Mistakes in Applying Computational Fluid Dynamics

Professor W.K. Chow Fellow, Hong Kong Academy of Engineering Sciences

Chair Professor of Architectural Science and Fire Engineering

Director, Research Centre for Fire Engineering

Head of Department, Department of Building Services Engineering

The Hong Kong Polytechnic University, Hong Kong, China

Founding President, Society of Fire Protection Engineers – Hong Kong Chapter

President, Asia-Oceania Association for Fire Science and Technology

CPD14-2e.ppt

CPD Lecture Room Z2-003 7:00 – 8:00 pm, 31 July 2014

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Topics Covered

1. Introduction

2. Heat Release Rate

3. Free Open Boundary Condition

4. Functional Analysis

5. Grid Size Variation

6. Example Cases: Time Line Analysis

7. Observations

8. Recommendations

9. Conclusion

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1. Introduction

4

With the rapid development of economics in the Far

East, many big construction projects were found.

The new designs have difficulties to comply with the

fire codes.

Performance-based design (PBD) was then applied

to determine fire safety provisions, particularly for

underground subway stations in urban areas

without much space.

Cost reduction is another key reason in using PBD.

5

Resources limitation: Free software of

Computational Fluid Dynamics (CFD) was applied

in hazard assessment in many construction projects.

No full-scale burning tests were carried out to

justify the CFD predicted results.

The free CFD software Fire Dynamics Simulator

(FDS) is commonly used to study fire-driven fluid

flow.

It was developed by National Institute of Standards

and Technology (NIST) and frequently used in

solving practical fire problems.

6

Authorities having jurisdictions (AHJ) are now

more knowledgeable in fire science and engineering.

Many officers are well-trained and possess a master

degree in fire engineering.

CFD Results are starting to be challenged.

Many new project submissions based on CFD for

fire hazard assessment were NOT approved.

Some validation works:

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Bad

8

Good

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10

Problems identified by fire officers in using CFD in

the Far East are:

Mainly accepted for smoke control design, many

doubts on fire simulation such as open kitchen in

small residential flats of very tall buildings and

wood houses.

FDS was commonly used, but not yet justified for

application in huge space with tall halls such as

public transport interchanges.

Air pressure, turbulence parameters seldom

presented.

Full-scale burning tests on typical scenarios with

similar conditions are necessary.

11

Similar criticisms for journal papers reviewing raised

by fire research workers on CFD results are:

Application to simulate fire needs to watch because

fire phenomena are substantially three-dimensional

and very unstable in the flow and temperature fields

of the buildings concerned.

Three-dimensionality and instability of the fire-

induced flow fields are not fully discussed.

Ability of CFD to resolve the flow in the turbulent

fire plume.

Ability of CFD to resolve the turbulent exchange

flow through the opening.

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Although consulting engineers believe in such CFD-

FDS predictions, a very tight inspection scheme was

implemented on new project applications based on

CFD.

Hot smoke tests required in atria of irregular shape

or taller than 12 m in some places.

13

Common mistakes made in PBD projects with

railway and subway systems based on CFD as

pointed out by different parties including fire

research workers, users, fire officers and journal

paper reviewers will be discussed in this

presentation.

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2. Heat Release Rate

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In fire safety design, the most important parameter

is the heat release rate (HRR), which is the single

most important variable in characterizing the

“flammability” of products and their consequent

fire hazard.

It gives information on fire size, fire growth rate,

available egress time and suppression system

impact.

The potential for ignition of nearby items, flashover

potential in a room, and the amount of water

needed to extinguish the fire can be estimated.

The evolution of HRR with time becomes the most

important input variable which must be estimated

properly for fire simulations.

16

Heat release rate of burning an object in the

enclosure should be agreed carefully in fire hazard

assessment.

Normally, only a small accidental fire was assumed

to break out in an empty enclosure.

Design Fire Down to 0.5 MW in a Railway Hall

years ago !

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Apple Daily 29 Jan 2011

Train cars as an example.

Beware of low values, say up to 6.5 MW for train

car was originally proposed even in the draft

version of the new fire safety (FS) code of Hong

Kong.

In Hong Kong, there are many parallel traders in

some stations.

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Pointed out by Chow well before 2004 that

combustible luggage should be reduced:

W.K. Chow, “Fire safety in train vehicle: Design based on

accidental fire or arson fire?”, The Green Cross,

March/April, 7 pages (2004).

19

Now up to 20 MW, good enough ?

At least 35 MW by SP recently !

Code of Practice for Fire Safety in

Buildings April 2012

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SP Report 2011: Scale model studies !

February 2014

New works on train fires by Ingason, SP

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3. Free Open Boundary Condition

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In applying CFD to building fire hazard assessment,

there are always windows and doors open to outside.

