microns.) The conclusions to be drawn from the published data regarding the effect of ingested dust on gas turbine performance are summarized as follows: Serious erosion of gas turbine components will be caused by ingestion of airborne dust generated by average military oper ations in unimproved areas. Efficient air filters will be re quired on all gas turbines to remove dust particles above 2 to 3 microns in diameter if improved erosion-resistant materials are not available. Anticipated gas turbine life varies inversely as the product of the maximum particle size and the dust concentration, and the erosion factor is independent of the rate of ingestion (i. e., dust concentration). The total erosion is independent of dust concentration within realistic limits, and is a direct function of the total weight of ingested dust of any given particle size for a given gas turbine. In the particle size range of airborne particles, there is negligible erosion of the inducer section of the com pressor impeller or other areas where the absolute velocity of the entrained dust is low. A simple cycle gas turbine demonstrates little or no external evidence (by observation or readout) of complete destruction by dust erosion prior to the point of complete failure. Vi bration of gas turbines does not appear to increase until almost total destruction of the rotating components by dust erosion has occurred. Field studies give only a generalized or end view of the erosion phenom ena, with limited potential for insight into the basic mechanisms of ero sion. This is because erosion in a turbine occurs over a wide and chang ing range of conditions that are difficult to control directly as vari ables. EROSION TESTING Recognizing the fact that erosion in a turbine is a product of many variables, then the development of a meaningful "bench" type test be comes monumental. Table 1 lists most of the variables that must be con sidered, incorporated, and controlled. It appears as if a full scale turbine engine is the only answer as a test vehicle; however, the cost consideration precludes this use as a routine test. Two of the more widely used erosion testing devices are the Roberts Jetabrader and the S.S. White erosion test. These are small, self-contained, primarily re search oriented testing equipment. The Solar Division of International Harvester has developed a new erosion test facility which can apply and evaluate most of the variables in Table 1 (Ref. 6). The primary draw back of all these test techniques is the inability to evaluate more than one specimen (material and/or coating) per test. The Naval Air Propulsion Test Center (NAPTC) has designed and operates a dynamic erosion facility. The test rig consists of a standard T58-GE 10 engine first stage rotor, front frame, bullet nose, and bellmouth (Fig. 4). The rotor contains 30 blades of cast AM-355, the standard alloy used in current model engines. An inlet cloud chamber, close-cou pled to the bellmouth (Fig. 5), allows a uniform, dispersed flow of meas ured abradant to enter the bellmouth. A calibrated feeder supplied the abradant through a funnel ejector to the inlet cloud chamber (Fig. 6). The abradant used for all testing to date is prepared Arizona Coarse Grade Road Dust, 0 to 200 microns, from the AC Spark Plug Division, GMC. |