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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

NASA GEOS-3/TRMM Re-analysis: Capturing Observed Tropical Rainfall Variability in Global Analysis for Climate Research

Understanding climate variability over a wide range of space-time scales requires a comprehensive description of the earth system. Global analyses produced by a fixed assimilation system (i.e., re-analyses) - as their quality continues to improve - have the potential of providing a vital tool for meeting this challenge. But at the present time, the usefulness of re-analyses is limited by uncertainties in such basic fields as clouds, precipitation, and evaporation - especially in the tropics, where observations are relatively sparse. Analyses of the tropics have long been shown to be sensitive to. the treatment of cloud precipitation processes, which remains a major source of uncertainty in current models. Yet, for many climate studies it is crucial that analyses can accurately reproduce the observed rainfall intensity and variability since a small error of 1 mm/d in surface rain translates into an error of approx. 30 W/sq m in energy (latent heat) flux. Currently, discrepancies between the observed and analyzed monthly-mean rain rates averaged to 100 km x 100 km resolution can exceed 4 mm/d (or 120 W/sq m ), compared to uncertainties in surface radiative fluxes of approx. 10-20 W/sq m . Improving precipitation in analyses would reduce a major source of uncertainty in the global energy budget. Uncertainties in tropical precipitation have also been a major impediment in understanding how the tropics interact with other regions, including the remote response to El Nino/Southern Oscillation (ENSO) variability on interannual time scales, the influence of Madden-Julian Oscillation (MJO) and monsoons on intraseasonal time scales. A global analysis that can replicate the observed precipitation variability together with physically consistent estimates of other atmospheric variables provides the key to breaking this roadblock. NASA Goddard Space Flight Center has been exploring the use of satellite-based microwave rainfall measurements in improving global analyses and has recently produced a multi-year, 1 x 1 TRMM re-analysis , which assimilates 6-hourly TMI and SSM/I surface rain rates over tropical oceans using a ID variational continuous assimilation (VCA) procedure in the GEOS-3 global data assimilation system. The analysis period extends from 1 November 1997 through 3 1 December 2002. The goal is to produce a multi-year global analysis that is dynamically consistent with available tropical precipitation observations for the community to assess its utility in climate applications and identify areas for further improvements. A distinct feature of the GEOS-3RRMh4 re-analysis is that its precipitation analysis is not derived from a short-term forecast (as done in most operational systems) but is given by a time- continuous model integration constrained by precipitation observations within a 6-h analysis window, while the wind, temperature, and pressure fields are allowed to directly respond to the improved precipitation and associated latent heating structures within the same analysis window. In this talk, I will assess the impact VCA precipitation assimilation on analyses of climate signals ranging from a few weeks to interannual time scales and compare results against other operational and reanalysis products.

Hou, Arthur Y.↗
NASA NTRS2002

Rudder/Fin Seals Investigated for the X-38 Re-Entry Vehicle

NASA is developing the X-38 vehicle that will demonstrate the technologies required for a potential crew return vehicle for the International Space Station. This vehicle would serve both as an ambulance for medical emergencies and as an evacuation vehicle for the space station. Control surfaces on the X-38 (body flaps and rudder/fin assemblies) require high temperature seals to limit hot gas ingestion and the transfer of heat to underlying low temperature structures. Working with the NASA Johnson Space Center, the Seals Team at the NASA Glenn Research Center completed a series of tests to further characterize baseline seal designs for the rudder/fin interfaces of the X-38. The structures of the rudder/fin assembly and its associated seals are shown in the the preceding illustration. Tests performed at Glenn indicated that exposure of the seals in a compressed state at simulated seal re-entry temperatures resulted in a large permanent set and loss of seal resiliency. This could be of concern because the seals are required to maintain contact with the sealing surfaces while the vehicle goes through the maximum re-entry heating cycle to prevent hot gases from leaking past the seals and damaging interior low-temperature structures. To simulate conditions in which the seals may become unloaded during use, such as when they take on a large permanent set, Glenn researchers performed room temperature flow and compression tests to determine seal flow rates, resiliency, and unit loads under minimal loads. Flow rates through an unloaded (i.e., 0-percent compression) double seal arrangement were twice those of a double seal compressed to the 20-percent design compression level. These flow rates are being used in thermal analyses to predict the effect of flow through the seals on over-all seal temperatures. Compression test results showed that seal unit loads and contact pressures were below the limits that Johnson had set as goals for the seals. In the rudder/fin seal location, the seals are in contact with shuttle thermal tiles and are moved across the tiles as the rudder is rotated during re-entry. Low seal unit loads and contact pressures are required to limit the loads on these tiles and minimize any damage that the seals could cause. A series of tests were performed on these seals in NASA Ames Research Center's arc jet facility. The arc jet facility approximates relevant thermal environments that a seal or other structure would be subjected to during extreme heating conditions such as those experienced during space vehicle re-entry. Eleven tests were completed, including one test in which no seal was installed in the gap to examine the flow of heat down into the gap. The seal was compressed between stationary insulation tiles and a movable elevon that was rotated during the test to deflect the arc jet exhaust into the seal gap. Peak seal temperatures as high as 2000 F were reached during the 5-min tests. Results of these tests indicate satisfactory performance of the seal for single-use (e.g., X-38) applications. The results of these tests were shared with the NASA Johnson Space Center and are being used to validate aerothermostructural analysis codes that predict seal temperatures under these conditions. The tests performed at Glenn have provided valuable information to Johnson about the performance of the seals that they are considering using in the rudder/fin location of the X-38 vehicle. Glenn and Johnson are currently defining what additional work needs to be done to develop the final rudder/fin seal design for the X-38 vehicle.

