Effect of nozzle upscaling on coaxial, gas-assisted atomization

Abstract

Mass flow scaling of gas-assisted coaxial atomizers from laboratory to industrial scale is of major interest for a wide field of applications. However, there is only scarce knowledge and research concerning the effect of atomizer scale-up on liquid breakup and spray characteristics. The main objective of this study is therefore to derive basic principles for liquid jet breakup using upscaled nozzles to increase the liquid mass flow rate M ˙ liq. For that purpose, atomizers with the same geometrical setup but increased sizes have been designed and experimentally investigated for M ˙ liq = 20, 50, 100, and 500 kg/h, while the aerodynamic Weber number Weaero and gas-to-liquid ratio GLR have been kept constant. The primary jet breakup was recorded via high-speed imaging, and the liquid core length LC and the frequency of the Kelvin-Helmholtz instability fK were extracted. Applying these results as reference data, highly resolved numerical simulations have been performed to gain a deeper understanding of the effect of mass flow scaling. In the case of keeping Weaero and GLR constant, it has been shown by both experiments and simulations that the breakup morphology, given by a pulsating liquid jet with the disintegration of fiber-type liquid fragments, remains almost unchanged with the degree of upscaling n. However, the normalized breakup length L C / d liq has been found to be considerably increased with increasing n. The reason has been shown to be the decreased gas flow velocity vgas at the nozzle exit with n, which leads to a decreased gas-to-liquid momentum flux ratio j and an attenuated momentum exchange between the phases. Accordingly, the calculated turbulence kinetic energy of the gas flow and the specific kinetic energy in the liquid phase decrease with n. This corresponds to a decreased fKHI with n or M ˙ liq, respectively, which has been confirmed by both experiments and simulations. The same behavior has been shown for two liquids with different viscosities and at different Weaero. The obtained results allow a first-order estimate of the liquid breakup characteristics, where the influence of nozzle upscaling can be incorporated into j and Reliq in terms of n.