.Greenhouses.Flows.HeatTransfer.FreeConvection .Greenhouses.Flows.HeatTransfer.FreeConvectionThe upward or downward heat exchange by free convection from an horizontal or inclined surface is modeled. In free convection, the Nusselt (Nu) number describing the exchange process can be defined as a function of the Rayleigh (Ra) number [1]. The heat exchange coefficients are therefore modeled based on the Nu-Ra relation. The model can be used for convection at the cover (upward flow, inclined surface), the floor (upward/downward flow, horizontal surface) or the screen (upward flow, horizontal surface). The bi-direction nature of the convective flow on the floor is due to the latter can be warmer or colder than the air above it. The different natures of the flows lead to different Nu-Ra relations for each surface. Therefore, the user should indicate (by means of the Boolean parameters) which surface is being modeled.
Depending on the status of the thermal screen, the heat flow to the cover can originate either from the top or the main air zone, and the heat flow to the screen can have a different magnitude. Therefore, when the model is used for the cover or the screen, the screen closure (control variable in the global system) is a required input.
[1] Luc Balemans. Assessment of criteria for energetic effectiveness of greenhouse screens. PhD thesis, Agricultural University, Ghent, 1989.