Date of Award

2025

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical and Aerospace Engineering

Advisor 1

Kazuya Tajiri

Committee Member 1

Fernando Ponta

Committee Member 2

Amitabh Narain

Committee Member 3

Paul L. Bergstrom

Abstract

Two-phase liquid-gas flow has many applications in the nuclear industry, including active heat exchangers, reactor core designs, secondary steam generators, and two-phase flow loop balance of the plant. The void fraction is a crucial parameter to determine the pressure drop, flow regimes and liquid levels inside the nuclear reactor core. For the past forty years, void fraction has been studied by many researchers by different measurement techniques. Each method has positive and negative attributes. In this study, after extensive literature reviews, the author has chosen a capacitance type void fraction sensor over other void fraction measuring techniques. The principal concept of a capacitance type void fraction sensor lies in the change in permittivity of a two-phase mixture that is caused by the change in void fraction. The capacitance is a proportional function of the permittivity of the dielectric. Capacitance is largest for all liquid, smallest for all gas and in between for a mixture. The first part of this research is to construct a self-made capacitance sensor and an unique vertical, air-water flow calibration loop. The sensor consists of rectangular acrylic duct made of four side plates with dimensions 6 inches x 4 inches x 0.2 inches (length x width x thickness) and one square base with the edge of 5 inches. Eighteen fully threaded copper screws served as electrodes are attached to two opposite plates of the sensor. The capacitance is measured by eighteen electrodes. A guard electrode is placed in between two measuring electrodes to minimize edge effects and stray capacitance. Vertical two-phase flow is generated with porous xiv plugs machined on the bottom surface of the test section. Capacitance signals are recorded by a high-speed capacitance meter that can measure small capacitance in the order of pF. Data was collected covering a wide range of void fractions, from approximately 0.0 to 1.0. Flow regimes encountered included bubble, slug and annular flow. The second part of this research is to build a finite element model (FEM) to simulate experimental environment for the evaluation of the capacitance. The FEM was built by Finite Element Heat Transfer (FEHT). The FEM consists of 6,256 triangular nodes. The measured capacitances for two boundary cases: empty test-section and filled with water are 8.31 pF and 29.97 pF, respectively. The FEM predicted capacitances for two boundary cases within 90% accuracy. Both simulation and experiment show a similar trend that capacitance monotonically decreases with void fraction. A third objective of this study is to create a new mathematical model capable of accurately predicting two-phase void fraction using only the channel geometry, liquid and vapor mass flow rates, and the properties of the working fluid. The predicted void fraction is validated by void fraction data collected using an in-house capacitance sensor and a unique vertical, air-water flow calibration loop. Compared to measured void fraction data, the new mathematical model has a better performance than commonly used models such as Lockhart Martinelli model, Rouhani-Axelsson model, Wallis model and homogeneous model.

Additional Files
PhD_Dissertation_CAPACITANCE SENSOR_final_good_3.pdf (9526 kB)
STUDY OF TWO-PHASE VOID FRACTION IN A RECTANGULAR CHANNEL USING CAPACITANCE SENSOR
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