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A comparison between four different ventilation systems
Conference Paper · September 2002
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Presented at the International conference Roomvent 2002, Copenhagen, September 2002.
A COMPARISON
BETWEEN FOUR DIFFERENT VENTILATION SYSTEMS
Y Cho1, H B Awbi1 , T Karimipanah2
1
The University of Reading, UK
2
Air Innovation AB, Sweden
y.j.cho@rdg.ac.uk h.b.awbi@rdg.ac.uk taghi.k@telia.com
Summary
Measurements and CFD simulations from four
systems are compared using the air change
index and a new Ventilation Parameter (VP).
VP combined with thermal comfort and indoor
air quality indices gives a good index for
comparison of four systems. Impinging Jet is
capable of achieving better air distribution in
the space than the other systems (mixing, wall
displacement and floor displacement)
particularly at higher heat loads.
Introduction
Traditional mixing ventilation (MV) systems
which are driven by high jet momentum force,
still occupy a large portion of the market
although in many cases has poor ventilation
efficiency and is less energy efficient.
In displacement ventilation (DV) fresh air is
usually supplied at floor level with an inlet
velocity < 0.5m/s and temperature ≥18 oC and
the cool air rises as it encounters heat sources
in the room thus creating a temperature
stratification. However, DV alone can only be
used for cooling.
Recently, a new method of air distribution
developed by Air innovation AB in Sweden that
is based on the impinging jet principle
(Karimipanah and Awbi 2002). As a medium
momentum supply device (DV < IJV
momentum < MV), impinging jet ventilation
(IJV) can combine the positive effects of both
mixing and displacement system. It produces
higher momentum than displacement
ventilation and can result in the jet spreading
evenly over the floor. As a result it can provide
a clean air zone in the lower part of the
occupied zone like displacement ventilation.
In this paper CFD and experimental results are
compared for four different ventilation systems
based using the mean velocity, comfort
parameters, local air change index at a
breading zone and ventilation parameters
including the heat and contaminant removal
effectiveness.
Experimental Set-up
The experiments were carried out in the
environmental chamber at the Univ. of Reading.
The chamber used for the experimental study
has external dimensions 4.0m x 3.0m x 2.52m
ceiling height. The dimensions of the test
compartment are 2.78m (length) x 2.78m
(width) x 2.3 m (height), i.e. representing a
small size office room.
In the case of the
wall displacement ventilation (DV), a semi
circular wall unit of overall dimensions 0.5m x
0.5m with a radius of 0.25m was used and the
air is supplied through many small diameter
holes. For floor displacement ventilation (DF),
two circular floor units of overall diameter
150mm was used and the air is supplied
through radial slots to create a swirl motion at
an angle to the face of the diffuser.
For
mixing ventilation, a slot with dimensions
400mm x 20mm was placed at a height of
2.16m above the floor projecting the air towards
the ceiling with an angle of 36o in order to avoid
the dumping of cold jet onto the lower zones.
For impinging jet ventilation, a semi circular
device with an inlet sectional area 0.0135m2
was used to supply air at a height of 0.84m
above the floor.
To provide a realistic office situation a light
(36W fluorescent), a computer box (a 400mm
cube with a 150W light bulb fitted inside), two
heated plates (2 x 500mm x 1000mm), a desk
and a chair were placed in the chamber. A
heated mannequin (see Fig. 1) was made from
1 mm aluminium sheet with the overall surface
area of 1.60m2. Heating elements inside the
body, head and legs of the mannequin were
controlled to provide a surface temperature
Presented at the International conference Roomvent 2002, Copenhagen, September 2002.
equal to that of a typical naked human
(Olesen,1982). A polyurethane tube was
attached to a copper tube (the nose) inside the
head and fed through the torso and out to the
gas sampler. This location represented the
sampling point for the breathing zone.
Four-wire Platinum Resistance Thermometer
(PRT) sensors (accuracy = ±0.15K) have been
used to measure the air temperature and the
inside and outside surface temperatures of the
chamber. Other measuring devices used in the
tests were an accurate Wattmeter, DANTEC
omni-directional velocity sensors and a Brüel
and Kjäer SF6 gas sampling system. The SF6
gas analysis system incorporated a sampling
box, gas analyser and a computer with analysis
and control software. Further details can be
found in Xing et al (2001).
calculated velocity, air temperature and radiant
temperature at each computational cell were
used in Fanger’s comfort equation (Fanger,
1972) for calculating the PD and PPD at the
cell. The calculation of local mean age of air,
τ p , in a room combines the air flow model with
the transport equation for age of air. The
transport equation for the local mean age of air,
τ , is:
Ui
∂τ p
∂xi
=
∂
∂xi
⎧⎪⎛ ν t ν ⎞ ∂τ p ⎫⎪
+ ⎟
⎨⎜⎜
⎬ +1
⎪⎩⎝ σ τ σ ⎟⎠ ∂xi ⎪⎭
(2)
where Ui is the mean velocity component in xi
direction, τ p is the local mean age of air,
ν and ν t are the laminar and turbulent
kinematic viscosity respectively, σ is the
laminar Scmidt (Prandtl) number and σ τ is the
Scmidt number for age of air (=1.0).
