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Cooled Fast-Response Wall Pressure Taps for Combustion Chamber Measurements

Periodic Reporting for period 4 - FAST TAPS (Cooled Fast-Response Wall Pressure Taps for Combustion Chamber Measurements)

Reporting period: 2020-05-05 to 2021-05-04

Environmental aspects have been historically the main driver for step changes in the gas-turbine technology. This is the case of NOx emissions which triggered, since the late 80s, a vivid research activity on the development of low (nowadays ultra-low) NOx combustion chambers. The basic philosophy of low-NOx burner consists in controlling the flame temperature to hinder the NOx formation at high-temperature, a result that can be obtained by processing a lean fuel-to-air mixture. This principle was indeed adopted by the 2-stages burner of the ABB GT24/GT26 land-based gas-turbines that, in the mid-90s, were setting the state-of-art of the low-NOx technology. Despite their extremely advanced design, their commercial diffusion rapidly stopped when the first reliability issues started to appear: the lean combustion process of the GT24/GT26 machines induced strong pressure fluctuations that, once amplified by the high-pressure turbine rotor and by the acoustic coupling with the second burner, were then leading to a severe heat release on the burner’s wall and to high amplitude structural vibrations. In 1999, the high rate of failures of the ABB GT24/GT26 machines and the extremely high related costs, forced ABB to sell its Power Generation segment to Alstom who finally solved the issue on the GT series only in 2003, by adopting combustion control techniques.
Improved low-NOx technologies will be one of the key strategies to meet the challenging targets set by the Strategic Research and Innovation Agenda for 2035 and beyond (2050). Nevertheless, a further maturation is required for the lean combustion approach that implies an even better emission performance, a reduced noise foot-print, a wider operational flexibility and an improved reliability. In this view, a more detailed knowledge is required on the formation mechanism of combustion instabilities and of their dynamics, leading to the need of more advanced and more performant measurement techniques that could support an experimental assessment of the phenomena at engine conditions.
The FAST TAPS project aims at developing and building high-bandwidth fast-response wall pressure taps for combustion chamber measurements. The availability of this type of instrumentation would be a fundamental tool to extend the knowledge around combustion instabilities, while they will directly support the development of more performant combustion control system. The final goal of the FAST TAPS project is to build eight cooled wall-mounted fast-response pressure taps exhibiting high reliability and wide bandwidth. Such target will be achieved through different objectives.
The state-of-the-art regarding cooled (and uncooled) fast-response pressure designs and the related design methodologies has been covered in “Probes Literature Survey and Design Methodology Report” (Deliverable D1.1).
The theoretical Hougen and Helmholtz models have been implemented and used for the design of tapped-recessed or screened-recessed sensor layouts. Depending on the geometrical parameters of these sensor layouts, resonance frequencies limiting the usable un-calibrated frequency bandwidth of the measurement device have been obtained. With the adequate sensor recess layout (screened vs tapped, small vs great recess distance) and through effective dynamic compensation, the specified frequency bandwidth of 40 [kHz] can be achieved with a rather high confidence level. A preference exists towards screened-recessed sensor layouts as they would offer a better protection of the fragile sensing element against radiative heat loads.
As soon as the aerothermal environment in which the probe will be used had been described and delivered by Safran Helicopter Engines, a simplified 1D conjugate heat transfer model has been implemented and used for the design of a (water) cooling layout around the fragile sensing element. Quite realistic results have been obtained regarding the probe surface temperatures, the coolant temperature and wall heat fluxes per unit area (whether convective or radiative). The latter will be used as thermal boundary conditions for state-of-the-art CFD/CHT simulations, which are expected to yield a more detailed and accurate picture of the cooling layout around the sensing element of the probe.
A list of potential materials for the measurement device has been delivered to Safran Helicopter Engines for approval. A preferred material widely used in gas turbine manufacturing has already been identified, namely Inconel 600, thanks to its good welding properties, its resistance against oxidation at high temperatures, caustic corrosion and corrosion by high-purity water and thanks to its overall good mechanical properties (mechanical strength and creep resistance at high temperature).
Through weekly communications with the topic leader(s) at Safran Helicopter Engines, a thorough description of the external cooling system supplying the measurement device with high-pressure distilled water has been delivered to Safran Helicopter Engines. Its requirements (pumps, filters, valves, pipes, availability of tap water in the engine test lab) as well as its rough dimensions (1.6x2x0.8 m) have been delivered as well. A more thorough design of the cooling supply system will follow once the measurement device design is at a more evolved stage.
The present project will contribute to the achievement of the targets set by the Clean Sky Joint Undertaking programme (CS2) by providing the means to achieve a better understanding of the combustion instabilities phenomenon in real engine conditions. Such enhanced knowledge can then be employed in the design of advanced combustion chambers with a reduced NOx production and noise foot-print. From this point of view, FAST TAPS will add a small by robust and helpful contribution.
The WP3 of the Engine ITD can be considered as a direct reaction to the hegemony of the US engine manufacturers in the turboprop market. In the 1800-2000 shp class, the market share of North American actors reaches 83%, a figure that can be reduced only by providing as efficient and cost-effective solutions as possible. At first instance, FAST TAPS will directly contribute to the achievement of greener and more quite 1800-2000 shp class turboprop. Such result will derive from the deeper understanding of the combustion mechanism that the current project will allow. The highly reliable, high-bandwidth, time-resolved pressure measurements will directly serve this target. Secondly, FAST TAPS will pave the way towards the development of air-cooled fast response pressure taps. The availability of such type of tools will enable in the future (“… going beyond 2020…”) their implementation in production engines, providing therefore faster and more precise combustion monitoring capabilities as well as more performant combustion instabilities control strategies. Moreover, the availability of such technology can be considered a business by its own and it will contribute to the strengthening of the European competitiveness both on the sensors packaging and monitoring-control markets, sectors still dominated by extra-European actors.
Description of FAST TAPS project (scope and advancement)