1. Introduction
Ventilation is one of the core systems that has a large impact on thermal comfort and indoor air quality (IAQ) in buildings. The design and implementation of air distribution systems require careful consideration, not only in terms of providing a good indoor environment, but also to be energy efficient. On top of these requirements the ventilation system needs to operate adequately both during cold and warm seasons.
One common air distribution system is called mixing ventilation (MV). This system is characterized by supplying air at high velocity into the room with the intended purpose of mixing the fresh supply air with the room air. This type of ventilation supply inlet is usually located high, close to the ceiling in the unoccupied area of the room. MV also creates a highly uniform vertical temperature field [
1,
2] which can result in slightly lower ventilation effectiveness when compared to other systems, e.g., impinging jet ventilation (IJV) and displacement ventilation (DV) [
3,
4,
5].
DV and IJV are usually categorized as stratified ventilation systems when utilized in cooling mode [
6,
7,
8]. The air from a DV supply device enters the room at a relatively low speed and at low height close to the floor when used for cooling. When entering the room, the fresh air will fall to the floor and continue flowing outward until it encounters a heat source. It will then start to heat up and start rising due to buoyancy effects, moving upwards to the upper parts of the room [
9,
10]. IJV works similarly in that respect but it uses impinging air jet with relatively high velocity and momentum which is discharged downwards close to a wall section at a distance from the floor area. IJV has been classified by several researchers as a hybrid system [
10,
11,
12] in that it combines the positive effects of both MV and DV to overcome the shortcomings of the DV system, e.g., the limitation in covering the entire floor area due to the low momentum. Another downside of DV is the difficulty of utilizing the system during cold season when heating is required [
6,
7,
8].
Most of the research around these ventilation systems has been conducted for cooling mode in a hot climate [
13,
14,
15]. However, it is interesting to evaluate the IJV system when heating is required in a cold climate. Very few studies have been done to examine the ventilation performance of the IJV system when it operates in heating mode. Some researchers have stated that IJV can be used for heating due to its high momentum [
9,
16].
In a numerical study done by Ye et al. [
17] they compared MV and IJV in order to evaluate the energy performance when used for heating in a large space with a high ceiling. Their results showed that IJV required less energy than MV for heating fresh air and re-circulating the return air. However, the fan power required more energy than MV. Adding these energy demands, the total heating energy usage for IJV was lower than that for MV. They concluded that the heating load index could be reduced by around 9–25 W/m
2 when the outdoor air temperature was in the range of −5 to 12 °C. Another study [
18] also concluded that IJV is more energy efficient than MV in heating mode. This study was carried out in a climate chamber 3.0 (L) x 3.6 (W) x 2.6 (H) m which was placed in a laboratory space. It is worth mentioning that this study also included intermittent opening of a door that caused cold outside air to invade the heating space. One interesting observation in these two studies [
17,
18] was that MV created greater thermal stratification than IJV, which is the opposite of when these systems are used in cooling mode [
11,
19].
There have not been many studies conducted to evaluate typical ventilation systems for heating mode. Some of these studies have been focused on building optimization and control system which also included the control of the air handling unit [
20,
21,
22]. Others have evaluated specific supply devices, such as stratum ventilation used for heating [
23] or a low-temperature all-air heating system in an office cubicle that was equipped with an active supply device on the ceiling [
24].
Due to the novelty of this research and to the authors’ best knowledge there has not been any experimental research carried out to evaluate multiple IJV devices places in the corners of an office room for heating mode.
In a recent study Ameen et al. [
19] evaluated and compared three different ventilation systems, corner impinging jet ventilation (CIJV), corner mixing ventilation (CMV) and DV. They evaluated heat removal effectiveness, local thermal comfort and indoor air quality in a mock-up medium-sized office room. The office contained two workstations, each with one mannequin and one piece of equipment. Nine different cases were examined with varying supply rates and heat sources. The results from this research showed that overall CIJV performed slightly better than the other two ventilation systems and there was a possibility of reducing the total energy usage. However, this research was conducted for summer cases, i.e., the systems were only evaluated for cooling mode.
