Engine Performance Analysis

Transcription

1 Engine Performance Analysis Introduction The basics of engine performance analysis The parameters and tools used in engine performance analysis

2 Introduction Parametric cycle analysis: Independently selected input: Flight condition M 0, T 0, P 0 Design choices c, T t4, e c, e t, etc. Determines =1 Performance output: F/ṁ 0, S, η, etc. 1 (a variable) Performance is determined at specific design flight conditions, values and limits Input design or reference point Limitations of Parametric Cycle Analysis Consider performance of engine built with selected design point When installed, its performance varies flight conditions throttle settings limited by engine control system When input is changed, component design changes (not possible with engine once built)

3 Engine Performance Analysis Looks at how specific engine perform at conditions other than design point (Off-design analysis) Engine built with selected design point Selected compressor pressure ratio c corresponding turbine temperature ratio (essentially constant) c is dependent on the throttle setting (T t4 ) and flight condition (M 0 and T 0 ) Off-design analysis: Changing through the throttle and indirectly by changing flight conditions Performance other than their design point Engine Performance Analysis Variable Parametric cycle Engine performance Flight condition (M 0, T 0, and P 0 ) Independent Independent Compressor pressure ratio c Independent Dependent (on engine speed and flight conditions) Main burner exit temperature T t4 Independent Independent (throttle input) Turbine temperature ratio Dependent (on flight conditions, c, and T t4 ) Constant Table 7-1 Comparison of Analysis Variables Source: Page 438

4 Design (Reference) Point Engine design point or reference point Specific set of input values that basic engine is designed to: Flight condition M 0, T 0, P 0 Design choices π c, T t4,, e c, e t, etc. Resulting F/ṁ 0, TSFC are valid only for the given engine cycle and values of T t4, M 0, T 0, etc. Normal design point Sea-level Static (SLS) Twin-Spool Gas Turbine Engine Low Pressure Spool (N1) High Pressure Spool (N2) Source: Page 439

5 Twin-Spool Gas Turbine Engine Subscript H denotes HP spool and L denotes LP spool Table 7-2 Additional Temperature and Pressure Relationships Source: Page 439 Reference Values and Assumptions Fixed-Area Turbine (FAT) conventional turbo machine - choked flow at Turbine inlet nozzles (HP turbine and LP turbine) Exhaust nozzle (core engine and fan duct, if separate) Unchanged from reference values:,,,,,,,,, Turbine cooling and leakage effects are neglected No power is removed from the turbine to drive accessories Constant and, and and that do not vary with power setting The term 1+ is considered as constant

6 Dimensional Analysis Impossible to test and record all operating conditions Dimensional analysis identifies correlating parameters that allow engine performance to be analysed under different conditions Dimensionless pressure and temperature represented by δ and θ where i is the station number Corrected mass flow rate at engine station i Corrected Parameters Table 7-3 Corrected Parameters Source: Page 443

7 Compressor and Fan Performance Map Steady operation above the surge line is impossible Entering the region even momentarily is dangerous to the gas turbine engine. Figure 7-2 Compressor Performance Map Source: Page 444 Compressor Stall Compressor stall may take one of two forms: High angle of attack (positive incidence) blade stall A front stage problem at low speeds Low AOA (negative incidence stall) Windmilling of blades Affects the rear stages at high speed Leads to blade vibration which can induce rapid destruction Watch video on Compressor Stall

8 Compressor Surge Caused by excessive demand for compressor pressure rise Beyond blades handling capability Results in: Instantaneous breakdown of flow through the engine High pressure air in the combustion system is expelled forward through the compressor Loud bang Resultant loss of engine thrust Watch video on Compressor Surge Compressor Stall / Surge Prevention Bleed valves to release air from middle stages Multi-spool compressors Optimum speeds for different spools Variable stator vanes, including the inlet guide vanes to guide the airflow at a desired angle Proper fuel scheduling to operate within Compressor Performance Map Limiting excessive pressure ratio during acceleration Source: Page 29, The Jet Engine (1986) by Rolls Royce plc,

9 Combustor Performance Map Figure 7-3 Combustor Efficiency Source: Page 445 Combustor Performance Map Burner inlet pressure Burner Temperature Rise Figure 7-4 Combustor Efficiency Source: Page 445

10 Turbine Performance Map To achieve high power to weight ratio, flow entering 1 st stage rotor is normally supersonic. Thus, sonic conditions are present at the minimum passage area of the inlet guide vanes Figure 7-5 Turbine Flow Map Source: Page 446 Turbine Performance Map Turbine efficiency assumed as constant during performance analysis Figure 7-6 Turbine Efficiency Map Source: Page 447

11 Single-Spool Turbojet w/o Afterburner Performance Analysis Variables for Single-Spool Turbojet Engine Source: Page 465 Single-Spool Turbojet w/o Afterburner Source: Page

12 Single-Spool Turbojet w/o Afterburner Source: Page 469 Single-Spool Turbojet w/o Afterburner Adapted from R-1, Example 8.4 Source: Page 473

13 Single-Spool Turbojet w/o Afterburner Adapted from R-1, Example 8.4 Source: Page 474 Single-Spool Turbojet w/o Afterburner Adapted from R-1, Example 8.4 Altitude = 20 kft Mach number = 0.8 As thrust is reduced from 100% (reading graph from right to left), o η P increases o η T decreases o η O increases to optimum at 40% power setting o TSFC decreases until about 40% of maximum thrust, then increases Turbojet Performance at Partial Throttle Effect due to increase in η P dominating the reduction of η T at throttle settings down till 40%. Source: Page 476

14 Summary The basics of engine performance analysis The need for design point and dimensional analysis Plot and critique component performance maps Reflection Question Review the component performance maps of a compressor and turbine: Explain the compressor stall/surge line. Discuss the rationale that the flow at the turbine inlet nozzle is assumed to be choked.