In recent years, air-bubble plumes have been applied as a hydraulic measure for water quality improvement to some Japanese water supply reservoirs. In order to optimize the hydraulic design of air-bubble systems, the relationship between mixing efficiency and system parameters must be clarified. Based on a review of previous studies, this paper discusses (1) analytical methods of air-bubble plumes in both homogeneous and nonhomogeneous environments, and (2) analytical methods of flow in lakes or reservoirs induced by air-bubble plumes. Examples of prototype designs for destratification and hypolimnetic aeration are presented. Future studies needed are summarized finally.
Flow reversal phenomena in natural-circulation boilers are studied analytically according to both the homogeneous-flow model and the slip-flow model. In the latter model Thom's pressure drop correlation is used. The pressure drop between the upper steam-water drum and the lower header is assumed not to be affected by inception of reverse flow in an evaporator tube which will be analyzed later. The mechanism of flow reversal is examined through analysis of the static flow characteristics of the evaporator tube in the non-dimensional form. Some important dimensionless parameters are introduced and their effects on inception of reverse flow are studied. It was found that the criterion of flow reversal is correlated practically with only three dimensionless parameters. Finally, an approximate method to predict flow reversal is put forward.
The velocity of single particles in air-water two-phase flows has been investigated. The geometrical shape of the test particles were sphere and cube; their size ranged in volume-equivalent diameter from 4 to 10mm; their density ranged from 1190 to 7850kg/m3. The volumetric fluxes of air and water varied within the ranges of 0≤jG<10m/s and 0≤jL<1.5m/s, respectively. The velocity of the particles in a vertical pipe of 26mm I.D. was measured using a pair of metal detectors. Both air and water velocities at which the particles float in the pipe were also measured. A simple model to correlate particle velocity with two-phase flow parameters is presented, based on the experimental data of spherical particles. The proposed model is compared with the data of cubic particles and shows good agreement.
When gas phase penetrates a liquid slug, velocity of the liquid slug deviates from that of Nicklin's correlation. Considering this phenomenon, a new concept of penetrant flow is introduced. Using penetrant flow as a flow pattern instead of churn flow, a flow, pattern map for gas-liquid two-phase flow was produced. Comparing published flow transition boundaries, slug to penetrant flow transition agreed with slug to churn flow transition. From the flow pattern maps for gas-liquid-solid three-phase flow, it was found that flow transition boundaries shifted with an increase in solid concentration.
Diesel spray injected into an atmosphere with high pressure and high temperature becomes, within a few milliseconds, a transient two-phase flow, entraining the surroundings. The motion of the surroundings is a significant phenomenon for the mixing of droplets and the entrained surroundings, and this phenomenon determines the combustion processes of diesel spray. In the experiment presented here, motion was visualized by the smoke-wire method. Moreover, the movement around a two-dimensional steady water spray (two-phase flow), and an unsteady water jet (single-phase flow), were also tested to generlize motion in the surroundings of the diesel spray. It was found that the entranining angle, the coefficient of entrainment velocity, and the entrainment coefficient of diesel spray at penetration part (where it shows characteristics of steady flow) are similar to those of two-dimensional steady water spray and the unsteady water jet, and that the ratio of the volume of the entrainment to the total volume of either the spray or the jet is nearly equal to 0.5.