1. Introduction
Ventilation systems play an important role in creating an acceptable microclimate in the indoor environment. Today, people spend more than 90% of their time in an artificial indoor environment [
1,
2]. An acceptable thermal environment and air quality in indoor spaces such as dwellings and workplaces have been linked to higher productivity and the general wellbeing of the occupants. Bako-Biro et al. [
3] found that adequate ventilation rates significantly improved thermal comfort and indoor air quality, which in turn enhanced the performance of pupils in schools. On the other hand, a poor indoor environment has been linked to problems such as the ‘sick building syndrome’ [
2]. Indoor air pollution which is greatly influenced by particulate matter has been shown to increase with insufficient ventilation rates and air flow patterns that create stagnation zones within the occupied zone of the indoor environment. The ability of particles to significantly affect indoor air quality is dependent on the airborne particle concentration, size distribution, and chemical or biological composition [
4]. The ventilation system is one of the components in a building that requires a lot of energy. This issue is important to address, since buildings accounted for roughly 22% of total world energy use in 2016 [
5]. Achieving thermal comfort and good health of the building occupants with minimized use of energy is the core principal of HVAC systems [
6]. It has been demonstrated that the use of advanced air distribution systems like stratum ventilation (SV) and displacement ventilation (DV) in specific configurations can reduce carbon emissions up to 31.7% and 23.3%, respectively [
7]. Increasing the ventilation’s effectiveness significantly reduces occupants’ exposure to particles in the indoor environment [
8]. The enhancement of ventilation, i.e., increasing the air change rate is an efficient measure to additionally reduce the pollutant load in indoor spaces [
1].
SV was proposed by Lin as a response to the requirements of some governments in East Asia for operating indoor spaces at elevated temperatures in order to conserve energy [
9,
10,
11]. The new recommended indoor air temperatures have been set to (26–28 °C) in the Republic of Korea, (26 °C) in Chinese mainland, (25.5 °C) Hong Kong, (27 °C) Taiwan and (28 °C) in Japan for summers [
12]. Since conventional ventilation systems are incapable of efficiently providing thermal neutrality in warm conditions, SV was devised to serve that purpose. The ventilation system is aimed at coping with higher room temperature and air movement and has been found suitable for cooling small to medium rooms [
13]. Lin et al. [
10] stated that with a properly designed supply air velocity and volume, location of diffusers and exhausts, SV has potential to maintain better thermal comfort with a smaller vertical temperature difference, lower energy use and better indoor air quality (IAQ) in the breathing zone. In addition, a comparison of the mean air temperatures in the occupied zone confirmed that SV offered the highest cooling efficiency, followed by DV, and in last place, mixing ventilation (MV) [
14].
SV draws on the strength of personalized ventilation systems. Personalized or task ventilation systems have been ranked as the most energy efficient and provide the best air quality in the breathing zone. However, such systems are inadequate because only limited ductwork can be installed in the occupied zone to avoid obstructions. Besides the limited ductwork, task ventilation systems cannot adequately cater for the mobile occupants within the occupied zone. SV supplies fresh air directly into the occupied zone in order to overcome the shortcomings of the task ventilation system while retaining the benefits of better indoor air quality and energy performance [
15]. For example, owing to its low nonlinearity and fast response, the SV system can be used to offer differentiated air velocity, temperature and predicted mean vote (PMV) distributions to cater for individual occupant preferences in shared spaces [
16,
17].
The underlying operation principle of the SV system is the supply of fresher air directly into the occupants’ breathing zone (i.e., between 0.9 m and 1.4 m from the floor surface). To achieve this purpose, air supply inlets are placed at the side wall of the room at locations slightly above the head height of the sitting person. The recommended air inlet height is 1.3 m from the floor which corresponds to head level of sitting sedentary worker [
17]. As a result of the typical discharge height, the air speed increases along the height, but a reverse temperature gradient (cool head and warm ankle) is formed in the occupied zone. Consequently, lower CO
2 concentration exist in the occupied zone than in the upper part of the room. The cooling effect which is strongest at the head level is due to both lower temperature and air movements of the supplied air [
12].
