THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 93-G-335 345 E. 47th St., New York, N.Y. 10017 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, © or printed in its publications. Discussion is printed only if the paper is pub- lished in an ASME Journal, Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright © 1993 by ASME Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A091/2403731/v03bt16a091-93-gt-335.pdf by guest on 26 September 2021 SIMUAIG IIEC US MEASUEME MEOS O IG YASS UOAS . Stvnn nd . Srvntt Department of Mechanical and Aerospace Engineering Carleton University Ottawa, Ontario, Canada ABSTRACT ABBREVIATIONS As yet, there is no known reliable method for directly measuring BPR bypass ratio the thrust of a turbofan in flight. Manufacturers of civil turbofans CDP HP Compressor delivery total pressure use various indirect thrust measurements to indicate performance EPR engine pressure ratio of an engine to the flight deck. Included among these are: Engine EGT LP Turbine exhaust gas total temperature Pressure Ratio (EPR), Integrated Engine Pressure Ratio (IEPR), Fan FPR fan pressure ratio mechanical speed (N,), and various Turbine Gas Temperatures such GG gas generator (core of turbofan) as ITT or EGT. Of key concern is whether these thrust indicators HDTO hot day take-off (ISA +15°C at SLS) give an accurate account of the actual engine thrust. The accuracy HPC,HPT high pressure compressor, turbine of these methods, which are crucial at take-off, may be IEPR integrated engine pressure ratio compromised by various types of common engine deterioration, to ISA international standard atmosphere the point where a thrust indicator may give a false indication of the ITT inter-turbine total temperature health and thrust of the engine. A study was done to determine the LPC,LPT low pressure compressor (Boosters), turbine effect of advanced engine cycles on typical values of these OPR overall pressure ratio parameters. A preliminary investigation of the effects of common PR pressure ratio kinds of turbofan deterioration was conducted to see how these RPR ram pressure ratio faults can affect both actual engine performance and the indirect SLS sea level static thrust indicators. TIT HP turbine inlet total temperature TOC top of climb (Mach 0.8 @ 35000 ft) NOMENCLATURE SUBSCRIPTS AND STATION NUMBERING A area bl HP Turbine cooling bleed (% core massflow) a air C airflow velocity c cold (fan or bypass) c, specific heat (at constant pressure) cc combustion chamber m massflow g gas h hot (core) N1 , N2 mechanical LP and HP spool speeds p, T static pressure, temperature i intake mech P. T. ambient pressure, temperature mechanical N nozzle plane PPb Combustion pressure drop (% CDP) po, To total pressure, temperature PN , TN nozzle plane static pressure, temperature Q non-dimensional massflow parameter corrected pressure (pa/l.01325, po in Bars) specific heat ratio 11_ polytropic efficiency 11 isentropic efficiency O corrected temperature (T0/288.16, To in K) Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cincinnati, Ohio May 242, This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 0, Hence, F = pN A CN +A (PN -p) (2) 0 2 a 8 A oe = PN A • 2 C (T - TN) + A (pn - p°) N 3 5 7Coe = p A PN P° p ° _ 1 CCoe 9 p - 1 p° p° TN C HP LPT +A ° — — - 1 ° Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A091/2403731/v03bt16a091-93-gt-335.pdf by guest on 26 September 2021 p°Figure 1 Twin-Spool Turbofan Station Numbering ° (3) INTRODUCTION Substituting the value of pN : P° Although jet engines have been used commercially for 40 years, there is still no direct method of measuring thrust in flight. 2 Indirect methods of indicating thrust are used, based on pressure =1 _L _ measurements or rotational speeds. The use of engine pressure A° Y-1 1l1 ° Y2 (4 ratio (EPR), which is defined as the ratio of total exhaust pressure + [(2 - over total inlet pressure, became dominant in early turbojet engines P. where there was a direct relationship between EPR and thrust. A problem arose with the introduction of high bypass turbofans, This gives, where a substantial portion of the total net thrust is provided by the cold or bypass stream; EPR considered only the core engine F_ P° 11 2 pressure ratio and hence the thrust contribution from the hot (Y+ 1) - 1 (5) stream. One alternative was to use integrated engine pressure ratio A P° P° Y+1 ) (IEPR) which is based on an area weighted average of the bypass and core nozzle pressure ratios. Another approach was to use fan It follows that rotational speed (N,) to indicate thrust. Advanced cycle turbofans may operate at significantly F = K