Influence of elbow curvature on flow and turbulence structure through a 90° elbow
Introduction
The pipeline elements in nuclear/fossil power plants, such as orifices, elbows, and tees, suffer from wall thinning caused by flow accelerated corrosion (FAC). Therefore, the flow through pipeline elements has been a topic of interest in the safety management of power plants (Keller, 1974, Sydberger and Lotz, 1982, Sanchez-Caldera, 1984, Dooley and Chexal, 2000). In 2004, a wall-thinning accident caused by FAC occurred in the Mihama Nuclear Power Plant in Japan when a pipeline break occurred downstream of an orifice. The break was caused by an interaction of the flow through the orifice and an elbow (NISA, 2005), where the elbow radius to diameter ratio was R/d = 1.2. Since then, FAC studies of various pipeline elements have become popular. These have covered not only single elements, but combinations of pipeline elements have been studied both experimentally and numerically (Hwang et al., 2009, El-Gammal et al., 2010, Ahmed et al., 2012, Fujisawa et al., 2012, Pietralik, 2012, Utanohara et al., 2012, Mazhar et al., 2013, Shan et al., 2014, Yamagata et al., 2014, Fujisawa et al., 2015a, Fujisawa et al., 2015b, Ikarashi et al., 2017). It should be mentioned that FAC is a diffusion phenomenon of the carbon steel wall material into the turbulent bulk flow through an oxide layer. Therefore, it is basically a mass transfer phenomenon that is highly affected by the flow turbulence. Furthermore, it is influenced by the temperature, pH, and oxygen concentration of the bulk flow. Therefore, experimental studies on the mass and momentum transfer in and downstream of pipeline elements have been carried out to understand the physical mechanisms of FAC.
In the past, mean flow and turbulence measurements in a 90° elbow were carried out at moderate Reynolds numbers, Re = (4.3–7) × 104 (Enayet et al., 1982, Sudo et al., 1998, Iwamoto et al., 2010, Yuki et al., 2011, Mazhar et al., 2016, Taguchi et al., 2018). These measurements are summarized in Table 1. On the other hand, there are fewer studies on elbow flow in the post-critical Reynolds numbers beyond Re = 4 × 105, where flow characteristics are expected to be independent of the Reynolds number (Ono et al., 2011, Iwamoto et al., 2012, Ebara et al., 2016). Numerical studies on elbow flows at high Reynolds numbers were carried out to understand the flow phenomena (Tanaka and Ohshima, 2012, Dutta et al., 2016). These past studies were conducted on elbows of different elbow radius to diameter ratios ranging from R/d = 1.0 to 2.8 and Reynolds numbers ranging from Re = (4.3 to 32) × 104. Among these past studies, the results of experiments on the flow in elbows with radius ratios between 1.0 and 1.5 are frequently applied to power plant pipelines. However, due to the limited number of experimental results, it is still difficult to explain the influence of the radius ratio on the flow field and the turbulence structure in an elbow. Furthermore, the influence of roughness on the mass transfer and FAC rate were investigated to understand the complex mechanism of wall thinning of elbow in relation to the wall turbulence (Wang et al., 2016, Abe et al., 2017, Fujisawa et al., 2017). It is noted that the welding at the upstream and downstream locations acts as a roughness in FAC.
Iwamoto et al., 2010, Iwamoto et al., 2012) measured the axial mean velocity and axial turbulence intensity in a short elbow (R/d = 1.0) at Re = 5 × 104 and 3.2 × 105 using laser Doppler velocimetry. This flow configuration was also examined by Yuki et al. (2011) using the matched refractive index PIV technique, which can remove the refractive index effect arising from the working fluid and surrounding wall material. It should be noted that the mean velocity field of Yuki et al. (2011) was shown by color contours focusing on an S-bend; therefore, the detailed distribution of the mean and fluctuating properties of the single elbow were less well described. These experimental results on the flow in a short elbow indicate that the flow approaches a separating condition near the elbow outlet and the turbulence intensity increases along the inner wall. On the other hand, the flow in a long elbow (R/d = 1.5) is not likely to indicate a flow separation along the inner wall (Mazhar et al., 2016, Taguchi et al., 2018). However, there is an axial decrease in the mean velocity and an increase in the turbulence intensity along the inner wall of the elbow, which is similar to the short-elbow case. These results indicate that the mean flow and turbulence characteristics are modified by the radius ratio, but the radius ratio effect on the mean flow and turbulence characteristics of the flow through the elbow are still not clear. Moreover, the variation of turbulence structure of the flow through the 90° elbow with the radius ratio was not studied in literature.
The purpose of this paper was to study the influence of the radius ratio of an elbow on the flow and turbulence characteristics by measuring the mean and fluctuating properties of the flow through the elbow at three different radius ratios (R/d = 1.0, 1.2, and 1.5) using planar PIV measurement. Furthermore, the secondary flow characteristics of the elbow were examined by measuring the cross-sectional velocity field near the elbow outlet using stereo PIV, and the results were examined by proper orthogonal decomposition (POD) analysis to understand the turbulence structure.
Section snippets
Experimental setup
An experimental study on the flow through an elbow was carried out in a closed-circuit water tunnel (Fujisawa et al., 2012). A schematic layout of the tunnel is shown in Fig. 1. It consisted of a pump, reservoir tank, honeycomb, flow-developing pipe section, and test section of the elbow, where the measurements of the velocity field were carried out using planar and stereo PIV. It should be noted that the length of straight circular pipe upstream of the elbow was set to 24d, where d is the pipe
Results and discussion
To complete as the database of the flow through elbows, the mean and fluctuating velocity 2d upstream of the elbow were measured by stereo PIV, and the results are summarized in Appendix B.
Conclusions
The velocity fields in and downstream of a 90° elbow were studied experimentally using planar, stereo PIV measurements to understand the influence of the elbow radius ratios in the range of 1.0–1.5 and Reynolds numbers in the range of Re = (3–10) × 104 on the flow field and turbulence structure. The results are summarized as follows:
- 1.
The experimental results showed that the axial mean velocity decreased along the inner wall in the second half of the elbow and the flow approached separation near
Acknowledgments
This work was supported by JSPS KAKENHI Grant Numbers JP24360391, JP18K04632. The authors are grateful to the helpful suggestion by Dr. F. Inada and Dr. K. Fujiwara from Central Research Institute of Electric Power Industry.
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