1. No category

Particle depletion in a differentially heated
cavity
Jarmo Kalilainen
SAFIR2014 FINAL SEMINAR, 20.3.2015
Introduction
• Aerosol behaviour in a containment building during an accident
scenario has an importance on the mitigation of the FP release to the
environment.
• Large scale SA experiments (e.g. Phebus FPT2-3) observed relatively
large deposition to vertical containment walls
Possible effects of turbulent natural convective flow to the particle wall
deposition?
• The aim of this study is to study fluid and heat transfer and particle
retention in a Differentially Heated Cavity (DHC). The experimental
results are used in validation of a large eddy simulation (LES) of the
DHC and Lagrangian particle tracking (LPT) simulations.
DIANA Facility
•
DIfferentially heated cavity with Aerosol in turbulent NAtural
convection (DIANA) facility.
•
The facility has two vertical isothermal aluminium walls and
four adiabatic glass walls.
•
The facility must allow optical access for laser-based
measurement devices used to determine the flow properties as
well as particle deposition rates
•
In order for the Boussinesq approximation, used in the
simulation work to be valid the heat difference between the
walls must remain below 50 K.
•
Fluid: air, T = 57 °C - 18 ° C = 39 ° C, Rayleigh number Ra ~ 109.
Turbulent flow.
PIV
Cavity
Two part study
• Experimental study:
Measurement of flow field and gas temperature in the
DIANA cavity
Measurement of particle deposition rates using
monodisperse SiO2 particles with diameters 1 µm and 2.5
µm.
• Computational study:
Validation of the LES model using experimental temp.
boundary conditions (BC).
Comparison of measurement to Lagrangian particle tracking
data obtained using the validated LES model.
Particle image velocimetry (PIV)
Mean and fluctuating velocity x-y components of the
flow field measured near the cavity lateral center
plane using a PIV.
•Imaging measurement of velocity fields of particles in a transparent fluid
•Flow visualization
•Double pulse laser and double frame CCD-camera
•Analysis of flow fields from two images taken at short interval
Fluid velocity
A series of 120 PIV measurements at the lower
left corner, 330 mm from the front adiabatic wall.
An average velocity, calculated from 1200 PIV
images at the lower left corner, 330 mm from the
front adiabatic wall.
Fluid velocity
Vmag=
+
=
•
Mean velocity magnitudes and the turbulence intensity at the cavity center plane.
•
Turbulent flow encircling a stagnant core next to the isothermal and horizontal walls.
LES of the cavity
•
Two LES with different BCs at the
horizontal wall were used in the particle
tracking simulations.
•
In the first LES (LES-WT), the thermal BCs
for isothermal and horizontal walls were
obtained from the wall temperature
measurement data.
•
The second simulations using idealized
adiabatic BCs on horizontal walls (LESIDEAL), was validated against the DNS
data by Puragliesi (PhD thesis, EPFL, 2010) with
similar conditions.
•
The LES-WT validated using the
measurement data.
•
Flow and temp. field differed with LESIDEAL – Flow geometry different, less
turbulent.
Horizontal and vertical temperature profiles measured with K-type
thermocouple, compared against LES-WT and LES-IDEAL simulation data.
Both show stratified temperature distribution at cavity core.
LES of the cavity
Comparison between PIV measurement of mean vertical and horizontal
velocity components and LES-WT simulation.
Rms of velocity along horizontal and vertical profiles from PIV
measurements and LES-WT simulations .
LES of the cavity
Velocity magnitude
LES-IDEAL
PIV
Turbulence kinetic
energy
LES-IDEAL
LES-WT
Particle deposition measurements
•
Monodisperse SiO2 (1 and 2.5 µm)
particles seeded from the bottom of
the cavity.
•
The change of particle concentration
was investigated by introducing a
laser sheet to the cavity through the
top or front glass wall and measuring
the intensity of the reflected light
from the particles by a CCD camera.
•
Tapered Element Oscillating
Microbalance (TEOM) was used in
addition to laser intensity
measurements.
Particle deposition measurements
Experiment matrix.
•
Exp.
