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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

An experimental study of high-pressure droplet combustion

The results are presented of an experimental study on suspended n-heptane droplet combustion in air for reduced pressures up to P(r) = 2.305. Transition to fully transient heat-up through the critical state is demonstrated above a threshold pressure corresponding to P(r) of roughly 1.4. A silhouette imaging technique resolves the droplet surface for reduced pressures up to about P(r) roughly 0.63, but soot formation conceals the surface at higher pressures. Images of the soot plumes do not show any sudden change in behavior indicative of critical transition. Mean burning rate constants are computed from the d-squared variation law using measured effective droplet diameters at ignition and measured burn times, and corrected burning times are computed for an effective initial droplet diameter. The results show that the burning rates increase as the fuel critical pressure is approached and decrease as the pressure exceeds the fuel critical pressure. Corrected burning times show inverse behavior.

Norton, Chris M.↗
NASA NTRS1991

Liquid-propellant droplet vaporization and combustion in high pressure environments

In order to correct the deficiencies of existing models for high-pressure droplet vaporization and combustion, a fundamental investigation into this matter is essential. The objective of this research are: (1) to acquire basic understanding of physical and chemical mechanisms involved in the vaporization and combustion of isolated liquid-propellant droplets in both stagnant and forced-convective environments; (2) to establish droplet vaporization and combustion correlations for the study of liquid-propellant spray combustion and two-phase flowfields in rocket motors; and (3) to investigate the dynamic responses of multicomponent droplet vaporization and combustion to ambient flow oscillations.

Yang, Vigor↗
NASA NTRS1991

N-Decane Droplet Combustion in the NASA-Lewis 5 Second Zero-Gravity Facility - Results in Test Gas Environments Other than Air

The burning rate of single droplets of n-decane in a microgravity environment of the NASA-Lewis 5 Second Zero-Gravity Facility was investigated as a function of time, together with the flame diameter/droplet diameter ratio, for a wide range of test environments other than normal air conditions, using an engineering model of the flight experiment. Oxygen mole fractions were varied from 18 to 50 percent, the total test chamber pressure was varied from 0.5 to 2 atmospheres, and the initial droplet diameter was varied from 0.98 to 2.41 mm. Measurements showed that the average burning rates for n-decane droplets exhibited the same qualitative trends as are found in two current models. Temporal analysis of the local burning rates showed variable rates of change in local burning as the droplet combustion progressed. The causes and implications of these findings are discussed.

Haggard, John B.↗
NASA NTRS1991

Uranium droplet core nuclear rocket

Uranium droplet nuclear rocket is conceptually designed to utilize the broad temperature range ofthe liquid phase of metallic uranium in droplet configuration which maximizes the energy transfer area per unit fuel volume. In a baseline system dissociated hydrogen at 100 bar is heated to 6000 K, providing 2000 second of Isp. Fission fragments and intense radian field enhance the dissociation of molecular hydrogen beyond the equilibrium thermodynamic level. Uranium droplets in the core are confined and separated by an axisymmetric vortex flow generated by high velocity tangential injection of hydrogen in the mid-core regions. Droplet uranium flow to the core is controlled and adjusted by a twin flow nozzle injection system.

Anghaie, Samim↗
NASA NTRS1992

Direct numerical simulation of a combusting droplet with convection

The evaporation and combustion of a single droplet under forced and natural convection was studied numerically from first principles using a numerical scheme that solves the time-dependent multiphase and multispecies Navier-Stokes equations and tracks the sharp gas-liquid interface cutting across an arbitrary Eulerian grid. The flow fields both inside and outside of the droplet are resolved in a unified fashion. Additional governing equations model the interphase mass, energy, and momentum exchange. Test cases involving iso-octane, n-hexane, and n-propanol droplets show reasonable comparison rate, and flame stand-off distance. The partially validated code is, thus, readied to be applied to more demanding droplet combustion situations where substantial drop deformation render classical models inadequate.

