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High Bypass Turbo Fan Engine Essay

Abstract

Turbofan engines have continued to gain popularity in aviation industry despite their complexity and cost. This because they have good range, fuel efficiency and low noise emission. They are mainly used on long range commercial airline plane and transport military planes. The design of these engines is based on gas turbine technology and fan engine. A typical high bypass turbofan engine design combines the principle of a gas turbine engine but in addition in has an intake fan in front of the engine. The gas turbine engine also called turbo take its power from burning fuel and it generates a spin that turns the ducted-fan that in return, push air rearwards. Marshall (2010) elucidates that air that is drawn in by the fan is divided into two parts, where one part passes through the turbine and the other part, go around it in defferent ratios. Depending on this ratio a turbofan engine can be referred to as high bypass or low bypass engine. In this paper I will examine the high bypass turbofan engines, what it is, how it work and why it is popular in commercial airlines planes. I will analyze its main parts and how they work and their contribution to the function of the engine..

High Bypass Turbo Fan Engine

Aviation industry has seen great development in engines. Since the first aeroplane, various engine designed have been made and used. Currently piston engine, turboprop engines turbojet, rocket engine and turbofan engine are in use on different aircrafts around the world.  Turbofan engines have continued to gain popularity because they have better propulsion efficiency at high speed compared to turboprop engines. They also have a better range than both turboprop and turbojet. Comparing these engines to turbojet engines, turbofan engines are more fuel efficient and emit less noise, because of this these engines are common on most commercial airline planes and military transport planes. Turbofan engines are divided into two, there is the high bypass ratio engine and the low bypass ratio engines depending on the amount of air that is allowed to go around the core and the amount allowed to go through the core of the engine. Although turbofan engines may appear fairly complex and may require advanced technology as well as expensive materials to manufacture, in design they are basically gas turbine engines with a fan in front. Marshall (2010)

High Bypass and Low Bypass Turbofan Engine

In the  modern aviation industry, most commercial and military planes are powered by different type of engines.  Although, there are other engines like turboprops and piston engines, Turbofan engine have continued to gain popularity because these engines produces more power compared to other engine of the same size such as piston engines.  In addition to this, they also have better fuel efficiency, better range and less noise emission than pure jet engines.  Spittle (2003) Turbofan engines are similar to traditional gas engines in many ways. Example of some of the gas engines are found on helicopters,  as well as some military tanks such as M-1 tank.

Electricity generating turbines, such as hydroelectric plants, nuclear plants, and wind mills work on the same principles of gas engine.  According to Spittle (2003), turbofan engines sucks in air, compress it then mixes the air with fuel and burns the mixture then the exploding exhaust is expelled through the nozzle rearwards and in return it gives the plane equal push in the opposite direction called the thrust. Unlike pure jet engines, the turbofan engine combines the gas turbine engine technology with a large air fan added in the front of the engine. Marshall (2010) posits that, the gas turbine part also called turbo, gets its power from the burning mixture of the compressed air and  the fuel  burning in the combustion chamber. When the air turbine is powered,it rotates and drives the fan via a connecting shaft.

The fan in turn, pulls in air through the inlet in front of the engine. This air mass  pulled in, splits into two parts, one part of the air mass goes through the air turbine while the other part bypasses the air turbine. Marshall (2010) further explains that , the two masses of air combines again at the nozzle and exit together as one mass.

The thrust of the turbofan engine is  a resultant of the combined velocity of the two portion of air masses as they exit  the engine rearwards. The ratio of air mass flow that bypasses air turbine to the mass of air that passes through the air turbine is called the bypass ratio.  As explained by Spittle (2003) the high bypass engines are designed in such a way that relatively more air mass bypasses the  air turbine and consequently the combustion chamber while low bypass engine allows more air mass to go through the air turbine and burn in the combustion chamber.

 figure 1: High bypass turbofan engine 

In a turbofan engine, the air mass that flows around the turbo travels at a lower speed compared to the air that go through the core.  This is because air that passes through the turbine, after being compressed and burnt, it explodes out of the combustion chamber through the nozzle at very high velocity. As this air mass exits the engine nozzle at high velocity, it makes a lot of noise because of the pop effect as on a pure jet engine or a gun. As stated by Spittle (2003), when this air mass combines with the low velocity air mass from the bypass portion, the resultant air velocity is reduced to a greater extend. Consequently, this makes the high bypass turbofan engine less noisy.  The results of this, is that some propulsion efficiency is lost because of reduced air mass exit velocity. For this reason, high bypass engines are used more on commercial airlines planes which are less noisy and which flies at a relatively lower speed while low bypass engines and pure-jet engines are more common used on military planes that enable them to fly at supersonic speeds a factor that very critical in military tactic despite the noise. Low bypass and pure-jet engine also have an after-burn capability which is not possible in high bypass engines as explained by Spittle (2003).

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High Bypass Turbofan Engine Parts

From the above analysis, it is clear that a high bypass turbofan engine is a turbofan engine that allows a large portion of air mass pulled in by the fan to go around the air turbine combustion chambers as compared to the part that is allowed to pass through the air turbine and combustion chamber. Generally the high bypass turbofan engine has five main sections which are; air intake unit, compressor unit, combustion section, turbine unit, and nozzle. Marshall (2010). This  parts are analysed as follows.

