Study on the discharge coefficient of wind-driven naturally ventilated Chinese solar greenhouses

Abstract

The Chinese solar greenhouse (CSG) is a prevalent feature in agricultural practices within China. Nevertheless, the regulation of natural ventilation within this architectural structure remains suboptimal. Consequently, the development of a natural ventilation model becomes imperative for the effective management of the greenhouse environment. Of particular significance within these models is the consideration of the discharge coefficient as a pivotal parameter. Conducting a multi-case investigation into the variable-dependent discharge coefficient is crucial for both practical application and model advancement. This research delved into the impact of various factors, including the upper-lower vents area ratio (A_up/A_low), vent-greenhouse area ratio (A_low/A_greenhouse), lower vent position height (h/H), the incident angle of the external wind, and altitude, on the discharge coefficient (C_d) of CSG. A CFD model was developed for a scaled CSG with validation conducted through field experiments and wind tunnel tests. Results indicated a 61.6% reduction in C_d on average corresponding to an 80% decrease in A_up/A_low. C_d levels remained consistent following the attainment of an A_up/A_low ratio of 1.0. Besides, there was an average increase of 52.5% in C_d levels for every 0.09 decline in h/H, attributed to the blocking effect of the cover. Moreover, the ventilation rate and the pressure coefficient difference were utilized to construct a model of C_d pertaining to greenhouse design and ventilation operation, exhibiting a notable accuracy level of R² = 0.95. Furthermore, the blocking effect of higher h/H was relieved as the incident angle θ decreased under the windward conditions. The increase in A_up/A_low and the decrease in A_low/A_greenhouse were identified as crucial factors contributing to the growth of C_d under leeward conditions. Ultimately, the high-altitude environment led to a rise in C_d levels in contrast to the low-altitude region. The increasing rate of C_d correlated positively with A_low/A_greenhouse and h/H initially, but exhibited a decline once A_low/A_greenhouse reached 0.036, remaining stable thereafter once h/H reached 0.18. In summary, a comprehensive examination of the discharge coefficient of CSG was undertaken, addressing a significant knowledge deficiency and laying the groundwork for advancements in the natural ventilation model and the intelligent control system for CSG.