1. Introduction
As humans spend more time indoors than outdoors, the importance of having a good indoor environment becomes crucial when designing or renovating buildings. Whether we are at home, at the office, or at school, a good indoor climate is important for several reasons, such as having fresh air, maintaining a high work performance [
1] and a good cognitive function [
2], and complying with state and local regulations. One of the most important systems for controlling the indoor climate is the ventilation system. This is also one of the components in a building that requires a lot of energy, which is also an important issue to address, since buildings accounted for roughly 22% of total world energy use in 2016 [
3]. There are several types of air distribution systems that work in different ways. Some of these have been researched extensively, while others are still in the evaluation stage.
One of these systems is called displacement ventilation (DV). Ventilation based on this type of device enters the room at a relatively low speed and at low height, close to the floor. When used for cooling the space, the inlet air will fall to the floor level and continue flowing out in the space until it encounters heat sources. It will then start to heat up and rise as a result of buoyancy effects, moving to the upper part of the space, where it will usually exit from an extraction point located close to the ceiling. There has been a lot of research on this type of ventilation. In the early 2000s, a research group examined whether the indoor air quality (IAQ) in the breathing zone was better than the average IAQ in the occupied zone, and the results showed that there was a 35–50% improvement in the breathing zone due to the buoyancy effect around the mannequin body [
4]. In a more recent publication, four different ventilation systems were compared in terms of ventilation efficiency, thermal comfort and energy-saving potential by using numerical simulations. The four systems compared were DV, mixing ventilation (MV), wall confluent jets ventilation (WCJ), and impinging jet ventilation (IJV). The comparison was done for an office environment. The results showed that DV was better than the other air delivery systems in all areas except when evaluating the vertical temperature gradient between ankle and neck levels for a standing person [
5]. Other research groups have also concluded that DV is more suitable for cooling by exhibiting higher ventilation and energy effectiveness. It also creates a temperature stratification which facilitates the concentration of pollutants in the lower strata of the occupied space [
6,
7,
8].
Another type of air distribution system is IJV, which has been the subject of much research [
9,
10,
11,
12,
13]. However, very few studies have examined multiple inlet devices based on IJV. In an early study, Karimipanah and Awbi [
14] compared IJV to wall displacement ventilation in a laboratory classroom. They tested several key parameters such as ventilation efficiency, local mean age of air, and other characteristic parameters both experimentally and by numerical simulations. One of the conclusions of their research was that the IJV system showed a slight improvement in mean age of air and velocity distributions due to a better balance between buoyancy and momentum forces. Similarly, Koufi et al. [
15] also reached the conclusion that IJV has higher ventilation effectiveness. They conducted a numerical simulation by comparing two types of MV, DV and IJV, investigated under isothermal conditions.
In another study, numerical simulations were carried out to evaluate IJV, MV, and DV. One of the results of that study showed that IJV has an advantage over DV in that it can also be used for heating in winter time; also, IJV was found to distribute air more efficiently to the occupied zone when compared to a top–top-configured MV [
16].
IJV has been classified by some researchers as a hybrid system [
5,
17,
18] in that it combines the positive effects of both MV and DV to overcome the shortcomings of the DV system, for example the limitation in covering the entire floor area due to low velocity.
An interesting placement of an IJV device is in the corners of a room. In this study, this configuration is called corner impinging jet ventilation (CIJV). One aspect of CIJV is the possibility of having a non-intrusive supply device, i.e., the device can almost be hidden in the corner of the room. This property increases the value of this device compared to others that are installed in the middle of a wall section. To the authors’ best of knowledge, there has not been any experimental research carried out to evaluate multiple IJV devices placed in the corners of an office room.
It is also important to consider some shortcomings of this type of system. The region in front of the jet impact area must be cleared from any furniture or objects for proper operation, and the possibility of stratification discomfort and draft might occur near the supply device [
19].
