Radial turbine

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A radial turbine is a turbine in which the flow of the working fluid is radial to the shaft. The difference between axial and radial turbines consists in the way the air flows through the components (compressor and turbine). Whereas for an axial turbine the rotor is 'impacted' by the air flow, for a radial turbine, the flow is smoothly orientated at 90 degrees by the compressor towards the combustion chamber and driving the turbine in the same way water drives a watermill. The result is less mechanical and thermal stress which enables a radial turbine to be simpler, more robust and more efficient (in a similar power range as axial turbines). When it comes to high power ranges (above 5 MW) the radial turbine is no longer competitive (heavy and expensive rotor) and the efficiency becomes similar to that of the axial turbines.

'Euler's Turbine Equation

Euler's Turbine Equation states that

W = (U2-U1)+(V2-V1)+(W2-W1),
where U = Blade Velocity = r*omega (omega - Angular Velocity)
     V = Absolute Velocity
     W = Relative Velocity

For an axial machine, you have Zero Degree of Reaction that makes U2-U1 = 0. This is one reason for the fact that axial turbines have low power output. For a radial turbine, along the flowpath, Radius reduces and hence U2 - U1 is not equal to zero, which proves that for similar turbines, radial turbine has higher power output.

Advantages compared to axial turbines

Thanks to lower thermal and mechanical stress on the turbine tips, it is possible to boost power quite significantly by increasing the turbine entry temperature (increasing fuel input) which results in an improved mechanical efficiency. The lower mechanical stresses also enable radial turbines to handle single stage compression and expansion. As a result, the radial turbine does not need to be air cooled, which means that all the air entering the compressor is used only to drive the turbine which gives the radial design a strong advantage for co-generation applications. Another result of avoiding air cooling is that power and efficiency are kept almost constant during the lifetime of the radial turbine whereas an axial gas turbine needs to be washed often to maintain ISO performance standards. The other advantage of such a simple rotor is that the bearings can be placed at the front, in the cold part, so less lubrication oil is needed, and there are no thermal losses due to lubrication of the hot parts of the rotor.

Nikola Tesla's Bladeless Radial Turbine

In the early 1900s, Nikola Tesla developed and patented his Bladeless Turbine. One of the difficulties with bladed turbines is the complex and highly precise requirements for balancing and manufacturing the bladed rotor which has to be very well balanced. The blades are subject to corrosion and cavitation. Tesla attacked this problem by substituting a series of closely spaced disks for the blades of the rotor. The working fluid flows between the disks and transfers its energy to the rotor by means of the boundary layer effect or adhesion and viscosity rather than by impulse or reaction. Tesla stated his turbine could realize incredibly high efficiencies by steam. As of yet there has been no documented evidence of Tesla Turbines achieving the efficiencies Tesla claimed.

References