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The purpose of this study is to characterize the swirling secondary flow in the downstream of a pipe bend using a numerical simulation of the flow. The CFD (Computational Fluid Dynamics) software OpenFOAM is used to simulate the turbulent flow in pipes with elbow. Various turbulence models are benchmarked with the existing experimental data and a comparative study is performed to select an appropriate turbulence model for the analysis. Predictions made by the selected turbulence model are compared with the LDA (Laser Doppler anemometer) measurements from the experiments currently conducted to find the dependency of the flows on the Reynolds number. It is found that the swirl intensity of the secondary flow is a strong function of the radius of curvature of the bend and a weak function of the Reynolds number. Additionally, it is found that the dissipation of the swirl intensity is exponential in nature. [ABSTRACT FROM AUTHOR]

The present study focuses on developing a database to investigate the effects of 90-deg vertical elbows on the transport and distribution of local two-phase flow parameters in air-water bubbly flows. The experimental facility consists of both vertical and horizontal sections made out of 50.8-mm inner diameter pipes and interconnected via 90-deg glass elbows. Six different flow conditions within or near the bubbly flow regime at the inlet are investigated in the current study. A multisensor conductivity probe is employed to measure detailed local two-phase flow parameters at ten axial locations along the test section, within which 90-deg elbows are installed at L/D = 63 and 244.7from the inlet. The data show that the elbow makes a significant impact on the two-phase pressure drop, bubble distribution, and bubble velocity. The bubbles moving across the vertical-upward elbow are entrained along the secondary flow streamlines leading to a bimodal distribution. For the test conditions investigated in the present study, this bimodal distribution is independent of the bubble distribution upstream of the vertical-upward elbow. In the case of the vertical-downward elbow, on the other hand, the large inertia of the axial liquid flow results in the bubbles migrating toward the inside of the elbow curvature. [ABSTRACT FROM AUTHOR]

The present study develops an interfacial area transport equation applicable to an air-water horizontal bubbly flow with a flow restriction. The experiments are performed in a round glass pipe of 50.3-mm inner diameter, along which a 90-deg elbow is installed at L/D= 206.6 from the two-phase mixture inlet. In total, 15 different flow conditions in the bubbly flow regime are studied. The detailed local two-phase flow parameters are acquired by a double-sensor conductivity probe at four different axial locations. The effect of the elbow is evident in the distribution of local parameters as well as in the development of interfacial structures. The elbow clearly promotes bubble interactions resulting in significant changes in both the void fraction and interfacial area concentration. In the present study, the elbow is found to promote the coalescence mechanism while reducing the disintegration mechanism. These geometric effects are also reflected in the axial development of one-dimensional two-phase flow parameters. In the present analysis, the interfacial area transport equation is developed in one-dimensional form via area-averaging based on the existing model for vertical flow. In the averaging process, characteristic nonuniform distributions of the two-phase flow parameters in horizontal two-phase flow are treated mathematically by covariance calculations. Furthermore, the change in pressure due to the minor loss of the elbow is taken into consideration by using a newly developed correlation analogous to Lockhart and Martinelli's. In total, 60 area-averaged data points are employed to benchmark the present model. The present model predicts the data well with an average percent difference of approximately ± 10%. [ABSTRACT FROM AUTHOR]