Parametric Cycle Analysis of Real Turbojet

Transcription

1 Parametric Cycle Analysis of Real Turbojet Introduction Parametric cycle analysis of a real turbojet Parametric cycle analysis of a real turbojet with afterburner

2 Introduction Parametric cycle analysis of ideal gas turbine engine: Engine components are idealized and Working fluid is assumed to behave as a perfect gas with constant c p Analysis of engine performance trends Component Performance: Variation of specific heat with temperature and fuel/air ratio More realistic assumptions, e.g. component losses Developed figures of merit of various engine components Real Engines Parametric cycle analysis of real engines: Equations for different engine cycles Determine their performance Determine the effects of real components with losses Source: The Jet Engine (1986) by Rolls Royce plc,

3 Parametric Cycle Analysis of Real Engines Assume one-dimensional flow Similar cycle analysis as ideal engine except Variation of specific heats : c pc and c pt for upstream and downstream of the combustor respectively Inclusion of component losses, Mass flow of fuel through components Pressure at exhaust nozzle is not necessarily atmospheric Real Turbojet Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

4 Ideal Turbojet T-s Diagram for Ideal Turbojet Engine Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Real Turbojet T-s Diagram for Real Turbojet Engine Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

5 Real Turbojet Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Real Turbojet Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

6 Real Turbojet Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Effect of e c on Turbojet Cycle R-1, Example 7.3 Mission consideration: Short range interceptor -Lowπ c preferred for high F/ṁ 0 as compressor size is small and weight is reduced As π c increases, the effect of engine losses increases. Optimum π c is affected by e c Long-range transport prefer high π c to give lower Sfor operating efficiency Source: Example 7.3 Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

7 Effect of Nozzle Off-Design Conditions R-1, Example 7.3 π c = 16; e c = 0.92 Effect of slight exit pressure mismatch is small Source: Example 7.3 Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Effect of Speed on Real Turbojet Note: thrust for real engine falls faster at high M 0 than f Source: Example 7.4 Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

8 Effect of Speed on Real Turbojet R-1, Example 7.4 Real cycle S increases exponentially since thrust for real engine falls to zero at high M 0 before fdoes Thermal efficiency at high M 0 is zero for real engine but increases in ideal engine Source: Example 7.4 Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Effects of Component Losses on Performance Performance Parameter Specific Thrust, Thrust Specific Fuel Consumption, S Fuel/Air Ratio, f Ideal Turbojet Maximum occur at a higher Goes to zero at high Lower S Decreases with increasing Lower f Real Turbojet Similar variation Similar magnitude Maximum occur at a lower (for same ) Goes to zero at high with lower Higher S Decreases to an minimum, then increases with increasing Increases with velocity, and rapidly towards infinity at high Higher f due to increase in c p, combustor inefficiency and higher combustor exit massflow ( + ) Propulsive Lower Slightly higher due to lower exhaust velocity Efficiency, Thermal Higher Lower Efficiency, Goes to zero at high for engine with high Overall Higher Lower due to lower Efficiency,

9 Real Turbojet with Afterburner Figure 6-4 Station Numbering of an Afterburning Turbojet Engine (adapted from R-1, page 399) Input: Output: Refer to Study Guide for equations Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly Effects of Afterburning Afterburner increases both specific thrust and fuel consumption Magnitude of increases depend on π c π c is important in gas turbine engine design Mission type dictates π c choice Figure 6-6 Performance of Turbojet Engine with and w/o Afterburner Source: Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly

10 Summary Concept and methodology of parametric cycle analysis of a real turbojet, without and with afterburning Effects of assumptions and losses on engine performance Effects of afterburning on turbojet output Reflection Question Perform a parametric cycle analysis on a turbojet engine, using the input from Example 7.1 (pages ) of the reference, Elements of Propulsion: Gas Turbines and Rockets by Jack D. Mattingly.