Bi-directional air flows were observed in

experimental studies, with hot gas flowing out and

cool air coming into the room.

The boundary conditions of the flow parameters,

particularly pressure, have to be specified carefully.

There are empirical correlations relating the

pressure profiles across the vertical openings such as

windows or doors, under different room geometries,

heat release rates of the fires, and opening sizes.

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In FDS simulations, the opening boundary

condition was taken as a passive opening to the

outside like a door or window.

An OPEN boundary was set on the exterior

boundary of the computational domain.

The outflow of opening boundary condition for the

momentum equation was determined by simplifying

the pressure in terms of the velocity vector, pressure

perturbation and density.

pp

~

2

2

u

The pressure is set to the ambient pressure by the

user, which is defaulted to zero.

p~

Outflow

Inflow

Inside

Outside

Neutral plane

Velocity profile on the free boundary

25

26

For inflow, the fluid element on the boundary has

been assumed to be accelerating from the state

along a streamline.

The flow field was calculated by Bernoulli

equation.

It assumes that pressure is zero infinitely far

away.

At the boundary between two grids, the pressure

boundary condition is similar to that at an

external open boundary.

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But pressure is taken from the adjacent grid

where the flow is incoming.

Outflow and inflow are separately set on the

opening vent.

This cannot simulate the real free boundary

condition because the position of neutral plane

cannot be determined.

Such boundary conditions might not give proper

specification.

A better approach is to extend the computing

domains outside.

Some pioneering work on fire modelling

demonstrated that the flow pattern in the vicinity of

doorway was entirely different if the free boundary

had not been extended sufficiently.

Markatos and co-workers (1984) extended the flow

domain to the ‘free boundary’ region outside the

doorway when studying the smoke flow in

enclosures and obtained results that agreed

reasonably with experimental data.

Markatos N.C. and Cox G., “Hydrodynamics and Heat Transfer in enclosures

containing a fire source,” PCH PhysicoChemical Hydrodynamics, 5 (1984)

53-66.

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Galea and associates (1994) pointed out in their

case study on simulating fire development in an

aircraft that it is desirable to extend the solution

domain outside the fire compartment in order to

find physically realistic behaviour in the vicinity

of the open doors.

Mawhinney R.N., Galea E.R., Hoffmann N. and Patel M.K., “A critical

comparison of a PHOENICS based fire field model with experimental

compartment fire data”, Journal of Fire Protection Engineering, 6 (1994)

137-152.

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Schaelin and co-workers (1992) pointed out that

extending the computing domains outside was a

better approach when simulating plume flow.

Schaelin A., van der Maas J. and Moser A., “Simulation of airflow through

large openings in buildings”, Proceedings of the ASHRAE Winter Meeting,

Anaheim, Calif, USA pp. 319-328, January 1992.

Schaelin A., van der Maas J. and Moser A. (1992), “Simulation of airflow through

large openings in buildings” ASHRAE Transactions, Vol. 98, No. 2, pp. 319-328.

Need to extend

very far to

outside

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32

In applying FDS, Hadjisophocleous and Ko (2009)

suggested that the impact of the open boundary at

the exterior of the computational domain was

minor when the boundary had been extended up

to 2 m outside a geometry of width 10 m.

Therefore, it may not be necessary to extend the

computational domain to some distance beyond

the opening to obtain good results while using

FDS version 4.07.

They also pointed out that this situation is rather

complicated and should be evaluated for different

cases.

Hadjisophocleous G. and Ko Y.J., “Impact of various parameters on the CFD

predictions of atrium smoke management systems”, ASHRAE Transactions

115 Part 1 (2009) 263-270.

A Local Example

Chow and Chow (2009)

A 5-level glass façade geometry of 15 m tall.

A fire was located in a compartment at level 3 with

heat release rate 1 MW or 5 MW.

The glass was broken with hot gases spreading out.

The computing domain was extended to 4 m outside.

The computing domain was divided into 80 by 40 by

120 or 384,000 parts.

Chow C.L. and Chow W.K., “A brief review on applying computational fluid

dynamics in building fire hazard assessment,” A Chapter in Fire Safety, I.

Søgaard and H. Krogh (Eds.), Nova Science Publishers, 2009.

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Free boundary

Fire

6 m

10 m

3 m

3 m

3 m

3m

3m

1 m

1 m

6 m

1 m

(a) Elevation (b) End view

The geometry of a glass façade

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(a) Initial (b) Intermediate (c) Steady

Velocity vectors for 1 MW fire with 4 m only outside

Results cut at here

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(a) Initial (b) Intermediate (c) Steady

Velocity vectors for 5 MW fire with only 4 m outside

More obvious for bigger fire

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(a) Initial (b) Intermediate (c) Steady

Velocity vectors for 1 MW fire with 20 m outside

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The computing domain was extended further by 20

m, having three times of the original grid size.

