Comparison and Analysis of the Condensation Benchmark Results

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Comparison and Analysis of the Condensation
Benchmark Results
W. Ambrosini, M. Bucci, N. Forgione, F. Oriolo,
S. Paci, J-P. Magnaud, E. Studer, E. Reinecke,
St. Kelm, W. Jahn, J. Travis, H. Wilkening, M.
Heitsch, I. Kljenak, M. Babic, M. Houkema, D.C.
Visser, L. Vyskocil, P. Kostka, R.
Huhtanen UNIPI (I), JSI (SLO), CEA Saclay (F),
NRG (NL) , FzJ (D), NRI Rez (CZ) , FzK IKET (D),
VEIKI (HU), JRC Petten (NL), VTT (FIN)
3rd European Review Meeting on Severe Accident
Research Nesseber, Hotel Vigo, Bulgaria 23 - 25
September, 2008
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Content

• Introduction
• Step 0 Condensation on an Isothermal Flat Plate
• Step 1 Condensation in the CONAN Facility
• Conclusions and Future Work

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Introduction

• Computational Fluid Dynamics codes are very
  promising for assessing the risk associated with
  the presence of hydrogen in power plant
  containments after a severe accident
• detailed description of flow patterns and gas
  distributions
• capability of a mechanistic approach to simulate
  basic phenomena
• Wall condensation is one of these relevant
  phenomena also influencing the levels of
  pressurization and atmosphere mixing
• engineering models based on the heat and mass
  transfer analogy
• CFD models
• completely mechanistic models based on vapour
  diffusion and low-Re models
• models based on wall functions and/or on
  engineering heat transfer coefficient evaluation

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Introduction (contd)

• The application of low-Re models needs a great
  detail in describing the near-wall region
• ? it is often impractical in describing
  real-life equipment
• Wall functions or engineering models for HTC are
  based on assumptions limiting their applicability
• ? an effort should be made to improve their
  reliability
• ? part of the useful information on flow fields
  from CFD is not used
• There is therefore strong motivation to go on in
  developing wall condensation models fro CFD
• In the frame of SARnet it was decided to analyse
  the state-of-the-art of models available to
  Partners and to propose a Benchmarking activity
  lead by the University of Pisa

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Introduction (contd)

• The activities on condensation in which the
  University of Pisa served as a coordinator where
  three
• Reporting on the models adopted within SARnet for
  condensation
• Proposing and analysing a 0th benchmark exercise
• code-to-code comparison in an idealised condition
• comparison with a heat and mass transfer
  correlation
• Proposing a 1st benchmark exercise in comparison
  with experimental data from the CONAN facility
• first data and spreadsheets already distributed
• more data to come in future steps

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Step 0 Condensation on an Isothermal Flat
Plate Problem Description

• This initial step dealt with an idealised version
  of the problem to be subsequently addressed on
  the basis of experimental data
• The objective of the 0th Step was to compare code
  results with correlations considered applicable
  to the addressed problem
• Reference was made to the 2D computational domain
  sketched in the Figure

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Step 0 Condensation on an Isothermal Flat
Plate Heat Transfer and Heat and Mass Transfer
Problems

• The domain had to be used in different ways
• in pure convective heat transfer calculations (no
  steam condensation), to assess the adequacy of
  the adopted turbulence models and numerical grids
  in front of the correlation
• in simultaneous heat and mass transfer
  calculations, whose results should be compared
  among each other and with the correlation

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Step 0 Condensation on an Isothermal Flat
Plate Different formulations for the Sherwood
number

• In heat and mass transfer cases, Participants
  were asked to calculate the Sherwood number at
  least according the two relationships
• As expected on the basis of the theories leading
  to these formulations, they provided very similar
  results
• Classical definitions were adopted for the
  Nusselt number

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Step 0 Condensation on an Isothermal Flat
Plate Boundary conditions

• The condensing plate is assumed to be kept at
  uniform temperature and saturated vapour
  concentration
• Atmospheric pressure conditions are addressed (as
  in the CONAN facility) for a saturated mixture
• Two values of mixture velocity are considered

