3.3.1 Condensation

Crop transpiration increases the percentage of water vapor in the environment, generating the possibility of obtaining saturated air. Environment saturation is an undesirable effect over the crops. There are some approximations in order to know condensation rate [8], which can be estimated as a difference between former quantity and the latter. Eq. (17) is used to estimate it:

$$\mathcal{Q}\_{cov} = \max(\mathbf{0}, \mathcal{M}\_{w, air} - \mathcal{M}\_{w, cov}) \text{ [kg]} \tag{17}$$

where Mw,air the humidity content of the greenhouse air (%) and Mw,cov\_in the saturated humidity content of air at the cover temperature (%).

#### 3.3.2 Water vapor

Water vapor transport is simulated with Eq. 18:

$$\frac{\partial(\acute{\varepsilon})}{\partial t} + \frac{\partial}{\partial \mathbf{x}\_{\acute{\jmath}}} \left( \acute{\mu}\_{i} \acute{\varepsilon} \right) = \frac{\partial}{\partial \mathbf{x}\_{i}} \left( D\_{w} + \frac{\mu\_{t}}{\acute{\rho} \mathfrak{s}\_{\acute{\varepsilon}t}} \right) \frac{\partial(\acute{\varepsilon})}{\partial \mathbf{x}\_{i}} + \mathsf{S}\_{w} \tag{18}$$

where C mass concentration of component in air (kg kg�<sup>1</sup> ); ui wind velocity in j direction (m s�<sup>1</sup> ); Dw water vapor diffusivity (m2 s �1 ); μt turbulence air viscosity �1 � � (kg m s <sup>1</sup> ); ρ density (kg m <sup>3</sup> ); S the average velocity module in the deformation (m s�<sup>2</sup> ) which is calculated with Eq. 19:

$$S\_w = \frac{ET}{Lv} LAB\tag{19}$$

where ET latent heat flux density (W m�<sup>2</sup> ); Lv evaporation latent heat (J kg�<sup>1</sup> ); and LAD leaf area density (m�<sup>1</sup> ).

### 4. CFD simulations in greenhouses

#### 4.1 Natural and mechanical ventilation

In a greenhouse crop production, the ventilation system is the most important auxiliary equipment for climate control. Natural or mechanical ventilation design accounts for the size of greenhouse to determine the vent dimension and position (Figure 1). Furthermore, new complementary devices have been adapted to enhance the efficiency of air renewal rates. For instance, the use of the back-wall vent dimension on solar greenhouse cooling was investigated by He et al. [10] using CFD. In this study, the average air temperature in a solar greenhouse with removable back walls (RG) was reduced by approximately 1.7°C with a back-wall vent of 1.4 m, thereby increasing ventilation efficiency.

The presence of screens in the lateral and roof windows reduces the ventilation rate. However, according to [7], screens promote uniform velocity distributions

#### CFD Simulation of Heat and Mass Transfer for Climate Control in Greenhouses DOI: http://dx.doi.org/10.5772/intechopen.86322

inside the greenhouse compared to no-screened greenhouses, especially near the crops. Figure 2 shows the results of a CFD simulation in a screenhouse, more specifically the exchange of air inside/outside near the screenhouse roof.

The advantages of numerical simulation are the possibilities to observe details in specific zones of the greenhouses (Figure 2A) and to convert a discrete phenomenon continuously. Figure 2B shows the mass air that enter and exit from a screenhouse under five exterior velocities simulated, when crop is simulated and empty. When the screenhouse is empty, mass balance is very similar; however, the crop reduces this flow until 200 kg s˜1 when exterior wind velocity ˜<sup>1</sup> is 5 m s .

In a greenhouse with combined mechanical and natural ventilation (Figure 3), the velocities' patron is marked different. For instance, when only mechanical ventilation (fist one) is simulated, temperatures' distribution is basically due to mechanical convection as a consequence of these air movements. In the second one, roof windows are 30% open and the wind patron changed. If just mechanical ventilation is working, under vents, the velocity is near to zero, but if the roof windows are open, the wind distribution is better than only mechanical ventilation.

CFD simulation of the ventilation systems, natural, mechanical, or combined, allows to observe the distribution of the air in a problematic zone and infer process of mass and energy transfer due to the interaction with the external climate conditions.

#### Figure 2.

(A) Top view of mass exchange in the roof of screenhouse and (B) scalar mass flow (kg s ˜1 ) side exchange (inside/outside) by CFD simulation with and without crop.

Figure 3. Wind velocity vectors' distribution in the central spans of greenhouse closed (A) and all-open (B).
