Gas Turbine Engine | Turbine Nozzle Inlet Guide Vane

Gas Turbine Engine

Gas Turbine Engine | Turbine Nozzle Inlet Guide Vane | Hottest part of gas turbine engine

Gas Turbine Engine

A gas turbine engine which is also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts of gas turbine engines are common to all types of gas turbine engines.

The gas turbine engine is unique in that each function has its own section, and all operations are carried out uninterruptedly and concurrently.

An typical gas turbine engine includes:

  1. An air inlet,
  2. Compressor Section,
  3. Combustion Section,
  4. Turbine Section,
  5. Exhaust Section,
  6. Accessory Section, and
  7. The systems required for fuel delivery, starting, lubrication, and auxiliary functions including de-icing, cooling, and pressurization.

The main sections of all gas turbine engines are essentially the same, but because each manufacturer uses a different language, there are some subtle nomenclature differences among the numerous engines that are currently in use. The relevant maintenance manuals account for these variations. The type of compressor or compressors for which the engine is designed is one of the most important single factors influencing the construction features of any gas turbine engine.

Aircraft are propelled and powered by one of four different types of gas turbine engines:

  • Turbofan
  • Turboprop
  • Turboshaft
  • Turbojet

Early gas turbine engine used in aviation was referred to as a "turbojet." These alternative engine designs were created to replace the pure turbojet engine as gas turbine technology advanced. When flying at aeroplane speeds, turbojet engines have issues with noise and fuel consumption (.8 Mach). Pure turbojet engine use is severely constrained as a result of these issues. So, turbofan engines are used by practically all aircraft of the airliner type. It was created to rotate a sizable fan or group of fans located at the front of the engine, which generates about 80% of the engine's thrust. In this speed range, this engine is quieter and uses less fuel. Many two-shaft turbofan engines have more than one shaft in the engine. This indicates that they are driven by two separate sets of compressors and turbines. Two spools are used in these two-shaft engines. A high-pressure spool and a low-pressure spool are both included in a two-spool engine. Typically, the fan(s) and the necessary turbine stages for their operation are included in the low-pressure spool. The high-pressure compressor, shaft, and turbines make up the high-pressure spool. The combustion section is located within this spool, which serves as the engine's core.

Low bypass and high bypass turbofan engines are both possible. The bypass ratio is based on how much air is passed around the engine's core. In most cases, the air that the fan pushes does not go through the engine's working core. The bypass ratio is the ratio of air flow, measured in lb/sec, from the fan bypass to the engine's core flow.

For instance, if the fan moves 200 lb/sec of air and the core moves 40 lb/sec, the by-pass ratio will be as follows:

turbine engine bypass ratio

Over .8 Mach, several low-bypass turbofan engines are employed (military aircraft). To improve thrust, these engines employ augmenters or afterburners. More fuel can be sprayed and burned by installing additional fuel nozzles and a flame holder in the exhaust system, which can result in significant increases in thrust for brief periods of time.

The turboprop engine, a gas turbine, drives a propeller through a gearbox that reduces speed. This sort of engine can operate on shorter runways than other aircraft and is best effective at speeds between 300 and 400 mph. The propeller is propelled by about 80 to 85 percent of the energy produced by the gas turbine engine. The remaining energy leaves the system as thrust through the exhaust. Equivalent shaft horsepower is obtained by combining the horsepower produced by the engine shaft and the horsepower in the outgoing thrust.

An onboard auxiliary power unit or a gas turbine engine designed to impart horsepower to a shaft that turns a helicopter transmission is an aircraft's turboshaft engine (APU). On turbine-powered aircraft, an APU is employed as a backup generator in flight as well as an electrical power source and bleed air on the ground. There are a variety of styles, shapes, and horsepower levels for turboshaft engines.

How Jet Engine Work

Jet engines create a powerful thrust that propels the aircraft forward with great force, making for relatively fast flight.

The basic operation of all jet engines, also known as gas turbines, is the same. A fan at the front of the engine pulls the air in. The air pressure is compressed by using a high pressure compressor. The shaft of the compressor fitted with several compressor blades. The air is compressed or squeezed as a result of the blades' rapid rotation. Fuel is then sprayed into the compressed air, and the combination is ignited by an electric spark. At the nozzle at the back of the engine, the expanding, burning gases exit at very high speed. The aircraft and engine are propelled forward as the gas jets exit rearward. The turbine is a different set of blades that the hot air goes through on its way to the nozzle. The compressor and turbine are mounted on the same shaft. The compressor spins as the turbine rotates.

