Abstract
General Electrics specialized 115B variant of its GE90 engines have perfected the processing of air molecules for commercial air travel efficiently and economically deliver flying capabilities. The aircraft engine exaggerates the qualities of the long-distance turboprop engines of its time, such as its 128-inch diameter inlet, which is the world’s largest at the time of creation. It also enables the aircraft that use it to perform with less emission, less noise pollution, and lower fuel to thrust ratios. Most statistical data within this research paper is derived from the engines European Union Aviation Safety Agency type certificate data sheet. The engines descriptive data is derived from a Stanford research article on the engine and the aircraft propulsion textbook. The findings of this paper result describe the unique characteristics of the GE90 engine. Additionally, the description of how an air molecule will flow through the engine in a tailored format that matches the engines specifications.
History of the GE90-115B
The company, General Electric, was introduced into the aircraft engine making business after the United States entered the first World War in 1917. The United States goal set to manufacturers was to create the first airplane engine booster or turbosupercharger on a piston engine. Of course, General Electric produced the turbosupercharger Liberty aircraft engine that produced 350 horsepower at 14,000 feet above sea level. General Electric turned their business into an aircraft engine manufacturer and continued to build engines, systems, and services with over $30 billion in revenue. The company went on to create Americas first Jet engine, one of the highest, fastest bombers, and became one of the leading manufacturers in the United States. (GE Aerospace, n.d.).
With such an extensive reputable background, it is no surprise that this engine was built by General Electric, alongside the Safran Aircraft Engines of France, IHI of Japan and FiatAvio of Italy. The objective of building this engine was to tailor it for the optimal use with the Boeing 777 aircraft which weighs about 500,000 lbs. The outcome of the matched engine is that the GE90 series have varying take-off thrust of calculations of 81,000 lbf to 115,000 lbf. Even at the lower range of 81,000 lbf, it only requires two GE 90 engines to depart and fly this massive aircraft (Cantwell, 2010).
One of the reasons the engine can have so much power in a single engine because of the General Electric and National Aeronautics and Space Administration’s collaboration in the Energy Efficient Engine program. This program has influences on this engine that has made it the most fuel efficient, silent, and environmentally friendly engine in the world. Which in return gives airliners, specifically ones that fly the Boeing 777, around a 5-6% fuel efficiency improvement, lower sound pollution, and 33% less emissions than other high-bypass ratio engines (Cantwell, 2010).
Design and Operation of Turbofan Engines
The GE 90-115B is a high-bypass turbofan aircraft engine that is capable of producing up to 115,000 lb-ft of thrust. (Cantwell, 2010). The design of this engine was meant to match the Boeing 777 and have optimal performance for the weight, size and center of gravity that comes with this plane. Specifically, the 115B derivative of the GE 90 was meant to serve the Boeing 777-200LR and 777-300ER, which are made for long range operations. The design of the engine holds the world’s largest (128 inch) fan with composite fan blades, and it has the highest engine bypass ratio (9:1) for optimal long-distance performance. The next plane that is in GE’s production is meant to improve on the GE90, called the GE9X. This one will have a 134-inch fan diameter and will only have 16 fan blades, instead of the GE90-115B’s 22 fan blades (GE Aerospace, n.d.).
Configuration and Performance Parameters of the GE90-115B
To get the exact configuration and performance parameters for this engine, let’s look at exactly what type of engine it is off its type certificate data sheet. The stated description is “Dual rotor, axial flow, high bypass ratio turbofan. The 10- stage [9-stage] high pressure compressor is driven by a 2- stage high pressure turbine. The single stage fan and 3- stage [4-stage] low pressure compressor is driven by a 6-stage low pressure turbine. The engine includes the starter and the engine mount and does not include the thrust reverser”. The dimensions are 286.7 inches overall length, 148.4 inches overall width and 154.6 inches overall height with a weight of 19,316 pounds. This aircraft has a maximum continuous low pressure rotor speed of 2,602 rotations per minute and high-pressure spool speed of 11,292 rotations per minute (EASA, 2019).
