1. Lecture Objectives Discuss: Project 1 Diffuser modeling
Ventilation effectiveness

2. Meshing (Project 1) Pat a) Numerical diffusion
The purpose of this project part is to analyze how mesh size and orientation affects
the accuracy of result.
outlet
inlet T1 T2
T1=30C
T2=20C
outlet
inlet

3. Concentration equation
Conservation of mass of considered gas (chemical species):
mgas=const
mgas=C∙mair= C∙ρ∙dxdydz=const
mgas,in
mgas,out dy
incompressible flow dz
C=const dx
Diffusion coefficient
C – concentration of: H2O , VOC, CO, CO2 , and other gasses
What about particles?

4. Inlets Diffuser Types Valve diffuser swirl diffusers ceiling diffuser
wall or ceiling
floor

5. Diffuser Types Grill (side wall) diffusers Linear diffusers Vertical
Horizontal one side

6. Displacement ventilation diffusers

7. Diffuser modeling Complex geometry - Δ~10-4m We can spend all our
momentum sources
Momentum method
Complex geometry - Δ~10-4m
We can spend all our
computing power for one small detail

8. Diffuser Modeling
Fine mesh or box method for diffuser modeling

9. 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
takes into account air distribution and source position and intensity

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

11. Single value IAQ indicators Ev 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

12. Air-change efficiency (Ev)
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

13. Air exchange efficiency for characteristic room ventilation flow types
Flow pattern
Air-change
efficiency
Comparison with average time of exchange
Unidirectional flow
1 - 2
tn < texc < 2tn
Perfect mixing 1
texc = tn
Short Circuiting
0 - 1
texc > tn

14. 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?

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

16. Thermal comfort
Temperature and relative humidity

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

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

19. 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 ( M ) ] 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 = exp [ - ( PMV PMV2)]
Empirical correlations
Ole Fanger
Further Details: ANSI/ASHRAE standard 55, ISO standard 7730