A naturally ventilated crop protection structure for tropical conditions.

Abstract

This study presents the theoretical and experimental results of natural ventilation rates induced by stack, wind and the combination of both stack and wind effects for a typical crop protection structure suitable for the tropics. The structure consists of simple structural frame, transparent roofing and insect screen side walls. It was found the relative importance of the stack and wind effects is dependent on the ratio between wind speed and the square root of the inside-outside temperature difference (u/AT°.5). In this study, the wind effect dominates over the stack effect when the ratio u/AT" becomes greater than 0.5. Ventilation rate induced by the stack effect was found to increase with increasing temperature difference between inside and outside of the crop protection structure according to a power law, with an index of 0.5. The wind effect ventilation rate was found to increase linearly with increasing outside wind speed measured at eaves level. In addition, the combination of the stack and wind effects could be represented as the vectorial sum of two the independent effects (40sw = [43)k2 + (1)„,nd21 0.5). However, the result of the wind effect in the combined effects was insignificant when the ratio of ventilator opening to the total wall area is higher than 20 %. Different methods have been used to determine the natural ventilation rates. The dynamic tracer gas was used as the control; direct airspeed measurement, energy balance and neutral plane methods were used to quantify ventilation induced by the stack effect. Pressure field measurements were used to quantify ventilation by wind effect. In addition, the dynamic tracer gas, energy balance, and stack and wind methods were used to quantify ventilation induced by the combined effects. However, these methods have their constraints and limitations because of statistically significant differences in the comparison between the methods. The tracer gas method was found very difficult to use in the highly porous structure. In addition, the ventilation rate measured by this method was 30-40 % less than the other methods. The energy balance method has the advantage that it estimates many important climatic and crop parameters, however, the errors were found to be the highest. The neutral plane method was suitable for measuring ventilation induced by stack effect, the simplest method, requiring only the measurement of the inside and outside temperatures. The direct airspeed measurement method was much easier to handle and the result was comparable to other methods suitable for determining the ventilation induced by the wind effect. The physical properties of the covering materials, namely light transmission, coefficient of discharge and airflow characteristics were also determined in this study. It was found that the light transmissions of transparent polythene film and insect screens were close to each other. The coefficient of discharge and light transmission were dominant parameters in the ventilation rate calculation. It was found that when air flows through a screen, the pressure drop increases linearly with the square of approach airspeed. Airflow distributions inside the crop protection structure induced by the stack and wind effects are also presented in this study. Finally, this study presents information on natural ventilation for tropical greenhouses that was not previously available.