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Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditions . Jean-Marie Le Corre Westinghouse Electric Sweden AB Carnegie Mellon University, Pittsburgh , USA. Outline. Introduction Visual experiments, Flow regime types and Flow regime map at DNB

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Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditions

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Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditions

Jean-Marie Le Corre

Westinghouse Electric Sweden AB

Carnegie Mellon University, Pittsburgh , USA


Outline

  • Introduction

  • Visual experiments, Flow regime types and Flow regime map at DNB

  • Postulated mechanistic modeling at DNB

  • Selected DNB model(s)

  • Model validation and applications (1D and 3D)

  • Conclusions and on-going work


Introduction

  • Boiling crisis is an important limiting parameter in boiling system

  • DNB = boiling crisis under subcooled boiling conditions

  • Physical modeling of DNB is not well established

  • Mechanistic DNB prediction is useful in development of new fuel design

  • Both 1D and 3D applications are desirable


Two-phase Flow Regimes at DNB


DNB visual experiments & flow regime map

  • Review of visual experiments available in the literature

  • Various flow regimes were reported

  • Dimensional analysis reveals relevant parameters

  • Consistent calculation of relevant local parameters

  • Preliminary map of flow two-phase flow regimes at DNB is established:

    • Low pressure

    • Limited geometric range

    • X and We as relevant parameters

  • More systematic experimental work is needed


DNB visual experiments & flow regime map

Three main types of flow regime at DNB:

Type 1: Bubbly flow

Type 2: Near-wall vapor clots

Type 3: Slug flow


DNB visual experiments


DNB flow regime map

Type 1

Type 2

Type 3


DNB Theoretical Modeling


Postulated mechanistic modeling at DNB

  • Various DNB physical modeling can be found in literature

  • Most used models relied on near-wall two-phase flow hydrodynamics only

  • Experimental evidence show that

    • Various flow pattern can exist at DNB (from “non-packed” bubbly flow to slug flow)

    • No near-wall macroscopic change at DNB

    • Wall effect (e.g. thickness) is important

  • Goal: Select model in agreement with experimental observations


DNB modeling in the literature

  • Theoretical studies:

    • Near-wall bubble crowding model (Weisman and Pei, 1983)

    • Liquid sublayer dryout model (Lee and Mudawar, 1988)

    • Many others…

  • Experimental studies:

    • Three main types of flow regime at DNB

    • Dry patch formation + quenching prevention has been mentioned for Type 1 and 3 (few theoretical studies)

    • Bubbly layer lift-off model (Mudawar) hypothesized for Type 2


Selected DNB model

  • Basic DNB modeling is based on a dry spot created under a nucleating bubble (1) or a vapor clots (2) or a vapor slug (3)

  • Temperature locally increases under dry area then decreases due to quenching

  • Quenching may be prevented in the limiting case (Leidenfrost)

  • Resulting dry patch may spread through radial conduction


DNB physical modeling (Type 1)

  • 2D transient wall thermal response to nucleation cycle is calculated (ADI scheme + transient heat flux boundary conditions)

  • Model consistent with wall boiling model is desirable

  • Most needed parameters are in use in wall partitioning model (e.g. RPI model)


DNB physical modeling (Type 1)

  • A limiting nucleation site is considered (stochastic nature)

  • Domain extend to as many “averaged” nucleation sites as necessary


DNB physical modeling (Type 1)

  • Needed (optional) constitutive relations:

    • Bubble departure diameter (& bubble growth rate)

    • Time of evaporation

    • Bubble departure frequency

    • Nucleation site density

    • Evaporation heat flux (transient form)

    • Quenching heat flux (transient form)

    • Limiting conditions


DNB Model Validation


Model validation and applications

  • Model validation

    • Limiting nucleation site is the key to the model

    • Use detailed boiling data at DNB (bypass most constitutive relations) to show Leidenfrost effect can happen

    • Model various CHF points (1D) to study the limiting nucleation site

  • Model applications

    • 1D

    • 3D


Model validation (Del Valle & Kenning data)

  • Detailed boiling information were reported form 70-95% of DNB (bubble departure diameter, bubble departure frequency, nucleation site density,…)

  • Parameters from most limiting nucleation site calculated from statistical distribution

  • Used for DNB model validations (Type 1)

  • Wall superheat around 100 C can be reached allowing for Leidenfrost effect (150 ± 50 C)


Model validation (Del Valle data)

  • 2D transient wall thermal response to Nucleation cycle

  • DNB occurrence

  • Wall thermal response immediately after DNB (dry patch spreading)


Model validation (Del Valle data)

  • 2D transient wall thermal response to Nucleation cycle

  • DNB occurrence

  • Wall thermal response immediately after DNB (dry patch spreading)

L

1

2

3


Model validation (Del Valle data)

Hot spot superheat

Dry patch radius

Time


Model applications (on going)

  • 1D applications: in progress…

    • Look-up CHF database

    • Study of limiting nucleation site

  • 3D applications:

    • Limited by current advances in the field

    • Prototype CFX-5.7.1 was used in simple geometry

      • Validated at high pressure

      • Validation at low pressure performed in this work

    • Applied to CFD experiments at high pressure (DeBortoli, 1958)

    • Approach to complex geometry and fuel assembly design


3-D CFX-5.7.1 validation (Bartel data)

Volumetric interfacial area

Void fraction


3-D DNB Model Applications (on-going)

  • DeBortoli data (1958)

  • Local We = 2426 at DNB, local x = -0.086

  • Type 2 region but probably Type 1 at high pressure

  • Modified Unal’s model was used

  • Limiting nucleation site = 2* “averaged” bubble diameter

  • DNB model application:

    • Peak wall superheat = 105 C for 0.5 mm SS heater

    • Peak wall superheat = 95 C for 1.0 mm SS heater


3-D application in complex geometries

  • Not in the scope of the current research program

  • No accurate prediction of CHF is expected

  • Correct parametric trends and correct treatment of 3D effects are expected

  • Approach:

    • Compute peak wall superheat in each near-wall computational cell (CFD post-processing)

    • Show local weak spot relative to DNB

    • Design goal is a low (& uniform) peak wall superheat


Conclusions

  • Different model of DNB can apply (compete) depending on conditions (pressure, We, x)

  • A “most likely” mechanism is identified for Type 1 (and Type 3)

    • Model is validated using detailed boiling data

    • Definition of limiting nucleation site is the key

    • Additional validations, 1D and 3D applications are on going

  • Accurate prediction of CHF is not expected

  • Help in increasing CHF performance of complex systems


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