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Two-Phase Flow Dynamics
Published in Neil E. Todreas, Mujid S. Kazimi, Nuclear Systems Volume I, 2021
Neil E. Todreas, Mujid S. Kazimi
The slip ratio itself is affected by the pressure (or density ratio) as well as by the profile of the void fraction within the cross-sectional area. As shown later, the macroscopic slip ratio can be obtained from a knowledge of the local phase velocity ratio (or microscopic slip) and the void distribution profile. The local slip can be obtained as a specified value or on the basis of a flow-regime-dependent approach.
Fundamentals of Two-Phase Flow in Nuclear Power Plants
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Sometimes air–water mixtures are used to simulate the behavior of steam–water mixtures. This is because it takes a great deal of heat to accurately replicate two-phase flow at the temperatures and pressures in a reactor core, while using air avoids this difficulty entirely because it is plentiful, easy to use, and cheap. However, air–water flows do not behave in exactly the same way as steam–water flows because steam is denser than air. At high vertical flow rates, the difference in their behavior is small, but at low vertical flow rates, it can be significant. Thus, the pressure drop correlations do not behave in exactly the same way for up-flow as they do for downflow, and it is not always appropriate to infer the motion of steam–water mixtures from the motion of air–water mixtures. Except for highly turbulent flow, where the Reynolds number is high, the liquid and vapor phases do not move at the same speed, and generally speaking, the vapor moves past the liquid. To describe this behavior, a slip ratio is used. In reactor work, the slip ratio S is defined as the ratio of the velocity of the vapor phase to the velocity of the liquid phase.
Boiling Water Reactors
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
Normally, the slip ratio has a value greater than 1 because the vapor (i.e., the steam) always moves faster than the colder liquid does. The slip ratio in a BWR fuel assembly can vary from 1.0 to about 4.0. When the slip ratio is greater than 1.0 (which is usually true near the top of the core), the equation relating the void fraction to the quality is
Assessment of the CUPID Code for Bubbly Flows in Horizontal Pipes
Published in Nuclear Technology, 2018
Dong Hun Lee, Seungjin Kim, Han Young Yoon, Jae Jun Jeong
Thus, the slip ratio should be less than or close to 1. On the other hand, the particle-averaged momentum equation predicts a slip ratio very close to 1. It is also noted that at r/R = 0.2, the void fraction in the results of the standard momentum equations is nearly zero, but the slip ratio is 1.023. This is physically unreasonable. Meanwhile, the particle-averaged momentum equations predict a slip ratio of nearly 1.0 at r/R = 0.2. Although the particle-averaged momentum equation cannot fully capture the aforementioned characteristics of a horizontal bubbly flow, it can deliver physically reasonable results in comparison with the standard one. Thus, the particle-averaged momentum equation was selected for the code assessment.
Void Fraction Measurement of Gas-Liquid Two-Phase Flow Based on Empirical Mode Decomposition and Artificial Neural Networks
Published in Heat Transfer Engineering, 2019
Weiwei Wang, Khellil Sefiane, Gail Duursma, Xiao Liang, Yu Chen
When there is a high gas–liquid slip ratio there is a large velocity difference between two phases and thus, a small variation of gas–liquid phase interface weakens the influence of bubbles on the liquid flow. Therefore, the void fraction is underestimated.