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The Efficiencies of Single-Stage Centrifugal Compressors for Aircraft

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The Efficiencies of Single-Stage Centrifugal Compressors for Aircraft THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 91-GT-77 35 E 7 S ew Yok Y 117 e Sociey sa o e esosie o saemes o oiios aace i aes o i is- ES cussio a meeigs o e Sociey o o is iisios o Secios o ie i is uicaios M iscussio is ie oy i e ae is uise i a ASME oua aes ae aaiae ] ® om ASME o iee mos ae e meeig ie i USA Copyright © 1991 by ASME Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1991/78989/V001T01A025/2400405/v001t01a025-91-gt-077.pdf by guest on 30 September 2021 e Eiciecies o Sige-Sage Ceiuga Comessos o Aica Aicaios C OGES Aeoemo a Coceua esig Cie Susa owe Sysems Sa iego CA 91 ASAC s Seciic See e us o mos ece aaces i sige—a wo—sage q Wok aco ceiuga comesso ecoogy y e aeosace commuiy P as ee moiae y iees i iceasig aieaig essue ousio sysem owe esiy, a imoig seciic ue R Gas Cosa cosumio wi ige sage essue aios. Aaces i e e eyos ume as ecae ae mae i aoiae o eiew e mao esig parameters influencing the efficiency levels of single—stage T Total Temperature centrifugal compressors for aircraft applications. U Impeller Tip Speed A simple efficiency correlation was derived for advanced W Flow Rate single—stage centrifugal compressors. It was based upon four critical parameters: A Difference • Inlet Specific Speed ° Density • Impeller Tip Diameter Tl Efficiency (Total—static) y Specific Heat Ratio • Inducer Tip Relative Mach Number v Kinematic Viscosity • Exit Discharge Mach Number The correlation was shown to predict attainable (0 Angular Velocity state—of—the—art efficiencies within a band width of ± 2 % points. k Shock Loss Constant This was considered acceptable for preliminary compressor and engine design work. Flow coefficient = Cm1 /U2 SUBSCRIPTS NOMENCLATURE 1 Impeller Inlet C Velocity 2 Impeller Tip CFS Volume Flow (based on inlet stagnation density) ad Adiabatic Cp Specific Heat at Constant Pressure c Compressor D Diameter e Diffuser Exit g Gravitational Constant h Hub J Joules Equivalent m Meridional H Isentropic Head s Shroud M Mach Number u Tangential N Rotational Speed pol Polytropic esee a e Ieaioa Gas uie a Aeoegie Cogess a Eosiio Oao ue 3- 1991 1.0 INTRODUCTION The correlation of efficiency levels of low and high pressure ratio centrifugal compressors and pumps with specific speed, have 0 produced relatively great uncertainty concerning the D'". 0. state-of-the-art in maximum attainable efficiency potential. zC.) This was demonstrated in Reference 1. This uncertainty was _w 80 caused by the practice of extrapolating the published U LL performances of widely differing impeller and diffuser designs. Ui Other factors were differences in determination of efficiency U Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1991/78989/V001T01A025/2400405/v001t01a025-91-gt-077.pdf by guest on 30 September 2021 measurement and computation, and basic data uncertainty and 0 repeatability. w t Reference 1 showed that improved efficiency correlation 80 could be achieved if the impeller and diffuser performances of centrifugal compressors and pumps could be separated. This allowed the impeller performance potential to be correlated separately in terms of peak polytropic efficiency versus specific 0 speed, based upon average flowpath density. Although the improved correlation of impeller efficiency 0.2 0. 0.4 0. .0 . 2.0 provided a more accurate definition of optimum impeller Ns SECIIC SEE (og scae s_oI features, its inherent shortcoming was that it did not quantitatively rate the efficiency potential of the overall stage, igue 1 Esimae Sage Eiciecies eeece 1 that is, of the impeller plus stationary diffusion system (diffuser). Furthermore, the average density specific speed of the impeller cannot be readily calculated. It requires iteration of the impeller The thrust of recent advances in single-stage centrifugal tip vector triangle. prr thnl h bn tvtd b th rp community in the interest of increasing power density through If it is assumed that the impeller and diffuser performances higher airflows per unit frontal area (higher specific speeds) and n b prtd, nd tht thr prfrn r nt improved thermal performances at higher (air) stage pressure interdependent, it is possible to apply the extensive data ratios. Single-stage pressure ratios of 15.0 with very reasonable generated for two-dimensional and conical diffusers to evaluate efficiency and adequate operating range, have been achieved by overall stage performance potential. Canadian Pratt & Whitney. Excellent performance for a small 4.0-inch diameter impeller at a pressure ratio of 6.0 is reported in Component separation works reasonably well for the impeller Reference 4. In parallel with aerodynamic improvements, (with undistorted inlet