space-based facility, understanding the fire properties of materials that will be used in such facilities is of critical importance. With this purpose, the flammability diagrams of the materials, as those produced by the Lateral Ignition and Flame Spread Test (LIFT) apparatus and by a new forced flow device, the Forced Flow Ignition and Flame Spread Test (FIST) apparatus, will be obtained. The specific objective of the program is to apply the new flammability apparatus, which will more accurately reflect the potential ambient conditions of space-based environments, to the characterization of the materials for space applications. This paper presents a parametric study of oxidizer flow effects on the ignition curve of the flammability diagrams of PMMA. The dependence of the ignition delay time on the external radiant flux and either the sample width (LIFT) or the flow velocity (FIST) has been studied. Although preliminary, the results indicate that natural and forced convection flow changes, affect the characteristics of the ignition curves of the flammability diagrams. The major effect on the ignition time appears to be due to convective transfer variations at the fuel surface. At high radiant fluxes or high flow velocities, however, it appears that gas phase processes become increasingly important, affecting the overall ignition delay time. A numerical analysis of the solid fuel heating and pyrolysis has also been developed. The theoretical predictions approximate the experiments well for conditions in which the gas phase induction time is negligible.

Cordova, J. L.↗
DOE OSTI2023

A comparative investigation of the effect of gas type on foam strength and flow behavior in tight carbonates

Foam generation technique has been practically applied to overcome gas mobility issue during enhanced oil recovery (EOR) processes. Incorporated with the utilized gas, the capability of foaming agent to generate persistent foam determines the flow performance under reservoir conditions. Here, in this study, we first identified the surfactants with excellent phase behavior and foaming properties and, second, evaluated their bulk foam performance with different gases, including N 2 , CO 2 , and hydrocarbon gas (CH 4 ) under high-pressure and high-temperature conditions. In bulk, the foam generated with the high-density gas, i.e., CO 2 , exhibited the greatest foamability but the lowest foam stability. With amidoamine oxide-based surfactant, CH 4 -foam was found to be more stable than N 2 -foam, while the opposite was observed for sulfobetaine-based surfactant. The most effective surfactant was later used to perform flow experiments on two tight carbonate rocks, including Minnesota Norther Cream Buff and Edwards limestone, with permeabilities of ~0.8 and 16 mD, respectively. The core-flooding experiments showed that the optimum quality of CO 2 -foam was lower than that of N 2 - and CH 4 -foams. For a given flow rate and fixed foam quality, the steady-state strength followed the descending order of CH 4 -, N 2 -, and CO 2 -foam. However, in general, the difference in foam strength between N 2 - and CH 4 -foams was insignificant when the total flow rate was varied. We also found that the variation in rock permeability had a greater effect on the strength of the CO 2 -foam compared to those generated with N 2 and CH 4 . In the presence of oil, the shear-thinning behavior of all foams was found more prominent. The increase in oil saturation had a more adverse effect on the strength of N 2 - and CH 4 -foams, particularly in the higher permeability rock. The results indicated that the foam strength, which is key in the effectiveness of foam EOR applications, is greatly influenced by several factors such as gas density, gas solubility in water, the presence of oil, and rock permeability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Fracture-matrix fluid exchange in oil-bearing unconventional mudstones

The poromechanical properties of unconventional reservoir materials are in large part dictated by their mineralogy. Since these properties govern the response to stress experienced during hydraulic fracturing, fluid production, and fluid injection, they play a central role in the formation of microcracks or bedding delaminations which ultimately dominate mass transport. In this work we study access to the porosity of end member unconventional reservoir materials, where the end members are predominantly dictated by carbonate content. Access to the porosity is quantified using state of the art 3D x-ray computed tomography coupled with physics informed data analytics. Xenon gas, which attenuates x-rays, provides a spatiotemporal map of access to the porosity. The accessible porosity is quantified over a range of net confining stress relevant to the manmade disturbances listed above. These experiments demonstrate that heavily carbonated mudstones are nearly impermeable at the core (~ cm) scale, while carbonate free analogues afford better access to the microstructure. Consistent with previous qualitative 2D radiographs, access to the interior of the clastic mudstones is first observed along planar microcracks, followed by slow penetration into the surrounding matrix. Physics informed data analytics of the 3D tomography measurements presented here show that these microcracks do not permit uniform access to the adjacent rock matrix. In addition, variation of the effective pressure elucidates the mechanisms that govern fracture/matrix fluid exchange. Under conditions consistent with hydrocarbon production fluid accumulates in the immediate vicinity of the nearest microcrack. While there is clear evidence that, as intended, part of this accumulation is from the more distant matrix, fluid is also squeezed out of the microcrack. The fluid build-up at the microcrack indicates that migration out of the rock is hindered by the coupled poroelastic response of the microcrack and adjacent rock matrix. We show that these mechanisms ultimately account for the meager oil recovery factors realized in practice. These insights have implications for making reservoir scale predictions based on core scale observations, and provide a basis for devising new asset development techniques to access more porosity, and enhance fluid extraction. Finally, these findings shed light on key features and mechanisms that govern shale storage capacity, with relevance to other important industrial processes, such as geologic CO 2 storage.

