AIR MOVEMENT AND CLIMATE UNIFORMITY IN VENTILATED GREENHOUSES

Air movement caused by ventilation in greenhouses is an important factor that affects the uniformity of greenhouse environment and consequently the uniformity of plant growth and quality. Natural ventilation systems have been widely adopted. The studies of the air movement in naturally-ventilated greenhouses have been performed by means of field experiments, laboratory tests such as wind tunnel tests using scale models, and numerical simulations using computational fluid dynamics (CFD) approach. The primary characteristics of the air movement caused by wind and stack effects in a single-span greenhouse are described. Recent studies on the air movement and the uniformity of environment under natural ventilation are reviewed and discussed with respect to the multi-span greenhouses, the effect of plants, the vent configuration, and the effect of wind direction. INTRODUCTION Greenhouse production has been expanding in the regions under mild climate. Year-round production in greenhouses is one of the primary concerns to increase the greenhouse efficiency and the plant productivity. The technologies to cool greenhouse air on hot sunny days have been becoming more important. Particularly, natural ventilation systems, which are significantly less energy intensive than fan ventilation systems, have been widely adopted. Under mild climate, a large amount of ventilation is required to prevent an excessive rise in internal air temperature and it is often difficult to realize such amount of ventilation less economically with fan ventilation. Ventilation is important through the year to supply CO2 from the external air and control humidity. Ventilation also affects the plant growth through the induced airflow, which promotes gas and energy exchange between the plants and the surrounding air. The greenhouse design and control technologies have made significant progress. The eaves (gutter) heights of greenhouses have been increased to ensure the uniformity and stability of environment. The wider vent openings are expected to improve the natural ventilation performance. One of the innovations in the greenhouse structures is the openroof design, which has been introduced as a solution in the middle of the 1990s (Roberts, 2001). In order to cool the internal temperature below the external temperature, evaporative cooling is only a method that is the most practical and inexpensive in operating cost. Although the pad-and-fan cooling has been widely utilized for fan ventilated greenhouses, the fog cooling in combination with natural ventilation has been introduced. Another method to prevent an excessive rise in greenhouse temperature is shading to decrease incoming solar radiation. Evaporative cooling and/or shading systems function well in combination with proper ventilation. Air movement caused by ventilation in greenhouses is an important factor that affects the uniformity of greenhouse environment and consequently the uniformity of plant growth and quality. The temperature distribution has a close relationship with internal airflow caused by ventilation. The studies of the air movement in greenhouses have been performed by means of field experiments, laboratory tests such as wind tunnel tests using scale models, and numerical simulations (Bailey, 2000; Boulard et al., 2002; Critten and Bailey, 2002). Advancements in velocity vector measurement techniques such as sonic anemometer systems for field experiments and particle imagery velocimetry Proc. IS on Greenhouse Cooling