The air flow through the engine is depicted in the graphic below. The engine's core is surrounded by air, which also passes through it. As a result, some of the air is quite hot, while other parts are colder. At the engine departure location, the cooler air then combines with the hot air.

Gases passes through four thermodynamic processes in an ideal gas turbine engine: isentropic compression, isobaric combustion (constant pressure), isentropic expansion, and heat rejection, this apply to all gas turbine engine.

When gas is compressed in a true gas turbine, mechanical energy is permanently transformed into pressure and thermal energy (owing to internal friction and turbulence) (in either a centrifugal or axial compressor). The combustion chamber is heated, which causes the gas's specific volume to rise and its pressure to slightly decrease. Again, irreversible energy change takes place as the turbine expands via the stator and rotor passageways. Instead of rejecting heat, fresh air is drawn in.

The exit pressure will be as close to the entry pressure as possible if the engine has a power turbine added to drive an industrial generator or a helicopter rotor. There will only be enough energy left to overcome the pressure losses in the exhaust ducting and expel the exhaust. For a turboprop engine to operate as economically as possible, there must be a specific balance between jet thrust and propeller power. Only enough pressure and energy from the flow are extracted in a turbojet engine to power the compressor and other parts. To create a jet to power an aircraft, the remaining high-pressure gases are accelerated through a nozzle.

To achieve the necessary blade tip speed, the shaft must rotate at a greater rate as the engine size decreases. The greatest pressure ratios that the turbine and compressor can achieve depend on the blade-tip speed. As a result, the engine's potential for maximum power and efficiency is constrained. If a rotor's diameter is cut in half, the rotational speed must increase by two times for the tip speed to remain constant. For instance, although tiny turbines can spin at speeds of up to 500,000 rpm, big jet engines run between 10,000 and 25,000 rpm.

Gas turbines can have less complicated mechanical designs than internal combustion piston engines. The compressor/shaft/turbine rotor assembly, the only major moving component of a simple turbine, may be accompanied by other moving components in the fuel system. This may then have an impact on cost. But this also meant that efficiency and dependability suffered. Modern jet engines and combined cycle power plants often use more sophisticated gas turbines, which can have two or three shafts (spools), hundreds of compressor and turbine blades, movable stator blades, and a large amount of external tubing for the fuel, oil, and air systems. These advanced gas turbines are built to precise specifications and use temperature-resistant alloys. Due to all of this, building a simple gas turbine is frequently more difficult than building a piston engine.

Additionally, the gas must be prepared to exact fuel specifications in order for modern gas turbine power plants to operate at peak efficiency. Prior to entering the turbine, the natural gas is treated by fuel gas conditioning systems to achieve the precise fuel specification in terms of pressure, temperature, gas composition, and the associated wobbe-index.

The power to weight ratio of a gas turbine engine is its main benefit.

Gas turbines are ideal for aircraft propulsion because they can produce a lot of useful work from a relatively light engine.

A design's thrust and journal bearings are essential components. They are rolling element bearings with oil cooling or hydrodynamic oil bearings. Foil bearings have a strong potential for use in small gas turbines and auxiliary power units, as well as in some other small machines like micro turbines.

The basic working principle of a gas turbine is a Brayton cycle with air as the working fluid: atmospheric air flows through the compressor, increasing its pressure; energy is then added by spraying fuel into the air and igniting it to produce a high-temperature flow; this high-temperature pressurised gas enters a turbine, producing a shaft work output in the process, used to drive the compressor; the unused energy is released in the exhaust gases. The design of the gas turbine is determined by its intended use in order to obtain the most ideal energy distribution between thrust and shaft work. Since gas turbines are open systems that do not reuse the same air, the fourth step of the Brayton cycle—cooling the working fluid—is skipped.

Aircraft, trains, ships, electricity generators, pumps, gas compressors, and storage tanks are all propelled by gas turbines.

Turbine Nozzle Inlet Guide Vane

The turbine disc and turbine inlet guide vanes are the two main components of the turbine assembly. Turbine inlet nozzle vanes, turbine inlet guide vanes, and nozzle diaphragm are three of the most frequently used names for the stator element.

