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NASA NTRS1985

Oxides of Nitrogen Emissions from the Combustion of Monodisperse Liquid Fuel Sprays

A study of NO sub x formation in a one dimensional monodisperse spray combustion system, which allowed independent droplet size variation, was conducted. Temperature, NO and NO sub x concentrations were measured in the transition region, encompassing a 26 to 74 micron droplet size range. Emission measurements of hydrocarbons, carbon monoxide, carbon dioxide and oxygen were also made. The equivalence ratio was varied between 0.8 and 1.2 for the fuels used, including methanol, isopropanaol, n-heptane and n-octane. Pyridine and pyrrole were added to n-heptane as nitrogen-containing additives in order to simulate synthetic fuels. Results obtained from the postflame regions using the pure fuels indicate an optimum droplet size in the range of 43 to 58 microns for minimizing NO sub x production. For the fuels examined, the maximum NO sub x reductions relative to the small droplet size limit were about 10 to 20% for lean and 20 to 30% for stoichiometric and rich mixtures. This behavior is attributed to droplet interactions and the transition from diffusive to premixed type of burning. Preflame vaporization controls the gas phase stoichiometry which has a significant effect on the volume of the hot gases surrounding a fuel droplet, where NO sub x is formed.

Sarv, H.↗

NASA NTRS2000

Experimental and Modeling Study of the Burning of an Ethanol Droplet in Microgravity

The microgravity ethanol droplet combustion experiments were performed aboard the STS-94/MSL-1 Shuttle mission within the Fiber-Supported Droplet Combustion-2 (FSDC-2) program. The burning histories and flame standoffs for pure ethanol and ethanol/water droplets were obtained from the images recorded with two 8 mm videocameras. The obtained results show that average gasification rate is related to the initial droplet size in a manner similar to n-alkanes and methanol and consistent with the results of Hara and Kumagai and the data taken recently in the NASA-Lewis 2.2 s droptower. A transient, moving finite-element chemically reacting flow model applied previously to sphero-symmetric combustion of methanol, methanol/water, n-alkane, and n-alkane binary mixture droplets was adopted for the problem of ethanol droplet combustion. The model includes detailed description of gas-phase reaction chemistry and transport, a simplified description of liquid phase transport, and non-luminous radiative heat transfer. Gas-phase chemistry was described with the detailed reaction mechanism of Norton and Dryer, which consists of 142 reversible elementary reactions of 33 species. Another recently published reaction mechanism of high-temperature ethanol oxidation was also considered. The model predictions were found to compare favorably with the experimental data. The model analysis also indicates that water condensation in the case of ethanol has smaller effect on average droplet gasification rate as compared with previously studied methanol cases. This effect is explained by non-ideal (azeotropic) behavior of binary ethanol-water mixtures. Further analysis of computational results and ethanol droplet radiative extinction behavior will be discussed.

Kazakov, Andrei↗

NASA NTRS1990

Properties of RBSN and RBSN-SiC composites

Strengths, fracture toughnesses, hardnesses, and dimensional changes have been measured for RBSN and RBSN/SiC composites. Samples were made from mixtures of Si and either Si- or C-rich SiC powders. For pure, 75 pct dense RBSN dispersed with octanol, strengths up to 858 MPa have been achieved. Improved strengths result from a combination of microstructural perfection and increased fracture toughness. The mechanical properties of the composites were approximately equal to those of methanol processed RBSN but not quite equal to those of the octanol-processed RBSN. Results are discussed in terms of observed microstructural features.