Many benefits are realized from the direct supply of air into the occupied zone such as shorter supply air path, younger mean age of air, higher ventilation effectiveness and better IAQ in the breathing zone. Other advantages include the smaller capacity required, smaller system size, smaller space requirement, lower initial costs, lower energy use and smaller carbon footprint compared with the MV, DV, impinging jet ventilation system (IJV) and task ventilation systems for a particular application [
7,
13]. It is recommended that the supply air path should not be longer than 9 m in order to achieve better performance [
13]. Ventilation inefficiencies resulting from the short cut ventilation phenomenon are minimized in this ventilation type. The main characteristics of the SV include: reverse temperature gradient in the occupied zone; higher air speed at the head–chest level for equal air supply volume; higher supply air temperature; higher room air temperature and higher evaporating temperature for the associated refrigeration plant, thus a higher coefficient of performance (COP) for the refrigeration machine(s) [
9,
10].
The energy saving potential of the SV lies in its use of low airflow rates because only the zones requiring cooling are serviced (head level). Additionally, energy is also saved by avoiding the overcooling of the lower part of the room [
18,
19]. The fan power is a function of airflow rate and the efficiency of the fan used in a ventilation system. Thus, lower airflow rate and higher efficiency of the fans lead to lower energy use [
20]. The cooling effect is obtained from the influence of both the low supply air temperature and air movement in the occupied zone [
12,
15]. When the annual energy use of the SV was compared with the MV and DV, substantial amount of savings was realized at 44% and 25%, respectively. Lin et al. [
18] attributed this energy saving to the reduction in ventilation and transmission loads coupled with increased COP of chillers used in SV systems.
According to the proposed performance evaluation and design guidelines for the SV, the recommended room temperatures are between 25.5 °C and 27 °C reliant on the activity level and clothing insulation value. For better performance, recommended supply air temperature of 21 °C can be utilized as the preliminary value. Depending on the level of thermal comfort, supply air temperatures of between 20 °C and 23 °C can also be used [
13]. To minimize the risk of draught and cross contamination, the supply air velocity and location of the air supply and exhaust terminal devices must be optimized to break the boundary layer around the occupant’s body. The location of the exhaust air terminal can be at elevation either below or above the supply air terminal [
19]. A study by Fong et al. [
19] to evaluate the thermal conditions in a classroom using three ventilation methods showed that SV could provide satisfactory thermal comfort level at room temperature up to 27 °C. The study also illustrated that SV used less energy due to lower ventilation load. SV achieved an energy saving of 12% and 9% compared with the MV and DV, respectively. Furthermore, the energy use by three ventilation systems was examined for an office, classroom and retail shop in Hong Kong. The results revealed that the year-round energy use by the SV was lower than that for MV and DV [
19]. To ascertain the thermal and ventilation performance of the SV, Tian et al. [
12] experimentally investigated the influence of air speed, temperature and CO
2 concentration in an office equipped with SV. The results of the study indicated that the values of PMV, predicted percentage of dissatisfied (PPD) and percentage dissatisfied due to draught (PD) conformed to the requirements of ISO 7730 and ASHRAE 55-2010 standards. The supply air temperature of 21 °C was found to provide better thermal comfort than air supplied at 19 °C. The ventilation effectiveness was close to 1.5 and the ventilation system was expected to provide better IAQ in an efficient way [
13,
21].
The overall aim of this study is to evaluate the influence of supply air temperature and supply airflow rates on the ventilation effectiveness and local thermal comfort of a corner-placed stratum ventilation system (CSV) in a medium sized office room. The goal of the study is to evaluate how SV operates when the supply inlets are placed in a corner configuration. This research is important since it will expand on previous research performed on this ventilation system which have yielded good results in terms of thermal comfort and ventilation effectiveness. This will be the first study to evaluate SV by placing the supply inlets in two corners of an office environment. The specific objectives are:
To conduct experimental study involving the tracer gas technique in order to determine different ventilation effectiveness indices: local air change index, air exchange efficiency and temperature effectiveness.
To carry out measurements of the air velocity and temperature in the office room in order to determine the thermal comfort conditions.
To conduct flow visualization to ascertain the airflow pattern in the office room.
This study evaluates the suitability of CSV for both cooling and heating applications. Additionally, this research is also a follow-up to two previous studies by Ameen et al. [
22,
23] in terms of evaluating different air distribution systems when their supply inlets are place in the corners of an office room.
