Mounting of a steam turbine produced by Siemens, Germany

Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid.

These two types of machines are governed by the same basic relationships including Newton's second Law of Motion and Euler's pump and turbine equation for compressible fluids. Centrifugal pumps are also turbomachines that transfer energy from a rotor to a fluid, usually a liquid, while turbines and compressors usually work with a gas.

The first use of turbomachines were technically water wheels between the 3rd and 1st century BCE, credited to the people in the Mediterranean region (see more at water wheel). The first real modern turbomachines did not appear until the late 1880’s. It was not until the industrial revolution, however, steam power started to be utilized with reciprocating engines and turbines, which opened up the potential of steam power. The first impulse type turbine was created by Carl Gustaf de Laval in 1883. This was closely followed by the first practical reaction type turbine in 1884, built by Charles Parsons. Parsons’ first design was a multi-stage axial-flow unit, which George Westinghouse acquired and began manufacturing in 1895, while General Electric acquired de Laval’s designs in 1897. Since then, development has skyrocketed from Parsons’ early design, producing 0.746 kW, to modern nuclear steam turbines producing upwards of 1500 MW. Today, steam turbines account for roughly 90% of electrical power generated in the United States. The first patents for gas turbines were filed in 1791 by John Barber. Then the first functioning industrial gas turbines were used in the late 1890’s to power street lights (Meher-Homji, 2000).

Classification

A steam turbine from MAN SE subdidiary MAN Turbo

In general, the two kinds of turbomachines encountered in practice are open and closed turbomachines. Open machines such as propellers, windmills, and unshrouded fans act on an infinite extent of fluid, whereas, closed machines operate on a finite quantity of fluid as it passes through a housing or casing.^[2]

Turbomachines are also categorized according to the type of flow. When the flow is parallel to the axis of rotation, they are called axial flow machines, and when flow is perpendicular to the axis of rotation, they are referred to as radial (or centrifugal) flow machines. There is also a third category, called mixed flow machines, where both radial and axial flow velocity components are present.^[2]

Turbomachines may be further classified into two additional categories: those that absorb energy to increase the fluid pressure, i.e. pumps, fans, and compressors, and those that produce energy such as turbines by expanding flow to lower pressures. Of particular interest are applications which contain pumps, fans, compressors and turbines. These components are essential in almost all mechanical equipment systems, such as power and refrigeration cycles.^[2]^[3]

Classification of fluid machinery in species and groups
machine type → group ↓	machinery	combinations of power and machinery	engines
open turbomachine	propeller		wind turbines
hydraulic fluid machinery (≈ incompressible fluids)	centrifugal pumps turbopumps and fans	Fluid couplings and clutches (hydrodynamic gearbox); Voith Turbo-Transmissions; pump-turbines (in pumped-storage hydroelectricity)	water turbines
thermal turbomachinery (compressible fluid)	compressors	gas turbines (inlet of GT consists of a compressor)	steam turbines ← turbine jet engines

Turbomachines

Definition

Any devices that extracts energy from or imparts energy to a continuously moving stream of fluid can be called a Turbomachine. Elaborating, a turbomachine is a power or head generating machine which employs the dynamic action of a rotating element, the rotor; the action of the rotor changes the energy level of the continuously flowing fluid through the machine. Turbines, compressors and fans are all members of this family of machines.^[4]

In contrast to positive displacement machines (particularly of the reciprocating type which are low speed machines based on the mechanical and volumetric efficiency considerations), the majority of turbomachines run at comparatively higher speeds without any mechanical problems and volumetric efficiency close to one hundred percent.^[5]

Categorization

Energy conversion

Turbomachines can be categorized on the basis of the direction of energy conversion:^[1]^[2]

Absorb power to increase the fluid pressure or head (ducted Fans, compressors and pumps).
Produce power by expanding fluid to a lower pressure or head (hydraulic, steam and gas turbines).

Fluid flow

Turbomachines can be categorized on the basis of the nature of flow path through the passage of the rotor:^[6]

Axial Turbomachine's Velocity Diagram^[1]

Axial flow turbomachines - When the path of the through-flow is wholly or mainly parallel to the axis of rotation, the device is termed an axial flow turbomachine.^[7] The radial component of the fluid velocity is negligible. Since there is no change in the direction of the fluid, several axial stages can be used to increase power output.
A Kaplan turbine is an example of an axial flow turbine.
In the figure:

U = Blade velocity,
V_f = Flow velocity,
V = Absolute velocity,
V_r = Relative velocity,
V_w = Tangential or Whirl component of velocity.

