cfd in ventilation design

Aalborg Universitet

CFD in Ventilation Design

Nielsen, Peter Vilhelm

Publication date:2009

Link to publication from Aalborg University

Citation for published version (APA):Nielsen, P. V. (2009). CFD in Ventilation Design: a new REHVA Guide Book. Aalborg: Department of CivilEngineering, Aalborg University.

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Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 1

CFD in Ventilation Design, a new REHVA Guide Book

Peter V. Nielsen,

Aalborg University

Development in Computer Speed

In 1970. 3D non-isothermal flow.1 mio. grid points

39 CPU years

In 2009. 3D non-isothermal flow.1 mio. grid points

A few CPU hours

My First Experience with CFD

The Developmentof CFD

The computation cost is decreasing by a factor of 10 each eighth year.

7 x 10

10,00031x31x31

CFD Prediction of Cross Infection

The Amoy Gardens SARS Outbreak

Index patientvisited Flats 7, 16 th Floor

Index patient visited a flat 7 on two nights in March 2003

Concentration Distribution with 45 degree Deflector

Airflow rate with 100m3/h

Airflow rate with 150m3/h Airflow rate with 200m3/h

Computational fluid dynamics in a nutshellSymbols and glossaryMathematical backgroundTurbulence modelsNumerical methodsBoundary conditionsQuality controlCFD combined with other prediction modelsApplication of CFD codes in building designCase studiesBenchmark tests

Authors: Peter V. Nielsen (ed.), Francis Allard, Hazim B. Awbi, Lars Davidson and Alois Schälin

About the design guide book

The user of the design book is mainly considered to be a consulting engineer who has to:

- order a CFD prediction

- consider and work with a CFD prediction

- discuss CFD and CFD quality with a supplier of a CFD prediction

The book is in principle not written for engineers who are making CFD predictions

Mathematical Background

From the general description

ΦΦ +ΦΓ=Φ+∂Φ∂ SgraddivVdivt

)()()( ρρ

to a two-dimensional time dependent transportequation

⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+∂∂

=∂∂

+∂∂

Turbulence Models

2D steady state laminar boundary layer flow/turbulent boundary layer flow

A discussion of different turbulence models asthe k-ε model, the k-ω model, the SST modeland the Reynolds Stress model

The Large Eddy Simulation is also discussed

2c cc c cu v Sx y y

ρ ρ∂ ∂ ∂+ = Γ +

∂ ∂ ∂

One-DimensionalCase

The case can be considered as a small part of a flow, whichin certain areas is one-dimensional, parallel with grid lines and steady.

cc Sdx

cddxdcu +Γ= 2

Steady One-Dimensional Convection -Diffusion Transport Equation

Analyticalexpression

One-Dimensional DiscretizationEquation

Control-volume formulation (of mass fraction transport Equation in x direction):

It is necessary to replace values at the cell surfaces e and w with values at the grid points WW, W, P, E and EE to have a final version of the discretization equation.

Different Discretization Equations, 1

The flow is studied in a case where the length x is equalto 4. The boundary values co and c3 are equal to 1.0 and 0.0.

Diffusion, u = 0.0

Central difference fx ce = (cP + cE)/2 and u = 0.1

Central difference fx ce = (cP + cE)/2 and u = 3.0

Wiggly for larger than 2c

uxPeΓΔ

History and Numerical Schemes, 1

The sixtiesThe central difference scheme becomes unstable (wiggly)when the Peclet number is large. The cure is to decreasethe grid size.

The seventiesUpwind difference opened the way for infinitely highReynolds numbers, but false diffusion could in manycases be larger than diffusion of physical kind.

The eighties and the ninetiesSecond order schemes decreased the effect of false diffusion.

Imperial College and the UpwindScheme

The upwind scheme was amongothers invented by Imperial College in the late sixties.

Upwind Scheme

Upwind scheme fx ce = cP and u = 3.0

False Diffusion

Second-OrderUpwind Scheme

Second-order discretization error. Unbounded (e-value can be larger than W, P, E-values).Non-physical wiggles may occur.

