Flow characteristics and turbulence analysis of a large-scale pressure-atomized spray

International audienceA typical water round-nozzle jet for agricultural applications is presented in this study. The dispersion of a liquid for irrigation or pesticides spraying is a key subject to reduce both water consumption and air pollution. A simplified study case is constructed to tackle both scenarios, where a round dn = 1.2mm nozzle of a length Ln = 50dn is considered. The water injection bulk velocity is equal to Uj = 35m/s, aligned with gravity, placing the liquid jet in a turbulent atomization regime. Experimental and numerical approaches are considered. LDV and DTV optical techniques are used to gather statistical information from both the liquid and the gas phases of the spray. The experimental campaign is carried out from x/dn = 0 to 800. Concerning the LDV, small (∼ 1µm) olive-oil tracers are used to capture the gas phase, where a distinction between the liquid droplets and tracers is achieved by a specific setup of the laser power source and the burst Doppler setting (BP-Filter and SNR). On the dispersed zone, DTV measurements are carried out to determine velocities and sizes of droplets. Special attention to the depth-of-field (DOF) estimation is taken in order to obtain a less biased droplet's size-velocity correlation. Finally, an optical probe (OP) was also used to determine the liquid volume fraction í µí± ̅ , liquid mass fraction í µí± ̃ , and mixture density í µí¼̅ , which are important features for such flows. These are key quantities for the determination of the mixture mean velocities and Reynolds stresses, and evaluation of the terms in their balance equations. Combining OP, LDV and DTV data allows to determine quantities such as the mixture mean density, í µí¼̅ = í µí± ̅ í µí¼ í µí°¿ + (1 − í µí± ̅)í µí¼ í µí°º , mixture mean velocity along the i direction, í µí±¢ ̃ í µí± = í µí± ̃ í µí±¢ ̃ í µí±,í µí°¿ + (1 − í µí± ̃)í µí±¢ ̃ í µí±,í µí°º , or mean slip velocity, í µí±¢ ̅ í µí±,í µí± = í µí±¢ ̅ í µí±,í µí°¿ − í µí±¢ ̅ í µí±,í µí°º = í µí±¢ í µí± ′′ í µí± ′′ ̃ í µí± ̃ (1−í µí± ̃) , where the notation '' denotes fluctuations with respect to the Favre averaged mean values. Similar relations hold for the Reynolds stresses. For such a flow, three dimensionless quantities can be constructed as a function of the forces that intervene in the atomization process. First, the nozzle Reynolds number, í µí±í µí± = í µí± í µí± í µí± í µí± í µí¼ í µí°¿ , allows to identify if the liquid flow inside the injector is turbulent. Then, the liquid Weber number, í µí±í µí± í µí°¿ = í µí¼ í µí°¿ í µí± í µí±í µí± í µí± 2 í µí¼ , and the gas Weber number, í µí±í µí± í µí°º = í µí¼ í µí°º í µí± í µí±í µí± í µí± 2 í µí¼ , which weights the importance of surface tension once the flow is in contact with the surrounding air. Finally, the Ohnesorge number, í µí± ℎ = í µí¼ í µí°¿ í µí¼ í µí°¿ √í µí¼ í µí°¿ í µí¼í µí± í µí± , characterizes the form of the liquid packets or droplets in the atomization process. Choosing Re = 41833 and Ln/dn = 50 makes the internal flow fully turbulent and ensures that the boundary layer inside the nozzle is fully developed for any upstream conditions. With WeL = 20158, WeG = 24.3 and Oh = 0.0034, the liquid phase turbulent kinetic energy should be the main responsible of the liquid-jet primary break-up, these flow conditions lying within the second wind-induced atomization regime