Yasunari Maeda

Time evolution of diameter of micro-bubbles generated by a pressurized dissolution method

Size distributions of micro-bubbles and concentrations of dissolved oxygen in water in a square duct downstream of the decompression nozzle were measured to investigate the time evolution of bubble diameter and the mass transfer of dissolved gas between bubbles and water after bubble generation in a pressurized dissolution method. A numerical simulation based on the Rayleigh-Plesset equation was also carried out to predict time evolutions of bubble diameter and concentration of dissolves gas. The validity of the prediction was discussed through the comparison between the predictions and the experiments. As a result, the following conclusions were obtained: (1) When cavitation does not occur in the decompression nozzle, few micro-bubbles are generated at the nozzle and the mass transfer rate between bubbles and water in the downstream region of the nozzle is low due to a low interface area concentration. The mass transfer due to bubble nucleation is negligibly small in the downstream region of the decompre...

Influence of Dissolved Gas Concentration on Diameter and Number Density of Micro-Bubbles

A micro-bubble generator based on a pressurized dissolution method can generate fine micro-bubbles with a high bubble number density. The mean diameter and the number density of micro-bubbles depend on the concentration of dissolved gas in the upstream region of a decompression nozzle. In this study, the mean diameter and the number density of micro-bubbles in the downstream region of the decompression nozzle are measured by using a phase Doppler anemometry (PDA) to examine effects of the dissolved gas concentration on the mean diameter and the number density of micro-bubbles generated by the pressurized dissolution method. Experiments are conducted for several dissolved gas concentrations in the upstream region of the decompression nozzle under bubble cavitation and sheet cavitation conditions in the decompression nozzle. The experimental results indicate that, (1) influence of the dissolved gas concentration on flow patterns in the decompression nozzle (bubble cavitation and sheet cavitation) is weak, (2) the Sauter mean diameter of micro-bubbles increases as dissolved gas concentration increases, (3) the gradient of Sauter mean diameter with respect to the dissolved gas concentration in a sheet cavitation case is higher than that in a bubble cavitation case, (4) micro-bubbles are generated even when the dissolved gas concentration is lower than the saturated concentration at the atmospheric pressure provided that a sheet cavitation takes place at the nozzle, and (5) the bubble number density increases with the dissolved gas concentration in bubble cavitation cases whereas it does not depend on the concentration in sheet cavitation cases.Copyright