Experimental and Numerical Model Investigations of Oxygen-Enriched Characteristics in Air-Conditioned Rooms

People who live and work in air-conditioned rooms with micro-hypoxia are prone to sick building syndrome (SBS). Enriching oxygen into an air-conditioned room to increase the oxygen concentration can improve indoor air quality (IAQ) and reduce ventilation to save building energy consumption. In the present paper, the number and diameter of the oxygen supply vents, the oxygen supply flow rate, the oxygen supply method and the air flow organization form were comparatively studied using a numerical model. The results were compared with the experiments results in un-air-conditioned rooms, which showed that this model can give a favorable prediction. The results show that the maximum axial velocity decreases with the increase of the axial distance under air-conditioned conditions. The relationship between the oxygen-enriched area and the oxygen flow rate is obtained by fitting. The diameter of the oxygen supply pipe is 0.006 m, and when oxygen supply methods 1# and 4# are adopted, the oxygen-enriched area is F = 0.4 + 0.383 Q and F = 0.237 + 0.8 Q, respectively.


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 microhypoxia 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.

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

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.  (19) Oxygen delivery port; (20)  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.

Physical Model
This article takes an artificial climate chamber as the research object and establishes a 3-D mathematical model; the specific physical model is shown in Figure 2. The model contains four seats and four tables, two air supply vents, two air return vents, and an oxygen supply vent. The x-axis is the length direction, the y-axis is the width direction, and the z-axis is the height direction. The oxygen supply under un-air-conditioned and air-conditioned conditions was studied separately. For the oxygen supply in an un-airconditioned environment, we studied the different indoor air flow characteristics and oxygen-enriched effects, such as the number of oxygen supply vents, the diameter of the oxygen supply vents, the oxygen flow rate, and the oxygen supply method. For the oxy-  (19) Oxygen delivery port; (20)  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.

Physical Model
This article takes an artificial climate chamber as the research object and establishes a 3-D mathematical model; the specific physical model is shown in Figure 2. The model contains four seats and four tables, two air supply vents, two air return vents, and an oxygen supply vent. The x-axis is the length direction, the y-axis is the width direction, and the z-axis is the height direction. The oxygen supply under un-air-conditioned and air-conditioned conditions was studied separately. For the oxygen supply in an un-airconditioned environment, we studied the different indoor air flow characteristics and oxygen-enriched effects, such as the number of oxygen supply vents, the diameter of the oxygen supply vents, the oxygen flow rate, and the oxygen supply method. For the oxygen supply under an air-conditioned environment, we studied the indoor air flow characteristics and oxygen-enriched area. The main structural parameters of the artificial climate chamber are shown in Table 1.
gen supply under an air-conditioned environment, we studied the indoor air flow characteristics and oxygen-enriched area. The main structural parameters of the artificial climate chamber are shown in Table 1.

Mathematical Model
To simplify the model, according to Wang et al. [41], the following assumptions were made: (1) The oxygen supply vent sends out oxygen-enriched gas in the form of a jet, which is mainly a binary ideal gas mixture of O2 and N2, and the oxygen volume fraction is 99%. (2) The oxygen supply vent has a small diameter and limited diffusion effect in a closed room after being sent out as a jet; therefore, the free jet theory can be used to describe this flow accordingly. (3) The fluid is considered incompressible and steady, and the turbulence model uses a dual equation model [42].
The governing equation can be expressed as follows: The continuity equation is expressed as Momentum balance is expressed as

Mathematical Model
To simplify the model, according to Wang et al. [41], the following assumptions were made: (1) The oxygen supply vent sends out oxygen-enriched gas in the form of a jet, which is mainly a binary ideal gas mixture of O 2 and N 2 , and the oxygen volume fraction is 99%. (2) The oxygen supply vent has a small diameter and limited diffusion effect in a closed room after being sent out as a jet; therefore, the free jet theory can be used to describe this flow accordingly. (3) The fluid is considered incompressible and steady, and the turbulence model uses a dual equation model [42].
The governing equation can be expressed as follows: The continuity equation is expressed as Momentum balance is expressed as Stakic et al. [43] simulated the characteristics of a two-phase free jet formed at the exit of a long vertical tube and believed that the k − ε model was in good agreement with the experimental results. Akbarzadeh et al. [44] used standard k − ε and k − w models to simulate the characteristics of the free jet formed by rectangular nozzles and believed that the standard k − ε can effectively predict the jet characteristics. Qu Yanpeng et al. [33] used different turbulence models to study the flow characteristics of circular jets and believed that the standard k − ε model could obtain more reasonable results. Therefore, the turbulence model in this paper uses the standard k − ε model.
The k − ε model takes the following form: The model constants were determined experimentally and were assigned a standard value of