2. Theory and Mathematical Models
This section provides the various key definitions of indoor climate indices which are used in this study. Since this study has a similar methodology and execution as two previous studies [
22,
23], a more in-depth explanation of these definitions can be found in those articles.
According to ISO 7730 [
24], the draught rate (DR) quantifies the discomfort a person experiences due to unwanted cooling of the human body. This index is a function of air velocity, air temperature and turbulent intensity.
Percentage dissatisfied (PD) is related to the local discomfort due to a high vertical air temperature difference between head and ankle. In this study, the temperature difference, ∆
T0.1–1.1 between the ankle level (0.1 m) and the neck level for a seated person (1.1 m) is used. Temperature effectiveness (
) [
22,
25,
26,
27] is a parameter that is used to evaluate the effectiveness of heat removal and is defined by
where
is the arithmetic mean air temperature of the heights 0.1, 0.6, and 1.1 m,
is the outlet air temperature, and
is the supply air temperature. In the case of evaluating how effective space heating is in a location during heating mode [
23,
28], the following equation can be used
If
> 1, this indicates that the temperature in the occupied zone is higher than the outlet. If
< 1, this indicates that the temperature in the occupied zone is lower than the outlet which means lower utilization of the heat from the ventilation system to the occupied zone. For a perfect mixing ventilation system
= 1.
The evaluation of ventilation effectiveness can be performed in several ways. Two frequently used indices related to IAQ are air exchange effectiveness (AEE) and air change effectiveness (ACE) [
29,
30,
31].
The inlet Archimedes number (Ar
i) [
22,
32] is a measure of the relative importance of buoyant and inertia forces. Ar
i is important in building airflows because it combines two important ventilation design parameters, i.e., supply air velocity and room temperature difference. A negative Ar
i value indicates that supply air is flowing downwards towards the floor and a positive value indicates that the it is rising towards the ceiling.
3. Experimental Set-up and Procedure
The study was conducted in one of the test rooms in a laboratory at the University of Gävle. The mock-up office closely resembled the features of the modern office with one exterior wall and three interior walls. The dimensions of the test room were 7.2 m × 4.1m × 2.7 m (L × W × H). The composition of the wall from inside to outside was: 15 mm wood sheet, 35 mm air gap, 15 mm wood sheet, 190 mm insulation, and 5 mm wood sheet. The floor and main ceiling were insulated by a 150 mm thick layer of mineral wool and covered by a layer of plastic sheet to minimize air infiltration. The test room had a suspended ceiling made of 60 cm × 60 cm fiberglass tiles which were hanging 31 cm below the main ceiling. It had three windows located on the northern wall of the test room built in direct connection with a climate chamber. The mock-up office mimicked a shared office with two workstations. The workstations, which were placed about 3.6 m from the air inlet terminals, comprised a table, chair and a seated thermal mannequin each. Each thermal mannequin was made of galvanized tube of 0.32 m diameter covered with fabric to emit the same radiation as an ordinary human being. It had the same area as a human body and produced 100 W. The computer at each workstation was simulated by a 75 W lamp placed inside metallic black cylinder. The total internal heat generated was 350 W.
Figure 1 shows the experimental set up of the mock-up office. As a side note, this setup has been evaluated with other types of air distribution systems by the same authors [
22,
23].
Figure 1 shows the positions of the workstations and the heat sources in relation to the air supply terminals. It also indicates the positions of the measurements for temperature and velocity along with the tracer gas sampling points. The tracer gas sampling points are denoted by letter T along with the position number, e.g., T1, while the points for temperature and velocity measurement are denoted by P such as P1.
The air distribution system had one main inlet which subsequently was divided into two final inlets in the test room. Rectangular straight grill air inlet terminals with dimensions 175 mm × 127 mm each placed at 1.3 m from floor surface to the centreline of the grilles. The effective area of each inlet terminal, excluding the surface area of the grill was 198 cm
2. The air inlet terminals were placed at the corners of the wall adjoining the south wall. There was only one exhaust terminal located in the ceiling near the northern wall of the room.
Figure 1 illustrates the details of the air supply terminals.