flow). This is not the case for the improved materials and structural design techniques are used, downstream diffuser. Here, as expected, the single most providing the capability of operating at higher tip speeds, with dominant parameter affecting the performance of a particular increased cyclical life. diffuser geometry is the inlet blockage. Therefore, diffuser The application of axial compressor transonic blading design performance with a centrifugal compressor or pump depends techniques to centrifugal compressor inducers has resulted in upon the particular impeller discharge flow conditions. further performance improvements. These include choke-flow capabilities, and also efficiency at inducer tip relative Mach Representative attainable compressor stage performances in numbers (Mis) exceeding approximately 1.2. The significance of 1980 were generated in Ref I by using the impeller data combined M 1 s on centrifugal compressor characteristics, particularly surge, with an assumed maximum diffuser static pressure recovery of has been shown in References 5 and 6. It is a critical design 0.78. This provided the results presented in Figure 1. The general parameter influenced by selection of specific speed and inducer efficiency trends of Reference I are still valid to date, but do not hub diameter ratio. reflect further improvements which have taken place in the last decade. These include: These advances in the last decade have made it appropriate to review the major design parameters influencing the attainable efficiencies of single-stage centrifugal compressors. • Extensive work of NASA (Ref 2), and Casey (Ref 3) on the effect of Reynold's number. 2.0 AIRCRAFT GAS TURBINE APPLICATIONS The simplicity and reduced cost features of the single-stage • The use of impeller shroud bleed, which generally increases compressor are ideal assets for small gas turbine auxiliary power efficiency between one and two percent points, together units (APUs); small, expendable turbojets; and small turboprop, with the more important attribute of increased flow range. turboshaft engines. The most prevalent applications are in engines up to approximately 500-hp output. Larger power • Extension of Centrifugal Compressor Technology to higher engines in the 500-3000 hp range feature two-stage centrifugal pressure ratios and Mach numbers. compressors, or combined axial centrifugal compressors, in single— or two—spool arrangements. Extensive small gas turbine cycle optimization studies, using single—stage centrifugal compressors and single—stage radial inflow turbines, were presented in Reference 7, and indicated that: • Optimum cycle SFC, and minimum cost/power occur at different specific speeds. • The optimum compressor specific speed for minimum cost /AU and weight is approximately unity. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1991/78989/V001T01A025/2400405/v001t01a025-91-gt-077.pdf by guest on 30 September 2021 • Optimum compressor specific speed for minimum SFC is approximately 0.7. • Cycle pressure ratios above 7.0 show diminishing gains in SFC and reduced cost. The weight and cost contraints of small intermittent duty APUs and short duration expendable turbojets usually mandate the selection of Ns close to unity. Specific fuel consumption (O/SA becomes more important in the design of longer duration small turboprops and turboshaft engines. In these cases, two axial turbine stages are typically selected, one to drive a higher pressure ratio (8.0) single—stage centrifugal compressor with a specific speed close to 0.7, and the second turbine stage to drive the propeller, or rotor. The three major avenues of single—stage centrifugal compressor technology for aircraft gas turbine applications are: i WO SAGE • APUs and Expendable Turbojets (Specific speeds of approximately unity with moderate pressure ratios of 4.0 to 5 • Small Turboprop and Turboshaft Engines (Specific speeds of approximately 0.7, with pressure ratios up to 8.0 as constrained by structural limitations.) • High Pressure Spool or Second —Stage Compressors (Applications require moderate specific speeds and pressure ratios, with larger inducer hub diameter ratios.) igue Ceiuga Comesso o Typical examples of such applications are shown in Figure 2. Aica Aicaios Note that in the first two applications, the consequence of high specific speed and moderate pressure ratio, or moderate specific speed and high pressure ratio can produce M1s levels greater that The shortfall of this approach was the need to define impeller tip 1.2. Efficient inducer design is critical in both applications. conditions in order to target the projected efficiency goal. Impeller tip vector conditions cannot be generated prior to 3.0 PERFORMANCE PARAMETERS engine cycle optimization, unless an iterative engine and component optimization technique is available.
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