Dunlap, Patrick H., Jr.↗
NASA NTRS2001

International Space Station as an Observation Platform for Hypersonic Re-Entry of its Visiting Vehicles

The International Space Station (ISS) will receive an armada of visiting supply vehicles during its life in orbit. Over 500 tons of material will be destroyed in targeted re-entries of these vehicles. Because all such re-entries lie in the same orbital plane of the station, and because the visiting vehicles typically deorbit within a few hours of departure, the ISS will usually be within sight of the re-entry process, at a range of only 300-600 kilometers. This vantage point offers an unprecedented opportunity for systematically measuring hypersonic destructive processes. This paper examines the integrated operational constraints of the ISS, its supply vehicles, and candidate sensors which can be employed in the scientific observation of the re-entry process. It is asserted the ISS program has the potential to reduce the worldwide risks from future deorbiting spacecraft, through systematic experimental characterization of the factors which affect the rupture, debris survival, and footprint size of its visiting vehicle fleet.

Bacon, John B.↗
NASA NTRS2011

Dust Destruction in the ISM: A Re-Evaluation of Dust Lifetimes

There is a long-standing conundrum in interstellar dust studies relating to the discrepancy between the time-scales for dust formation from evolved stars and the apparently more rapid destruction in supernova-generated shock waves. Aims. We re-examine some of the key issues relating to dust evolution and processing in the interstellar medium. Methods. We use recent and new constraints from observations, experiments, modelling and theory to re-evaluate dust formation in the interstellar medium (ISM). Results. We find that the discrepancy between the dust formation and destruction time-scales may not be as significant as has previously been assumed because of the very large uncertainties involved. Conclusions. The derived silicate dust lifetime could be compatible with its injection time-scale, given the inherent uncertainties in the dust lifetime calculation. The apparent need to re-form significant quantities of silicate dust in the tenuous interstellar medium may therefore not be a strong requirement. Carbonaceous matter, on the other hand, appears to be rapidly recycled in the ISM and, in contrast to silicates, there are viable mechanisms for its re-formation in the ISM.

Jones, A. P.↗
NASA NTRS2012

NASA Integrated Model Centric Architecture (NIMA) Model Use and Re-Use

This whitepaper accepts the goals, needs and objectives of NASA's Integrated Model-centric Architecture (NIMA); adds experience and expertise from the Constellation program as well as NASA's architecture development efforts; and provides suggested concepts, practices and norms that nurture and enable model use and re-use across programs, projects and other complex endeavors. Key components include the ability to effectively move relevant information through a large community, process patterns that support model reuse and the identification of the necessary meta-information (ex. history, credibility, and provenance) to safely use and re-use that information. In order to successfully Use and Re-Use Models and Simulations we must define and meet key organizational and structural needs: 1. We must understand and acknowledge all the roles and players involved from the initial need identification through to the final product, as well as how they change across the lifecycle. 2. We must create the necessary structural elements to store and share NIMA-enabled information throughout the Program or Project lifecycle. 3. We must create the necessary organizational processes to stand up and execute a NIMA-enabled Program or Project throughout its lifecycle. NASA must meet all three of these needs to successfully use and re-use models. The ability to Reuse Models a key component of NIMA and the capabilities inherent in NIMA are key to accomplishing NASA's space exploration goals. 11

