Assessment of the applicability of different turbulence models in CFD simulations for the description of turbulent free jets during biomass combustion

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22 Φεβ 2014 (πριν από 3 χρόνια και 6 μήνες)

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Assessment of the applicability of different turbulence models in CFD
simulations for the description of turbulent free jets during biomass
combustion



M. Miltner
1
, C
.
Jordan
1
, M. Harasek
1

1
Vienna University of Technology
,

Institute of Chemical Engineerin
g
,

Thermal Process
Engineering
-

Computational Fluid Dynamics
, G
etreidemarkt 9/166
,
A
-
1060 Vienna, Austria
;
tel: +43
1

58801

166276
; email:
martin.miltner
@tuwien.ac.at


Straight or ro
tating turbulent free jets

are of great relevance in flows encountered in

modern
process industry. In a current research project, such free jets are applied to stimulate the
heterogeneous combustion of baled herbaceous biomass. Thus, the combustion of the
biogenous fuel is strongly intensified resulting in improved efficiency a
nd higher carbon
burnout.

As the developed combustion concept is highly innovative and strongly depending
on the flow in the combustor, computational fluid dynamic simulations (CFD) are extensively
used

during the initial development phase.

One of

the mo
st fundamental and important factor
s

in CFD simulations is the adequate
modelling of turbulent flow.
Various

turbulence models with numerous
sub
-
options are already available in
modern CFD solvers.

All these
models have their distinct application
range and

show significant
contribution to the overall
computational efforts.

The current
wor
k analyses the applicability of
seven popular turbulence models for
the description of a straight and a
slightly rotating turbulent free jet.
Thus, an extensive analysis ap
plying
the commercial CFD
-
solver
FLUENT


has been performed on
t
w
o different jet geometries
.

The simulation results
of the analysed

models
for the

simple case example
s

show substantial discrepancies

(see
Fig. 1

and Fig. 2
) for velocities

and
turbulence par
ameters.

These
simulation
results are comprehensively compared to own measurements (performed
on
a
lab
-
scale test
-
bench
with Laser
-
Doppler
-
anemometry) and
to literature data in order to elaborate
the applicability of the analysed turbulence models and thei
r sub
-
options. Additionally, the
degree of uncertainty in
jet
flow predictions and the errors to be expected
for

less appropriate
models are determined.

This work gives significant basic information for a
sound
CFD simulation of the complex
biomass combust
ion apparatus. Furthermore, it provides
sufficient data to estimate the trade
-
off between accuracy and computational efforts
a
nd proves to be a

helpful guideline for CFD
-
engineers during their
examinations of flow problems in industrial practise.

Fig.1
:
J
e
t bou
n
dary surface predicted with Standard
-
k
-

-
浯摥氠mle晴f⁡湤⁓ST
-
k
-

-
浯me氠⡲ig桴h

摩de湳楯n汥獳⁡硩慬x
癥汯捩瑹⁶
x
/v
x
(0,0) [
-
]

dimensionless jet length x/d [
-
]

0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
0
5
10
15
20
25
30
Fig. 2
:
D
imensionless axial velocity vs. jet lengt
h

(measurements and simulations)