Liang, Pak-Yan↗
NASA NTRS1991

Some further observations on droplet combustion characteristics - NASA LeRC-Princeton results

Experimental and numerical studies are reviewed which are designed to examine the effects of droplet/gas motion, product-intermediate absorption, extinction, and sooting on droplet combustion. The experimental work at the NASA-Lewis Research Center involves a 2.2-s droptower for investigating microgravitational effects of droplet combustion over a relatively extended range. The droplet-gas velocities are very low because the spherosymmetrical nature of the major combustion processes produces a quiescent environment. The refined experimental results are combined with numerical modeling based on a technique that is fully transient, comprehensive, and has few empirical simplifications. The combination of techniques improves the present understanding of convection-induced effects, reducing soot formation, and promoting quiescent droplet combustion.

Choi, Mun Y.↗
NASA NTRS1991

Extinction of a Free Methanol Droplet in Microgravity

The combustion of isolated, unsupported methanol droplets under microgravity conditions has been studied in the NASA-Lewis 2.2-sec drop tower. The substitution of O2/He mixtures for O2/N2 mixtures results in a substantial increase in the oxygen index required for ignition, the rate of gasification, and the diameter at extinction. As a result, droplets with initial size larger than 1 mm can be produced, deployed, ignited, and burned to extinction, all under microgravity conditions in this facility. A comprehensive time dependent numerical model incorporating detailed multicomponent molecular transport mechanisms and complex elementary chemical kinetics was developed and applied to simulate the entire transient droplet combustion processes (ignition/quasi-steady burning/extinction). Model outputs show good agreement with the experimental measurements, without parameter adjustment. Analysis of modeling results shows that the thermal conductivity of the gas phase under combustion environments increased by a factor of two when the 50-percent O2/50 percent He mixture is used instead of air. The increased thermal conductivity accelerates the heat transfer from the flame front to the droplet surface, resulting in a factor of two increase in the burning rate.

Cho, S. Y.↗
NASA NTRS1993

Experiments on the effect of initial diameter in spherically symmetric droplet combustion of sooting fuels

The effect of initial droplet diameter on the burning rate of sooting fuels (n-heptane and 1-chloro-octane) is studied experimentally at low gravity. A 1.2 s drop tower provided a low gravity environment to minimize buoyancy and achieve spherically symmetric flames for stationary droplets. Free-floating and fiber supported droplets were burned, and both methods gave matching results for droplets of similar initial diameter.

Jackson, G. S.↗
NASA NTRS1993

Numerical Study of the Transient Vaporization of an Oxygen Droplet at Sub- and Super-Critical Conditions

Unsteady vaporization of a droplet in a high-pressure quiescent environment has been studied. With spherical symmetry and constant pressure, the process is diffusion controlled. At low pressures droplet heating is significant for most of the droplet lifetime, and an unsteady analysis of the gas phase is required. Then, the vaporization of a liquid oxygen droplet in gaseous hydrogen at moderate and high pressures is considered. At super-critical pressures, the surface temperature reaches the computed critical mixture value where a model for super-critical combustion is needed. The details of the diffusion layer of dissolved hydrogen do not significantly affect the results. Various methods are explored to compute the gas-phase density. Under the model's assumptions, homogeneous nucleation is not likely to occur.

Delplanque, J.-P.↗
NASA NTRS1954

Impingement of Water Droplets on an Ellipsoid with Fineness Ration 10 in Axisymmetric Flow

The presence of radomes and instruments that are sensitive to water films or ice formations in the nose section of all-weather aircraft and missiles necessitates a knowledge of the droplet impingement characteristics of bodies of revolution. Because it is possible to approximate many of these bodies with an ellipsoid of revolution, droplet trajectories about an ellipsoid of revolution with a fineness ratio of 10 were computed for incompressible axisymmetric air flow. From the computed droplet trajectories, the following impingement characteristics of the ellipsoid surface were obtained and are presented in terms of dimensionless parameters: (1) total rate of water impingement, (2) extent of droplet impingement zone, and (3) local rate of water impingement. These impingement characteristics are compared briefly with those previously reported for an ellipsoid of revolution with a fineness ratio of 5.

