Development of a planar velocity measurement technique for gas-solid particles in liquid flows using the characteristics of scattered light on phase surfaces

When measuring mixtures of three phase flows, such as bubbly liquid flows with tracer particles, a typical approach is the analysis of light projection images of the gaseous and solid phases (e.g. [1,2]). However, the discrimination of the phases for the extraction of their velocity distributions can become a difficult task when the bubbles and the tracer particles are similarly sized. A new non-intrusive measurement technique for the measurement of the liquid and gaseous velocity distributions of a three-phase mixture flows where the bubble and tracer size is comparable is proposed. The measurement arrangement aims for simplicity and employs a single-camera arrangement that receives light simultaneously from a Light Emitting Diode (LED) and a continuous-wave laser (Fig. 1). Due to the light scattering characteristics, continuous light illumination and constant exposure, the resulting images contain black streaks due to the shadow of the tracer particles and white streaks due to the imaging of the glare points [2] generated by the bubble-scattered light (Fig. 2). Interrogation of the streaks between consecutive images provides the velocity vector information for both the gaseous and liquid phase. The bubble size information can also be obtained via the glare point separation for each bubble. The technique was successfully applied into a microbubble-mixture transiently flowing in a container with velocity vectors exhibiting the expected behavior for each phase with an error margin of less than 2% for the gaseous phase rising to the top and less than 3% for the liquid phase sinking towards the bottom. The technique was also utilized for measurements in a device (Fig. 3) that uses swirling motion to achieve bubble void-fraction separation from the liquid mixture. Measurements close to the rim of the internal outlet gave qualitatively sound results, with bubbles accelerating towards the entrance of the outlet faster than the liquid phase (Fig. 4). However, further comparisons to established measurement techniques need to be made to solidify the applicability of the method for the measurement of the liquid phase velocity and bubble velocity and droplet size distributions.