In most of these practical applications, bubbles are formed not at single orifices or nozzles, but at multiple orifice plates or porous plates. A review of the literature showed that although the present single orifice theory is well established, the lack of knowledge and physical understanding of the process of gas injection through multiple injectors is striking. The stable single bubbling regime is observed for very low gas flow rates and it is characterised by the formation of individual bubbles at a single orifice. The alternate single bubbling regime occurs at higher gas flow rates. At even higher gas flow rates, the so-called simultaneous bubbling regime is observed.
Bubbles form simultaneously at more than one orifice of the plate.
The developed theoretical formulation is based on the extension of a single orifice model to a multiple orifice configuration. It is assumed that the interorifice distance is sufficiently large to avoid coalescence at the surface of the plate. The model predictions have been validated by comparison with the experimental results, covering inviscid and viscous liquids within a Morton number range from 10- 11 to 102. Figure 1 shows the comparison between the model predictions and the experimental results for air bubbles formed at a plate with 2 circular holes of 1 mm diameter separated by 6.9 mm and submersed in stagnant glycerol. The formation of bubbles at a multiple orifice plate is strongly influenced by the distance between the orifices as it plays a critical role on the coalescence process.