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Step 0 Condensation on an Isothermal Flat
Plate Participants and models
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Step 0 Condensation on an Isothermal Flat
Plate Results for Heat Transfer Cases HT-30-3
Reasonable agreement with the correlation
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Step 0 Condensation on an Isothermal Flat
Plate Results for Heat Transfer Cases HT-30-6
Reasonable agreement with the correlation
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Step 0 Condensation on an Isothermal Flat
Plate Results for Heat and Mass Transfer Cases
HTM-30-3
Greater spread around the correlation
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Step 0 Condensation on an Isothermal Flat
Plate Results for Heat and Mass Transfer Cases
HTM-30-6
Greater spread around the correlation
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Step 1 Condensation in the CONAN
Facility Experimental Apparatus (1)

• The primary loop circulates an air-steam mixture,
  with a variable speed blower
• The secondary loop extracts water from a mixing
  vessels and circulates it into a 5 mm deep, flat
  rectangular channel behind the cooled plate
• The tertiary loop injects cold water from a large
  reservoir into the mixing vessel, returning an
  equal flow of warm water

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Step 1 Condensation in the CONAN
Facility Experimental Apparatus (2)
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Step 1 Condensation in the CONAN
Facility Addressed domain

• In the purposes of the exercise, the apparatus
  can be modeled as in the figure
• A 1D domain representing the center plane of the
  channel was suggested

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Step 1 Condensation in the CONAN
Facility Modelling the cooled plate

• Concerning the simulation of the cooled plate,
  two possible strategies were suggested, as
  already experimented by the University of Pisa,
  through internal preliminary calculations
• adopting a conjugated heat transfer approach,
  including heat conduction in the heated plate and
  energy transport in the channel
• imposing an equivalent heat transfer conductance
  between the cooled surface and the secondary
  fluid, obtained as reciprocal of the series of
  the heat transfer resistances of the plate and of
  the secondary fluid

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Step 1 Condensation in the CONAN
Facility Proposed approaches
Conjugated heat transfer Equivalent heat
transfer conductance
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Step 1 Condensation in the CONAN
Facility Experimental Uncertainties
Additional uncertainties are expected by the
limited steadiness of the process, requiring time
averaging of the relevant variables
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Step 1 Condensation in the CONAN
Facility Boundary Conditions
Five experimental data points were proposed
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Step 1 Condensation in the CONAN
Facility Obtained Results
A group of models (CEA, NRG, UJV, UNIPI and
VEIKI) provided very similar results, in
reasonable agreement with experimental data
negligible effect of plate representation
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Step 1 Condensation in the CONAN Facility Sample
Detailed Comparisons CEA Results

The observed tendency to underestimate the
overall condensation rate seems to be due to
limited accuracy in simulating boundary layer
development
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Step 1 Condensation in the CONAN Facility Sample
Detailed Comparisons UJV Results


This feature is common to all the models based
on low-Re capabilities and a mechanistic approach
for vapour diffusion the effect of simulation of
the falling film was weak
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Step 1 Condensation in the CONAN Facility Sample
Detailed Comparisons FzK Results



The near-wall model adopted by FzK in GASFLOW
provides a different heat flux distribution at
channel entrance
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Conclusions and Future Work General Outcome from
the Activity

• The two steps of the benchmarking activity
  performed up to now allowed assessing the
  behaviour of the models adopted in the frame of
  the Severe Accident Research Network
• The proposed problems were related to forced
  convection condensation, which is often of lower
  interest for containment analyses with respect to
  free convection
• However, the availability of the CONAN facility
  offered an opportunity to address in a systematic
  way the capabilities of codes in predicting
  experimentally observed behaviour

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Conclusions and Future Work Quantitative Aspects

• Though at different extents, most of the adopted
  CFD models are reasonably in agreement with the
  information at the basis of well known
  correlations
• Even in the idealised case of condensation over
  an isothermal flat plate, most codes provided a
  reasonable prediction of the expected behaviour
• The application to actual experiments revealed
  more details on the behaviour of models,
  highlighting a general tendency to underestimate
  entrance effects, whose reason is still matter of
  analysis (3D effects, boundary conditions)

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Conclusions and Future Work Challenges and Future
Perspectives

• The use of low-Re models is promising but still
  relatively expensive from the computational point
  of view if applied to large facilities or a full
  scale plant.
• Further efforts must be therefore spent in order
  to develop accurate but affordable techniques for
  predicting near-wall behaviour without loosing
  too much about the necessary quantitative details
  (e.g., FzK GASFLOW model)
• The rather enthusiastic adhesion in both steps of
  the activity encourages the University of Pisa to
  go ahead in proposing new experimental data, also
  on free convection