Measurement and simulation of climate inside Almerı́a-type greenhouses using computational fluid dynamics

In this paper, the effect of wind speed on the natural ventilation of an Almerı́a-type greenhouse is analysed by means of computational fluid dynamics (CFD), using the commercial program ANSYS/FLOTRAN v6.1 based on the finite elements method. The experiment was carried out in an Almerı́a-type greenhouse equipped with top and side ventilation. The vertical profiles of air velocity were measured with a hot bulb anemometer and visualised by means of a smoke tracing technique. The airflow through the insect-proof screens and the crop were described by means of an approach to porous media. Two-dimensional simulations in a steady state, which described the turbulent transfers by means of the standard K−ε turbulence model, were also carried out. Cooler, denser air entered through sidewall openings and left the greenhouse through the roof window when the wind speed exceeded 1 m s−1. The importance of roof ventilators for efficient ventilation in Almerı́a-type greenhouses was observed. The air temperature distribution shows a gradient from the sidewalls towards the centre of the greenhouse due to the movement of the hot air rising towards the roof vent, and a vertical gradient due to the movement of the air above the surface of the ground absorbing solar energy at floor-level. Maximum air velocity inside the greenhouse was reached near the side vents, with the lowest values observed in the middle of the greenhouse. The velocity decrease produced in the windward opening between the outside and inside of the greenhouse was 75–85% in every case. The air velocity in the leeward area remained more or less constant around 0.3 m s−1, as result of the “chimney effect”. The model was verified by comparing the numerical results with experimental data. The differences between values predicted by the CFD models and those measured were from 0.0 to 0.36 m s−1 for air velocities, and from 0.1 to 2.1 °C for air temperatures.