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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Effects of Gravity on the Transition to Turbulence of Gas Jet Diffusion Flames

An ongoing study of the transition to turbulence of gas jet diffusion flames conducted in a reduced microgravity environment at the NASA Lewis 2.2 second drop tower is described. The experiments were based on propane injected into quiescent air. Several differences with the corresponding normal gravity process were observed. In particular, large scale structures appeared intermittently in microgravity which are manifested at the flame base and convected down stream. As the Reynolds number increased, a continuous train of structures was observed along the flame front. These observations are explained by instability of the shear layer associated with injection of flow into quiescent surroundings.

Bahadori, M. Y.↗
NASA NTRS1987

The near field behavior of turbulent gas jets in a long confinement

The near-field behavior of a turbulent gas jet (8.73 mm in diameter) in a long confinement was studied using a test rig with a confinement area ratio of about 205 and a length-to-jet diameter ratio of about 1700. Experiments were carried out with CO2, air, and He/air jets at different jet velocities, using a laser Doppler velocimeter for velocity and turbulence measurements and hot-wire anemometers for a detailed examination of the turbulent shear field of an air jet. The air column inside the tunnel was seen to be first compressed by the jet and then to be slowly pushed out of the tunnel, causing the jet to spread rapidly and to decay quickly. As a result, an equilibrium turbulence field is established in the first two diameters of the jet which bears a similarity to that found in self-preserving turbulent free jets and jets in short confinement. However, in contrast to the cases of the two latter jet types, the near field of jets in a long confinement is independent of jet fluid densities and velocities.

So, Ronald M. C.↗
DOE OSTI2023

In-gas-jet laser spectroscopy with S 3 -LEB

The Super Separator Spectrometer-Low Energy Branch (S 3 -LEB) is a low-energy radioactive ion beam experiment under commissioning as part of the GANIL-SPIRAL2 facility. It will be used for the production and study of exotic nuclei by in-gas laser ionization and spectroscopy (IGLIS), decay spectroscopy, and mass spectrometry. Here, we report recent results from the off-line commissioning of S 3 -LEB, including first laser spectroscopy measurements in both the gas cell and the supersonic gas jet, the determination of the transport efficiency of laser ions from the gas cell through the RFQ chain, and time-of-flight measurements with the multi-reflection time-of-flight mass spectrometer PILGRIM. The measurements were performed using erbium, introduced by evaporation from a heated filament in the gas environment. The reported laser spectroscopy results include a characterization of the pressure broadening in the gas cell, proof-of-principle isotope shift measurements, and hyperfine-structure measurements.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1985

Effects of buoyancy on gas jet diffusion flames - Experiment and theory

Theoretical and experimental research on the effects of buoyancy on gas-jet diffusion flames is described. Part of this research involves an assessment of existing data obtained under reduced-gravity conditions. The results show that uncertainties in the current understanding of flame structure exist and further research is required before reliable predictions of ignition, stabilization, and propagation of flames under microgravity conditions can be made. Steady-state and transient theories have been developed and used in the analysis of existing drop-tower data and new data obtained from a stationary experiment involving inverted flames. The result of this research has led to the definition of a microgravity experiment to be performed in space.

Edelman, R. B.↗
NASA NTRS2000

Transitional Gas Jet Diffusion Flames in Microgravity

Drop tower experiments were performed to identify buoyancy effects in transitional hydrogen gas jet diffusion flames. Quantitative rainbow schlieren deflectometry was utilized to optically visualize the flame and to measure oxygen concentration in the laminar portion of the flame. Test conditions consisted of atmospheric pressure flames burning in quiescent air. Fuel from a 0.3mm inside diameter tube injector was issued at jet exit Reynolds numbers (Re) of 1300 to 1700. Helium mole percentage in the fuel was varied from 0 to 40%. Significant effects of buoyancy were observed in near field of the flame even-though the fuel jets were momentum-dominated. Results show an increase of breakpoint length in microgravity. Data suggest that transitional flames in earth-gravity at Re<1300 might become laminar in microgravity.

