Effect of the bend geometry on the two-phase frictional pressure drop and flow behaviour in the vicinity of the bend

The effect of a sharp return bend on the two-phase flow frictional pressure drop close up-and downstream of the bend is investigated. The measurements are performed for refrigerant R134a at mass fluxes ranging from 200 to 500 kg/m(2) s. Four bend geometries are tested, with inner channel diameters ranging from 4.93 mm to 8.1 mm and the curvature ratio between 2.55 and 4.43. For both up-and downward oriented flow the main bend effect on the frictional pressure drop is observed close downstream of the return bend. However, for downward oriented flow a decrease in pressure drop is observed and for upward flow an increase is found. To assess the effect the underlying flow behaviour, the void fraction time trace is analysed. The average void fraction and wavelet variance of the void fraction signal can be used to explain the trends in the frictional pressure drop. The correlation between the frictional pressure drop and the flow behaviour indicators is investigated quantitatively for the different geometries. (C) 2016 Elsevier Ltd. All rights reserved.