Two-phase frictional pressure drop in return bends and contiguous straight tubes with ammonia for low-temperature applications

The performance of a refrigeration system depends on the refrigerant pressure drop and heat transfer. While single-tube void fraction and pressure drop data are plentiful, research on U-bends is limited, especially for ammonia at low saturation temperatures. Ammonia’s comparatively higher liquid-to-vapor density ratio results in a higher void fraction and thus affects the local pressure drop. This paper presents two-phase flow pressure-drop characteristics of ammonia in return bends and contiguous straight tubes at a saturation temperature of -24 °C. Experiments were conducted on tubes with nominal outer diameters ( d o ) of 9.5 and 15.9 mm, and bend radii-to-diameter ratios ( R/d o ) of 1.2 and 2.5 for both tubes. Three orientations of the return bend were studied, i.e., horizontal, vertical upward, and vertical downward. Mass flux was varied between 30 and 60 kg m⁻² s⁻¹ for the small tube and 15–30 kg m⁻² s⁻¹ for the medium tube at various vapor quality. The pressure drop increased with mass flux and vapor quality, as well as with lower tube diameters. For the return bends, a higher pressure drop occurred at a smaller R/d o ratio; however, bend orientation had a mixed effect on the pressure drop. The return bends primarily showed higher pressure drops than straight tubes under similar experimental conditions. Pressure drop measured in the return bend of a small-diameter tube at low saturation temperatures was 40–80% higher than that of the authors’ previous work at medium saturation temperature due to higher liquid viscosity and vapor specific volume.