Initial and Boundary Conditions
No penetration and non-slip conditions were imposed at all solid wall boundaries. The air supply vent was defined as the velocity supply (V in ). The air supply velocity was distributed evenly; V in = Q M −1 , where Q is the supply flow of the air supply vent and M is the cross-sectional area of the air supply vent. The input velocities of the air supply vents came from experimental measurements. The pressure of the outlet boundary was imposed at the air outlet vent (pressure outlet), 101,325 Pa. The supply air was assumed to be sent into the room with a constant air temperature of 298 K. Initial indoor air temperature was 294 K.
The oxygen supply vent sent oxygen out with an oxygen volume fraction of 99% and entered into the confined building spaces in the form of a jet. The diameters of the oxygen supply vents were 0.006 m and 0.01 m, respectively. The oxygen supply methods were carried out by a single oxygen supply vent and double oxygen supply vents, and the oxygen-enriched characteristics and effects in the confined building spaces under the un-air-conditioned and the air-conditioned were studied, respectively.
The single oxygen supply vent was located in the center of the air supply vent on the side wall and was 1.5 m above the ground; the single oxygen supply vent was vertical forward (represented by the symbol 1#) and the double oxygen supply vents were located in the center of the air supply vents on the side wall. The three oxygen supply methods were vertical forward (symbol 2#), relative 45 • (symbol 3#), and opposite 45 • (symbol 4#); the initial conditions of oxygen supply vents methods are shown in Table 2. Initial conditions of the oxygen supply parameters are shown in Table 3. The velocity of air flow in the air-conditioned environment is shown in Table 4.

Meshing and Method of Solution
The mathematical model described above was realized in CFD software Fluent using the control volume method.
To obtain reasonable and accurate calculation results more efficiently with limited computing resources, this paper first uses three sets of grids for pre-calculation and unstructured grid division of the artificial climate chamber model. Due to the large difference between the geometric dimensions of the artificial climate chamber model and the oxygen supply port, local grid encryption is performed near the oxygen supply port. The pre-calculation sets the same boundary conditions and only differs in the number of grid cells. The verification result of grid independence is shown in Figure 3. The number of grids is changed from 42 million to 53 million and the change in axial velocity is very small, which satisfies the requirement of independence. Therefore, considering the accuracy and computational efficiency, selecting a model with a mesh number of 42 million can meet the requirements.
The computational grid of the model is shown in Figure 4. CFD simulations were

Meshing and Method of Solution
The mathematical model described above was realized in CFD software Fluent using the control volume method.
To obtain reasonable and accurate calculation results more efficiently with limited computing resources, this paper first uses three sets of grids for pre-calculation and unstructured grid division of the artificial climate chamber model. Due to the large difference between the geometric dimensions of the artificial climate chamber model and the oxygen supply port, local grid encryption is performed near the oxygen supply port. The pre-calculation sets the same boundary conditions and only differs in the number of grid cells. The verification result of grid independence is shown in Figure 3. The number of grids is changed from 42 million to 53 million and the change in axial velocity is very small, which satisfies the requirement of independence. Therefore, considering the accuracy and computational efficiency, selecting a model with a mesh number of 42 million can meet the requirements.
The computational grid of the model is shown in Figure 4. CFD simulations were

