Turbines

The turbine of gas turbine engine transforms a portion of the kinetic (velocity) energy of the exhaust gases into mechanical energy to drive the gas generator compressor and accessories. The sole purpose of the gas generator turbine is to absorb approximately 60 to 70 percent of the total pressure energy from the exhaust gases and convert into mechanical energy. The exact amount of energy absorption at the turbine is determined by the load the turbine is driving (i.e., size and type of compressor, number of accessories installed, and the load applied by the other turbine stages). These turbine stages can be used to drive a low-pressure compressor (fan), propeller, and shaft in aircraft/helicopters. The turbine section of a gas turbine engine is located aft of the combustion chamber. Specifically, turbine is directly behind the combustion chamber outlet.

Turbine Assembly

The turbine assembly consists of two basic elements which is necessary to extract power from exhaust gases: these two components are the turbine inlet guide vanes and turbine disk. The stator element is known by a variety of names by different of manufacturer, of which turbine inlet nozzle vanes, turbine inlet guide vanes, and nozzle diaphragm are three of the most commonly used by the industry. The turbine inlet nozzle vanes (nozzle guide vanes) are located directly aft of the combustion chambers and immediately forward of the turbine wheel. Turbine inlet nozzle vanes is the components which comes in contact with highest or hottest temperature in the engine. The turbine inlet temperature must be controlled, or damage will occur to the turbine inlet vanes, leading to complete failure of the engine.

After the combustion chamber the heat energy converted into the mass airflow and delivered it evenly to the turbine inlet nozzles, the nozzles must prepare the mass air flow to drive the turbine rotor blade. The stationary vanes of the turbine inlet nozzles are contoured and set at such an angle that they form a number of small nozzles discharging gas at an extremely high speed. The nozzle converts a varying portion of the heat and pressure energy to velocity energy that can then be converted to mechanical energy through the turbine blades of an gas turbine engine.

Turbine Disk with Blade installed

Types of Turbine Blade

There are three types of turbine blades use in gas turbine engine: the impulse turbine blade, reaction turbine blade, and the reaction-impulse turbine blade.

Impulse Turbine

The bucket is another name for the impulse turbine blade. This is because the direction of the energy changes as the air stream contacts the middle of the blade, spinning the disc and rotor shaft. To improve the effectiveness of the air stream striking the turbine blades or buckets, the turbine nozzle guiding vanes can typically be changed during engine refurbishment and assembly.

Reaction Turbine

Reaction turbine blades causes the disk to rotate by the aerodynamic action of the hot airstream directed to flow past the blade at a particular angle in order to produce the most efficient power from the turbine engine.

Reaction-Impulse Turbine

The action of the impulse and response blade designs are combined in the reaction-impulse turbine blade. At the blade root, the blade has a more bucket-like impulse blade shape, while on the second half of the blade, at the blade's outer end, it has a more airfoil-like reaction blade shape.

The gases are directed at a specified 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 tunnel while it is still revolving.

Turbine Inlet Nozzle Assembly

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 layout and design of the actual components may differ slightly, all turbine inlet nozzles must have one unusual 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 fits 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.

Turbine Shaft & Turbine Wheel

Turbine shaft & Turbine Disk

The turbine section's rotor component essentially consists of a shaft and a wheel. The turbine wheel, which has blades attached to a revolving disc, is a dynamically balanced component. The disc is then fastened to the engine's main power-transmitting shaft. The turbine assembly rotates at a very high speed as a result of the exhaust gases acting on the turbine wheel's blades as they exit the turbine inlet nozzle vanes. The turbine wheel experiences heavy centrifugal loads as a result of the high rotational speed, and the material's strength is also decreased by the high temperatures. In order to maintain safe turbine operation, the engine speed and temperature must be managed.

Without blades, the turbine disc is referred regarded as such. The disc transforms into the turbine wheel after the turbine blades are mounted. The disc serves as a part that anchors the turbine blades. The blades may transfer the energy they capture from the exhaust fumes to the rotor shaft because the disc is fastened or welded to the shaft.

The disc rim is in contact with the hot gases moving through the blades and picks up a lot of heat from them. In addition, the rim conducts heat away from the turbine blades. Therefore, disc rim temperatures are typically high and significantly higher than the temperatures of the further away

a part of the disk's inside. These temperature variations lead to the addition of thermal stresses to the rotational stresses. Additionally, due to exposure to high temperatures, turbine blades are typically more prone to operating damage than compressor blades. The aforementioned pressures can be at least partially relieved using a variety of techniques. Bleeding cooled air back onto the disk's face is one such technique.

As a byproduct of installing the blades, the disc can also be relieved of its heat strains in this way. The rim of the disc has a number of broached grooves or notches that follow the design of the blade root. In addition to providing space for the disk's thermal expansion, these grooves enable the connection of the turbine blades to the disc. The turbine blade can move when the disc is cold because there is enough space between the blade root and the notch. As the engine runs, the disc expands, reducing the clearance. Due to this, the blade root adheres closely to the disc rim.

