Turbine blades and vanes often come in prime-number counts for a reason
Look closely at a turbine and the rotor blades and stator vanes rarely match in number. Designers often choose two different primes, which spreads out vibration harmonics. It is one small trick in a machine whose name, borrowed from the Latin for a spinning top, entered engineering in an 1822 French memo.
Claude Burdin, a French mining engineer, coined the term in a paper on high-speed rotary water machines that he sent to France's royal academy of sciences. French already used the word for certain spiral seashells. A committee took two years to approve his memo, and it was Burdin's former student Benoit Fourneyron who built the first practical water turbine. Windmills and waterwheels were much older ancestors, and Vitruvius mentioned turbine-like devices around 70 BC.
Every turbine does the same basic job: moving fluid pushes on blades fixed to a shaft or drum, and the spinning rotor delivers work, often to a generator. There are two ways to harvest the fluid's energy. An impulse machine, like a Pelton wheel, fires a fast jet at its blades and turns it, with all the pressure drop happening in fixed nozzles. A reaction machine, like a Francis water turbine or a wind turbine, feels the fluid's pressure change as it passes the moving blades. For steam, a Parsons reaction design needs roughly twice as many blade rows as a de Laval impulse design, making it longer and heavier but slightly more efficient. Most modern designs mix the two.
Engineers choose among designs using a figure called specific speed, which describes how fast a turbine should spin at peak efficiency for a given power and flow, independent of its size. That lets a proven design be scaled up or down reliably. Classical calculations from the mid 19th century relied on graphical methods and simplifying assumptions; computational fluid dynamics has steadily improved designs over the past forty years.
Some variants stretch the idea. Swedish brothers Fredrik and Birger Ljungström patented a contra-rotating radial steam turbine in 1894, with two nested rotors spinning opposite ways and four times the heat drop per stage of a Parsons machine. Experimental ceramic blades could let gas turbines run hotter without air cooling, though their brittleness raises the risk of sudden failure.
Source: Turbine
