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Abstract

Abbreviations BSD, bubble size distribution; PET, positron emission tomography; PIV, particle image velocimetry, RPT; radioactive particle tracking; NMR, nuclear magnetic resonance imaging IntroductionMultiphase flow systems have been applied extensively in chemical, physical, petrochemical and biochemical processing industries.1-3 Among different multiphase flow systems, bubbly flow is a two phase flow where small bubbles are dispersed or suspended in a liquid continuum. Typical features of this flow are moving and deformable interfaces of bubbles in time and space domains and complex interactions between the interfaces, and also between the bubbles and the liquid flow. Bubbly flows occur frequently in natural systems and are also used for different applications in energy-producing and chemical and petroleum industries. Some of the common applications involve bubble columns which are used as reactors in a variety of chemical and biochemical processes, e.g. the Fischer-Tropsch process for hydrocarbon synthesis, hydrogenation of unsaturated oil, coal liquefaction, fermentation, waste water treatment etc. 4,5 Bubbly flows are also ubiquitously found in flotation cells 6 and spargers 7 and have recently been investigated extensively for aeration studies. [8][9][10][11][12] Concentration, size and velocity distributions of solid or liquid particles suspended in gas or liquid are important in many processes such as meteorological research, biology, environmental protection, chemistry, medicine and agricultural engineering. Further, commercial activities such as conversion of natural gas to fuels and chemicals have prompted further fundamental research in fluid and bubble dynamics, transport phenomena and the effects due to scale up of three-phase fluidization systems.The prediction of pressure drop and wall heat transfer is necessary in many of these processes and these are strongly dependent upon the concentration, morphology and spatial distribution of the bubbles. 13 Similarly, in many liquid-gas systems, gases are dispersed in liquids to obtain large interfacial area for chemical reactions, heat and mass transfer processes. The rate of these processes is determined by bubble surface area flux which is closely linked to the bubble size distribution. 14 DiscussionDifferent techniques have been employed to measure bubble size distributions. Broadly, it can be divided into two categoriesintrusive and non-intrusive techniques.15 Both these methods have been extensively reported in the literature and a brief description of different intrusive and non-intrusive techniques is provided below, Intrusive techniquesConsiderable intrusive techniques have been developed to study bubble behavior in gas-liquid and gas-liquid-solid fluidized systems. These intrusive techniques include impedance (conductivity or resistivity) probes, capillary suction probes, 16 optical fiber probes, ultrasound probes, endoscopic probes, wire-mesh sensors 17 and hot film anemometry.The impedance probe has been applied to measure the bubble volume f...