An Air Molecules Journey Through the GE 90-115B
In this research papers presented situation, there is an air molecule that is floating around the exact time and in the airspace that a Boeing 777-200LR or 777-300ER aircraft would be flying. This molecule will begin its journey around 100 feet in front of the aircraft, before the aircraft flies into it and sucks it up. The presented journey below is what the air molecule will experience and the way that the engine will process the air molecule during its normal cruising airspeed and cruising altitude. This process will begin by being sucked up into the engine, to being pressurize while going through the 13 stages of compression, and then used to light and explosion that rapidly expands to be thrown out of the back as exhaust thrust. The journey will end as the newly minted molecule decelerates into the surrounding air, 100 feet behind the aircraft.
First 100 Feet
There is not much that should be happening at the beginning stages of this journey. The Boeing 777 is certified to fly as high as 43,000 feet in the air but is most efficient around 35,000 to 39,000 feet so it is where you will normally see them for long distance flights. Therefore, the air molecule will be floating around relatively stagnant around the starting altitude, right in the middle at 37,000 feet.
At this altitude, there are some characteristics that are different than an air molecule at ground level. The aircraft is just exiting the troposphere at this altitude and entering the stratosphere. In the troposphere as altitude increases, the temperature of the air will decrease, but the stratosphere starts the line where the opposite starts to occur. This is because the sun’s ultraviolet radiation begins warming the air up, the same radiation that is responsible for reduced turbulence and shutters. Here, the temperature is around negative 30 degrees Celsius, the absolute pressure is around 1000 newtons per meter squared, and the density is about 0.25 kilograms per meter cubed (Engineering Toolbox, n.d.).
Even though Nitrogen is a lighter gas than Oxygen, the composition of air does not vary with altitude. However, the oxygen percentage will decrease as altitude increases and at this altitude, the oxygen percentage is lower than 6.3%, which is around the percentage at the max altitude training facility and about one percent less than what it would be on Mount Everest. The reason it does this is because as pressures decrease with altitude, the air molecules widen the gap between each other, which gives less oxygen, but not less percentage of composition (Oxygen Chart, n.d.).
Intake
Whenever the air molecule first interacts with the engine, it will be from air diffusion or the forced air flow into or away from the aircraft’s subsonic inlet. The airflow forced away from the inlet is external diffusion, while the air that is captured in the steam tube continue into the internal segments. The air molecule we have for this example will inter the inlet capture area (Figure 1). One of the most prominent and important aspects of the GE 90-115B aircraft is the world’s largest fan at 128 inches with 22 composite fan blades on it. This means that the inlet capture area of this engine will have one of the largest inlet capture areas of any commercial jet in use (GE Aerospace, n.d.)
Figure 1. From Aircraft propulsion (2nd ed.), by Farokhi, S.
Compression
Once the air molecule passes through the air intake section of the engine, it is then time to pass through a few compressors to get the maximum air pressure possible for combustion. From the type certificate data sheet, the GE90-115B has a 1 fan 4-stage low-pressure compressor and a 9-stage high-pressure compressor (EASA, n.d.). This engine uses a multistage axial compressor because it sends the compressed air flow parallel to the axis of rotation. Which is the most modern and efficient way for large turbojets to perform.
The theory behind the GE 90-115B’s multistage axial compressor, and what also makes it different from centrifugal and single stage axial compressors, is that it builds pressure over time or more precisely over stages. So instead of a robust compression of air in a single centrifugal or axial compressor these engines multistage compressor will use all of its 13 stages to slowly and incrementally build the air pressure within the engines. When looking into how this will affect this situations air molecule scenario, the air molecule can expect to flow through the engine and all the stages at a relatively slow rate, gaining pressure and get closer to other air molecules as it goes through the process.
The purpose of the compressor is to increase the fluid pressure efficiently so that is what will happen to the air molecule during this stage. The expectation is that the air molecule will be compressed to high pressures and temperatures before being sent to the next stage of engines cycle, combustion.
Combustion
The GE90-115B engine runs a dual dome annular combustor that has qualities that have been derived from multiple advanced military programs. The design allows less unburned hydrocarbon, carbon monoxide, smoke, and has exhausted nitrous oxide emission levels as low as 10 ppm. This improved operability ensures longer life of the engine, longer time between maintenance, and an increased efficiency throughout the entire engine. Another highlight of this engine is that it has a re-light capability all the way up to 30,000 feet, which is big because multistage axial compressor engines are known for stalling and having starting problems (Cantwell, 2010).
