Aerosol Atomization from Transonic Air Jets in Cross Flow

We investigate the droplet size distributions and concentrations produced by an aerosol generator driven by a high velocity air jet delivered in cross flow to an oil tube at Mach numbers of 0.1-1.0, and Weber numbers based on the air flow parameters between 1000 to 40000. Measured Sauter mean droplet diameters (SMD) from 1μm to 3.5μm were determined by an Electronic Low Pressure Impactor over the range of Weber numbers. The droplet sizes are shown to scale with the Weber number based on the air jet velocity according to SMD (μm) = 12.4 We, where We is based on the air properties. The measured diameters were little sensitive to other variables tested under controlled conditions. Comparisons with previous correlations failed to capture the dependence on Weber number, or predicted very different diameters. The number density was found to increase with oil and air flow rates, with values clearly limited by the oil flow rate delivered. The narrow variation in diameters over the high range of velocities tested demonstrates why the present technique is effective in delivering suitable aerosols for a range of applications. Introduction The reliable generation of aerosols with a given range of droplet sizes is important in a number of industrial applications, such as fuel injection, spray coating, particle and gas analysis, as well as cleaning equipment and pollution control. Much of the information on methods of aerosol and droplet generation via shear has come from fuel injection studies, where droplets diameters of the order of tens of microns are desirable [1, 2, 3]. Aerosol generation for submicron diameter ranges often relies on growth from particles in slowly condensing environments. In the present case, we consider the high rate generation of micron-size droplets, which are often used in the measurement of flow velocities via light scattering methods. Here we consider a shear breakup method, where the liquid sheet is disrupted by high speed air colliding in cross flow. This is a common method for nebulisation of aerosols [4, 5]. Surprisingly, there has been to date no published quantification of the typical droplet size or rate generated by such methods. Moreover, there has been no assessment of the sensitivity of droplet size or delivery rate to geometry or operating conditions. This gap in the knowledge base may be in part due to difficulties in the measurement of droplets in the submicron range. In this study, we investigate the rate of formation of droplets by high speed air in cross flow, as a function of the various physical parameters involved, and propose a correlation for the mean diameter obtained. The breakup of a liquid stream or jet is traditionally described as composed of two phases: primary, where the liquid column or sheet becomes unstable due to the propagation of wave disturbances, and disintegrates through the stripping of ligaments into large droplets; and secondary, where the primary droplets or filaments break up into multiple smaller droplets, typically under the action of shear. In the latter, as described in this work, shear forces acting on the liquid-air interface are very high, so that the liquid tends to break up promptly, rather than in stages [1], [6], [7], [8]. This prompt atomization means that the liquid viscosity and the injector geometry are expected to have little impact on the final droplet size relatively to the velocity of the air jet [1]. The atomization is controlled by the ratio of drag forces supplied by the cross flow air at density ρa and velocity Va on the liquid stream of characteristic diameter Do, to the surface forces keeping the liquid intact via the liquid-air surface tension, σ. This can be expressed as a Weber number based on the density and velocity of the air and a characteristic length scale, which can be the integral turbulence length scale or, as in the present case, the diameter of the air jet, Da,