Abstract
Greenhouses are used extensively in both agricultural and horticultural industries. Cultivating crops in greenhouses is an energy-intensive process, especially as far as ventilation is concerned. The use of natural ventilation as an alternative to control the indoor climate of greenhouses has become increasingly popular. Natural ventilation in greenhouses is due to the air exchange process between the internal and external environment of the greenhouse mainly due to thermal buoyancy and wind-induced forces.
The main objective of this thesis is to determine the effect of dimensional and geometrical influences on the heat transfer characteristics of greenhouses using Computational Fluid Dynamics. This entails an investigation into the effect of geometric design changes on the thermal and flow performance of greenhouses due to buoyancy driven flow. The study also includes a numerical investigation into the effect of plant benches on the indoor climate of greenhouses.
This thesis commences with a review of the available literature, and identified areas in the research that require more attention. A background on the numerical tool used (Computational Fluid Dynamics) was given, as well as a background on the experimental methods used in this thesis. In this research study, the investigation was initiated with analysis of a simple, zero-degree roof angle cavity, which represents a square greenhouse. Results were compared to experimental results found in the literature and a good comparison was found. Confidence was established in the two-dimensional CFD model. A three-dimensional zero-degree roof angle cavity was also created and verified against data found in the literature. A good comparison between Nusselt numbers was found, and therefore confidence in the three-dimensional CFD model was also established. The three-dimensional CFD model approach was used to investigate the multi-dimensional heat transfer effects of the zero-degree greenhouse cavity. A significant difference between the two- and three-dimensional cases was observed.
The next step was to modify the zero-degree roof angle cavity, by adding an angled roof. This represented a simple, single-span greenhouse. The heat transfer in the cavity subject to various roof angles was investigated, and Nusselt-Rayleigh number relationships were established. A three-dimensional cavity of similar dimensions was investigated and compared to the two-dimensional cavity. Temperatures were found to be higher, and velocities much lower compared to the two-dimensional case. A roof ventilator was added to the one side of the greenhouse, and it was found that adding a ventilator does have a noticeable effect on the heat transfer inside the cavity. Nusselt-Rayleigh number relationships were also established for the...
D.Ing.