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Lecture Objectives. Finish Thermal Comfort and Air Quality analyses in CFD Start particle modeling. Thermal comfort. Temperature and relative humidity. Thermal comfort. Velocity Can create draft Draft is related to air temperature, air velocity, and turbulence intensity.

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Lecture Objectives

Finish

  • Thermal Comfort and

  • Air Quality analyses in CFD

    Start particle modeling


Thermal comfort
Thermal comfort

Temperature and relative humidity


Thermal comfort1
Thermal comfort

Velocity

Can create draft

Draft is related to air temperature,

air velocity, and turbulence intensity.


Thermal comfort2
Thermal comfort

Mean radiant

temperature

potential problems

Asymmetry

Warm ceiling (----)

Cool wall (---)

Cool ceiling (--)

Warm wall (-)


Prediction of thermal comfort
Prediction of thermal comfort

  • Predicted Mean Vote (PMV)

  • + 3 hot

  • + 2 warm

  • + 1 slightly warm

  • PMV = 0 neutral

  • -1 slightly cool

  • -2 cool

  • -3 cold

  • PMV = [0.303 exp ( -0.036 M ) + 0.028 ] L

  • L - Thermal load on the body

  • L = Internal heat production – heat loss to the actual environment

  • L = M - W - [( Csk + Rsk + Esk ) + ( Cres + Eres )]

  • Predicted Percentage Dissatisfied (PPD)

  • PPD = 100 - 95 exp [ - (0.03353 PMV4 + 0.2179 PMV2)]

Empirical correlations

Ole Fanger


Iaq parameters
IAQ parameters

Number of ACH

quantitative indicator

ACH - for total air

- for fresh air

Ventilation effectiveness

qualitative indicator

takes into account air distribution in the space

Exposure

qualitative indicator

takes into account air distribution and source position and intensity


Iaq parameters1
IAQ parameters

  • Age-of-air

    air-change effectiveness (EV)

  • Specific Contaminant Concentration

    contaminant removal effectiveness e


Single value iaq indicators e v and
Single valueIAQ indicatorsEv and ε

  • Contaminant removal effectiveness (e)

  • concentration at exhaust

  • average contaminant concentration

    Contamination level

  • 2. Air-change efficiency (Ev)

  • shortest time for replacing the air

  • average of local values of age of air

    Air freshness


Air change efficiency e v

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
Contaminant removal effectiveness ( flow typese)

  • 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 e v and e

E flow typesv= 0.41

e= 0.19

e= 2.20

Differences and similarities of Evande

Depending on the

source position:

- similar or

- completely different

air quality


Particulate matters pm
Particulate matters (PM) flow types

  • Properties

    • Size, density, liquid, solid, combination, …

  • Sources

    • Airborne, infiltration, resuspension, ventilation,…

  • Sinks

    • Deposition, filtration, ventilation (dilution),…

  • Distribution

    - Uniform and nonuniform

  • Human exposure


Particles properties and sources
Particles flow typesProperties and sources

ASHRAE Transaction 2004


Properties flow types

ASHRAE

Transaction 2004


Two basic approaches for modeling of particle dynamics
Two basic approaches flow typesfor 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 model particle tracking

  • m flow types∙a=SF

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 model particle tracking1
Lagrangian Model flow typesparticle 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
Algorithm for CFD and flow typesparticle 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


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