Mesure de taille de gouttes in situ en soufflerie

International audienceA series of wind tunnel measurements were performed in Irstea Montpellier wind tunnel combining different nozzles types (flat fan, flat fan air induction and flat fan air induction twin jets), wind speeds (2, 4 and 7.5 m s-1), boom orientations (perpendicular or parallel to the wind direction) and boom heights (0.4, 0.6 and 0.8 m). The wind tunnel is equipped with a distribution test bench that collects the sedimented droplets (Douzals & Alheidary, 2014). Results are analyzed in terms of the remaining fraction of the sprayed volume not sedimented at a distance of 5 m downwind. This criteria is called the drift ratio value (Dr) (Alheidary et al., 2016). The same study also showed the correspondence between the drift ratio values and the ratio between the travelling distance of droplets to the sampling position and the wind velocity called Time of Flight (ToF). The present study aimed at verifying whereas drift ratio values observed for similar ToF can be explained by in situ droplet size distribution. A Malvern Spraytec (Malvern, UK) device (632 nm 5 mW HeNe laser beam, 750 mm lens, optical path of 1.5 m, acquisition frequency of 1 Hz) was installed in Irstea Montpellier wind tunnel. Different ToF situations were obtained while combining different distances from the boom with a range of wind speeds resulting in a range of ToF from 0.25 s (distance 1 m - wind 4 m s-1) to 2.67s (distance 4 m - wind 1.5 m s-1). The domain of investigation was mainly dictated by the capability of the laser diffraction device to make a measurement, i.e. having a sufficiently high droplet density for a measurement. A short boom equipped with four flat fan nozzles (AXI 110 02, Albuz) at 2.5 bar was installed at a height of 0.6 m above the laser beam level, e.g. 1.2 m from the tunnel ground and placed in a frontal or lateral position with respect to the wind direction. In the case of a lateral position, the distance considered for the ToF calculation is the centre of the boom. Each measurement consisted of a sampling time of minimum 30 s (i.e. minimum 30 measurements) under continuous spraying and a standard deviation lower than 2% was observed for the Dv50. Each modality was replicated three times. A comparison of the results is proposed for the frontal and lateral position showing a power law correlation between the droplet size distribution (DSD) expressed in terms of Dv10- Dv50-Dv90 with the ToF. Compared to the DSD measured under the spray without wind, the effect of the wind logically involved a discrimination of the droplet population according to their diameter and the sedimentation distance. The dependence of the sedimentation to the ToF was also verified for a frontal and lateral drift situation. These results also confirmed the robustness of the drift ratio parameter as a relevant and stable indicator of potential drift in a wind tunnel