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29 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

The nature of melt process changes in microgravity

The effects on melt impurities of the lack of buoyancy-driven convection in melts and the gas phase are outlined. Probable effects noted are: retardation of convective flow by a thin adsorbed impurity layer because of surface tension gradients (Marangoni flow); changes in the transport of volatile impurities to the melt surface and the consequent reduction of evaporation; and the change in dependence of evaporation rates on pressure due to the absence of convective flow in the gas next to the melt.

Wilcox, W.↗
NASA NTRS1981

Review of literature surface tension data for molten silicon

Measurements of the surface tension of molten silicon are reported. For marangoni flow, the important parameter is the variation of surface tension with temperature, not the absolute value of the surface tension. It is not possible to calculate temperature coefficients using surface tension measurements from different experiments because the systematic errors are usually larger than the changes in surface tension because of temperature variations. The lack of good surface tension data for liquid silicon is probably due to its extreme chemical reactivity. A material which resists attack by molten silicon is not found. It is suggested that all of the sessile drip surface tension measurements are probably for silicon which is contaminated by the substrate materials.

Hardy, S.↗
NASA NTRS1982

Thermocapillary flows and their stability: Effects of surface layers and contamination

The fluid mechanics and heat transfer of motions driven by surface tension gradients (Marangoni convection) were analyzed theoretically to obtain an understanding of the convection accompanying the process of growing high quality single crystals from the melt in a mu-g environment. The geometries considered include two dimensional liquid filled slots and axisymmetric float zone configurations.

Davis, S. H.↗
NASA NTRS1982

Use of thermocapillary migration in a controllable heat valve

In accordance with the Marangoni effect, immiscible droplets in a host fluid in which a temperature gradient exists move in the direction of increasing temperature. It is proposed that this thermocapillary migration could be used to construct a 'liquid wick' that would return the condensed vapor at the condenser end of a heat pipe back to the evaporator, thus completing the fluid circuit. The droplets would be formed by capillary pressure forcing the condensate through a perforated diaphragm whose temperature would control the droplet flux, and hence the heat flux between the two ends of the heat pipe, thus making it a controllable heat valve.

Schmid, L. A.↗
NASA NTRS1983

Thermocapillary migration of bubbles and droplets

A discussion is presented concerning the motion of droplets in a surrounding fluid induced by a temperature gradient. Such 'thermocapillary migration' may be characterized as the tangential stress at the interface that is induced by the temperature gradient's variation of interfacial tension, causing motion in the neighboring fluid both inside and outside the droplet. As a result, the droplet moves in the direction of the pole with the lower interfacial tension. An analysis is conducted in the creeping flow limit, for the case of small Marangoni number values. The method of matched asymptotic expansions is used for the solution of the field equations.

Subramanian, R. S.↗
NASA NTRS1983

Instabilities of dynamic thermocapillary liquid layers. I Convective instabilities

A steady shear flow is set up in a planar liquid layer bounded below by a rigid plate, and above by an interface with a passive gas, by imposing a temperature gradient along the layer and driving the motion by thermocapillarity. The thermal-convective instabilities to which this dynamic state is susceptible include stationary longitudinal rolls involving the classical Marangoni instability, and unsteady hydrothermal waves, whose new instability mechanism derives its energy from the horizontal temperature gradients. The thermal stability characteristics for liquid layers with and without return flow profiles are presently given as functions of the Prandtl number for the liquid and the Biot number for the interface, and comparisons are made with available experimental observations.

Smith, M. K.↗
NASA NTRS1984

Thermocapillary flows and their stability: Effects of surface layers and combination

The theoretical analysis of the fluid mechanics and heat transfer of motions driven by surface tension gradients (Marangoni convection) was researched. Convection accompanying the process of growing high quality single crystals from the melt in a micro-g environment was examined. The geometries considered include two dimensional liquid filled slots and axisymmetric float-zone configurations.

Davis, S. H.↗
NASA NTRS1995

The Benard Problem: A Comparison of Finite Difference and Spectral Collocation Eigen Value Solutions

The application of spectral methods, using a Chebyshev collocation scheme, to solve hydrodynamic stability problems is demonstrated on the Benard problem. Implementation of the Chebyshev collocation formulation is described. The performance of the spectral scheme is compared with that of a 2 nd order finite difference scheme. An exact solution to the Marangoni-Benard problem is used to evaluate the performance of both schemes. The error of the spectral scheme is at least seven orders of magnitude smaller than finite difference error for a grid resolution of N = 15 (number of points used). The performance of the spectral formulation far exceeded the performance of the finite difference formulation for this problem. The spectral scheme required only slightly more effort to set up than the 2 nd order finite difference scheme. This suggests that the spectral scheme may actually be faster to implement than higher order finite difference schemes.

J Raymond Lee Skarda↗