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An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system
Accepted Manuscript Title: An experimental study on a full-scale indoor thermalenvironment using an Under-Floor Air Distribution system Authors: Lin, Y.J.P. Tsai, T.Y. PII: DOI: Reference:
S0378-7788(14)00432-0 http://dx.doi.org/doi:10.1016/j.enbuild.2014.05.028 ENB 5064
To appear in:
ENB
Received date: Revised date: Accepted date:
26-3-2014 20-5-2014 22-5-2014
Please cite this article as: Lin, Y.J.P., Tsai, T.Y., An experimental study on a full-scale indoor thermalenvironment using an Under-Floor Air Distribution system, Energy & Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.05.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Research Highlights on ‘An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system’ include
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• Characteristics of an indoor space using an Under-Floor Air Distribution system are presented. • Indoor air stratification in the steady state for three flow rate options is presented.
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• This research focuses on the influence of supply air flow rate on the indoor environment.
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• Temperature profile is highly dependent on the distance away from the supply diffuser position.
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• The flow rate, and its corresponding momentum and buoyancy fluxes, have different effects on stratification.
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Lin, Y. J. P.∗, Tsai, T. Y.
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An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system
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Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Section 4, Keelung Rd., Taipei 106, Taiwan.
Abstract
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Characteristics of thermal environment in a full-scale indoor space utilizing an Under-Floor Air Distribution (UFAD) system are investigated by
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conducting and analyzing experimental measurements. The indoor air strat-
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ification of different locations in the steady state for three supply flow rate options is presented. This research focuses on the influence of supply air flow
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rate, and its corresponding momentum and buoyancy fluxes, on the vertical temperature profile in the indoor environment. Temperature measurements show that the indoor vertical temperature profile is influenced highly by the distance away from the supply diffuser position. Experimental results also show that the supply air flow rate has a strong effect on the vertical temperature profile. When the supply air flow rate of a given diffuser increases, and then the supply air momentum flux increases as well, the gradient of the vertical temperature profile becomes gentler. The stratification height in ∗
Corresponding author. Tel: +886-2-2730-3235; Fax: +886-2-2737-6460. Email address: [email protected] (Lin, Y. J. P.)
Preprint submitted to Energy and Buildings
May 20, 2014
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the indoor environment ascends with a higher total flow rate. The almost
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same throw height at the supply diffuser is observed for the identical flow rate option in two sets of experiments with different total flow rates.
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Keywords:
Under-Floor Air Distribution (UFAD) system, vertical temperature profile,
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stratification height, throw height, jet flow, gravity current Nomenclature :
opening area of the supply diffuser (m2 )
D
:
distance away from the supply diffuser (m)
F0
:
buoyancy flux of the supply jet at the source (m4 s−3 )
g
:
gravitational acceleration (m s−2 )
g0
′
:
reduced gravity of the supply jet at the source (m s−2 )
Kt
:
proportional constant for estimating the throw height (-)
lm
:
thermal length scale of the supply air jet (m)
M0
:
momentum flux of the supply jet at the source (m4 s−2 )
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Ae
Q0
:
volumetric flow rate of the supply jet at the source (m3 s−1 )
Qi
:
flow rate option, i=1, 2, 3.
S.H.
:
stratification height (m)
T.H.
:
throw height (m)
T0
:
temperature of the supply jet at the source (o C or K)
Tr
:
reference temperature for normalization (o C or K) Greek symbols
∆
:
magnitude of the difference (-)
ρ0
:
density of the supply jet at the source (kg m−3 )
ρr
:
reference density for normalization (kg m−3 ) 2
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1. Introduction Ventilation (cooling and heating) in buildings consumes a large amount
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of energy nowadays. Particularly, air conditioning systems have become in-
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creasingly popular. Such systems consume a huge amount of energy. Ac-
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cording to Refs. [1, 2], non-industrial buildings contribute 30 – 50% of all
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primary energy consumption in Organization for Economic Cooperation &
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Development countries, and ventilation operation consumes as much as 50%
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of the amount attributed to the non-industrial buildings section.
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The Under-Floor Air Distribution (UFAD) system originally was intro-
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duced in the 1950s. Its main purpose was to remove high heat loads in the
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spaces. In the 1970s, UFAD system was introduced into office buildings, in
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West Germany, to remove heat load in the office. Recently UFAD systems
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have achieved considerable acceptance in Europe, South Africa, Japan and
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North America.
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UFAD systems differ from a traditional overhead ventilation system. UFAD systems usually use an under-floor supply plenum and a raised access floor
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system to supply the conditioned air through floor supply diffusers. The
conditioned air is delivered into the occupied zone of the indoor environment directly (mostly near the floor level). Room air is extracted through return
vents (mostly on the ceiling and sometimes on the floor), and then returns
to the air conditioner.
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Alajmi and El-Amer [3] investigated the energy consumption of UFAD
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systems in commercial buildings for various types of application and at dif-
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ferent air supply temperatures in a hot climate. Their EnergyPlus simulation
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results show that UFAD systems have a significant energy saving compared 3
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to ceiling-based air distribution (CBAD, i.e. overhead ventilation system),
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especially for high ceiling buildings.
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Bauman and Webster [4] showed that UFAD has several benefits, such
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as reduced life cycle building costs, improved thermal comfort, improved
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ventilation efficiency and indoor air quality (IAQ), reduced energy use, re-
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duced floor to floor height in new construction and improved productivity
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and health. A design guide book by Bauman [5] provides very comprehensive
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information on UFAD systems.
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Webster et al. [6] showed the impact of air flow rate and supply air
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temperature on the thermal stratification for UFAD systems. As the air
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flow rate increases, room air stratification decreases. When the supply air
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temperature is varied, the temperature profile almost keeps the same shape,
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but the temperature profile moves to higher or lower temperature.
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Lin and Linden [7] presented a study on the steady-state flow driven by
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a heat source and an UFAD cooling diffuser in a ventilated space having a
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ceiling vent. The model was based on plume theory for the heat source and
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the displacement case and lower layer temperature increases. The interface
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height is controlled by the ventilation rate and momentum flux, and the
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temperature contrast between the two layers is set by the momentum flux.
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a fountain model for the diffuser flow and assumed a steady-state two-layer stratification in the room. The control parameters are the buoyancy flux
of the heat source, the volume flux and the momentum flux of the cooling diffuser. The results showed that when the vertical momentum is small, the entrainment mechanism at interface is not significant. When the vertical
momentum increases, the interface is raised above the height obtained by
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The heat load determines the temperature in the space for given ventilation
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condition. This simple model was extended by Liu and Linden [8] to consider
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more general conditions.
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Wan and Chao [9] found that the temperature stratification in the enclo-
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sure with UFAD systems highly depended on the thermal length scale of the
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floor supply jet issued from the supply diffuser. When the thermal length
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scale is greater than 1, temperature stratification is minor for all tested heat
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densities and air distribution methods. When the thermal length scale is less
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than 1, there is a significant vertical temperature gradient.
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Kong and Yu [10] showed that the combination effect of three parameters,
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i.e. heat load, supply air flow rate and supply air velocity, on room air
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temperature stratification would be expressed by the length scale of the floor
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supply jet, which is the same as that shown in [9]. Their numerical simulation
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results, by using computational fluid dynamics, show that there is an interface
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formed, when there is only one local heat source in the room. The interface
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height is about 1.42 times the length scale. The maximum height of the
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In the lower narrow zone, the temperature changes abruptly and increases
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linearly. In the transitional zone, it can be considered as the transitional
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region between the lower cooler zone and the upper warmer zone. In the
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supply air jet is about 1.56 times the length scale. Wang et al. [11] investigated the thermal stratification characteristics in
a full-scale UFAD experimental space. Their results show that 4 zones are
considered to compose the vertical thermal stratification. They are bottom cooler zone, lower narrow zone, transitional zone, and upper warmer zone.
In the bottom cooler zone, the temperature gradient is relatively unclear.
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upper warmer zone, the temperature is much higher than other zones. Characteristics of thermal environment in a test chamber utilizing an
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UFAD system are investigated through experimental measurements in this
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research. The research examines the influence of supply air flow rate, or
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its corresponding momentum and buoyancy fluxes, on the vertical temper-
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ature profile in the indoor environment. The paper presents experimental
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results of a full-scale indoor thermal environment in a test chamber. We
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focus on the connections between temperature stratification and supply air
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conditions, and neglect the role of humidity in this paper. Dimensions of the
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experimental space, measuring instruments and experimental arrangements
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are introduced in Section 2. Experimental results of measured velocity and
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temperature are presented and discussed in Section 3. Section 4 summarizes
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the findings in this research.
