Experimental study of coaxial atomizers scaling. Part I: Dense core zone

Coaxial injection is a very common system in industrial burners or rocket engines. Theoretical studies involving different hypotheses have been devoted to this subject. Experimental studies have been performed in several individual configurations. Subsequently, many correlations are proposed in the literature. All these expressions do not take into account the same basic quantities, which may be troublesome for injector designers. In the present work, we have performed experiments in a system where both geometry and liquid characteristics could be varied over a wide range. Changes in atomization regimes, liquid core length, and spray angle are especially examined in the present paper. They were estimated using fast shadowgraphy. We show that the momentum flux ratio plays a key role in all these quantities in the dense core zone. Gas flow Reynolds number is also important for disintegration mode classification. Empirical correlations show that liquid's physical properties (viscosity or surface tension) have little influence on the atomization process in the range studied. The Rayleigh instability growth rate model, based on the existence of a shear layer at the interface, seems to be the most suitable in predicting the dense core zone properties, showing that primary atomization in coaxial injectors is essentially ruled by aerodynamic effects.