Lightfoot, A.↗

NASA NTRS2011

All-Solid-State 2.45-to-2.78-THz Source

Sources in the THz range are required in order for NASA to implement heterodyne instruments in this frequency range. The source that has been demonstrated here will be used for an instrument on the SOFIA platform as well as for upcoming astrophysics missions. There are currently no electronic sources in the 2 3- THz frequency range. An electronically tunable compact source in this frequency range is needed for lab spectroscopy as well as for compact space-deployable heterodyne receivers. This solution for obtaining useful power levels in the 2 3- THz range is based on utilizing power-combined multiplier stages. Utilizing power combining, the input power can be distributed between different multiplier chips and then recombined after the frequency multiplication. A continuous wave (CW) coherent source covering 2.48 2.75 THz, with greater than 10 percent instantaneous and tuning bandwidth, and having l 14 W of output power at room temperature, has been demonstrated. This source is based on a 91.8 101.8-GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid-state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source has enabled wideband, high-resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by other existing systems, providing new insight in the physics of these molecules. Further - more, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 1012, and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 108. The harmonic purity is better than 25 dB. This source can serve as a local oscillator for a variety of heterodyne systems, and can be used as a method for precision control of more powerful but much less frequency-agile quantum mechanical terahertz sources.

Mehdi, Imran↗

NASA NTRS2000

The Structure of Ice Nanoclusters: Implications for Interstellar Ice Grains

Crystalline ice nanoclusters and thin-films of pure and impure water ice were deposited and studied in order to evaluate the extent to which surface-related effects control bulk properties. In pure water ice nanoclusters and thin-films of impure water ice, the cubic to hexagonal phase transformation occurs at lower temperatures than in thin-film deposits of pure water ice. In laboratory-grown crystalline ice nanoclusters, approx. 20 nm diameter, a significant proportion of water molecules exists in surface and near-surface environments which have an amorphous or nearly amorphous character. These disordered regions, which are highly reactive, serve to promote transformations or reactions which would otherwise be kinetically hindered. Likewise, dilute impurities such as methanol, sequestered to the surface of thin-films on crystallization, introduce defects into the ice network, thereby allowing sluggish structural transitions to proceed. These structural effects, which are surface phenomena, are believed to play an important role in promoting molecular reactions known to occur within interstellar ice grains in cold molecular clouds, where the first organic compounds are formed.

Delzeit, L.↗

NASA NTRS2005

Sol-Gel Process for Making Pt-Ru Fuel-Cell Catalysts

A sol-gel process has been developed as a superior alternative to a prior process for making platinum-ruthenium alloy catalysts for electro-oxidation of methanol in fuel cells. The starting materials in the prior process are chloride salts of platinum and ruthenium. The process involves multiple steps, is time-consuming, and yields a Pt-Ru product that has relatively low specific surface area and contains some chloride residue. Low specific surface area translates to incomplete utilization of the catalytic activity that might otherwise be available, while chloride residue further reduces catalytic activity ("poisons" the catalyst). In contrast, the sol-gel process involves fewer steps and less time, does not leave chloride residue, and yields a product of greater specific area and, hence, greater catalytic activity. In this sol-gel process (see figure), the starting materials are platinum(II) acetylacetonate [Pt(C5H7O2)2, also denoted Pt-acac] and ruthenium(III) acetylacetonate [Ru(C5H7O2)3, also denoted Ru-acac]. First, Pt-acac and Ru-acac are dissolved in acetone at the desired concentrations (typically, 0.00338 moles of each salt per 100 mL of acetone) at a temperature of 50 C. A solution of 25 percent tetramethylammonium hydroxide [(CH3)4NOH, also denoted TMAH] in methanol is added to the Pt-acac/Ruacac/ acetone solution to act as a high-molecular-weight hydrolyzing agent. The addition of the TMAH counteracts the undesired tendency of Pt-acac and Ru-acac to precipitate as separate phases during the subsequent evaporation of the solvent, thereby helping to yield a desired homogeneous amorphous gel. The solution is stirred for 10 minutes, then the solvent is evaporated until the solution becomes viscous, eventually transforming into a gel. The viscous gel is dried in air at a temperature of 170 C for about 10 hours. The dried gel is crushed to make a powder that is the immediate precursor of the final catalytic product. The precursor powder is converted to the final product in a controlled-atmosphere heat treatment. Desirably, the final product is a phase-pure (Pt phase only) Pt-Ru powder with a high specific surface area. The conditions of the controlled- atmosphere heat are critical for obtaining the aforementioned desired properties. A typical heat treatment that yields best results for a catalytic alloy of equimolar amounts of Pt and Ru consists of at least two cycles of heating to a temperature of 300 C and holding at 300 C for several hours, all carried out in an atmosphere of 1 percent O2 and 99 percent N2. The resulting powder consists of crystallites with typical linear dimensions of <10 nm. Tests have shown that the powder is highly effective in catalyzing the electro-oxidation of methanol.

