Experimental Study of Discharge Coefficients for Radial Orifices in High-Speed Rotating Shafts

In modern multi shaft gas turbine engines the cooling air taken from the compressor passes through various stationary or rotating holes or orifices to the turbine stage. The discharge coefficient of rotating orifices may vary considerably depending on the operating point and the geometry of the orifice and its surroundings. The predictability of this behavior is crucial for safe and efficient operation and for the life specification and the reliability of the components. This paper reports on measurements of the characteristics of flow discharge through radial orifices in a rotating system, with cross-flow included and on parameters suitable for correlation of the test data. A test rig was designed to simulate shaft diameters, rotation speeds and pressure ratios typical for real gas turbine designs. The pressure ratio across the orifices varied from 1.05 to 1.5 and the Mach number of the flow approaching the holes was 0;0.1;0.15 and 0.2. Hole configurations with different L/d (length-to-diameter) ratio and with round and sharp edges were examined. Inserts representing the different hole types are used to achieve flexibility of the rig. The upstream and downstream static pressure and the pressure drop across the radial holes are measured by sub miniature pressure transducers rotating with the outer shaft. A correlation parameter optimal for presentation of the test data is derived and data are compared to literature.