Effects of Buoyancy and Wind Direction on Airflow and Temperature Distribution in a Naturally Ventilated, Single-Span Greenhouse using a Wind Tunnel

摘要

Experiments were performed in a wind tunnel to study the effects of buoyancy and wind direction on airflow patterns and temperature distribution within an empty, naturally ventilated, arched-roof, single-span greenhouse (open roof, open screened sidewalls). Archimedes Number was used for similarity to determine the necessary wind tunnel air speed and greenhouse floor-to-outside air temperature difference (AT) for the 1/15 scale model. Buoyancy effects were studied for three AT (10, 20, 30 deg C) andtwo wind directions (90 deg , 210 deg). Particle Image Velocimetry (PIV) was used to measure airflow vectors. Dimensionless velocity [U(x), V(x)] and dimensionless temperature (Or) were analyzed. At 90 (roof opening facing wind), the velocity of the downward flowing air from the roof (V = -0.232) created a strong circular airflow pattern that caused the influx of air through the windward sidewall to be significantly small (U = 0.06). At 270 deg (roof opening opposite to wind direction), the velocity of air entering the windward sidewall (U=0.158) was greater than from the roof (V=0.010), creating a more horizontal airflow pattern. Although there was no significant difference in the mean theta_t inside the greenhouse model based on wind direction, the center and leeward sections were significantly greater for the 270 deg wind direct/on (p<0.001). Also, AT did not have a significant effect on mean theta_T inside the model. theta_t was significantly lower at the windward side of the model for all treatments. For the 90 deg wind direction, the center of the model had the significantly highest mean theta_t . For 270 DEG wind direction the center and leeward sections were significantly higher than the windward side. Although the floor was heated, thehighest temperatures were observed in the middle of the model, where airflow was minimal. If insect screens had a coarser mesh, more air could enter through the sidewalls to cool the center zone. Results from this experiment will be used to study positions for high-pressure fog cooling nozzles in the full-scale greenhouse.