Finally, the most common ventilation system used in buildings today is MV. This system can be installed in various configurations. This type of ventilation has been researched extensively. In one study [
20], an experimental investigation was conducted comparing MV to confluent jet ventilation and a floor-mounted underfloor air distribution system. This investigation was conducted in a medium-sized open-plan office which included six workstations. The study focused on the thermal comfort performance of these systems, and the conclusion reached was that MV had some shortcomings in terms of higher draught in the occupied zone and lower heat removal capacity compared to the other systems. Similar investigative comparisons between MV and other types of ventilation system to evaluate thermal comfort and/or ventilation effectiveness have been made by many research groups [
6,
21,
22,
23,
24,
25,
26,
27,
28].
This article focuses on the experimental evaluation of the potential benefits of a newly designed impinging jet ventilation system located in the corners of an office-type environment. The CIJV system was compared to two other separate systems based on DV and MV, to evaluate heat removal effectiveness, local thermal comfort, and indoor air quality in the occupants’ breathing zone. The DV supply device used in this study was designed for relatively high momentum compared to traditional DV devices. The supply devices for MV were located high in the room corners, providing a type of ventilation hereby called corner mixing ventilation (CMV).
2. Theory and Mathematical Models
This section provides an overview and explanation of the key definitions of indoor climate indices which are used in this study. According to ISO 7730 [
29], the draught rate (DR) describes the discomfort a person experiences due to unwanted cooling of the human body. This index is a function of air temperature, air velocity, and turbulent intensity, and predicts the percentage of dissatisfaction due to draft. This is estimated by
where
ua is the mean air velocity,
Ip is the local turbulence intensity, and
Ta is the local temperature.
Another index, the 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) was used. According to ISO 7730, PD is estimated by
In ISO 7730, the local thermal comfort is categorized into three levels (A, B, and C) for office environments.
Table 1 shows the criteria for each category.
Temperature effectiveness (
) [
30,
31,
32,
33] is a parameter that can be 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.
The evaluation of ventilation effectiveness can be done in multiple ways. Two commonly used indices related to IAQ are air exchange effectiveness (AEE) and air change effectiveness (ACE) [
34,
35,
36]. The guidelines in ASHRAE Standard 129-1997 [
34] require measuring ACE in 25% of the workstations or measuring a minimum of 10 locations throughout the evaluated space. Another way to calculate AEE is to make measurements at the exhaust location. These indices have been utilized by many researchers for evaluating indoor environments using different tracer gas techniques [
21,
22,
37,
38,
39,
40,
41,
42,
43,
44].
The definition of local mean age of air is the average time it takes for fresh air to travel from an inlet to any place in the room [
40,
45,
46]. The air at the examined place is a mixture of components of the air present in the room for different lengths of time. The supply air should be distributed in such a way that the occupants are “flushed” with fresh air at the breathing level. The local mean age of air,
, at a specific point is calculated from sampled tracer gas concentration histories. By utilizing the step-down tracer method, the local mean age of air is obtained by
where the start of the decay corresponds to a time of zero with initial concentration
, and
is the tracer gas concentration at time
t.
The mean age of air for the entire space,
, is calculated by
AEE is calculated by
where
is the nominal time constant defined as the reciprocal of the nominal air exchange rate.
represents the mean age of air at the ventilation outlet obtained from tracer gas measurements [
45].
is calculated by
where
is the volume of the room, and
is the ventilation flow rate.
ACE relates to local air change effectiveness (ACE
p) or average spatial air change effectiveness in a region (ACE
avg) within the breathing level against the nominal time constant of the ventilation system. They are defined by
and
Here, is the local age of air in a considered point, and is the arithmetic average age of air for several points. A reference case (ACE = 1.0) is used, which is correspond to perfect air mixing.
The inlet Archimedes number (Ar
i) [
47,
48] 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. Ar
i is defined by
where
g is the gravitational acceleration,
[K] is the mean air temperature in the center of the room at 1.7 m above floor level,
[K] is the mean supply air temperature,
is the inlet supply opening area, and
is the nominal inlet air velocity.