Meshing and Method of Solution
The mathematical model described above was realized in CFD software Fluent using the control volume method.
To obtain reasonable and accurate calculation results more efficiently with limited computing resources, this paper first uses three sets of grids for pre-calculation and unstructured grid division of the artificial climate chamber model. Due to the large difference between the geometric dimensions of the artificial climate chamber model and the oxygen supply port, local grid encryption is performed near the oxygen supply port. The pre-calculation sets the same boundary conditions and only differs in the number of grid cells. The verification result of grid independence is shown in Figure 3. The number of grids is changed from 42 million to 53 million and the change in axial velocity is very small, which satisfies the requirement of independence. Therefore, considering the accuracy and computational efficiency, selecting a model with a mesh number of 42 million can meet the requirements.
The computational grid of the model is shown in Figure 4. CFD simulations were performed using a double-precision unsteady-state implicit solver, with the coupled algorithm to solve the pressure-velocity coupling problem in the momentum equations. A SIMPLE discretization scheme was used for the pressure and velocity. To reduce numer-

Meshing and Method of Solution
The mathematical model described above was realized in CFD software Fluent using the control volume method.
To obtain reasonable and accurate calculation results more efficiently with limited computing resources, this paper first uses three sets of grids for pre-calculation and unstructured grid division of the artificial climate chamber model. Due to the large difference between the geometric dimensions of the artificial climate chamber model and the oxygen supply port, local grid encryption is performed near the oxygen supply port. The pre-calculation sets the same boundary conditions and only differs in the number of grid cells. The verification result of grid independence is shown in Figure 3. The number of grids is changed from 42 million to 53 million and the change in axial velocity is very small, which satisfies the requirement of independence. Therefore, considering the accuracy and computational efficiency, selecting a model with a mesh number of 42 million can meet the requirements.
The computational grid of the model is shown in Figure 4. CFD simulations were performed using a double-precision unsteady-state implicit solver, with the coupled algorithm to solve the pressure-velocity coupling problem in the momentum equations. A SIMPLE discretization scheme was used for the pressure and velocity. To reduce numerical diffusion, a second-order upwind scheme was selected for the discretization of the momentum and energy equations. The solution was assumed to converge when the different residuals had reached sufficiently low steady values of 10 −4 . The time step was 0.01 s, and the overall calculation time was 1200 s.    Figure 5 shows the comparison between the simulated and the experimental values of the oxygen-enriched area in the un-air-conditioned environment. The cross-section of the oxygen-enriched area is placed at a height of 1.5 m, which is consistent with the position of the oxygen sensor in the experiment. The oxygen-enriched effect is achieved when the oxygen volume fraction predicted by the model is greater than 22%. Liu et al. [46] pointed out that the hypoxic environment formed by factors such as elevated altitude, closed places and environmental pollution affects people's normal life and occupational safety, and the standard for supplying oxygen to the hypoxic environment is that the volume fraction of oxygen reaches about 22%. The simulation values of the oxygen-enriched area formed in the axial and radial directions are larger than the experimental values, regardless of the single oxygen supply vent or the double oxygen supply vents. This is because, during the experiment, there is low wind speed in the confined building room in addition to human interference. Thus, the diffusion effect of indoor oxygen is strengthened, and the loss of oxygen during the diffusion process is increased. It can also be seen    Figure 5 shows the comparison between the simulated and the experimental values of the oxygen-enriched area in the un-air-conditioned environment. The cross-section of the oxygen-enriched area is placed at a height of 1.5 m, which is consistent with the position of the oxygen sensor in the experiment. The oxygen-enriched effect is achieved when the oxygen volume fraction predicted by the model is greater than 22%. Liu et al. [46] pointed out that the hypoxic environment formed by factors such as elevated altitude, closed places and environmental pollution affects people's normal life and occupational safety, and the standard for supplying oxygen to the hypoxic environment is that the volume fraction of oxygen reaches about 22%. The simulation values of the oxygen-enriched area formed in the axial and radial directions are larger than the experimental values, regardless of the single oxygen supply vent or the double oxygen supply vents. This is because, during the experiment, there is low wind speed in the confined building room in addition to human interference. Thus, the diffusion effect of indoor oxygen is strengthened, and the loss of oxygen during the diffusion process is increased. It can also be seen  Figure 5 shows the comparison between the simulated and the experimental values of the oxygen-enriched area in the un-air-conditioned environment. The cross-section of the oxygen-enriched area is placed at a height of 1.5 m, which is consistent with the position of the oxygen sensor in the experiment. The oxygen-enriched effect is achieved when the oxygen volume fraction predicted by the model is greater than 22%. Liu et al. [46] pointed out that the hypoxic environment formed by factors such as elevated altitude, closed places and environmental pollution affects people's normal life and occupational safety, and the standard for supplying oxygen to the hypoxic environment is that the volume fraction of oxygen reaches about 22%. The simulation values of the oxygen-enriched area formed in the axial and radial directions are larger than the experimental values, regardless of the single oxygen supply vent or the double oxygen supply vents. This is because, during the experiment, there is low wind speed in the confined building room in addition to human interference. Thus, the diffusion effect of indoor oxygen is strengthened, and the loss of oxygen during the diffusion process is increased. It can also be seen that the axial and radial errors of the simulated and experimental values of the oxygen-enriched area are less than 0.25 m; the difference between the simulated value and the experimental value of the axial velocity is small. Therefore, the mathematical model can better predict the oxygen-enriched characteristics and effects in a confined building room.  