Talk:Jet Propulsion/Thermodynamic Cycles

Ideal Jet‐Propulsion Cycle
Gas-turbine engines are widely used to power aircrafts because of their light-weight, compactness, and high power-to-weight ratio. Aircraft gas turbines operate on an open cycle called jet-propulsion cycle. Some of the major differences between the gas-turbine and jet-propulsion cycles are:  gases are expanded in the turbine to a pressure where the turbine work is just equal to the compressor work plus some auxiliary power for pumps and generators i.e. the net work output is zero  since the gases leave at a high velocity, the change in momentum that the gas undergoes provides a thrust to the aircraft  the fluid passes through a diffuser first where it is decelerated (gas pressure increases)  typically operate at higher pressure ratios, often in the range of 10 to 25 Fig.1: Schematic of a turbojet engine. Fig. 2: T-s diagram for ideal turbojet cycle. Diffuser Compressor Burner Turbine Nozzle 1 3 4 5 6 2 Vinlet T s 1 2 3 4 5 6 Qin Qout P = const. P = const. VexitM. Bahrami                    ENSC 461 (S 11)              Jet Propulsion Cycle                                         2 Diffuser (1-2)  decelerates the incoming flow relative to the engine  a pressure rise known as a ram effect occurs, V (↓), P (↑). It can be explained through the Bernoulli’s equation: Const. 2 1 2 P  V  gh  Compressor, Burner and Turbine (2-5) 2-3: isentropic compression 3-4: constant pressure heat addition 4-5: isentropic expansion through the turbine during which work is developed  turbine power just enough to drive the compressor  air and fuel are mixed and burned in the combustion chamber at constant pressure  air velocity leaving the turbine is small and can be neglected Nozzle (5-6)  isentropic expansion through the nozzle, air accelerates and the pressure deceases  gases leave the turbine significantly higher in pressure than atmospheric pressure  gases are expanded to produce a high velocity, Ve >> Vi results in a thrust The pressure at the inlet and the exist of a turbojet engine are identical (the ambient pressure); thus the net thrust developed by the engine is: F mV mV m    Vexit Vinlet N exit inlet                    For an aircraft cruising in still air, Vinlet is the aircraft velocity. The power developed from the thrust of the engine is called the propulsive power: WP  FVaircraft  m    Vexit Vinlet Vaircraft kW   The thermal efficiency is defined based on the propulsive power. This then becomes a measure of how efficiently the energy released during the combustion process is converted to propulsive energy.     in P P Q W Input energy (Cost) Propulsive power (Benefit)  Afterburner Afterburner is popular in military aircrafts and it is used whenever a need for extra thrust arises, such as for short takeoffs or combat conditions. Afterburner is similar to a reheat device; it is located after the turbine and before the nozzle. It produces a higher M. Bahrami                    ENSC 461 (S 11)              Jet Propulsion Cycle                                         3 temperature (and pressure) at the nozzle inlet, results in  an increase in velocity (and thrust). Fig. 3: T-s diagram for an ideal turbojet with afterburner cycle. Fig. 4: Schematic diagram for a turbojet engine with afterburner. Turboprop  gas turbine drives the compressor and the propeller  most of the thrust is from the propeller  works by accelerating large volumes of air to moderate velocities  propellers are best suited for low speed (less than 500 mph) flight  by-pass ratio of 100:1 or more massflow through the combustion chamber massflow bypassing the combustion chamber Bypassratio  T 1 s 2 3 4 5 6 Qin Qout P = const. P = const. 7 Qafterburner Diffuser Compressor 2 Burner Nozzle 1 3 4 5 6 Vinlet 7 Vexit Afterburner TurbineM. Bahrami                    ENSC 461 (S 11)              Jet Propulsion Cycle                                         4 Turbofan The most popular engine in aircrafts is the turbofan (or fanjet) where fan driven by the turbine forces a considerable amount of air through a duct (cowl) surrounding the engine.  best choice for fuel economy and speed  high speed exhaust gases are mixed with the lower speed air in the by-pass resulting in a considerable noise reduction  by-pass ratio can be adjusted  by-pass provides thrust for takeoff the core provides thrust for cruising  typically used for speeds up to 600 mph  typical by-pass ratios are 5-6 Fig. 5: Schematic of a turbofan engine. Ramjet A ramjet is properly shaped duct with no compressor or turbine. It is used for high-speed propulsion and missiles. Compression is achieved by decelerating the high-speed incoming air in the diffuser aircraft must already be in flight at a high speed. Ramjet is typically used in aircraft flying above Mach 1.