3. Experimental Set-Up
The study was carried out in a medium-sized mock-up of an open-plan office room, 7.2 (L) × 4.1 (W) × 2.67 (H) m. The room resembled a medium-sized open office space with three interior walls and one exterior wall. The room was divided into two sections that were occupied by two workstations, each containing a desk, a chair, and a seated thermal mannequin, as can be seen in
Figure 1. The composition of the side walls from the inside to the outside were as follows: 15 mm wood sheet, 35 mm air gap, 15 mm wood sheet, 190 mm insulation, and 5 mm wood sheet. The floor and the main ceiling were insulated by a 150 mm-thick layer of mineral wool and covered by a layer of plastic sheet to reduce air infiltration. The suspended ceiling consisted of 60 cm × 60 cm fiberglass tiles and was located 31 cm below the main ceiling. The location of the room was inside a large laboratory hall with a steady temperature of 24.6 °C ± 0.9 °C during the measurement periods. The supply inlets were installed in the corners of the south wall. The main air inlet was in the middle of the wall section, and air was delivered to each device through well-insulated (20 mm mineral wool) ventilation tubes. The first supply devices (A) were installed in the lower section of the corners. The height location of these devices was similar to those used by many other researchers [
4,
8,
14,
17,
49,
50]. These devices represent DV. The second set of supply devices (B) represents CIJV. The shape of the outlet was an equilateral triangle, and the air entered the room at the height of 80 cm above the floor level. The last supply devices (C) were suspended 15 cm from the ceiling. These devices represent CMV, and the air entered the room from an inlet which had a circular shape and a diameter of 80 mm. In addition to the two mannequins, two heat sources were also placed on the side of the tables. There was only one outlet, which was located on the ceiling close to the north wall. A climate chamber was built up in connection to the north wall of the test room where three windows were located. For velocity and temperature measurements, 27 low-velocity omnidirectional thermistor anemometers were used. The thermistor and the logger system, CTA88, were designed and calibrated for velocities between 0.0 and 1.0 m/s. The turbulence intensity was calculated by the following equation
where
is the root mean square of the turbulent velocity fluctuations, and
is the mean velocity. Measurements were performed at seven locations in the room. For position P1 through P4, the heights used were 0.1, 0.6, 1.1, and 1.7 m. For P5, 0.1, 0.3, 0.6, 0.8, 1.1, 1.4, and 1.7 m were used, and for P-6 and P-7, only 1.1 and 1.7 m were used. The sampling interval for all measurements was set to 600 s. The velocity was measured with an accuracy of ±0.05 m/s excluding the 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 temperatures of the supply inlets, exhaust, and surrounding laboratory were measured by using T-type (copper–constantan) thermocouples connected to an Agilent 34970A data logger and a computer. In order to confirm that the accuracy of the thermocouples and logger were in the expected range, all measuring devices were calibrated before and after the measurements.
Nine cases were studied which are listed in
Table 2. The primary supply air temperature was maintained around 17.6 °C except for case C1-CMV which was 0.3 °C lower. It is important to mention that the comparisons were done in a non-dimensional form for all cases. The mannequins used in the experiments had the same surface area as a human, and each produced 100 W of heat in a sitting position. They were made of galvanized tube 0.32 m in diameter and covered with fabric to emit the same level of radiation as a normal person. Two black painted metal cylinders containing a halogen lamp generated 75 W of heat each when used in the experiments. There was also heat generation from the measuring equipment that amounted to 39 W. The nominal inlet air velocity and inlet Archimedes number were calculated on the basis of
Ae for each supply device and case. The
Ae for DV, CIJV, and CMV were 299, 133, and 50 cm
2, respectively.
The measurement positions and other components of the experimental set-up are shown in
Figure 2. Sulfur hexafluoride (SF
6) was used as the tracer gas in this study. The measurements were performed at six locations in the room, at a height of 1.1 m. These were labeled T1 up to T5. The sixth location was at the outlet. During the experiments, the test room was exposed to about 350 ppm of SF
6. Gas chromatography (GC) 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 GC and analyzed from five fixed locations in the room and one in the outlet. The measurements were repeated three times to ensure the validity of the results. The average deviation between the three measurements ranged between 1 to 3%. The uncertainty of measurements of mean age of air was ±2.5%, but, when including airflow variation, pressure balancing, air leakage, etc., this value might increase. The uncertainty of ACE in this study was estimated to be in compliance with Appendix E of ASHRAE Standard 129 [
34].
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. This result is close to those reported by some other laboratory studies [
42,
43,
44,
45].