Turbine Nozzle Inlet Guide Vane

Hottest Component in Turbine Engine

Directly behind the combustion chambers and immediately in front of the turbine wheel are the turbine inlet nozzle vanes. This is the greatest or hottest temperature that metal engine parts can experience. The turbine inlet temperature must be kept under control to prevent damage to the inlet vanes.

The mass airflow must be ready to drive the turbine rotor after the heat energy has been evenly distributed to the turbine inlet nozzles by the combustion chamber and introduced into the mass airflow. The turbine inlet nozzles' stationary vanes are shaped and angled in such a way that they create a number of tiny nozzles that discharge gas at incredibly high speeds. As a result, the nozzle transforms varying amounts of heat and pressure energy into velocity energy, which can then be transformed into mechanical energy by the turbine blades.

The gases are directed at a specific angle in the direction of the turbine wheel rotation as the turbine inlet nozzle's secondary function. It is crucial to aim the gas flow from the nozzle in the general direction of turbine rotation because it must enter the turbine blade passageway while it is still rotating.

The inner and outer shrouds between which the nozzle vanes are fastened make up the turbine inlet nozzle assembly. Varied types and sizes of engines use different numbers and sizes of inlet vanes.

There are numerous ways to assemble the turbine inlet nozzle's vanes between the outer and inner shrouds or rings. Although the configuration and design of the actual components may differ slightly, all turbine inlet nozzles must have one peculiar feature: the nozzle vanes must be made to accommodate thermal expansion.

Otherwise, the fast temperature variations would cause the metal components to twist or distort severely.

There are numerous ways to achieve the thermal expansion of turbine nozzles. One technique requires the supporting inner and outer vane shrouds to be assembled loosely.

Each vane slips into a shroud slot that has been shaped to meet the vane's airfoil shape. These slots provide a loose fit since they are somewhat larger than the vanes. Inner and outer support rings, which increase strength and rigidity, are encased by the inner and outer shrouds to offer additional support. The removal of the nozzle vanes as a whole is also made easier by these support rings. Without the rings, as the shrouds were taken off, the vanes might fall out.

Fitting the vanes into the inner and outer shrouds is another technique of thermal expansion construction; however, in this manner, the vanes are welded or riveted into place. It is necessary to give a way for thermal expansion, so either the inner or outer shroud ring is divided into sections. The segments are divided by saw cuts, which allow for enough expansion to prevent stress and warping of the vanes.

Construction of Nozzle guide vanes (NGVs)

Gas turbines' crucial structural components known as nozzle guide vanes (NGVs). Considering that NGVs must withstand extremely high temperatures and hostile environments, superalloys based on nickel are typically used to create them. The only commercially viable method of producing these parts, which have extremely complex shapes, is investment casting in vacuum, also known as the lost-wax process. With improved NGV designs, which typically result in more complex shapes and thinner geometries, significant gains in turbine efficiency can be made. However, the complexity of the production process, which increases the frequency of flaws (mostly porosity) during the investment casting of components with complex geometries and thin elements, hinders these improvements. As a result, the new generation of NGVs' investment casting routes are being developed via a "trial and error" method, or, in other words, through experimental casting trials. However, this approach is very costly and time-consuming, which significantly slows the rate of invention.

Turbine Nozzle Inlet Guide Vane Inspection

The first stage turbine blades and turbine nozzle vanes can be examined after the necessary components have been removed. The blade restrictions listed in the overhaul and service instruction handbook provided by the engine manufacturer should be followed. If the damage is minor and doesn't go above certain depths, minor nicks and dents are acceptable.

Look for nicks or cracks in the nozzle vanes. Vane rejection is not warranted by minor nicks as long as they blend in seamlessly.

Check the nozzle vane supports for damage brought on by foreign particles impinging on them. Any nicks that aren't sure to be there can be smoothed out using a stone. Similar to turbine blades, certain engines allow for the replacement of a certain number of turbine nozzle vanes.

A replacement turbine nozzle vane assembly must be replaced if more than the maximum number of vanes are damaged. It is possible to check the rear turbine stage for cracks or signs of blade stretch after removing the tailpipe (exhaust nozzle). Strong light inspection of further nozzle stages is also possible through the rear-stage turbine.

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