Air Intake Unit

According to Sirignano and Liu (1999), air intake unit is a large fan located at the front of the engine. The fan is connected to the turbine by a shaft and rotates together with the turbine. Refer to figure 3. When the turbine is powered it spins the fan that in turn draws in air. The fan forces the air into the engine compressor section. Part of the air goes through the turbo combustion system where it compressed and burned together with fuel in the turbo unit. The other part of air mass travels around the combustion chamber and exit rearwards as bypass mass. As per Sirignano and Liu (1999) this bypass air generates a thrust force as it exit through the nozzle. This thrust force of exiting air mass is derived from the intake fan. Therefore, the fan has two functions, first to pull in air and second to generate thrust force through the bypass air mass.

Compressor Unit

This is cone-shaped cylindrical part of the turbofan engine core with vanes blades around it that pushed air into combustion chamber. The compressor is driven is by the turbine via a connecting shaft. It rotates at high speed adding energy to the air from the inlet increasing pressure and temperature. When designed a turbofan engine the challenge is maximize compression ratio and compression efficiency while control weight and avoiding too complexity in the engine design.  The more air is compressed into the combustion chamber the more power is extracted from the fuel to drive the turbine. The compressor has to two parts: low and high pressure sections. At various points some compressed air is bleed to perform secondary activities such as cooling, pressurize the passenger compartment, and de-icing. Earlier compressors were able to generate pressure up to a ratio of about 5:1. Today because of improve aerodynamic achieved by splitting the compressor into two and use of guiding vanes and blades angled to reduce air flow losses, the turbofan engine can achieve a pressure ratio of 44:1 and above. Refer to figure 2 below.

figure 2: Compressor part of high bypass turbofan engine 

The compressor unit forces air at high pressure into the combustion chamber. The purpose of this is to supply as much air as possible into the chamber to support combustion. Marshall (2010)

Combustion Section

This is an area where fuel burn to generate power. High pressurized air is push in the combustion area by the compressor turbine and a ring of fuel injectors injects fuel. As the mixture of fuel and air burns it generates gasses that rush through the turbine is spins it at high speed. The combustion in turbofan engine is continuous flow process unlike in piston engines. The combustion chamber is windy because of the air enters the chamber at high pressure and speed at hundreds of miles per hour. It is hard to keep a steady flame burning continuously in such environment. To solve this problem the chamber has a device called “flame holder” this is a can like perforated piece of strong metal. Refer to figure 3. The “flame holder” is a perforated can rapped inside another can. When compressed air enters the compustion unit it is guide through the perforation where it is mixed with fuel and burn without blowing off the flame. Care must taken to ensure that combustion is maintain within the turbin temperature limits, to achieve this, normally only about 25% of air is used in combustion. Marshall (2010)

Figure 3: Combustion chamber 

Turbine Unit

This is the spinning unit that convert energy of gas to mechanical spinning motion. The turbine is cone-shape cylinder with vanes around it similar to compressor but relatively smaller in size. It is located behind the compressor and immediately after the combustion chamber. The turbine is powered by the escaping gasses from the combustion chamber. According to Sirignano and Liu (1999), as a mass of gasses pass through its vanes it is spin around at a very high speed. The turbine is connected to the compressor and to the ducted fan by a shaft and they operate as a single unit. Therefore, as the turbine spins, it drives the compressor and the fan. Refer to figure 4 below

Figure 4: Turbine

In a gas turbine engine, there is a turbine at the rear final stage of the turbine system. This may have a single vane or multiple vanes depending on the type of the engine. This part of the turbine is separated from the rest of the turbine system by freewheeling and bearing and spin as a stand-alone unit completely separated without connection to the rest of the engine. The exhaust gasses that flow through this generate huge power. Example in In M-1 tanks this generates 1500 horsepower that propels 63 tons mass of the tank. Spittle (2003)

Nozzle (exhaust) unit.

After the combustion the exhaust explode rearwards through a nozzle. Consider the figure 5 below. The escaping gasses, blow out at very high velocity and generates forward thrust. Spittle (2003)

This thrust force pushes the plane foreword. The speed at which the gasses escape determines the power and the speed of the power. High speed generates more power and makes the graft very fast. However, high speed also generates a lot of noise due to pop effect for example subsonic military crafts. To reduce the noise turbofan engine combines the exhaust from the turbine which air through the bypass. The combined mass of air will have a relative lower speed and consequently generate less noise although this reduces thrust. According to Sirignano, Delplanque, and Liu (1997) this is why low bypass turbofan engines are used on military jets and high bypass engine are more on commercial air crafts and military transport. Generally low bypass turbofan engines have higher propulsion efficiency but more noise compared to high bypass turbofan engines.

The Future of High Bypass Engine

Manufacturer in aviation industry like other industry face stiff competition from each other. Because of this, designers continually revise their technology to remain ahead of competition. The focus of improvement of high bypass turbofan engine is to produce a more efficient and powerful engine. For example Rolls-Royce has commission research in designing a utra-high bypass engine. This engine expected in the market in the year 2025 with designed base on 3- spool model. The engine is expected to improve propulsion efficient and improve the range of the planes.  Spittle (2003) similarly, as the world move towards green technology, the future high bypass ratio engines are expected to improve in reduction of engine emissions and noise.  The applicability the engine is also an area of concern. The engines are generally used in aircraft, but in future they could be used in other transport modes like skycar. Czyzewski (2011)

Conclusion

High bypass turbofan is a genius piece of engineering. It combines technology developed in aviation industry over time. The technology and material used are all cut-edge. The engine is ideal in commercial airline industry. It has more range efficiency than other airplane engines such as turboprop and piston engines. The discussion in this paper is obviously not exhaustive about the turbofan engines. The engines operate at high speed, temperature, and pressure consequently demands major engineering considerations in the area of bearing and lubrication, this is not covered in this paper.  Nevertheless, it is clear that turbofan engines are very successful and will continue to be used in commercial airline into the future.

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