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## PowerPoint Slideshow about ' Lecture Objectives' - pearl-wilkinson

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Lecture Objectives

- Finish with age of air modeling
- Introduce particle dynamics modeling
- Analyze some examples related to natural ventilation

Depends only on airflow pattern in a room

We need to calculate age of air (t)

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

Air-change efficiency (Ev)Contaminant removal effectiveness (e)

- 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?

e= 0.19

e= 2.20

Differences and similarities of EvandeDepending on the

source position:

- similar or

- completely different

air quality

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

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=SF
- 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 spherical

collector is governed by the Newton’s equation

Forces that affect the particle

- (rVvolume) particle∙dvx/dt=SFx

- (rVvolume) particle∙dvy/dt=SFy

- (rVvolume) particle∙dvz/dt=SFz

System of equation for each particle

Solution is velocity and direction of each particle

Lagrangian Modelparticle tracking

Basic equations

- momentum equation based on Newton\'s second law

Drag force due to the friction

between particle and air

- 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:

For finite time step

Algorithm for CFD and particle tracking

Unsteady state airflow

Steady state airflow

Airflow (u,v,w) for time step

Airflow (u,v,w)

Steady state

Injection of particles

Injection of particles

Particle distribution for time step

Particle distribution for time step

Airflow (u,v,w) for time step +

Particle distribution for time step +

Particle distribution for time step +

Particle distribution for time step +2

…..

…..

One way coupling

Case 1 when airflow is not affected by particle flow

Case 2 particle dynamics affects the airflow

Two way coupling

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

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