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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Buoyant subduction on Venus: Implications for subduction around coronae

Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

Burt, J. D.↗
NASA NTRS2001

Large-Scale Flow Structure in Turbulent Nonpremixed Flames under Normal- And Low-Gravity Conditions

It is well known that buoyancy has a major influence on the flow structure of turbulent nonpremixed jet flames. Buoyancy acts by inducing baroclinic torques, which generate large-scale vortical structures that can significantly modify the flow field. Furthermore, some suggest that buoyancy can substantially influence the large-scale structure of even nominally momentum-dominated flames, since the low velocity flow outside of the flame will be more susceptible to buoyancy effects. Even subtle buoyancy effects may be important because changes in the large-scale structure affects the local entrainment and fluctuating strain rate, and hence the structure of the flame. Previous studies that have compared the structure of normal- and micro-gravity nonpremixed jet flames note that flames in microgravity are longer and wider than in normal-gravity. This trend was observed for jet flames ranging from laminar to turbulent regimes. Furthermore, imaging of the flames has shown possible evidence of helical instabilities and disturbances starting from the base of the flame in microgravity. In contrast, these characteristics were not observed in normal-gravity. The objective of the present study is to further advance our knowledge of the effects of weak levels of buoyancy on the structure of transitional and turbulent nonpremixed jet flames. In later studies we will utilize the drop tower facilities at NASA Glenn Research Center (GRC), but the preliminary work described in this paper was conducted using the 1.25-second drop tower located at the University of Texas at Austin. A more detailed description of these experiments can be found in Idicheria et al.

Clemens, N. T.↗
NASA NTRS1999

Effects of g-Jitter on Diffusion in Binary Liquids

The microgravity environment offers the potential to measure the binary diffusion coefficients in liquids without the masking effects introduced by buoyancy-induced flows due to Earth s gravity. However, the background g-jitter (vibrations from the shuttle, onboard machinery, and crew) normally encountered in many shuttle experiments may alter the benefits of the microgravity environment and introduce vibrations that could offset its intrinsic advantages. An experiment during STS-85 (August 1997) used the Microgravity Vibration Isolation Mount (MIM) to isolate and introduce controlled vibrations to two miscible liquids inside a cavity to study the effects of g-jitter on liquid diffusion. Diffusion in a nonhomogeneous liquid system is caused by a nonequilibrium condition that results in the transport of mass (dispersion of the different kinds of liquid molecules) to approach equilibrium. The dynamic state of the system tends toward equilibrium such that the system becomes homogeneous. An everyday example is the mixing of cream and coffee (a nonhomogeneous system) via stirring. The cream diffuses into the coffee, thus forming a homogeneous system. At equilibrium the system is said to be mixed. However, during stirring, simple observations show complex flow field dynamics-stretching and folding of material interfaces, thinning of striation thickness, self-similar patterns, and so on. This example illustrates that, even though mixing occurs via mass diffusion, stirring to enhance transport plays a major role. Stirring can be induced either by mechanical means (spoon or plastic stirrer) or via buoyancy-induced forces caused by Earth s gravity. Accurate measurements of binary diffusion coefficients are often inhibited by buoyancy-induced flows. The microgravity environment minimizes the effect of buoyancy-induced flows and allows the true diffusion limit to be achieved. One goal of this experiment was to show that the microgravity environment suppresses buoyancy-induced convection, thereby mass diffusion becomes the dominant mechanism for transport. Since g-jitter transmitted by the shuttle to the experiment can potentially excite buoyancy-induced flows, we also studied the effects of controlled vibrations on the system.

