Measurements of the secondary velocity vector in a 180° U-tube using a 2-D LDA system with a refractive index matching technique
Conference
Muguercia, I, Cazanas, B, Li, W et al. (1993). Measurements of the secondary velocity vector in a 180° U-tube using a 2-D LDA system with a refractive index matching technique
. Proceedings of SPIE - The International Society for Optical Engineering, 2052 103-110.
Muguercia, I, Cazanas, B, Li, W et al. (1993). Measurements of the secondary velocity vector in a 180° U-tube using a 2-D LDA system with a refractive index matching technique
. Proceedings of SPIE - The International Society for Optical Engineering, 2052 103-110.
An experimental study using Laser Doppler Anemometry (LDA) has been carried out to investigate the developing laminar flow inside a 180° U-tube. A previous experimental loop was modified for the purpose of measuring both the axial and secondary velocities. The uncertainties of velocity, the Reynolds number, and the Dean number were found to be under 1.52%, 2.37% and 2.60%, respectively. The U-tube test section consists of a glass tube with an inner diameter (d) of 22.0 mm. The curvature ratio of the coil centerline of the curved diameter to the pipe diameter (Dc/d) is 5.77. A refractive index matching technique was applied in this investigation in order to measure the secondary velocity vectors. A water-like chemical solution, which has the same refractive index as the glass test section, as well as a low and reasonable cost, was developed and utilized in this study, and variations of the refractive index with the concentration and temperature are summarized in this paper. Other properties, such as the viscosity and density of the chemical properties are also reported. In the present investigation, the axial velocity distributions at the entrance of the U-tube for Reynolds numbers of 1,000 and 2,000 are verified. The axial velocity distribution and secondary velocity vectors at cross sections of 90° and 180° from the entrance of the U-tube were measured and are reported for the same Reynolds numbers. The secondary velocity measurement confirms that two vortices are superimposed on the axial flow, and that the magnitude of the secondary flow is proportional to the axial flow or Reynolds number. As a result of centrifugal force acting on the fluid flow in the curved pipe, inspection of the velocity contours reveals that the axial velocity profile shifts toward the outer wall of the U-tube pipe. Finally, as a result of the secondary velocity in the curved pipe, the axial velocity exhibits a double-peak behavior.