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2. Full-scale experiments
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2.1. The test chamber
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Measurements are conducted in a full-scale test chamber on the sixth floor
of Taiwan Building Technology Center (T.B.T.C.) building, which is located in the campus of National Taiwan University of Science and Technology (NTUST) in Taipei. Taipei is in a monsoon-influenced humid subtropical climate. Using air-conditioners to cool the indoor air temperature is common from May to October in Taipei.
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The measurement space comprises the Main area and the area in front of
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Display stand, and is marked in Figure 1(a). The space of the Main area is
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810 cm (length) × 625 cm (width) × 325 cm (height) and the space of the 6
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area in front of Display stand is 190 cm (length) × 230 cm (width) × 325 cm
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(height). The total floor area of the space is 55 m2 and the measurement
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space is empty of furniture.
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The conditioned air is delivered into the space through floor supply dif-
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fusers. The room air exits by two square return vents on the ceiling, then
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moves to the space behind the return air wall and finally comes back to the
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air conditioners which are placed beneath the floor. Either of the return
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vents has the size of 60 cm × 60 cm. The air motion is cyclic as shown in
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Figure 1(b).
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The conditioned cool air is delivered by 8 rectangular supply diffusers
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for the CAC & WAC set and 4 diffusers for the CAC set. Here CAC and
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WAC denote the central and the western air conditioners respectively. Each
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diffuser has the same size of 40 cm (length) × 20 cm (width) on the floor and is
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located in the perimeter zone of the measurement space as shown in Figure 1.
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Each diffuser with linear bar grilles is a passive type and is connected to an
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air conditioner via a duct. Two sets of temperature measurements, the CAC
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& WAC and CAC sets, are presented in this paper, and their arrangements are described in Section 2.3. 2.2. Experimental instruments The measuring instruments include a hot-wire anemometer for measuring
the air supply velocity, DICKSON TK550 thermistors for measuring the
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outdoor weather temperature and the supply conditioned air temperature,
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and T-type thermocouples for measuring the indoor temperature at different
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vertical levels.
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A separate temperature measurement experiment was carried out to calibrate the DICKSON TK550 thermistors and the thermocouples by using a testo
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Pt100 probe as a benchmark. Measurement deviations from this benchmark
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were all less than 0.5o C, and mostly 0.3o C, for the DICKSON TK550 thermistors
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and the thermocouples used in this research.
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The conditioned air supply velocity is measured at 6 measuring points
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just above a supply diffuser by an anemometer, as sketched and shown in
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Figure 2. Each measuring point has 3 recording data by using the eyeball
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reading method, and 90 recording data during the span of three minutes by
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using the data logger recording method. The time-averaged velocities of 6
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measurement points are used to estimate the flow rate of the supply diffuser.
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Experiments of temperature measurements are carried out during the
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night and the window blind is used to reduce the radiation effect on the
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indoor environment. The outdoor temperature is measured by a thermistor
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which is placed on the balcony, and the supply conditioned air temperature
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is measured by a thermistor which is placed in the air supply duct. Each
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2.3. Experimental sets
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thermistor has the acquisition frequency of every two consecutive measure-
ments between an interval of 4 minutes. There are 16 levels, i.e. 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, 2.7, 2.9 and 3.1 m above the floor, for measuring the indoor temperature. T-type thermocouple has
the acquisition frequency of every two consecutive measurements between an interval of 2 seconds.
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According to the air conditioners used in experiments, experiments of
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temperature measurements are classified as 2 sets; (1) the central and the 8
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western air conditioners are used (CAC & WAC set), as shown in Figure 3,
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and (2) only the central air conditioner is used (CAC set), as shown in
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Figure 4. Two sets of experimental conditions are presented in Table 1.
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Average values of supply air temperature and outdoor air temperature from
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20:46 to 23:00 in experiments are listed in the table and regarded as the
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conditions in the steady state.
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A typical experiment usually takes about 1 to 2 hours after turning on
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the air conditioners to make the space from the initial state to the steady
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state. Results of the measured temperature against the time for a CAC &
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WAC experiment with the Q1 option are presented in Figure 5, and there
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are temperature measurements at 3 different levels, i.e. 0.5, 1.5 and 2.5 m,
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in this figure. The air conditioner is turned on at 19:00 (i.e. 1140 min., and
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00:00 as 0 min.) and is turned off at 23:00 (i.e. 1380 min.).
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Figure 5 shows that after turning on the air conditioner there is a transient
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state before the measured temperature at a fixed level becomes steady. This
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experiment has approached the steady state at 20:46 (i.e. 1246 min). After
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near the conditioned air supply area, or the clear zone. The locations B, C,
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F and G are in the free area. The locations D and E are in the air return
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area.
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this time, the temperature measuring pole is moved to different locations, as
shown in Figures 3 and 4, to collect the indoor temperature distribution data.
The time span at every measuring location for this experiment is shown in Figure 5 as well.
There are 9 locations for measuring the vertical temperature profile in
the CAC & WAC set, as shown in Figure 3. The locations A, H and I are
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The CAC set is conducted to investigate variations in the vertical temper-
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ature profiles due to different distances away from the conditioned air supply
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diffuser. The locations for measuring the temperature profile are respectively
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75 cm, 150 cm and 225 cm away from the center of 4 supply diffusers in the
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CAC set, as shown in Figure 4.
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3. Results and discussion
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3.1. Conditioned air flow rate
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Experimental measurements of the supply air velocity at the supply dif-
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fuser give the supply air flow rate for a single supply diffuser and are used to
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estimate the total supply air flow rate in the experimental space. Experimen-
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tal results of conditioned air supply velocity give the flow rate for one supply
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diffuser as 0.022 m3 s−1 for the Q1 option, 0.024 m3 s−1 for the Q2 option
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and 0.033 m3 s−1 for the Q3 option by using the eyeball reading method.
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Experimental results using the data logger recording method show that the
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deviation on velocity measurements between two methods is less than 10%.
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The supply diffuser has the size of 40 cm (length) × 20 cm (width) and the grille porosity (or the void ratio) of 50%. Experimental results of air supply velocity are presented in Table 2. The total supply air flow rate of the CAC & WAC set, using 8 supply
diffusers, is estimated to be 0.18 m3 s−1 for the Q1 option, 0.20 m3 s−1 for
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the Q2 option and 0.27 m3 s−1 for the Q3 option. The total supply air flow
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rate of the CAC set, using 4 supply diffusers, is half of that for the CAC &
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WAC set.
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3.2. Vertical temperature profiles for 2 sets of experiments Temperature measurements presented hereinafter are regarded as in the
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steady state. Figure 6 shows the vertical temperature profiles of the CAC
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& WAC set in the steady state with three different flow rate options. There
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is a clear stratified interface formed in the room and its level changes with
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the flow rate. When the supply air flow rate increases, indoor temperature
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stratification decreases because of disturbance by the stronger supply inflow
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momentum.
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When the measuring location (A, H or I) is near the supply diffuser, as
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shown in Figure 3, the temperature profile in Figure 6 is influenced clearly by
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the supply conditioned air jet flow, especially the bottom part of the profile.
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However, the vertical temperature profile is not affected significantly by the
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position of return vent (D or E) and in the free area (B, C, F or G), as shown
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in Figure 6.
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Figure 7 shows the vertical temperature profiles of the CAC set. Experi-
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mental results show the relationship between the vertical temperature profile
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and the distance from the measuring location to the supply diffuser with three different flow rate options. When the distance is far enough from the
supply diffuser, the vertical temperature profile is unaffected by the supply conditioned air jet flow. Experimental results indicate that when the distance from the supply diffuser is larger than 150 cm, the vertical temperature pro-
file seems unaffected by the supply conditioned air jet flow.
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Figures 6 and 7 show that the location near the supply diffuser has quite
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similar vertical temperature profiles for the same flow rate option. The con-
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ditioned air jet flow, or the fountain flow as described in Ref. [7], is dominant 11
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in this air supply area. As the location is away from the supply diffuser,
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the gravity current flow, or the horizontal flow due to density difference as
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described by Simpson [12] or Ungarish [13], becomes important in the free
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area. This gravity current flow pattern was also observed in the space using
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displacement ventilation as shown by Li et al. [14].
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The conditioned air flow rate for the CAC set is only half of that for the
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CAC & WAC set. The locations in the free area, (D=150 and 225 cm for the
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CAC set) have different characteristics in the bottom occupied zone between
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2 sets of experiments, but similar results in the top warm zone for the same
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flow rate option. The top warm zone has a gentle temperature gradient in all
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experiments. The bottom occupied zone of the CAC set with a lower flow rate
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has a thinner layer compared to the CAC & WAC set. Because the bottom
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occupied zone is formed mainly by properties of the supply conditioned air,
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as shown in Ref. [7], the less supply flow rate gives a shallower layer of the
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bottom occupied zone.
