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 [email protected] 1
CFD in Ventilation Design, a new REHVA Guide Book
by
Peter V. Nielsen,
Aalborg University
Peter V. Nielsen, Aalborg University [email protected] 2
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
Peter V. Nielsen, Aalborg University [email protected] 4
The Developmentof CFD
The computation cost is decreasing by a factor of 10 each eighth year.
7 x 10
10,00031x31x31
Peter V. Nielsen, Aalborg University [email protected] 6
The Amoy Gardens SARS Outbreak
F
E
C
DA
B
N
65 4
32
6
17 8
Index patientvisited Flats 7, 16 th Floor
F
E
C
DA
B
F
E
CD
A
B
4
Index patient visited a flat 7 on two nights in March 2003
Peter V. Nielsen, Aalborg University [email protected] 7
Peter V. Nielsen, Aalborg University [email protected] 8
Peter V. Nielsen, Aalborg University [email protected] 9
Concentration Distribution with 45 degree Deflector
Airflow rate with 100m3/h
Airflow rate with 150m3/h Airflow rate with 200m3/h
Peter V. Nielsen, Aalborg University [email protected] 10
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 11
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
Peter V. Nielsen, Aalborg University [email protected] 12
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 13
Mathematical Background
From the general description
ΦΦ +ΦΓ=Φ+∂Φ∂ SgraddivVdivt
)()()( ρρ
to a two-dimensional time dependent transportequation
⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂
+∂∂
=∂∂
+∂∂
+∂∂
2
2
2
2
yc
xcD
ycv
xcu
tc
Peter V. Nielsen, Aalborg University [email protected] 14
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 15
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
2
2c cc c cu v Sx y y
ρ ρ∂ ∂ ∂+ = Γ +
∂ ∂ ∂
Peter V. Nielsen, Aalborg University [email protected] 16
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 17
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
2
ρ
Peter V. Nielsen, Aalborg University [email protected] 18
Steady One-Dimensional Convection -Diffusion Transport Equation
Analyticalexpression
Peter V. Nielsen, Aalborg University [email protected] 19
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.
Peter V. Nielsen, Aalborg University [email protected] 20
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
Peter V. Nielsen, Aalborg University [email protected] 21
Different Discretization Equations, 2
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.
Central difference fx ce = (cP + cE)/2 and u = 0.1
Peter V. Nielsen, Aalborg University [email protected] 22
Different Discretization Equations, 3
Central difference fx ce = (cP + cE)/2 and u = 3.0
Wiggly for larger than 2c
uxPeΓΔ
=ρ
Peter V. Nielsen, Aalborg University [email protected] 23
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.
Peter V. Nielsen, Aalborg University [email protected] 24
Imperial College and the UpwindScheme
The upwind scheme was amongothers invented by Imperial College in the late sixties.
Peter V. Nielsen, Aalborg University [email protected] 26
Different Discretization Equations, 4
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.
Upwind scheme fx ce = cP and u = 3.0
Peter V. Nielsen, Aalborg University [email protected] 27
History and Numerical Schemes, 2
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.
Peter V. Nielsen, Aalborg University [email protected] 29
Second-OrderUpwind Scheme
Second-order discretization error. Unbounded (e-value can be larger than W, P, E-values).Non-physical wiggles may occur.
Peter V. Nielsen, Aalborg University [email protected] 30
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
Peter V. Nielsen, Aalborg University [email protected] 31
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.
Svidt
Dispersive error
Diffusive error
Peter V. Nielsen, Aalborg University [email protected] 32
History and Numerical Schemes, 3
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.
Peter V. Nielsen, Aalborg University [email protected] 33
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 34
Boundary Conditions
• Wall boundary• Free boundary• Plane of symmetry• Air supply opening• Air exit opening• Obstacle boundary
Air supply opening
Peter V. Nielsen, Aalborg University [email protected] 35
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 36
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
Peter V. Nielsen, Aalborg University [email protected] 37
Quality Control, Sources of Errors and Uncertainties
Some examples: 2D treatment instead of 3D
Simplification, modelling level
Peter V. Nielsen, Aalborg University [email protected] 38
Cell Quality
Monitoring velocitiesVersus number of cellsin the prediction
Peter V. Nielsen, Aalborg University [email protected] 40
Turbulence Model
3D wall jet in a room simulated by a k-epsilon Modeland a Reynolds Stress Model.