Figure 6 shows the maximum axial concentration distribution of oxygen with different oxygen supply flow rates in the single oxygen supply vent. When the axial distance gradually increases, the maximum axial oxygen concentration decreases; the axial concentration of oxygen decreases rapidly within a range of 0.55 m and then becomes close to the oxygen concentration in the environment. This is due to the convection diffusion process between pure oxygen and the surrounding air, with the difference in oxygen concentration as a driving force to mix oxygen quickly with indoor air.    Figure 6 shows the maximum axial concentration distribution of oxygen with different oxygen supply flow rates in the single oxygen supply vent. When the axial distance gradually increases, the maximum axial oxygen concentration decreases; the axial concentration of oxygen decreases rapidly within a range of 0.55 m and then becomes close to the oxygen concentration in the environment. This is due to the convection diffusion process between pure oxygen and the surrounding air, with the difference in oxygen concentration as a driving force to mix oxygen quickly with indoor air.  Figure 6 shows the maximum axial concentration distribution of oxygen with different oxygen supply flow rates in the single oxygen supply vent. When the axial distance gradually increases, the maximum axial oxygen concentration decreases; the axial concentration of oxygen decreases rapidly within a range of 0.55 m and then becomes close to the oxygen concentration in the environment. This is due to the convection diffusion process between pure oxygen and the surrounding air, with the difference in oxygen concentration as a driving force to mix oxygen quickly with indoor air.     Figure 8 shows the oxygen-enriched area when the number of oxygen supply vents and the oxygen supply method are different under un-air-conditioned conditions at t = 120 s. With the different pipe diameters and different oxygen supply modes (in the form 1#, 2#, 3#, and 4#), the oxygen-enriched area varies widely. This is because when 3# is used for oxygen supply, after the oxygen is ejected from the oxygen supply vent at a certain outflow speed, the two gases meet, and the gas molecules directly collide with each other, making the flow velocity of the gas weaken. When the 4# is used for oxygen supply, the oxygen-enriched area formed by the double oxygen supply vents with a pipe diameter of 0.006 m roughly consists of two fan-shapes.  Figure 8 shows the oxygen-enriched area when the number of oxygen supply vents and the oxygen supply method are different under un-air-conditioned conditions at t = 120 s. With the different pipe diameters and different oxygen supply modes (in the form 1#, 2#, 3#, and 4#), the oxygen-enriched area varies widely. This is because when 3# is used for oxygen supply, after the oxygen is ejected from the oxygen supply vent at a certain outflow speed, the two gases meet, and the gas molecules directly collide with each other, making the flow velocity of the gas weaken. When the 4# is used for oxygen supply, the oxygen-enriched area formed by the double oxygen supply vents with a pipe diameter of 0.006 m roughly consists of two fan-shapes.  Figure 8 shows the oxygen-enriched area when the number of oxygen supply and the oxygen supply method are different under un-air-conditioned conditions 120 s. With the different pipe diameters and different oxygen supply modes (in the 1#, 2#, 3#, and 4#), the oxygen-enriched area varies widely. This is because when 3# is for oxygen supply, after the oxygen is ejected from the oxygen supply vent at a c outflow speed, the two gases meet, and the gas molecules directly collide with each making the flow velocity of the gas weaken. When the 4# is used for oxygen suppl oxygen-enriched area formed by the double oxygen supply vents with a pipe diame 0.006 m roughly consists of two fan-shapes.  Figure 9 shows the calculation of the oxygen-enriched area under different oxygen pipe diameters and different oxygen supply methods. It can be seen from Figure 9 that as the oxygen flow rate increases, the corresponding oxygen-enriched area also increases. The relationship between the oxygen-enriched area and the oxygen flow rate obtained by fitting is a linear relationship, as shown in Table 5. The oxygen-enriched area with a pipe diameter of 0.006 m is larger than that formed by a diameter of 0.01 m with the same oxygen flow rate. Therefore, the research will mainly study the oxygen supply pipe diameter of 0.006 m. It also can be seen that because the oxygen-enriched area of 2# and 3# is relatively small, the oxygen-enrichment effect is poor, so it is no longer considered 2# and 3#. Figure 9 shows the calculation of the oxygen-enriched area under different oxygen pipe diameters and different oxygen supply methods. It can be seen from Figure 9 that as the oxygen flow rate increases, the corresponding oxygen-enriched area also increases. The relationship between the oxygen-enriched area and the oxygen flow rate obtained by fitting is a linear relationship, as shown in Table 5. The oxygen-enriched area with a pipe diameter of 0.006 m is larger than that formed by a diameter of 0.01 m with the same oxygen flow rate. Therefore, the research will mainly study the oxygen supply pipe diameter of 0.006 m. It also can be seen that because the oxygen-enriched area of 2# and 3# is relatively small, the oxygen-enrichment effect is poor, so it is no longer considered 2# and 3#.   Figure 10 shows the oxygen-enriched area with the oxygen supply method and oxygen supply flow rate is different under air-conditioning (Vin = 0.85 m·s −1 ) at t = 120 s in the form 1# and 2#. Under air-conditioning, it is appropriate to use a single oxygen supply vent and an air flow supply on the opposite side. Thus, in the subsequent analysis, we will focus on the analysis of the single oxygen supply vent with a diameter of 0.006 m and the air flow supply on the opposite side.