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3.3. Throw height and stratification height
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Figure 8 shows the temperature profiles in the air supply area and the
free area respectively for the CAC & WAC set with three different flow rate options. It is obvious that the bottom part of the temperature profile in the
air supply area is influenced by the supply conditioned air jet flow. Experi-
mental results show that the temperature profile above a certain level in the
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air supply area approximates to that in the free area. This specific level is
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regarded as the throw height (T.H.) for the supply diffuser. The throw height
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is considered as the maximum height which the supply air jet flow can reach.
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Experimental results show that the throw height is about 0.9 m for the Q1 12
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option, 0.9 m for the Q2 option, and 1.9 m for the Q3 option. Using the thermal length scale of the supply conditioned air jet introduced
249
by Refs. [9, 10, 15, 16], or similarly the fountain model by Ref. [17], gives
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estimations of the throw height as shown in Table 3. Two proportional
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constants of Kt , 1.56 in Ref. [10] and 1.85 in Ref. [17], are used to calculate
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the throw height as T.H. = Kt lm .
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(1)
This simple approach has a reasonable agreement with experimental results.
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The thermal length scale is defined as
3
,
M
lm =
M04
1
| F0 | 2
(2)
where M0 is the momentum flux and F0 is the buoyancy flux of the supply
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jet source as
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and
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te
Q20 M0 = Ae
F0 =
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∆ρ0 Q0 g ′ = g0 Q0 . ρr
(3)
(4)
Q0 is the volume flux and Ae is the opening area of the supply source. F0
has a negative value because ∆ρ0 = ρr − ρ0 is the density difference between the supply source conditioned air and the indoor air, and the density of the supply source conditioned air, ρ0 , is larger than that of the indoor air, ρr , for the cooling cases. ρr is the reference air density and the indoor air density is
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used in this research. g is the gravitational acceleration. For an ideal gas with
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a moderate temperature difference ∆T0 , a linear approximation is applicable
265
and the normalized density difference
∆ρ0 ρr
0 can be expressed as − ∆T . Either Tr
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of two normalized terms can be expressed as the reduced gravity as ′
g0 =
∆ρ0 ∆T0 g=− g. ρr Tr
(5)
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Figure 9 shows the profiles of averaged temperature in the free area and
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the air return area, and the profiles of the corresponding gradient for three
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different flow rate options. The level having the maximum vertical temper-
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ature gradient is regarded as the stratification height (S.H.). The stratifica-
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tion height is regarded as the level which separates a room into two zones.
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There is a more uniform temperature with a gentle gradient in the upper
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zone above the stratification height and the temperature of the lower zone
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beneath the stratification height is clearly influenced by the conditioned air
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flow. Properties of the lower layer highly depend on the conditions of the
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supply conditioned air, as discussed in Section 3.2. Experimental results
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show that the stratification height is about 0.7 m for the Q1 option, 0.7 m
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for the Q2 option, and 1.3 m for the Q3 option.
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When the supply air flow rate increases, the stratification height rises as
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well as the throw height. The ratio of the stratification height to the throw height for the CAC & WAC set in this research is between 0.68 and 0.78, which is slightly less than the value, 0.9, reported by Ref. [10]. 4. Conclusions
An experimental study on the space using an UFAD system is conducted
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and the indoor vertical temperature profiles due to different supply condi-
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tions are analyzed in this research. The indoor air stratification of different
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locations in the steady state for three flow rate options is presented in the pa-
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per. According to experimental results, the supply air flow rate has a strong 14
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effect on the vertical temperature distribution in the steady state. When the
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supply conditioned air flow rate increases for a given diffuser, i.e. the momen-
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tum flux increases as well, the vertical temperature gradient becomes gentler
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and the indoor air stratification reduces. The higher supply flow rate pro-
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vides a thicker layer of the bottom occupied zone. The throw height and the
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stratification height both ascend with the higher supply air flow rate for the
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same set of experiments. The vertical temperature profile is also influenced
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by the distance away from the supply diffuser. When the location is near
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the supply diffuser, the vertical temperature profile is influenced significantly
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by the conditioned air jet flow. The vertical temperature profile seems unaf-
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fected by the position of return air vent. When the distance from the supply
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diffuser is reasonably far, the vertical temperature profile seems unaffected
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by the supply conditioned air jet flow and the gravity current flow becomes
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important. The relationship between the spread of the gravity current flow
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and the stratification height in the space requires further investigation to be
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clarified.
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315
[9] M. P. Wan, C. Y. Chao, Numerical and experimental study of velocity and temperature characteristics in a ventilated enclosure with underfloor ventilation systems, Indoor Air 15 (2005) 342–355.
[10] Q. Kong, B. Yu, Numerical study on temperature stratification in a room with underfloor air distribution system, Energy and Buildings 40 (2008) 495–502.
330
[11] X. Wang, C. Huang, W. Cao, X. Gao, W. Liu, Experimental study on
331
indoor thermal stratification in large space by under floor air distribution
332
system (UFAD) in summer, Engineering 3 (2011) 384–388. 16
Page 17 of 31
335
336
2nd Edition, Cambridge University Press, 1997.
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334
[12] J. Simpson, Gravity Currents in the Environment and the Laboratory,
[13] M. Ungarish, An Introduction to Gravity Currents and Intrusions, CRC Press, 2009.
cr
333
[14] Y. Li, M. Sandberg, L. Fuchs, Vertical temperature profiles in rooms
338
ventilated by displacement: full-scale measurement and nodal modeling,
339
Indoor Air 2 (1992) 225–243.
an
341
[15] D. Etheridge, M. Sandberg, Building Ventilation: Theory and Measurement, John Wiley and Sons, 1996.
M
340
us
337
[16] W. Rodi, Turbulent Buoyant Jets and Plumes, Pergamon Press, 1982.
343
[17] J. S. Turner, Buoyancy Effects in Fluids, Cambridge University Press,
te
1973.
Ac ce p
344
d
342
17
Page 18 of 31
Figure 1: (a) The plan of the test chamber and the measurement space. (b) The perspec-
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tive view of the test chamber.
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Figure 2: (a) The schematic diagram of 6 measurement points just above a supply diffuser
for measuring the supply velocity of the conditioned air. (b) A photo of the anemometer
us
above a diffuser.
an
Figure 3: The measuring locations in the CAC & WAC set of experiments.
M
Figure 4: The measuring locations in the CAC set of experiments.
Figure 5: Temperature measurements against the time for a CAC & WAC experiment
te
d
with the Q1 option.
Figure 6: Vertical temperature profiles of the CAC & WAC set with the flow rate option
Ac ce p
of (a) Q1 , (b) Q2 and (c) Q3 .
Figure 7: Vertical temperature profiles of the CAC set having different measuring distances from the supply diffuser with three different flow rate options.
Figure 8: The temperature profiles in the air supply area and the free area with the (a) Q1 , (b) Q2 and (c) Q3 option.
Figure 9: The average temperature profiles in the free area and the air return area and their corresponding gradient profiles with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
19
Page 19 of 31
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Figures and Tables (tex file)
The option
Average Outdoor
Temp.(o C)
Temp.(o C)
11.6
29.2
2012/09/03
11.5
27.8
2012/09/04
14.2
28.5
2012/10/17
9.8
22.6
Q2
2012/10/25
12.4
25.5
Q3
2012/10/24
14.6
23.9
Q2 Q3
2012/08/20
d
Q1 CAC & WAC
Ac ce p
Q1
CAC
Date
M
Average Supply
te
Run.
an
Table 1: Experimental conditions for the CAC & WAC and CAC sets.
1
Page 20 of 31
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us
Table 2: Experimental results of conditioned air supply velocity above a supply diffuser
an
and estimations of the total flow rate for the CAC & WAC set.
A single diffuser The option Average
Approach
Velocity
Q1
reading
0.56
(m3 s−1 )
(m3 s−1 )
(hr−1 )
0.022
0.18
3.6
te
eyeball
d
(m/s)
0.024
data logger
Q1
0.52
0.021
0.17
recording
Q2
0.55
0.022
0.18
Q3
0.82
0.033
0.27
Q2
0.61
0.20
4.0
Q3
0.83
0.033
0.27
5.4
Ac ce p ∗
Flow Rate Total Flow Rate A.C.H.∗
M
Experimental
CAC & WAC set
It denotes Air Change rate per Hour.
2
Page 21 of 31
ip t cr us an
Table 3: Experimental results and predictions on the throw height for the CAC & WAC set.
Predictions
F0
M0
T.H.
(m4 s−3 )
(m4 s−2 )
(m)
(m)
(m)
0.0123
0.9
0.82
0.7
d
The option
M
Experimental results
-0.0069
Q2
-0.0063
0.0146
0.9
Q3
-0.0072
0.0274
1.9
Ac ce p
te
Q1
Kt
1.85
T.H.