Duval, Walter M. B.↗
NASA NTRS1990

Laboratory experiments on stability and entrainment of oceanic stratocumulus. Part 2: Entrainment experiment

A stratified interface is stable to the buoyancy reversal instability for surprisingly large values of D (buoyancy reversal parameter). A new instability mechanism is proposed, which considers the mixing process at the interface. For the type of density curves studied here, under strong perturbations, the mixed parcel must have a buoyancy reversal comparable to the initial stratification before the interface is unstable. This is in accord with a simple model of the interface mixing process, as well as aircraft observations of long-live marine stratocumulus clouds. These clouds' remarkable longevity in the face of finite D indicates that they can be stable (Hanson, 1984; Albrecht et al., 1985; Siems et al., 1989). It is suggested that buoyancy reversal as well as the disturbance must be large for Cloudtop Entrainment Stability. The effect of buoyancy reversal (evaporative cooling) does not always enhance the entrainment rate over that in the inert case, but it may be negligible if Ri (Richardson number) is large (Ri is larger than 50) and D is small (D is smaller than 0.5). This work may shed some light on the fundamental mechanism of the breakup process of the subtropical stratocumulus clouds into tradewind cumulus. These results may also be related to the instability in the Weddell Sea off of Antarctica.

Shy, Shenqyang S.↗
NASA NTRS1997

Structure and Stability of Burke-Schumann Diffusion Flames

The general goal of this NASA Grant is twofold: to improve our understanding of (1) the influence of buoyancy on the stability and structure of Burke-Schumann type diffusion flames, and (2) the effects of buoyancy on vortex-flame interactions in co-flow diffusion flames. A numerical code with a higher order accuracy for spatial discretization is developed in this project for simulation of time-dependent diffusion flames by Sheu and Sheu and Chen, and an extended reduced mechanism is incorporated for prediction of methane oxidation and NO(x)(NO, NO2, and N2O) formation and emission from methane Burke-Schumann diffusion flame (BSDF) as reported in Sheu, and Sheu and Chen. Initial investigation of vortex and flame interaction within the context of fast chemistry is reported. Experiments are conducted in reduced pressure to study the lift-off and stabilization of methane-fueled BSDF in reduced buoyancy environments due to reduced pressure. Measurements of temperature and species concentrations are made in normal and reduced pressure environments to study the effects of buoyancy on the structure of BSDF, and will be reported in this paper. To study the buoyancy effects on the lift-off and stabilization of methane-fueled jet diffusion flames in coflowing air, a glovebox investigation, Enclosed Laminar Flames (ELF), has been proposed and approved for space-based testing on the fourth United States Microgravity Payload (USMP-4) mission, scheduled for October 1997. A brief description of the ELF investigation is also presented.

Lee, Yong G.↗
NASA NTRS1999

Single Bubble Sonoluminescence in Low Gravity and Optical Radiation Pressure Positioning of the Bubble

Several groups of researchers have demonstrated that high frequency sound in water may be used to cause the regular repeated compression and luminescence of a small bubble of gas in a flask. The phenomenon is known as single bubble sonoluminescence (SBSL). It is potentially important because light emitted by the bubble appears to be associated with a significant concentration of energy within the volume of the bubble. Unfortunately, the detailed physical mechanisms causing the radiation of light by oscillating bubbles are poorly understood and there is some evidence that carrying out experiments in a weightless environment may provide helpful clues. In addition, the radiation pressure of laser beams on the bubble may provide a way of simulating weightless experiments in the laboratory. The standard model of SBSL attributes the light emission to heating within the bubble by a spherically imploding shock wave to achieve temperatures of 50,000 K or greater. In an alternative model, the emission is attributed to the impact of a jet of water which is required to span the bubble and the formation of the jet is linked to the buoyancy of the bubble. The coupling between buoyancy and jet formation is a consequence of the displacement of the bubble from a velocity node (pressure antinode) of the standing acoustic wave that drives the radial bubble oscillations. One objective of this grant is to understand SBSL emission in reduced buoyancy on KC-135 parabolic flights. To optimize the design of those experiments and for other reasons which will help resolve the role of buoyancy, laboratory experiments are planned in simulated low gravity in which the radiation pressure of laser light will be used to position the bubble at the acoustic velocity node of the ultrasonic standing wave. Laser light will also be used to push the bubble away from the velocity node, increasing the effective buoyancy. The original experiments on the optical levitation and radiation pressure on bubbles in water by Unger and Marston noted above were carried out using a continuous wave (CW) beam of an Argon laser. For lateral stability the beam had a intensity minimum along its axis. Calculations of the optical radiation force on an SBSL bubble indicate that ion laser technology is a poor choice for providing the magnitude of the average optical radiation force required. Consequently it is necessary to examine various diode-pumped solid state laser technologies. The approach for this part of the research will be to achieve optical levitation of a quiescent bubble based on contemporary laser technology and then to strobe the laser synchronously with the SBSL bubble oscillations.

