Application of Semi-Empirical Ventilation Models in A Mediterranean Greenhouse with Opposing Thermal and Wind Effects. Use of Non-Constant Cd (Pressure Drop Coefficient Through the Vents) and Cw (Wind Effect Coefficient)

Abstract

The present work analyses the natural ventilation of a multi-span greenhouse with one roof vent and two side vents by means of sonic anemometry. Opening the roof vent to windward, one side vent to leeward, and the other side vents to windward (this last vent obstructed by another greenhouse), causes opposing thermal GT (m3 s−1) and wind effects Gw (m3 s−1), as outside air entering the greenhouse through the roof vent circulates downward, contrary to natural convection due to the thermal effect. In our case, the ventilation rate RM (h−1) in a naturally ventilated greenhouse fits a second order polynomial with wind velocity uo (RM = 0.37 uo2 + 0.03 uo + 0.75; R2 = 0.99). The opposing wind and thermal effects mean that ventilation models based on Bernoulli’s equation must be modified in order to add or subtract their effects accordingly—Model 1, in which the flow is driven by the sum of two independent pressure fields GM1=GT2±Gw2, or Model 2, in which the flow is driven by the sum of two independent fluxes GM2=GT±Gw. A linear relationship has been obtained, which allows us to estimate the discharge coefficient of the side vents (CdVS) and roof vent (CdWR) as a function of uo [CdVS = 0.028 uo + 0.028 (R2 = 0.92); CdWR = 0.036 uo + 0.040 (R2 = 0.96)]. The wind effect coefficient Cw was determined by applying models M1 and M2 proved not to remain constant for the different experiments, but varied according to the ratio uo/∆Tio0.5 or δ [CwM1 = exp(-2.693 + 1.160/δ) (R2 = 0.94); CwM2 = exp(−2.128 + 1.264/δ) (R2 = 0.98)].

Impact of Balcony Configuration on Natural Ventilation Performance in Indoor Spaces of Social Buildings (Rusunawa) in Indonesia

“…Pada konfigurasi lebar balkon 20% aliran udara dari luar ruangan mengalir secara langsung menuju inlet ruangan sedangkan dalam konfigurasi lebar balkon 40% terdapat dinding yang berada tegak lurus dengan arah datangnya aliran udara sehingga menyebabkan terjadinya pembelokan aliran yang akan mengarahkan aliran udara menuju inlet ruangan sehingga memberikan peningkatan nilai yang signifikan dibandingkan konfigurasi lebar balkon 20%. (López-Martínez et al, 2019) menjelaskan bahwa efek Bernoulli melalui modifikasi ventilasi mampu menambah aliran udara akibat efek angin dan termal secara presisi.…”

“…Sebagai gantinya, efek Bernoulli berperan penting dalam mempengaruhi peningkatan dan penurunan aliran udara dalam ruangan. (López-Martínez et al, 2019) di dalam penelitiannya menjelaskan bahwa efek Bernoulli melalui modifikasi ventilasi tidak hanya mampu menambah aliran udara, namun juga bisa mengurangi jumlah aliran udara akibat efek angin dan termal secara presisi.…”

Estimation of the aerodynamic tortuosity of woven/wire screens

“…When the flow passes through a WS, a pressure drop is produced. This is expressed as a function of the velocity of the air passing through the WS according to ∆P = a 1 U 2 + a 2 U [8,38,39]. In this equation, a 1 and a 2 are two modelling coefficients that depend on two important mechanical characteristics of the mesh: the permeability of the porous medium (K p ), which depends on the geometry of the porous medium; and the inertial factor (Y ), which depends on the nature of the porous media [40,41].…”

On the estimation of three-dimensional porosity of insect-proof screens

“…The physical description of the ventilation flow inside a greenhouse depends on the window geometry and the woven geometry of the insect-proof screen, as studied in [11,15,16,17,18] . This produces a pressure drop due to the circulation of air flow through a porous media.…”