1. Introduction
A Chinese solar greenhouse (CSG) is a unique energy-saving and environment-friendly horticultural facility in Northern China which mainly relies on solar energy as the heat source [
1,
2]. This facility can achieve the overwintering production of vegetables without or with less heating in the winter. CSGs generally adopt the form of closed management during the winter to ensure a suitable interior temperature, which leads to airflow stagnation. The airflow organization can break the boundary layer of the leaves to promote photosynthesis and makes the temperature, humidity, and CO
2 concentration in the greenhouse uniformly distributed, which facilitates crop growth [
3,
4]. The suitable airflow range for crop growth is 0.2–1.0 m/s [
4,
5]. Therefore, the greenhouse air circulation is an important part of improving the growth environment and increasing the crop yield.
The circulation fan could enhance the convective airflow near the crop canopy during the operation condition, thus increasing the boundary layer conductance [
6,
7]. The application of circulation fans in greenhouses can be traced back to 1961 in order to regulate interior environmental uniformity. There are few specific studies on the effects of the different airflow distributions formed by the circulation fan on crops in the greenhouse industry. The first study on the different air supply methods of circulation fans was in 1974. Walker et al. [
8] discussed the effects of a vertical air supply, horizontal air supply, and certain-angle air supply on the air velocity in a greenhouse. The results showed that the air distribution was optimal when the fan was deflected at 15° horizontally, which could meet the design requirements. In addition, the number, location, deflection angle, and operation combination form of circulating fans have a significant influence on the greenhouse microclimate [
9,
10]. Ishii et al. [
11] found that 10 to 15 fans were required to generate an airflow above 0.3 m/s in a greenhouse of 1000 square meters. Tokairin et al. [
12] suggested that arranging the additional fan when the air supply speed of the circulating fan dropped below 0.1 m/s could effectively improve the velocity attenuation problem. On this foundation, Tokairin et al. [
13] concluded that the coupling effect of the height of the circulation fan from the ground and the air supply speed also affected the environmental conditions in the greenhouse. The experimental results showed that the disturbance effect was not obvious when the circulating fan system was installed at an excessive height, which was unfavorable to the regulation of the microclimate within the crop canopy. Previous studies have adopted a single circulating fan system to create a relatively appropriate growth environment for greenhouse crops. In recent years, more research has focused on the combined utilization of recirculating fans with other environmental control devices to achieve the improved regulation of the greenhouse environment. Air conditioning and circulation fans are the most widely employed environmental control devices, which are mainly used in plant factories. The combined application of circulation fans and air-conditioning equipment dramatically improves the airflow rates. There was one study that revealed that the combined usage of both devices increased the air flow rate by 24% compared to the separate usage of air conditioning, which contributed to the improvement of the temperature deviation between different culture beds. Furthermore, the lettuce indicators were determined through the experiment, and the fresh weight, leaf number, and leaf length increased by 40.6%, 41.1%, and 11.1%, respectively [
14]. In the investigation of the combined use of internal and external circulation fans and air conditioning on the improvement of crop growth indicators, it was found that the effect of external circulation fans on the improvement of crop growth indicators was limited and that only the combined use of the three devices could optimally improve the effect [
15]. For multispan glass and plastic greenhouses, there have been some studies on the combined use of circulating fan systems and evaporative cooling pad–fan systems [
12,
16,
17]. These studies focused on the air supply direction of the two systems. When the circulating fan system and the evaporative cooling pad–fan system of the air supply direction were opposite, the air mixture on different planes was better, and the temperature distribution was more uniform. Additionally, the greenhouses with polycarbonate sheets as a covering material employed a combination of the circulating fan and the box evaporative cooler for heat removal [
18]. In a nutshell, these studies have found that the air circulation from the circulating fans promoted the uniformity of the crop growth parameters in the whole greenhouse and effectively improved the relative humidity and temperature range of the crop-growing environment.
