Two-phase frictional pressure drop measurements in U-type wavy tubes subject to horizontal and vertical arrangements

Two-phase frictional pressure gradient is significantly affected by the flow patterns. Though most of the U-type wavy tubes are commonly vertically installed in the air-conditioning and refrigerating systems, yet none investigations had reported the two-phase frictional pressure loss in vertical U-type wavy tubes. This study presents the measurements of R-134a two-phase frictional pressure gradient subject to vertical and horizontal arrangements of a U-type wavy tube with inner diameter of 5.07 mm and a curvature ratio of 5. The ratio between two-phase pressure gradients of U-bend and straight tube is about 2.5-3.5. For the straight tube, the frictional two-phase pressure gradient ratio between the vertical and horizontal arrangements is marginally higher (1.0-1.2) for annular flow pattern at x > 0.5, and is 1.0-1.4 for the U-bend in the wavy tube. The higher resistance in the vertical arrangement is due to the buoyancy force against the flow inertia. However, for x < 0.5, this ratio is gradually increased due to the difference of flow pattern. The ratio is increased to 1.8 in the straight tube. For the U-bend, the ratio is 2.1 for flow entering the upper tube and is 1.5 for flow entering the lower tube at x = 0.1 and G = 200 kg/m² s. For the vertical wavy tube, additional effects like the flow pattern transition, liquid flow reversal, and freezing slug may cause additional pressure drops.