Numerical simulation of ventilation efficiency with perforated air tubes for controlling canopy environment in a plant factory based on CFD

Abstract: Much more cultivation layers and high canopy density have posed great challenges to the even distribution of airflow in plant factories. It is highly required for the optimal design of airflow distribution systems, thereby improving the physiological orders and post-harvest quality for a high commercial value of the products. In this study, a ventilation system was designed with the three parallel lines of perforated air tubes, further to generate a constant airflow inside the plant canopy. The air tube was fabricated in a diameter of 20 mm with a length of 1 500 mm using the layout and size of a cultivation shelf. Each air tube was equipped with two rows of jets. A three-dimensional computational fluid dynamics (CFD) model was developed to analyze the distribution of airflow in the plant's canopy under different inlet velocities. Furthermore, the lettuce leaves were assumed as the porous medium with the drag coefficient of 0.02, resulting in a viscous resistance factor and the inertial resistance factor of 25 and 1.3, respectively. A LED lamp (?20 mm) was assumed as an aluminum substrate and a glass part. An energy term with a constant heat generation of 297 525 W/m3 was also defined as the heat source lost by the volumetric convection from the LED lamp aluminum substrate part for the heat dissipation. The average error of simulated air velocity was 16%, compared with the measured. Three groups of air velocity (lower than 0.1 m/s, between 0.1 and 1.0 m/s, and higher than 1.0 m/s) were selected to calculate the proportion of the areas of the plant's canopy surface and the volume proportion inside the plant's canopy. The results showed that the percentage of air velocity at the lettuce canopy surface between 0.1 and 1 m/s increased from 33.33% to 68.57%, while, the percentage of volumes inside the plant's canopy increased from 42.84% to 77.08%, respectively, when the air velocity increased from 6 to 9 m/s. By contrast, the percentage of volumes inside the plant's canopy increased from 0.01% to 0.06%, with the air velocity higher than 1.0 m/s. The maximum distance between the ventilation pipe and high airflow increased from 50 to 70 mm. Therefore, the air velocity was recommended as 8 m/s, further to avoid the air velocity higher than 1 m/s plants canopy. At the inlet air flow velocity, the percentage of volumes inside the plant's canopy with the air velocity between 0.1 and 1 m/s was 67.58%, the percentage of areas on the plant's canopy surface with the air velocity between 0.1 and 1.0 m/s was 55.23%, and the volume-weighted air velocity in the canopy was 0.14 m/s. This finding can provide a new solution to improve the canopy environment in a plant factory.