METEOROLOGY↗
NASA NTRS1953

Free fall and evaporation of N-Octane droplets in the atmosphere as applied to the jettisoning of aviation gasoline at altitude

In connection with proposed rapid jettisoning of aviation gasoline at altitude, a theoretical study was made of the free fall and evaporation of N-octane droplets in the atmosphere ; N-octane was selected for study because of the expected similarity of over-all evaporation behavior of N-octane and gasoline. It was concluded that gasoline (or N-octane) droplets larger than 2000 microns in diameter would be unstable ; terminal speeds and Reynolds numbers were obtained for droplets not larger than 2000 microns at altitudes to 11,000 feet. The motion data were used in the calculation of N-octane evaporation rates under various conditions. It was found that a droplet having an original diameter of 2000 microns would fall about 4000 feet from 6000 feet under NACA standard atmosphere conditions before coming virtually to rest. Finally, it was concluded that temperature effects are of paramount importance; at highest air temperatures a 1000-foot ground clearance would probably be adequate to prevent ground contamination, whereas at lowest air temperatures only an atomizing spray arrangement would prevent ground contamination, irrespective of ground clearance.

FIRE HAZARDS↗
NASA NTRS1995

Kinetics of diffusional droplet growth in a liquid/liquid two-phase system

A new powerful experimental technique based on holographic observations, developed at the NASA Marshall Space Flight Center, now permits observation of small liquid droplets coarsening. This technique was developed and used for mixed-dimensional coarsening studies. Experiments were conducted on an isopycnic two-phase alloy of succinonitrile and water, annealed isothermally over a four-month period. The succinonitrile-rich droplets precipitate from a water-rich liquid matrix having a density very close to that of the droplets. The matrix and droplets, however, have different optical indices. The results of these experiments, along with the results of computer simulation based on the quasi-static diffusion approximation developed at Rensselaer are reported. These results were published recently. Copies of these papers are attached to this report.

Glickman, M. E.↗
NASA NTRS1995

The effects of sooting in droplet combustion

The study of the burning of a single droplet is an ideal problem from which to gain fundamental understanding of diffusion flame characteristics. Droplet combustion is a complex physico-chemical process that involves a chemically-reacting two-phase flow with phase changes and yet simple experiments and analysis can be used to attain important insights into the burning rate, flame dynamics, kinetic extinction and disruption processes. It is a subject that has been actively studied for the past 40 years with most of the fundamental experiments being performed under reduced-gravity conditions for direct comparisons with theoretical/computational analyses that invoke spherical symmetry assumptions. In the earlier studies, the effects of sooting on the overall burning characteristics were not considered. However, recent microgravity investigations performed at the NASA-LeRC droptowers (Droplet Combustion Experiment) and others indicate that effects of soot and sootcloud formation may be significant during the lifetime of the droplet and therefore must be included in the analysis.

Lee, Kyeong-Ook↗
NASA NTRS1995

Laser diagnostics for microgravity droplet studies

An instrument has been designed, built, and tested for performing laser diagnostic measurements of droplet combustion in low-gravity-flight aircraft. Nonintrusive measurements are of particular importance for droplet combustion (the simplest example of non-premixed combustion) and transport in microgravity environments, where physical contact would introduce an unacceptable level of perturbations. The resolution of these diagnostics can also isolate transport to length scales much smaller than the droplet diameter. These techniques can be configured to instantaneously map an entire flow field in two and three dimensions, providing either qualitative or quantitative information on the distribution of a desired scalar or vector quantity. Detailing the gas-phase flow field and position of the flame front can be achieved using planar laser-induced fluorescence (PLIF) of OH or another flame front marker. An alternative approach is to obtain LIF from a diagnostic seed included in the liquid phase fuel; it would be consumed at the flame front. The main advantage to this approach is that it is easier to choose the wavelength of the molecular absorption which coincides with convenient laser wavelengths rather than finding lasers which can be configured to access OH. Our present method uses a nitrogen-pumped dye laser tuned to a sodium absorption and addition of small concentrations of NaCl to the fuel. Particle image velocimetry (PIV) is a laser-based technique which has recently had its practicality greatly enhanced by the development of high-resolution CCD cameras and the increase in speed and capacity of computer systems. With this technique, a seeded flow is illuminated with a double-pulsed laser sheet to generate a double exposure image on a film or CCD camera. Computer analysis of the image is used to determine the particle velocity vectors and, thus, the gas velocity within the plane of the laser sheet. Our current experiment uses PIV for measuring relative droplet gas-phase velocities in a parabolic flight environment. The results show successful application of PLIF relative to luminosity and radical emission near 308 nm. PIV results are less successful due to gas thermal expansion deflecting the seed particles away from the measurement volume. PIV is applicable for measurements in the absence of combustion.