Oxygen-Enriched Flow Characteristics and Oxygen-Enriched Area under
Air-Conditioned State Figure 10 shows the oxygen-enriched area with the oxygen supply method and oxygen supply flow rate is different under air-conditioning (V in = 0.85 m·s −1 ) at t = 120 s in the form 1# and 2#. Under air-conditioning, it is appropriate to use a single oxygen supply vent and an air flow supply on the opposite side. Thus, in the subsequent analysis, we will focus on the analysis of the single oxygen supply vent with a diameter of 0.006 m and the air flow supply on the opposite side. Figure 11 shows the oxygen-enriched area with different oxygen supply flow rate under air-conditioning (V in = 0.85 m·s −1 ) at t = 120 s in the form 1# and 4#. The relationship between the oxygen-enriched area and the oxygen flow rate obtained by fitting is a linear relationship, as shown in Table 6. Under the same conditions, 4# has a larger oxygenenriched area than 1#, and the oxygen enrichment effect is better. Therefore, the two are suitable for different office occasions: 1# is suitable for a centralized office area where office personnel is concentrated in the middle of the office, and 4# is suitable for a personalized oxygen delivery method in an office where personnel are dispersed. Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15  Figure 11 shows the oxygen-enriched area with different oxygen supply flow rate under air-conditioning (Vin = 0.85 m·s −1 ) at t = 120 s in the form 1# and 4#. The relationship between the oxygen-enriched area and the oxygen flow rate obtained by fitting is a linear relationship, as shown in Table 6. Under the same conditions, 4# has a larger oxygenenriched area than 1#, and the oxygen enrichment effect is better. Therefore, the two are suitable for different office occasions: 1# is suitable for a centralized office area where office personnel is concentrated in the middle of the office, and 4# is suitable for a personalized oxygen delivery method in an office where personnel are dispersed. Oxygen supply flow rate (m 3 ·h -1 ) Figure 11. Oxygen-enriched area with different oxygen supply modes and oxygen flow rates under air-conditioning.

