Convective heat transfer in a heated greenhouse tunnel
Convective transfers determine for a large part the micro-climate inside a greenhouse. These processes are very complicated because they combine free and forced convection modes, most often turbulent with very different characteristic scales. The aim of the present study is to analyze the internal convective flows generated in a closed greenhouse by thermal buoyancy forces. A commercially available computational fluid dynamics (CFD) code (CFX-5) was used for this purpose in a tunnel greenhouse with crop heated by pipes. Three common heating pipes positions were studied: (a) heating pipes positioned on the soil close to the root of the crop; (b) heating pipes positioned at the middle height of the crop and (c) heating pipes positioned overhead of the crop. All the simulations were carried out in three dimensions. The standard k-σ model was adopted to describe the turbulent transport. The crop was modelled by means of the concept of porous medium and the Darcy-Forcheimer equation. For the same energy released from the heating pipes, results show that there were not significant differences in mean air temperature for the three tested positions of heating pipes. Higher air velocities, in the crop cover, were achieved with the first configuration resulting in an increase on crop aerodynamic resistance and thus in an increase to the heat and mass transfer between crop and air.