Fifteen cases were investigated involving three nominal supply air temperature setpoints: 17.7 °C, 21 °C and 25 °C. At each supply air temperature, five different airflow rates were investigated, i.e., 30, 40, 50, 60 and 70 L/s. The details of the inputs to the different cases are shown in
Table 1. The temperature set points are similar to previous studies made with the same setup [
22,
23]. Setpoints 17.7 and 21 °C are evaluated through case C1–10 where ‘C’ denotes cooling mode and 25 °C is evaluated through case H1-5 where ‘H’ denotes heating mode.
The concentration decay tracer gas method using SF
6 gas was used in the study. The tracer gas measurements were conducted using the INNOVA 1302 gas monitor and INNOVA 1303 multi-channel sampling unit being augmented by the INNOVA 7260 software installed on the laboratory computer. The INNOVA multi-channel sampling unit has six channels which enabled the measurement of the concentration of the tracer gas at six different positions in the room. The sampling points were strategically selected with one point located in the exhaust. The remaining sampling points were all placed at the breathing zone level, i.e., height of 1.1 m from the floor surface. Positions T1, T2, and T5 were placed in a straight line along the centreline of the room. Position T4 was on the table in front of the thermal mannequin and position T3 was placed on the right-hand side of one thermal mannequin. Position T6 was placed in the exhaust. Due to the limitation of the measuring equipment, no sampling was done near the other mannequin. However, this shortfall has no effect on the results since symmetry of the room was assumed. Before any tracer gas measurements, all the visible air passages were sealed and the room was tested for any leakages—it was found to be acceptably airtight for tracer gas measurements.
Figure 2 shows the configuration of the tracer gas sampling points in the test room.
The SF
6 gas was injected into the test room at a concentration of around 350 ppm and the automatic circulating movable fans were operated for the first three minutes to ensure thorough mixing. In order to enhance mixing, the SF
6 gas was injected at multiple points in the room at a height of about 1.8 m. Gas chromatography was used to measure the concentration of the gas in air samples. In each tracer gas test, air samples were collected via a pump connected to the gas chromatography unit. Measurements were performed two times for about four hours each session and the average deviation between the two measurements was less than 2%. The uncertainty of measurements of mean age of air was ± 2.5%. Although, this value can increase when including airflow variation, pressure balancing and air leakage. The uncertainty of ACE in this study was estimated to be in compliance with Appendix E of ASHRAE Standard 129 [
29]. The final estimated uncertainty of measured values of ACE was around 7% which was based on the negligible air leakage and the measuring accuracy of the equipment. Other laboratory studies [
22,
23,
33,
34] have shown similar results in estimated uncertainty for tracer gas measurements.
The temperature and velocity at selected points were measured using the low-velocity Omni-directional thermistor anemometer type CTA88. The thermistors were connected to a multi-channel logger and were recorded on a personal computer on which LabVIEW program was installed. A total of seven different positions in the room were used for the measurement of the temperature and velocity using the thermistors. For positions represented by points P-1 to P4, measurements were performed at four different heights, i.e., 0.1, 0.6, 1.1 and 1.7 m from the floor surface. For locations P6 and P7, measurements were taken at heights 1.1 and 1.7 m from the floor surface. At point P5, measurements were taken at 0.1, 0.3, 0.6, 0.8, 1.1, 1.4 and 1.7 m from the floor surface.
Figure 2 shows the positions in the room at which the air temperature and velocity were measured. All the thermistors were calibrated in a low-velocity calibration unit before use to ensure accurate results. The sampling interval for all measurements was 600 s. The velocity was measured with an accuracy of ± 0.05 m/s excluding directional error with the response time of 0.2 s to 90% of a step change. The uncertainty of temperature measurements was ± 0.2 °C with the response time of 12 s to 90% of value in still air. The surface temperatures for the wall, ceiling, and window were measured using the T-type (copper-constantan) thermocouples connected to an Agilent 34970A data logger and computer. The same type of thermocouples was used to measure the temperature supply air in the main inlet and the two final supply inlets. The calibration of the thermocouples and logger was carried out before and after the measurements to ensure that the accuracy was within the expected range. The climate chamber was maintained at −6.2 ± 0.3 °C during measurement periods for the heating cases. Two cooling units were used to provide an even cooling of the air inside the chamber. The unintended heat losses from the office to the surroundings has been already evaluated in a previous study and has shown to be within acceptable limits [
23].