||Development of Two-phase Velocity Measurement Using PIV Technique
||Department of Aeronautics & Astronautics
由於影像上的粒徑尺寸差距小，如要明顯辨識需要運用灰階(gray level)來進行篩選。在此採用不同的灰階值標準並搭配中值濾波器(median mask)，來區分同一張影像中的連續(carrier-phase)與分散相(dispersed-phase)顆粒，並運用此影像辨識法來獲得可靠的連續相與分散相速度量測。
PIV演算法在演算窗格中(interrogation windows)需要具備15顆粒子以上才能達上統計上的穩定，但在迴流泡區域(recirculation bubble zone)粒子通常很難滲入非常上游的核心區域(near-wake region)，因此本研究欲藉由實驗中發展出的灰階值門檻法去判斷是否達到統計上的穩定，進而取代尾流區後方顆粒不足的PIV判斷演算法。
欲縮小大小顆粒倍數差距，更進ㄧ步把灰階值門檻判斷法應用在兩相紊流場速度PIV量測中，同時加入兩組粒徑分佈之SiO2粉末，其中分散相之平均粒徑為 54.9 μm，連續相之平均粒徑為 2.7 μm，從不同分散相顆粒濃度(α = 0, 1, 3, 5%)中，探討其複雜的兩相紊流交互效應。
流場實驗條件分別為雷諾數3,856 和 9,959，經由改變不同分散相顆粒濃度0(單相), 1, 3, 5%中，根據論文研究結果統整發現方均根擾動速度隨著近尾流區域中顆粒的加載會有增強或衰減的現象。雷諾應力卻隨著顆粒濃度增加以至於增加了紊流抑制力進而呈現遞減的現象。
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.
第一章 緒論 iii
第二章 實驗設備量測系統 iv
第三章 實驗規劃與PIV單相流結果比較 vi
第四章 兩相PIV量測法 vii
第五章 結論與未來建議 viii
LIST OF TABLES xv
CHAPTER 1 INTRODUCTION 1
1.1 Literature review 4
1.1.1 X-type hot wire anemometer 4
1.1.2 PIV (single-phase) 6
1.1.3 PIV (two-phase) 8
1.2 Motivation and Objective 15
CHAPTER 2 EXPERIMENTAL EQUIPMENT AND MODELS 21
2.1 Experimental Facilities 21
2.1.1 Wind tunnel 21
2.1.2 Seeding system 22
2.1.3 Traverse mechanism 23
2.2 Velocity Measurement Instrumentation 23
2.2.1 Particle Image Velocimetry (PIV) 23
2.2.2 Image analysis software 25
2.2.3 X-type hot-wire anemometer 26
2.2.4 Experimental conditions 26
2.3 Error Analysis 27
CHAPTER 3 EXPERIMENTAL PLAN AND PROCESS 43
3.1 Experimental plan 43
3.2 Wind tunnel quality inspection 44
3.3 Experimental process 44
3.3.1 X-type hot-wire anemometer 44
3.3.2 Single-Phase PIV 44
3.3.3 Two-Phase PIV 45
3.4 Single-phase data analysis 45
3.4.1 X-type hot-wire anemometer measurements angle 46
3.4.2 Calculation methods of probe 46
3.4.3 Cross wire angle restriction filter 47
3.4.4 PIV measurements 48
3.5 Results and discussion 51
CHAPTER 4 METHODOLOGY OF TWO-PHASE PIV MEASUREMENT 67
4.1 Two-Phase Discrimination Process For the cases with 5% Mass Loading Ratio 68
4.2 The Phase Discrimination Results for Various Mass Loading Ratios 74
4.3 Two-phase Interactions under Various Mass Loading Ratios 76
4.3.1 Turbulence statistics of the carrier phase 77
4.3.2 Turbulence statistics of the dispersed phase 80
4.4 Summary 81
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 135
5.1 Concluding remarks 135
5.2 Recommendations for Future Work 136
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