This study investigated the influence of solid particles on primary breakup and resulting droplet size for different process parameters. Two sets of Newtonian fluids (each consisting of one pure liquid and one suspension at the same respective viscosity) were used, for isolated investigation of solid particles on spray formation independent of liquid viscosity. The spray was recorded by a high-speed camera and a SpraySpy^® system based on the time-shift effect, while a commonly used Spraytec^® laser diffraction analyzer was employed for validation. An external-mixing twin-fluid atomizer was operated at different gas velocities and corresponding GLR at constant liquid mass flow. For the investigated suspensions an increased Sauter mean diameter was detected, compared to the pure liquids with identical dynamic viscosity. This effect was explained by the tensile strength stabilizing the suspension droplets.

This study compares three different nozzle flow configurations—central liquid jet with surrounding gas phase, liquid sheet with central gas phase, and liquid sheet with inner and outer gas phases. To guarantee constant velocities, as well as momentum flows at the nozzle orifice, a nozzle with identical orifice areas (a central tube with inner and outer slits) was utilized in the experiments. The influence of gas velocity (GLR), dynamic viscosity of the liquid, and nozzle configuration on the resulting droplet sizes (D₃₂, ID_32,m) and primary breakup was investigated at constant liquid mass flow. A high-speed camera (HSC) was used for the detection of primary breakup, whereas droplet size measurements were performed with a phase Doppler anemometer. The variation of nozzle configuration exhibited distinct influence on the resulting breakup morphology and droplet size. Especially, for atomizing high-viscosity liquids, application of sheet configurations led to smaller droplet sizes compared to liquid jet configuration.

This study aims to derive basic principles for liquid mass flow scaling of gas-assisted coaxial nozzles. Four liquid mass flow steps were investigated in the range of ${\dot{M}}_{liq}$ = 20–500 kg·h${}^{-1}$, applying four atomizers with similar geometry designed at $W{e}_{aero}=$ const. High-speed camera and phase Doppler anemometer were utilized to detect the local droplet size distribution. To estimate a reliable measurement plane, a detection method and determination according to the free jet theory was used. The resulting droplet size was analyzed, applying the aerodynamic Weber number, as well as the gas momentum flow. An empirical model was derived out of the measured data, which allows for liquid mass flow scaling when process parameters such as $GLR$, liquid mass flow, and required Sauter mean diameter are specified. The model was developed as a first step towards liquid mass flow scaling of gas-assisted coaxial atomizers within the investigated range of operating conditions.

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 ${\dot{M}}_{\mathit{liq}}$ . For that purpose, atomizers with the same geometrical setup but increased sizes have been designed and experimentally investigated for ${\dot{M}}_{\mathit{liq}}=20$ , 50, 100, and 500 kg/h, while the aerodynamic Weber number We_aero 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 L_C and the frequency of the Kelvin–Helmholtz instability f_K 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 We_aero 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 $L_C/d_{\mathit{liq}}$ has been found to be considerably increased with increasing n. The reason has been shown to be the decreased gas flow velocity v_gas 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 f_KHI with n or ${\dot{M}}_{\mathit{liq}}$ , 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 We_aero. 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 Re_liq in terms of n.