Higher Order SchemesSecond order upwind scheme is an example of a newscheme developed in the middle of the seventies.

The one-dimensional case with the velocity u = 3.0 and1st and 2nd order upwind scheme

False Diffusion and Order of the Schemes

Flow from an opening, which is inclined at ~30 deg. to the mesh.Three-dimensional flow.

Profile at the upper surface at a distance of 1 m from the opening.

Dispersive error

Diffusive error

Boundary Conditions

• Wall boundary• Free boundary• Plane of symmetry• Air supply opening• Air exit opening• Obstacle boundary

Air supply opening

Quality Control

Quality control consists of these major steps:- recognize possible sources of errors, - check for them in your own simulations, - estimate the accuracy of the simulations,- improve the simulations, if possible.

Main items in this chapter are:- Steps in a CFD simulation - Sources of errors and uncertainties- How to ensure high quality predictions (recommendations)

- Questions to ask the CFD engineer about the work reported

- Additional advice and remarks- A short check list

Quality Control, Sources of Errors and Uncertainties

Some examples: 2D treatment instead of 3D

Simplification, modelling level

Cell Quality

Monitoring velocitiesVersus number of cellsin the prediction

Cell Quality

Turbulence Model

3D wall jet in a room simulated by a k-epsilon Modeland a Reynolds Stress Model.

Schälin and Nielsen

CFD Combined with other Prediction Models

The Regional Library of Northern Jutland

The Regional Library of Northern Jutland is used as a testbuilding where a combination of BEPS and CFD is used forprediction of energy consumption and indoor climate.

Contaminant Distribution

Application of CFD Codes in Building Design

The applications of CFD in buildings may be grouped under the following headings:

• Prediction of air jet diffusion

• Room air movement analysis

• Prediction of contaminant dispersal

• Modelling emission from materials and equipment in buildings

• Indoor air quality prediction

• Thermal comfort assessment

• Mean age of air and ventilation effectiveness predictions

• Prediction of fire and smoke spread

• Wind flow around buildings

Case studies on Different Air Distribution Systems

Five air distribution systems are compared with each other. They are all installed in the same room, and they all handle the same situation and the same load.

Mixing ventilation generated by a ceiling mounted radial diffuser. End wall mounted return opening below ceiling.

Displacement ventilation. End wall mounted low velocity diffuser. End wall mounted return opening below ceiling.

Vertical ventilation with a textile terminal. End wall mounted return opening at floor level.

Mixing ventilation with end wall mounted diffuser. Return opening below the supply.

The Test Room

The test room is the IEA Annex 20 room with length, width and height equal to 4.2 m, 3.6 m and 2.5 m.The heat load consists of two PCs, two desk lamps and two manikins producing a total heat load of 480 W. One work placeis used in some of the experiments (240 W).

CFD Model

• Radiation is ignored

– Convection is estimate tobe 50 % of total heat flux

• Wall, ceiling and floor

– Surface temperature• Mannequins, PC´s and lamps

– Fixed heat flux

Case Study, Mixing Ventilation withWall Mounted ATD

Simulation of the Diffuser

Mixing Ventilation with End WallMounted Diffuser

Case Study, Vertical Ventilation

Vertical Ventilation

Radiation ignored, plane-symmetrical solution domain used.(Stratified flow = plane-symmetrical flow, Momentum driven flow may give unsymmetrical flow)

Observations Measurements

Vertical Ventilation, Diffuser

1st order steady state equations, k-ε turbulence model, 300,000 cells

BC: Diffuser F

Vertical Ventilation, Quality Control

Monitoring points

Velocity iny-direction

Predictions in the wholeroomn = 5 h-1

218.400 grid points

Case Study, Displacement Ventilation

Diffuser for Displacement Ventilation

A/cos θ

u Non-prepenticular flow must be compensatedin flow area to ensure fixed flow

Displacement Ventilation1st order steady state equations, k-ε turbulence model, 220,000 cells,Radiation ignored, plane-symmetrical solution domain used.(Stratified flow = plane-symmetrical flow, Momentum driven flow may give unsymmetrical flow)