Conroy, Mike↗
NASA NTRS2012

Development of Resistive Electrode Gas Electron Multiplier (RE-GEM)

We successfully produced Resistive-Electrode Gas Electron Multiplier (RE-GEM) which has resistive electrodes instead of the metal ones which are employed for the standard GEM foils. RE-GEM has a resistive electrode of 25 micron-thick and an insulator layer of 100 micron-thick. The hole structure of RE-GEM is a single conical with the wider and narrower hole diameters of 80 micron and 60 micron, respectively. A hole pitch of RE-GEM is 140 micron. We obtained the maximum gain of about 600 and the typical energy resolution of about 20% (FWHM) at an applied voltage between the resistive electrodes of 620 V, using a collimated 8 keV X-rays from a generator in a gas mixture of 70% Ar and 30% CO2 by volume at the atmospheric pressure. We measured the effective gain as a function of the electric field of the drift region and obtained the maximum gain at an drift field of 0.5 kV/cm.

detectors↗
NASA NTRS2014

Stellar Laboratories: 3. New Ba 5, Ba 6, and Ba 7 Oscillator Strengths and the Barium Abundance in the Hot White Dwarfs G191-B2B and RE 0503-289

Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. Reliable Ba 5-7 oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191−B2B and the DO-type white dwarf RE 0503−289 and to determine their photospheric Ba abundances. Methods. We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191−B2B and RE 0503−289. Results. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503−289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g=7.5. The Ba abundance is 3.5 +/- 0.5 × 10(exp−4) (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191−B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 +/- 0.5 × 10(exp−6) (about 265 times solar). Conclusions. Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191−B2B and RE 0503−289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely.

New Ba V↗
NASA NTRS2011

Airborne Observation of the Hayabusa Sample Return Capsule Re-Entry

NASA Ames Research Center and the SETI Institute collaborated on an effort to observe the Earth re-entry of the Japan Aerospace Exploration Agency's Hayabusa sample return capsule. Hayabusa was an asteroid exploration mission that retrieved a sample from the near-Earth asteroid Itokawa. Its sample return capsule re-entered over the Woomera Prohibited Area in southern Australia on June 13, 2010. Being only the third sample return mission following NASA's Genesis and Stardust missions, Hayabusa's return was a rare opportunity to collect aerothermal data from an atmospheric entry capsule returning at superorbital speeds. NASA deployed its DC-8 airborne laboratory and a team of international researchers to Australia for the re-entry. For approximately 70 seconds, spectroscopic and radiometric imaging instruments acquired images and spectra of the capsule, its wake, and destructive re-entry of the spacecraft bus. Once calibrated, spectra of the capsule will be interpreted to yield data for comparison with and validation of high fidelity and engineering simulation tools used for design and development of future atmospheric entry system technologies. A brief summary of the Hayabusa mission, the preflight preparations and observation mission planning, mission execution, and preliminary spectral data are documented.

Spectroscopy↗
NASA NTRS2016

Risk-Hedged Approach for Re-Routing Air Traffic Under Weather Uncertainty

This presentation corresponds to: our paper explores a new risk-hedged approach for re-routing air traffic around forecast convective weather. In this work, flying through a more likely weather instantiation is considered to pose a higher level of risk. Current operational practice strategically plans re-routes to avoid only the most likely (highest risk) weather instantiation, and then tactically makes any necessary adjustments as the weather evolves. The risk-hedged approach strategically plans re-routes by minimizing the risk-adjusted path length, incorporating multiple possible weather instantiations with associated likelihoods (risks). The resulting model is transparent and is readily analyzed for realism and treated with well-understood shortest-path algorithms. Risk-hedged re-routes are computed for some example weather instantiations. The main result is that in some scenarios, relative to an operational-practice proxy solution, the risk-hedged solution provides the benefits of lower risk as well as shorter path length. In other scenarios, the benefits of the risk-hedged solution are ambiguous, because the solution is characterized by a tradeoff between risk and path length. The risk-hedged solution can be executed in those scenarios where it provides a clear benefit over current operational practice.

air traffic routing↗
NASA NTRS2020

Calorimetric Measurements of the Thermodynamic Properties of RE-Silicate Coating Materials

