A CFD greenhouse night-time condensation model

Abstract

A computational fluid dynamics (CFD) model for simulating greenhouse night-time climate and condensation is presented. The model was applied to a four-span plastic covered greenhouse. Film condensation was simulated by applying a user defined function (UDF) added to the commercial CFD package. The CFD model was verified by comparing CFD predictions with experimental measurements. The effect of cover temperature on greenhouse humidity was then determined by the CFD model and compared to experimental results. Root mean square values for the differences between the CFD and experimental values of internal temperature and humidity showed there was good agreement between. The results showed the importance of heat transfer losses by radiation, particularly for low values of soil heat flux (SHF). They also showed the roof was the coolest surface in the greenhouse, and therefore the sink for the water vapour produced by the crop. For each configuration (SHF 10, 25, 50 and 100 W m−2 and equivalent sky temperature 263 K, 273 K and 276 K), the condensation rate curves and relative humidity (RH) evolution are presented. It was observed that all the condensation rate curves had the same characteristic shape and could be represented by a single logistic function. The response of the CFD model to a step-change in the water vapour source (night-time transpiration from the crop) was then analysed. It was observed that the model predicted the same steady-state temperature, RH and condensation rate independent of the time when the water vapour source was enabled. The CFD condensation model is intended to be used for the design of strategies for humidity control, particularly in unheated greenhouses.