Ventilation Parameters
Local Air Change Index ( ε ap )
ε ap , the ratio of nominal time constant ( τ n ) to
the local mean age of air ( τ p ), is expressed as
ε ap =
Fig. 1: Mannequin and Computer box
Two cooling loads for the case I and case II
was 36 W/m2, 60W/m2 respectively and air
change rate was 5ach, i.e. air supply rate of
25l/s.
Using tracer decay gas technique, SF6 gas
was injected from venturi in order to obtain the
contaminant removal effectiveness and the age
of air at a number of points in the room. The
local age of air at any point in the room can be
calculated using following expressions:
∞
∫ C p (t )dt
τ p= 0
C (0)
(3)
Ventilation Parameter (VP)
To assess the effectiveness of a ventilation
system in both measurement and CFD
simulation, the effectiveness for heat removal
( ε t ) and contaminant removal ( ε c ) are used
together with the predicted percentage of
dissatisfied (PPD) for thermal comfort and
percentage of dissatisfied (PD) for air quality.
ε t and ε c are defined by:
(1)
εt =
CFD Calculations
The CFD program VORTEX (Gan and Awbi,
1994) has been used to predict the air flow,
heat transfer and mean age distribution in the
chamber. This is a three dimensional program
which solves, using a Cartesian grid, the
continuity equation, the Navier –Strokes
equation, the thermal energy equation, the
concentration of species equation, the two
equations for k and ε in the k − ε turbulence
model and room surface radiation. The
τn
τp
To − Ti
Tm − Ti
and ε c =
Co − Ci
Cm − Ci
(4)
In equations (4), T is temperature (oC), C is the
contaminant concentration (ppm), subscripts o,i
and m denote outlet, inlet and mean value for
the occupied zone (to a height of 1.8m). ε t is
similar to a heat exchanger effectiveness and is
a measure of the heat removing ability of the
system. ε c is a measure of how effectively the
contaminant is removed. The values for
and
ε c are determined by heat and
εt
Presented at the International conference Roomvent 2002, Copenhagen, September 2002.
contaminant sources, the method of room air
distribution, room characteristics, etc. However,
high values do not always give a good
indication of the thermal comfort and air quality
in the occupied zone.
Fanger (1972) has developed expressions for
the percentage of dissatisfied (PD) with the
indoor air quality and the predicted percentage
of dissatisfied (PPD) with the thermal
environment given by Eqs. (5) and (6).
PD = 395 ⋅ exp(−1.83 v& )
(5)
4
PPD = 100 – exp -{0.03353 (PMV) + 0.2179
(6)
(PMV)2}
0.25
Where v& is the ventilation rate (ls-1) and PMV
is the Predicted Mean Vote as defined in ISO
7730(1994) and the recommended PPD limit
for ideal thermal environment is 10%,
corresponding to -0.5 ≤ PMV ≤ 0.5. Thus, low
values for both indices guarantee a good
indoor air quality and thermal comfort.
The comfort number, Nt , and the air quality
number, Nc , (Awbi 1998) combined with PPD
and PD respectively are useful to examine the
quality of a ventilation system. These are
defined as:
Nt =
εt
PPD
,
Nc =
εc
PD
(7)
These two numbers can be combined into a
single parameter which determines the
effectiveness of an air distribution system in
providing air quality and thermal comfort in the
form of a Ventilation Parameter defined as:
VP =
Nt × Nc
(8)
Results and Discussions
Tests with the four types of system (MV, DV,
DF, IJV) were carried out for 5ach and two heat
loads (36 W/m2 without heated plates and
60W/m2 with heated plates).
Table1 summarises the test conditions and the
results obtained from the tests and the CFD
simulations at the same conditions for the 8
case. The overall agreement between the
measured and predicted (CFD) are generally
good. The discrepancies between the
measured and CFD can be due to a limited
number of measuring points, a poor accuracy in
measuring velocities less than 0.1m/s and the
changes in shape of air inlets when a Cartesian
grid is used in the CFD solutions.
