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Laminar–Turbulent Transitional Heat Transfer
Published in Je-Chin Han, Lesley M. Wright, Analytical Heat Transfer, 2022
For turbine blade film cooling, relatively cool air is injected from the inside of the blade to the outside surface, which forms a protective layer between the blade surface and hot mainstream. Film cooling can be modeled as coolant jet in crossflow at a given inclined angle as sketched in Figure 15.21. Film cooling performance depends primarily on the coolant-to-hot-mainstream pressure ratio (Pc/Pt), temperature ratio (Tc/Tg), film cooling hole location (leading edge, trailing edge, pressure and suction sides, end-wall, and blade tip), and geometry (hole size, spacing, shape, angle from the surface, and number of rows) under representative engine flow conditions such as Reynolds number, Mach number, combustion-generated high free-stream turbulence, stator-rotor unsteady wake flow, etc. The coolant-to-mainstream pressure ratio is related to the coolant-to-mainstream mass flux ratio (blowing ratio), while the coolant-to-mainstream temperature ratio is related to the coolant-to-mainstream density ratio. In a typical gas turbine airfoil, the Pc/Pt ratios vary from 1.02 to 1.10 with the corresponding blowing ratios vary approximately from 0.5 to 2.0. Whereas the Tc/Tg values vary from 0.5 to 0.85, the corresponding density ratios vary approximately from 2.0 to 1.5 [13].
Lubrication and Coating Challenges in Extreme Conditions
Published in Ahmed Abdelbary, Extreme Tribology, 2020
Surface coatings are commonly applied in components and devices in order to improve tribological properties, material performance, durability, strength and resistance in harsh operating conditions. For example, turbine blades used in jet engines can be made of materials like nickel-based superalloys. In some cases, the blades may have to be coated with a thermal barrier coating (TBC) in order to minimize exposure of the blade material to high temperatures. Also, the effectiveness and performance of hard to superhard coatings, such as diamond-like carbon, WC/C, TiC/C, VC, diamond films and other solid-film lubricants, have been validated in a variety of severe contact conditions. Table 9.6 introduces some properties of high-temperature coating materials.
Liquid Jet-Cutting
Published in Ian L. Spain, Jac Paauwe, High Pressure Technology, 2017
Another way in which high, local, contact-stresses can be produced is by collapse of cavitation bubbles in a liquid near the solid surface. When vapor-bubbles collapse, there is an implosion of liquid, which can produce high shock-wave pressures. Also, asymmetrical collapse can produce microjets, some of which may be directed toward the surface and produce high contact-pressure. These effects cause erosion of hydrofoils, propellers and hydraulic turbine blades. Research is underway to exploit these cavitation phenomena for disintegration of solids [1, 2, 3]. Special nozzles are used to produce cavitation-bubbles in the flow, some of which will collapse near the surface causing granular erosion. The local stresses depend on the hydrodynamics of the jet-flow in the advancing cavity or slot.
Cavitation erosion behaviour and mechanism of HVOF-sprayed NiCrBSi–(Cr3C2–NiCr) composite coatings
Published in Surface Engineering, 2018
Cavitation erosion often occurs when hydraulic structural components, such as ship propeller or turbine blades, are impacted by a fluid moving at a high speed and a pressure changing state, the bubbles are carried by the fluid to areas of high pressure where they collapse and damage the coating. This leads to the formation of holes [1–3]. Such phenomenon acts as an origin of destruction, leading to the component damage or even the equipment failure. To ensure the safety and enhance the service life, many new materials, including polymer materials, corrosion-resistant steel and titanium alloy, have been tried as hydraulic components. Chen [4] found that epoxy-modified phenolic resin had good corrosion resistance in the acidic environment and alkaline environment. Yao et al. [5] introduced a repairing scheme, using the Belzona polymer material, to paint the impeller, which efficiently thwarts chemical corrosion and cavitation, with an advantage of short repair and maintenance time as well as prolongation of the life cycle of the impeller. Mccaul and Fumagalli [6] invented a new metastable austenitic steel by changing the composition content of C, Mn, N, Si, Ni, Cr in the alloy which can be used to make impellers to resist the cavitation erosion and which possesses cavitation resistance several times better than that of existing standard impeller materials. In general, the corrosion resistance can be effectively enhanced by using new materials. However, the higher cost is still an obstacle, limiting the applications of the questioned materials on a broad scale.
A port-Hamiltonian formulation of coupled heat transfer
Published in Mathematical and Computer Modelling of Dynamical Systems, 2022
Jens Jäschke, Matthias Ehrhardt, Michael Günther, Birgit Jacob
To improve the thermal efficiency, modern gas turbines are operated at very high temperatures (1200°C to 1500°C). Since these temperatures greatly exceed the range in which the turbine blade’s metal can be used safely, active cooling of the turbine blades is necessary for them to withstand these temperatures. One of the techniques used for this purpose is convection cooling, i.e. cooling channels are installed within the blade. These internal cooling channels are small ducts within the turbine blade that are filled with a stream of cooling fluid, usually (comparatively cool) air extracted from the compressor. Figure 1 gives a graphical representation of the cooling channels within a turbine blade, as well as other cooling techniques not discussed here.
Gas turbine performance enhancement for naval ship propulsion using wave rotors
Published in Journal of Marine Engineering & Technology, 2022
The quantity of the air mass flow rate to be used in the mixer in order to moderate the exhaust gas temperature can be calculated using an energy balance equation based on the inlet air flow rate, power output, efficiency, gas turbine outlet temperature and TIT according to the method proposed by Prasad et al. (2016) and by Jonsson et al. (2005). Turbine blades are normally made of nickel or rhenium alloys capable of withstanding high heat without distortion. The High Pressure Turbine airfoils as well as the Power Turbine airfoils can also be cooled. They can be made of INCO 738 coated with a silicon aluminide coating (Shepard et al. 1994).