Most of the existing research on circulating fans has been conducted in glass and plastic greenhouses, and little research has been performed in CSGs. Duan et al. [
19,
20] arranged two internal circulation fans in a CSG and simulated the airflow and temperature fields formed by the fans at different locations using CFD techniques. It was found that the system could realize the air circulation and significantly improve the wind speed and temperature. In the follow-up research, Zhuang et al. and Zhang et al. [
21,
22] analyzed the installation height of a circulation fan in a CSG. The results showed that the application of circulating fans could improve the airflow velocity, and the installation height of the fans installed at 1 m to 2 m above the crop canopy produced the highest percentage of the airflow velocity in the effective disturbance range. However, the installation height was not determined through a detailed simulation analysis and was only selected due to the percentage of the effective disturbance velocity. Li et al. [
23] proposed a fan–coil unit (FCU) active heat collection and release system that integrated a circulation fan with a water circulation heat storage and release module for their combined application. The FCU system was installed in a ridge inside a plastic greenhouse or CSG. The heat was collected and released through the coupling of the water-vapor heat exchange and forced convection. Therefore, the FCU system supported the effect of collecting and releasing heat while having a disturbing airflow to provide a certain amount of air speed for crop growth. After system optimization and upgrading, He et al. [
24,
25] concluded that the FCU system had a remarkable energy-saving effect and that the coefficient of performance was excellent. Different heat collection modes could be selected for different weather conditions. Zong et al. [
26] also confirmed that the FCU system ensured the safe overwinter production of crops in cold weather after the experimental studies. However, the circulation fan inside the FCU system has the effect of a uniform airflow field during the process of air supply, but no attention has been attached to the homogeneity of the airflow field formed by this system.
The existing research on circulating fans has focused on its utilization to provide the appropriate air velocity to improve the greenhouse microclimate environment, but little research has been done on the uniformity of the airflow field. A uniform airflow field can provide a relatively consistent airflow for the crop canopy, which is conducive to stimulating the growth and development of the crops. Therefore, the main objective of this paper was to propose a novel predictive scenario-based installation parameter model using CFD techniques to aid in optimizing airflow uniformity. Solidworks software was used to construct the physical model, and the Fluent platform was applied to simulate the airflow fields. The numerical model was validated by the experimental measurements. By optimizing four factors, the tilt angle, swing angle, height above the ground, and shape of the outlet baffle, an installation scheme with an effective uniform airflow field was finally obtained. The aim of improving the airflow uniformity in the greenhouse could be achieved without any additional equipment or energy consumption simply by optimizing the installation parameters. This study could provide theoretical guidance for the installation and design of the system and could provide ideas for a uniform greenhouse environment.
4. Results and Discussion
Based on the validated simulation model, the proposed layout parameters were determined systematically. The optimization results showed that the best uniform airflow field was formed when the fan tilt angle was −2.5°, the fan swing angle was 60°, the fan height above the ground was 2.9 m, and the side opening angle was 60°.
4.1. Analysis of Coefficient of Variation
For the proposed and traditional empirical system layouts, changing the placement strategy of the FCU systems would influence the overall uniformity as shown in
Table 6. The variation regularity of the coefficient of variation when the system layout was not optimized could be expressed in the range of 0.63–0.91. This occurred as the fan was tilted upwards, and the circulating airflow blew directly from the outlet to the front of the greenhouse film then gradually down the film to the bottom corner of the south side as also seen clearly in
Figure 9. Therefore, the concentration of the plane velocity in the south zone of the greenhouse was stronger as the horizontal height rose compared to the north side, where there was no significant airflow organization. This pattern of air circulation distribution led to extremely poor uniformity.
In the proposed optimization scheme based on the simulation model, the variation regularity of the coefficient of variation could be expressed in the range of 0.22–0.44. There was a stable prevailing airflow at a level of 1.6 m from the ground, which resulted in the highest coefficient of variation and the worst uniformity. However, compared with the empirical layout, the coefficient of variation of this canopy plane decreased from 0.91 to 0.44, and the uniformity improved by 51.65%. The monitoring plane that was 0.7 m from the ground was closest to the greenhouse soil, and the airflow organization was unstable because the vortical flow fields acted in a complicated manner. Meanwhile, the uniformity at 1 m and 1.3 m from the ground also improved significantly. Therefore, the proposed system layout strategy facilitated the improvement of the crop microclimate, with the average coefficient of variation reduced from the conventional 0.76 to 0.33, and its overall uniformity in the canopy area improved by 56.58%.
The main purpose of the circulation fan applied in the CSG earlier was to provide a certain airflow for the crops [
19,
20]. However, there were few detailed studies on the effect of the installation factors of the circulation fans. This paper used ANSYS Fluent 2020 R1 to optimize the installation factors of the FCU systems based on the research of the literature [
23,
24,
25,
26] to seek a scheme that could form a more uniform airflow field. The coefficient of variation was used as a quantitative evaluation index to compare the airflow field homogeneity of different schemes. The results showed that the optimized scheme had a great improvement in the uniformity of the airflow field. It could guide the installation of FCU systems and could also provide a new idea for optimizing the greenhouse microclimate.
