Turbulence predicting criterion based on shear forces at the boundaries in a two-phase flow

The use of traditional Re number turbulence indicator can be extended for a variety of non-pipe flow channel geometries by the use of appropriately defined equivalent diameters. A modified equivalent diameter can be defined also for open channel flow, however, it fails when condensation is present at the open channel interface. This is shown by experimental measurements performed at the LEI (Lithuanian Energy Institute) two-phase test facility for condensing separated flows. It was observed that for such conditions the liquid region can become fully turbulent even when Ref < 1370. This is documented using axial temperature measurements, and illustrated in more detail by thermo-visual images, obtained with an IR camera. Experimental evidence from other stratified flow facilities is reviewed, which confirm the importance of shear at the vapour-liquid interface for the determination of liquid region turbulence. A turbulence indicator is proposed which takes into account potentially different shear stresses occurring at segments of the flow channel boundaries. It can be used to predict turbulence for separated flows, including flows for which phase change occurs. Compared to the Re number indicator it uses additional readily available information to estimate shear magnitude, it can therefore be advantageously applied for single phase flows.