Schälin and Nielsen
Peter V. Nielsen, Aalborg University [email protected] 41
Peter V. Nielsen, Aalborg University [email protected] 42
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 44
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.
=
Peter V. Nielsen, Aalborg University [email protected] 46
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 47
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
Peter V. Nielsen, Aalborg University [email protected] 48
CFD in Ventilation Design
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
Peter V. Nielsen, Aalborg University [email protected] 49
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.
Peter V. Nielsen, Aalborg University [email protected] 50
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).
Peter V. Nielsen, Aalborg University [email protected] 51
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
Peter V. Nielsen, Aalborg University [email protected] 52
Case Study, Mixing Ventilation withWall Mounted ATD
Peter V. Nielsen, Aalborg University [email protected] 54
Mixing Ventilation with End WallMounted Diffuser
Peter V. Nielsen, Aalborg University [email protected] 56
Vertical Ventilation
Radiation ignored, plane-symmetrical solution domain used.(Stratified flow = plane-symmetrical flow, Momentum driven flow may give unsymmetrical flow)
Observations Measurements
Peter V. Nielsen, Aalborg University [email protected] 58
Vertical Ventilation, Diffuser
1st order steady state equations, k-ε turbulence model, 300,000 cells
Peter V. Nielsen, Aalborg University [email protected] 60
Vertical Ventilation, Quality Control
Monitoring points
Velocity iny-direction
Peter V. Nielsen, Aalborg University [email protected] 61
Vertical Ventilation
Predictions in the wholeroomn = 5 h-1
218.400 grid points
Peter V. Nielsen, Aalborg University [email protected] 63
Diffuser for Displacement Ventilation
θ
A
A/cos θ
u Non-prepenticular flow must be compensatedin flow area to ensure fixed flow
Peter V. Nielsen, Aalborg University [email protected] 64
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
5
10
15
20
25
30
35
40
0 50000 100000 150000 200000 250000
Antal netpunkter
Fejl
i for
hold
til m
ålte
væ
rdie
r [%
]
Monitorpunkt 1 Monitorpunkt 2 Monitorpunkt 3 Monitorpunkt 4
Grid convergence
Peter V. Nielsen, Aalborg University [email protected] 67
Case Study, Mixing Ventilation withCeiling Mounted ATD
Peter V. Nielsen, Aalborg University [email protected] 68
Mixing Ventilation with Ceiling Diffuser, Diffuser Models
– 4-way diffuser– Fixed-flow diffuser– Diffuser with
horizontal surface
Peter V. Nielsen, Aalborg University [email protected] 69
Diffuser Models
Measurements 4-way diffuser
Peter V. Nielsen, Aalborg University [email protected] 70
Diffuser Models
Fixed-flow diffuser Diffuser with horizontalsurface
Peter V. Nielsen, Aalborg University [email protected] 72
CFD Simulations• Maximum velocity at 1.80 m
(top boundary of the occupied zone)
Peter V. Nielsen, Aalborg University [email protected] 73
Steady and/or Transient FlowMeasurements in ceiling region
n = 6.02 h-1 n = 3.25 h-1
Peter V. Nielsen, Aalborg University [email protected] 74
Simulation of Transient Flow
(Time Dependent Equations)
y-velocity at 1.80 m
y-velocity at different positions
n = 3.25 h-1
Peter V. Nielsen, Aalborg University [email protected] 75
Direct Description of a Diffuser, 1
Diffuser Rectangular cells and multigridstructure, case 1
Unstructured grid,case 2
Peter V. Nielsen, Aalborg University [email protected] 77
Direct Description of a Diffuser, 3
CFD simulation of the diffuserbased on fineunstructured grid.
Prediction based onthe PV method. Velocityvalues are given from the diffuser simulation.
Kondo et al
Peter V. Nielsen, Aalborg University [email protected] 78
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.