4.2. Analysis of Disturbance Characteristics of the Optimized System Layout
The disturbance characteristics of the optimized system layout were evaluated using the percentage of the velocity distribution in the different horizontal profiles as shown in
Table 7. Zhuang et al. [
21] used a wind speed range between 0.15 m/s and 0.50 m/s as the effective disturbance airflow. In this paper, we referred to this wind speed range and mainly analyzed the percentage of the airflow velocity distribution that was in this range.
Table 7 demonstrates that the airflow velocity range before and after optimization was mostly distributed below 0.35 m/s. This was mainly because the fan rotation speed in the FCU systems was low and because the air supply was small; thus, the overall velocity level of the greenhouse was in a moderate range. The four horizontal planes before and after optimization were compared sequentially. It could be found that the percentages of planes 1 and 2 in the range of the effective disturbance airflow speed after optimization (58.68% and 43.73%) were improved compared with the empirical scheme (42.73% and 41.02%) and that plane 1 had the best enhancement effect. Meanwhile, the improvement of the effective disturbance airflow speed in planes 3 and 4 was not significant. This reason could be analyzed through the uniformity of the velocity distributions in the four horizontal profiles. As shown in
Table 6, the velocity uniformity of plane 1 was the best among the four planes when unoptimized. It was more conducive to the formation of the effective disturbance wind speed when optimizing the uniformity, and plane 2 was the same. The velocity uniformity of planes 3 and 4 was poor before being optimized, while, with the optimized results of the coefficient of variation, it could be seen that the optimization effect of these two planes was relatively better. This was because the air supply volume of the system was constant, and a better disturbance speed could not be guaranteed while improving the uniformity. Therefore, the average velocity of planes 3 and 4 was in the relatively lower-speed range. This problem could be improved by adjusting the air supply volume of the FCU systems in the future.
Zhuang et al. and Zhang et al. [
21,
22] analyzed the installation height of the circulation fans in a CSG using the percentage of the velocity distribution as a quantitative evaluation index. However, the effects of other installation factors on the internal airflow field of the CSG have not been studied in depth. This paper analyzed the percentage of the velocity distribution after optimizing the four installation factors and compared the airflow velocity distribution range formed by the proposed and empirical schemes. The results of the optimized wind speed distribution percentages showed that the scheme proposed in this paper has the potential to improve the effective disturbance wind speed range while ensuring the optimized uniformity. Therefore, by comparing the percentage and coefficient of variation of the airflow velocity distribution, it could be concluded that the uniformity of the airflow field was considerably improved and that the disturbance effect of the system was improved to a certain extent when the four installation factors of the fan were optimized.
5. Conclusions
In this study, in order to solve the dilemma that various environmental parameters in solar greenhouses are less homogeneous, the numerical simulation method was used to analyze the airflow environment generated by a fan–coil unit (FCU) system in a greenhouse. Four critical installation parameters, the fan tilt angle, fan swing angle, fan height above the ground, and fan outlet baffle shape, were optimized to establish a uniform airflow field.
The greenhouse airflow organization before optimization was mainly concentrated in the south area with a high velocity, while the airflow velocity near the north side was very small and uneven. In addition, there was no significant airflow disturbance between the two identical fans. The overall coefficient of variation for the horizontal profiles was 0.76. The effective percentages of the airflow disturbance velocity in horizontal planes 1 and 2 were 42.73% and 41.02%, respectively. Based on the validated simulation model, the proposed layout parameters were determined systematically. The optimization results showed that the best uniform airflow field was formed when the fan tilt angle was −2.5°, the fan swing angle was 60°, the fan height above the ground was 2.9 m, and the side opening angle was 60°. The airflow velocity near the greenhouse’s north side increased significantly, and the airflow disturbance at the crop canopy height was enhanced to a certain extent. The average coefficient of variation was reduced to 0.33, and its overall uniformity in the canopy area improved by 56.65%. The effective percentages of the airflow disturbance velocity in horizontal planes 1 and 2 were enlarged to 58.68% and 43.73%. The active multifunctional fan–coil system achieved a stable airflow transition without any additional devices or energy requirements, confirming the dominant role of the proposed installation parameters in the coefficient of variation and homogeneous characteristics and thereby generating a suitable thermal environment for the horticulture facility.
This paper provided a solution for improving the airflow field in Chinese solar greenhouses based on the proposed FCU system. The method for constructing the fan model is universal and can be applied to different types of greenhouse facilities. The optimization results are applicable to low-growing leafy vegetables. This paper contributed to guiding the installation of the FCU system and provided theoretical guidance for the optimization of the airflow field. The future development direction of this study is to consider the effect of different crop heights and growth stages on the airflow field. It is also necessary to further optimize and scientifically design an FCU system associating the temperature and airflow fields.