Radial Turbomachine's Velocity Diagram^[1]

Radial flow turbomachines - When the path of the through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbomachine.^[7] Therefore, the change of radius between the entry and the exit is finite. A Radial turbomachine can be inward or outward flow type depending on the purpose that needs to be served. Outward flow type increases the energy level of the fluid and vice versa. Due to continuous change in direction, several radial stages are generally not used.
A centrifugal pump is an example of a radial flow turbine.
In the figure:

U = Blade velocity,
V_f = Flow velocity,
V = Absolute velocity,
V_r = Relative velocity,
V_w = Tangential or Whirl component of velocity.

Mixed flow turbomachines – When axial and radial flow are both present and neither is negligible, the device is termed a mixed flow turbomachine.^[7] It combines flow and force components of both radial and axial types.
A Francis turbine is an example of a mixed-flow turbine.

Physical action

Turbomachines can finally be classified on the relative magnitude of the pressure changes that take place across a stage:

An Impulse Turbomachine Stage^[1]

Impulse Turbomachines operate by accelerating and changing the flow direction of fluid through a stationary nozzle (the stator blade) onto the rotor blade. The nozzle serves to change the incoming pressure into velocity, the enthalpy of the fluid decreases as the velocity increases. Pressure and enthalpy drop over the rotor blades is minimal. Velocity will decrease over the rotor.^[1]^[7]
Newton's second law describes the transfer of energy. Impulse turbomachines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor.
A Pelton wheel is an impulse design.

A Reaction Turbomachine Stage^[1]

Reaction Turbomachines operate by reacting to the flow of fluid through aerofoil shaped rotor and stator blades. The velocity of the fluid through the sets of blades increases slightly (as with a nozzle) as it passes from rotor to stator and vice versa. The velocity of the fluid then decreases again once it has passed between the gap. Pressure and enthalpy consistently decrease through the sets of blades.
Newton's third law describes the transfer of energy for reaction turbines. A pressure casement is needed to contain the working fluid. For compressible working fluids, multiple turbine stages are usually used to harness the expanding gas efficiently.
Most turbomachines use a combination of impulse and reaction in their design, often with impulse and reaction parts on the same blade.

Dimensionless ratios to describe turbomachinery

Pelton wheel being installed into Walchensee Hydroelectric Power Station.

The following dimensionless ratios are often used for the characterisation of fluid machines. They allow a comparison of flow machines with different dimensions and boundary conditions.

Pressure range ψ
Flow number φ (including delivery or volume number called)
Performance numbers λ
Run number σ
Diameter Number δ

Applications

Power Generation

Hydro electric- Hydro-electric turbomachinery uses potential energy stored in water to flow over an open impeller to turn a generator which creates electricity
Steam turbines- Steam turbines used in power generation come in many different variations. The overall principle is high pressure steam is forced over blades attached to a shaft, which turns a generator. As the steam travels through the turbine, it passes through smaller blades causing the shaft to spin faster, creating more electricity.
Gas turbines- Gas turbines work much like steam turbines. Air is forced in through a series of blades that turn a shaft. Then fuel is mixed with the air and causes a combustion reaction, increasing the power. This then causes the shaft to spin faster, creating more electricity.
Windmills- Also known as a wind turbine, windmills are increasing in popularity for their ability to efficiently use the wind to generate electricity. Although they come in many shapes and sizes, the most common one is the large three-blade. The blades work on the same principle as an airplane wing. As wind passes over the blades, it creates an area of low and high pressure, causing the blade to move, spinning a shaft and creating electricity. It is most like a steam turbine, but work with an infinite supply of wind.

Marine

Steam turbine- Steam turbines in marine applications are very similar to those in power generation. The few differences between them are size and power output. Steam turbines on ships are much smaller because they don’t need to power a whole town. They aren’t very common because of their high initial cost, high specific fuel consumption, and expensive machinery that goes with it.
Gas turbines- Gas turbines in marine applications are becoming more popular due to their smaller size, increased efficiency, and ability to burn cleaner fuels. They run just like gas turbines for power generation, but are also much smaller and do require more machinery for propulsion. They are most popular in naval ships as they can be at a dead stop to full power in minutes (Kayadelen, 2013), and are much smaller for a given amount of power. Flow of air through a turbocharger and engine

Auto

Air and exhaust flow through engine and turbocharger

Turbochargers- Turbochargers are one of the most popular turbomachines. They are used mainly for adding power to engines by adding more air. It combines both forms of turbomachines. Exhaust gases from the engine spin a bladed wheel, much like a turbine. That wheel then spins another bladed wheel, sucking and compressing outside air into the engine.
Superchargers- Superchargers are used for engine-power enhancement as well, but only work off the principle of compression. They use the mechanical power from the engine to spin a screw or vein, some way to suck in and compress the air into the engine.