Agrawal, Ajay K.↗
NASA NTRS1993

Attitude stabilization of a rigid spacecraft using gas jet actuators operating in a failure mode

The attitude stabilization of a rigid spacecraft using control torques supplied by gas jet actuators about only two of its principal axes is considered. First, the case where the uncontrolled principal axis of the spacecraft is not an axis of symmetry is considered. In this case, the complete spacecraft dynamics are small time locally controllable. However, the spacecraft cannot be asymptotically stabilized to an equilibrium attitude using time-invariant continuous feedback. A discontinuous stabilizing feedback control strategy is constructed which stabilizes the spacecraft to an equilibrium attitude. Next, the case where the uncontrolled principal axis of the spacecraft is an axis of symmetry is considered. In this case, the complete spacecraft dynamics are not even accessible. However, the spacecraft dynamics are strongly accessible and small time locally controllable in a reduced sense. The reduced spacecraft dynamics cannot be asymptotically stabilized to an equilibrium attitude using time-invariant continuous feedback, but again a discontinuous stabilizing feedback control strategy is constructed. In both cases, the discontinuous feedback controllers are constructed by switching between one of several feedback functions.

Krishnan, Hariharan↗
NASA NTRS2001

Modeling Coma Gas Jets in Comet Hale-Bopp

We present an analysis of OH, CN, and C2 jets observed in Comet Hale-Bopp. The relative contributions from and composition of the coma gas sources, and the parameters describing the active areas responsible for the gas jets will be discussed. Additional information is contained in the original extended abstract.

Lederer, S. M.↗
NASA NTRS2000

Buoyancy Effects on Flow Transition in Hydrogen Gas Jet Diffusion Flames

Experiments were performed in earth-gravity to determine how buoyancy affected transition from laminar to turbulent flow in hydrogen gas jet diffusion flames. The jet exit Froude number characterizing buoyancy in the flame was varied from 1.65 x 10(exp 5) to 1.14 x 10(exp 8) by varying the operating pressure and/or burner inside diameter. Laminar fuel jet was discharged vertically into ambient air flowing through a combustion chamber. Flame characteristics were observed using rainbow schlieren deflectometry, a line-of-site optical diagnostic technique. Results show that the breakpoint length for a given jet exit Reynolds number increased with increasing Froude number. Data suggest that buoyant transitional flames might become laminar in the absence of gravity. The schlieren technique was shown as effective in quantifying the flame characteristics.

Albers, Burt W.↗
NASA NTRS1982

Aerodynamic levitation of laser-heated solids in gas jets

The aerodynamic levitation technique is developed for studies of high-temperature material properties and gas/condensed-phase reaction kinetics. Stable levitation is demonstrated in a supersonic jet from a 0.081 cm nozzle with 0.03-0.20 g 0.24-0.47 cm diameter solid spheres at a height between 0.7-2.0 cm above the nozzle and ambient pressures between 1.1-18 Torr. A model of supersonic jet levitation is developed which accurately predicts the values of height vs pressure over the full range of conditions investigated. It is found that the efficiency with which jet momentum is converted into levitation force decreases with the jet/specimen diameter ratio and the jet Reynolds number, and the rate of jet spreading with distance from the nozzle is found to agree with that measured by pitot tube traverses of the jet. In addition, laser heating is shown to reduce the jet momentum required for levitation at a given height and to increase levitation stability. Measurements of sphere levitation in subsonic gas jets show that the required jet momentum flow rate exceeds the specimen weight by about 2/the specimen drag coefficient at its terminal free-fall speed under ambient conditions.

