Abstract
Microbubbles are bubbles below 1mm in size and have been extensively deployed in industrial settings to improve gaseous exchange between gas and liquid phases. The high surface to volume ratio offered by microbubbles enables them to enhance transport phenomena and therefore can be used to reduce energy demands in many applications including, waste water aeration, froth flotation, oil emulsion separations and evaporation dynamics. Microbubbles can be produced by passing a gas stream through a micro-porous diffuser placed at the gas–liquid interface. Previous work has shown that oscillating this gas steam can reduce the bubble size and therefore increase energy savings. In this work we show that it is possible to further reduce microbubble size (and consequently maximise the number of bubbles) by varying the frequency of the oscillating gas supply. Three different microbubble generation systems have been investigated; an acoustic oscillation system and a mesh membrane, a fluidic oscillator coupled to a single orifice membrane and a fluidic oscillator coupled to a commercially available ceramic diffuser. In all three bubble generation methods there is an optimum oscillation frequency at which the bubble size is minimised and the number of microbubbles maximised. In some cases a reduction in bubble size of up to 73% was achieved compared with non-optimal operating frequencies. The frequency at which this optimum occurs is dependent on the bubble generation system; more specifically the geometry of the system, the type micro-porous diffuser and the gas flow rate. This work proves that by tuning industrial microbubble generators to their optimal oscillation frequency will result in a reduction of microbubble size and increase their number density. This will further improve gaseous exchange rates and therefore improve the efficiency of the industrial processes where they are being employed to produce bubbles, leading to a reduction in associated energy costs and an increase in the overall economic and energetic feasibility of these processes.
Highlights
Bubbling systems have regularly been employed in industrial processes in order to achieve gaseous exchange of both mass and heat from gaseous phases to the liquid phase and vice versa
The two other microbubble generation systems use microfluidic devices known as a Tesar–Zimmerman fluidic oscillator (Jilek, 2013; Tesar, 2012; Tesarand Bandalusena, 2011; Zimmerman et al, 2011a,b, 2010) to generate the oscillation before the gas stream passes through two different diffusers, one with a single orifice and another with multiple orifices
The input flow rate used as standard through the fluidic oscillator is 65 l/min with over 99.9% being vented in order to be used with the single orifice membrane and the inlet pressure is maintained at 0.5 bar (g)
Summary
Bubbling systems have regularly been employed in industrial processes in order to achieve gaseous exchange of both mass and heat from gaseous phases to the liquid phase and vice versa. The two other microbubble generation systems use microfluidic devices known as a Tesar–Zimmerman fluidic oscillator (Jilek, 2013; Tesar, 2012; Tesarand Bandalusena, 2011; Zimmerman et al, 2011a,b, 2010) to generate the oscillation before the gas stream passes through two different diffusers, one with a single orifice and another with multiple orifices (mesoporous diffusers). The latter is most typical of the large scale microbubble generators being used in industry. This work investigates how frequency control affects microbubble generation using three different bubble generation systems
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