Use of a flexible drop counter for a better comparability of potential spray drift measurement protocols in wind tunnels

摘要

In Europe, three main low wind speed wind tunnels (cross-section 3 m x 2 m) are used for regulatory authorization of drift reducing nozzles to implement three different measurement protocols to assess the potential spray drift. Both German and UK protocols use short duration sprays including a dye tracer and implements the ISO 22856 standard protocol based either on a vertical array of horizontal nylon strings placed 2 m downwind and every 0.1 m vertically (Herbst, 2001) or on a horizontal array of nylon strings placed every meter at 0.1 cm above ground cited by Taylor et al. (2004) respectively. Both protocols consider a wind speed of 2 m s1. The French protocol is based on a long duration process where sedimentation is collected on a 9 m long distribution test bench with 3 m x 0.05 m grooves connected to 500 mL collecting tubes mounted on weight cells, under a wind speed of 7.5 m s'(Douzals, 2014). The major differences highlighted in previous protocols offer a limited comparability of the driftreducing performance of nozzles. However, a preliminary study conducted by Douzals et al. (2017) demonstrated the interest of an in situ droplet characterization in a wind tunnel by using a laser diffraction device although the experimental domain is limited by an insufficient amount of intercepted droplets to get a measurement. The context of the future revision of the ISO 22856 standard on the assessment of potential drift in a wind tunnel stresses the question of a better comparability of the previous protocols. The purpose of the present study aims at better defining the spatial qualitative distribution of droplets in a wind tunnel based on the reference sampling positions of each protocol. A portable Drop Counter (Billericay Farm Services, UK) wasused to qualify the droplet distribution in a 3D sampling grid. Experimental parameters include different reference nozzles used in ad hoc countries (FF 03, three bar and FF02 2.5 bar) and air induction nozzles (BFS Air Bubble Jet, Albuz CVI) at a pressure of two bar and under wind speeds of 2, 4, 6 and 7.5 m s'. Results in terms of local droplet size distribution profiles are discussed for various wind speeds and nozzle types for a better comparability of the results between methodologies.