atw 2018-09v3


atw Vol. 63 (2018) | Issue 8/9 ı August/September



| | Fig. 3.

Comparison of simulations with empirical contact angle model and the laboratory experiments by Becker

Technologies [5] (in false color representation) with mass flow rate ṁ = 11 g/s (ṁ =12 g/s for inclination

of 10°), three different inclinations (left 2°, middle 10°, right 20°) and without aerosol loading.

given water velocity parallel to the

surface such that a specified mass flow

rate is achieved. The flat plate is

bounded by vertical sidewalls and

has an inclination angle α. Material

properties of water and air are used.

For snapshots of the resulting flow

fields see Figure 3.

The simulations are conducted

with the empirical contact angle

­model and the filtered initial

­randomized contact angle field [6].

The contact angle is specified in the

boundary conditions of the water field

and is taken into account to calculate

the curvature of the water-air interface.

The contact angle has a huge

impact on the formation of rivulets

and their stability as shown in previous

studies [7]. The empirical contact

angle model accounts for the

wetted history and therefore enforces

a spatially and temporally stable

­rivulet flow.

4 Simplified geometry

This study also considers a simplified

geometry with dimensions of 6 cm

x 5 cm, 60° inclination and different

water loadings. As a first step the

­simplified geometry, for which additional

benchmark data from CFD

simulations and experiments are

available, is used for the parameter

variation to save computational effort

and time. Later the findings are

transferred to the larger laboratory

geometry. Also the experimental

data can be used to investigate the

empirical contact angle model [6] in

another scenario than the laboratory

geometry where it was developed. For

the simplified geometry Singh et al.

[8] provide results of CFD simulations,

as do Hoffmann [9] and Iso et.

al [10]. Experiments are conducted by

Ausner [11]. All of the latter use the

identical geometry, but different inlet

conditions (overflow weir and feed

tube) and various simulation tools

(Singh and Iso Fluent, Hoffmann

CFX). In the present study simulations

with constant contact angle and with

empirical contact angle model are

performed. The results for different

Weber numbers are evaluated and

compared to the results of the studies

mentioned above for validation. Five

different Weber numbers (We = 0.02,

We = 0.24, We = 0.47, We = 0.76 and

We = 1.10) are investigated, which

correspond to an increasing water

mass flow rate:

We =

with liquid density ρ l , inclination

angle α, volumetric mass flow rate Q,

surface tension σ, plate width W

and viscosity μ. As the water load

­increases, the flow pattern changes

from a thin rivulet to a more pronounced

rivulet to a fully wetting

­water film (see Figure 4).

The influence of the side walls

is also clearly visible and was also

observed by Hoffmann [9] and Ausner

[11]. With a constant contact angle of

70° (which is the value frequently

quoted in the literature for the material

combination water on steel) the

percentage of wetted area in the

present CFD calculations and in the

calculations of Hoffmann and Iso

tends to be underestimated, whereas

the similar setup of Singh yields, for

an unknown reason, larger values

of wetted area. In Figure 5 the

measurements are shown as blue

triangles, the results of Hoffmann, Iso

and Singh in purple, yellow and green,

respectively, and the current calculations

with the different contact angles

in red, gray and black. The simulations

with constant contact angle and

empirical contact angle model with

70° for a dry surface and 50° for a wet

one still slightly underestimate the

wetted surface. With the empirical

contact angle model 30°/70° the

results are very well within the variation

of the experiments.

5 Wash-off model

The particle wash-off consists of a

two-stage process. First the sedimented

particles on the plate floor are

| | Fig. 4.

Comparison of simulations with empirical contact angle model with

θ dry = 70° and θ wet = 30° for different Weber numbers We. The water height

is indicated by color.

| | Fig. 5.

Normalized wetted surface A wn for different Weber numbers. Blue triangles

indicate the experimental results; results of CFD simulations are displayed

with differently colored lines.

AMNT 2018 | Young Scientists' Workshop

Development and Validation of a CFD Wash-Off Model for Fission Products on Containment Walls ı Katharina Amend and Markus Klein

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