VOID FRACTION AND BUBBLE SIZE MEASUREMENT INSIDE CLOUDS OF AIR BUBBLES IN FLOWING WATER

The mechanics of bubble clouds are essential to many industrial processes in the energy and chemical realms. In the specific case of hydroelectric turbines, bubble clouds are present when air is injected into the water to increase dissolved oxygen levels in the water flowing through the powerplant. Understanding the physics of the phenomena driving the mixing of bubbles is still needed to improve the modeling of such flows. Even in less complicated geometries such as those found in many laboratory flows, measurements of bubble cloud evolution are sparse and validation of numerical models is difficult. Descriptions of the evolution of bubble clouds in simple geometries can then be used to predict the distribution in more complex geometries.

This study aims at providing a dataset of bubble cloud evolutions in a simple flow geometry. The experimental setup consists of a rectangular test section of 20 cm height by 10 cm width wherein water flows can reach velocities up to 2 m.s^-1. Air is injected transverse to the liquid flow through air injection slots at the bottom of the test section. The air volumetric flow rate is from 0% up to 30% of the liquid volumetric flow rate. This range of air to water volumetric flow rate ratio encompasses what is required in hydroelectric turbines in order to increase dissolved oxygen to an adequate level. The test section is instrumented with several pressure transducers and a double optical probe is used to measure void fractions, air velocities and bubble sizes. The double optical probe can be located at any place inside the two-phase flow. This proven technology has been developed at Polytechnique Montreal over several decades. Each optical fiber is calibrated and manufactured at our laboratory. Our probes are also used on a daily basis at Atomic Energy of Canada Limited’s Chalk River laboratories.

Measurements are made on several planes normal to the flow velocity. The upstream plane is located just downstream of the air admission slot. Measurements of void fractions, bubble velocities and sizes in these planes are presented in the form of maps. Comparison of these maps allows us to characterize the evolution of bubble clouds for this geometry. The flow maps can then be used in the validation of numerical models of bubble clouds evolution.