Nordine, P. C.↗
NASA NTRS1986

Thermal environment around the Space Shuttle with hot-gas jets for ice suppression

To prevent prelaunch ice formation on the external tank of the Space Shuttle, the final selected approach is to heat the surrounding air with vertical hot-gas jets located at the launch pad. This approach was considerably more cost-effective than other ice suppression methods considered, although its feasibility was not easily discernible due to the complex flowfield interactions. This paper describes how the use of vertical jets was first evaluated with the aid of a computational fluid dynamics (CFD) technique. An existing general-purpose CFD program (PHOENICS) was used to predict the thermal environment around the Shuttle under various jet configurations and wind conditions. The program accounts for effects of buoyancy, turbulence, and structural obstructions in the flowfield. The computed results showed physically plausible and consistent trends. The wind wake effects were found to be significant, and normally resulted in higher temperatures on the leeward side of the tank. High wind conditions were found to be more severe than calm wind conditions. The use of four jets with two different temperatures was identified as a promising option in which the air temperatures were raised sufficiently to prevent ice formation on the external tank, without excessive increase in Orbiter surface temperature. The use of the numerical model also facilitated the selection of test configurations and a test matrix for verifying the approach. Selected results of an experimental verification (by using a 2 percent scale model in a wind tunnel) are also presented.

Singhal, A. K.↗
DOE OSTI2025

Longitudinal tapering in gas jets for increased efficiency of 10-GeV class laser plasma accelerators

Modern laser plasma accelerators often require plasma waveguides tens of centimeters long to propagate a high-intensity drive laser pulse. Tapering the longitudinal gas density profile in 10 cm scale gas jets could allow for single stage laser plasma acceleration well beyond 10 GeV with current petawatt-class laser systems. Via simulation and interferometry measurements, we show density control by longitudinally adjusting the throat width and jet angle. Density profiles appropriate for tapering were calculated analytically and via particle-in-cell simulations and were matched experimentally. Further, these simulations show that tapering can increase electron beam energy using 19 J laser energy from ∼9 GeV to > 12 GeV in a 30 cm plasma and the accelerated charge by an order of magnitude.

43 PARTICLE ACCELERATORS↗
NASA NTRS1995

Effects of energy release on near field flow structure of gas jets

The primary objective is to understand how buoyancy affects the structure of the shear layer, the development of fluid dynamic instabilities, and formation of the coherent structures in the near-nozzle regions of gas jets. The secondary objectives are to study the role of buoyancy in lifting and reattachment process of diffusion flames, to evaluate the scaling behavior of diffusion flames, and to aid development and/or validation of theoretical models by providing quantitative data in the absence of buoyancy. Fast reacting hydrogen or hydrogen-inert fuels are used to isolate the effects of buoyancy on fluid dynamics without masking the flame behavior by soot and radiative heat transfer. This choice of fuel also permits an evaluation of simulating low gravity in low pressure ground experiments because the similarity constraints are relaxed for the fast reacting, nonsooting diffusion flames. The diagnostics consists primarily of a color schlieren system coupled with computer generated rainbow filters, video recording, and image analysis. The project involves (1) drop tower experiments, (2) ground experiments, and (3) theoretical analysis.

Agrawal, Ajay K.↗
NASA NTRS1991

Radiation from Gas-Jet Diffusion Flames in Microgravity Environments

This paper presents the first demonstration of quantitative flame-radiation measurement in microgravity environments, with the objective of studying the influences and characteristics of radiative transfer on the behavior of gas-jet diffusion flames with possible application to spacecraft fire detection. Laminar diffusion flames of propane, burning in quiescent air at atmospheric pressure, are studied in the 5.18-Second Zero-Gravity Facility of NASA Lewis Research Center. Radiation from these flames is measured using a wide-view angle, thermopile-detector radiometer, and comparisons are made with normal-gravity flames. The results show that the radiation level is significantly higher in microgravity compared to normal-gravity environments due to larger flame size, enhanced soot formation, and entrapment of combustion products in the vicinity of the flame. These effects are the consequences of the removal of buoyancy which makes diffusion the dominant mechanism of transport. The results show that longer test times may be needed to reach steady state in microgravity environments.

