lecture objectives -finish with age of air modeling -introduce particle dynamics modeling -analyze...
TRANSCRIPT
Lecture Objectives
- Finish with age of air modeling
- Introduce particle dynamics modeling
- Analyze some examples related to natural ventilation
Air-change efficiency (v)
• Depends only on airflow pattern in a room• We need to calculate age of air ()
Average time of exchange
• What is the age of air at the exhaust?
Type of flow– Perfect mixing– Piston (unidirectional) flow – Flow with stagnation and short-circuiting flow
2
2
2
2
2
2
z)(
y)(
x)()(
τtttzyx z
Vy
Vx
τV
[sec] ACH/1 τn
Contaminant removal effectiveness ()
• Depends on:- position of a contaminant source- Airflow in the room
• Questions
1) Is the concentration of pollutant in the room with stratified flow larger or smaller that the concentration with perfect mixing?
2) How to find the concentration at exhaust of the room?
Differences and similarities of Ev and Depending on the source position:
- similar or - completely different
air quality
v = 0.41
= 0.19 = 2.20
Particulate matters (PM)
• Properties– Size, density, liquid, solid, combination, …
• Sources – Airborne, infiltration, resuspension, ventilation,…
• Sinks- Deposition, filtration, ventilation (dilution),…
• Distribution- Uniform and nonuniform
• Human exposure
ASHRAE Transaction 2004
Properties
Particle size distribution
ASHRAE Transaction 2004
Ventilation system affect the PM concentration in indoor environment !
Human exposure
ASHRAE Transaction 2004
Two basic approaches for modeling of particle dynamics
• Lagrangian Model– particle tracking– For each particle ma=F
• Eulerian Model – Multiphase flow (fluid and particles)– Set of two systems of equations
Lagrangian Modelparticle tracking
A trajectory of the particle in the vicinity of the sphericalcollector is governed by the Newton’s equation
m∙a=F(Vvolume) particle ∙dvx/dt=Fx
(Vvolume) particle ∙dvy/dt=Fy
(Vvolume) particle ∙dvz/dt=Fz
System of equation for each particle
Solution is velocity and direction of each particle
Forces that affect the particle
Lagrangian Modelparticle tracking
Basic equations- momentum equation based on Newton's second law
eFF
tiV
PPd drag
3
6
- dp is the particle's diameter, - p is the particle density, - up and u are the particle and fluid instantaneous velocities in the i direction,- Fe represents the external forces (for example gravity force).
This equation is solved at each time step for every particle.
The particle position xi of each particle are obtained using the following equation:
ii Vdt
dx
puufFdrag
Drag force due to the friction between particle and air
For finite time step
tdt
tdt
Algorithm for CFD and particle tracking
Airflow (u,v,w)
Steady state airflow Unsteady state airflow
Particle distribution for time step
Particle distribution for time step +
Particle distribution for time step +2
Steady state
Injection of particles
…..
Airflow (u,v,w) for time step
Particle distribution for time step
Particle distribution for time step +
Injection of particles
…..
Airflow (u,v,w) for time step +
Case 1 when airflow is not affected by particle flowCase 2 particle dynamics affects the airflow
One way coupling Two way coupling
Natural Ventilation:Science Park, Gelsenkirchen, Germany
Natural Ventilation and CFD simulation
• Wind driven outdoor flow• Buoyancy driven indoor flow
Solution approach– Model boundary condition in-between outdoor and indoor
domain– Couple CFD with
• 1) energy simulation program (buoyancy driven flow) • 2) multi-zone modeling program (inter-zonal flow)
External flow
Wind profile
Buoyancy driven indoor flow
Important parameters• Geometry• Heat sources
– Intensity (defined temperature or heat flux)– Distribution– Change (for unsteady-state problem)
• Openings Defined – Pressure– Velocity
Natural Ventilation:Stack-driven flow in an atrium
Natural Ventilation:Wind scoop
Natural Ventilation:Solar-assisted ventilation
Window Design
Natural Ventilation:
Natural Ventilation: