THE SCALE-UP OF LARGE BUBBLES ATTACHED TO SPARGERS IN DOWNWARD 2-PHASE FLOW

A large stationary air bubble hanging beneath an air sparger in a down ward liquid flow can exist at flow rates substantially greater than fl ow 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 o f bubble was studied experimentally using a vertical flow circuit of i nternal diameter 0.105 m, which is twice the size of the apparatus use d by Bacon et al. (1995) for similar experiments. Liquid flow rates us ed ranged between 0.003 and 0.015 m(3)/s. Air supply was achieved usin g four different sparger designs-two spargers consisted of horizontal brass pipes, with downward-facing air holes, and two more, within a se ction of full-bore pipe, injecting air from either six or eight inlets evenly distributed in a horizontal plane along the pipe periphery. Ex periments 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 perip heral spargers showed superior characteristics compared to the horizon tal ones, bur suffered from sudden bubble length fluctuations. The kin d of unbounded bubble growth reported by Bacon et al. (1995) was obser ved for all the sparger designs investigated. These bubble runaway res ults have been successfully analysed along with those of Bacon et al. (1995) using dimensional analysis, and the following scale-up correlat ions 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.