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Finish Thermal Comfort and Air Quality analyses in CFD Start particle modeling

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.

erasmus-guthrie
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Finish Thermal Comfort and Air Quality analyses in CFD Start particle modeling

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  1. Lecture Objectives Finish • Thermal Comfort and • Air Quality analyses in CFD Start particle modeling

  2. Thermal comfort Temperature and relative humidity

  3. Thermal comfort Velocity Can create draft Draft is related to air temperature, air velocity, and turbulence intensity.

  4. Thermal comfort Mean radiant temperature potential problems Asymmetry Warm ceiling (----) Cool wall (---) Cool ceiling (--) Warm wall (-)

  5. 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

  6. 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

  7. IAQ parameters • Age-of-air air-change effectiveness (EV) • Specific Contaminant Concentration contaminant removal effectiveness e

  8. 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

  9. 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)

  10. Air exchange efficiency for characteristic room ventilation flow types

  11. 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?

  12. Ev= 0.41 e= 0.19 e= 2.20 Differences and similarities of Evande Depending on the source position: - similar or - completely different air quality

  13. 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

  14. ParticlesProperties and sources ASHRAE Transaction 2004

  15. Properties ASHRAE Transaction 2004

  16. 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

  17. m∙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

  18. 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

  19. 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

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