Αριθμητική προσομοίωση των συνθηκών περιβάλλοντος θερμοκηπίου με συστήματα φυσικού αερισμού και δροσισμού υγρής παρειάς

Abstract

The present Ph.D. study was carried out in the Faculty of Agriculture of Aristotle University of Thessaloniki. The main target was to simulate numerically the greenhouse environment and its surrounding area concerning natural ventilation and cooling conditions with pad and fan evaporative cooling system. Both the experimental procedures and the theoretical investigation were carried out at the facilities of the Center of Agricultural Structures Control located at the farm of Aristotle University. The experiments were carried out in a single-span, 8m x 15m greenhouse with an arched roof (Fig. 1); it’s orientation was 30° from North and its position was at: Latitude 40.54 N, Longitude: 22.99 E. The greenhouse had FRP (fiberglass reinforced plastic) sidewalls and a tetrafluoroethylene copolymer 60 microns film roof. The gutter height was 2.6 m and the ridge height was 4.2 m. A cooling pad of width 6.0 m and height 1.0 m was positioned at the center of the north-wall, at 1.0 m above the ground. On the south wall, two fans, with propeller diameter of 0.76 m and 0.60 respectively, were placed at 1.32 m above the ground. Four different treatments concerning the greenhouse environment were investigated during the summer months of the period 2004-2007: natural ventilation, natural ventilation with internal shading, evaporative cooling system and evaporative cooling system with external shading. The periods of measurement coincided with the nature stage of tomato crop cultivated using the common one stem technique. During experiments the following measurements were recorded by a data logger system. Outside the greenhouse: air temperature, air humidity, air speed and direction at 10.0 m height from the ground, global and diffuse radiation. Inside the greenhouse: air temperature at 23 points 1.2 m from the ground, air humidity at 8 points in the same horizontal level, solar radiation above the canopy, leaf wetness, soil moisture content and leaf temperature. Measurements were obtained also with 2D Sonic anemometer at inside the greenhouse (19 points), at foreside of pad (15 points) and outside of fans (9 points). Every week the leaf area index was recorded at 30 locations with a Sun Scan canopy analysis system (Delta-T Devices Ltd). Pad water rate was measured by an integrated flow meter which was connected to the makeup water line to the pad storage tank and the operation time of the pump was recorder with an electrical on/off sensor. Analysis of experimental data was carried out in order to determine in detail the greenhouse environment. The experimental data sets, obtained from specific points in the greenhouse, were analysed according to Kriging method using a commercial geostatistical software. By this approach temperature and humidity maps were produced for different height levels. For the numerical approach, the experimental greenhouse was designed and meshed with the geometrical processor Gambit® as a 3D full scale model. The main characteristics of the experimental greenhouse, such as pad, fans, frame, covering materials and individual plants, were thoroughly integrated in the geometrical model. The commercially available CFD code Fluent® used a finite volume numerical scheme to solve the equations of conservation for the different transported quantities in the flow (mass, momentum, energy and water vapor concentration). The set up of simulation model was determined in detail, especially concerning boundary conditions. Finally 35 simulations cases were solved corresponded to different experimental conditions. The results showed in general a qualitatively good agreement with experimental data. For all the simulation models the correlation coefficient ranged between 0.54-0.97. Both analysis of experimental data and simulation models showed that the tested cooling system was able to keep the greenhouse temperature only few degrees below outside air temperature for the specific ventilation rate. Although the length of the greenhouse it is not too long, important thermal gradients were observed in the direction from evaporative pads to exhaust fans. A thermal gradient was also observed in the vertical direction, from greenhouse ground to greenhouse roof. The simulation results showed not only the key role of the ventilation rate, but also how the specific characteristics of the air flow inside the greenhouse influences the performance of the fan and pad evaporative cooling system. The cooling efficiency, which expressed as a function of outside air temperature and inside wet bulb temperature entering the greenhouse, can not provide detailed information about the heat removal due to system’s operation. The numerical models proved to be a very useful tool not only to investigate the greenhouse environment but also to improve the efficiency of the environmental control systems.

Η παρούσα διδακτορική διατριβή, εκπονήθηκε στη Σχολή Γεωπονίας του Α.Π.Θ. Οι κύριοι στόχοι που τέθηκαν κατά τον ορισμό της παρούσας διατριβής, που εντάσσεται στο επιστημονικό πεδίο του ελέγχου του περιβάλλοντος των θερμοκηπίων, αφορούν α) τη λεπτομερή περιγραφή του περιβάλλοντος του θερμοκηπίου υπό διαφορετικές συνθήκες δροσισμού, β) τη χωρική στοχαστική επεξεργασία των κλιματικών παραμέτρων του θερμοκηπίου, γ) την ανάπτυξη μεθοδολογίας για την κατασκευή ενός μοντέλου αριθμητικής προσομοίωσης του εσωτερικού και εξωτερικού περιβάλλοντος του θερμοκηπίου και δ) την ενεργειακή ανάλυση του συστήματος δροσισμού με υγρή παρειά και ανεμιστήρες. Πέραν των παραπάνω στόχων, βασικές επιδιώξεις της παρούσας διατριβής, που ταυτόχρονα αποτελούν και κριτήρια αξιολόγησης, υπήρξαν α) η μεγαλύτερη δυνατή ποσοτικοποίηση των αποτελεσμάτων, β) η εξαγωγή συμπερασμάτων που μπορούν να γενικευθούν και να χρησιμοποιηθούν στον αποδοτικότερο σχεδιασμό των συστημάτων ελέγχου του περιβάλλοντος ενός θερμοκηπίου και γ) η ανάδειξη των προβλημάτων του συγκεκριμένου επιστημονικού πεδίου που χρήζουν μελλοντικής ερευνητικής προσπάθειας.