Particle
diam.
[µm]
Laser
window
placem.
Exp.
Particle
diam.
[µm]
Laser
window
placem.
1
1.0
xz hot
6
2.5
xz hot
2
1.0
xy hot
7
2.5
xy hot
3
1.0
xy
centre
8
2.5
xy
centre
4
1.0
xy cold
9
2.5
xy cold
5
1.0 (dry
atm.)
xz hot
Particle distribution in the cavity direction
(experiments 1 & 6).
•
Particles approx. uniformly distributed to
the cavity at the lateral direction.
•
The results indicated uniform deposition
rates throughout the cavity atmosphere.
•
No effect of dry atmosphere on particle
depletion rate was observed in exp. 5.
Particle tracking simulations
Simulation matrix.
1 µm particles
LES-IDEAL-CRW tp ON
LES-IDEAL-CRW tp OFF
LES-WT-CRW tp ON
LES-WT-CRW tp OFF
LES-WT tp ON
2.5 µm particle
LES-IDEAL-CRW tp ON
LES-IDEAL-CRW tp OFF
LES-WT-CRW tp ON
LES-WT-CRW tp OFF
LES-WT tp OFF
number
of particle
thermophoretic
force considered
computation
ended [s]
10000
10000
10000
10000
10000
Yes
No
Yes
No
Yes
~6300
~6300
~6300
~6300
~1000
10000
10000
10000
10000
100000
Yes
No
Yes
No
No
~2300
~2300
~2300
~2300
~1700
•
In Continuous Random Walk (CRW) simulations the fluctuating
fluid velocity is modelled using a Markov chain based on the
normalized Langevin equation which takes into account the
inhomogeneities of the turbulence (Dehbi, 2008). The mean flow and
temperature fields, along with the average Reynolds stresses were
extracted from LES and used to calculate the fluctuating velocity
component.
•
CRW compared against pure LES particle tracking. Good match.
•
Particle tracking simulations made
using the LES-WT with realistic BCs
and LES-IDEAL with ideal adiabatic
BCs at horizontal walls.
•
Thermophoretic force (tp)
considered in some simulations.
Particle tracking simulations
•
In the stirred settling case, particles are kept uniformly
distributed in a cubic volume with side length L, and are
deposited only through gravitational settling to the
enclosure floor (Hinds, 1999).
•
The inclusion of thermophoretic force to the simulation
has only relatively small effect on the deposition speed.
Only effects on distribution of deposit on different
surfaces.
Average decay constants from the laser intensity and mass
concentration measurements, particle tracking simulations and
form the stirred settling calculation.
Comparison of particle decay from the cavity
atmosphere with dp = 1 µm particles.
dp = 1 µm
dp = 2.5 µm
Average , TEOM [s]
5220 ± 190 s
1100 ± 90 s
Average , laser intensity [s]
4970 ± 60 s
1800 ± 80 s
LES-WT-CRW tp ON
4890 s
1510 s
LES-WT-CRW tp OFF
5060 s
1520 s
LES-WT tp (1 µm ON) (2.5 µm OFF)
5530 s
1610 s
LES-IDEAL-CRW tp ON
6360 s
1860 s
LES-IDEAL-CRW tp OFF
6920 s
1840 s
Theoretical Stirred Settling (Hinds, 1999)
10210 s
1780 s
Summary
•
Flow and temperature measurements were performed in DIANA cavity with
turbulent natural convective flow.
LES model using DIANA cavity BCs produced valid representation of the flow and temp.
fields.
•
Aerosol depletion in DIANA cavity was investigated experimentally using
monodisperse SiO2 particles with dp = 1 µm and dp = 2.5 µm.
Depletion rates from Lagrangian particle tracking with realistic BCs at the cavity walls
agreed well with the measurement data.
•
Comparison to theoretical stirred settling indicated large discrepancy with 1 µm
particle.
LPT with ideal boundary conditions resulted on slower particle depletion due to altered
flow geometry and decreased level of turbulence in the cavity.
Stirred settling not applicable in DHC geometry even in the cavity core where the flow is
almost stagnant.