Bahadori, M. Yousef↗
NASA NTRS1946

A preliminary study of a propeller powered by gas jets issuing from the blade tips

Computations are made of the performance of a propeller designed to develop 56 thrust horsepower at 100 miles per hour. The fuel consumption of the jet-operated propeller would be considerably higher than that of a reciprocating engine and a propeller. The lighter weight of the jet-operated propeller will result in a lighter weight of engine plus fuel for short-range flights. A theoretical analysis is made of a propeller powered by gas jets issuing from the blade tips. In the propeller considered, the air is drawn through the hub and passes through the hollow propeller blades to the tips, where propellers heat the air and expel it through the nozzles in the blade tips. The reaction of the tips rotates the propeller. For long range flights, the weight of the jet-operated propeller with its fuel would be greater than the weight of a reciprocating engine with its propeller and fuel.

Sanders, J C↗
NASA NTRS1995

Detailed modeling analysis for soot formation and radiation in microgravity gas jet diffusion flames

Radiation heat transfer in combustion systems has been receiving increasing interest. In the case of hydrocarbon fuels, a significant portion of the radiation comes from soot particles, justifying the need for detailed soot formation model and radiation transfer calculations. For laminar gas jet diffusion flames, results from this project (4/1/91 8/22/95) and another NASA study show that flame shape, soot concentration, and radiation heat fluxes are substantially different under microgravity conditions. Our emphasis is on including detailed soot transport models and a detailed solution for radiation heat transfer, and on coupling them with the flame structure calculations. In this paper, we will discuss the following three specific areas: (1) Comparing two existing soot formation models, and identifying possible improvements; (2) A simple yet reasonably accurate approach to calculating total radiative properties and/or fluxes over the spectral range; and (3) Investigating the convergence of iterations between the flame structure solver and the radiation heat transfer solver.

Ku, Jerry C.↗
NASA NTRS1984

An experimental investigation of gas jets in confined swirling air flow

The fluid dynamics of jets in confined swirling flows which is of importance to designers of turbine combustors and solid fuel ramjets used to power missiles fired from cannons were examined. The fluid dynamics of gas jets of different densities in confined swirling flows were investigated. Mean velocity and turbulence measurements are made with a one color, one component laser velocimeter operating in the forward scatter mode. It is shown that jets in confined flow with large area ratio are highly dissipative which results in both air and helium/air jet centerline velocity decays. For air jets, the jet like behavior in the tube center disappears at about 20 diameters downstream of the jet exit. This phenomenon is independent of the initial jet velocity. The turbulence field at this point also decays to that of the background swirling flow. A jet like behavior in the tube center is noticed even at 40 diameters for the helium/air jets. The subsequent flow and turbulence field depend highly on the initial jet velocity. The jets are fully turbulent, and the cause of this difference in behavior is attributed to the combined action swirl and density difference. This observation can have significant impact on the design of turbine combustors and solid fuel ramjets subject to spin.

Mongia, H.↗
NASA NTRS1999

Fuel Preheat Effects on Soot-Field Structure in Laminar Gas Jet Diffusion Flames Burning in 0-g and 1-g

An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to quantify the influence of moderate fuel preheat on soot-field structure within 0-g laminar gas jet diffusion flames. Parallel work in 1-g is also presented to delineate the effect of elevated fuel temperatures on soot-field structure in buoyant flames. The experimental methodology implements jet diffusion flames of nitrogen-diluted acetylene fuel burning in quiescent air at atmospheric pressure. Fuel preheat of approximately 100 K in the 0-g laminar jet diffusion flames is found to reduce soot loadings in the annular region, but causes an increase in soot volume fractions at the centerline. In addition, fuel preheat reduces the radial extent of the soot field in 0-g. In 1-g, the same fuel preheat levels have a more moderated influence on soot loadings in the annular region, but are also seen to enhance soot concentrations near the axis low in the flame. The increased soot loadings near the flame centerline, as caused by fuel preheat, are consistent with the hypothesis that preheat levels of approximately 100 K enhance fuel pyrolysis rates. The results show that the growth stage of particles transported along the soot annulus is shortened both in 1-g and 0-g when elevated fuel temperatures are used.

Konsur, Bogdan↗