Some high bypass turbofan engines, e.g., the Rolls-Royce Trent 900 that powers the A380, carry this specialization of compressor-turbine pairs further so as to have three concentric drive shafts with the very inner one dedicated to the fan and its driving turbine. The high bypass turbofan, characterized by a large fan diameter, is illustrated in Figure 4.21. High by pass turbofan engines may be considered to be those with β greater than 4 or 5 current engines have β values as high as 10. Turbofans have a bypass ratio describing the ratio of the mass flow passing through the fan (the “cold” flow) to that passing through the core engine (the “hot” flow) and is expressed as follows: As a consequence all modern commercial airliner engines are turbofan engines the major difference between such engines is in the degree of bypass utilized. In this fashion the turbofan combines the high speed capability of the pure turbojet with the fuel efficiency and good acceleration characteristics of the propeller. The fan accelerates the air passing through it, called the bypass air, by directly doing mechanical work on the air without any appreciable heating of that bypass air. However, the power supplied to the fan is transmitted to the air and therefore acts much like a propeller. The hot gases of combustion are again accelerated through a nozzle to produce pure jet thrust as in the turbojet. In the turbofan engine, as illustrated in Figure 4.21, the turbines extract power to drive not only the compressors but also the fan. The engineering principles of the geared turbofan are essentially the same as those of advanced turboprops and are discussed in detail in Section 10.9. Allowing both the fan and the high-pressure sections of the engine to run at speeds closer to their optimum provides reductions in specific fuel consumption of up to 10%. This type of engine is illustrated in Fig.
#WHAT IS PROJECT M TURBO MODE SERIES#
To deal with the problem of increasing fan-turbine speed mismatch, the P&W PW1000G series applies a gearbox to a mid-size engine in the 25,000–35,000 lb (111–155 kN) thrust class for application to popular single-aisle aircraft like the Boeing 737 and the Airbus A320. Current engines have fan diameters on the order of 3 m, for example, the General Electric GE90 and the Rolls-Royce Trent. In the case of turboprop engine the use of large diameter propellers made it necessary to employ a gearbox between the output shaft of the gas turbine and the input shaft of the propeller to reduce the rotational speed of the propeller by a factor of at least 10. This dichotomy between requiring high rotational speeds to keep compressor diameters, and therefore frontal area, low and increasing the size of the fan to improve fuel efficiency has been dealt with in the past by reaching a compromise position between the two, thereby making optimization questionable. If the compressor rotational speed is such that at the tip of a representative blade u ~ M, then the relative velocity w at the tip of the fan may be double that, but a speed close to M = 2 is not acceptable from either a performance or noise perspective. Therefore a high value of u is beneficial but the limiting factor in this instance is the growth of compressibility effects because u at the tip of a blade can approach the local sound speed. The study of turbomachinery in Chapter 7 showed that the pressure rise in a compressor rotor depends upon the square of the relative velocity w and Eq. The linear speed of rotation u is proportional to Nr, the product of the rotational speed and the radius at which the speed is to be measured. They begin to face the difficulty that was always apparent in turboprop engines, as discussed in Section 10.9. The turbofan engine has distinct limitations as the bypass ratio increases because of larger diameter fans. Sforza, in Theory of Aerospace Propulsion (Second Edition), 2017 9.2.7 Dual-Shaft High Bypass Geared Turbofan