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.I The high efficiencies of mall radial turbines have led to their application in space power systems and numerous APU and shaft power engines. Experi- ment+ and analytical work associated with these sys- tef as included examination of blade-shroud clear- * 51 'P s INTRODUCTION The high efficiencies of small radial inflow turbines have resulted in their use in many APU' s, small turboshaft engines, and space power systems. The selection and development of components for specific applications has required harledge of Rey- nolds number effects, internal losses, blade-shroud clearance penalties, and other characteristics to assure adequate and safe operation. . The Lewis Research Center of NASAbegan in 1962 to investigate components designed under contract for Brayton cycle space parer systems. The studies in- cluded internal flow analyses as well as experimental operation of compressors, turbines, alternators, and various combinations of these. The work in radial turbines from 1962 to1967was highlighted andsum- maxized in Ref 1, a paper presented at the Second Intersociety Energy Conversion Engineering Conference in August of 1967. Since that time work has continued in the areas of blade-shroud clearance, blade loading, exit diffusers, and a specific speed investigation with various stator and rotor areas in a single tur- bine. Experiments have been made with cold air and argon and the high precision instrumentation and pro- cedures described i n Ref. 2. This paper includes the major results obtained in several experimental programs concluded since 1967 and others s t i l l i n progress. Detailed descriptions of the research hardware and the investigations may be found in the references listed. TEST EQUIPMENT The apparatus used in the performance evaluations of the turbines to be discussed is shown in Fig. 1. P r, w m ance ABSTRACT Argon or air, from a high pressure supply system, was passed through a calibrated orifice plate for flow measurement upstream of the turbim. Turbine inlet and exit pressures were controlled with automatic valves during data taking. An airbrake dynamometer, cradled on air bearings, was used to absorb parer, control speed,.and measure torque. Internal instru- mentation included static pressure taps, thermo- couple rakes, and survey probes for flow angle and local total pressures and temperatures. Figure 2 is a photograph of one of the turbines. It is typical of those investigated, with an inlet volute, conventional stator blades, and a rotor with fill and splitter blades. Its tip diameter is 15.3 cm while other test turbines discussed herein had diam- eters of ll.7 cm and 12.6 a. These were all low pressure ratio machines, 1.5 to 1.8. BLADE-SHROUD CLEARANCE EFFECTS The effect of blade-shroud clearance on turbine performance is of interest because of mass flow, blade loading, and efficiency considerations. The clear- ance must be adequate to avoid contacts during speed and thermal transients, but minimized to avoid blade unloading and excessive loss generation. Besides t h i s operational concern, howledge is also desirable for better understanding of the flow and comparison with axial turbine clearance effects. A test program in- volving varying clearance and clearance distribution was carried out with a 15.3 cm turbine designed to drive the compressor of a two-shaft space power sys- tem. This investigation, described in Ref. 3, in- cluded step changes in clearance in the inlet portion of the rotor and also in the exit portion. These were carried out so that separate trends could be established with local clearance change as well as with uniform incremental changes in terms of percent of passage height. Minimun clearance was established by coating the shroud with lacquer and hand finishing. Measurements shaved about 0.25 percent of passage height from inlet to outlet for this minimum clearance case. Figure 3 shows that peak static efficiency dropped from 0.84 t o 0.81 and then 0.77 as clearance was increased uniformlyfrom0.25 to3.0 and7.0. Theeffecton total efficiency was about the same. The increased clearances were obtained by removing the lacquer from the shroud and grinding material from the blade tips. The trend shown in Fig. 3 may be compared with similar trends in axial flow turbines. Figure 4 shows the effect of clearance on the static effi- ciencies of this radial turbine and two axial flow TM X-67903 RECENT R A D I X TURBINE RESEARCH AT THE NASA LEWIS RESElLRCH CENTER by Harold E. Rohlik and Milton G. Kofskey Lewis Research Center National Aeronautics and Space Administration Cleveland, Ohio , blade loading, and exit diffuser design. Re- sults indicate high efficiency over a wide range of specific speed and also insensitivity to clearance and blade loading in the radial part of the rotor. The exit diffuser investigation indicated that a conventional conical outer wall may not provide the velocity variation consistent with minimum overall diffuser loss. A list of recently published NASA radial turbine reports is included. 1PDF Image | RECENT RADIAL TURBINE RESEARCH AT NASA Lewis
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