Thermodynamic quantities of coatings materials and siliceous debris-induced corrosion products are crucial to understand in order to develop mitigation strategies necessary to improve the durability of gas-turbine engines. Siliceous induced corrosion can occur when debris consisting mainly of CaO-MgO-Al2O3-SiO2 (CMAS) is ingested by aircraft engines during and after take-off, which sticks to hot surfaces and forms calcium rare-earth silicate oxyapatites. In this work, high-temperature oxide melt drop solution calorimetry (HT drop solution calorimetry) was used to obtain the enthalpies of formation for RE silicate (RE2Si2O7, RE2SiO5 where RE = Yb, Er, Y, Dy, Nd, Lu and Gd) environmental barrier coatings (EBCs) and the calcium RE silicate oxyapatite Ca2RE8(SiO4)6O2 (RE = Yb, Er, Y, Dy, Nd, Gd and Sm) corrosion products. Trends in the enthalpy of formation as a function of the ionic potential of the rare-earth cations in their related crystallographic sites are discussed.

Costa, Gustavo↗
NASA NTRSDate not supplied

Metal-silicate Partitioning of Re, Ru, Pt, Os, Ti, Nb, and Ta in Reduced Differentiated Planetary Bodies

Siderophile (iron-loving) elements are strongly fractionated during differentiation of planetary bodies into core and mantle [1]. Because the fractionation is controlled by the pressure, temperature, redox conditions, and composition, this group of elements can provide important constraints on the conditions of accretion and core formation in early solar system bodies (planetesimals) and planets (Earth, Mercury, Venus)[2]. At the reducing conditions thought to prevail in the early solar system, Si is known to alloy with FeNi metallic liquids (e.g., [3]) affecting the activity coefficients of siderophile elements in FeNi liquids and thus ultimately their detailed partitioning between metal and silicate melt. The effect of Si can be significant for some siderophile elements, as demonstrated previously by (e.g., [4]: Ni, Co; [5,6]: Ge, As, Sb, Pd, Pt, Au). The effect of Si has not yet been determined for several key groups of siderophile elements including the highly siderophile Re, Ru and Os, and the weakly siderophile Ta, Nb, and Ti. Here, we report new experiments designed to quantify the effect of Si on the partitioning of Re, Pt, Os, Ru, Ti, Ta and Nb between metal and silicate melts. The results will be used to evaluate metal/silicate equilibrium for Nb, Ta, Ti and Nb/Ta ratios in planetary mantles, mantle concentrations of Ru, Re, Pt, Os during accretion, the evolution of Re/Os, Pt/Os ratios in magma oceans, and the role of late veneer in establishing Re and Ru abundances in the terrestrial mantle.

core formation↗
DOE OSTI2020

Materials Data on Re(AgCl3)2 by Materials Project

Re(AgCl3)2 crystallizes in the trigonal R-3 space group. The structure is two-dimensional and consists of three Re(AgCl3)2 sheets oriented in the (0, 0, 1) direction. Re4+ is bonded in an octahedral geometry to six equivalent Cl1- atoms. All Re–Cl bond lengths are 2.38 Å. Ag1+ is bonded in a distorted trigonal planar geometry to three equivalent Cl1- atoms. All Ag–Cl bond lengths are 2.57 Å. Cl1- is bonded in a distorted water-like geometry to one Re4+ and one Ag1+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Re(Te4Cl3)2 by Materials Project

ReCl6Re(TeCl3)2(Te)14 crystallizes in the monoclinic C2/c space group. The structure is zero-dimensional and consists of four hexachlororhenium molecules, fifty-six tellurium molecules, and four Re(TeCl3)2 clusters. In each Re(TeCl3)2 cluster, Re4+ is bonded in an octahedral geometry to six Cl1- atoms. There are four shorter (2.37 Å) and two longer (2.38 Å) Re–Cl bond lengths. Te+0.25+ is bonded in a single-bond geometry to one Cl1- atom. The Te–Cl bond length is 3.09 Å. There are three inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a single-bond geometry to one Re4+ atom. In the second Cl1- site, Cl1- is bonded in a single-bond geometry to one Re4+ atom. In the third Cl1- site, Cl1- is bonded in a distorted water-like geometry to one Re4+ and one Te+0.25+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Re(TeBr2)2 by Materials Project

