Two phase frictional pressure drop in horizontal and vertical return bends with Ammonia

Air handling units in refrigeration systems have U-bends that significantly affect pressure drops but have little impact on heat transfer. This study experimentally investigated the two-phase pressure drop performance of ammonia in horizontal and vertical return bends. The study was performed on three tubes with nominal outer diameter of 9.5, 15.9, and 22.2 mm. The bend radius to tube diameter ratio (R/do) for each tube was kept at 1.2, 1.9 and 2.5. Experiments were conducted at two saturation temperatures of -15 and 10 degrees C with mass flux between 10 and 50 kg m-2 s-1 and vapor quality between 0.1 and 0.9. A classical trend of increase in pressure drop with mass flux and vapor quality was observed. Results showed a decrease in pressure drop with saturation temperature, tube diameter and bend curvature ratio. In vertical orientations, the influence of gravity was significant, leading to higher pressure drop particularly at low mass flux and vapor quality regions, especially for medium and large tubes. Moreover, bend curvature ratio exhibited minimal effect on pressure drop at high saturation temperature, low mass flux, and vapor quality in vertical orientations. Overall, the pressure drop in vertical upward flow was found to be higher than the vertical downward and horizontal flows. Additionally, the effect of tube diameter was less effective at higher mass flux and vapor quality particularly at low saturation temperatures. These findings contribute to a better understanding of pressure drop behavior in U-bends, which is crucial for the design and optimization of refrigeration systems.