Liquid interface dynamics at primary breakup in close-coupled gas atomization

Detailed measurements of the liquid jet interface dynamics close to the nozzle exit in close-coupled gas atomization focused on “filming” and “no-filming” conditions and their transitional behavior, were performed using digital inline holography. Experiments covered four Weber numbers, We g , three apex angles, θ , for a range of momentum ratios, M . The JPDFs of the instantaneous liquid jet interface positions revealed strikingly different interface behavior depending on the combination of We g , M , and θ . A spectral analysis identified coherent axial frequency bands, associated with the radial movement of the jet interfaces. Based on analysis of (i) reconstructed snapshots, (ii) JPDFs of instantaneous jet positions, and (iii) spectral analysis, four different “flow” regimes were proposed, namely “no-filming”, “filming”, and two transitional regimes (“periodic flapping” and intermittent “switching”). Flow regime maps (Re l versus We g ) constructed for different apex angles, show that “filming” occurred at low Re l for all investigated We g . Increasing θ , increased the value of Re l for which transitional behavior was observed. In addition, keeping M constant while increasing We g (implies increasing Re l ) may cause transition from “filming” to “no-filming”. Despite the different proposed flow regimes, peak Strouhal numbers mostly ranged between 2 ≤ St p ≤ 3, irrespective of M , We g , and θ (excluding “no-filming” conditions). This study has provided a detailed spectral characterization of the transition to filming in CCGA, quantitatively expressed as regime maps that are essential for predicting primary breakup behavior and optimizing atomizer design. • Liquid jet interface dynamics in closed-coupled gas atomization was investigated. • High-speed digital, inline holography measurements for a range of We g , M and θ . • Spectral analysis of radial interface positions revealed coherent frequency bands. • Four flow regimes were detected with transitional behavior in between “filming” and “no-filming”. • Flow regime maps were presented showing the effect of different apex angles.