Most often, alloy steel is used to manufacture the turbine shaft. The large torque loads that are applied to it must be able to be absorbed by it.

There are various ways to attach the shaft to the turbine disc. One technique involves welding the disc to the shaft, which features a butt or protrusion to serve as the joint. Another approach uses bolting. For this technique to work, the disc face must have a machined surface on the shaft's hub. The bolts are then fixed in tapped holes in the disc after being inserted via holes in the shaft hub. Bolting is the more prevalent of the two connection strategies.

The compressor rotor hub must have a way to attach the turbine shaft. Typically, a spline cut on the shaft's forward end is used to do this. The coupling mechanism between the compressor and turbine shafts accepts the spline. The turbine shaft's splined end may fit into a splined recess in the compressor rotor hub if no coupling is utilised. As opposed to axial compressor engines, which can employ either of the two described techniques, centrifugal compressor engines generally exclusively use this splined coupling configuration.

Turbine blade attaching method

Turbine blades can be attached in a variety of methods, some of which are comparable to compressor blade attachment. The fir-tree design is the one that works best.

There are other ways to keep the blades in their specific grooves, but the most popular ones are peening, welding, lock tabs, and riveting.

Numerous applications of the peening method of blade retention are common. One of the most popular peening applications is for a small notch to be ground into the blade's edge near the fir-tree root before the blade is installed. The metal from the disc "flows" into the notch once the blade is put into it through a small punch mark that was produced in the disc next to the notch. This task requires an instrument that resembles a centre punch.

Another technique for blade retention is to build the blade's root so that it has all the components required for retention. In this technique, the tang is located on one end of the blade root and acts as a stop to prevent the blade from being inserted or removed in any other direction. To keep the blade in the disk, the tang is bent.

Depending on the makeup of the metals, turbine blades may be either forged or cast. The majority of blades are precisely cast and shape-finished ground. Since several turbine blades are made as a single crystal, the blades' strength and heat conductivity are improved. The turbine blades and inlet nozzles are kept colder by heat barrier coating, such as ceramic coating, and air flow cooling. As a result, the engine's efficiency can be increased by raising the exhaust temperature.

Although a second form of turbine known as the shrouded turbine is occasionally employed, the majority of turbines have open blade perimeters. In reality, the shrouded turbine blades create a band around the turbine wheel's outer edge. As a result, stage weights can be reduced and efficiency and vibration characteristics are improved. However, it restricts turbine speed and necessitates larger blades.

It is occasionally required to use more than one stage of turbines while building turbine rotors. It is frequently necessary to add extra turbine stages since a single turbine wheel is frequently incapable of absorbing sufficient power from the exhaust gases to drive the components that depend on the turbine for rotative power.

A row of still vanes or nozzles is followed by a row of revolving blades in a turbine stage. Up to five turbine stages have been effectively used in various turboprop engine variants. It should be kept in mind that there is always a turbine nozzle before each wheel, regardless of the number of wheels required to drive engine components.

The employment of more than one turbine wheel is occasionally justified in situations of high rotational loads, as was highlighted in the discussion of turbine stages that came before it. Additionally, it should be noted that numerous compressor rotors are frequently advantageous when used in conjunction with the same loads that call for multistage turbines.

In a one-stage rotor turbine, a single turbine rotor generates all of the power, and this wheel drives all engine-driven components. On engines when the demand for low weight and compactness is paramount, this design is used. The pure turbojet engine in this form is the most basic.

Each spool in an engine with several spools has a unique set of turbine stages. The compressor connected to each turbine stage rotates. The majority of turbofan engines feature two spools: a low pressure turbine and a high pressure turbine to drive the fan shaft.

The turbine housing or casing is the final topic to be covered in relation to familiarising yourself with turbines. The stator components of the turbine section are either directly or indirectly supported by the turbine casing while also enclosing the turbine wheel and nozzle vane assembly. It is always equipped with front and rear flanges for attaching the assembly to the exhaust cone assembly and the combustion chamber housing, respectively.

Turbine Design Considerations

Both high rotor speeds and high temperatures are used to turbines. Due to the high temperatures and high centrifugal forces produced by high rotor speeds, turbines must operate within a range of temperatures that, if exceeded, will weaken the materials used to construct them. Turbine blades experience a distortion known as creep over time. Creep refers to the blade's stretching or lengthening. The strength of the blade, which is influenced by the temperature in the turbine, and the load placed on the turbine both affect how quickly this situation creeps.

The turbine wheel, which has blades made of metal alloy linked to a revolving disc, is a dynamically balanced component. To enable it to be firmly attached to the disc and still provide room for growth, the base of the blade is often of the so-called "fir tree" form. The tips of the rotating blades of some turbines are open, but they are covered in others. Shrouded blades surround the turbine's edge in a band, which lessens blade vibrations. Because the shrouds allow for thinner, more effective blade sections than would otherwise be allowed due to vibration concerns, the weight of the shrouded tips is offset. Additionally, shrouds reduce gas loss around the turbine blade tips.