58 GEOSCIENCES↗
DOE OSTI2023

Neutron Characterization of 3D Printed Gas Turbine Alloy With Internal Cooling Structure

The objective of this research work is to investigate the residual strains induced by the presence of a exit hole and internal hollow geometry present in a 3D printed IN718 similar to that seen in a gas turbine blade. The specimens were manufactured with an internal hollow structure that ended with an exit hole, and fabricated by direct metal laser sintering additive manufacturing technique. Using this technique, metal powder was melted by a laser source and rapidly cooled to form the part layer by layer. Rapid heating and cooling generate large thermal gradients and residual stresses in the 3D printed material. To be aligned with industry standards, specimens were heat-treated to relieve most of the residual stresses as well as to investigate whether IN718 still preserved a nonhomogeneous stress state after the thermal treatment. Roughness and microstructure analysis was performed to understand the heat-treated sample properties. To investigate the volumetric residual strain, the neutron diffraction technique was used with a definite neutron wavelength. Bragg’s law was used to determine lattice spacing. Interplanar lattice spacing and stress-free state lattice spacings were measured to calculate the strains. Residual strains were measured in the x and y direction to understand the impact of the presence of a exit hole and internal hollow geometry. The results of volumetric residual strain in the x and y directions showed no significant variations in the distribution of the strain due to the presence of a hollow structure as well as the exit hole. Overall, no significant stress concentration was observed in the heat treated sample. Sample geometry, instrumentation for neutron diffraction, and material properties are discussed in detail in this paper.

Raju, Nandhini↗
NASA NTRS1949

Gas Motion in a Local Supersonic Region and Conditions of Potential-Flow Breakdown

For a certain Mach number of the oncoming flow, the local velocity first reaches the value of the local velocity of sound (M = 1) at some point on the surface of the body located within the flow. This Mach number is designated the critical Mach number M(sub cr). By increasing the flow velocity, a supersonic local region is formed bounded by the body contour and the line of transition from subsonic to supersonic velocity. As is shown by observations with the Toepler apparatus, at a certain flow Mach number M > M(sub cr) a shock wave is formed near the body that closes the local supersonic region from behind. The formation of the shock wave is associated with the appearance of an additional resistance defined as the wave drag. In this paper, certain features are described of the flow in the local supersonic region, which is bounded by the contour of the body and the transition line, and conditions are sought for which the potential flow with the local supersonic region becomes impossible and a shock wave occurs. In the first part of the paper, the general properties of the potential flow in the local supersonic region, bounded by the contour of the profile and the transition line, are established. It is found that at the transition line, if it is not a line of discontinuity, the law of monotonic variation of the angle of inclination of the velocity vector holds (monotonic law). An approximation is given for the change in velocity at the contour of the body. The flow about a contour having a straight part is studied. In the second part of the paper, an approximation is given of the magnitudes of the accelerations at the interior points of the supersonic region. With the aid of these approximations, it is shown that for profiles convex to the flow the breakdown of the potential flow,associated with an increase of the Mach number of the oncoming flow, cannot be due to the formation of an envelope of the characteristics within the supersonic region. On the basis of the monotonic law, the transitional Mach number M is found, beyond which the potential flow with local supersonic region becomes impossible.

Nikolskii, A. A.↗
DOE OSTI2025

Greenhouse gas emissions of global construction material production

Abstract Global production of building materials is a primary contributor to greenhouse gas (GHG) emissions, but the production of these materials is necessary for modern infrastructure and society. Understanding the GHG emissions from building materials production in the context of their function is critical to decarbonizing this important sector. In this work, we present estimates of global production, approximate ranges of GHG emissions, and ranges of material properties of 12 critical building material classes to provide a unified dataset across material types. This dataset drew from industry analyses of production and emissions, ranges of emission factors within a material type, and broad reporting of thermal and mechanical properties to compare both within and between material types. Globally, in 2019, we estimate 42.8 Gt of these 12 materials were produced, with 38.6 Gt used in the building and construction industry. As a result of this production, 9.3 Gt of CO 2 was emitted, or 25% of global fossil GHG emissions, with 5.8 Gt CO 2 (16% of global GHG emissions) due to materials used in construction applications. Both construction material production and emissions are primarily driven by structural materials, such as concrete and steel. Material selection can play a key role in reducing emissions in the context of the function, with variation in emissions of structural materials per unit strength between 0.001–0.1 kg CO 2 /kg/MPa and in insulation materials per R -value/thickness of 0.018–0.14 kg CO 2 /kg/(K⋅m 2 W −1 ))). The developed dataset can play a key role in supporting decision-making in materials by providing a unified source for examining emissions, material properties, and quantity of material produced.