As you can expect, this part of the process comes with a lot of kinetic energy and results in a high temperature. This engine is rated for maximum continuous temperature at 1,922 degrees Fahrenheit, but it can grow higher for small stints of time for unique situations (EARA, n.d.). As an air molecule heats up, it will move faster and further apart from other air molecules which is just saying a group of air molecules will expand when it is heated. During this stage, the already pressurized air molecule will be heated to extreme temperatures causing a rapid expansion of the air molecules, which will be forced to go somewhere, which is out of the back of the engine.
The already highly pressurized air molecule will be used in the combustion chamber to produce an explosion, resulting in more pressure and a rapid release of kinetic energy blasting toward the next stage in the process. However, before this molecule of air is able to be turned into the aircraft exhaust, it will first be sent through turbine assembly. This assembly is attached through the engine to the fan blades within the beginning stages of the engine near the inlet. Therefore, before the combusted expanding air is being forced out of the back of the engine, it is first being used to give more energy to the inlet turbine blades to continue sucking up new air for the engine.
Exhaust and the Last 100 Feet
As mentioned above, the last stage in the engines cycle is blowing all of the pressurized high energy air out of the back of the engine. Part of this process includes pushing air through the high-pressure turbine fan and producing additional energy for the inlet fans through torquing a connected rod. This portion was explained above in the combustion section on what the air molecule will go through before passing to the nozzle. For the exhaust of the aircraft, the 115B model of the GE90 can produce 85,000 lbs of thrust on takeoff and around 115,000 lbs of thrust during cruise (Cantwell, 2010).
The GE90-115B has a simple geometry convergent co-annular nozzle that is meant to simply converge the hot gasses from the engine into a central point, compressing the air one last time before exhausting it into ambient air. The co-annular portion of the nozzle is used to let two separate streams of airflow escape the engine. The outer annular nozzle will allow the fan flow air to escape, while the central nozzle allows normal operations of air to flow. The operation of these two nozzles allows the air to mix on exit and it also serves as a great way to achieve noise reduction.
The air molecule for this situation will be flowing out of the central convergent nozzle. Once the air molecule has been forced out of the engine it will meet other air molecules that have been subject to the aircrafts forces like drag, but they are relatively stagnant air molecules. The situations air molecule will push up against the stagnant air like a wall before disepating and losing its energy.
The escaping air molecules interaction with ambient air molecules is not unlike a human jumping off a 10-foot cliff into water or a 100-foot cliff into water. The human jumping off a 10-foot cliff into water will not garner a lot of energy in the short distance to the water. Therefore, the water molecules will separate and allow the human to flow into the water without much resistance. The human jumping off a 100-foot cliff into water will garner a lot of energy and will enter the water at a high speed. Once the human contacts the water, the water will not have as much time to separate to allow the human in. Therefore, the human will abruptly stop at the surface of the water, creating a harder impact and more energy.
This air molecule will have a high escape velocity so it will be abruptly met with the ambient air and force the aircraft in the opposite direction, creating thrust. The air will rapidly lose energy until it settles in with the surrounding air molecules. The situations air molecule can be expected to also influence the surrounding air and cause all the air to move, but eventually settle.
Conclusions and Lessons Learned
The 115B variation of the GE90 engine was one of the aviation world’s greatest contributions to efficient commercial air travel, especially when paired with a long-range model of the Boeing 777 aircraft. The engine, having one of the biggest inlet diameters, is able to take an enormous amount of stationary air molecules and begin cycling it through its engine. The 13 stages of compression that it has is able to compress the air and create a high amount of air pressure and temperature to prepare it for combustion. Once processed through the compression, the air enters the combustion chamber, where it is used to ignite an explosion and release kinetic energy. This energy is then processed through turbines, that torque the inlet fan blades, before being ejected out of the back of the engine as thrust. The air molecule then settles and is ready to be processed by another aircraft engine.
References
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