Thiessen, D. B.↗
NASA NTRS2023

Influence of Lower Tropospheric Moisture on Local Soil Moisture-Precipitation Feedback Over the U.S. Southern Great Plains

Land-atmosphere coupling (LAC) has long been studied focusing on land surface and atmospheric boundary layer processes. However, the influence of lower tropospheric (LT) humidity on LAC remains largely unexplored. In this study, we use radiosonde observations from the U.S. Southern Great Plains (SGP) site and an entrained parcel buoyancy model to investigate the impact of LT humidity on LAC there during the warm season (May–September). We quantify the effect of LT humidity on convective buoyancy by measuring the difference between the 2–4 km vertically integrated buoyancy with and without the influence of background LT humidity. Our results show that, under dry soil conditions, anomalously high LT humidity is necessary to produce the buoyancy profiles required for afternoon precipitation events (APEs). These APEs under dry soil moisture cannot be explained by commonly used local land-atmosphere coupling indices such as the convective triggering potential/low-level humidity index (CTP/HI Low ), which do not account for the influence of the LT humidity. On the other hand, consideration of LT humidity is unnecessary to explain APEs under wet soil moisture conditions, suggesting the boundary layer moisture alone could be sufficient to generate the required buoyancy profiles. These findings highlight the need to consider the impact of LT humidity, which is often decoupled from the humidity near the surface and largely controlled by moisture transport, in understanding land-atmospheric feedbacks over dry soil conditions, especially during droughts or dry spells over the SGP.

Gaoyun Wang↗
NASA NTRS2017

Contribution of Sub-Mesoscales to the Vertical Velocity: The Omega-Equation

The oceans ability to regulate carbon dioxide depends on biogeochemical processes that are influenced strongly by eddies. Eddy-resolving simulations have shown that sub-mesoscales (SM) (110 km) generate the highest magnitude vertical velocity and that mesoscales (M) also enhance their contribution to the vertical velocity but to a lesser extent. In this study, we consider the question: can analogous results be obtained using the less numerically demanding -equation? Previously, this question has not been answered because of two reasons: 1) the canonical Hoskins form of the - equation does not include the buoyancy vertical fluxes caused by M and SM; and 2) Giordani et al. (2016) showed how to include an arbitrary vertical buoyancy flux, but no parameterizations were available for the M and SM vertical fluxes. How- ever, the latter are now available together with their assessments, so we consider SM because they make the largest contribution to the vertical velocity. The resulting vertical velocity depends on the extent of the SM regime, the horizontal buoyancy gradient (representing baroclinic instabilities), and the SM eddy kinetic energy. The vertical velocity depends in a linear manner on the wind stress and it may exhibit seasonal variations. The wind stress has two effects on the -equation: indirectly via its contribution to the sub-mesoscale buoyancy flux and directly through the wind stress itself. The results of our sensitivity analysis highlight the range of SM-induced vertical velocities obtained using different input data.

oceans↗
NASA NTRS1991

Rotating reactor studies

Undesired gravitational effects such as convection or sedimentation in a fluid can sometimes be avoided or decreased by the use of a closed chamber uniformly rotated about a horizontal axis. In a previous study, the spiral orbits of a heavy or buoyant particle in a uniformly rotating fluid were determined. The particles move in circles, and spiral in or out under the combined effects of the centrifugal force and centrifugal buoyancy. A optimization problem for the rotation rate of a cylindrical reactor rotated about its axis and containing distributed particles was formulated and solved. Related studies in several areas are addressed. A computer program based on the analysis was upgraded by correcting some minor errors, adding a sophisticated screen-and-printer graphics capability and other output options, and by improving the automation. The design, performance, and analysis of a series of experiments with monodisperse polystyrene latex microspheres in water were supported to test the theory and its limitations. The theory was amply confirmed at high rotation rates. However, at low rotation rates (1 rpm or less) the assumption of uniform solid-body rotation of the fluid became invalid, and there were increasingly strong secondary motions driven by variations in the mean fluid density due to variations in the particle concentration. In these tests the increase in the mean fluid density due to the particles was of order 0.015 percent. To a first approximation, these flows are driven by the buoyancy in a thin crescent-shaped depleted layer on the descending side of the rotating reactor. This buoyancy distribution is balanced by viscosity near the walls, and by the Coriolis force in the interior. A full analysis is beyond the scope of this study. Secondary flows are likely to be stronger for buoyant particles, which spiral in towards the neutral point near the rotation axis under the influence of their centrifugal buoyancy. This is because the depleted layer is thicker and extends all the way around the reactor.

