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Part 1 of this paper which appeared in the August 1973 issue of Cryogenics dealt with nucleate pool boiling. This paper describes an experimental research programme to study the flow boiling of helium I with a view to its potential use in cooling systems for superconductive devices. This research includes investigation of flow boiling at both macroscopic and microscopic levels; the macroscopic studies being concerned mainly with saturated convective boiling and the microscopic studies with sub-cooled convective boiling.

Flow boiling crisis can be categorized into at least two types: Departure from Nucleate Boiling (DNB) in the subcooled and low-quality region and dry-out in the high-quality region. To analyze the ...

A semianalytic model is presented for the prediction of void fractions in subcooled flow boiling at elevated pressures. The model is based on the formation and growth of a bubble boundary layer adjacent to the heated surface at a rate determined by the difference between the imposed surface heat transfer and the heat removal capability of the subcooled liquid core of the flow. The latter heat transfer rate is determined by the analogy between heat and momentum transfer in the liquid employing empirical friction-factor data for low-quality two-phase flow. The analysis is compared to experimental results.

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.