Capacitance sensor measurements of upward and downward two-phase flow in vertical return bends

• Capacitance time traces used to asses bend effect on two-phase flow.
• Visual comparison of time traces showed effect downstream of the bend for slug flow.
• Wavelet variance of the signals showed bend effect for intermittent and annular flow.
• Bend effect was observed at least 21.5 tube diameters downstream of the bend.

In this work, the feasibility of using capacitance sensors to assess the two-phase flow behaviour up- and downstream of a sharp return bend is evaluated. A return bend with a radius of 11 mm and an inner diameter of 8 mm is used. The refrigerant is R134a, the mass flux is varied between 200 and 400 kg/m2 s and the vapour quality x is varied between 0 and 1. The bend orientation is vertical. Upward flow as well as downward flow through the bend is studied. Eight capacitance sensors are placed at several locations upstream and downstream of the return bend. The capacitance time traces for these sensors are compared with the aim to determine the presence and the extent of the bend effect. It was concluded that the disturbance due to the return bend stretches out to at least 21.5 tube diameters downstream of the bend. Upstream of the bend, an effect was absent for downward flow, for upward flow a limited effect was observed.