Vapor Generation in the Two-Group Two-Fluid Model in Boiling Flow

Bottini, Joseph Larkin

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https://hdl.handle.net/2142/117676 Copy

Description

Title

Vapor Generation in the Two-Group Two-Fluid Model in Boiling Flow

Author(s)

Bottini, Joseph Larkin

Issue Date

2022-12-02

Director of Research (if dissertation) or Advisor (if thesis)

Brooks, Caleb S

Doctoral Committee Chair(s)

Brooks, Caleb S

Committee Member(s)

• Uddin, Rizwan
• Kozlowski, Tomasz
• Fischer, Paul

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Two-Fluid Model
• boiling
• phase change
• two-group

Abstract

Multiphase flows are utilized in a number of applications where phase change is necessary to provide high rates of heat transfer, such as in the cores of light-water reactors. Boiling flows in particular consist of complex, multiscale phenomena that are critical to model for the optimal design and safe operation of current and next-generation nuclear reactors. The Two-Fluid Model is a rigorous representation of multiphase flow systems, obtained by aggregating bubble behavior from the Local-Instant Formulation, eliminating the need for individual bubble interface tracking. As a result, the Two-Fluid Model is the framework upon which thermal-hydraulic and system-analysis codes rely. In lieu of interface tracking, the Two-Fluid Model depends on interfacial models for mass, momentum, and energy transfer between the phases. The most important of these is the mass transfer term as it determines the quantity of vapor in the system—represented through the void fraction—which in turn is present in nearly every term in the Two-Fluid Model. The mass transfer must be accurate if any other component of the Two-Fluid Model is to be used reliably. Boiling flows are characterized by a number of flow regimes which have different heat-transfer and pressure-drop characteristics. Historically, these have been modeled through flow regime transition criteria and smoothing functions between flow regimes. One alternate approach is the implementation of the two-group Two-Fluid Model which separates bubbles based on size, enabling different heat and mass-transfer correlations based on the bubble group. The additional flexibility in modeling comes at the cost of increased complexity, necessitating the modeling of intergroup transfer as well as interphase transfer. The two-group Two-Fluid Model mass transfer models have been developed for condensing and evaporating flows but not for boiling flows where the application is greatest. Local two-group two-phase and temperature data are collected in a boiling annulus facility for a range of pressure, inlet subcooling, mass flux, and wall heat flux. The data are collected at eleven radial locations and five axial locations over a three-meter heated length for eighty-eight conditions. In addition, the Point of Net Vapor Generation is identified for each condition, and high-speed videos are recorded for a subset of the conditions. The two-phase data and the images show a gradual progression from low-void-fraction, one-group flow—characterized by spherical bubbles—to two-group cap and slug flow containing both large and small bubbles. The area-averaged two-phase data are used to develop and evaluate a two-group boiling model. The boiling model builds from existing one-group models and partitions the contributions from the wall to the two groups. Combined with the intergroup models and the two-group Interfacial Area Transport Equation, the two-group boiling model predicts the formation and rise of group-2 vapor in the boiling channel without the need for flow regime transitions. The formation of group-2 vapor is driven by the intergroup term as bubbles coalesce and expand; the presence of group-2 vapor interface then drives the transfer to group-2 vapor directly. The model is most consistent with experimental results for high-pressure, low-heat-flux conditions where covariances between subcooling and void fraction are smallest. The coupled continuity-IATE equations with the two-group wall boiling model can predict total void fraction within ±25% well into the two-group region—up to 60–70% void fraction. The model also improves the results in the one-group region where the existing one-group models struggle to predict the location of significant void fraction increase. The vapor mass transfer model builds on the improvements to the two-group Two-Fluid Model beyond adiabatic flows to better predict the vapor mass transfer in two-phase flows.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Joseph L Bottini

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