Re(TeBr2)2 crystallizes in the tetragonal I-4 space group. The structure is three-dimensional. Re7+ is bonded in a 6-coordinate geometry to four Te+1.50- and two Br1- atoms. There are a spread of Re–Te bond distances ranging from 2.64–2.78 Å. There are one shorter (2.67 Å) and one longer (2.68 Å) Re–Br bond lengths. There are two inequivalent Te+1.50- sites. In the first Te+1.50- site, Te+1.50- is bonded in a 2-coordinate geometry to one Re7+ and four Br1- atoms. There are a spread of Te–Br bond distances ranging from 2.59–2.92 Å. In the second Te+1.50- site, Te+1.50- is bonded in a 10-coordinate geometry to three equivalent Re7+ and seven Br1- atoms. There are a spread of Te–Br bond distances ranging from 3.48–3.84 Å. There are four inequivalent Br1- sites. In the first Br1- site, Br1- is bonded in a distorted single-bond geometry to three Te+1.50- atoms. In the second Br1- site, Br1- is bonded in a 1-coordinate geometry to one Re7+ and three Te+1.50- atoms. In the third Br1- site, Br1- is bonded in a 1-coordinate geometry to one Re7+ and three Te+1.50- atoms. In the fourth Br1- site, Br1- is bonded in a distorted single-bond geometry to two Te+1.50- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Re(Te2Mo)2 by Materials Project

Re(MoTe2)2 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are four inequivalent Mo+2.50+ sites. In the first Mo+2.50+ site, Mo+2.50+ is bonded to five Te2- atoms to form MoTe5 square pyramids that share corners with two MoTe5 square pyramids, corners with two ReTe5 square pyramids, edges with two ReTe5 square pyramids, and edges with three MoTe5 square pyramids. There are a spread of Mo–Te bond distances ranging from 2.74–2.91 Å. In the second Mo+2.50+ site, Mo+2.50+ is bonded to five Te2- atoms to form MoTe5 square pyramids that share a cornercorner with one ReTe5 square pyramid, corners with three MoTe5 square pyramids, edges with two ReTe5 square pyramids, and edges with three MoTe5 square pyramids. There are a spread of Mo–Te bond distances ranging from 2.72–2.86 Å. In the third Mo+2.50+ site, Mo+2.50+ is bonded to five Te2- atoms to form MoTe5 square pyramids that share a cornercorner with one ReTe5 square pyramid, corners with three MoTe5 square pyramids, edges with two ReTe5 square pyramids, and edges with three MoTe5 square pyramids. There are a spread of Mo–Te bond distances ranging from 2.75–2.87 Å. In the fourth Mo+2.50+ site, Mo+2.50+ is bonded to five Te2- atoms to form MoTe5 square pyramids that share corners with two MoTe5 square pyramids, corners with two ReTe5 square pyramids, edges with two ReTe5 square pyramids, and edges with three MoTe5 square pyramids. There are a spread of Mo–Te bond distances ranging from 2.73–2.90 Å. There are two inequivalent Re3+ sites. In the first Re3+ site, Re3+ is bonded to five Te2- atoms to form ReTe5 square pyramids that share a cornercorner with one ReTe5 square pyramid, corners with three MoTe5 square pyramids, an edgeedge with one ReTe5 square pyramid, and edges with four MoTe5 square pyramids. There are a spread of Re–Te bond distances ranging from 2.73–2.88 Å. In the second Re3+ site, Re3+ is bonded to five Te2- atoms to form ReTe5 square pyramids that share a cornercorner with one ReTe5 square pyramid, corners with three MoTe5 square pyramids, an edgeedge with one ReTe5 square pyramid, and edges with four MoTe5 square pyramids. There are a spread of Re–Te bond distances ranging from 2.73–2.90 Å. There are eight inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a 4-coordinate geometry to three Mo+2.50+ and one Re3+ atom. In the second Te2- site, Te2- is bonded in a 4-coordinate geometry to two Mo+2.50+ and two Re3+ atoms. In the third Te2- site, Te2- is bonded in a 4-coordinate geometry to three Mo+2.50+ and one Re3+ atom. In the fourth Te2- site, Te2- is bonded in a 4-coordinate geometry to three Mo+2.50+ and one Re3+ atom. In the fifth Te2- site, Te2- is bonded in a 4-coordinate geometry to three Mo+2.50+ and one Re3+ atom. In the sixth Te2- site, Te2- is bonded in a 4-coordinate geometry to two Mo+2.50+ and two Re3+ atoms. In the seventh Te2- site, Te2- is bonded in a 6-coordinate geometry to one Mo+2.50+ and two Re3+ atoms. In the eighth Te2- site, Te2- is bonded in a 6-coordinate geometry to three Mo+2.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Re(TeCl2)2 by Materials Project

