The Gas Turbine Engine Engineering Essay
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According to Marshall Brain, pressurized gas spins the turbine in a gas turbine engine. In recent times gas turbine engines produces their own pressurized gas by burning propane, natural gas, kerosene, diesel or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed charge of this hot air causes the turbine to spin (Brain). Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks (contributors, Gas turbine).
Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed without an increase or decrease in the entropy, combustion occurs at constant pressure, and expansion over the turbine occurs without an increase or decrease in the entropy back to the starting pressure (contributors, Gas turbine). They have a very simple design and consist of three main parts the compressor assembly, combustion assembly, and the turbine assembly. According to Marshall Brian the first step is for the Compressor to compress the incoming air to high-pressure. Then the Combustion area burns the fuel and produces high-pressure, high-velocity gas. Next the Turbine extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber (Brain). Finally the turbine drives controllable pitch propellers through a conventional main reduction gear, shaft and clutch (Sherman).
Gas turbine engines have some advantages over diesel engines. As said by Marshal Brain, they have a great power-to-weight ratio compared to other internal combustion engines. That is, the power you get out of the engine compared to the weight of the engine itself is excellent. Gas turbine engines are smaller than their internal combustion engine counterparts of the same power (Brain). This engine also has fewer moving parts than internal combustion engines and low lubricating oil cost and use. In agreement with Robert Sherman, another advantage of a gas turbine engine is its ability to be removed from the ship for major repair in a small period of time roughly 72 hours. A set of rails is permanently fixed in the intakes of each engine. Another set of transition rails, are also temporarily fixed to allow the disconnected engine to move from its mounting attachments in the module onto the rails in the intakes (Sherman).
Consistent with Marshall Brain, the main disadvantage of gas turbines is that, they are expensive compared to an internal combustion engine of the same size. Because they turn at such high speeds and because of the high working temperatures, designing and manufacturing gas turbines is a hard-hitting dilemma from both the materials and engineering point of view. Gas turbines work better under a constant rather than a fluctuating load. They also have a tendency to use more fuel when they are idle (Brain).
In accordance with Wikipedia, The first U.S. gas-turbine powered ships were the U.S. Coast Guard's Hamilton-class High Endurance Cutters in 1967. In modern time they have powered the U.S. Navy's Perry-class frigates, Spruance-class and Arleigh Burke-class destroyers, and Ticonderoga-class guided missile cruisers. The Wasp-class amphibious assault ship USS Makin Island (LHD 8) is to be the U.S. Navy's first amphibious assault ship powered by gas turbines (contributors, Gas turbine).
The gas turbine of choice for the U.S. Navy is the General Electric LM2500. The LM2500 incorporates many features which get the most out of shipboard maintenance and minimizes parts replacement downtime, in-place blade and vane replacement, such as a split compressor casing, in-place hot section maintenance and easy to get to external fuel nozzles because of its straightforward modular design (LM2500 Marine Gas Turbine).
According to the LM2500 marine gas turbine datasheet this engine has a simple-cycle, two-shaft, high performance engine. It is developed from GEâ€™s TF39 and CF6-6 aircraft engines. The LM2500 has a gas generator, power turbine, attached fuel and lube oil pumps, a fuel control and speed governing system, associated inlet and exhaust sections, lube and scavenge systems as well as controls and devices for starting and monitoring engine operation (LM2500 Marine Gas Turbine).
Consistent with LM2500 marine gas turbine datasheet:
The four major parts of the LM2500 include a 16-stage, 18:1 pressure ratio compressor with seven stages of changeable stators and inlet guide vanes; a fully annular combustor with externally mounted fuel nozzles; a two stage, air-cooled high-pressure turbine which drives the compressor and the accessory-drive gearbox; and a six-stage, aerodynamically coupled, low-pressure power turbine which is driven by the gas generatorâ€™s high-energy exhaust gas flowâ€¦ the LM2500 has a simple modular design, marine corrosion-resistant materials and minimum size, weight and space requirements per horsepower. The engine delivers high thermal efficiency and low fuel and airflow per horsepower (LM2500 Marine Gas Turbine).
According to GE Aviation, The LM2500 is located in a high-shock resistant and thermal enclosure. The enclosure also weakens the noise level in the engine room and assists the resilient shock mounts in reducing the shipâ€™s overall noise on the water. It also provides sensors for possible problems such as inlet icing and fire (GE Aviation; LM2500 Marine Gas Turbine).
The modules come to the ship yard ready for installation. The LM2500 module weighs 45,000lbs. plus 3,000lbs when shock mounts are added. It requires only 2430 cubic feet of ship space and the inlet and exhausts duct flow area and is less than 48 square feet. (LM2500 Marine Gas Turbine).
Because of high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly Gas turbines are used in many naval vessels. This engine design has proven itself to be very reliable over its long history.
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