Numerical simulations of air-assisted primary atomization at different air-to-liquid injection angles

Numerical simulations have been performed for a coaxial, twin-fluid nozzle to study the influence of the angle a between the central liquid jet and the annular airflow on the primary atomization process. A glycerol/water mixture with a high dynamic viscosity of 200 mPas is used and the gas-to-liquid ratio is 0.6. The simulations show good agreement with experiments for the breakup morphology. The liquid jet breaks up quickly and its core length LC decreases with a from 0 degrees to 30 degrees, which is attributable to a reinforced aerodynamic interaction. The flow velocity of the gas phase close to the liquid jet increases with a in this case, which is confirmed by corresponding PIV measurements. This is due to the formation of a high pressure zone at the base of the liquid jet, which results in a favorable pressure gradient in the bulk flow direction. However, further increase of a from 30 degrees to 60 degrees leads to a decreased gas flow velocity along the liquid jet and an increase of LC. The same behavior has been found for the integral specific kinetic energy kL in the liquid phase, which represents a measure for the momentum transfer between the gas and liquid phases. kL increases from a = 0 degrees to 30 degrees and it decreases again with higher a. Moreover, kL yields a similar distribution compared with the turbulent kinetic energy (TKE) of a typical turbulent flow in the spectral domain. This is attributed to the local concentration of TKE of the liquid phase in a small region around the tip of the liquid jet. The results reveal that, in addition to the common dimensionless operating parameters, the flow direction has an essential impact on the atomization process. According to the current work, the best atomization performance is achieved at an angle of a = 30 degrees. The spectral correlation of kL with TKE of the gas flow may be used to assess the dynamics of the liquid phase.

Automated summary

Numerical simulations were conducted to investigate the impact of the angle a between the central liquid jet and annular airflow on the atomization process. The results showed that the core length LC of the liquid jet decreased as the angle a increased from 0 to 30 degrees due to reinforced aerodynamic interaction. However, for angles greater than 30 degrees, the gas flow velocity along the liquid jet decreased and LC increased. Flow direction was found to have a significant impact on the atomization process, in addition to the common dimensionless operating parameters.