Kane, Seth (ORCID:0000000269401369)↗
DOE OSTI2020

CHARACTERISTICS OF CYCLE-TO-CYCLE COMBUSTION VARIABILITY AT PARTIAL-BURN LIMITED AND MISFIRE LIMITED SPARK TIMING UNDER HIGHLY DILUTED CONDITIONS

Diluted combustion with exhaust gas recirculation (EGR) has been widely employed to improve the fuel economy of spark ignition engines. The combustion kinetics, however, are affected and the flame propagation speed is decreased. In order to compensate for this adverse effect, the spark timing needs to be recalibrated to achieve maximum brake torque (MBT). At high levels of EGR dilution, the spark timing is constrained by two ignition limits: 1) the partial-bum limit where the spark timing is retarded from MBT and 2) the misfire limit where the spark timing is too advanced. This work uses a probabilistic framework to capture the differences between both ignition limits. In particular, it introduces the concept of a nominal indicated mean effective pressure (IMEP) distribution based on the stochastic properties of the cycle-to-cycle variability (CCV) at nominal stable conditions. By defining a nominal band where fully burned cycles occur with high probability, we introduce a cycle classification method that can be used to 1) determine the level of randomness of misfire and partial-burn events, and 2) measure CCV. The new CCV metric based on the density of the nominal band is compared with the traditional coefficient of variation of IMEP (CoVIMEP). It is shown that the nominal band concept, together with the CoVIMEP, can help to discern between partial-burn limited and misfire limited conditions. Furthermore, the Kullback-Leibler divergence is used to demonstrate that the IMEP distribution is significantly different between nominal and partial-burn/misfire limited conditions. Experiments are carried at various EGR levels and spark timings while recoding in-cylinder pressure at steady state. Although the emphasis of this work is to characterize the differences of both ignition limits from a probabilistic point of view, similarities between partial-burn cycles at either limiting conditions are also discussed.

Maldonado, Bryan↗
NASA NTRS2011

Intraseasonal Variations in Tropical Energy Balance: Relevance to Climate Sensitivity?

Intraseasonal variability of deep convection represents a fundamental mode of organization for tropical convection. While most studies of intraseasonal oscillations (ISOs) have focused on the spatial propagation and dynamics of convectively coupled circulations, here we examine the projection of ISOs on the tropically-averaged heat and moisture budget. One unresolved question concerns the degree to which observable variations in the "fast" processes (e.g. convection, radiative / turbulent fluxes) can inform our understanding of feedback mechanisms operable in the context of climate change. Our analysis use daily data from satellite observations, the Modern Era analysis for Research and Applications (MERRA), and other model integrations to address these questions: (i) How are tropospheric temperature variations related to that tropical deep convection and the associated ice cloud fractional amount (ICF), ice water path (IWP), and properties of warmer liquid clouds? (ii) What role does moisture transport play vis-a-vis ocean latent heat flux in enabling the evolution of deep convection to mediate PBL - free atmospheric temperature equilibration? (iii) What affect do convectively generated upper-tropospheric clouds have on the TOA radiation budget? Our methodology is similar to that of Spencer et al., (2007 GRL ) whereby a composite time series of various quantities over 60+ ISO events is built using tropical mean tropospheric temperature signal as a reference to which the variables are related at various lag times (from -30 to +30 days). The area of interest encompasses the global oceans between 20oN/S. The increase of convective precipitation cannot be sustained by evaporation within the domain, implying strong moisture transports into the tropical ocean area. The decrease in net TOA radiation that develops after the peak in deep convective rainfall, is part of the response that constitutes a "discharge" / "recharge" mechanism that facilitates tropical heat balance maintenance on these time scales. However, water vapor and hydrologic scaling relationships for this mode of variability cast doubt on the utility of ISO variations as proxies for climate sensitivity response to external radiatively forced (e.g. greenhouse gas-induced) climate change.

