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Numerical Study of Cavitation Within Orifice Flow

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Numerical Study of Cavitation Within Orifice Flow NUMERICAL STUDY OF CAVITATION WITHIN ORIFICE FLOW A Thesis by PENGZE YANG Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Chair of Committee, Robert Handler Committee Members, David Staack Prabir Daripa Head of Department, Andreas Polycarpou December 2015 Major Subject: Mechanical Engineering Copyright 2015 Pengze Yang ABSTRACT Cavitation generally occurs when the pressure at certain location drops to the vapor pressure and the liquid water evaporates as a consequence. For the past several decades, numerous experimental researches have been conducted to investigate this phenomenon due to its degradation effects on hydraulic device structures, such as erosion, noise and vibration. A plate orifice is an important restriction device that is widely used in many industries. It serves functions as restricting flow and measuring flow rate within a pipe. The plate orifice is also subject to intense cavitation at high pressure difference, therefore, the simulation research of the cavitation phenomenon within an orifice flow becomes quite essential for understanding the causes of cavitation and searching for possible preventing methods. In this paper, all researches are simulation-oriented by using ANSYS FLUENT due to its high resolution comparing to experiments. Standard orifice plates based on ASME PTC 19.5-2004 are chosen and modeled in the study with the diameter ratio from 0.2 to 0.75. Steady state studies are conducted for each diameter ratio at the cavitation number roughly from 0.2 to 2.5 to investigate the dependency of discharge coefficient on the cavitation number. Meanwhile, a study of the flow regime transition due to cavitation is also carried out based on the steady state results. Moreover, a transient study is done to clarify the relationship between cavitation at the orifice and cavitation at the downstream pipe wall. Several conclusions can be made from this numerical study. The discharge coefficient of the orifice plate is independent of cavitation number; As cavitation number decreases, cavitation tends to be more intense and the flow regime transit to super cavitation eventually; In addition to cavitation occurring at the orifice edge, it also initiates at downstream pipe wall. ii TABLE OF CONTENTS Page ABSTRACT ....................................................................................................................... ii TABLE OF CONTENTS .................................................................................................. iii LIST OF FIGURES............................................................................................................ v LIST OF TABLES ........................................................................................................... vii CHAPTER I INTRODUCTION ........................................................................................ 1 CHAPTER II OBJECTIVES ............................................................................................. 6 CHAPTER III APPROACH .............................................................................................. 7 III.1 Theoretical Models ............................................................................................... 7 III.1.1 Continuity Equation ................................................................................... 7 III.1.2 Conservation of Momentum ...................................................................... 9 III.1.3 Cavitation Model ...................................................................................... 11 III.2 Fluent Adapted Momentum and Cavitation Model ............................................ 12 III.2.1 k-epsilon Model ....................................................................................... 12 III.2.2 Schnerr-Sauer Model ............................................................................... 13 III.3 Numerical Algorithm .......................................................................................... 14 III.4 Geometry and Boundary Condition Setup in FLUENT ...................................... 15 CHAPTER IV TURBULENT AND CAVITATION MODEL VALIDATION ............. 16 IV.1 Turbulence Model Validation ............................................................................. 16 IV.1.1 Case Description ...................................................................................... 16 IV.1.2 Results and Comparison .......................................................................... 18 IV.2 Cavitation Model Validation ............................................................................... 28 IV.2.1 Case Description ...................................................................................... 28 IV.2.2 Results and Comparison ........................................................................... 30 CHAPTER V NON-DIMENSIONALIZATION STUDY .............................................. 35 CHAPTER VI RESULTS AND DISCUSSIONS............................................................ 38 VI.1 Steady State Study of Cavitation Effect on Discharge Coefficient .................... 38 VI.1.1 Case Description ...................................................................................... 40 iii Page VI.1.2 Results and Discussion ............................................................................ 41 VI.2 Steady State Study of Flow Regime Transition due to Cavitation ..................... 50 VI.2.1 Case Discription ....................................................................................... 52 VI.2.2 Results and Discussion ............................................................................ 52 VI.3 Transient Study of Cavitation Inception and Development ................................ 57 VI.3.1 Case Description ...................................................................................... 57 VI.3.2 Results and Discussion ............................................................................ 57 CHAPTER VII CONCLUSION ...................................................................................... 62 REFERENCES ................................................................................................................. 64 iv LIST OF FIGURES Page Figure 1 The shock-wave mechanism and the micro-jet mechanism of the cavitation erosion ............................................................................................................... 2 Figure 2 Restriction orifices. .............................................................................................. 4 Figure 3 ASME standard sharp edge orifice with D and D/2 pressure tapping. ................ 5 Figure 4 CV for continuity equation .................................................................................. 8 Figure 5 CV for net momentum flux analysis .................................................................... 9 Figure 6 CV for net stress analysis ................................................................................... 10 Figure 7 CV for vapor transport equation ........................................................................ 12 Figure 8 Geometry of fluid domain and boundary condition setup ................................. 15 Figure 9 Geometry and boundary condition setup ........................................................... 17 Figure 10 Mesh of flow domain ....................................................................................... 18 Figure 11 Axial velocity along the radial direction at variable locations in the pipe comparison. ..................................................................................................... 19 Figure 12 Normalized mean axial velocity profile comparison ....................................... 21 Figure 13 Non-dimensional turbulence kinetic energy profile comparison ..................... 22 Figure 14 Non-dimensional vorticity profile comparison ................................................ 23 Figure 15 Geometry and mesh of 3D model .................................................................... 24 Figure 16 Normalized pressure and velocity comparison ................................................ 25 Figure 17 Normalized mean axial velocity profile comparison ....................................... 26 Figure 18 Non-dimensional TKE comparison ................................................................. 26 Figure 19 Non-dimensional vorticity comparison ........................................................... 27 v Page Figure 20 Geometry and mesh of the orifice .................................................................... 29 Figure 21 Experiment and simulation results of downstream wall pressure vs inlet pressure ............................................................................................................ 31 Figure 22 Discharge coefficient vs cavitation number ..................................................... 31 Figure 23 Normalized pressure and axial velocity distribution along centerline ............. 33 Figure 24 Vapor volume fraction contour comparison .................................................... 34 Figure 25 Simulation result of the discharge coefficient in terms of cavitation number and diameter ratio ............................................................................... 45 Figure 26 Experiment result of the discharge coefficient in terms of cavitation number and diameter ratio .............................................................................
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