Thermal Comfort Local Thermal Discomfort

Thermal Comfort
Local Thermal Discomfort
Arsen Melikov
October 2010, CTU in Prague
International Centre for Indoor Environment and Energy
Department of Civil Engineering, Technical University of Denmark
www.ie.dtu.dk
1

Thermal Comfort
Human thermal comfort is defined by ASHRAE
as the state of mind that expresses satisfaction
with the surrounding environment.
Maintaining thermal comfort for occupants of
buildings or other enclosures is one of the
important goals of HVAC design engineers
2

Factors for Thermal Comfort
• Environmental parameters
- air temperature (essential factor)
- mean radiant temperature (important)
- air velocity (important)
- relative humidity (relatively small effect)
• Personal variables
- activity level
- clothing insulation
• Secondary factors
- non-uniformity of the environment
- adaptation
-age
- outdoor climate
-etc.
3

Conditions for Thermal Comfort
• Environmental parameters
- air temperature (essential factor)
- mean radiant temperature (important)
- air velocity (important)
- relative humidity (relatively small effect)
• Personal variables
- activity level
- clothing insulation
• Secondary factors
- non-uniformity of the environment
- adaptation
-age
- outdoor climate
-etc.
4

Thermal comfort
• General thermal comfort:
- Summer temperature range
- Winter temperature range
• Local thermal discomfort:
- Draught
- Vertical temperature difference
- Radiant asymmetry
- Cold/warm floor
5

Local Thermal Discomfort
• Asymmetric Thermal Radiation
Caused by cold windows, uninsulated walls, cold products,
cool or warm machinery, heated or cold ceiling, etc.
6

Local Thermal Discomfort
• Asymmetric Thermal Radiation
Requirements in standards and guidelines
DISSATISFIED
%
80
60
40
20
10
86
4
LOCAL DISCOMFORT CAUSED BY RADIANT
TEMPERATURE ASYMMETRY
WARM CEILING
COOL WALL
COOL CEILING
WARM WALL
2
1
0 5 10 15 20 25 30 35 o C
RADIANT TEMPERATURE ASYMMETRY
7

Local Thermal Discomfort
• Vertical Air Temperature Difference
Vertical temperature difference may cause warm discomfort at
the head and cold discomfort at the feet. May occur in rooms with
displacement ventilation, etc.
8

Local Thermal Discomfort
• Vertical Air Temperature Difference
Requirements in standards and guidelines
9

Local Thermal Discomfort
• Warm or Cold Floors
Discomfort due to direct contact between the feet and floor with a
too high or too-low surface temperature. Important for people with
bare feet.
To save energy, floor materials with a low contact coefficient (cork,
wood, carpets), floor heating systems can be used to eliminate the
desire for higher ambient temperatures caused by cold feet.
Textiles 21 to 28˚C
Oak floor 24.5 to 28˚C
Concrete 26 to 28.5˚C
10

Local Thermal Discomfort
• Warm or Cold Floors
Requirements in standards and guidelines
DISSATISFIED
%
80
60
40
20
10
86
4
LOCAL DISCOMFORT CAUSED BY
WARM AND COOL FLOORS
2
1
5 10 15 20 25 30 35 40
FLOOR TEMPERATURE
o C
11

Local Thermal Discomfort
• Draught
Undesired local cooling of the body caused by air movement.
Serious problem in buildings and vehicles
Leads to demand for higher air temperatures in rooms and thus
use of more energy
May cause people to stop ventilation systems which leads to
decreased perceived indoor air quality, health effects and
decrease performance of occupants
12

Local Thermal Discomfort
• Airflow characteristics in rooms
13

Local Thermal Discomfort
• Airflow characteristics in rooms
Turbulence intensity
Air Velocity(m/s)
0,4
0,3
0,2
0,1
Vm=0.14 m/s Tu=71%
Vm=0.139 m/s Tu=36%
Vm=0.126 m/s Tu=6.6%
0
10 12 14 16 18 20 22
Time(s)
14

Local Thermal Discomfort
Frequency of velocity flutuations
15

Local Thermal Discomfort
Frequency of velocity flutuations
16

Local Thermal Discomfort
• Equivalent frequency
The equivalent frequency, f e , of a randomly fluctuating velocity is
defined as the frequency of sinusoidal velocity fluctuations with
the same ratio of the standard deviation of acceleration to the
standard deviation of air velocity as in the random velocity
fluctuations. It is an integral measure for the frequency of the
velocity fluctuations of a turbulent flow.
0,3
f
e
=
1
2π
⋅
SD
SD
a
v
Air Velocity (m/s)
0,2
0,1
a = ΔV/Δτ s
ΔV
Δτ s
0
15 17 19 21 23 25
Time(s)
17

Local Thermal Discomfort
• Factors affecting draught
- air temperature
- mean velocity
- turbulence intensity
- airflow direction
- frequency of velocity fluctuation
- person’s general thermal sensation
18

Local Thermal Discomfort
• Draught Rating Model
PD = (34 – t a )(v – 0.05) 0.62 (0.37vTu + 3.14)
For v < 0.5 m/s insert v = 0.05 m/s;
for PD > 100%, insert PD = 100%
Valid for: 20 < t a < 26˚C, v < 0.05 m/s
19

Local Thermal Discomfort
Impact of general thermal sensation on draught discomfort
20

Local Thermal Discomfort
Impact of airflow direction on draught discomfort
21

Local Thermal Discomfort
Impact of airflow direction on draught discomfort
22

Local Thermal Discomfort
Impact of velocity flutuations on draught discomfort
Human subjects exposed to periodial velocity fluctuations
at the same mean velocity and air temperature
23

Local Thermal Discomfort
Impact of equivalent frequency draught discomfort
DISSATISFIED (%)
40
30
20
10
Mean Velocity
0.1 m/s
0.2 m/s
0.3 m/s
T a =20C, T u ~35%
0
0 0,2 0,4 0,6 0,8 1 1,2
EQUIVALENT FREQUENCY (Hz)
24