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Turbine: principles, types, history and applications

A turbine converts the energy of moving fluids into rotational mechanical power. This article explains how turbines work, their main parts, types (steam, gas, water, wind), history and common uses.

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Overview

A turbine is a rotary machine that extracts energy from a moving fluid and transforms it into usable mechanical work. The mechanical output is typically delivered by a rotating shaft that may drive a generator, ship propeller, compressor, pump or other machinery. In broad usage, the word "turbine" applies to devices driven by gases, steam, water, wind or other fluids. For a basic definition see turbine and for the working medium see fluid. The produced power is commonly converted to electricity via an alternator or generator.

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Turbinenläufer Gasturbine G24
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Design and main components

Most turbines are turbomachines with one principal moving part: the rotor, a shaft or drum fitted with blades or buckets that intercept the flow. A rotor assembly is often accompanied by stationary components (stators or nozzles) that direct and accelerate the fluid before it reaches the moving blades; for more on the rotor see rotor. Other important parts include the casing, bearings, seals and a control system. The casing shapes and confines the flow for efficiency and safety, while bearings support the shaft and maintain alignment.

Types and operating principles

Turbines operate by transferring momentum or by converting pressure into velocity. Two fundamental principles are impulse and reaction. In an impulse turbine, high-speed jets of fluid strike the blades and change momentum, producing torque. In a reaction turbine, the fluid expands as it passes over the blades and generates lift in much the same way as an aircraft wing, producing rotation through pressure differences. Many modern machines combine elements of both. Examples of common turbine categories include:

  • Steam turbines — used widely in thermal power plants.
  • Gas turbines — operate with combustion gases and power aircraft, ships and power stations.
  • Hydraulic turbines (water turbines) — used in dams and run-of-river hydroelectric plants.
  • Wind turbines — convert kinetic energy of the wind into rotational power for electricity.
  • Tidal and wave turbines — emerging technologies that harness ocean movements.

History and development

Devices that extract energy from fluid motion date back millennia — waterwheels and horizontal-axis windmills are early, pre-industrial examples. The steam turbine as a high-speed rotary prime mover was developed in the late 19th century; credit for inventing influential steam turbine designs is commonly given to Sir Charles Parsons (reaction turbine) and Gustaf de Laval (impulse turbine), whose work enabled compact, high-speed generation of electricity and marine propulsion. Since then, advances in materials, aerodynamics, seals and control systems have greatly improved efficiency, reliability and power density.

Applications and importance

Turbines are a cornerstone of modern energy and transport systems. Steam and gas turbines generate the majority of the world's electricity when coupled to generators; water turbines supply large amounts of renewable energy in hydroelectric schemes. In transportation, gas turbines power jet engines and some naval ships, while wind turbines form a central part of renewable electricity portfolios. Industrial turbines drive compressors, pumps, and other rotating equipment in oil, chemical, and manufacturing plants.

Notable distinctions and practical considerations

Design choices depend on fluid properties, desired speed, efficiency and operating range. Axial-flow machines are common in large steam and gas turbines; radial-flow and mixed-flow designs appear in smaller or specialized units. Efficiency varies with size and loading; large, multi-stage turbines generally reach higher efficiencies than small, single-stage devices. Maintenance issues include blade erosion, fouling, vibration and bearing wear. Modern control systems and predictive maintenance extend life and improve availability.

For further reading and technical references, consult introductory resources on turbomachinery and engineering handbooks. Historical and technical summaries often link the conceptual definitions above to practical design examples and performance charts; see a general overview at steam turbine and biographical contexts at Swedish engineer.

Three types of water turbines: Kaplan blade (front), Pelton wheel (middle), Francis turbine (back left).
Three types of water turbines: Kaplan blade (front), Pelton wheel (middle), Francis turbine (back left).

Basics

This section does not separate well enough between a single impeller and an overall turbine, which includes other components involved in power conversion - for example, the guide vanes or the flow channel cross-section; in addition, turbines are sometimes multi-stage. The section needs some revision. More details should be given on Discussion:Turbine. Please help improve it, and then remove this tag.

Theory

The theoretical foundations for calculating any type of turbine were laid as early as the 18th century by Leonhard Euler.