Narayanan, Sekharipuram↗

NASA NTRS2014

A Predictive Bubble Point Pressure Model for Porous Liquid Acquisition Device Screens

This article presents a simplified model for porous screen channel liquid acquisition devices based on a maximum bubble point pressure method from Adamson and Gast (1997). To validate the model, three 304 stainless steel (325 × 2300, 450 × 2750, and 510 × 3600) mesh samples were tested in methanol, acetone, isopropyl alcohol, and water. Screen pores are estimated based on analysis from scanning electron microscopy, historical data, and current test data. Results show that the bubble point pressure is proportional to the surface tension of the fluid only when accounting for nonzero contact angles. The previous assumption that bubble point pressure scales inversely with effective pore diameter is shown to be invalid, as the second finest 450 × 2750 produced the highest bubble point of the three screens. The simplified bubble point model can be used to make predictions for any pure fluid when pore diameters are based on bubble point tests and not SEM analysis.

Liquid Acquisition Device↗

NASA NTRS2026

CH3OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH3OH/HCN Production Rate Ratio

We report the detection of methanol (CH3OH) toward interstellar comet 3I/ATLAS using the At-acama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances (r H ) of 2.6 – 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH3OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH3OH included production from coma sources at Lp > 258 km at 99% confidence. However low signal-to-noise on long baselines, which sample emission on small spatial scales closest to the nucleus, prevented definitively ruling out CH3OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH3OH production rate increased sharply from August through October, including an uptick near the inner edge of the H2O sublimation zone at rH = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH3OH/HCN ratios in 3I/ATLAS of 124+30−34 and 79+11−14on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS)

Comae↗

NASA NTRS2008

Nano-Engineered Catalysts for Direct Methanol Fuel Cells

Nano-engineered catalysts, and a method of fabricating them, have been developed in a continuing effort to improve the performances of direct methanol fuel cells as candidate power sources to supplant primary and secondary batteries in a variety of portable electronic products. In order to realize the potential for high energy densities (as much as 1.5 W h/g) of direct methanol fuel cells, it will be necessary to optimize the chemical compositions and geometric configurations of catalyst layers and electrode structures. High performance can be achieved when catalyst particles and electrode structures have the necessary small feature sizes (typically of the order of nanometers), large surface areas, optimal metal compositions, high porosity, and hydrophobicity. The present method involves electrodeposition of one or more catalytic metal(s) or a catalytic-metal/polytetrafluoroethylene nanocomposite on an alumina nanotemplate. The alumina nanotemplate is then dissolved, leaving the desired metal or metal/polytetrafluoroethylene-composite catalyst layer. Unlike some prior methods of making fine metal catalysts, this method does not involve processing at elevated temperature; all processing can be done at room temperature. In addition, this method involves fewer steps and is more amenable to scaling up for mass production. Alumina nanotemplates are porous alumina membranes that have been fabricated, variously, by anodizing either pure aluminum or aluminum that has been deposited on silicon by electronbeam evaporation. The diameters of the pores (7 to 300 nm), areal densities of pores (as much as 7 x 10(exp 10)sq cm), and lengths of pores (up to about 100 nm) can be tailored by selection of fabrication conditions. In a given case, the catalytic metal, catalytic metal alloy, or catalytic metal/ polytetrafluoroethylene composite is electrodeposited in the pores of the alumina nanotemplate. The dimensions of the pores, together with the electrodeposition conditions, determine the sizes and surface areas of the catalytic particles. Hence, the small features and large surface areas of the porosity translate to the desired small particle size and large surface area of the catalyst (see figure). When polytetrafluoroethylene is included, it is for the purpose of imparting hydrophobicity in order to prevent water from impeding the desired diffusion of gases through the catalyst layer. To incorporate polytetrafluoroethylene into a catalytic-metal/polytetrafluoroethylene nanocomposite, one suspends polytetrafluoroethylene nanoparticles in the electrodeposition solution. The polytetrafluoroethylene content can be varied to obtain the desired degree of hydrophobicity and permeability by gas.

