• Ketoja, Jukka A.
• Al-Qararah, Ahmad
• Hjelt, Tuomo
• Harlin, Ali
• Koponen, Antti Ilmari

Abstract

The air content and bubble size distribution in purefoams and in fibre foams were determined in axial mixingfor varied rotation speed, liquid surface tension andwood fibre consistency. Onset of flow macro-instabilitieswere observed at high rotation speeds both with andwithout fibres. Before the instability, the bubble sizedecreased and air content increased with increasingrotation speed. After the instability, the air contentwas not only strongly affected by rotation speed but,interestingly, both the air content and bubble sizeincreased with reduced surface tension. In both regions,the average bubble size could be explained based on thecapillary number after the air content was explicitlyincluded in the model. With kraft fibres included in thefoam, the behaviour of the air content is very similar tothat without fibres but the macro-instability behaviourbegins a bit earlier, roughly at N=5000 RPM for thesurfactant concentration of 0.2 g/l. At this rotationspeed, the air content finds its maximum for both 0.33and 0.66% consistency.The main effect of the fibres is to reduce the bubblesize in the stable region. The higher the consistency offibres, the greater the reduction in the mean radiusturns out to be. This is probably due to the added localshear forces due to fibre inertia enhancing the breakingtendency of the larger bubbles. However, it seems that inthe unstable regime the fibres do not have a similareffect. For very high rotation speeds, the bubble size ispractically equivalent with and without fibres. Thismight be caused by the change in the turbulence spectrumat the beginning of instabilities - the shear forces dueto the fluid phase may become strong enough to dominatethose caused by the fibre inertia.

Topics

• impedance spectroscopy
• surface
• phase