General

Pumps- Pumps are another very popular turbomachine. Although there are very many different types of pumps, they all do the same thing. Pumps are used to move fluids around using some sort of mechanical power, from electric motors to full size diesel engines. Pumps have thousands of uses, and are the true basis to turbomachinery (Škorpík, 2017).
Air compressors- Air compressors are another very popular turbomachine. They work on the principle of compression by sucking in and compressing air into a holding tank. Air compressors are one of the most basic turbomachines.
Fans- Fans are the most general type of turbomachines. They work opposite of wind turbines. Mechanical power spins the blades, forcing air through them and forcing out. Basic desk-top fans to large turbofan airplane engines work this way.

Aerospace

Gas turbines- Aerospace gas turbines, or more commonly know jet engines, are the most common gas turbines. They are the most like power generation turbines because the electricity used on the airplane is from the turbines, while also providing the propulsion. These turbines are the smallest out of the industrial turbines, and are most often the most advanced.

Partial list of turbomachine topics

Many types of dynamic continuous flow turbomachinery exist. Below is a partial list of these types. What is notable about these turbomachines is that the same fundamentals apply to all. Certainly there are significant differences between these machines and between the types of analysis that are typically applied to specific cases. This does not negate the fact that they are unified by the same underlying physics of fluid dynamics, gas dynamics, aerodynamics, hydrodynamics, and thermodynamics.

Turboprop

Schematic diagram showing the operation of a turboprop engine

A Fairchild F-27 representative of the 2nd generation of modern Rolls-Royce Dart turboprop powered propjet aircraft, after the initial success of the 1950s era Vickers Viscount.

An ATR-72, a typical modern turboprop aircraft.

A turboprop engine is a turbine engine that drives an aircraft propeller.^[1] In contrast to a turbojet, the engine's exhaust gases do not contain enough energy to create significant thrust, since almost all of the engine's power is used to drive the propeller.

In its simplest form a turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.

Technological aspects

Flow past a turboprop engine in operation

Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (up to that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low.^[2]^[3]^[4] Consequently, the exhaust jet typically produces around or less than 10% of the total thrust,^[5] a higher proportion of the thrust come from the propeller at low speeds and less at higher speeds.^[6] Turboprops can have bypass ratios up to 50-100^[7]^[8]^[9] although the propulsion airflow is less clearly defined for propellers than for fans.^[10]

The propeller is coupled to the turbine through a reduction gear that converts the high RPM, low torque output to low RPM, high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines.^{[citation needed]}

Unlike the small diameter fans used in turbofan jet engines, the propeller has a large diameter that lets it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree,^[11]^[12] a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.^[13]^[14]

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft^[2]^[3]^[4] above Mach 0.6-0.7.^[5] However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching Mach 0.75. To increase propeller efficiency, a mechanism can be used to alter their pitch relative to the airspeed. A variable-pitch propeller, also called a controllable-pitch propeller, can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine outage, the pitch can be adjusted to a vaning pitch (called feathering), thus minimizing the drag of the non-functioning propeller.^{[citation needed]}

While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression. Centrifugal compressors have the advantage of being simple and lightweight, at the expense of a streamlined shape.

While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed.^[15] Residual thrust on a turboshaft is avoided by further expansion in the turbine system and/or truncating and turning the exhaust 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.

A Rolls-Royce RB.50 Trent on a test rig at Hucknall, in March 1945

Kuznetsov NK-12M Turboprop, on a Tu-95

Rolls-Royce Dart turboprop engine

Alan Arnold Griffith had published a paper on turbine design in 1926. Subsequent work at the Royal Aircraft Establishment investigated axial turbine designs that could be used to supply power to a shaft and thence a propeller. From 1929, Frank Whittle began work on centrifugal turbine designs that would deliver pure jet thrust.^[16]

The world's first turboprop was designed by the Hungarian mechanical engineer György Jendrassik.^[17] Jendrassik published a turboprop idea in 1928, and on 12 March 1929 he patented his invention. In 1938, he built a small-scale (100 Hp; 74.6 kW) experimental gas turbine.^[18] The larger Jendrassik Cs-1, with a predicted output of 1,000 bhp, was produced and tested at the Ganz Works in Budapest between 1937 and 1941. It was of axial-flow design with 15 compressor and 7 turbine stages, annular combustion chamber and many other modern features. First run in 1940, combustion problems limited its output to 400 bhp. In 1941,the engine was abandoned due to war, and the factory was turned over to conventional engine production. The world's first turboprop engine that went into mass production was designed by a German engineer, Max Adolf Mueller, in 1942.^[19]