0.98 1.47
Kt
1.56
T.H.
0.83 1.24
3
Page 22 of 31
ip t cr us an M d
(a)
Ac ce p
625
56
te
Return air vents
Western air conditioner
325
Return air wall
56
1230
Unit: cm
Supply diffusers
Central air conditioner (b)
Figure 1: (a) The plan of the test chamber and the measurement space. (b) The perspective view of the test chamber.
4
Page 23 of 31
C4-4
(10,12)
(20,12)
(30,12)
C4-1
C4-2
C4-3
(10,6)
(20,6)
(30,6)
C4
6
us
x
cr
y
12
20
C4-5
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C4-6
an
10 20
M
30 40
Unit: cm
Ac ce p
te
d
(a)
(b) Figure 2: (a) The schematic diagram of 6 measurement points just above a supply diffuser for measuring the supply velocity of the conditioned air. (b) A photo of the anemometer above a diffuser.
5
Page 24 of 31
ip t cr us an M d te Ac ce p
Figure 3: The measuring locations in the CAC & WAC set of experiments.
6
Page 25 of 31
ip t cr us an M d te Ac ce p
Figure 4: The measuring locations in the CAC set of experiments.
7
Page 26 of 31
ip t 34 locatition (C)
( A ) ( B )( D ) ( E ) ( F ) ( G ) ( H )( I )
an
32 30
M
26 24 22 20
d
Temperature ( oC )
28
te
18
H = 2.5 m H = 1.5 m H = 0.5 m
16
Ac ce p
14 12
cr
us
2012-08-20
Turn on A.C.
Turn off A.C.
10 1100 1120 1140 1160 1180 1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Time (min)
Figure 5: Temperature measurements against the time for a CAC & WAC experiment with the Q1 option.
8
Page 27 of 31
(a) 2012-08-20 Vertical Temperature Distribution 3
2
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Height (m)
2.5
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Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
1.5
0.5
0 10
15
20
25
Temperature (oC)
30
an
(b)
us
1
2012-09-03 Vertical Temperature Distribution Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
3
M
2
1.5
1
te
0.5
d
Height (m)
2.5
0 10
15
20
25
Temperature (oC)
30
Ac ce p
(c)
2012-09-04 Vertical Temperature Distribution
3
Height (m)
2.5
2
Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
1.5
1
0.5
0 10
15
20
25
Temperature (oC)
30
Figure 6: Vertical temperature profiles of the CAC & WAC set with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
9
Page 28 of 31
2.5
0.5
0.5
25
2012-10-25 Vertical Temperature Distribution C-1 C-2 C-3 C-4 C-5 C-6 C-7
2
2.5
2
1.5
1
0.5
0.5
25
30
C-1 C-2 C-3 C-4 C-5 C-6 C-7
Height ( m )
2
1.5
20
2.5
25
0 10
30
C-8 C-9 C-10 C-11 C-12
2.5
2
1.5
0 10
20
25
Temperature ( oC )
30
C-13 C-14 C-15 C-16 C-17
3
2
Q3
1.5
0.5
30
15
2012-10-24 Vertical Temperature Distribution
0.5
25
Q2
0.5
0.5
20
30
2
1
Temperature ( oC )
25
C-13 C-14 C-15 C-16 C-17
3
1
15
20
Temperature ( oC )
2012-10-25 Vertical Temperature Distribution
1
0 10
15
1
2012-10-24 Vertical Temperature Distribution
2.5
0 10
30
1.5
Temperature ( oC )
3
Ac ce p
2.5
15
te
2012-10-24 Vertical Temperature Distribution 3
0 10
Height ( m )
20
Temperature ( oC )
d
1
15
25
C-8 C-9 C-10 C-11 C-12
3
1.5
0 10
20
Temperature ( oC )
Height ( m )
Height ( m )
2.5
15
2012-10-25 Vertical Temperature Distribution
Height ( m )
3
0.5
0 10
30
Q1
1
an
20
Temperature ( oC )
1.5
M
15
2
cr
1.5
1
0 10
2.5
2
1
C-13 C-14 C-15 C-16 C-17
3
us
1.5
2012-10-17 Vertical Temperature Distribution
Height ( m )
2
C-8 C-9 C-10 C-11 C-12
3
Height ( m )
Height ( m )
2.5
2012-10-17 Vertical Temperature Distribution
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C-1 C-2 C-3 C-4 C-5 C-6 C-7
3
Height ( m )
2012-10-17 Vertical Temperature Distribution
15
20
25
Temperature ( oC )
D≤ 75 cm
30
D= 150 cm
0 10
15
20
25
Temperature ( oC )
30
D= 225 cm
Figure 7: Vertical temperature profiles of the CAC set having different measuring distances from the supply diffuser with three different flow rate options.
10
Page 29 of 31
(a)
2012-08-20 Vertical Temperature Distribution
Height (m)
2
1.5
1
0 10
25
30
2012-09-03 Vertical Temperature Distribution Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
3
M
2.5
2
Height (m)
20
Temperature (oC)
an
(b)
15
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T.H. = 0.9 m
0.5
cr
2.5
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Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
3
1.5
d
1
te
0.5
0 10
(c)
20
25
Temperature (oC)
30
Ac ce p
2012-09-04 Vertical Temperature Distribution
3
2.5
Height ( m )
15
T.H. = 0.9 m
Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
2
T.H. = 1.9 m
1.5
1
0.5
0 10
15
20
o
25
30
Temperature ( C )
Figure 8: The temperature profiles in the air supply area and the free area with the (a) Q1 , (b) Q2 and (c) Q3 option.
11
Page 30 of 31
(a) 2012-08-20_Q1_Vertical Average Temperature Distribution
2012-08-20_Q1_Vertical Temperature Gradient
2.5
2
2
1.5
1
S.H.= 0.7 m
0.5
0 10
0.5
15
20
25
Average temperature (oC)
0
30
Location (C), (B), (D), (E), (F)
M Height (m)
0.5
15
2
1.5
1
S.H.= 0.7 m
20
25
d
Height (m)
1.5
0
30
Ac ce p
2.5
2
2
Height (m)
Height (m)
2.5
S.H.= 1.3 m
1
0.5
0.5
20
25
20
0
30
S.H.= 1.3 m
1.5
1
Average temperature (oC)
10
Gradient (oC/m)
3
Location (C), (B), (D), (E), (F)
15
0
2012-09-04_Q3_Vertical Temperature Gradient
2012-09-04_Q3_Vertical Average Temperature Distribution
0 10
S.H.= 0.7 m
0.5
te
Average temperature (oC)
(c)
1.5
20
2.5
2
3
10
Gradient (oC/m)
3
2.5
0 10
0
2012-09-03_Q2_Vertical Temperature Gradient
2012-09-03_Q2_Vertical Average Temperature Distribution
1
S.H.= 0.7 m
an
(b)
3
1.5
us
1
cr
2.5
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3
Location (C), (B), (D), (E), (F)
Height (m)
Height (m)
3
0
10
Gradient (oC/m)
20
Figure 9: The average temperature profiles in the free area and the air return area and their corresponding gradient profiles with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
12
Page 31 of 31
S0378-7788(14)00432-0 http://dx.doi.org/doi:10.1016/j.enbuild.2014.05.028 ENB 5064
To appear in:
ENB
Received date: Revised date: Accepted date:
26-3-2014 20-5-2014 22-5-2014
Please cite this article as: Lin, Y.J.P., Tsai, T.Y., An experimental study on a full-scale indoor thermalenvironment using an Under-Floor Air Distribution system, Energy & Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.05.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Research Highlights on ‘An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system’ include
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• Characteristics of an indoor space using an Under-Floor Air Distribution system are presented. • Indoor air stratification in the steady state for three flow rate options is presented.
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• This research focuses on the influence of supply air flow rate on the indoor environment.
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• Temperature profile is highly dependent on the distance away from the supply diffuser position.
Ac ce pt e
d
M
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• The flow rate, and its corresponding momentum and buoyancy fluxes, have different effects on stratification.
1
Page 1 of 31
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Lin, Y. J. P.∗, Tsai, T. Y.
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An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system
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Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Section 4, Keelung Rd., Taipei 106, Taiwan.
Abstract
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Characteristics of thermal environment in a full-scale indoor space utilizing an Under-Floor Air Distribution (UFAD) system are investigated by
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conducting and analyzing experimental measurements. The indoor air strat-
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ification of different locations in the steady state for three supply flow rate options is presented. This research focuses on the influence of supply air flow
Ac ce p
rate, and its corresponding momentum and buoyancy fluxes, on the vertical temperature profile in the indoor environment. Temperature measurements show that the indoor vertical temperature profile is influenced highly by the distance away from the supply diffuser position. Experimental results also show that the supply air flow rate has a strong effect on the vertical temperature profile. When the supply air flow rate of a given diffuser increases, and then the supply air momentum flux increases as well, the gradient of the vertical temperature profile becomes gentler. The stratification height in ∗
Corresponding author. Tel: +886-2-2730-3235; Fax: +886-2-2737-6460. Email address: [email protected] (Lin, Y. J. P.)
