Turbofan Engines Essay (Essay Sample)

Turbofan Engines
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Turbofan Engines
Turbofans are airbreathing jet engines that are used to propel aircrafts. Turbofans are the most up-to-date variation of the simple gas turbine engines, providing a high thrust and excellent fuel efficiency. Turbofans have a core engine surrounded by an anterior fan and an extra turbine at the rear (NASA, 2014). The fan, fan turbine, core turbine and core compressor have many blades and are linked to an additional shaft. Some of the fan blades turn in unison with the shaft, whereas others remain stationary. The shaft of the fan traverses the core shaft, creating an arrangement referred to as a two spool engine (NASA, 2014).
Turbofan engines use a combination of a jet exhaust nozzle and a ducted fan to provide thrust. The oxygen that is needed to burn fuel for the creation of power comes from a portion of air streaming from the ducted fan and passing thought the core. The other bigger portion of the air flow bypasses the core of the engine to mix with the faster stream from the core. Thrust is produced more efficiently by a significantly slower bypass airflow than a high-speed air from the core. The ratio of the air bypassing the engine core to that going through the core gives the bypass ratio. Since the rate of fuel flow for the core only changes to small quantities by the addition of the fan, more thrust is generated by a small amount of fuel used by the core (NASA, 2014). Therefore, turbofans have a high fuel efficiency, with those that have a high bypass ratio being the most efficient.
Turbofans are used in high speed transports because of their efficient operation at higher speeds compared to simple propellers. The many blades on the fan that is enclosed by the inlet are responsible for this high efficiency. Turbofan engines are used in the current commercial jet aircrafts owing to their high efficiency and are quieter than turbojets. Moreover, turbofans are used in many military aircraft, including the F-15 Eagle, and in RQ-4 Global Hawk, an unmanned aerial vehicle (Husain, 2010).
Early turbofans
Limited overall pressure ratio and inlet turbine temperature for turbojet engines owing to low technology came with a high fuel-inefficiency. This necessitated the need for highly fuel-efficient jet engines for propulsion of aircrafts. The first turbofan, Daimler-Benz DB 670, was made in Germany and tested on April 1st, 1943, though it was abandoned due to the problems that were presented by the war (Husain, 2010). The first British turbofan, Metrovick F.3, was made late in 1943, by giving a fan to the Metrovick F.2 axial flow jet. The overall pressure ratio was increased by the continued improvement of materials and the introduction of two compressors such as in the Bristol Olympus. Though these improvements led to more thermodynamic sufficiency of the engines, propulsion efficiency was yet to be achieved.
Figure 1: Early turbo fan (source: pilotfriend.com)
Designing of engines called for improved propulsion efficiency through the reduction of the exhaust velocity, as in the manufacture of the original low-bypass turbofans. The first turbofan, Rolls-Royce Conway, had a bypass ratio of 0.3 whereas civilian turbofans of the 1960s, such as the Rolls-Royce Spey and Pratt & Witney JT8D had bypass ratios nearer to 1. A bypass ratio of 2.0 was achieved with the General Electric CFT700 engine which was since used to propel many aircrafts. Another highly improved early design of the turbofan was the CJ05-23 which was larger.
Low-bypass turbofan
Low bypass turbofans have a high ratio of specic thrust to low bypass due to the presence of multiple fans in different stages, and develop a relatively high pressure ratio to yield a high exhaust velocity. The flow of air in the core should be big enough to provide power to drive the fan by the core. A smaller ratio, core flow to higher bypass, is achieved by increasing the turbine rotor inlet temperature (Husain, 2010).
Figure 2: Low bypass turbofan (source: britanica.com, 2014)
Unlike turbojet engines, low bypass turbofans operate at a higher nozzle pressure ratio though they employ lower exhaust temperature. There is improved specific fuel consumption with the reduction of fuel flow owing to temperature rise across the engine. Some low-bypass military turbofans such as F404 direct the flow of air across the rotor stage using variable inlet guide vanes. This improves the surge margin of the fan in the mid-flow range.
Afterburning turbofan
A big number of the jet fighter engines that have been used since the 1970s are low or medium turbofans having an assorted exhaust, adjustable area final nozzle and afterburner. Located upstream of the nozzle and downstream of the turbine blades, an afterburner burns injected fuel (Huang, & Ji, 2011). The afterburner burns copious amounts of fuel to generate high temperatures of the exhaust gases. Higher exhaust velocity results, generating specific thrust. The adjustable geometry nozzle should open to a larger throat accommodating the extra volume flow during combustion by the afterburner.
A good design for afterburning should lead to generation of substantial thrust during take off, combat maneuvers and transonic acceleration, at high fuel efficiency. Afterburning is only used for a small percentage of a mission (Lin & Zhen, 2012). Modern low-bypass turbofans that have a mixed exhaust, adjustable area propelling nozzle and afterburner, include the Eurojet EJ200, the Pratt & Whitney F119 and the General Electric F110, to mention a few (Husain, 2010).
High-bypass turbofan
High-bypass turbofans have a low ratio of specific thrust to high bypass and are mostly used in modern civil jetliners, including some military aircrafts. To generate a high specific thrust, the multi-stage fan is replaced by a single-stage unit. Modern military engines have stationary inlet guide vanes at the anterior of the fan rotor, this feature is absent in modern civil turbofans (Husain, 2010). Enough core power should be should be generated by the gas generator of the engine to drive the fan. Technological improvements in turbine cooling material, higher turbine inlet temperature, facilitates the improvement of the thermal efficiency. Reduction of the mass of the core increases the load on the LP turbine, therefore leading to the requirement of more stages that reduce stage loading and bring more efficiency of the LP turbine.
Core flow reduction also plays a significant role in the reduction of the bypass ratio (Linke-Diesinger, 2008). Core thermal efficiency can further be improved by increasing the overall pressure of the core. The number of compressor stages required is reduced by improving blade aerodynamics. A significantly raised overall pressure ration is achieved by the use of many compressors whereas an adjustable geometry enables high efficiency for the high-pressure-ratio compressors.
High-bypass turbofans are used in the modern jet airliners due to their high fuel efficiency and reduced noise. Military aircrafts use high-bypass turbofans though due to fuel economy, though modern combat aircraft use low-bypass turbofans (Adams et al., 2012). Higher bypass ratios of turbofans come with less jet outlet velocity leading to decreased thrust with increased speed. High thrust at low speeds...