Investigation Into the Effect of Tip Clearance on Centrifugal Compressor Performance

The axial clearance between the tip of the blades of an unshrouded impeller and its stationary shroud has been varied to study its effect on overall compressor performance. The compressor under investigation consisted of an inlet nozzle, a 3D open impeller with full inducer, a parallel-wall vaneless diffuser and a collector. High-accuracy overall performance data were obtained for this compressor. The experiments were carried out in a closed-loop centrifugal compressor test rig with the impeller running at a rotational Mach number u2/a0 = 1.39. The impeller tip diameter was 0.516 m, its tip width 0.021 m and the impeller blade exit angle was 30 degrees from radial. Assuming a linear relationship, the experimental data indicates a pressure ratio decrease of 0.77 percent, an efficiency loss of 0.31 points, an input head reduction of about 0.25 percent and an output head reduction of about 0.65 percent for each percent increase in clearance ratio. However, the data seems to indicate a non-linear effect showing stronger performance sensitivity at smaller clearances. The test data are compared against a clearance loss model. Improved performance prediction is obtained by including the effect of clearance on impeller work input.