Roberts, Glyn O.↗
NASA NTRS1991

Heat transfer in rotating serpentine passages with trips normal to the flow

Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multipass, heat transfer model with both radially inward and outward flow. Trip strips on the leading and trailing surfaces of the radial coolant passages were used to produce the rough walls. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature ratio, Rossby number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from stationary and rotating similar models with trip strips. The heat transfer coefficients on surfaces, where the heat increased with rotation and buoyancy, varied by as much as a factor of four. Maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels obtained with the smooth wall model. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation, varied by as much as a factor of three due to rotation and buoyancy. It was concluded that both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips and that the effects of rotation were markedly different depending upon the flow direction.

Wagner, J. H.↗
NASA NTRS1992

Heat transfer in rotating serpentine passages with trips skewed to the flow

Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multi-pass, heat transfer model with both radially inward and outward flow. Trip strips, skewed at 45 deg to the flow direction, were machined on the leading and trailing surfaces of the radial coolant passages. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature, rotation number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from similar stationary and rotating models with smooth walls and with trip strips normal to the flow direction. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation and buoyancy, decreased to as low as 40 percent of the value without rotation. However, the maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels previously obtained with the smooth wall models. It was concluded that (1) both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips, (2) the effects of rotation are markedly different depending upon the flow direction, and (3) the heat transfer with skewed trip strips is less sensitive to buoyancy than the heat transfer in models with either smooth or normal trips. Therefore, skewed trip strips rather than normal trip strips are recommended and geometry-specific tests are required for accurate design information.

Johnson, B. V.↗
NASA NTRS1991

Heat transfer in rotating serpentine passages with trips normal to the flow

Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multipass, heat transfer model with both radially inward and outward flow. Trip strips on the leading and trailing surfaces of the radial coolant passages were used to produce the rough walls. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature ratio, Rossby number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from stationary and rotating similar models with trip strips. The heat transfer coefficients on surfaces, where the heat increased with rotation and buoyancy, varied by as much as a factor of four. Maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels obtained with the smooth wall model. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation, varied by as much as a factor of three due to rotation and buoyancy. It was concluded that both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips and that the effects of rotation were markedly different depending upon the flow direction.

Wagner, J. H.↗
NASA NTRS1991

Dynamical behavior of strong magnetic fields in the solar convection zone

Magnetic buoyancy is thought to play an important role in the dynamical behavior of the sun's magnetic field in the convection zone. Magnetic buoyancy is commonly thought to cause inescapable rapid loss of toroidal flux from much of the convection zone, thereby suppressing effective operation of a solar dynamo. This paper reexamines the detailed character of magnetic buoyancy, especially as it is influenced by the magnetic field's effect on heat transport and temperature gradients in the convection zone. It is suggested that suppression of convective heat transport across strong magnetic flux tubes can alter the temperature within the tubes and can subdue, or even reverse, the effect of magnetic buoyancy.

Vainshtein, S.↗
NASA NTRS1993

The structure of three-dimensional convection beneath oceanic spreading centres

The combined effects of buoyancy forces due to thermal expansion and compositional gradients in the mantle beneath spreading centers have been investigated in a series of numerical experiments. Compositional buoyancy leads to three-dimensional flow beneath the spreading axis, with along-axis variations in upwelling and melt production. Thermal buoyancy drives upwelling beneath the axis and convective overturn, which forms rolls aligned perpendicular to the spreading axis. The distance from the axis at which these rolls form is controlled by the spreading rate, mantle viscosity, and relative strength of compositional buoyancy. Implications of the results for the spreading center structure are examined.