Preprint submitted to Energy and Buildings
May 20, 2014
Page 2 of 31
the indoor environment ascends with a higher total flow rate. The almost
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same throw height at the supply diffuser is observed for the identical flow rate option in two sets of experiments with different total flow rates.
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Keywords:
Under-Floor Air Distribution (UFAD) system, vertical temperature profile,
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stratification height, throw height, jet flow, gravity current Nomenclature :
opening area of the supply diffuser (m2 )
D
:
distance away from the supply diffuser (m)
F0
:
buoyancy flux of the supply jet at the source (m4 s−3 )
g
:
gravitational acceleration (m s−2 )
g0
′
:
reduced gravity of the supply jet at the source (m s−2 )
Kt
:
proportional constant for estimating the throw height (-)
lm
:
thermal length scale of the supply air jet (m)
M0
:
momentum flux of the supply jet at the source (m4 s−2 )
Ac ce p
te
d
M
an
Ae
Q0
:
volumetric flow rate of the supply jet at the source (m3 s−1 )
Qi
:
flow rate option, i=1, 2, 3.
S.H.
:
stratification height (m)
T.H.
:
throw height (m)
T0
:
temperature of the supply jet at the source (o C or K)
Tr
:
reference temperature for normalization (o C or K) Greek symbols
∆
:
magnitude of the difference (-)
ρ0
:
density of the supply jet at the source (kg m−3 )
ρr
:
reference density for normalization (kg m−3 ) 2
Page 3 of 31
1
1. Introduction Ventilation (cooling and heating) in buildings consumes a large amount
3
of energy nowadays. Particularly, air conditioning systems have become in-
4
creasingly popular. Such systems consume a huge amount of energy. Ac-
5
cording to Refs. [1, 2], non-industrial buildings contribute 30 – 50% of all
6
primary energy consumption in Organization for Economic Cooperation &
7
Development countries, and ventilation operation consumes as much as 50%
8
of the amount attributed to the non-industrial buildings section.
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2
The Under-Floor Air Distribution (UFAD) system originally was intro-
10
duced in the 1950s. Its main purpose was to remove high heat loads in the
11
spaces. In the 1970s, UFAD system was introduced into office buildings, in
12
West Germany, to remove heat load in the office. Recently UFAD systems
13
have achieved considerable acceptance in Europe, South Africa, Japan and
14
North America.
16
17
18
19
20
21
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UFAD systems differ from a traditional overhead ventilation system. UFAD systems usually use an under-floor supply plenum and a raised access floor
Ac ce p
15
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9
system to supply the conditioned air through floor supply diffusers. The
conditioned air is delivered into the occupied zone of the indoor environment directly (mostly near the floor level). Room air is extracted through return
vents (mostly on the ceiling and sometimes on the floor), and then returns
to the air conditioner.
22
Alajmi and El-Amer [3] investigated the energy consumption of UFAD
23
systems in commercial buildings for various types of application and at dif-
24
ferent air supply temperatures in a hot climate. Their EnergyPlus simulation
25
results show that UFAD systems have a significant energy saving compared 3
Page 4 of 31
to ceiling-based air distribution (CBAD, i.e. overhead ventilation system),
27
especially for high ceiling buildings.
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26
Bauman and Webster [4] showed that UFAD has several benefits, such
29
as reduced life cycle building costs, improved thermal comfort, improved
30
ventilation efficiency and indoor air quality (IAQ), reduced energy use, re-
31
duced floor to floor height in new construction and improved productivity
32
and health. A design guide book by Bauman [5] provides very comprehensive
33
information on UFAD systems.
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28
Webster et al. [6] showed the impact of air flow rate and supply air
35
temperature on the thermal stratification for UFAD systems. As the air
36
flow rate increases, room air stratification decreases. When the supply air
37
temperature is varied, the temperature profile almost keeps the same shape,
38
but the temperature profile moves to higher or lower temperature.
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M
34
Lin and Linden [7] presented a study on the steady-state flow driven by
40
a heat source and an UFAD cooling diffuser in a ventilated space having a
41
ceiling vent. The model was based on plume theory for the heat source and
47
Ac ce p
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39
48
the displacement case and lower layer temperature increases. The interface
49
height is controlled by the ventilation rate and momentum flux, and the
50
temperature contrast between the two layers is set by the momentum flux.
42
43
44
45
46
a fountain model for the diffuser flow and assumed a steady-state two-layer stratification in the room. The control parameters are the buoyancy flux
of the heat source, the volume flux and the momentum flux of the cooling diffuser. The results showed that when the vertical momentum is small, the entrainment mechanism at interface is not significant. When the vertical
momentum increases, the interface is raised above the height obtained by
4
Page 5 of 31
The heat load determines the temperature in the space for given ventilation
52
condition. This simple model was extended by Liu and Linden [8] to consider
53
more general conditions.
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51
Wan and Chao [9] found that the temperature stratification in the enclo-
55
sure with UFAD systems highly depended on the thermal length scale of the
56
floor supply jet issued from the supply diffuser. When the thermal length
57
scale is greater than 1, temperature stratification is minor for all tested heat
58
densities and air distribution methods. When the thermal length scale is less
59
than 1, there is a significant vertical temperature gradient.
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54
Kong and Yu [10] showed that the combination effect of three parameters,
61
i.e. heat load, supply air flow rate and supply air velocity, on room air
62
temperature stratification would be expressed by the length scale of the floor
63
supply jet, which is the same as that shown in [9]. Their numerical simulation
64
results, by using computational fluid dynamics, show that there is an interface
65
formed, when there is only one local heat source in the room. The interface
66
height is about 1.42 times the length scale. The maximum height of the
72
Ac ce p
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60
73
In the lower narrow zone, the temperature changes abruptly and increases
74
linearly. In the transitional zone, it can be considered as the transitional
75
region between the lower cooler zone and the upper warmer zone. In the
67
68
69
70
71
supply air jet is about 1.56 times the length scale. Wang et al. [11] investigated the thermal stratification characteristics in
a full-scale UFAD experimental space. Their results show that 4 zones are
considered to compose the vertical thermal stratification. They are bottom cooler zone, lower narrow zone, transitional zone, and upper warmer zone.
In the bottom cooler zone, the temperature gradient is relatively unclear.
5
Page 6 of 31
76
upper warmer zone, the temperature is much higher than other zones. Characteristics of thermal environment in a test chamber utilizing an
78
UFAD system are investigated through experimental measurements in this
79
research. The research examines the influence of supply air flow rate, or
80
its corresponding momentum and buoyancy fluxes, on the vertical temper-
81
ature profile in the indoor environment. The paper presents experimental
82
results of a full-scale indoor thermal environment in a test chamber. We
83
focus on the connections between temperature stratification and supply air
84
conditions, and neglect the role of humidity in this paper. Dimensions of the
85
experimental space, measuring instruments and experimental arrangements
86
are introduced in Section 2. Experimental results of measured velocity and
87
temperature are presented and discussed in Section 3. Section 4 summarizes
88
the findings in this research.
89
2. Full-scale experiments
90
2.1. The test chamber
92
93
94
95
96
Ac ce p
91
te
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M
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77
Measurements are conducted in a full-scale test chamber on the sixth floor
of Taiwan Building Technology Center (T.B.T.C.) building, which is located in the campus of National Taiwan University of Science and Technology (NTUST) in Taipei. Taipei is in a monsoon-influenced humid subtropical climate. Using air-conditioners to cool the indoor air temperature is common from May to October in Taipei.
97
The measurement space comprises the Main area and the area in front of
98
Display stand, and is marked in Figure 1(a). The space of the Main area is
99
810 cm (length) × 625 cm (width) × 325 cm (height) and the space of the 6
Page 7 of 31
area in front of Display stand is 190 cm (length) × 230 cm (width) × 325 cm
101
(height). The total floor area of the space is 55 m2 and the measurement
102
space is empty of furniture.
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100
The conditioned air is delivered into the space through floor supply dif-
104
fusers. The room air exits by two square return vents on the ceiling, then
105
moves to the space behind the return air wall and finally comes back to the
106
air conditioners which are placed beneath the floor. Either of the return
107
vents has the size of 60 cm × 60 cm. The air motion is cyclic as shown in
108
Figure 1(b).
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103
The conditioned cool air is delivered by 8 rectangular supply diffusers
110
for the CAC & WAC set and 4 diffusers for the CAC set. Here CAC and
111
WAC denote the central and the western air conditioners respectively. Each
112
diffuser has the same size of 40 cm (length) × 20 cm (width) on the floor and is
113
located in the perimeter zone of the measurement space as shown in Figure 1.
