Lecture Objectives Ventilation Effectiveness Thermal Comfort Meshing.
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Transcript of Lecture Objectives Ventilation Effectiveness Thermal Comfort Meshing.
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