The first mention of turboprop engines in the general public press was in the February 1944 issue of the British aviation publication Flight, which included a detailed cutaway drawing of what a possible future turboprop engine could look like. The drawing was very close to what the future Rolls-Royce Trent would look like.^[20] The first British turboprop engine was the Rolls-Royce RB.50 Trent, a converted Derwent II fitted with reduction gear and a Rotol 7 ft 11 in (2.41 m) five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 — the sole "Trent-Meteor" — which thus became the world's first turboprop-powered aircraft, albeit a test-bed not intended for production.^[21]^[22] It first flew on 20 September 1945. From their experience with the Trent, Rolls-Royce developed the Rolls-Royce Clyde, the first turboprop engine to be fully type certificated for military and civil use,^[23] and the Dart, which became one of the most reliable turboprop engines ever built. Dart production continued for more than fifty years. The Dart-powered Vickers Viscount was the first turboprop aircraft of any kind to go into production and sold in large numbers.^[24] It was also the first four-engined turboprop. Its first flight was on 16 July 1948. The world's first single engined turboprop aircraft was the Armstrong Siddeley Mamba-powered Boulton Paul Balliol, which first flew on 24 March 1948.^[25]

The Soviet Union built on German World War II development by Junkers Motorenwerke, while BMW, Heinkel-Hirth and Daimler-Benz also developed and partially tested designs.^{[citation needed]} While the Soviet Union had the technology to create the airframe for a jet-powered strategic bomber comparable to Boeing's B-52 Stratofortress, they instead produced the Tupolev Tu-95 Bear, powered with four Kuznetsov NK-12 turboprops, mated to eight contra-rotating propellers (two per nacelle) with supersonic tip speeds to achieve maximum cruise speeds in excess of 575 mph, faster than many of the first jet aircraft and comparable to jet cruising speeds for most missions. The Bear would serve as their most successful long-range combat and surveillance aircraft and symbol of Soviet power projection throughout the end of the 20th century. The USA would incorporate contra-rotating turboprop engines, such as the ill-fated twin-turbine Allison T40 — essentially a twinned up pair of Allison T38 turboprop engines driving contra-rotating propellers — into a series of experimental aircraft during the 1950s, with aircraft powered with the T40, like the Convair R3Y Tradewind flying boat never entering U.S. Navy service.

The first American turboprop engine was the General Electric XT31, first used in the experimental Consolidated Vultee XP-81.^[26] The XP-81 first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. The technology of the Allison's earlier T38 design evolved into the Allison T56, with quartets of the T56s being used to power the Lockheed Electra airliner, its military maritime patrol derivative the P-3 Orion, and the widely produced C-130 Hercules military transport aircraft. One of the most produced turboprop engines used in civil aviation is the Pratt & Whitney Canada PT6 engine.^{[citation needed]}

The first turbine-powered, shaft-driven helicopter was the Kaman K-225, a development of Charles Kaman's K-125 synchropter, which used a Boeing T50 turboshaft engine to power it on 11 December 1951.^[27]

Usage

Propulsive efficiency comparison for various gas turbine engine configurations

Compared to turbofans, turboprops are most efficient at flight speeds below 725 km/h (450 mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low. Modern turboprop airliners operate at nearly the same speed as small regional jet airliners but burn two-thirds of the fuel per passenger.^[28] However, compared to a turbojet (which can fly at high altitude for enhanced speed and fuel efficiency) a propeller aircraft has a lower ceiling.

The most common application of turboprop engines in civilian aviation is in small commuter aircraft, where their greater power and reliability offsets their higher initial cost and fuel consumption. Turboprop-powered aircraft have become popular for bush airplanes such as the Cessna Caravan and Quest Kodiak as jet fuel is easier to obtain in remote areas than avgas. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required.

Turboprop engines are generally used on small subsonic aircraft, but the Tupolev Tu-114 can reach 470 kt (870 km/h, 541 mph). Large military and civil aircraft, such as the Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop power. The Airbus A400M is powered by four Europrop TP400 engines, which are the third most powerful turboprop engines ever produced, after the eleven megawatt-output Kuznetsov NK-12 and 10.4 MW-output Progress D-27.^{[citation needed]}

Some commercial aircraft with turboprop engines include the Bombardier Dash 8, ATR 42, ATR 72, BAe Jetstream 31, Beechcraft 1900, Embraer EMB 120 Brasilia, Fairchild Swearingen Metroliner, Dornier 328, Saab 340 and 2000, Xian MA60, Xian MA600, and Xian MA700, Fokker 27, 50 and 60.