All free

In comparing with the case by extending out by 20 m

more, the flow pattern of the entire window flame.

Note that the steady state results would be very

different as shown in these figures.

Higher heat release rates would give a longer range,

need to extend to longer distance away.

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(a) Initial (b) Intermediate (c) Steady

Velocity vectors for 5 MW fire with 20 m outside

Long enough !

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4. Functional Analysis

PM

P

M

PM

2 parameters

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In order to quantify this comparison precisely,

functional analysis proposed on zone modeling

was applied to evaluate the CFD results.

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Transient predicted and measured data are

expressed as vectors and .

The Euclidean norm and secant inner product

cosine between and are calculated:

Norm =

Cosine =

M

P

P

M

P

MP

MP

MP

Values of norm and cosine are used to compare

CFD predicted results with measured data.

Values of norm should be 0, and cosine should be

close to 1 for good agreement.

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5. Grid Size Variation

Grid size denoted by x, y and z is the most

important numerical parameter in CFD simulations.

Quality of the mesh was assessed by a non-

dimensional parameter rather than an absolute mesh

cell size.

For simulations involving buoyant plumes, a

measure of how well the flow field is resolved is given

by the non-dimensional expression on R*:

*

* ),,max(

D

zyxR

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The nominal size of the mesh cell δx and the

characteristic fire diameter D* given by fire power ,

air density , temperature T , specific heat of air

cp , and gravitational acceleration g are important in

simulating buoyant plumes:

Q

5/2

*

gTc

QD

p

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The ratio D*/δx can be taken as the number of

computational cells spanning the characteristic

diameter of the fire.

A refined grid system can improve the accuracy of

results of LES.

It was suggested that the value of D*/δX should be

larger than 10 to guarantee a reliable operation of

FDS.

Zou and Chow got reasonable FDS predictions of

temperature and radiation data with D*/δX of

about 14.

Study by Hietaniemi et al. on pool fire showed

that having at least 20 cells within the diameter of

the pool would give predictions agreed with

experiments.

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Validation study given by Hill et al. suggests that

R* should be between 0.06 to 0.25.

It is also found that the optimum resolution of a

pool fire simulation R* is around 0.05 by Ma and

Quintiere, the centerline temperature and velocity

in the non-combusting region is also predicted

well.

However, for the non-combustion flow field

prediction, FDS did not give any suggestion about

the grid size.

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6. Example Cases: Time Line

Analysis

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The most important impact of CFD is on

evacuation study in crowded halls with ASET-

RSET approach.

It is criticized to be a FLAWED Concept.

Evacuation was studied by the timeline analysis

with Available Safe Egress Time (ASET) and

Required Safe Egress Time (RSET) calculated.

The safety margin SM is:

ASET

RSET

Safety Margin

SM = ASET - RSET

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Problems identified in Southeast Asia in projects on

estimating ASET are:

Predicted by CFD fire models, with very few

Validation & Verification works for large halls.

Small fire scenarios without experimental

justification.

Seldom used big fires.

Other toxicants, irritants and asphyxiants were not

yet specified because there is no fire engineering tool

available to predict the chemical species liberated

from combustion accurately.

There are far too many chemical reactions involved

in the burning process.

(in fire retardants)

Sensory irritation

– Irritation of eyes

Pulmonary irritation

– Upper respiratory tract

– coughing

Smoke Toxicant

Smoke toxicants

Asphyxiants Irritants

CO HCN CO2 HBr Halogen acid Organic

HCl Acrolein Central nervous system

depression

Loss of consciousness

Ultimately death

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Tenability limits commonly used (following partly

CIBSE Guide E 2010 are:

Lift safety for occupants and firemen

Radiative heat flux: 2.5 kWm-2

Smoke layer temperature: 120 oC

Smoke layer interface height: 2.5 m

Carbon monoxide concentration [CO]: 6000 to

8000 ppm for 5 minutes exposure

Toxicity effect of many toxic gases such as

hydrogen chloride HCl is very severe.

Neglecting them will give problems in estimating

ASET.

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Again, only heat and [CO] are specified.

All heat and toxic gases are assumed to be within

the stratified smoke layer at high levels.

This would not hold for tall atria.

These approaches are only applicable for those

fuels emitting only carbon monoxide upon

burning.

If toxicity of fire gases is included in the

tenability limit, ASET is highly reduced.

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Such problems were even found in the draft new

code on building fire safety issued by the Hong

Kong authority for consultation in September

2011, and implemented in April 2012.

