Two-phase pressure drops and flow pattern observations in 90° bends subject to upward, downward and horizontal arrangements

• Published in: Experimental thermal and fluid science Vol. 68; pp. 484 - 492
• Main Authors: Hsu, Li-Chieh, Chen, Ing Youn, Chyu, Chien-Min, Wang, Chi-Chuan
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.11.2015
• Subjects: 90° bend; Bends; Flow reversal; Fluid dynamics; Flux; Frictional pressure drop; Horizontal; Inertia; Mathematical analysis; Positioning; Pressure drop; Tubes; Two-phase flow; 90° bend; Flow reversal; Two-phase flow; Frictional pressure drop
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2015.06.012

•We examined friction and flow patterns of 90° bends for various tubes/arrangements.•The bends have inner diameters of 5.5 and 9.5mm, and curvature ratio of 5.4 and 4.2.•The two-phase pressure drop for the upward flow exceeds other arrangements.•Flow reversal may occur for upward flow and it is not seen for downward flow.•A flow pattern map is proposed to describe the flow reversal subject to tube diameter. In this study, single-phase/two-phase frictional characteristics and flow patterns of 90° bends are experimentally investigated. The two bends have inner diameters (D) of 5.5 and 9.5mm with tube curvature ratio (2R/D) of 5.4 and 4.2, respectively. The bends are installed either an upward, horizontal or downward arrangement. For results concerned with single-phase flow, the bend friction factor (fB) is always greater than that of the straight tubes at the same liquid Reynolds number. On the other hand, the fB values for D=9.5mm with smaller curvature ratio are greater than those of D=5.5mm. The resultant two-phase frictional pressure drops of 90° bend are well predicted by the Sánchez Silva et al. [14]’s correlation. For the two-phase frictional pressure gradient, the upward arrangement is greater than that of the horizontal arrangement due to swirled motion and liquid flow reversal. The flow reversals are observed for upward flow arrangement at specific flow pattern and pressure gradient. However, no swirled motion and liquid flow reversal is seen in the downward arrangement. As mass flux and quality increase, the differences of pressure drop among the three arrangements are getting smaller due to the appreciable offset by the rising flow inertia. Meanwhile, the liquid flow reversal is more evidently observed in the large diameter bend. Flow pattern map showing the region of liquid flow reversal is shown on the diagram of the superficial gas velocity (UGS) versus superficial liquid velocity (ULS). The region for liquid flow reversal for D=9.5mm is much greater than that of D=5.5mm. For D=5mm, there is no liquid flow reversal when UGS is above 2m/s. On the other hand, the threshold value is raised to 4m/s for the 9.5mm internal diameter tube.