Myung, Nosang↗

NASA NTRS1998

Fiber-Supported Droplet Combustion Experiment-2

A major portion of the energy produced in the world today comes from the burning of liquid hydrocarbon fuels in the form of droplets. Understanding the fundamental physical processes involved in droplet combustion is not only important in energy production but also in propulsion, in the mitigation of combustion-generated pollution, and in the control of the fire hazards associated with handling liquid combustibles. Microgravity makes spherically symmetric combustion possible, allowing investigators to easily validate their droplet models without the complicating effects of gravity. The Fiber-Supported Droplet Combustion (FSDC-2) investigation was conducted in the Microgravity Glovebox facility of the shuttles' Spacelab during the reflight of the Microgravity Science Laboratory (MSL- 1R) on STS-94 in July 1997. FSDC-2 studied fundamental phenomena related to liquid fuel droplet combustion in air. Pure fuels and mixtures of fuels were burned as isolated single and duo droplets with and without forced air convection. FSDC-2 is sponsored by the NASA Lewis Research Center, whose researchers are working in cooperation with several investigators from industry and academia. The rate at which a droplet burns is important in many commercial applications. The classical theory of droplet burning assumes that, for an isolated, spherically symmetric, single-fuel droplet, the gas-phase combustion processes are much faster than the droplet surface regression rate and that the liquid phase is at a uniform temperature equal to the boiling point. Recent, more advanced models predict that both the liquid and gas phases are unsteady during a substantial portion of the droplet's burning history, thus affecting the instantaneous and average burning rates, and that flame radiation is a dominant mechanism that can extinguish flames in a microgravity environment. FSDC-2 has provided well-defined, symmetric droplet burning data including radiative emissions to validate these theoretical models for heptane, decane, ethanol, and methanol fuels. Since most commercial combustion systems burn droplets in a convective environment, data were obtained without and with convective flow over the burning droplet (see the following photos).

Colantonio, Renato O.↗

NASA NTRS1999

Experimental Study of Nonane and Nonane/Hexanol Fuel Droplet Combustion in Microgravity

In this presentation we review experiments carried out on nonane droplets, and a nonane/hexanol droplet, burning in microgravity to promote spherical symmetry. The nonane/hexanol combination was selected for the following reasons: 1) the spherically symmetric burning history of nonane and nonane/hexanol mixtures has not been previously studied; 2) measurements of the burning history of pure nonane droplets in air extend the existing data base of spherical droplet flames of soot-producing fuels which are useful for testing detailed chemical kinetic models of the spherically symmetric droplet burning process; 3) nonane and hexanol have almost identical boiling points so heterogeneous nucleation on a support fiber is unlikely; 4) hexanol does not have a strong propensity for water vapor absorption; 5) hexanol produces less soot than nonane so that mixtures of nonane and hexanol should show an effect of composition on soot formation. The far-field gas was atmospheric pressure air at room temperature. The evolution of droplet diameter was measured using high speed cine photography of spark-ignited droplets within a confined volume in a drop tower. The importance of soot formation during droplet combustion is derived from the fact that soot is the basic component of the particulate emission process that occurs in spray combustion. The complexity of soot formation motivates a one-dimensional transport condition which is advantageous for modeling. Recent numerical studies of droplet combustion have assumed spherical symmetry when incorporating such aspects as detailed chemistry and radiation, though soot formation itself has not yet been included in any droplet combustion modeling effort. If radiation is not important as would be the case for'small' droplets (i.e., droplets with initial diameters less than about ]mm), soot formation can lead to a nonlinear burning process and a time-varying burning rate, (non-linear burning of a non-sooting fuel like methanol is due to water vapor absorption on the droplet. The classical quasi-steady droplet burning theory predicts a linear evolution of droplet diameter in scaled coordinates for any fuel type.

Avedisian, C. T.↗