114
Each diffuser with linear bar grilles is a passive type and is connected to an
115
air conditioner via a duct. Two sets of temperature measurements, the CAC
117
118
119
120
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Ac ce p
116
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109
& WAC and CAC sets, are presented in this paper, and their arrangements are described in Section 2.3. 2.2. Experimental instruments The measuring instruments include a hot-wire anemometer for measuring
the air supply velocity, DICKSON TK550 thermistors for measuring the
121
outdoor weather temperature and the supply conditioned air temperature,
122
and T-type thermocouples for measuring the indoor temperature at different
123
vertical levels.
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Page 8 of 31
124
A separate temperature measurement experiment was carried out to calibrate the DICKSON TK550 thermistors and the thermocouples by using a testo
126
Pt100 probe as a benchmark. Measurement deviations from this benchmark
127
were all less than 0.5o C, and mostly 0.3o C, for the DICKSON TK550 thermistors
128
and the thermocouples used in this research.
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125
The conditioned air supply velocity is measured at 6 measuring points
130
just above a supply diffuser by an anemometer, as sketched and shown in
131
Figure 2. Each measuring point has 3 recording data by using the eyeball
132
reading method, and 90 recording data during the span of three minutes by
133
using the data logger recording method. The time-averaged velocities of 6
134
measurement points are used to estimate the flow rate of the supply diffuser.
135
Experiments of temperature measurements are carried out during the
136
night and the window blind is used to reduce the radiation effect on the
137
indoor environment. The outdoor temperature is measured by a thermistor
138
which is placed on the balcony, and the supply conditioned air temperature
139
is measured by a thermistor which is placed in the air supply duct. Each
145
Ac ce p
te
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M
an
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129
146
2.3. Experimental sets
140
141
142
143
144
thermistor has the acquisition frequency of every two consecutive measure-
ments between an interval of 4 minutes. There are 16 levels, i.e. 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, 2.7, 2.9 and 3.1 m above the floor, for measuring the indoor temperature. T-type thermocouple has
the acquisition frequency of every two consecutive measurements between an interval of 2 seconds.
147
According to the air conditioners used in experiments, experiments of
148
temperature measurements are classified as 2 sets; (1) the central and the 8
Page 9 of 31
western air conditioners are used (CAC & WAC set), as shown in Figure 3,
150
and (2) only the central air conditioner is used (CAC set), as shown in
151
Figure 4. Two sets of experimental conditions are presented in Table 1.
152
Average values of supply air temperature and outdoor air temperature from
153
20:46 to 23:00 in experiments are listed in the table and regarded as the
154
conditions in the steady state.
us
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149
A typical experiment usually takes about 1 to 2 hours after turning on
156
the air conditioners to make the space from the initial state to the steady
157
state. Results of the measured temperature against the time for a CAC &
158
WAC experiment with the Q1 option are presented in Figure 5, and there
159
are temperature measurements at 3 different levels, i.e. 0.5, 1.5 and 2.5 m,
160
in this figure. The air conditioner is turned on at 19:00 (i.e. 1140 min., and
161
00:00 as 0 min.) and is turned off at 23:00 (i.e. 1380 min.).
d
M
an
155
Figure 5 shows that after turning on the air conditioner there is a transient
163
state before the measured temperature at a fixed level becomes steady. This
164
experiment has approached the steady state at 20:46 (i.e. 1246 min). After
170
Ac ce p
te
162
171
near the conditioned air supply area, or the clear zone. The locations B, C,
172
F and G are in the free area. The locations D and E are in the air return
173
area.
165
166
167
168
169
this time, the temperature measuring pole is moved to different locations, as
shown in Figures 3 and 4, to collect the indoor temperature distribution data.
The time span at every measuring location for this experiment is shown in Figure 5 as well.
There are 9 locations for measuring the vertical temperature profile in
the CAC & WAC set, as shown in Figure 3. The locations A, H and I are
9
Page 10 of 31
The CAC set is conducted to investigate variations in the vertical temper-
175
ature profiles due to different distances away from the conditioned air supply
176
diffuser. The locations for measuring the temperature profile are respectively
177
75 cm, 150 cm and 225 cm away from the center of 4 supply diffusers in the
178
CAC set, as shown in Figure 4.
179
3. Results and discussion
180
3.1. Conditioned air flow rate
an
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174
Experimental measurements of the supply air velocity at the supply dif-
182
fuser give the supply air flow rate for a single supply diffuser and are used to
183
estimate the total supply air flow rate in the experimental space. Experimen-
184
tal results of conditioned air supply velocity give the flow rate for one supply
185
diffuser as 0.022 m3 s−1 for the Q1 option, 0.024 m3 s−1 for the Q2 option
186
and 0.033 m3 s−1 for the Q3 option by using the eyeball reading method.
187
Experimental results using the data logger recording method show that the
188
deviation on velocity measurements between two methods is less than 10%.
190
191
192
193
d
te
Ac ce p
189
M
181
The supply diffuser has the size of 40 cm (length) × 20 cm (width) and the grille porosity (or the void ratio) of 50%. Experimental results of air supply velocity are presented in Table 2. The total supply air flow rate of the CAC & WAC set, using 8 supply
diffusers, is estimated to be 0.18 m3 s−1 for the Q1 option, 0.20 m3 s−1 for
194
the Q2 option and 0.27 m3 s−1 for the Q3 option. The total supply air flow
195
rate of the CAC set, using 4 supply diffusers, is half of that for the CAC &
196
WAC set.
10
Page 11 of 31
197
3.2. Vertical temperature profiles for 2 sets of experiments Temperature measurements presented hereinafter are regarded as in the
199
steady state. Figure 6 shows the vertical temperature profiles of the CAC
200
& WAC set in the steady state with three different flow rate options. There
201
is a clear stratified interface formed in the room and its level changes with
202
the flow rate. When the supply air flow rate increases, indoor temperature
203
stratification decreases because of disturbance by the stronger supply inflow
204
momentum.
an
us
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198
When the measuring location (A, H or I) is near the supply diffuser, as
206
shown in Figure 3, the temperature profile in Figure 6 is influenced clearly by
207
the supply conditioned air jet flow, especially the bottom part of the profile.
208
However, the vertical temperature profile is not affected significantly by the
209
position of return vent (D or E) and in the free area (B, C, F or G), as shown
210
in Figure 6.
te
d
M
205
Figure 7 shows the vertical temperature profiles of the CAC set. Experi-
212
mental results show the relationship between the vertical temperature profile
213
214
215
216
217
218
Ac ce p
211
and the distance from the measuring location to the supply diffuser with three different flow rate options. When the distance is far enough from the
supply diffuser, the vertical temperature profile is unaffected by the supply conditioned air jet flow. Experimental results indicate that when the distance from the supply diffuser is larger than 150 cm, the vertical temperature pro-
file seems unaffected by the supply conditioned air jet flow.
219
Figures 6 and 7 show that the location near the supply diffuser has quite
220
similar vertical temperature profiles for the same flow rate option. The con-
221
ditioned air jet flow, or the fountain flow as described in Ref. [7], is dominant 11
Page 12 of 31
in this air supply area. As the location is away from the supply diffuser,
223
the gravity current flow, or the horizontal flow due to density difference as
224
described by Simpson [12] or Ungarish [13], becomes important in the free
225
area. This gravity current flow pattern was also observed in the space using
226
displacement ventilation as shown by Li et al. [14].
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222
The conditioned air flow rate for the CAC set is only half of that for the
228
CAC & WAC set. The locations in the free area, (D=150 and 225 cm for the
229
CAC set) have different characteristics in the bottom occupied zone between
230
2 sets of experiments, but similar results in the top warm zone for the same
231
flow rate option. The top warm zone has a gentle temperature gradient in all
232
experiments. The bottom occupied zone of the CAC set with a lower flow rate
233
has a thinner layer compared to the CAC & WAC set. Because the bottom
234
occupied zone is formed mainly by properties of the supply conditioned air,
235
as shown in Ref. [7], the less supply flow rate gives a shallower layer of the
236
bottom occupied zone.
237
3.3. Throw height and stratification height
239
240
241
242
an
M
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te
Ac ce p
238
us
227
Figure 8 shows the temperature profiles in the air supply area and the
free area respectively for the CAC & WAC set with three different flow rate options. It is obvious that the bottom part of the temperature profile in the
air supply area is influenced by the supply conditioned air jet flow. Experi-
mental results show that the temperature profile above a certain level in the
243
air supply area approximates to that in the free area. This specific level is
244
regarded as the throw height (T.H.) for the supply diffuser. The throw height
245
is considered as the maximum height which the supply air jet flow can reach.
