Experimental study of bubble size distribution in bubbly and bubbly-to-slug transition flow

Abstract

The current work experimentally investigates the evolving characteristics of bubble size distribution (BSD) in bubbly and bubbly-to-slug transition flow in a 25.4 mm inner diameter pipe. Four-sensor conductivity probes used in present research enables accurate acquisition of bubble chord length in the flows with void fraction varying from 0.1 to 0.6. The flow regime transition is implemented by increasing superficial gas velocity while keeping superficial liquid velocity constant. Six right-skewed probability distribution functions (PDFs) commonly used for predicting bubble chord length distribution (CLD) are selected and used to fit the chord length data. The goodness of fit for all PDFs is evaluated by R-square, root-mean-square error (RMSE) and corrected Akaike information criterion. The results show that the PDF which reasonably fits the CLD in bubbly flow fails to fit the CLD in bubbly-to-slug transition flow. The evolving characteristics of CLD in flow regime transition process are qualitatively and quantitatively discussed based on bubble coalescence and break-up mechanisms. It is found that the invalidation of uniform morphology assumption and different mechanisms of bubble coalescence and break-up may result in the failed fitting. To properly predict real bubble size distribution (BSD) in bubbly-to-slug transition flow from CLD, an analytical backward transformation method is proposed to relate the CLD to BSD based on the bubble geometrical characteristics in the transition flow. The predicted BSD can be applied to calculate the interfacial area concentration (IAC) in the transition flow. The calculated IAC shows good agreements with experimental data which demonstrates the validity of predicted BSD in the transition flow.