Euler's turbine equation

The basis of Euler's turbine equation is found in the conservation of angular momentum of a material flow in a closed system:

{\displaystyle D=m\cdot v\cdot r}

The change of momentum within a subsystem (here: the turbine blades) generates a torque around the centre of the turbine:

{\displaystyle M={\frac {dD}{dt}}={\frac {dc}{dt}}\cdot r\cdot m}

It makes sense that only those parts of the flow velocity of the fluid can contribute to the torque that are perpendicular to the turbine's centre of rotation in the sense of the law of levers. Such components are marked with the index u.

An integration of the formula gives the following result:

{\displaystyle \int _{t_{1}}^{t_{2}}M\cdot dt=m\cdot r\cdot \int _{1}^{2}dc_{u}}

From the relationship between torque, the speed nand the power Pis calculated:

{\displaystyle P=M\cdot 2\cdot \pi \cdot {n}=M\cdot \omega }

{\displaystyle P={\frac {m\cdot r\cdot \omega \cdot dc_{u}}{dt}}={\frac {m\cdot u\cdot dc_{u}}{dt}}}

with u as the maximum possible circumferential velocity in a cross section under consideration.

A new integration provides

{\displaystyle P={\dot {m}}\cdot {\int _{1}^{2}u\cdot dc_{u}}}resp.

{\displaystyle {\frac {P}{\dot {m}}}={\int _{1}^{2}u\cdot dc_{u}}=\mathbf {Y} }

The last equation is called Euler's turbine equation. Its solution is given by:

Y=u_{2}\cdot c_{{u2}}-u_{1}\cdot c_{{u1}}

Y is here the specific blade work, u the circumferential velocity of the rotating blade tip at the inlet (index 1) and outlet (index 2), likewise the useful fluid velocity c_{u}at the inlet and outlet.

In reality, the friction losses of the flowing fluid must also be taken into account for the approximate turbine design.

Application of Euler's turbine equation to axial flow machines
Application of Euler's turbine equation to axial flow machines
Perspective representation of the physical quantities for Euler's turbine equation
Perspective representation of the physical quantities for Euler's turbine equation

Engineering

This section applies to gaseous fluid turbines only and contains physical errors; needs revision. More details should be given on the discussion page in the Revert section as of 5/6/2019. Please help improve it, and then remove this marker.

As a rule, several blades are mounted on a hub, so that a blade or impeller is formed. The blades are curved profiled, similar to an aircraft wing.

If turbines are mounted in a flow-through casing, there is a guide wheel in front of each impeller stage. The guide vanes project from the casing into the flowing medium and impart an angular momentum (twist) to it. The twist (kinetic energy) generated in the guide wheel is dissipated as completely as possible in the following impeller in order to drive the shaft on which the impeller blades are mounted via the hub. The rotation of the shaft can be used to drive a generator, for example. Ultimately, this is how the mechanical flow energy of water power, steam or air is converted into electrical energy. The guide wheel and impeller together are called a stage. In gas turbines and especially in steam turbines, several such stages are connected in series, while water turbines have a single stage. Since the stator is stationary, its vanes can be attached to either the inside or the outside of the casing, thus providing a bearing for the shaft of the impeller. Free-standing turbines (e.g. wind turbines) usually have no guide vanes and only one stage. The impellers are decisive for the stage classification - each is the basis of its own stage.

Turbines can be directly coupled to high-speed generators that convert the mechanical rotational energy into electrical energy. These high-speed, low-pole generators are also called turbogenerators. A combination of turbine and turbogenerator is called a turbo set.

If a turbine is driven by means of a compressor and a combustion system for gas or oil, the overall system is called a "gas turbine". Gas turbines are used, for example, in aircraft, ships or in gas and oil-fired power plants. Turbine jet engines are gas turbines that power aircraft wholly or partly by the recoil of their accelerated exhaust gases (thrust). Jacketed jet engines generate a larger part of the thrust by means of a turbine-driven fan ("fan").

Special cases

There were wind turbines that were designed with only one rotor blade (and one counterweight), the so-called single-winged turbines.

The Ljung flow turbine is a design of steam turbine without guide vanes. The turbine, through which the flow passes radially from the inside to the outside, consists of two intermeshing halves which rotate in opposite directions. The impeller blades of one half act as guide vanes for the other half. In the Pelton and Laval turbines, the guide vanes are reduced to one or more nozzles.

Assembly of a steam turbine
Assembly of a steam turbine

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