Lecture Objectives

• Ventilation Effectiveness

• Thermal Comfort

• Meshing

IAQ parameters

- Age-of-air

air-change effectiveness (EV)

- Specific Contaminant Concentration

contaminant removal effectiveness

Single value IAQ indicators Ev and ε

1.Contaminant removal effectiveness ()

concentration at exhaust average contaminant concentration

Contamination level

2. Air-change efficiency (v)

shortest time for replacing the air average of local values of age of air

Air freshness

C

Cε e

τ2

τEv n

[sec] ACS/1τn

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 τ,τ2τ nexe

Air exchange efficiency for characteristic room ventilation flow types

Flow patternAir-changeefficiency

Comparison with average time of exchange

Unidirectional flow 1 - 2 n < exc < 2n

Perfect mixing 1 exc = n

Short Circuiting 0 - 1 exc > n

τ2τexe

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

Thermal comfort

Temperature and relative humidity

Thermal comfort

VelocityCan create draft

Draft is related to air temperature, air velocity, and turbulence intensity.

Thermal comfort

Mean radianttemperature

potential problems

AsymmetryWarm ceiling (----)Cool wall (---)Cool ceiling (--)Warm wall (-)

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

Further Details: ANSI/ASHRAE standard 55, ISO standard 7730

Meshing (Project 1)

T1=30C

T2=20C

outletinlet

outlet

outletinlet

inlet

T1

T2

Pat a) Numerical diffusion The purpose of this project part is to analyze how mesh size and orientation affects the accuracy of result.

Grid type and resolution

Hexa– Uniform hexa– Nonuniform hexa– Unstructured hexa

Body-fitted coordinate hexa - Structured – Unstructured–

Tetra mesh – Structured – Unstructured

Polyhedral mesh

Grid type and resolution hexa

boundary-fitted, structured grid

Uniform

Nonuniform (2-D)

Unstructured hexa (2-D)

Grid type and resolutionTetra

Structured

Unstructured

Grid type and resolutionPolyhedral mesh

Computational resource saving by mesh type