1. Introduction
With high-efficiency building energy-saving requirements and the improvement of the sealing performance of indoor spaces, the amount of fresh air in these spaces has continually decreased, and the building space has shown a closed trend. This leads to two IAQ problems: on the one hand, insufficient infiltration and inadequate natural ventilation can increase indoor pollutant concentration [
1,
2], and on the other hand, micro-hypoxia problems are more common in air-conditioned rooms. These two issues have led to a reduction in work efficiency and SBS. In recent years, the problem of indoor air pollutants in confined building spaces has become more prominent. Therefore, there have been more studies and applications on indoor air pollutants’ removal in confined building spaces [
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13]. However, the removal of air pollutants does not solve the problem of micro-hypoxia in indoor environments. Although the problem of a micro-hypoxia environment is more prominent compared with air pollutants, studies focusing on this issue are still rare. The problem of micro-hypoxia in confined building spaces is serious but easily overlooked. People who have lived and worked in a micro-hypoxia closed building space for a long time are prone to many physical problems, such as headaches, chest tightness, fatigue, irritability, insomnia, and skin allergies [
14,
15,
16,
17]. Liu Yingshu [
18] pointed out that the use of oxygen generated by special oxygen equipment increases the oxygen concentration in closed buildings and is expected to provide a new way to improve indoor air quality.
Oxygen-enriched technology is widely used in various places, such as tunnel construction, mine operations, and underwater submarine environments [
19,
20,
21,
22,
23,
24,
25,
26], and has achieved certain results in scientific research and practical applications. However, research on micro-hypoxia in air-conditioned rooms, such as office buildings, shopping malls, and supermarkets, is seldom conducted. Yang Guoping et al. [
27] carried out field measurements of the oxygen concentration inside a typical room in Beijing, and the test results showed that the O
2 concentration inside a densely populated and closed room was generally low.
The oxygen supply vent is used to inject a certain concentration of oxygen into closed building spaces, and the oxygen flows out by a gas jet. For jet problems, there are usually three research methods. The first method is to obtain practical empirical relations through experimental data. Liu Zhenyi et al. [
28] carried out a small-scale experimental study of CO
2 pipeline leakage, obtaining the peak concentration curve of the monitoring point by power function fitting and determining the safety distance of the production site. Jin Wufeng et al. [
29] studied a combustible refrigerant R32 indoor air-conditioner. The leakage diffusion characteristics were experimentally studied. Jianmin Fu et al. [
30] proposed a method for calculating the stable pressure of small hole leakage by constructing a small hole leakage experimental system for a liquid phase pipeline, which effectively solved the pressure solution problem in the classic calculation formula. The second method is to theoretically analyze the Gaussian distribution of the cross-sectional velocity of the jet [
31,
32]. The third method is Computational Fluid Dynamics (CFD) numerical simulation. With the continuous development of computer technology, the CFD method has been adopted by more and more researchers. Qu Yanpeng et al. [
33] used different turbulence models to study the flow characteristics of circular jets based on fluent and considered that the standard model could obtain reasonable results. Ke Daoyou et al. [
34] studied the hydrogen leakage process based on CFD simulation, and the results accurately predicted that the diffusion and the law of motion is suitable for emergency treatment. Wang Jiansheng et al. [
35] studied the heat transfer characteristics of impinging jets based on numerical simulation. Shiying Cai et al. [
36] investigated rarefication effects on jet impingement loads at an inclined plate surface, with the jet firing from a planar or round nozzle. The simulation results were compared with recently developed jet impingement flow formulas at the collisionless flow limits. Intarat Naruemon et al. [
37] employed an improved one-dimensional diesel spray model to analyze the potential of varying injection rate shapes to lower the pollutant emissions of diesel engines and find the optimized injection strategy.
The most effective way to improve the IAQ of traditional closed air-conditioned rooms is to introduce fresh air. But the energy used to process the fresh air during the air-conditioning process accounts for approximately 25% to 30% of the total building energy consumption, and 40% [
38] for hotels and office buildings. Therefore, it is particularly important to study the alternative or supplementary schemes of closed air-conditioned room ventilation to improve IAQ in closed air-conditioned rooms. In order to effectively improve the micro-hypoxia environment to meet people’s physiological needs and improve work efficiency, local oxygen supply can be provided to closed air-conditioned rooms. This method achieves the dual effects of improving indoor air quality and reducing energy consumption by eliminating micro-hypoxia and reducing fresh air supply. Therefore, based on experiments and CFD numerical simulations, this paper studies the number of oxygen supply vents, the diameter of the oxygen supply vents, the oxygen supply flow rate, the oxygen supply method, and the different air flow organization forms on the oxygen enrichment of a closed air-conditioned room. This article has reference significance for establishing oxygen-enriched safety standards, increasing the amount of fresh air in air-conditioned rooms, and reducing air-conditioning energy consumption.
2. Experimental Investigation
A schematic view of the experimental system used in the present study is shown in
Figure 1. The system consists of two parts. One is the oxygen generator system, which consisted of a pressure swing adsorption (PSA) unit, a zirconia sensor oxygen measurement system, a buffer tank, a control panel, a rotometer, and control valves. The other is the oxygen delivery system, consisting of a oxygen delivery port, 22 oxygen sensors, a data collector, and a computer.
All experiments were carried out in an artificial climate chamber; the dimensions were 6.8 m in length, 4 m in width, and 2.9 m in height. The PSA oxygen generator is a two-stage PSA oxygen system developed by our research group [
39,
40], and the obtained oxygen volume fraction is 99%. The zirconia sensor oxygen measurement system (ZO-101T) was used to measure the oxygen concentration produced by the PSA oxygen generator, with an accuracy of ±2.0% of full scale and a measuring range of 0.1~100%. The volumetric flow rate of the oxygen was controlled by the rotameter (DK800-6F), with a measuring range of 0~2 m
3·h
−1. The data collector (USB5936) was used to collect the oxygen concentration signal and record it in real time. The oxygen sensors (Figaro, KE-25) were used to measure oxygen concentration at different locations, with an accuracy of ±1% and a measuring range of 0~100%. The oxygen sensor was placed at a height of 1.5 m from the ground.
5. Conclusions
The oxygen-enriched flow characteristics and oxygen-enriched area were investigated experimentally and numerically in the un-air-conditioned and air-conditioned room. The following conclusions can be derived.
The maximum axial velocity decreases with the increase in the axial distance under un-air-conditioned conditions. Within the axial distance of 0.6 m, there is a maximum axial velocity gradient.
The number of oxygen supply vents, the diameter of the oxygen supply vent, the oxygen supply flow rate, and the oxygen supply method are different, and the formed oxygen-enriched areas vary greatly.
The relationship between the oxygen-enriched area and the oxygen flow rate is obtained by fitting. Under the un-air-conditioned state, when the diameter of the oxygen supply pipe is 0.006 m and the oxygen supply methods 1# and 4# are adopted, the oxygen-enriched area is F = 0.149 + 0.33 Q and F = 0.215 + 0.313 Q. When the diameter of the oxygen supply pipe is 0.01 m and the oxygen supply methods 1# and 4# are adopted, the oxygen-enriched area is F = −0.016 + 0.182 Q and F = 0.183 + 0.312 Q.
Under the air-conditioned state, the best way to supply oxygen to the room is by using an airflow supply on the opposite side with a diameter of 0.006 m. When the diameter of the oxygen supply pipe is 0.006 m and the oxygen supply methods 1# and 4# are adopted, and the oxygen-enriched area is F = 0.4 + 0.383 Q and F = 0.237 + 0.8 Q.