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Results for “THERMAL CONVECTION”

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 NTRS1992

Convective effects during the physical vapor transport process. I - Thermal convection

The effects of convection on diffusive-convective physical vapor transport process are examined computationally. We analyze conditions ranging from typical laboratory conditions to conditions achievable only in a low gravity environment. This corresponds to thermal Rayleigh numbers Ra(T) ranging from 1.80 to 1.92 x 10 exp 6. Our results indicate that the effect of the sublimation and condensation fluxes at the boundaries is 10 increase the threshold of instability. For typical ground based conditions time dependent oscillatory convection can occur. This results in nonuniform temperature and concentration gradients at the crystal interface. Spectral analysis of the flow field shows regions of both periodic and quasi-periodic states. Low gravity conditions can effectively reduce convective effects, thus resulting in uniform temperature and concentration gradients at the interface, a desirable condition for crystal growth.

Duval, Walter M. B.↗
NASA NTRS2006

Computer Modeling of Thermal Convection in Melts to Explain Glass Formation in Low Gravity and on Earth

Experiments conducted up to this time on glass forming melts in the low gravity environment of space show that glasses prepared in low-g are more chemically homogeneous and more resistant to crystallization than the comparable glasses prepared at 1-g on Earth. This result is somewhat surprising and opposite to the accepted concept on glass formation for a melt. A hypothesis based on "shear thinning" of a melt, a decrease in viscosity with increasing shear stress, is proposed as an explanation for the observed low-gravity results. This paper describes detailed simulation procedures to test the role of thermal convection in introducing shear stress in glass forming melts, using a lithium disilcate melt as a model. The simulation system in its idealized version consists of a cylinder that is heated at one end and cooled at the other with gravity acting in a transverse direction to the thermal gradient. The side wall of the cylinder is assumed to be insulating. The governing equations of motion and energy are solved using variable properties for viscosity (Arrehenius and non-Arrehenius behaviors) and density (constant and temperature dependent). Other parametric variables in the calculations include gravity level and gravity vector orientation. The shear stress in the system are then computed as a function of gravity from the calculated values of maximum melt velocity, and its effect on melt viscosity (shear thinning) is predicted. Also included and discussed are the modeling efforts related to other potential convective processes in glass forming melts and their possible effects on melt viscosity.

Ray, Chandra S.↗
NASA NTRS2003

Control of Thermal Convection in Layered Fluids Using Magnetic fields

Immiscible fluid layers are found in a host of applications ranging from materials processing, for example the use of encapsulants in float zone crystal growth technique and a buffer layer in industrial Czochralski growth of crystals to prevent Marangoni convection, to heat transfer phenomena in day-to-day processes like the presence of air pockets in heat exchangers. In the microgravity and space processing realm, the exploration of other planets requires the development of enabling technologies in several fronts. The reduction in the gravity level poses unique challenges for fluid handling and heat transfer applications. The present work investigates the efficacy of controlling thermal convective flow using magnetic fluids and magnetic fields. The setup is a two-layer immiscible liquids system with one of the fluids being a diluted ferrofluid (super paramagnetic nano particles dispersed in carrier fluid). Using an external magnetic field one can essentially dial in a volumetric force - gravity level, on the magnetic fluid and thereby affect the system thermo-fluid behavior. The paper will describe the experimental and numerical modeling approach to the problem and discuss results obtained to date.

Ramachandran, N.↗
NASA NTRS1984

Thermal convection during Bridgman crystal growth

Numerical experiments are used to study thermally driven flows which occur during vertical Bridgman crystal growth of a single component fluid. The solid-liquid interface was specified as parabolic and flow patterns were calculated for various insulation thicknesses, Grashof, Prandtl, and Biot numbers. When the melt is on top and the gravity vector is axially downward it is shown that flow persists as long as a radial temperature gradient is present. If the interface is convex, as viewed from the liquid, a single cell is observed. A concave interface exhibits multiple counterrotating cells. The insulation thickness and Grashof, Prandtl, and Biot numbers influence the flow in a quantitative manner.

Carlson, Frederick M.↗
NASA NTRS1996

Experiments on Thermal Convection in Rotating Spherical Shells With Radial Gravity: The Geophysical Fluid Flow Cell

Experiments designed to study the fluid dynamics of buoyancy driven circulations in rotating spherical shells were conducted on the United States Microgravity Laboratory 2 spacelab mission. These experiments address several aspects of prototypical global convection relevant to large scale motions on the Sun, Earth, and on the giant planets. The key feature is the consistent modeling of radially directed gravity in spherical geometry by using dielectric polarization forces. Imagery of the planforms of thermally driven flows for rapidly-rotating regimes shows an initial separation and eventual merger of equatorial and polar convection as the heating (i.e. the Rayleigh number) is increased. At low rotation rates, multiple-states of motion for the same external parameters were observed.