ε c are not correlated with ε t
since ε c is affected by convection and ε t is
The results for
mainly affected by convection and radiation as
Heiselberg and Sandberg (1990) found. The
heat removal effectiveness for all four
ventilation systems is generally satisfactory.
The mixing ventilation for high load (case II)
has a high ε t , however PPD is also high
because of dumping of the cold jet into the
occupied zone. Thus MV cannot guarantee
thermal comfort and energy saving for large
heat loads.
For the floor displacement ventilation, the air
does not flow over the floor as expected but
actually spreads from the unit at an angle to
the floor. Thus the PPD is too high which also
affects the ventilation parameter (VP) as shown
in Fig. 2. Also, the air entrained by the
mannequin is a mixture of the supply air and
room air, hence the air change index at the
breathing zone for case I is lower than what is
expected, see Fig.3. For wall displacement
system, case I (36 W/m2), ε ap and VP are
generally good but for case II (60 W/m2) there
is difficulty in satisfying the thermal comfort
criterion.
The impinging jet acts as displacement
ventilation and produces a higher velocity in a
thin layer over the floor. It also gives the
highest values of VP and an air change index
which is similar to displacement systems (DV
and DF systems).
Conclusions
1. The new Ventilation Parameter (VP) can
provide useful information on ventilation
performance of a system with respect to
thermal comfort and indoor quality.
2. Although mixing ventilation can remove a
high heat load, the thermal comfort may not be
adequate as high velocity cold air can produce
draught and a high PPD.
3. The impinging jet system produced higher
values of the ventilation parameter (VP) than
the other systems examined. It is still capable
of achieving better air distribution in the space
than the other three systems at the higher heat
load.
References
Awbi, H B (1998) Energy Efficient Room
Air Distribution, Renewable Energy, Vol.15, pp293299.
Fanger, P.O.(1972) Thermal comfort.
Presented at the International conference Roomvent 2002, Copenhagen, September 2002.
McGraw-Hill New York.
Gan, G and Awbi, HB (1994): Numerical
simulation of the indoor environment, Building and
Environment, Vol. 29, No. 4, pp 449-459.
Heiselberg, P. and Sandberg, M (1990)
Convection from a slender cylinder in a ventilated
room, Roomvent 90’,Norway
ISO/CEN 7730 (1994) Moderate thermal
environments: Determination of PMV and PPD
indices and specification of the conditions for thermal
comfort.
Karimipanah, T. and Awbi, H.B. (2002)
Theoretical and experimental investigation of
impinging jet ventilation and comparison with wall
displacement ventilation, to be published in Building
and Environment.
Olesen, B.W. (1982) Thermal comfort,
Bruel and Kjaer Technical Review, No 2
Xing, H Hatton, A. and Awbi H.B. (2001) A
study of air quality in the breathing zone in a room
with displacement ventilation, Building and
Environment, 2001;Vol 36: 809-820.
Fig 2: Ventilation Parameters (VP) at
different systems
Fig 3: Local air change index ( ε ap ) at
the breathing zone at 36W/m2
Table 1: Data for Experiments and CFD
Pv
I
DV
DF
MV
IJV
II
DV
DF
MV
IJV
Tin
Vm
T0.1
T1.1
εc
( %)
(%)
PPD
PD
(%)
(%)
Nt
Nc
VP
2
36W/m
EXP
CFD
EXP
CFD
EXP
CFD
EXP
CFD
60W/m2
EXP
CFD
EXP
CFD
EXP
CFD
EXP
CFD
24
18
31
18
53
13
35
18
43
18
41
18
67
12
37
18
0.07
0.03
0.05
0.05
0.10
0.08
0.07
0.06
23.9
24.4
23.8
24.1
24.5
24.5
23.3
23.5
26.0
25.9
25.0
24.9
25.3
25.1
25.3
25.2
126
106
95
93
115
121
114
124
99
101
111
113
0.06
0.05
0.05
0.04
0.12
0.09
0.07
0.06
26.2
25.6
26.3
25.4
26.9
26.6
23.3
24.3
27.9
27.3
28.8
29.2
28.1
27.6
26.1
26.9
137
159
97
103
117
118
97
102
101
105
108
112
2
Ti = Inlet temp. (oC)
Pv =Ventilation load(W/m )
Vm = Mean velocity in the occupied zone (m/s)
View publication stats
εt
7.8
6.4
16.1
18.0
17.0
12.4
6.6
8.5
18.7
12.7
7.7
6.4
12.1
15.7
13.8
7.1
6.5
17.0
17.4
17.2
13.9
6.5
9.8
14.9
12.1
28.3
6.5
5.6
15.7
9.4
22.2
6.4
4.6
16.4
8.7
7.9
6.5
14.9
17.2
16.0