Re(TeCl2)2 crystallizes in the tetragonal I-4 space group. The structure is three-dimensional. Re7+ is bonded in a 6-coordinate geometry to four Te+1.50- and two Cl1- atoms. There are a spread of Re–Te bond distances ranging from 2.63–2.76 Å. There are one shorter (2.51 Å) and one longer (2.52 Å) Re–Cl bond lengths. There are two inequivalent Te+1.50- sites. In the first Te+1.50- site, Te+1.50- is bonded in a 10-coordinate geometry to three equivalent Re7+ and seven Cl1- atoms. There are a spread of Te–Cl bond distances ranging from 3.38–3.82 Å. In the second Te+1.50- site, Te+1.50- is bonded in a 2-coordinate geometry to one Re7+ and five Cl1- atoms. There are a spread of Te–Cl bond distances ranging from 2.39–4.07 Å. There are four inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted single-bond geometry to three Te+1.50- atoms. In the second Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two Te+1.50- atoms. In the third Cl1- site, Cl1- is bonded in a 1-coordinate geometry to one Re7+ and three Te+1.50- atoms. In the fourth Cl1- site, Cl1- is bonded in a 1-coordinate geometry to one Re7+ and four Te+1.50- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Enhancing durability and activity toward oxygen evolution reaction using single-site Re-doped NiFeO x catalysts at ampere-level

NiFeO x materials are known as among the most active catalysts toward oxygen evolution reaction (OER) for hydrogen generation in alkaline media. Nevertheless, the long-term durability of NiFeO x catalysts for OER is still too far to the industrial application. Herein, we prepared a NiFeReO x catalyst with single-site Re dopants and observed that the single-site Re dopants could significantly enhance the durability without compromising the activity. A cell voltage of 1.82 V without iR correction is noted at the current density of 3000 mA cm –2 in anion-exchange membrane water electrolyzer (AEMWE) with NiFeReO x catalyst, and a very small degradation is observed under 2000 and 1000 mA cm –2 , which remarkably outperforms the pristine NiFeO x . Additionally, the overpotential of 305 mV at 10 mA cm –2 is achieved with the NiFeReO x catalyst, which is lower than 50 mV compared with the pristine NiFeO x catalyst, together with a smaller Tafel slope of 54.3 mV dec -1 . The boosted OER durability and activity of the NiFeReO x catalyst could be attributed to the strong electron-withdrawing property of Re 7+ single atoms leading to the electronic structure optimization and stabilization of Ni/Fe active sites. Our insights propose a new path for designing NiFeO x catalysts with high durability and activity toward OER.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Computational analysis of flame initiation, quenching, and re-ignition in a prechamber natural gas engine under varying EGR-dilution levels

The on-road natural-gas (NG) fueled transportation relies on stoichiometric spark-ignition engines for the advantages of simple after-treatment system despite the efficiency penalty relative to lean-burn combustion strategies. Exhaust gas recirculation (EGR) has the potential to reduce this efficiency gap at low to moderate loads without the need for complex lean-exhaust aftertreatment systems. However, EGR dilution leads to reduced combustion stability and increased cycle-to-cycle variability. A promising technology that has the potential to achieve reliable operation under diluted conditions is the prechamber ignition (or turbulent jet ignition) which uses chemically active turbulent jets generated from combustion inside a prechamber to initiate, stabilize and accelerate combustion of the mixture inside the main chamber. The present work focusses on developing a RANS-based CFD approach to accurately reproduce in-cylinder phenomena in a stoichiometric NG prechamber-assisted heavy-duty engine without relying on complex combustion models that account for turbulence-chemistry interactions. This is necessary because reactive prechamber jets at high EGR dilution tend to extinguish while emerging into the main chamber, which is followed by a phase of re-ignition — a phenomenon that conventional G-equation or well-stirred reactor combustion models cannot reproduce. With addition of a damping multiplier to the well-stirred reactor model, the predictions are seen to show good agreement with experimental pressure evolution and combustion images acquired from a single cylinder Cummins N-14 optical diesel engine retrofitted with a prechamber ignition system. Model predictions of local heat release in the flame and temperature evolution inside the flame are used to investigate combustion dynamics in the prechamber and the main chamber. It is seen that the well-stirred reactor model with the inclusion of damping is able to reproduce the temporary reduction in heat release within the flame, which can be considered equivalent to quenching of jets, and the subsequent re-ignition of the flame inside the main chamber. The delay between quenching and re-ignition depends on the amount of dilution, as explained by an illustration of flame evolution in a Borghi diagram.

Prechamber ignition↗