Twisted Wall Research Articles

Twist walls in nematic liquid crystals

Abstract An approximate solution is derived for the structure of a twist wall between two regions of opposite 180° twist in a nematic liquid crystal film. The result is used to show that a twist wall is unstable unless the elastic constants of the nematic are such that K22≤ ½(K11 + K33). Thus the observation of such walls in nematic liquid crystals is evidence of this elastic anisotropy. Experimental evidence is given for the validity of the approximate solution obtained.

Domain walls in bubble films. II. Static properties of thick films

The structure of static walls is analyzed assuming that the film thickness is much larger than the wall width. It is shown that several different types of wall structure can exist. For the normal wall type the twist angle (angle between the wall magnetization at the midplane of the film and at the film surface) is approximately π/2. For the first anomalous type this angle is approximately −3π/2, for the second type approximately 5π/2, etc. The width and energy of such walls are calculated using (i) a Bloch-line approximation and (ii) a sinusoidal variational trial function (as in Paper I of this series). The ratio of the energy of a twisted wall (E) to the energy of an untwisted wall (E0) can in all cases be represented as E/E0=(1−βσ)1/2, where σ=4πM0/Ha [M0 is the saturation magnetization, Ha is the anisotropy field]. According to the first method β=0.692 for the normal wall type and β=0.692− (π/c)(D/2πM)1/2 for the first anomalous type. Here 2c is the film thickness, D=2A/M0, and A is the exchange stiffness constant. According to the second method β=0.654 for the normal wall type, and β=0.319 for the first anomalous type.

Subcooled Swirl-Flow Boiling and Burnout With Electrically Heated Twisted Tapes and Zero Wall Flux

A series of swirl-flow tests was conducted in which all of the heat was generated in twisted-tape swirl generators. This is in contrast to past ORNL swirl-flow tests with twisted tapes, in which ∼99 percent of the heat was generated in the metallic tube wall. In the present study, water from a constant-head tank flowed by gravity at 5 to 8 fps through a vertical 0.27-in.-ID glass tube ∼13 in. long, in which was located a resistance-heated, 16-mil-thick A-nickel tape. Tape-twist ratios were varied from 2.7 to ∞ inside tube diameters/180-deg twist, inlet water temperatures from 63 to 173 F, and heat fluxes from 0.21 × 106 to 1.20 × 106 Btu/hr·ft2. The water head above the top of the tube was held at 30.7 in. In all cases, the critical wall superheat increased with decrease of tape-twist ratio, whereas the critical heat fluxes for the twisted tapes fell between 93 percent and 122 percent of those for flat tapes, maximizing in all cases at a tape-twist ratio of 7 to 10. It is postulated that the deleterious effect of centripetal acceleration with this geometry, which tends to hold the vapor on the heated surface, is compensated in the swirl-flow entrance region by inertial impingement of the liquid onto the tape surface, and along the remainder of the length by a double-vortex secondary flow pattern in the plane normal to the tube wall. The power density of a swirl-flow tube assembly may therefore be significantly increased by generating heat in the twisted tape as well as in the tube wall.