Taylor-Couette flow in a compressible perfect gas is studied. The onset of instability is examined as a function of the Reynolds numbers of the inner and outer cylinder, the Mach number of the flow and the Prandtl number of the gas. We focus on the case of a wide gap, with radius ratio 0.5. We find new modes of instability at high Prandtl number, which can allow oscillatory axisymmetric modes to onset first. We also find that onset can occur even when the angular momentum increases outwards, so that the classical Rayleigh criterion can be violated in the compressible case. We have also considered the case of counter-rotating cylinders, where the m = 0 and m = 1 modes can onset simultaneously to give a codimension 2 bifurcation, leading to the formation of complex flow patterns. In compressible flow we also find codimension 3 points. The Mach number and the critical inner and outer Reynolds numbers can be adjusted so that the two neutral curves for the m = 0 and m = 1 modes touch rather than cross. Complex codimension 3 points occur more readily in the compressible case than in the Boussinesq case, and they are expected to lead to a rich nonlinear behaviour.

Countercurrent flow limitation (flooding), two-phase flow patterns, and void fraction in countercurrent flow of gas−pseudoplastic liquids in a vertical and an inclined channel (with a 68° inclination angle with respect to the horizontal plane) were experimentally studied. The test section was a transparent tube, 2.36 m long with a 1.9 cm inner diameter. Aqueous 500 and 1000 ppm solutions of PERCOL 727 (a copolymer of sodium acrylate and acrylamide) with flow behavior indices of 0.58 and 0.35, respectively, were used as the liquid phase, and air constituted the gas phase. The gas and liquid superficial velocity ranges in the experiments were 0.5−198 and 0.8−34 cm/s, respectively. The flooding curves of both polymer solutions were similar for both angles of inclination and were similar to the flooding curves of low-viscosity Newtonian liquids. They thus indicate little effect of pseudoplasticity on flooding. The visually observed two-phase flow regimes were bubbly/slug, slug, slug/churn, and churn. Annular flow occurred only at near-flooding conditions. The flow regime transition lines showed significant differences with data representing a Newtonian liquid with a viscosity of the same order of magnitude as the polymer solution apparent viscosities. The slug flow pattern, characterized by Taylor bubbles, was the most dominant flow pattern. The measured void fractions were correlated based on the drift flux model, using different correlation parameters for bubbly/slug, and slug or slug/churn flow regimes.