The refrigerant two-phase flow void fraction in the vicinity of a sharp return bend.

In this work, the void fraction is measured for two-phase refrigerant flow at several locations up-and downstream of a sharp return bend. The tube diameter D ranges between 4.83 mm and 8 mm and the curvature radius of the bend (2R/D) ranges between 2.5 and 4.5. The void fraction measurement is based on the electrical capacitance of the flow. The refrigerant used is R134a, the mass flux was varied between 200 and 600 kg/m²s and the vapour quality varied between 0 and 1. The tubes up-and downstream of the return bend are horizontal and the plane of the bend is placed vertical. Downward as well as upward oriented flow through the bend was tested. Three flow regimes were observed: slug flow, intermittent flow and annular flow. A significant effect is observed downstream of the return bend, upstream the result was limited. Furthermore, the influence of the bend decreases with decreasing channel diameter.