Development of Two-phase Velocity Measurement Using PIV Technique

  • 陳 威呈

Student thesis: Doctoral Thesis

Abstract

Optical techniques such as Phase Doppler Anemometry (PDA) and particle image velocimetry (PIV) have been applied to two-phase turbulent flow measurements PDA is a rather mature technique as compared to PIV allowing collection of statistical information of both fluid and dispersed-phase velocities as well as particle size However PDA is limited to a temporal pointwise measurements technique which is difficult to interpret the data into meaningful physical mechanisms controlling the inter-phase dynamics To reach a good understanding of two-phase flow the microscale spatial information of the inter-phase dynamics is essential In contrast PIV is an instantaneous whole-field measuring technique which makes it possible to detect spatial flow structure and provide information of the spatial differential quantities of turbulence such as vorticity dissipation rate of turbulent kinetic energy and even the direct indication of inter-phase couplings However the existing two-phase PIV measurement methods require high ratios of the large particles to the small particles (exceeding 30 times) in the two-phase flow in order to discriminate clearly between them in the images This limits the application of current PIV diagnostics to two-phase flow measurements A two-phase PIV measurement method is developed in the study to further minify the ratio of the large to small particles and upgrade the measurement capability A double-discriminating process in terms of gray level and size of image pattern together with the median mask technique is employed for discriminating the image patterns of the carrier and dispersed phases in the wake flow field To assure a statistically stationary result in PIV measurement it needs no less than 15 randomly located seedings in an interrogation window of the image However in the near-wake region of turbulent flow over an obstacle there exists a recirculation bubble zone right behind the obstacle The seedings (small particles) are usually difficult to penetrate into the core zone of very upstream near-wake subregion so that the statistically stationary measurement cannot assured The criteria based on gray level ratio of image are developed in the PIV measurements to check the attainment of statistically stationary results Since PIV must be made of seedings there actually exist two groups of particles in the investigated particle-laden flow in the study Two group of SiO2 particles are fed into the upstream flow region in front of the cylinder The finer one (ranging from 0 4 to 9 8 μm with a nominal mean size of 2 7 μm) serves as seeding role while the coarse group (ranging from 43 7 to 83 9 μm with a nominal mean size of 55 0 μm) which is incapable of following the carrier fluid motion faithfully represents the dispersed phase Two thresholds of gray level ratio are respectively set for the seedings (representing carrier fluid) and the dispersed phase (the laden particles) to assure statistically stationary results Two cases of wake flow with Red = 3 856 and 9 959 are separately investigated with four mass loading ratios of coarse particles including 0% (single-phase) 1% 3% and 5% It is found that the root-mean square fluctuating velocity for both stream-wise and lateral components of the carrier phase are either enhanced or attenuated through the loadings of particles in the near-wake regions In contrast the Reynolds stress of the carrier fluid is suppressed by loading the particles in the flow; and the suppression level is monotonously increased with the increment of mass loading ratio
Date of Award2018 Aug 2
Original languageEnglish
SupervisorKeh-Chin Chang (Supervisor)

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