Wet fibre-laden foams in axial mixing with macro-instabilities

The air content and bubble size distribution in pure foams and in fibre foams were determined in axial mixing for varied rotation speed, liquid surface tension and wood fibre consistency. Onset of flow macro-instabilities were observed at high rotation speeds both with and without fibres. Before the instability, the bubble size decreased and air content increased with increasing rotation speed. After the instability, the air content was not only strongly affected by rotation speed but, interestingly, both the air content and bubble size increased with reduced surface tension. In both regions, the average bubble size could be explained based on the capillary number after the air content was explicitly included in the model. With kraft fibres included in the foam, the behaviour of the air content is very similar to that without fibres but the macro-instability behaviour begins a bit earlier, roughly at N=5000 RPM for the surfactant concentration of 0.2 g/l. At this rotation speed, the air content finds its maximum for both 0.33 and 0.66% consistency. The main effect of the fibres is to reduce the bubble size in the stable region. The higher the consistency of fibres, the greater the reduction in the mean radius turns out to be. This is probably due to the added local shear forces due to fibre inertia enhancing the breaking tendency of the larger bubbles. However, it seems that in the unstable regime the fibres do not have a similar effect. For very high rotation speeds, the bubble size is practically equivalent with and without fibres. This might be caused by the change in the turbulence spectrum at the beginning of instabilities - the shear forces due to the fluid phase may become strong enough to dominate those caused by the fibre inertia.