The bubble-slug flow pattern transition and instabilities of void fraction waves

The propagation of spontaneous void fraction disturbances in a nitrogen-water flow has been studied through the statistical analysis of conductivity probe signals, for void fractions ranging from 0.1 to 0.5 and including the bubble-slug transition. The power spectral density function and the standard deviation of the void fraction have been computed for each probe, as well as the system phase factor (related to the wave velocity), the coherence function and the system gain factor between each pair of consecutive probes as functions of frequency. For bubble flow, the results are compatible with the results obtained by other authors. The transition from bubble to slug flow is associated with void fraction wave instabilities. Two kinds of instabilities seem to occur simultaneously: amplitude increase (system gain factor > 1) and wave-breaking.