Browsing by Author "Bailey, B. J."
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Item Open Access A naturally ventilated crop protection structure for tropical conditions.(Cranfield University, 1999-06) Kamaruddin, Rezuwan; Douglass, M. P.; Bailey, B. J.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.Item Open Access Seawater greenhouse for arid lands(1997-09) Raoueche, Abdelkrim; Stenning, B. C.; Bailey, B. J.The growth of the world’s population and the consequent food shortage, requires the expansion of agriculture into arid zones which constitute about 60 % of the earth’s land area and are characterized by high levels of solar radiation and shortage of freshwater. The objective of the seawater greenhouse for arid lands was to develop and demonstrate a cost effective means of producing both crops and pure water in hot, arid coastal regions. The project exploited both the high solar radiation and prevailing wind to drive most of the energy exchange processes in the greenhouse. In addition to a crop grown inside the greenhouse, a shade tent provided shelter for nursery plants and an outdoor planting scheme was maintained with the supply of freshwater as well as protected by the structure of the greenhouse itself. The greenhouse is cooled with an evaporative cooling pad (Celdek™) through which an air flow is promoted by the prevailing wind. The water vapour transpired by the plants combines with the cooled and humidified ventilation air stream to generate a high relative humidity in the exhaust air. A second evaporation pad (Celdek™) is used to further humidify the exhaust air which passes through a condenser cooled by seawater to produce fresh water. The wind also promotes an air flow through a roof cavity in the greenhouse where more seawater is evaporated and the humid air passes through the condenser. A computer program was modified to describe the greenhouse, which at first was triangular in plan, and models were written to describe the evaporative cooling systems, the selectively absorbing roof, the humidification of the air in the roof and the seawater condenser. These models were incorporated into the existing greenhouse model. Analytical techniques were used to create the hourly values of solar radiation, air temperature, humidity, cloud cover and wind speed required by the simulation model. This simulation model was used to predict system performance and determine the sensitivity of the greenhouse environment and fresh water output to air and water flowrates and the climatic conditions. The sensitivity analysis showed that adding the second (rear pad) evaporative cooling pad increased the fresh water production compared to using only the front evaporative cooling pad. The use of cooling water at the wet bulb temperature gave a lower condenser output than using surface seawater. The results showed that an effective desalination system would consist of an evaporating cooling pad coupled directly to the seawater condenser. Data were recorded in the greenhouse in Tenerife in order to determine the heat and mass transfer coefficients of the second Celdek™; these established the program of the system and provided data for validating the model. The experiments performed on a prototype Celdek™ heat exchanger in the laboratory were used to design the Celdek™ condenser. The validity of the seawater greenhouse simulation model was checked using experimental data recorded on the prototype seawater greenhouse in Tenerife, in December 1994 and June 1995. The model was revised as a result of the validation exercise.