10 th Pacific Symposium on Flow Visualization and Image Processing Naples, Italy, 15-18 June, 2015 Paper ID:34 1 Velocity measurements within an elbow type draft tube of a Kaplan turbine Kaveh Amiri 1,* , Berhanu Mulu 2 , Mehrdad Raisee 3 , Michel Cervantes 1,4 1 Department of Engineering Science and Mathematics, Luleå University of Technology, Luleå, Sweden 2 Vattenfall Research and Development, Älvkarleby, Sweden 3 Mechanical Engineering Department, University of Tehran, Tehran, Iran 4 Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norway * corresponding author: kaveh.amiri@ltu.se *corresponding author: email@address.gggg.hh.kkk Abstract Flow condition in a Kaplan turbine draft tube is investigated using particle image velocimetry. The draft tube is composed of a conical diffuser, an elbow and a straight diffuser. The three velocity components were measured after the elbow at two different locations across the straight diffuser; after the draft tube bend and at the draft tube outlet. The objective is to investigate the effect of the swirl living the runner on the flow asymmetry in the straight diffuser. The results are presented at three operating points of the turbine. The swirl strength at the draft tube inlet and the centrifugal force induced by the elbow, forming Dean vortices, are found to be the main parameters affecting the flow condition and performance of the straight diffuser. The flow condition after the draft tube bend was shown to be highly dependent on the vortex structures within the straight draft tube; namely Dean vortices and the main swirl entering the draft tube. At operating points with high flow rates and low swirl, Dean vortices dominate the upstream swirl; hence, a symmetric flow resembling flow after a pipe bend forms inside the straight diffuser. However, at part load operating points with high swirl and low flow rate, the flow after the bend is dominated by the upstream swirl resulting in asymmetric flow within the draft tube. About 80% of the draft tube is occupied by strong vortices inducing centrifugal force to the flow. This results in formation of low axial velocity region where the swirling flow exists while the major part of the flow rate passes through the other part of the draft tube because of the centrifugal force. Keywords: Kaplan turbine, Draft tube, Particle image velocimetry, Dean vortices, Swirl 1. Introduction Draft tube is an important part of reaction turbines. It is a diffuser converting the kinetic energy of the flow leaving the runner to pressure and increasing the effective head of the turbine. It is usually composed of a conical diffuser, an elbow and a straight diffuser, see Fig. 1b. Most of the pressure recovery, about 70%, occurs in the conical diffuser also named draft tube cone. The elbow has usually a converging cross section to avoid separation on its inner section due to the centrifugal forces induced to the flow by the elbow curvature. A straight diffuser follows the elbow ejecting the flow to the tailrace water. In low head reaction turbines, i.e., propeller and Kaplan, the turbine efficiency is significantly affected by the performance of the draft tube because the kinetic energy leaving the runner represents a substantial part of the turbines available energy [13]. At the same time, numerical and experimental investigation of the flow inside the draft tubes is a challenge for researchers due to the flow complexity. Adverse pressure gradient, highly unsteady large scale vortices, turbulence, separation and swirling flow are the main challenges. Also, the presence of the runner hub in the middle of the draft tube inlet as a rotating bluff body affects the flow condition inside the draft tube. The large dimensions of the prototypes and even the industrial scaled models are another challenge for performing measurements and numerical simulations. Different measurement techniques have been used to analyse the flow inside the draft tube of reaction turbines. Draft tube wall pressure measurements may be referred as the oldest method for draft tube flow analysis. The pressure recovery and pulsations of turbine models at different operating conditions are the main issues addressed in such studies [6,9,13,14,16,17]. An extensive work was performed by Arpe and Avellan [1] to characterize the pressure fields on the draft tube walls of a Francis turbine model. Special attentions have been