Slip factor

Hasil gambar untuk animasi mobil bergerak

In turbomachinery, the slip factor is a measure of the fluid slip in the impeller of a compressor or a turbine, mostly a centrifugal machine. Fluid slip is the deviation in the angle at which the fluid leaves the impeller from the impeller's blade/vane angle. Being quite small in axial impellers(inlet and outlet flow in same direction), slip is a very important phenomenon in radial impellers and is useful in determining the accurate estimation of work input or the energy transfer between the impeller and the fluid, rise in pressure and the velocity triangles at the impeller exit.
A simple explanation for the fluid slip can be given as :Consider an impeller with z number of blades rotating at angular velocity ω. A difference in pressure and velocity during the course of clockwise flow through the impeller passage can be observed between the trailing and leading faces of the impeller blades. High pressure and low velocity is observed at the leading face of impellers blade as compared to lower pressure with high velocity at the trailing face of the blade. This results in a circulation in the direction of ω around the impeller blade which prevents the air from acquiring the whirl velocity equivalent to impeller speed with non-uniform velocity distribution at any radius.
This phenomenon reduces the output whirl velocity, which is a measure of the net power output from a turbine or a compressor. Hence, the slip factor accommodates for a slip loss which affects the net power developed which increases with increasing flow-rate.

Factors accounting for slip factor

Relative eddy.
Back eddy.
Impeller design or geometry

Mean blade loading.
Thickness of blade.
Finite number of blades.

Fluid entry conditions.
Working fluid's viscosity.
Effect of boundary layer growth.
Flow separation.
Friction forces on the walls of flow packages.
Boundary layer blockage.

Mathematical Formulae for Slip factor

Fig 1. Ideal and Actual velocity triangles at impeller exit

Mathematically, Slip factor denoted by 'σ' is defined as the ratio of the actual & ideal values of the whirl velocity components at the exit of impeller. The ideal values can be calculated using analytical approach while the actual values should be observed experimentally.

\sigma ={\frac {V'_{w2}}{V_{w2}}}

where,

V'_w2 : Actual Whirl Velocity Component ,

V_w2 : Ideal Whirl Velocity Component

Usually,σ varies from 0-1 with an average ranging from 0.8-0.9 .
The Slip Velocity is given as:
V_S = V_w2 - V'_w2 = V_w2(1-σ)
The Whirl Velocity is given as:
V'_w2 = σ V_w2

Slip Factor correlations[edit]

Stodola's Equation: According to Stodola, it is the relative eddy that fills the entire exit session of the impeller passage. For a given flow geometry, the slip factor increases with the increase in the number of impeller blades, thus, accounts for one of the important parameter for losses.

\sigma =1-{\frac {\pi }{z}}{\frac {\sin \beta _{2}}{(1-\phi _{2}\cot \beta _{2})}}

where, z = number of blades and

\phi _{2}={\frac {V_{r2}}{U_{2}}}

For Radial tip, β₂ = 90⁰ ∴

\sigma =1-{\frac {\pi }{z}}

Theoretically, In order to get the perfect ideal flow guidance, one can infinitesimally increase the number of thin vanes so that the flow should leave the impeller at an exact vane angle.

However, later experiments proved that beyond a particular value, further increase in number of blades results in reduction of slip factor due to increase in blockage area.

Stanitz's Equation: Stanitz found the slip velocity does not depend upon of the blade exit angle and hence, gave the following equation.

\sigma =1-{\frac {1.98}{z(1-\phi _{2}\cot \beta _{2})}}

where, z = number of blades,

β₂ varies from 45⁰ to 90⁰.

For radial tip: β₂ = 90⁰ ∴

\sigma =1-{\frac {1.98}{z}}

Balje's formula: An approximate formula given by Balje for radial-tipped (β₂=90⁰) blade impellers:

\sigma =[1+{\frac {6.2}{z.n^{2/3}}}]^{-1}

where, z = number of blades , n =

{\frac {Impeller\ Tip\ Diameter}{Eye\ Tip\ Diameter}}

The above explained models clearly states that the Slip factor is solely a function of geometry of Impeller. However, later studies proved that Slip factor depends on other factors as well namely 'mass flow rate',viscosity