Code of Practice for Fire Safety in

Buildings April 2012

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7. Observations

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The following problems were identified in using CFD

for Fire Engineering Approach (or known as

performance-based design elsewhere) in determining

fire safety provisions.

Air pressure and turbulence parameters are

seldom presented, and only the velocity vector

patterns and temperature distribution are shown.

Grid sensitivity criteria are only deduced from

temperature and velocity predictions, but not

pressure or turbulence parameters.

There is no justification on fire phenomena by

scale-models or full-scale burning tests.

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The following criticisms were raised in applying CFD

for fire simulations:

All phenomena are substantially three-

dimensional; the flow and temperature fields of

the smoke exhaust in the hall are very unstable.

The three-dimensionality and the instability of the

flow fields are not fully discussed.

Ability of CFD to resolve the flow in the turbulent

fire plume is in doubt.

Uncertainty in the ability of CFD to resolve the

turbulent exchange flow across the opening.

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8. Recommendations

59

The ‘stability criteria’ and ‘free boundary

conditions’ must be inspected in CFD reports.

There are always windows and doors in a

building. Bi-directional flow would be resulted,

with hot gas flowing out and cool air coming into

the room.

The boundary conditions of flow parameters,

pressure in particular, have to be specified

carefully to predict such bi-directional flow.

There are empirical correlations among the

pressure profiles across the vertical openings such

as windows or doors, and room geometries, heat

release rates of the fires, and opening sizes.

60

However, the empirical parameters might not be

appropriate for the boundary conditions of the

particular room fire.

From the results of this project, a better approach

is to extend the computing domain outside as

reported in the above study.

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More caution is needed in the application of CFD

to simulate phenomena in a building fire.

Therefore, CFD might only be good for practical

smoke control design in big halls to avoid putting

in fire simulation.

Full-scale burning tests on typical scenarios with

similar conditions are still necessary to justify the

CFD predictions.

Hot smoke tests must be carried out during the

testing and commissioning of smoke exhaust

system in tall halls.

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The following should be justified in the CFD

simulations in construction project submission:

The three groups of parameters on physical

models, numerical parameters and physical

systems in the CFD models should be watched.

Details of the grid systems. Note that in a large

airport terminal, a 10 m grid means that velocity

and temperature are the same within 10 m.

The convergence and stability criteria, turbulent

parameters.

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The boundary conditions on velocity components,

pressure and temperature.

The extension of free boundaries to outside

domain.

The prediction and presentation in the following

three groups [18] should be evaluated:

- Velocity-temperature

- Pressure

- Turbulent parameters

Justification on the above three groups of CFD

predictions by empirical formula and analytical

expressions.

Experiments on scale models of the building to

compare with CFD predictions.

In-situ field tests to evaluate the system

performance.

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The following should be discussed in fire hazard

assessment reports using CFD:

The choice of grid that gives suitably grid-

independent results.

The location of the external boundary conditions

that influence the predictions.

Quality control checks for the CFD predictions to

give details of :

- exchange flow

- neutral pressure planes

Experimental verification using data from full-

scale burning tests.

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9. Conclusion

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Consulting engineers pushed hard to promote the

use of CFD, and always believe in the CFD-FDS

predictions.

However, in-depth verification and validation

work is necessary.

Free opening boundary condition should be

evaluated before applying in CFD simulations,

especially when the combustion process is included

while simulating fires in tall or supertall buildings.

Extending the computational domain to a

sufficient distance beyond the opening is

recommended.

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Further related work on validation and

verification of liquid fuel model in FDS should be

conducted.

As raised by Chen (2009) on indoor aerodynamics,

it is difficult to have the whole set of CFD

predicted results agreed with experiments.

Macroscopic flow parameters predicted by CFD

are very useful.

Chen Q., “CFD for simulating air distribution in buildings: The state of the

art, challenges, and opportunities”, Proceedings of the 11th International

Conference on Air Distribution in Rooms (ROOMVENT 2009), 24-27 May

2009, Busan, Korea, pp. 23-31.

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Note that different results were predicted by

different CFD software packages.

Further, strongly buoyant flow should be

predicted more carefully.

It is useful to compare the prediction with the

Reynolds Averaged Navier Stokes equation

method (RANS).

However, this is quite labour intensive to develop

a new CFD software and very expensive to

purchase commercial CFD license.

Earlier studies on thermal plume suggested that

results are similar.

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Acknowledgment

This work described in this presentation was partially

supported by the Construction Industry Institute (Hong

Kong) / PolyU Innovation Fund for the project “Assessment

of a fire model in simulating combustion for construction

projects with fire engineering approach”.