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Implementation of HVAC heat transfer
research into CFD software
Implementation of HVAC heat transfer
research into CFD software
Dr Yehuda SinaiProject Manager Fire, Safety & HVACANSYS UK Ltd.
yehuda.sinai@ansys.com
Dr Yehuda SinaiProject Manager Fire, Safety & HVACANSYS UK Ltd.
yehuda.sinai@ansys.com
CIBSE NVG seminar, London,18 November 2008CIBSE NVG seminar, London,18 November 2008
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Contents
Introduction
Window sub-grid model Web-enabled CFD
Conclusions
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Introduction
ANSYS.
Developer of ANSYS family of products. Family of products includes
ANSYS Mechanical (structural FE)
CFX (general-purpose CFD and FSI) FLUENT (general-purpose CFD)
Airpak (customised for HVAC)
AUTODYN (blast, structural response) and more.
This presentation will focus on CFX.
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Window sub-grid model
Window assemblies involve solid & fluid
conduction, fluid convection, and radiation in gasand semi-transparent solids.
CFX offers CHT (Conjugate Heat Transfer).
It is possible to model conduction and radiation(grey or non-grey) in glass using CFX.
However, the computational cost can be
excessive for multiple glazing, largewindows/facades, or for a large number ofwindows.
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Window sub-grid model
For this reason, the author has been developing
1-D sub-grid window models, for incorporation asboundary conditions in CFD.
The model
takes the analysis further than the obviousapproximations such as U + transmissioncoefficient;
is a combined-mode theory. Sinai, Int. J. Ventilation, Vol. 2. No. 2, 2003.
The work is ongoing.
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Window sub-grid model
Schematic - double glazing
T3Exterior
T4
Ta
T2T1
Solid 1Air gap
Solid 2
Fluid
domain
L1 L2 x
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Window sub-grid model
Grey or non-grey.
Panes 1 & 2 subject to volumetric heating = Q1 &Q2 W/m
3.
In Solid 1 & Solid 2: Conduction + radiation.
In gap and outside: Radiation and correlation forconvection.
)1().( QTk = Aim: Produce Cauchy relationship between
convective flux and T1 (i.e. heat transfer
coefficient).
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Window sub-grid model
Solving the conduction equation for each solid, and
applying continuity of temperature & convective flux,leads to
)2(
)2
(
2
)2
(
)(0
)(23
023)(
224
22
11
1
11
4
3
2
4
2
2
2
2
2
223
2
2
23
1
1
+
+
=
+
+
+
LQTh
LQ
LQ
L
Tk
T
T
T
hL
k
L
k
L
kh
L
kh
hhL
k
aaa
h23and h4aare the convective + longwave radiationHTCs between solids 1 & 2 and between surface 4
and the ambient.
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Window sub-grid model
Solving these three equations leads to
where
TTUq s )3()(~~
1=
aa
rras
a
hk
LR
hhk
L
k
LR
RRTTTT
hhk
L
k
LRU
42
22
4232
2
1
11
2211
4232
2
1
11
1
2;
11
2
)4(;
11~~
+=+++=
+=+=
+++==
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Window sub-grid model
R is recognizable as the single-mode series
resistance. Ts is the effective sink/source temperature, equal
to the ambient temperature plus a radiant
temperature displacement. This embodies themultimode effects.
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Window sub-grid model
Radiant absorption in slabs.
Model can be grey or no-grey, but mustaccount for spectral effects.
Allowing for multiple reflections between slabs
(E denotes due to external radiant source, qEis the incident external flux),
)5()1(...)1()( 21
212
2
2
121211
=+++=
E
EE
q
q
)6()1(
1)(21
2122
+=
EE q
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Window sub-grid model
Similar expressions for absorption due to
radiation arriving from the interior. Verification:
Transparent slabs: Classical overall resistance
obtained for single-mode resistances in series. Transparent slabs and vanishing slab
resistances (e.g. thin slabs): T1 T2 , as it
should.
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Window sub-grid model
Example:
Two slabs, L = 6 mm, k = 1.4 W/m/K h23 = 5 W/m
2/K, h4a = 30 W/m2/K, = 0.
Qi=0, Qe=1000 W/m^2
0
10
20
3040
50
60
70
80
90
0 0.2 0.4 0.6 0.8 1
Absorptivity
T
r(K)
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Web-enabled CFD
Flkt Woods has commissioned ANSYS UK Ltd.
to develop two customized web-enabled CFDtools:
Room ventilation (software namedSimulateAir).
Car park ventilation and fire.
This followed validation of ANSYS-CFX againstFlkt Woods own experiments (Sinai et al,
ROOMVENT 2004). ANSYS UK has customized a web tool named
EASA, in collaboration with EASA Ltd.
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Web-enabled CFD
Problem definition is performed locally.
Small quantity of data sent to server over internet,and entire process performed in batch there.
ANSYS has developed a customized geometry
tool. Pre-selected results, in html format, returned over
web.
Flkt Woods has coupled SimulateAir to IDA (azone model).
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Web-enabled CFD
Example from SimulateAir. Chilled beams (Box Method).
CourtesyFlktWoods
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Web-enabled CFD
Example from SimulateAir.
CourtesyFlktWoods
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Web-enabled CFD
Example from SimulateAir.
CourtesyFlktWoods
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Web-enabled CFD
Car park ventilation & fire. Postulated example.
CourtesyFlktWoods
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Acknowledgements
The web-enabled tool, based on the EASA
software, has been developed in collaborationwith Flkt Woods, with the aid of EASA Ltd.
The customization was carried out by my
colleague M. Owens.
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Conclusions
Whilst software and hardware improvements
continue apace, challenges remain. Sub-models such as that described here will
therefore continue to play a role.
Web-enabled CFD simplifies the simulationprocess and facilitates wider usage.
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