Energy Consumption and Economic Analysis in Conventional HVAC
Under the same conditions, the traditional method of increasing fresh air volume for oxygen supplementation and PSA oxygen generation is used to supply oxygen to the   Figure 11 shows the oxygen-enriched area with different oxygen supply flow rate under air-conditioning (Vin = 0.85 m·s −1 ) at t = 120 s in the form 1# and 4#. The relationship between the oxygen-enriched area and the oxygen flow rate obtained by fitting is a linear relationship, as shown in Table 6. Under the same conditions, 4# has a larger oxygenenriched area than 1#, and the oxygen enrichment effect is better. Therefore, the two are suitable for different office occasions: 1# is suitable for a centralized office area where office personnel is concentrated in the middle of the office, and 4# is suitable for a personalized oxygen delivery method in an office where personnel are dispersed. Oxygen supply flow rate (m 3 ·h -1 ) Figure 11. Oxygen-enriched area with different oxygen supply modes and oxygen flow rates under air-conditioning.

Energy Consumption and Economic Analysis in Conventional HVAC
Under the same conditions, the traditional method of increasing fresh air volume for oxygen supplementation and PSA oxygen generation is used to supply oxygen to the

Energy Consumption and Economic Analysis in Conventional HVAC
Under the same conditions, the traditional method of increasing fresh air volume for oxygen supplementation and PSA oxygen generation is used to supply oxygen to the same air-conditioned room (two adult men are working in the room), and the energy savings and economic efficiency of these two oxygen supplement methods are compared.

Energy Consumption in Traditional Method of Increasing Fresh Air Volume for Oxygen Supplementation
According to the Design Code for Heating, Ventilation and Air Conditioning of Civil Buildings (GB50736-2012) [47], the fresh air volume per person in the working environment is determined to be 30 m 3 /h. The indoor and outdoor meteorological parameters in summer and winter in Beijing are shown in Table 7. The fresh air load is calculated according to Formula (6) [48]. Through calculation, it can be concluded that the traditional air-conditioning system, with increased fresh air volume, consumes 0.502 kW/Day and 1.098 kW/Day in summer and winter, respectively. The air-conditioning system runs 24 h a day and runs for three months in summer and winter. The total energy consumption for the whole year is 3456 kWh. The annual operating cost of the traditional air-conditioning system with fresh air is 1728 RMB.

Energy Consumption in PSA Oxygen Generation for Oxygen Supplementation
According to the national standard [48], the oxygen consumption of an adult man in a light physical work state is 0.03 m 3 /h, and the oxygen consumption of two adult men in an air-conditioned room is 0.06 m 3 /h. Using the PSA oxygen production system to produce 1 m 3 /h of oxygen, the power consumption is about 0.35 kWh, and the oxygen consumption of two individuals per hour requires the power consumption of the oxygen production system to be 0.021 kWh. With the electricity fee set to 0.5 RMB/(kWh), the annual operating cost of the oxygen generation system is 0.021 × 24 × 365 × 0.5 = 91.98 RMB. The PSA oxygen supplementing air-conditioning system consumes 5.05 × 10 −4 kW in summer and 1.1 × 10 −3 kW in winter, so the annual load cost is 3.47 RMB. The PSA oxygen-making air-conditioning system can save 1632.55 RMB per year compared with the traditional airconditioning system with additional fresh air. According to the current oxygen production level of the PSA oxygen generator, the average cost of oxygen production for 1 m 3 /h is about 2500 RMB [47], and the equipment investment and the installation costs required to meet the oxygen enrichment for two people is about 150 RMB. The payback period of the equipment is 0.1 year, which is 33 days.
The combination of PSA oxygen production technology and air-conditioning systems can deliver a certain concentration of oxygen to air-conditioned rooms, which can not only improve indoor air quality, but also reduce building energy consumption. Compared with the traditional increase of fresh air volume to supplement oxygen in air-conditioned rooms, PSA oxygen generation technology has certain advantages in terms of operating costs and energy consumption.

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.