1. Introduction
Individuals spend as much as 90% of their time within buildings where air pollutant concentrations can be several times higher than outdoor levels, posing significant health risks [
1,
2,
3]. With the rapid development of the economy and the continuous improvement of people’s living standards, the requirements for the indoor environment are becoming more stringent. From the initial focus on thermal comfort to the concern of indoor humidity conditions, increasing attention has been given to the quality of indoor air quality (IAQ), such as the concentration of indoor particulate matter with a diameter of less than 2.5 µm (PM
2.5), and carbon dioxide (CO
2) [
4,
5]. In the summer, to pursue a more comfortable and cleaner office environment, the most commonly used methods are using air conditioners for indoor cooling [
6], employing air purifiers to purify indoor polluting particles [
1,
7], and utilizing fresh air systems to supply fresh outdoor air to dilute the indoor CO
2. [
8]. Many researchers have indicated that one of the major factors affecting indoor air quality is the ventilation system, which is associated with the distribution and dilution of indoor pollutants. The air distribution and its impact on the indoor air quality have often been considered as an important consideration [
9]. For rooms that combine different systems in the same zone, a more complex organization of indoor airflow is formed, which in turn affects the purification efficiency of the air purifier and the effectiveness of the ventilation system [
10]. Thus, the airflow and its impact on the indoor quality of the combined system is of great significance for creating a good indoor air quality environment.
The airflow organization generated by air conditioners, air purifiers or fresh air systems has been recognized as crucial in evaluating indoor air quality. Using both field tests and simulation methods, Sekhar and Willem [
9] studied the impact of airflow profiles on the indoor air quality within an office building in Singapore. The results show that the airflow pattern significantly influences the distribution path of pollutants, which provides important scientific evidence for improving and enhancing the building environment. Dai and Zhao [
11] emphasized the importance of properly arranging portable air cleaners (PACs) in indoor environments to reduce the risk of COVID-19 transmission through the air, and they provided a simple and easy-to-use model to guide the placement strategy of PACs in actual engineering projects. Luis et al. [
12] studied the impact of the varying positions and orientations of air purifiers on the ventilation efficiency and air age within elevator cabins. The results demonstrate that selecting the optimal position can significantly improve ventilation efficiency. However, determining the correct air purification flow rate is even more critical for ensuring proper air renewal. Placing the air purifier on the larger sidewall and directing the airflow downward is considered the best position, with a flow rate in the range of 0.4 to 0.6 m
3/min being sufficient to ensure adequate ventilation.
With increasing concern over indoor air quality issues, air purifiers or fresh air systems have been used together with air conditioners in office buildings. The airflow organization became much more complicated due to the interaction between them. Zhang et al. [
13] employed a combination of experimental measurements and computational fluid dynamics (CFD) modeling to study the impact of various air conditioner and air purifier placement methods, air conditioner diffuser type, cooling or heating mode, and pollution source location on the effectiveness of air purification. The results indicated that when the air conditioner and the air purifier are positioned opposite to each other, the efficacy of air purification in the human breathing zone is enhanced. In addition, when an air conditioner utilizes a grid grille diffuser, compared to a dual-slot diffuser, the air conditioner is more compatible with portable air purifiers, leading to improved air purification outcomes. Shang et al. [
14] emphasized the significant impact of occupants’ window-opening behavior on the indoor PM
2.5 concentration, and a reinforcement learning-based control strategy was proposed to provide better indoor air quality with lower energy consumption. Zhang et al. [
15] investigated the impact of air purifiers and window operation on the indoor air quality. The study revealed that air purifiers, when used in conjunction with appropriate window operation, can effectively reduce indoor PM
2.5 levels. Additionally, the study highlighted changes in window operation behavior during the COVID-19 pandemic due to increased demand for ventilation, which could affect the thermal comfort and air quality of the indoor environment. Karam et al. [
16] investigated the impact of combining an intermittent air jet system (IAJS) with portable air cleaners (PACs) on the indoor air quality within a classroom. The results demonstrated that the use of an IAJS and PAC integrated system in classrooms can reduce cross-contamination, and the optimal operating flow rate is 350 m
3/h for PACs, ensuring protection against cross-contamination without increasing energy consumption. Shi and Li [
6] compared two indoor air purification strategies—the air purifier (AP) mode with open-window ventilation (AP-Mode) and the fresh air unit (FAU) mode with positive pressure control (FAU-Mode)—to determine which strategy is most suitable for split air-conditioned (SAC) buildings. The results show that the FAU-Mode has an advantage in terms of energy consumption, especially in conditions with more severe outdoor PM
2.5 pollution. However, the FAU-Mode requires the room to be airtight to maintain positive pressure control. For rooms with less than 1 ACH fresh air requirement, the AP-Mode should be used. For rooms with a fresh air requirement greater than 1 ACH, the FAU-Mode is the appropriate and energy-efficient choice.
