Jean N. Koster

Thermocapillary convection in two immiscible liquid layers with free surface

Thermocapillary convection is studied in two immiscible liquid layers with one free surface, one liquid/liquid interface, and differential heating applied parallel to the interfaces. An analytical solution is introduced for infinite horizontal layers. The defining parameter for the flow pattern is λ, the ratio of the temperature coefficient of the interfacial tension to that of the surface tension. Four different flow patterns exist under zero gravity conditions. ‘‘Halt’’ conditions which halt the fluid motion in the lower encapsulated liquid layer have been found. A numerical experiment is carried out to study effects of vertical end walls on the double layer convection in a two‐dimensional cavity. The halt condition obtained from the analytical study is found to be valid in the limit of small Reynolds numbers. The flow in the encapsulated liquid layer can be suppressed substantially.

Visualization of liquid-solid interface morphologies in gallium subject to natural convection

Abstract Melting and solidification of pure gallium is studied using real-time radioscopy methods. Sharp contrast between melt and solid phases with a 3% density difference is illustrated. The interfacial shape reflects convective flow in the liquid phase. Comparisons are made between real-time non-intrusive X-ray observations and results gained from probing techniques and numerical simulations reported in the literature. Similar qualitative trends in temperature distribution, yet discrepancies in interfacial location and shape, are exhibited.

Density inversion effect on transient natural convection in a rectangular enclosure

Transient natural convection in a water layer subjected to density inversion is studied numerically by a finite element method. The results illustrate that the temperature difference which determines the position of the maximum density plane in the water layer, can alter flow field and heat transfer substantially. The significant effect of aspect ratio on transient natural convection is also investigated. The heat transfer is maximized in a square enclosure and is less at other aspect ratios.

Radioscopic visualization of indium antimonide growth by the vertical Bridgman-Stockbarger technique

Abstract A new vertical Bridgman-Stockbarger furnace with the capability for real-time melt/crystal interface visualization is demonstrated. Radioscopic visualization requirements are considered in the furnace design. The semiconductor material indium antimonide (InSb) is melted and then solidified from below. Sharp contrast of the 10.3% density difference for liquid and solid InSb is obtained in a cylindrical ampoule.

Visualization study of melting and solidification in convecting hypoeutectic Ga-In alloy

Natural convection flow visualization experiments during melting and solidification of a hypoeutectic gallium five weight percent indium were performed using a real time X-ray radioscopy technique. The experiments revealed a conductive-convective threshold for the onset of natural convection in the alloy. The threshold appears to be caused by a substantial chemical stratification of indium in the melt. The observed gravitational segregation has been evaluated and compared to theoretical models, which, however, do not predict the observations.

In situ visualization of constitutional supercooling within a Bridgman-Stockbarger system

Abstract Constitutional supercooling is studied within the semiconductor material indium antimonide (InSb) during vertical crystal growth from the melt. The starting mixture is stoichiometric In 50 Sb 50 . Translation rates are as low as 2 μm/s. Solid-liquid interface morphology and axial interface positions are monitored in real-time with radioscopic X-ray visualization. The solid-liquid interface temperature is assessed from calibrated thermocouple measurements at the ampoule wall. Results indicate substantial constitutional supercooling of the melt. Elemental segregation is assessed from the visualization of the interface location in a measured temperature profile and from the phase diagram. Temperature data suggest that the interface is in contact with a melt composition close to In 62 Sb 38 (based on the 525.7°C melting temperature of α-InSb in a specified phase diagram). That composition reflects significant gravitational segregation in the melt.

Steady natural convection in a two-layer system of immiscible liquids

Abstract Thermal convection in two horizontal layers of immiscible liquids that are differentially heated from the side is studied analytically, numerically and experimentally. The flow in the two layers is thermally and mechanically coupled. Flow in both layers is visualized and the flow pattern is found to be in agreement with theoretical predictions. Flow velocities in the core region of the cavity are found to be in quantitative agreement with theory.

Natural convection of water in a rectangular cavity including density inversion

Abstract Two-dimensional natural convection in water with density inversion is studied numerically in a rectangular cavity. The non-Boussinesq parabolic density-temperature relationship is incorporated in a finite element model. Numerical results are obtained for Rayleigh numbers up to 10 6 . The evolution of the temperature field and flow pattern show that density inversion and initial location of the maximum density surface within the liquid have a determining effect on convection character. The investigation of aspect ratio on flow character is also presented. It is found that interactive convection across the density inversion surface is dependent on aspect ratio and Rayleigh number.

Hybrid Electric Integrated Optimized System (HELIOS) Design of a Hybrid Propulsion System for Aircraft

This paper discusses select design aspects and early testing results for the HELIOS project. The objective of this project was to complete preliminary research and design of a hybrid electric propulsion system for unmanned aerial vehicles. Efficiency models showed that a mechanically additive system would provide the greatest benefits when compared to other additive system designs. Utilizing a clutch-less gearing system to mechanically combine the output power of an internal combustion engine with an electric motor, a 15% increase in fuel efficiency over traditional gas powered UAVs of similar size was projected. Preliminary efficiency tests using a custom-built dynamometer have shown that this model will be satisfactorily accurate. The technology provides also additional operational safety in case of engine failure. Operating the aircraft in electric mode only allows for low noise operation during any phase of the operation. Preliminary testing of the HELIOS system was performed on the 13 ft wingspan aircraft being constructed by students from Daniel Webster College located in Nashua, New Hampshire.

Steady natural convection in a double layer of immiscible liquids with density inversion

Natural convection of a layered fluid system composed of two immiscible liquids, silicone oil on top of water, is studied numerically. The flow in the two layers is viscously and thermally coupled. Two counter-rotating natural convection roll cells of opposite vorticity develop when one side wall temperature in the density inversion fluid is above and the other below the density inversion temperature. In contrast, only one roll cell develops in a liquid layer with Boussinesq properties. The viscous coupling between both immiscible layers strengthens the roll cell with opposite vorticity in the density inversion layer. At large Rayleigh number the flow pattern in the density inversion layer becomes very complex. The largest heat transport occurs in the upper Boussinesq layer. The two-roll cell pattern in the density inversion layer is impeding the total horizontal heat transfer. A vertical heat transport exists across the interface from the density inversion layer into the encapsulating upper layer. The moving interface between the immiscible liquids improves the heat transfer in each layer when compared to the cavity cases.