The scale-up of large bubbles attached to spargers in downward two-phase flow

A large stationary air bubble hanging beneath an air sparger in a downward liquid flow can exist at flow rates substantially greater than flow rates which would wash away large free bubbles (sometimes known as slugs). This is detrimental because the driving force for circulation is reduced, potentially leading to the fermenter stalling. This kind of bubble was studied experimentally using a vertical flow circuit of internal diameter 0.105 m, which is twice the size of the apparatus used by Bacon et al. (1995) for similar experiments. Liquid flow rates used ranged between 0.003 and 0.015 m(3)/s. Air supply was achieved using four different sparger designs-two spargers consisted of horizontal brass pipes, with downward-facing air holes, and two more, within a section of full-bore pipe, injecting air from either six or eight inlets evenly distributed in a horizontal plane along the pipe periphery. Experiments have been carried out to measure the stable bubble length at various liquid flow rates. The two horizontal pipe spargers gave very similar results; so did the two peripheral sparger designs. The peripheral spargers showed superior characteristics compared to the horizontal ones, bur suffered from sudden bubble length fluctuations. The kind of unbounded bubble growth reported by Bacon et al. (1995) was observed for all the sparger designs investigated. These bubble runaway results have been successfully analysed along with those of Bacon et al. (1995) using dimensional analysis, and the following scale-up correlations developed: Horizontal sparger: Fr(G,max)Eo(-0.5) = 0.0271 (Fr-L)(0.1525) - 0.0213. Peripheral sparger: FrG,maxE-0.5 N-mu(0.07) = 0.0039 Fr-L + 0.0002. (C) 1997 Elsevier Science Ltd.