Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditio...
Download
1 / 26

Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditions - PowerPoint PPT Presentation


  • 284 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Flow Regimes and Mechanistic Predictions of Critical Heat Flux under Subcooled Flow Boiling Conditions' - parvani


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Slide1 l.jpg

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 l.jpg
Outline Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
Introduction Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg

Two-phase Flow Regimes at DNB Flux under Subcooled Flow Boiling Conditions


Dnb visual experiments flow regime map l.jpg
DNB visual experiments & flow regime map Flux under Subcooled Flow Boiling Conditions

  • 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 map6 l.jpg
DNB visual experiments & flow regime map Flux under Subcooled Flow Boiling Conditions

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 l.jpg
DNB visual experiments Flux under Subcooled Flow Boiling Conditions


Dnb flow regime map l.jpg
DNB flow regime map Flux under Subcooled Flow Boiling Conditions

Type 1

Type 2

Type 3


Dnb theoretical modeling l.jpg

DNB Theoretical Modeling Flux under Subcooled Flow Boiling Conditions


Postulated mechanistic modeling at dnb l.jpg
Postulated mechanistic modeling at DNB Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
DNB modeling in the literature Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
Selected DNB model Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
DNB physical modeling (Type 1) Flux under Subcooled Flow Boiling Conditions

  • 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 114 l.jpg
DNB physical modeling (Type 1) Flux under Subcooled Flow Boiling Conditions

  • A limiting nucleation site is considered (stochastic nature)

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


Dnb physical modeling type 115 l.jpg
DNB physical modeling (Type 1) Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg

DNB Model Validation Flux under Subcooled Flow Boiling Conditions


Model validation and applications l.jpg
Model validation and applications Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
Model validation Flux under Subcooled Flow Boiling Conditions (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 l.jpg
Model validation (Del Valle data) Flux under Subcooled Flow Boiling Conditions

  • 2D transient wall thermal response to Nucleation cycle

  • DNB occurrence

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


Model validation del valle data20 l.jpg
Model validation (Del Valle data) Flux under Subcooled Flow Boiling Conditions

  • 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 data21 l.jpg
Model validation (Del Valle data) Flux under Subcooled Flow Boiling Conditions

Hot spot superheat

Dry patch radius

Time


Model applications on going l.jpg
Model applications (on going) Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
3-D CFX-5.7.1 validation (Bartel data) Flux under Subcooled Flow Boiling Conditions

Volumetric interfacial area

Void fraction


3 d dnb model applications on going l.jpg
3-D DNB Model Applications (on-going) Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
3-D application in complex geometries Flux under Subcooled Flow Boiling Conditions

  • 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 l.jpg
Conclusions Flux under Subcooled Flow Boiling Conditions

  • 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


ad