A New Experimental Technique to Simulate Secondary Erosion in Turbine Cascades

In refineries, power-recovery turbines are widely used in the fluid catalytic cracking process to extract power from particle-laden gases. The gas particles impinging on the blades cause blade and platform erosion. This erosion can be broadly classified into primary and secondary erosion according to the dynamics of the particle/flow interactions and whether the damage is caused by direct inertial impingement or by recirculation of fine particles (1–2 μm) through the blade secondary flows, by the vortices induced by these flows. This paper reports on a new experimental method devised to simulate the secondary erosion patterns without the use of a dust-laden stream. In this method, blades coated with a sublimating material such as naphthalene are tested in a wind tunnel. The secondary flow vortices tend to increase the local rate of sublimation in those areas where surface momentum gradients are high. This simulates the condition of preferential erosion induced by the dust loading present in the vortices. Tests show the induced patterns to be quite akin to secondary erosion seen on field run blades especially in the critical erosion patterns seen near the blade roots. Design modifications were then successfully developed to minimize this secondary erosion. These were shown to have a capability of reducing the erosion by 50 to 75 percent, by utilizing platform step control and naturally induced boundary layer suction. Conversely, many other features were found surprisingly ineffective. The method was also shown to be a very effective surface flow visualization technique for internal and external surfaces. Of the approaches found to be successful in minimizing erosion damage, control of the geometry of the stator to rotor steps along the hub flow path blade platforms was critical. Steps in general have the effect of making erosion worse. The most successful approach is the introduction of an inherent suction slot and the suction flow passage between individual blade platforms.