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Bubble Formation from Sub-millimetre Orifices

Mohseni, E.

Efficient contacting of gas and liquid is an important problem in many industrial processes. Examples can be found (bio-) chemical engineering, medicine, water and wastewater treatment among others. To operate these processes in a more efficient manner, usually the gas phase is dispersed into the continuous phase in the form of bubbles. In this case, it is often required to reduce the bubble size to enhance the mass transfer. This is often done by scaling down the opening from which bubbles are generated. Hence, gas bubble generation from sub-millimetre orifices has been an interesting topic in research and technology. The present thesis is an endeavour to bring more physical insights on the mechanism of bubble formation and detachment from sub-millimetre orifices as this phenomenon is not yet well-understood.

Formation of bubbles from a submerged orifice depends on various parameters such as orifice diameter dor, gas flow rate Q, volume of the gas reservoir upstream of the orifice Vc and others. On the basis of these parameters, the bubble growth dynamics, bubbling regime, and bubble volume strongly vary. To further advance the fundamental understanding of the bubble formation at sub-millimetre orifices, two series of experiments were conducted, and the nfluence of the design parameters namely dor, Q, and Vc were investigated in detail. In the experiments, pressurized air is used to generate bubbles in eionized water using stainless-steel orifices in the range of 0.03 mm< dor <1 mm. Based on the findings of these experiments, a mechanism is designed to control the formation process and therefore the bubble volume. Accordingly, in a separate series of experiments, periodic modulations in the gas reservoir is utilised to control the formation and detachment of bubbles from the orifice.

In the first series of experiments, the influence of dor and Q on bubble formation dynamics are investigated. It is observed that the mechanism of bubble formation is not uniform throughout the sub-millimetre range. For orifices with dor > 0.4 mm, the bubble volume Vb progressively increases with both dor and Q. For smaller orifices, however, Vb is determined by the number of subsequent bubbles that merge with the leading bubble after its departure. This is eferred to as in-rush bubbling and it even occurs in the lowest range of Q, at which larger orifices generate bubbles with a single detachment. The bubble formation from orifices with dor < 0.4 mm is highly affected by the gas kinetic energy. Hence, in the balance of forces acting on the bubble, the gas momentum force and the liquid inertia force substantially contribute to the bubble growth rate and therefore Vb.

In the second series of experiments the influence of Vc on bubble formation is investigated. Experimental results showed that, by increasing Vc, the magnitude of the pressure variations in the gas reservoir reduces. Moreover, enlarging the gas reservoir, not only results in a variable gas flux q into the bubble, but also the average gas flux elevates with Vc. Consequently, larger bubbles at lower bubbling frequencies are generated. The impact of Vc on bubble formation for dor < 0.4 mm is, however, limited as the gas kinetic energy readily affects the bubble formation. According to the findings of the first two series of experiments, new models are developed to calculate Vb. The models agree well with experimental results. Moreover, new correlations for bubble detachment are delivered which correlate the non-spherical bubble shape from the observations in the experiments to the spherical bubble volumes calculated from the models.

Last but not least, a control mechanism by means of continuous pressure modulation of the gas phase in the reservoir upstream of the orifice is experimentally investigated. The modulation comes from a loudspeaker, which is mounted in the gas reservoir. The modulation propagates through the pipe to an orifice with a 0.5 mm diameter. Consequently, the onset and termination of the formation process is forced by each successive compression and rarefaction of the acoustic waves. Using the control mechanism, the bubble diameter can be reduced by a factor of four of that generated by uncontrolled formation process. Studying the force balance on the bubble revealed that the detachment of the bubble in the controlled method is determined by the hydrodynamic forces. Moreover, applicability of the Rayleigh-Plesset equation for calculation of the bubble size is examined for the presented bubble generator.

Keywords: Bubble formation; Sub-millimetre orifice; Bubble dynamics; Modelling; Bubble in-rush; Controlled bubble formation

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Publ.-Id: 34062