This experimental work reports, for the first time, observations of internal flow field involving an DARPA SUBOFF afterbody design aerator body in an inside-out type of effervescent atomizer. The effect of operating parameters like air-to-liquid ratio (ALR), operating pressure, aerator orifice diameter, aeration area and mixing chamber diameter on internal flow within the effervescent atomizer is studied. The effect of increasing ALR on the internal flow is quantified by identifying different gas injection mechanisms at the aerator orifice (into the mixing chamber) and two-phase mixing chamber flow regimes using high-speed shadowgraphy. In particular, it is observed that as ALR is systematically increased the gas injection mechanism transits in the following sequence: single bubbling, pulsed bubbling, elongated jetting, atomized jetting and evacuated chamber. The range of ALRs within which these mechanisms are observed are employed to draw up a flow regime map. Similar analysis on two-phase mixing chamber flow regimes yielded corresponding regime map for internal two-phase stabilized flow in the mixing chamber. The flow regime transited from bubbly flow to slug flow to churn flow and finally to annular flow as the ALR was increased. The spray characteristics (size and velocity) at the nozzle exit are reported using Phase Doppler Anemometry (PDA) measurements. It is observed that dense bubbly and bubbly-slug flow regimes produce stable sprays with droplet sizes in the range of 50-80 µm in the range of 0.25%-1.50% ALR.Dependence of internal flow on other parameters such as orifice aerator diameter (with constant total aerator orifice area), mixing chamber diameter and operating pressure are also studied.

The present experimental work is concerned with the study of effervescent atomisation, a two-phase gas-liquid spray generation technique that offers many advantages over conventional atomisers. This study shows the advantage of streamlined aerator design over flat-end aerator type with respect to formation of gas void in the aerator wake in the interior of an inside-out type of effervescent atomizer. The experiments are performed employing high-speed shadowgraphy visualizations. It is observed that in the conventional flat-end type of aerator design the formation of gas void is undesirable and leads to spray characterized by instabilities, causing fluctuating spray properties. The existence of gas void also prevents the formation of bubbly flow inside the effervescent atomizer which is actually preferred in these types of atomizers to enable stable spray generation and fine atomization. The formation and existence of gas void is found to be a result of aerator bluff body recirculation and gas phase buoyancy effects.Four different streamlined aerator designs with tips in the shape of circular arc, circular arc/conical hybrid, conical and DARPA SUBOFF afterbody design (which is common in the conventional ship designs) are evaluated to determine the best among them with respect to mitigating the unwanted gas-void in the interior of an effervescent atomizer. These are evaluated with respect to ability to produce bubbly flow over comparatively large operating range and the ability to impart minimum wake (of aerator body) effect. It is concluded, upon careful experimental observations, that DARPA SUBOFF afterbody design is the best among the streamlined aerator designs.