According to the aforementioned literature review, most studies have focused on the effects of different flow patterns of air conditioning systems and the placement of air purifiers on the indoor air quality. The combined system of air purifiers and air conditioning has also been studied, and the optimal relative placement and supply air volume have been recommended. However, there is a lack of research on the synergistic effects of an integrated system of air conditioners, air purifiers and fresh air units on indoor air quality, which is commonly used in large office buildings. By integrating the three systems in one room, the indoor airflow pattern becomes more complex. The mutual influence mechanism of flow filed and mass transportation under the combine of three systems is not clear. Due to the existence of this gap, the combined operation system lacks theoretical guidance and cannot achieve optimal operational effects. Thus, further research is required to maintain high indoor air quality efficiently.
To fill the gap mentioned, this study adopted a combined approach of experimentation and numerical simulation to investigate the synergistic effects on the indoor air quality of an integrated air conditioner, air purifier, and fresh air system. The IAQ under different operation conditions was analyzed. Since the PM
2.5 was found to be related to lung cancer and cardiovascular diseases and overall mortality [
17,
18]. In term of CO
2, although it does not have a direct adverse effect, it is considered a good indicator of pollutants emitted by humans and ventilation efficiency, which have a significant impact on IAQ [
19,
20]. Thus, in this study, we employ the PM
2.5 and CO
2 as the indicators of IAQ and the indoor PM
2.5 and CO
2 concentrations were analyzed and compared under different operation conditions. The result of this study would provide guidance for the operation and placement of the combined system, and it would also serve as the basis for an intelligent control system to maintain high indoor air quality efficiently.
5. Discussion
This study explores the synergistic impact of the combined use of an air purifier, air conditioning unit, and fresh air system on indoor air quality. The constraints of our measurement capabilities, we have elected to focus on PM2.5 and CO2 as indicators for IAQ, acknowledging that the exclusion of VOCs represents a limitation of our research. To analysis the synergistic effects of the integrated system, we have employed a hybrid approach combining experimental and computational methods. It is acknowledged that computational simulations are inherently subject to errors, which may arise from model simplifications and deviation from real case in boundary and initial conditions. For instance, the RANS (Reynolds-Averaged Navier-Stokes) model and the species transport models, while foundational to our approach, are not exempt from such limitations. In regard of boundary condition, the velocity of the outlet normally assumed to be uniform, which is also the potential error of the simulation method. The simulation models utilized in this study have been validated against experimental data, demonstrating that the associated errors are within acceptable thresholds. Base on the validated model, we have expanded our simulations to a large office room using the same governing equations for heat, mass, and multi-species transfer. It is recognized that this expansion may introduce additional deviations from absolute accuracy. The findings from this study are intended to offer strategic insights into the optimal operation and positioning of the integrated system. Furthermore, our work lays the groundwork for the development of an intelligent control system designed to sustain superior indoor air quality with high efficiency. It is important to note that the current study does not investigate the long-term operational performance and maintenance requirements of the combined system. In an integrated system where the air purifier, air conditioning units, and fresh air systems operate in concert, the interplay between airflow patterns and species transport dynamics is complex. Consequently, the operational dynamics and long-term performance of the combined system are of significant consequence to its practical application. Therefore, a comprehensive investigation into the long-term performance and maintenance requirements is one of the critical areas for future research.