Good vibrations: Simulation prevents turbine blades from cracking
Turbine blade vibration is bad for the long-life fatigue strength of gas turbines. Design engineers have for many years been using simulation processes to determine the blade shapes that are least susceptible to vibration. But for the tight blade arrangements used in the latest gas turbines, traditional simulation can take days or even weeks. A new method drastically reduces the calculation time required to enable fast simulations – with previously unachievable results.
By Hubertus Breuer and Frank Krull
Turbine blades in modern gas-fired power plants are high-performance components. They not only need to be capable of withstanding temperatures of up to 1,500° Celsius and rapid temperature changes, they also need to handle other extremes: Hot gas sweeps past the blades at supersonic speeds, and the rotary forces are enormous when speeds reach several thousand rpm. Under these inferno-like conditions, the blades face a major risk of material fatigue, especially when intrinsic vibration increases susceptibility to cracking. Over time, cracks can become so widespread that blade fragments ultimately break off, destroying the turbine.
Don’t monitor – prevent
“That kind of damage is incredibly expensive,” says Stefan Schmitt, a turbine expert at Siemens. “Each blade costs as much as an automobile, and if the turbine is out of action for several days for repairs, the costs can quickly escalate.” Even regular inspections can’t totally prevent cracks from developing. “One factor is that cracks form and spread too quickly,” Schmitt explains. “And the combustion chamber is too hot for sensors.” This means the best protection against cracking is to use blades with a shape that will decrease the intrinsic vibration that causes material fatigue.
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“As long as the vibration forces are below the threshold, intrinsic vibration isn’t problematic,” affirms simulation expert Arianna Bosco at Siemens Corporate Technology. Siemens Corporate Technology. She and her colleague Krishna Chaitanya are working with Schmitt and other turbine experts at Siemens to develop simulation solutions to be used in the search for low-vibration blade shapes. Their main tool is TRACE, the simulation program developed by the Institute of Propulsion Technology at the German Aerospace Center (DLR). Bosco has adapted it in recent years to simulate vibration in power station turbines.
Not an easy task, because turbine blades in modern gas-fired power plants are arranged in very close formation to save space and material. Because the flows around the blades are extremely complex in these tight arrangements, simulating them poses a unique challenge. For each blade, in fact, thousands of slightly varying shapes would have to be simulated in many different arrangements in order to allow the shape with the lowest intrinsic vibration to be selected from all the simulation results. “But,” says Bosco, “That’s far too labor-intensive. “A simulation like that would take many days or even weeks, and it just isn’t practical during the turbine design stage.”
Lean simulation solution
With this in mind, Bosco and Chaitanya began the search for a less time-consuming alternative, and they soon found an attractive solution. The solution cleverly reduced the number of influencing factors included in the simulation calculations without distorting the result. This cut down the amount of time needed for the simulation process so much that this step can now be conveniently included in the turbine design stage.
At Siemens, a number of new HL-Class gas-fired turbines and retrofits for existing turbines use blades whose vibration characteristics have been optimized using the simulation method developed by Bosco and Chaitanya. “Our observations so far have shown us that the intrinsic vibration of these blades is much lower than that of earlier blades, exactly as we predicted,” Schmitt confirms.
These positive results have encouraged Bosco to expand the new simulation method even more. “The perfect turbine blade is like a target you get closer and closer to but never quite reach,” she comments. For her next step, she’s working to integrate the current solution into a program package that will also make it possible to optimize the blade shape by taking thermodynamic and other factors into account.
January 21, 2020
Hubertus Breuer is a science and technology journalist and works as a freelancer in Munich.
Frank Krull is a physicist and journalist and works in the communications department of Siemens.
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