The overall objective of this study is to continue the research done by Ameen et al. [
19] and evaluate the same three types of air distribution systems for heating mode, i.e., winter conditions. The supply device for the DV evaluated in that study was a modified version that provided slightly higher supply velocity compared to traditional DV systems. This DV system is called hybrid displacement ventilation (HDV) in this study. These three different systems will be examined and compared to one another to evaluate their performance based on local thermal comfort and ventilation effectiveness in order to make an overall evaluation of their usability for both cooling and heating.
2. Theory and Mathematical Models
This section provides a brief overview and explanation of the key definitions of indoor climate indices which are used in this study. Since this study is a continuation of the experimental work done by Ameen et al. [
19], a more in-depth explanation of these definitions can be found in that article.
According to ISO 7730 [
25], 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 dissatisfied due to draft. Another index, the percentage dissatisfied (PD), is related to the local discomfort due to high vertical air temperature between head and ankle. In this study the temperature difference, ∆
T0.1–1.1 is used which is between ankle level (0.1 m) and neck level for a seated person (1.1 m).
Temperature effectiveness (
) [
24] is an index that can be used to evaluate how effective space heating is in a space or location for heating mode. This is defined by
where
is the supply air temperature,
is the arithmetic mean air temperature of the heights 0.1, 0.6 and 1.1 m and
is the outlet air temperature. 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. This index is different from the one used in the cooling mode article [
19] in that it can be used for heating mode.
The evaluation of ventilation effectiveness can be done in several ways. Two commonly used indexes related to IAQ are air exchange effectiveness (AEE) and air change effectiveness (ACE) [
26,
27,
28]. The guidelines in ASHRAE Standard 129-1997 [
26] require measuring ACE in 25% of the workstations or measuring a minimum of ten locations throughout the evaluated space. Another way to calculate AEE is to make measurements at the exhaust location. These indexes have been utilized by many researchers for evaluating indoor environments using different tracer gas techniques [
29,
30,
31].
Inlet Archimedes number (Ar
i) [
32,
33] 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, supply air velocity and room temperature difference.
3. Experimental Set-Up
This study was conducted in a room 7.2 (L) × 4.1 (W) × 2.67 (H) m. The room resembled a medium-sized open-plan office space with three interior walls and one exterior wall. A climate chamber was built up in connection to the exterior wall of the test room as shown in
Figure 1. For an in-depth description of the office wall materials, design setup, supply device dimensions, measuring equipment, etc. see [
19].
Six cases were studied which are listed in
Table 1. The primary supply air was maintained between 25.1–25.2 °C for C1 cases and 24.8 °C for C2 cases. It is important to mention that the comparisons were done in a non-dimensional form for all cases.
The wall facing the climate chamber contained three windows. The size of each window was 1.61 m × 0.91 m with a frame to glass ratio of 31.7%. Single pane windows were used with a total U-value of 4.6 W/m
2 °C. The surface temperatures of the windows were maintained at 10.1 ± 0.3 °C during the measurement periods. The location of the room was inside a large laboratory hall with a steady temperature condition of 22.3 ± 0.3 °C during the measurement periods. The heat transfer between the cold climate chamber and the office room was measured by several heat flux sensors of the type HFP01 made by Hukseflux. A total of three heat flux sensors were used. One was placed on the external wall 130.5 cm above the floor level and 47.2 cm from the west wall. Another sensor was placed in the dead center of the window located closest to the west wall. The last sensor was placed on the westside lower frame corner of the same window. The uncertainty of the heat flux sensor is ±3%. The total heat transfer from the office to the climate chamber and the surrounding surface is presented in
Table 2. The climate chamber was maintained at −6.2 ± 0.3 °C during measurement periods. Two cooling units were used, one on each side of the climate chamber to provide an even cooling of the air inside the chamber. The heat loss through the rest of the office surfaces, excluding the external wall, amounted to −0.2 ± 0.1 W/m
2. The measurement positions and a top view layout of the experimental set-up are shown in
Figure 2.