Hart, John E.↗
NASA NTRS1979

Effects of truncation in modal representations of thermal convection

The Galerkin (including single-mode and Lorenz) equations were examined for convection in a sphere to determine which physical processes are neglected when the equations of motion are truncated too severely. The conclusions were tested by calculating solutions to the equations of motion for different values of the Rayleigh number and for different values of the limit of the horizontal spatial resolution. It was shown that the transitions from steady state to periodic, then to aperiodic convection depend not only on the Rayleigh number but also very strongly on the horizontal resolution. One of the effects of truncation is to enhance the high wavenumber end of the kinetic energy and thermal variance spectra. The numerical examples indicate that as long as the kinetic energy spectrum decreases with wavenumber, a truncation gives a qualitatively correct solution.

Marcus, P. S.↗
DOE OSTI2025

AIVT: Inference of turbulent thermal convection from measured 3D velocity data by physics-informed Kolmogorov-Arnold networks

We propose the artificial intelligence velocimetry-thermometry (AIVT) method to reconstruct a continuous and differentiable representation of the temperature and velocity in turbulent convection from measured three-dimensional (3D) velocity data. AIVT is based on physics-informed Kolmogorov-Arnold networks and trained by optimizing a loss function that minimizes residuals of the velocity data, boundary conditions, and governing equations. We apply AIVT to a set of simultaneously measured 3D temperature and velocity data of Rayleigh-Bénard convection, obtained by combining particle image thermometry and Lagrangian particle tracking. This enables us to directly compare machine learning results to true volumetric, simultaneous temperature and velocity measurements. We demonstrate that AIVT can reconstruct and infer continuous, instantaneous velocity and temperature fields and their gradients from sparse experimental data at a high resolution, providing an additional approach for understanding thermal turbulence.

Science & Technology - Other Topics↗
NASA NTRS1981

Effects of truncation in modal representations of thermal convection

The Galerkin equations for convection in a sphere are examined to determine which physical processes are neglected by the severe truncation of the equations of motion. It is demonstrated that the gross features of the flow are affected by truncation in the horizontal direction, with all of the models considered being well resolved in the vertical direction. One of the effects of truncation is to enhance the high-wave number end of the kinetic energy and thermal variance spectra. The examples cited indicate that as long as the kinetic energy spectrum decreases with wave number, a truncation gives a qualitatively correct solution. Conclusions are tested by calculating solutions to the equations of motion for several values of the Rayleigh number and the limit of horizontal spatial resolution.

Marcus, P. S.↗
DOE OSTI2021

Boundary zonal flows in rapidly rotating turbulent thermal convection

Recently, in Zhang et al. ( Phys. Rev. Lett. , vol. 124, 2020, 084505), it was found that, in rapidly rotating turbulent Rayleigh–Bénard convection in slender cylindrical containers (with diameter-to-height aspect ratio $\varGamma =1/2$ ) filled with a small-Prandtl-number fluid ( ${Pr}\approx 0.8$ ), the large-scale circulation is suppressed and a boundary zonal flow (BZF) develops near the sidewall, characterized by a bimodal probability density function of the temperature, cyclonic fluid motion and anticyclonic drift of the flow pattern (with respect to the rotating frame). This BZF carries a disproportionate amount ( ${>}60\,\%$ ) of the total heat transport for ${Pr} < 1$ , but decreases rather abruptly for larger ${Pr}$ to approximately $35\,\%$ . In this work, we show that the BZF is robust and appears in rapidly rotating turbulent Rayleigh–Bénard convection in containers of different $\varGamma$ and over a broad range of ${Pr}$ and ${Ra}$ . Furthermore, tirect numerical simulations for Prandtl number $0.1 \leq {\textit {Pr}} \leq 12.3$ , Rayleigh number $10^7 \leq {Ra} \leq 5\times 10^{9}$ , inverse Ekman number $10^{5} \leq 1/{\textit {Ek}} \leq 10^{7}$ and $\varGamma = 1/3$ , 1/2, 3/4, 1 and 2 show that the BZF width $\delta _0$ scales with the Rayleigh number ${Ra}$ and Ekman number ${\textit {Ek}}$ as $\delta _0/H \sim \varGamma ^{0} Pr^{\{-1/4, 0\}} {Ra}^{1/4} {\textit {Ek}}^{2/3}$ ( $\{{\textit {Pr}}<1, {\textit {Pr}}>1\}$ ) and with the drift frequency scales as $\omega /\varOmega \sim \varGamma ^{0} Pr^{-4/3} {Ra}\,{\textit {Ek}}^{5/3}$ , where $H$ is the cell height and $\varOmega$ the angular rotation rate. The mode number of the BZF is 1 for $\varGamma \lesssim 1$ and $2 \varGamma$ for $\varGamma = \{1,2\}$ independent of ${Ra}$ and ${Pr}$ . The BZF is quite reminiscent of wall mode states in rotating convection.