Robertson, Franklin R.↗
NASA NTRS1994

Belt-zone variations in the Jovian cloud structure

Voyager Infrared Interferometer Spectrometer (IRIS) observations of Jupiter in the far infrared (180-1200/cm) are sensitive to emission originating from pressures less than 2 bars, while the 5-micrometer (1800-2300/cm) observations are primarily sensitive to emission originating from pressures greater than 2 bars. We use these differences in the location of the peak emission level to constrain the properties of the upper tropospheric cloud structure from the far-infrared observations and then use the 5-micrometer observations, with the upper tropospheric cloud structure fixed, to constrain the deep cloud structure. The relationship between observed 45- and 5-micrometer brightness temperatures reveals three distinct regions between +/- 25 deg latitude: North Equatorial Belt hot spots, which are 'hot' at both 45 and 5 micrometers; Equatorial Zone spectra, which are 'warm' at 45 micrometers and 'cold' at 5 micrometers; and North Tropical Zone spectra, which are 'cold' at both 45 and 5 micrometers. We find that the hot extreme spectral ensemble is unique to belts, and the cold extreme spectral ensembles are unique to zones, but that all other intermediate spectral ensembles are common to both regions. Analyses of these spectra using an anisotropic multiple scattering radiative transfer model reveal that the primary difference between belts and zones is the increased opacity and vertical extent of clouds in zones relative to their belt counterparts. In addition, we find a shift in the location of the para hydrogen gradient toward lower pressures in zones, with an increase in the 'equilibrated' cloud-top para fraction. We suggest that all of the variations in gas abundance profiles, temperature, and cloud structure are consistent with the effects of a simple mean circulation and large-scale wave motions.

Carlson, Barbara E.↗
DOE OSTI2025

Temperature Measurements in Hypersonic Wind Tunnels via Femtosecond Coherent Anti-Stokes Raman Scattering

A femtosecond coherent anti-Stokes Raman scattering (fs CARS) instrument is developed to perform gas-phase thermometry in cold-flow hypersonic wind tunnels. Measurements are reported for Mach 8 and 14 pure-nitrogen flows. The fs CARS instrument includes a 100 fs pump/Stokes pulse and a spectrally narrow probe pulse from a second harmonic bandwidth compressor. Important experimental considerations such as limits on the pump/Stokes pulse energy are discussed. The fs CARS focusing and collimating optics are mounted on a two-axis translation stage system to scan the measurement location during a 30 second wind tunnel run. Single-laser-shot rotational CARS spectra are recorded at the laser repetition rate of 1 kHz in the wind tunnel freestream and near simple cone models. Spectral fitting is used to determine quantitative gas temperatures. Freestream temperatures at Mach 8 and 14 spanned ranges of 40–75 and 35–50 K, respectively, depending on tunnel operating conditions. Temperature variations across the central 100 mm span of the wind tunnel were quantified. Measured temperature jumps across conical bow shocks from various models varied by less than 1% from predicted values. Hypersonic boundary layer measurements were demonstrated. In conclusion, these measurements illustrate the utility and robustness of this instrument for the study of complex fluid flow phenomena in challenging ground test facilities.

Aerodynamics↗
DOE OSTI2021

Large Eddy Simulation of Lean Mixed-Mode Combustion Assisted by Partial Fuel Stratification in a Spark-Ignition Engine

Partial fuel stratification (PFS) is a promising fuel injection strategy to improve the stability of lean combustion by applying a small amount of pilot injection right before spark timing. Mixed-mode combustion, which makes use of end-gas autoignition following conventional deflagration-based combustion, can be further utilized to speed up the overall combustion. In this study, PFS-assisted mixed-mode combustion in a lean-burn direct injection spark-ignition (DISI) engine is numerically investigated using multi-cycle large eddy simulation (LES). A previously developed hybrid G-equation/well-stirred reactor combustion model for the well-mixed operation is extended to the PFS-assisted operation. The experimental spray morphology is employed to derive spray model parameters for the pilot injection. The LES-based model is validated against experimental data and is further compared with the Reynolds-averaged Navier–Stokes (RANS)-based model. Overall, both RANS and LES predict the mean pressure and heat release rate traces well, while LES outperforms RANS in capturing the cycle-to-cycle variation (CCV) and the combustion phasing in the mass burned space. Liquid and vapor penetrations obtained from the simulations agree reasonably well with the experiment. Detailed flame structures predicted from the simulations reveal the transition from a sooting diffusion flame to a lean premixed flame, which is consistent with experimental findings. LES captures more wrinkled and stretched flames than RANS. Finally, the LES model is employed to investigate the impacts of fuel properties, including heat of vaporization (HoV) and laminar burning speed (SL). Combustion phasing is found more sensitive to SL than to HoV, with a larger fuel property sensitivity of the heat release rate from autoignition than that from deflagration. Moreover, the combustion phasing in the PFS-assisted operation is shown to be less sensitive to SL compared with the well-mixed operation.

fuel property sensitivity↗