Flow pattern identification for two-phase flow in a U-bend and its contiguous straight tubes

Gas-liquid two-phase flow in curved channels is widely found in various heat exchangers and transport pipes. The fluid flow and heat transfer characteristics are closely related to flow patterns. This work experimentally investigated flow patterns for air-water flows in a U-bend and its contiguous straight tubes at a pressure of 0,1 MPa and a temperature of 25 degrees C, and proposed a new flow pattern identification method. A flow visualization technique along with frequency spectral (power spectral density, PSD) and chaos analysis (multi-scale entropy, MSE) of pressure drop time series were used. The bend had an inner diameter of 16 mm with curvature radius of 100 mm, and it was installed with either a downward or an upward arrangement. The superficial air and water velocities were set to the range of 0.17-16.58 m/s and 0.10-2.03 m/s, respectively. Five flow patterns were identified on the basis of captured flow images, pressure drop time series and PSD distributions. Flow pattern maps for both flow orientations were proposed. Two unique flow patterns (stratified-plug flow in downward flow and unstable flow in upward flow) were obtained under low superficial gas and liquid velocities. Combining the skewness of PSD and the rate of MSE enabled us to objectively identify flow patterns in the U-bend unit, which facilitates identification of flow patterns in other non-transparent curved channels.