246
Experimental results show that the throw height is about 0.9 m for the Q1 12
Page 13 of 31
247
option, 0.9 m for the Q2 option, and 1.9 m for the Q3 option. Using the thermal length scale of the supply conditioned air jet introduced
249
by Refs. [9, 10, 15, 16], or similarly the fountain model by Ref. [17], gives
250
estimations of the throw height as shown in Table 3. Two proportional
251
constants of Kt , 1.56 in Ref. [10] and 1.85 in Ref. [17], are used to calculate
252
the throw height as T.H. = Kt lm .
254
(1)
This simple approach has a reasonable agreement with experimental results.
an
253
us
cr
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248
The thermal length scale is defined as
3
,
M
lm =
M04
1
| F0 | 2
(2)
where M0 is the momentum flux and F0 is the buoyancy flux of the supply
256
jet source as
d
255
and
Ac ce p
257
te
Q20 M0 = Ae
F0 =
258
259
260
261
262
∆ρ0 Q0 g ′ = g0 Q0 . ρr
(3)
(4)
Q0 is the volume flux and Ae is the opening area of the supply source. F0
has a negative value because ∆ρ0 = ρr − ρ0 is the density difference between the supply source conditioned air and the indoor air, and the density of the supply source conditioned air, ρ0 , is larger than that of the indoor air, ρr , for the cooling cases. ρr is the reference air density and the indoor air density is
263
used in this research. g is the gravitational acceleration. For an ideal gas with
264
a moderate temperature difference ∆T0 , a linear approximation is applicable
265
and the normalized density difference
∆ρ0 ρr
0 can be expressed as − ∆T . Either Tr
13
Page 14 of 31
of two normalized terms can be expressed as the reduced gravity as ′
g0 =
∆ρ0 ∆T0 g=− g. ρr Tr
(5)
ip t
266
Figure 9 shows the profiles of averaged temperature in the free area and
268
the air return area, and the profiles of the corresponding gradient for three
269
different flow rate options. The level having the maximum vertical temper-
270
ature gradient is regarded as the stratification height (S.H.). The stratifica-
271
tion height is regarded as the level which separates a room into two zones.
272
There is a more uniform temperature with a gentle gradient in the upper
273
zone above the stratification height and the temperature of the lower zone
274
beneath the stratification height is clearly influenced by the conditioned air
275
flow. Properties of the lower layer highly depend on the conditions of the
276
supply conditioned air, as discussed in Section 3.2. Experimental results
277
show that the stratification height is about 0.7 m for the Q1 option, 0.7 m
278
for the Q2 option, and 1.3 m for the Q3 option.
280
281
282
283
284
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an
M
d
te
When the supply air flow rate increases, the stratification height rises as
Ac ce p
279
cr
267
well as the throw height. The ratio of the stratification height to the throw height for the CAC & WAC set in this research is between 0.68 and 0.78, which is slightly less than the value, 0.9, reported by Ref. [10]. 4. Conclusions
An experimental study on the space using an UFAD system is conducted
285
and the indoor vertical temperature profiles due to different supply condi-
286
tions are analyzed in this research. The indoor air stratification of different
287
locations in the steady state for three flow rate options is presented in the pa-
288
per. According to experimental results, the supply air flow rate has a strong 14
Page 15 of 31
effect on the vertical temperature distribution in the steady state. When the
290
supply conditioned air flow rate increases for a given diffuser, i.e. the momen-
291
tum flux increases as well, the vertical temperature gradient becomes gentler
292
and the indoor air stratification reduces. The higher supply flow rate pro-
293
vides a thicker layer of the bottom occupied zone. The throw height and the
294
stratification height both ascend with the higher supply air flow rate for the
295
same set of experiments. The vertical temperature profile is also influenced
296
by the distance away from the supply diffuser. When the location is near
297
the supply diffuser, the vertical temperature profile is influenced significantly
298
by the conditioned air jet flow. The vertical temperature profile seems unaf-
299
fected by the position of return air vent. When the distance from the supply
300
diffuser is reasonably far, the vertical temperature profile seems unaffected
301
by the supply conditioned air jet flow and the gravity current flow becomes
302
important. The relationship between the spread of the gravity current flow
303
and the stratification height in the space requires further investigation to be
304
clarified.
306
307
308
309
310
cr
us
an
M
d
te
Ac ce p
305
ip t
289
References
[1] M. W. Liddament, M. Orme, Energy and ventilation, Applied thermal engineering 18 (1998) 1101–1109.
[2] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy and Buildings 40 (2008) 394–398.
[3] A. Alajmi, W. El-Amer, Saving energy by using underfloor-air-
15
Page 16 of 31
distribution (UFAD) system in commercial buildings, Energy Conver-
312
sion and Management 51 (2010) 1637–1642.
314
[4] F. Bauman, T. Webster, Outlook for underfloor air distribution, ASHRAE Journal 43 (6) (2001) 18–27.
cr
313
ip t
311
[5] F. Bauman, Underfloor Air Distribution (UFAD) Design Guide, Amer-
316
ican Society of Heating, Refrigerating and Air-Conditioning Engineers,
317
Inc., 2003.
321
322
323
324
325
326
327
328
329
an
M
[7] Y. J. P. Lin, P. F. Linden, A model for an under floor air distribution system, Energy and Buildings 37 (4) (2005) 399–409.
d
320
stratification, ASHRAE Journal 44 (5) (2002) 28–36.
[8] Q. A. Liu, P. F. Linden, The fluid dynamics of an underfloor air distri-
te
319
[6] T. Webster, F. Bauman, J. Reese, Underfloor air distribution: Thermal
bution system, Journal of Fluid Mechanics 554 (2006) 323–341.
Ac ce p
318
us
315
[9] M. P. Wan, C. Y. Chao, Numerical and experimental study of velocity and temperature characteristics in a ventilated enclosure with underfloor ventilation systems, Indoor Air 15 (2005) 342–355.
[10] Q. Kong, B. Yu, Numerical study on temperature stratification in a room with underfloor air distribution system, Energy and Buildings 40 (2008) 495–502.
330
[11] X. Wang, C. Huang, W. Cao, X. Gao, W. Liu, Experimental study on
331
indoor thermal stratification in large space by under floor air distribution
332
system (UFAD) in summer, Engineering 3 (2011) 384–388. 16
Page 17 of 31
335
336
2nd Edition, Cambridge University Press, 1997.
ip t
334
[12] J. Simpson, Gravity Currents in the Environment and the Laboratory,
[13] M. Ungarish, An Introduction to Gravity Currents and Intrusions, CRC Press, 2009.
cr
333
[14] Y. Li, M. Sandberg, L. Fuchs, Vertical temperature profiles in rooms
338
ventilated by displacement: full-scale measurement and nodal modeling,
339
Indoor Air 2 (1992) 225–243.
an
341
[15] D. Etheridge, M. Sandberg, Building Ventilation: Theory and Measurement, John Wiley and Sons, 1996.
M
340
us
337
[16] W. Rodi, Turbulent Buoyant Jets and Plumes, Pergamon Press, 1982.
343
[17] J. S. Turner, Buoyancy Effects in Fluids, Cambridge University Press,
te
1973.
Ac ce p
344
d
342
17
Page 18 of 31
Figure 1: (a) The plan of the test chamber and the measurement space. (b) The perspec-
ip t
tive view of the test chamber.
cr
Figure 2: (a) The schematic diagram of 6 measurement points just above a supply diffuser
for measuring the supply velocity of the conditioned air. (b) A photo of the anemometer
us
above a diffuser.
an
Figure 3: The measuring locations in the CAC & WAC set of experiments.
M
Figure 4: The measuring locations in the CAC set of experiments.
Figure 5: Temperature measurements against the time for a CAC & WAC experiment
te
d
with the Q1 option.
Figure 6: Vertical temperature profiles of the CAC & WAC set with the flow rate option
Ac ce p
of (a) Q1 , (b) Q2 and (c) Q3 .
Figure 7: Vertical temperature profiles of the CAC set having different measuring distances from the supply diffuser with three different flow rate options.
Figure 8: The temperature profiles in the air supply area and the free area with the (a) Q1 , (b) Q2 and (c) Q3 option.
Figure 9: The average temperature profiles in the free area and the air return area and their corresponding gradient profiles with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
19
Page 19 of 31
us
cr
ip t
Figures and Tables (tex file)
The option
Average Outdoor
Temp.(o C)
Temp.(o C)
11.6
29.2
2012/09/03
11.5
27.8
2012/09/04
14.2
28.5
2012/10/17
9.8
22.6
Q2
2012/10/25
12.4
25.5
Q3
2012/10/24
14.6
23.9
Q2 Q3
2012/08/20
d
Q1 CAC & WAC
Ac ce p
Q1
CAC
Date
M
Average Supply
te
Run.
an
Table 1: Experimental conditions for the CAC & WAC and CAC sets.
