THE TURBINE BLADE: ROOTS, SHROUD AND AIRFOIL

The load that the blade would enduredetermines the design too. *Static loads in the form of centrifugal forces aswell as torque and bending forces (in the form of air resistance) as well asdynamic loads of the moving blades.

Other factors like blade elongation (due toheat), hot temperatures, erosion, corrosion, and oxidation will determine thedesign too.

First, everything is designed to make surethat the product is in spec but manufactured in the most cost-effective waywith minimal waste and maximum performance. The root, shroud and airfoil designdepend on the environmental conditions the blade will need to perform in.

High, intermediate, or low-pressureapplications determine the blade size and material choice. For example, withhigh-pressure turbines, the blades tend to be smaller, due to an environment ofhigh heat and pressure. It also needs heat-resistant materials likenickel-based alloys. While with low-pressure turbines with lower temperaturesthe blades tend to be exceptionally long and made from less heat resistantmaterials like common stainless turbine steel. But with their high mechanicalloads high mass of blades at high turning speeds lead to high centrifugalforces and mechanical loads, which leads to more complex roots that isimperative for safety reasons.

The Root mounts the blade to the rotor orin the case of a vane it will be mounted into the casing. By mounting theblades/vanes next to each other a big “circle” of blades/vanes is created fullyand interact with the gas passing through the turbine or compressor. The shapeof the root depends on the thermal and mechanical load it will need to withstandover periods of time. The design engineers must keep manufacturing costs downbut at the same time enhance performance. A more complicated root design meansa higher safety rating due to more “shoulders” taking the load but that willmean higher manufacturing costs because the root’s shape requires higherprecision and tools and techniques in the manufacturing process.

The connection of the blade/vane to therotor/casing is usually created via a form fit or via a welded connection.

The Shroud’s main purpose is to stabilizethe blades/vanes via a loose connection between the neighboring shrouds. Afterassembly this closed ring of shrouds lowers the flow leakage of the gas or airthat is bypassing the airfoil. Additionally, on the top of the shroud there canbe a labyrinth design to further decrease leakage. To increase stabilitybetween the shrouds they can be connected to each other with interlockinggeometrical features.

When stabilizing and leakage are not afactor in the performance above features are not necessary.

The Airfoil is the part that interacts withthe gas or steam or fluid passing by. In the case of a turbine blade itsfunction is to convert the energy of the bypassing gas into a rotation of therotor. In the case of a compressor blade the forced rotation of the rotor (viaan engine) leads to air being sucked in and compressed by the airfoils ofmultiple stages. Therefore, the compressor works with a reserved principle.

The function of a vane’s airfoil (turbineor compressor) is to redirect the flow of the gas at the perfect angle tointeract with the next blade. This is important for optimal performance andmaking maximum use of the gas. Both blades and vane’s airfoil profiles show aleading-edge, trailing edge which is a, suction side, and pressure side. Theleading edge is where the gas flow enters the profile while the trailing edgeis where the gas leaves the profile. These neighboring blades build up thechannel of gas flow that pass over the profiles and performs in the way it wasdesigned to.

An airfoil profile is curved from theleading to trailing edge to ensure the flow enters the profile in a certaindirection and gets redirected within the channel to interact in the desired wayand leaves the profile in another direction. The entrance and exit angles ofthe airfoil profiles must be synchronized with the gas flow directions to be asefficient as possible. The suction and pressure side in-between have to bedesigned as smoothly and homogeneously as possible to match the high-efficiencyrequirements.

The profiles of the airfoil changeconstantly from the root to the shroud depending on the local boundaryconditions (especially flow speeds). This results in a continuous change ofairfoil shape and lean and twist from root to shroud.

The concrete radii, shapes, lengths, andthicknesses of a blade are determined via complex simulation software thatcalculates speeds, temperatures, pressures, and reducing imperfections in theareas where they must perform optimally.

These geometrical elements must be designed,manufactured, and measured on high-performance levels to match the highestgrades of efficiency as well as for future improvements. Measurement plays anintricate part in determining what is the best performing design and where toimprove what.

Quality control and quality assurance are abig part of ensuring safety. In conclusion we will look at measuring the bladewith WENZEL’s Core machine which was developed to measure more parts faster.Especially in aviation where the blades have an extremely narrow leading edge.The spot of light from the Core can measure the smallest of radii and pick upthe smallest of defects.   This is true inmedical device manufacturing too, which is known for their intricate and complicatedparts.

Unlike other optical systems, the CORE canmeasure recently machined, polished, and even mirror-finished parts without theneed to spray them white. The fully integrated optical sensors provide thedexterity and measurement capability needed to rapidly inspect the complex geometryof today`s parts. CORE’s focus is measurements on the manufacturing floor. TheCORE construction has been FEM optimized. Linear Guide-Ways with wide bearingspreads ensure long-life and stable operation even in the productionenvironments from 18° – 30°C.

An integral, high resolution, direct driveRotary Table provides synchronous part positioning to the measuring sensorfurther optimizing inspection cycle through a complete simultaneous 5 axismotion. Maximum scanning speed is up to 400 mm/s across the surface of thepart. CORE has a travel range of 300 x 200 x 450 mm and is optimized formeasurement of turbine blades, orthopedic implants, and other small high-volumequality critical manufactured parts