Winter, Michael↗
NASA NTRS1999

Some Recent Observations on the Burning of Isolated N-Heptane and Alcohol Droplets

In a joint program involving Prof F.A. Williams of the University of California, San Diego and Dr. Vedha Nayagam of the National Center for Microgravity Research on Fluid and Combustion, the combustion of liquid fuel droplets having initial diameters between about 1 mm and 6 mm is being studied. The objectives of the work are to improve fundamental knowledge of droplet combustion dynamics through microgravity experiments and theoretical analyses. The Princeton contributions to the collaborative program supports the engineering design, data analysis, and data interpretation requirements for the study of initially single component, spherically symmetric, isolated droplet combustion studies through experiments and numerical modeling. The complementary UCSD contributions apply asymptotic theoretical analyses and are described in the published literature and in a companion communication in this volume. Emphases of the Princeton work are on the study of simple alcohols (methanol, ethanol), alcohol/water mixtures, and pure alkanes (n-heptane, n-decane) as fuels, with time dependent measurements of drop size, flame-stand-off, liquid-phase composition, and finally, extinction. Ground based experiments have included bench-scale studies at Princeton and collaborative experimental studies in the 2.2 and 5.18 second drop towers at NASA-Glenn Research Center. Spacelab studies have included fiber-supported droplet combustion (FSDC) experiments in the Glovebox facility with accompanying numerical analyses. Experiments include FSDC-1, performed on the USML-2 mission in October, 1995 (STS-73) and FSDC-2, on the second flight of the MSL-1 mission in July, 1997 (STS-94).

Dryer, F. L.↗
NASA NTRS1999

Pressure Effects on Combustion of Methanol and Methanol-Docecanol Droplets

The objective of this research is to improve understanding of the combustion of binary fuel mixtures in the vicinity of the critical point. Fiber-supported single droplets and two-droplet arrays of methanol and of mixtures of methanol and 1-dodecanol, initially 0.9 mm in diameter, were burned in room-temperature air at pressures from 0.1 MPa to 9.0 MPa in the NASA Lewis 2.2-second drop tower. The work is a continuation of a collaborative Japan-US research effort designed to increase knowledge of high-pressure combustion of fuel sprays, relevant to application in propulsive and power-production devices such as Diesel engines. Some previous publications from this cooperative program may be cited. All of the previous experiments concerned alkanes and alkane mixtures. The new research reported here addresses alcohols and alcohol mixtures, to ascertain the degree to which previous results for alkanes extend to alcohols. There have been many previous experimental studies of methanol droplet combustion and a few of alcohol mixtures, but not at the high pressures of interest here. There is some experimental information on methanol droplet combustion at elevated pressure but none on the alcohol mixtures extending to critical pressures, as in the present study.

Okai, K.↗
NASA NTRS1999

Interference of Backscatter from Two Droplets in a Focused Continuous-Wave CO2 Doppler Lidar Beam

With a focused continuous-wave CO2 Doppler lidar at 9.1-microns wavelength, the superposition of backscatter from two approximately 14.12-micron-diameter silicone oil droplets in the lidar beam produced interference that resulted in a single backscatter pulse from the two droplets with a distinct periodic structure. This interference is caused by the phase difference in backscatter from the two droplets while they are traversing the lidar beam at different speeds, and thus the droplet separation is not constant. The complete cycle of interference, with periodicity 2(pi), gives excellent agreement between measurements and lidar theory.

Jarzembski, Maurice A.↗
NASA NTRS1999

Droplet Growth in Undercooled Cu-Co Alloys

High undercooling usually leads to the refinement of microstructures, while an opposite tendency occurs in liquid immiscible alloys. In the Cu-Co system, there is a metastable liquid miscibility gap which may allow the study of droplet nucleation and growth by undercooling experiments. In the present work, two alloys of Cu50Co50 and Cu68Co32 (atomic) were undercooled using a melt fluxing technique in which the liquid separation temperature could be directly measured. It was observed that after separation the resultant melts were further undercooled by 315 K prior to solidification. The relationship between the largest droplet radius and undercooling was found to exhibit three regimes: a power law growth, linear growth, and saturation stage. In addition to dendrites and droplet-like morphology reported previously, an interconnected structure was formed for the Cu50Co50 alloy at intermediate undercooling. It played a crucial role in droplet coarsening at high undercooling.

Robinson, Michael B.↗