Simulation research on airflow distribution performance of solar greenhouse heating system in northwest China

. Combined with the structure of the solar greenhouse, this paper proposed a new type of solar greenhouse heating and ventilation system with the framework of the solar greenhouse as the carrier of heating and ventilation. The system's return air mode, tuyere type, tuyere installation position and aperture ratio were determined by computational fluid dynamics (CFD). The results showed that: 1) When the inlet wind speed was 10m/s and the temperature was 37°C, using the upper return air method, the average temperature of the greenhouse crop area increases by 14.7°C, which was greater than 1.3°C of the lower return air and 2.3°C of the middle return air, the heating effect was the best. 2) Based on the nozzle and slot-type tuyere, five kinds of air outlets were designed. It was found that the use of the nozzle-type air outlet could promote the circulation effect of the airflow in the greenhouse and send more heat to the crop area. 3) When the aperture ratio was controlled at 1.5, the number of air outlets has little effect on the greenhouse heating effect.


introduction
The research on the regulation of the humid and hot environment of solar greenhouses in northwest China is relatively weak, and the problem of crop freezing damage has not been effectively solved [1,2]. Taking into account factors such as heating costs and environmental protection, the use of solar energy and auxiliary heating facilities to heat greenhouses has become the mainstream [3]. At present, many scholars have done a lot of research on the heat storage part of the heating system, but less research on the greenhouse airflow organization during the heat release process, resulting in uneven heat distribution in the greenhouse, and some crops cannot be heated. In order to solve the above problems, the airflow organization of the greenhouse should be designed. In order to improve the growth environment of the plant factory, Zhang [4] and Liu [5] established a pipeline air supply system in the plant factory to provide a good wind speed environment for the growth of crops, but did not consider the temperature factors that affect the growth of crops. Therefore, this paper established a pipeline heating system to control the wind speed environment and temperature environment of the greenhouse.

Experimental setup
The experiment was carried out in the campus of Lanzhou Jiaotong University (latitude 36°05′N, longitude 103°88′E). The solar greenhouse is facing south, extending from east to west, with a length of 7m and a span of 6.88m. The height of the ridge is 3.4m, the thickness of the back wall is 1.32m, and the height is 2.6m. The front sloping roof is covered with a polyethylene-vinyl acetate (EVA) film with a thickness of 0.12m.A variable frequency fan was installed inside the greenhouse to drive the circulation of the airflow.

Measuring point layout
In the experiment, T-type thermocouples were arranged in three directions, as shown in Fig 1, and the data was recorded every five minutes. In order to obtain accurate outdoor weather parameters, the PC-4 weather station was arranged on the south side of the greenhouse 2m away from the greenhouse.

CFD modeling and meshing
Use Gambit6.3 to create a 1:1 3D model according to the actual greenhouse size, and the grid independence of the model was verified under three grid numbers of 255470, 567125, and 1025457, and the air temperature error analysis in the greenhouse was selected. After calculation, the maximum relative errors were 9%, 7% and 6% respectively, which ensures the reliability of the calculation results.

Parameter design and calculation method
A typical sunny day was used to verify the test, and the measured data from 8:00 am to 20:00 pm on Sept. 28, 2021 was used as the boundary condition. On the test day, the shed was opened for ventilation at 11:00 in the morning and closed at 16:00 in the afternoon. On the day of the test, the highest temperature of the outdoor environment was 29.3°C, which appeared at 3:45 pm, and the lowest temperature was 11.3°C, which appeared at 7:20 am. The maximum illuminance of solar radiation was 819W/㎡. and 10W/(m 2 ·K). The outdoor temperature was 7°C, and the indoor initial temperature was 10°C.
It can be seen from Fig.3. that the simulated temperature in the greenhouse was consistent with the measured temperature change curve with time, in which the maximum absolute error of temperature was 2.6℃, and the average absolute error was 0. 6℃.

Comparison of tuyere types
The slot type tuyere and spout were suitable for public buildings with large space or tall workshops with allowable fluctuation range of room temperature greater than or equal to 1℃. Therefore, based on these two types of tuyeres, we designed five kinds of tuyere, as shown in Fig. 5, and the size of the tuyere in Table 1. It can be seen from the Table 1

Comparison of different aperture ratios
This Study had indicated that when the aperture ratio was about 1.5, it was conducive to the uniform air supply of the pipeline. Therefore, in this design, the aperture ratio was controlled around 1.5. By changing the number and diameter of the air outlets, four air supply schemes were determined, and the dimensions are shown in Table   3. Fig. 6 showed a schematic diagram of the layout of the air outlet.

4.Conclusions
The main conclusions are as follows: 1) The location of the return air outlet had a great influence on the heating effect. and it was better to set the return air outlet above