Improvement in Turbine Inlet Temperature

Increasing the temperature of the turbine inlet is the most alluring way to improve thrust and turbine power output. The requirement for better turbine blade materials and effective cooling techniques is directly related to increases in turbine inlet temperature.

Materials Considerations

The features of metals and alloys' high-temperature strength have been the subject of extensive research and are still being improved. As a result of this endeavour, a number of cobalt- and nickel-based alloys have been developed that significantly outperform iron-based alloys in terms of high temperature strength. Newer, exotic materials offer even more potential. Blades formed of materials that have been directionally solidified, exposed to high solidification rates, or even made of a single crystal can now be produced thanks to advancements in metallurgical processes.

Improved Temperatures Due to Improved Metallurgical Techniques

Ceramic materials are being looked into by several manufacturers for usage in turbines. While ceramic blades have a substantial advantage over metal ones in terms of their capacity to endure high temperatures, issues related to stresses brought on by centrifugal loads brought on by fast rotation rates present additional challenges.

Turbine Blade Cooling

There are numerous ways to cool turbine blades, but they all essentially involve passing a cooling fluid through the blade to keep the metal's temperature within acceptable operating ranges. In aviation gas turbine engines, this cooling fluid is air drained from the compressor and directed onto the turbine wheel's blades after travelling through internal engine channels. The standards for achieving good cooling efficiency directly derive from the concepts of fluid heat transfer in a closed duct. It is important to fulfil two fundamental requirements in order to achieve high heat transfer rates in such a system, namely,

(1) flow the cooling fluid with a high Reynolds number, and

(2) provide a large surface area for the energy flow path.

Given these considerations, it is clear why a finned blade is far superior to an open hollow blade. Because a boundary layer forms across the inner surface of the open hollow blade, acting as a superb heat transfer insulator, very little cooling occurs. When fins or tubes are inserted into the blade, cooling air is forced to pass across a larger surface area while being very turbulent, creating a turbulent boundary layer that easily transfers energy.

The open hollow blade's limited structural capacity is another drawback. The open hollow blade vibrates freely and with great magnitude at its resonance frequency in the absence of fins or supporting parts, resulting in a "breathing action" and fatigue failure.

There are four general methods employed for blade cooling. These are:

Convection
Impingement
Film
Transpiration
Turbine Cooling Methods

The simplest way of cooling turbine blades was convection cooling, which was also the first to be utilized. In the convection cooling process, internal passageways in the turbine blade allow coolant air to flow outward from the base to the tip. The size of the internal passageways in the blade and the limitation on the amount of cooling air accessible limit the efficacy of convection cooling.

As a variation on convection cooling, impingement cooling involves turning the air normal to the radial direction and passing it through a series of holes such that it impinges on the inside of the blade at the desired cooling location. Impingement cooling works incredibly well in small spaces and may be applied to stator blades with ease. The leading edge of the blade, where the highest temperatures are anticipated, is where this technique is typically used.

Compressor bleed air is injected along the surface of the blades and vanes to cool the film. This creates a layer of colder air that acts as insulation between the metal of the blades and the hot gases coming from the combustor. Convection cooling and impingement cooling are less efficient than film cooling.

The turbine blades can function at higher temperatures thanks to a practical and innovative technique called transpiration cooling. This technique uses a sintered wire mesh to build the airfoil, which results in a blade with thousands of cooling holes through which cooling air flows to shield the blade from high-temperature gases. The airfoil needs an internal strut to carry the structural and centrifugal loads because the material cannot support itself.

Cooling Effectiveness Comparison

Like any other device that improves engine performance, cooling turbine blades has both benefits and drawbacks. A superficial analysis of the turbine blade cooling issue makes it appear as though the problem may be solved by simply blowing compressor bleed air through hollow turbine blades. However, a more thorough investigation of the matter will reveal how intricate the whole turbine blade cooling issue is. Because the cooling air within the blades is accelerated by centrifugal forces as it absorbs huge amounts of energy and there is a tendency for gas choking, the fundamental problem of heat transmission in a duct is rendered more challenging and intricate. Because the coolant air bled from the compressor does not contribute to power extraction in the turbine, an engine with cooled blades will perform significantly worse than one with uncooled blades at a given turbine inlet temperature. Due to the intricacy of the manufacturing process, pricing is arguably the biggest drawback of turbine blade cooling.

As has already been said, basic, open, hollow blades do not provide appropriate cooling; however, blades with fins, inserts, and bundles of tubes are more complicated to build but do so. These intricate cooled blades need to be produced correctly. They must be able to endure the tremendous strains placed on them by the centrifugal loads in addition to providing proper cooling. Manufacturing the turbine rotor needed for cooled blades is equally challenging. The air-sealing issue at the area where the coolant is delivered into the rotor hub further complicates a rotor that supplies cooling air.