Measurement of interstitial velocity of homogeneous bubbly flows at low to moderate void fraction

We develop a new methodology to examine the conditional and unconditional vertical velocity induced by high-Reynolds-number bubbles rising in a uniform flow, at low to moderate void fraction a (up to 15%). These statistics provide a local description of the perturbation of the liquid velocity around a test bubble in the swarm. In particular, the attenuation of the length of the wakes with increasing void fraction is measured for a large range of void fraction. The strong attenuation of the wakes is related to wake intermingling mechanisms. The methodology also enables a definition of the interstitial liquid flow. The velocity of the fluid averaged over all the interstitial volume far away from the bubbles is introduced. It is a useful concept, in particular to define the relative velocity, or for drift models. Our experimental results allow a discussion of the predictions of irrotational drift models. For low void fraction (alpha <= 2 %), potential flow models provide practical estimates of the interstitial velocity field. At higher void fractions, the effect of vorticity is important. A simple phenomenological model is proposed to include the effect of the flow generated by the bubble wakes.