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Figure 3. Comparison between experimental and numerical results of temperature for the 3 cases.
and velocity profiles numerically obtained are shown on Figure 5. Is observed that the relative temperature is approximately constant, and the airflow velocity is fluctuate between 0.4 m/s on the left side and 0.1 m/s near the first heater tube location.
The difference in the average temperature value between cases B and C allows verifying that ventilation provokes sufficient energy loss to decrease the temperature in 3˚C. The airflow velocity, in case C is higher than in the other two cases, but only in a certain part of the path. This is due to the fact of the exterior air flow (right to left direction), which feeds the descending convection flow.
In Figure 5 is clearly showed the influence of the AHT and natural ventilation in temperature (about 7˚C in case B and about 3˚C in case C). This influence is almost insignificant in the airflow velocity profile.
As far as temperature is concerned a special note should be made for the thermal inversion that happens in all the vertical paths but is more visible in case B. This behaviour has its cause in the AHT system set at 0.125 m from the soil and the wind orientation.
3.3. Air Velocity inside Greenhouse
In Figures 6-8 the velocity vectors of fluid flow are represented, respectively in situations A, B and C.
The turbulent regime is lower in a greenhouse without heating and no ventilation. Figure 6 shows a main convection flow in anti-clockwise.
The polyethylene heating tubes change substantially the turbulent regime inside the greenhouse.
In this situation, behind the main convection flow, it is possible to identify six other flows generated by the heating tubes.
The turbulent regime increases even further in the situation in which there is air flux between exterior and interior areas. In this situation it is possible to identify secondary convection flows. Another important observation is the fact that, the descending cold airflow velocity is greater than in the situations without air exchange with the exterior, caused by the addition of cold exterior air.
In the literature, many researchers are working on the analysis of the wind effect on ventilation [3-5,12], but no work has been done on the wind effect for the greenhouse with AHT implanted in the soil. The influence of AHT in the temperature and air velocity was examined numerically. Three cases were studied, the closed greenhouse without heater tubes, the heater tubes implanted in
Figure 4. Temperature and velocity profiles on path AB.
Figure 5. Temperature and velocity profiles on path CD.
Figure 6. Vector of velocity for case A using FLOTRAN.
Figure 7. Vector of velocity for case B using FLOTRAN.
Figure 8. Vector of velocity for case C using FLOTRAN.
the soil in closed greenhouse and finally, the second case with opening windows. Temperatures obtained numerically were compared with experimental data. These demonstrate a good concordance and allow to validate the used numerical code to simulate heat and mass transfer in this studied domain. Then temperatures distribution in a horizontal plane situated 1.125 m from the ground are presented. For each case, the distribution of temperature inside the greenhouse was quite different and the resultant temperature profile was mainly affected by airflow. When the wind enters with a velocity equal to 1 m/s and a temperature equal to 1˚C, the temperature inside the greenhouse decrease significantly (from 19˚C to 16˚C). This decrease depends deeply not only on the wind velocity and its temperature but also on its direction, as studied.
Air velocity distribution along the greenhouse presents a main circulation in the middle of the greenhouse for all situations. In respect to the openings, both air velocity and temperature had a uniform distribution along the greenhouse and air velocity varied between 0.3 and 0.4 m/s. When air flow was parallel to the openings and each opening acted as an inlet and an outlet, we observed regions inside the greenhouse, mainly in the middle of greenhouse, with very low air velocities (0.05 - 0.1 m/s). Consequently, temperature gradually increased between the two openings up to 5˚C higher than the outside air.
This paper describes and evaluates the computational facilities using the finite element method to study the effects of heating tubes and natural ventilation on greenhouses indoor air properties especially during the night.
In opposite to earlier works, usually based on thermal loads, in this study the incompressible Reynolds averaged Navier-Stokes equations with the k-e model was performed. This numerical procedure avoids the use of empirical heat transfer coefficients and provides adequate CPU (Computational Processing Unit) time and residual values. This mathematical model was implemented in FLOTRAN module of ANSYSâ, which is based on finite element method. Good agreement has been observed between the numerical and experimental values. This allows to validate the Computational Fluid Dynamics code used in this work.
Results shown that the heating tubes increases the temperature in about 6.7˚C. If both heating tubes and natural ventilation are introduced this increase reduces to about 3.5˚C. Turbulent regime is lower in case A, and it increases slightly when the heating system is introduced (case B), and it increases significantly in case C due to the effect of natural ventilation.
The simulation of these processes using ANSYS can be a good path to explore, namely in the simulation of three dimension resolution and optimizing the size of the element mesh in order to reduce the computation time.