Simulation of the effects of ventilation on canopy environment using cultivation bed with draft tube in a plant factory

Air conditions are usually used to control the environmental parameters in plant factories with artificial light. Among them, the air velocity is generally lower than the optimum range of 0.1-1 m/s that is required for plant growth. The occurrence of tipburn behavior is often observed on the inner and newly developed leaves of lettuce plants, especially during the last days of transplanting. It is very necessary to properly design the airflow in the indoor cultivation systems, thus promoting the growth for less occurrence of tipburn in the lettuce plants. The perforated air tubes can be used to improve the airflow. However, this ventilation scheme is required by the installation of pipelines in the plant factory, inevitably leading to an increase in the installation process and the complexity of the project. In this study, a kind of air-conducting cultivation tank was designed to integrate with the cultivation bed (CBT). The airflow disturbance increased inside the crop canopy in the multi-layer cultivation mode of the plant factory, indicating the simple construction of the ventilation pipe. A model of CBT was also constructed using three-dimensional computational fluid dynamic software (CFD). The airflow velocity was simulated and measured inside the plant canopy with the inlet velocity of 5 m/s. The root mean square error was 0.22 m/s between the calculated and measured airflow velocity. There was a consistent distribution of the simulated and measured geometric center section in the cultivation area, indicating the accurate simulation of the airflow velocity. The validated model was used to simulate the effects of different air inlet velocities on the airflow direction and distribution inside the crop canopy. The simulation results showed that the airflow entered the crop canopy area via the holes of the cultivation pipe, thus forming a regular airflow beam profile. There was a larger airflow area of high velocity in the crop canopy with the increase of the inlet airflow velocity. Once the inlet velocity was 6 m/s, the percentage of volumes was 56.3% inside the plant canopy with the air velocity between 0.1 and 1 m/s, and the volume-weighted average air velocity was 0.15 m/s in the canopy interior. Correspondingly, 6 m/s was selected as the entrance speed of CBT, where the mature lettuce was used as the test material. The microenvironment was compared in the inner part of the lettuce canopy under CBT and traditional ventilation control mode (TVC) under the same environmental conditions. A systematic investigation was carried out to test the effect of ventilation and temperature regulation on the air flow velocity. The results showed that the air temperature was 22.4 ℃ inside the canopy under CBT treatment during the light period, which was lower than the TVC treatment of 23.7 ℃. There was a relatively less significant difference in air temperature in the dark period. The average air relative humidity in the light and dark periods inside the canopy under the CBT treatment was 65.8% and 71.6%, respectively, which were 11.4 and 3.0 percentage points lower than those under the TVC treatment. As such, the CBT greatly contributed to the regulation and control of various micro-environmental parameters inside the crop canopy, compared with the traditional ventilation mode. The finding can provide the promotion value to reduce the requirements of environmental control for the high efficiency of air conditioning temperature.