yehuda sinai

8/2/2019 Yehuda Sinai

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2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

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.

[email protected]

Dr Yehuda SinaiProject Manager Fire, Safety & HVACANSYS UK Ltd.

[email protected]

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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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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Schematic - double glazing

T3Exterior

T4

Ta

T2T1

Solid 1Air gap

Solid 2

Fluid

domain

L1 L2 x


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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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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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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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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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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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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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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.

yehuda sinai

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