Ventilation of Greenhouses

Abstract

The primary need for ventilation of greenhouses is to prevent excessive rise of temperature in sunny days, and the other needs are to replenish the carbon dioxide and to lower an excessive level of humidity. In the design stage of a greenhouse equipped with natural ventilation, it is very useful if the natural ventilation rate can be calculated theoretically as a function of (i) greenhouse structures, (ii) arrangements of ventilators or other openings and (iii) wind speed and wind direction.It is possible to estimate the ventilation rate from the pressure differences across openings and the resistances to air flow through openings. Pressure and discharge coefficients of openings for a single-span greenhouse with continuous side and ridge ventilators, and entrance/exit at gable end were determined experimentally. A one-fifteenth model was installed in a boundary layer wind tunnel for the experiments. The wind speed profile in the wind tunnel was adjusted to be the logarithmic profile with the roughness length of 0.067mm, which is a representative value observed often in nature. Based on the measurements of internal pressures and mean air speeds through openings, pressure and discharge coefficients were determined from the equations (10), (14) and (15).The results are presented in Fig. 6, Table 1 and Fig. 7. Some noticeable features are as follows:(1) All pressure coefficients across openings were not equal to those over surfaces at the ventilators closed, so that the calculation of ventilation rate should be based on the values for the ventilators open.(2) Pressure coefficients of side ventilator were dependent only on wind direction, not on angle of opening. The maximum value of 0.3 was observed at the wind angles of 0-30° and the minimum value of -0.4 at the wind angles of 150-180°. It was assumed that the wind angle is 0° when the wind blows perpendicular to the ridge line facing the ventilator open.(3) When only one side of ridge ventilators was open, pressure coefficients of the ventilator showed similar patterns to those of side ventilator, although the absolute values of them on the windward side increased slightly with increasing angle of opening. The minimum value of -0.4 was obtained at the wind angle of 135°(4) When both ridge ventilators were open, pressure coefficients of the ventilators were negative at any wind angle and angle of opening, and dependent on both wind direction and angle of opening. The minimum value was -0.4 at the wind angle of 30° when the ventilators were positioned horizontally.(5) Pressure coefficients of entrance/exit at gable end changed from -0.5 to 0.6 with the change of wind angle of 0 to 90° on the windward side, and on the leeward side they were -0.2 at the wind angles larger than 30°.(6) Discharge coefficients of ventilators under windless condition were approximately 0.1, 0.4 and 0.6 at the angles of opening 10, 30 and 50°, respectively, and that of entrance/exit at gable end was 0.7. However, those values changed slightly with the differences in types and mounted positions of ventilators.(7) Due to the influences of circumambient wind around the greenhouse, discharge coefficients changed with the change of wind direction and ν/V (mean air speed through opening to wind speed ratio). They reached the same values as those under windless condition with the increase of ν/V.(8) Discharge coefficients of side ventilator and entrance/exit at gable end were independent of wind direction and ν/V.(9) When only one side of ridge ventilators was open, discharge coefficients of the ventilator were dependent both on wind direction and ν/V at the angles of opening larger than 30°.(10) When both ridge ventilators were open, discharge coefficients of the ventilators were notably dependent on wind direction and ν/V, and the maximum values were observed at the wind