This paper reports on an experimental and numerical study of detailed flow structures in a transonic centrifugal compressor impeller at various operating conditions. Experimental data were obtained from conventional and laser-two-focus measurements inside the impeller. Numerical results were obtained from steady, three-dimensional Reynolds-averaged Navier-Stokes calculations. Both the experimental data and the numerical solutions at the design condition indicate that the flow incidence is high near the hub and flow separation exists near the leading edge. Due to the flow separation, low momentum fluid migrates rapidly to the tip area resulting in further loss generation through increased shock/boundary layer interaction. At a higher flow rate, a second passage shock develops near the leading edge of the splitter blade, further increasing shock/boundary layer interaction.
Numerical studies were performed to explore possible design modifications for better efficiency and higher pressure rise. First, the blade camber near the leading edge was modified to improve the incidence. Second, the blade thickness was reduced by 50 percent. The incidence modification eliminates the flow separation near the leading edge and makes a more uniform flow split between the two channels, resulting in better flow distribution at the impeller exit. The simulated blade thickness reduction, along with the modified incidence, improves the efficiency by about 5 percent and increases the impeller pressure rise from 6.1:1 to 7.1:1.
Flow Separation Associated with 3-D Shock-Boundary Layer Interaction (SBLI)