42 ENGINEERING↗
NASA NTRS1974

Mass transfer in a 1370 C (2500 F) lithium thermal convection loop

Experimental results from a test to evaluate interstitial element mass transfer effects on T-111, ASTAR 811C, and ASTAR 1211C after 5000 hours in flowing lithium at 1370 C (2500 F) are presented. No gross corrosion effects were observed. However, hafnium and nitrogen transfer to cooler regions within the loop were noted. Oxygen was in general removed from test specimens, but there was no evidence to indicate that it was a major factor in the mass transfer process. Carbon and hydrogen transfer were not detected.

Scheuermann, C. M.↗
NASA NTRS1973

Numerical simulation of small-scale thermal convection in the atmosphere

A Boussinesq system is integrated numerically in three dimensions and time in a study of nonhydrostatic convection in the atmosphere. Simulation of cloud convection is achieved by the inclusion of parametrized effects of latent heat and small-scale turbulence. The results are compared with the cell structure observed in Rayleigh-Benard laboratory conversion experiments in air. At a Rayleigh number of 4000, the numerical model adequately simulates the experimentally observed evolution, including some prominent transients of a flow from a randomly perturbed initial conductive state into the final state of steady large-amplitude two-dimensional rolls. At Rayleigh number 9000, the model reproduces the experimentally observed unsteady equilibrium of vertically coherent oscillatory waves superimposed on rolls.

Somerville, R. C. J.↗
NASA NTRS1980

Infinite Prandtl number thermal convection in a spherical shell

A Galerkin technique is used to calculate the steady-state axisymmetric nonlinear convective motions in an infinite-Prandtl-number Boussinesq fluid in a relatively thick spherical shell heated from below. A reasonably complete study of the properties of the even and general axisymmetric steady states is carried out for a range of moderately supercritical Rayleigh numbers. In addition, stability analyses are conducted to determine which form of axisymmetric steady convection is the preferred one and whether the axisymmetric steady flows are unstable to azimuthal perturbations.

Zebib, A.↗
NASA NTRS1980

Thermal convection of an internally heated infinite Prandtl number fluid in a spherical shell

A Galerkin technique is used to study the finite-amplitude axisymmetric steady convective motions of an infinite Prandtl number Boussinesq fluid in a spherical shell. Two types of heating are considered: in one case, convection is driven both by internal heat sources in the fluid and by an externally imposed temperature drop across the shell boundaries; in the other case, only internal heat sources drive convection and the lower boundary of the shell is adiabatic. Two distinct classes of axisymmetric steady states are found to be possible: states characterized by temperature and radial velocity fields that are symmetric about an equatorial plane; and a class of solutions that does not possess any symmetry properties about the equatorial plane.

Schubert, G.↗
NASA NTRS1983

Character and stability of axisymmetric thermal convection in spheres and spherical shells

The influence of shell size and mode of heating on the behavior and stability of axisymmetric, infinite Prandtl number convection in a spherical geometry is studied. Heating from within and below features convection onset governed by a self-adjoint system of equations and boundary conditions. For heating only from within or from below, linearized equations and boundary conditions are non-self-adjoint. Identification of the parameter which initiates the departure from self-adjointness, together with the properties of the self-adjoint solution, provide a basis for calculating the heat transfer characteristics of the non-self-adjoint situations. The investigations are an effort to develop a model for heat transfer in planetary interiors. Further development of the technique by modifying the Galerkin method by the introduction of diagonal mode truncation is suggested to permit the consideration of higher values of the Rayleigh numbers, i.e., those more commensurate with terrestrial planet mantles.

Zebib, A.↗
NASA NTRS1985

Numerical simulation in three space dimensions of time-dependent thermal convection in a rotating fluid

Three-dimensional time-dependent convection in a plane layer of fluid, uniformly heated from below and subject to vertical shear and to rotation about an axis tilted from the vertical, was simulated by the numerical solution of the Boussinesq equations, including all Coriolis terms. Rotation about a vertical axis produces smaller convection cells with diminished heat fluxes and considerable vorticity. When the rotation axis is tilted from the vertical to represent tropical latitudes, the convection cells become elongated in a N-S direction. Imposed flows with constant vertical shear produce convective rolls aligned with the mean flow. When the rotation vector is tilted from the vertical, the competing effects due to rotation and shear can stabilize the convective motions.

Hathaway, D. H.↗
NASA NTRS1986

Space-laboratory and numerical simulations of thermal convection in a rotating hemispherical shell with radial gravity

The flight of the Spacelab 3 microgravity laboratory onboard the Space Shuttle Challenger in May 1985 enabled electroconvection experiments to be conducted using the goephysical fluid flow cell instrument. Experimental results are presented which illustrate the variety of convection achieved by varying the imposed radial and latitudinal temperature gradients, rotation rates, and the strength of the electrostatic gravity. These results are compared with those obtained from nonlinear three-dimensional simulations and good agreement is found.

Hart, John E.↗