0 50000 100000 150000 200000 250000

Antal netpunkter

Monitorpunkt 1 Monitorpunkt 2 Monitorpunkt 3 Monitorpunkt 4

Grid convergence

Displacement Ventilation

Case Study, Mixing Ventilation withCeiling Mounted ATD

Mixing Ventilation with Ceiling Diffuser, Diffuser Models

– 4-way diffuser– Fixed-flow diffuser– Diffuser with

horizontal surface

Diffuser Models

Measurements 4-way diffuser

Diffuser Models

Fixed-flow diffuser Diffuser with horizontalsurface

CFD Simulations

• Temperatures

CFD Simulations• Maximum velocity at 1.80 m

(top boundary of the occupied zone)

Steady and/or Transient FlowMeasurements in ceiling region

n = 6.02 h-1 n = 3.25 h-1

Simulation of Transient Flow

(Time Dependent Equations)

y-velocity at 1.80 m

y-velocity at different positions

n = 3.25 h-1

Direct Description of a Diffuser, 1

Diffuser Rectangular cells and multigridstructure, case 1

Unstructured grid,case 2

CFD simulation of the diffuserbased on fineunstructured grid.

Prediction based onthe PV method. Velocityvalues are given from the diffuser simulation.

Kondo et al

LiteratureP. V. Nielsen, The Selection of Turbulence Models for Prediction of Room Airflow. ASHRAE Transactions. 1998; Vol. 104, Part 1B. -

pp. 1119-1127.

P. V. Nielsen, Indoor Climate Modelling, Chapter in: Per Erik Nilsson, Achieving the Desired Indoor Climate – Energy Efficiency Aspects of System Design. The Commtech Group, Studentlitteratur, Lund, 2003.

D. N. Sørensen and P. V. Nielsen, Quality Control of Computational Fluid Dynamics in Indoor Environments. International Journal of Indoor Environment and Health, Vol. 13, no. 1, pp. 2-17, March 2003.

A. Schälin and P. V. Nielsen, Impact of Turbulence Anisotropy near Walls in Room Air Flow. Indoor Air, International Journal of Indoor Environment and Health, Vol. 14, No. 3, pp. 159-168, 2004.

P. V. Nielsen, A. Restivo and J. H. Whitelaw, Buoyancy-Affected Flows in Ventilated Rooms, Numerical Heat Transfer, Vol. 2, 1979.

B. Bjerg, K. Svidt, G. Zhang, S. Morsing, J.O. Johnsen, Modeling of air inlets in CFD prediction of airflow in ventilated animal houses, Computers and Electronics in Agriculture, 34, 223–235, 2002

P. V. Nielsen, The Description of Supply Openings in Numerical Models for Room Air Distribution. ASHRAE Transactions, Vol. 98, Part 1, 1992.

P. V. Nielsen, The Box Method - A Practical Procedure for Introduction of an Air Terminal Device in CFD Calculation. - Aalborg: AAU, 1997. - 15 p. (R9744. - ISSN: 1395-7953).

P. V. Nielsen, The Prescribed Velocity Method - A Practical Procedure for Introduction of an Air Terminal Device in CFD Calculation. - Aalborg: AAU, 1998. - 13 p. (R9827. - ISSN: 1395-7953).

Bjerg B., Svidt K., Morsing S., Zhang G, Comparion of Methods to Model a Wall Inlet in Numerical Simulation of Airflow in Livestock Rooms, Proceedings of AgEng2000, International Conference on agricultural engineering, Warwick, UK, 2000.

L. Davidson, P. V. Nielsen and A. Sveningsson, Modifications of the Model for Computing the Flow in a 3D Wall Jet. Submitted to THMT-03, International Symposium on Turbulence, Heat and Mass Transfer, October 12 – 17, 2003, Antalya, Turkey.

L. Davidson, P. V. Nielsen and C. Topp, Low-Reynolds Number Effects in Ventilated Rooms: A Numerical Study. In: Proceedings of ROOMVENT 2000, Reading, 2000.

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