Sparks, David W.↗
NASA NTRS1993

Marangoni effect induced convection in material processing under microgravity (M-18)

On the ground the upper layer of the liquid in a vessel heated from below becomes hotter than the lower layer. This well-known phenomenon results from buoyancy-induced convection. Though this means that the buoyancy-induced convection is not dominant under microgravity conditions in space, another convection called Marangoni convection may possibly occur. This convection results from an intermolecular force which acts on a free surface, that is surface tension. Since it is stronger at lower temperatures, the liquid surface near the heated wall is pulled to the cooled side as shown. This surface movement causes the inner convection. Marangoni convection, however, may be negligible on the Earth, for the molecular force is generally smaller than buoyancy. Various tests on material processing were recently conducted in space and some high quality crystals free from buoyancy convection were obtained. But, at the same time, others proved to be less uniform than expected in the components' distribution. This nonuniformity seems to be mainly caused by Marangoni convection. It is, therefore, very important to know how to control the convection by studying its characteristics, but the problem is that on the ground it is impossible to carry out the experiment without gravity. That is what a space experiment aboard the space shuttle is planned as the First Material Processing Tests (FMPT) of Japan. The experiment on Marangoni flow visualization is being performed in order to investigate the characteristics of convection in uni-dimensional melt growth under microgravity conditions.

Enya, S.↗
NASA NTRS1994

Pool and flow boiling in variable and microgravity

As is well known, boiling is an effective mode of heat transfer in that high heat flux levels are possible with relatively small temperature differences. Its optimal application requires that the process be adequately understood. A measure of the understanding of any physical event lies in the ability to predict its behavior in terms of the relevant parameters. Despite many years of research the predictability of boiling is currently possible only for quite specialized circumstances, e.g., the critical heat flux and film boiling for the pool boiling case, and then only with special geometries. Variable gravity down to microgravity provides the opportunity to test this understanding, but possibly more important, by changing the dimensional and time scales involved permits more detailed observations of elements involved in the boiling process, and perhaps discloses phenomena heretofore unknown. The focus here is on nucleate boiling although, as will be demonstrated below, under but certain circumstances in microgravity it can take place concurrently with the dryout process. In the presence of earth gravity or forced convection effects, the latter process is usually referred to as film boiling. However, no vapor film as such forms with pool boiling in microgravity, only dryout. Initial results are presented here for pool boiling in microgravity, and were made possible at such an early date by the availability of the Get-Away-Specials (GAS). Also presented here are some results of ground testing of a flow loop for the study of low velocity boiling, eventually to take place also in microgravity. In the interim, variable buoyancy normal to the heater surface is achieved by rotation of the entire loop relative to earth gravity. Of course, this is at the expense of varying the buoyancy parallel to the heater surface. Two questions which must be resolved early in the study of flow boiling in microgravity are (1) the lower limits of liquid flow velocity where buoyancy effects become significant to the boiling process (2) the effect of lower liquid flow velocities on the Critical Heat Flux when buoyancy is removed. Results of initial efforts in these directions are presented, albeit restricted currently to the ever present earth gravity.

Merte, Herman, Jr.↗
NASA NTRS1994

Electrophoretic interactions and aggregation of colloidal biological particles

The separation of cells or particles from solution has traditionally been accomplished with centrifuges or by sedimentation; however, many particles have specific densities close to unity, making buoyancy-driven motion slow or negligible, but most cells and particles carry surface charges, making them ideal for electrophoretic separation. Both buoyancy-driven and electrophoretic separation may be influenced by hydrodynamic interactions and aggregation of neighboring particles. Aggregation by electrophoresis was analyzed for two non-Brownian particles with different zeta potentials and thin double layers migrating through a viscous fluid. The results indicate that the initial rate of electrophoretically-driven aggregation may exceed that of buoyancy-driven aggregation, even under conditions in which buoyancy-driven relative motion of noninteracting particles is dominant.

Davis, Robert H.↗
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.↗