1
Page 20 of 31
ip t cr
us
Table 2: Experimental results of conditioned air supply velocity above a supply diffuser
an
and estimations of the total flow rate for the CAC & WAC set.
A single diffuser The option Average
Approach
Velocity
Q1
reading
0.56
(m3 s−1 )
(m3 s−1 )
(hr−1 )
0.022
0.18
3.6
te
eyeball
d
(m/s)
0.024
data logger
Q1
0.52
0.021
0.17
recording
Q2
0.55
0.022
0.18
Q3
0.82
0.033
0.27
Q2
0.61
0.20
4.0
Q3
0.83
0.033
0.27
5.4
Ac ce p ∗
Flow Rate Total Flow Rate A.C.H.∗
M
Experimental
CAC & WAC set
It denotes Air Change rate per Hour.
2
Page 21 of 31
ip t cr us an
Table 3: Experimental results and predictions on the throw height for the CAC & WAC set.
Predictions
F0
M0
T.H.
(m4 s−3 )
(m4 s−2 )
(m)
(m)
(m)
0.0123
0.9
0.82
0.7
d
The option
M
Experimental results
-0.0069
Q2
-0.0063
0.0146
0.9
Q3
-0.0072
0.0274
1.9
Ac ce p
te
Q1
Kt
1.85
T.H.
0.98 1.47
Kt
1.56
T.H.
0.83 1.24
3
Page 22 of 31
ip t cr us an M d
(a)
Ac ce p
625
56
te
Return air vents
Western air conditioner
325
Return air wall
56
1230
Unit: cm
Supply diffusers
Central air conditioner (b)
Figure 1: (a) The plan of the test chamber and the measurement space. (b) The perspective view of the test chamber.
4
Page 23 of 31
C4-4
(10,12)
(20,12)
(30,12)
C4-1
C4-2
C4-3
(10,6)
(20,6)
(30,6)
C4
6
us
x
cr
y
12
20
C4-5
ip t
C4-6
an
10 20
M
30 40
Unit: cm
Ac ce p
te
d
(a)
(b) Figure 2: (a) The schematic diagram of 6 measurement points just above a supply diffuser for measuring the supply velocity of the conditioned air. (b) A photo of the anemometer above a diffuser.
5
Page 24 of 31
ip t cr us an M d te Ac ce p
Figure 3: The measuring locations in the CAC & WAC set of experiments.
6
Page 25 of 31
ip t cr us an M d te Ac ce p
Figure 4: The measuring locations in the CAC set of experiments.
7
Page 26 of 31
ip t 34 locatition (C)
( A ) ( B )( D ) ( E ) ( F ) ( G ) ( H )( I )
an
32 30
M
26 24 22 20
d
Temperature ( oC )
28
te
18
H = 2.5 m H = 1.5 m H = 0.5 m
16
Ac ce p
14 12
cr
us
2012-08-20
Turn on A.C.
Turn off A.C.
10 1100 1120 1140 1160 1180 1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Time (min)
Figure 5: Temperature measurements against the time for a CAC & WAC experiment with the Q1 option.
8
Page 27 of 31
(a) 2012-08-20 Vertical Temperature Distribution 3
2
cr
Height (m)
2.5
ip t
Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
1.5
0.5
0 10
15
20
25
Temperature (oC)
30
an
(b)
us
1
2012-09-03 Vertical Temperature Distribution Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
3
M
2
1.5
1
te
0.5
d
Height (m)
2.5
0 10
15
20
25
Temperature (oC)
30
Ac ce p
(c)
2012-09-04 Vertical Temperature Distribution
3
Height (m)
2.5
2
Free area (C) Air supply area (A) Free area (B) Air return area (D) Air return area (E) Free area (F) Free area (G) Air supply area (H) Air supply area (I)
1.5
1
0.5
0 10
15
20
25
Temperature (oC)
30
Figure 6: Vertical temperature profiles of the CAC & WAC set with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
9
Page 28 of 31
2.5
0.5
0.5
25
2012-10-25 Vertical Temperature Distribution C-1 C-2 C-3 C-4 C-5 C-6 C-7
2
2.5
2
1.5
1
0.5
0.5
25
30
C-1 C-2 C-3 C-4 C-5 C-6 C-7
Height ( m )
2
1.5
20
2.5
25
0 10
30
C-8 C-9 C-10 C-11 C-12
2.5
2
1.5
0 10
20
25
Temperature ( oC )
30
C-13 C-14 C-15 C-16 C-17
3
2
Q3
1.5
0.5
30
15
2012-10-24 Vertical Temperature Distribution
0.5
25
Q2
0.5
0.5
20
30
2
1
Temperature ( oC )
25
C-13 C-14 C-15 C-16 C-17
3
1
15
20
Temperature ( oC )
2012-10-25 Vertical Temperature Distribution
1
0 10
15
1
2012-10-24 Vertical Temperature Distribution
2.5
0 10
30
1.5
Temperature ( oC )
3
Ac ce p
2.5
15
te
2012-10-24 Vertical Temperature Distribution 3
0 10
Height ( m )
20
Temperature ( oC )
d
1
15
25
C-8 C-9 C-10 C-11 C-12
3
1.5
0 10
20
Temperature ( oC )
Height ( m )
Height ( m )
2.5
15
2012-10-25 Vertical Temperature Distribution
Height ( m )
3
0.5
0 10
30
Q1
1
an
20
Temperature ( oC )
1.5
M
15
2
cr
1.5
1
0 10
2.5
2
1
C-13 C-14 C-15 C-16 C-17
3
us
1.5
2012-10-17 Vertical Temperature Distribution
Height ( m )
2
C-8 C-9 C-10 C-11 C-12
3
Height ( m )
Height ( m )
2.5
2012-10-17 Vertical Temperature Distribution
ip t
C-1 C-2 C-3 C-4 C-5 C-6 C-7
3
Height ( m )
2012-10-17 Vertical Temperature Distribution
15
20
25
Temperature ( oC )
D≤ 75 cm
30
D= 150 cm
0 10
15
20
25
Temperature ( oC )
30
D= 225 cm
Figure 7: Vertical temperature profiles of the CAC set having different measuring distances from the supply diffuser with three different flow rate options.
10
Page 29 of 31
(a)
2012-08-20 Vertical Temperature Distribution
Height (m)
2
1.5
1
0 10
25
30
2012-09-03 Vertical Temperature Distribution Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
3
M
2.5
2
Height (m)
20
Temperature (oC)
an
(b)
15
us
T.H. = 0.9 m
0.5
cr
2.5
ip t
Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
3
1.5
d
1
te
0.5
0 10
(c)
20
25
Temperature (oC)
30
Ac ce p
2012-09-04 Vertical Temperature Distribution
3
2.5
Height ( m )
15
T.H. = 0.9 m
Free area (C) Free area (B) Free area (F) Free area (G) Air supply area (I)
2
T.H. = 1.9 m
1.5
1
0.5
0 10
15
20
o
25
30
Temperature ( C )
Figure 8: The temperature profiles in the air supply area and the free area with the (a) Q1 , (b) Q2 and (c) Q3 option.
11
Page 30 of 31
(a) 2012-08-20_Q1_Vertical Average Temperature Distribution
2012-08-20_Q1_Vertical Temperature Gradient
2.5
2
2
1.5
1
S.H.= 0.7 m
0.5
0 10
0.5
15
20
25
Average temperature (oC)
0
30
Location (C), (B), (D), (E), (F)
M Height (m)
0.5
15
2
1.5
1
S.H.= 0.7 m
20
25
d
Height (m)
1.5
0
30
Ac ce p
2.5
2
2
Height (m)
Height (m)
2.5
S.H.= 1.3 m
1
0.5
0.5
20
25
20
0
30
S.H.= 1.3 m
1.5
1
Average temperature (oC)
10
Gradient (oC/m)
3
Location (C), (B), (D), (E), (F)
15
0
2012-09-04_Q3_Vertical Temperature Gradient
2012-09-04_Q3_Vertical Average Temperature Distribution
0 10
S.H.= 0.7 m
0.5
te
Average temperature (oC)
(c)
1.5
20
2.5
2
3
10
Gradient (oC/m)
3
2.5
0 10
0
2012-09-03_Q2_Vertical Temperature Gradient
2012-09-03_Q2_Vertical Average Temperature Distribution
1
S.H.= 0.7 m
an
(b)
3
1.5
us
1
cr
2.5
ip t
3
Location (C), (B), (D), (E), (F)
Height (m)
Height (m)
3
0
10
Gradient (oC/m)
20
Figure 9: The average temperature profiles in the free area and the air return area and their corresponding gradient profiles with the flow rate option of (a) Q1 , (b) Q2 and (c) Q3 .
12
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