p° - 1 where K = f (1) (6) ige aues o yass aio ( a oea essue aio (OPR) which may influence the choice of parameter for indicating A ° ° thrust. The effects of in-service deterioration may also be This can be extended to work with EPR instead, and noting that important in selecting the most suitable parameter. = P_' P°t This paper analyses the performance of a typical high i.e. nozzle PR = EPR RPR performance turbofan in the hope of providing objective data for P. P°I P. comparing the usefulness of various thrust setting parameters. Then, THE ORIGINS OF EPR F + 1l I RPR = K(EPR) (7) AP, J It is instructive to consider the basis of EPR as a means of L indicating the thrust of a simple, fixed nozzle turbojet: (where K = 1.2594 for y = 1.333) Thus, for a fixed nozzle turbojet, the ideal thrust can readily P0 Pa be found from the EPR. In the case of a turbofan, however, the To EPR will depend on both the bypass ratio and fan pressure ratio ------------- :i N sice o o ese ae a eec o e oa essue o e LPT ^ TN Turbine exhaust, and there is no longer a direct relationship Exit between total net thrust and EPR. The concept of IEPR attempts — — -^ to include the substantial thrust contribution from the fan or bypass Figure 2 nozzle through an area weighted average defined as follows: For a choked nozzle, IEPR = po2A0 + po7At' (8) pot (A, + Ah) P Y F = m CN + A (PN - p°) , andd = (2 )Fr (1) This requires extensive pressure measurements in the fan duct but p° Y+1 appears to be a more logical way to indicate the thrust of a high Also: bypass turbofan. The thermodynamic derivation of EPR is straightforward, PN 2 but to pilots, EPR may appear to be an abstract concept which = pAC C =2 CP (° N) PN = RN 7 ; does not give an instinctive "feel" for thrust developed. Rotational 2 speeds, such as N1 , can readily be related to thrust; anyone who has ever driven a car equipped with a tachometer can readily recognize COMPUTER SIMULATION METHODS USED red line limits and instinctively keeps within them. a) On-Design Model CHOICE OF DESIGN POINT The off-design performance calculation procedure used required detailed knowledge of several design point parameters of Modern high-bypass turbofans are designed around several the chosen engine cycle, particularly the engine cycle parameters flight conditions or design points, and not just the Sea Level Static such as FPR, BPR, OPR and TIT and fixed nozzle areas. A (SLS) condition as was perhaps the common design point in early straightforward thermodynamic cycle calculation was used based on engines when adequate take-off thrust was the primary concern. LP and HP spool work balances, polytropic compression and Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A091/2403731/v03bt16a091-93-gt-335.pdf by guest on 26 September 2021 Current modem designs are usually optimized around three expansion processes, and separate nozzles. While the precise common flight conditions (Philpot, 1992): simulation of a particular engine was not the aim of this study, reference was made to manufacturers' information on similar high Sea Level Static (SLS) Takeoff: Usually at ISA +15°C, 1) bypass designs so that reasonable on-design performance could be to allow a flat rated take-off thrust up to some hot ambient simulated. This, in general, required a number of iterations since temperature condition (Hot Day Take-off, or HDTO). Most many of the design parameters (such as component efficiencies) are modem engines automatically control the take-off fuel flow to essentially educated guesses, especially since TOC engine cycle prevent the TIT from exceeding a specified safety limit. In parameters are rarely quoted. practical terms, the Inter-turbine Temperature (ITT) is used as the limiter due to difficulties with directly measuring the high TITS b) Off- common in modem turbofans. The HDTO condition is important, Design Simulation Model limiting the maximum allowable TIT and hence influencing the ultimate thermodynamic cycle choice. Note that the ITT is The off-design simulation procedure used in this study was the twin-spool turbofan matching procedure based on the use of sometimes called the gas generator Exhaust Gas Temperature (EGT) by many engine manufacturers. In this paper, EGT will be component characteristics or performance maps. This straight- used to denote the LP Turbine exhaust temperature. forward procedure is outlined only briefly below; a more detailed description can be found in Gas Turbine Theory (Cohen, Rogers and Saravanamuttoo, 1987). 2) Cruise: A majority of the fuel burned during a typical civil airliner flight will be during cruise, especially for long range.