As global air traffic continues to grow, the need for cleaner, more efficient airplanes is rising right along with it. In an effort to reduce the global impact of pollution attributable to aviation, the International Civil Aviation Organization (ICAO) adopted new CO2 emissions standards in 2017 for commercial aircraft, requiring new aircraft type designs to meet these standards before delivery. These regulatory requirements, coupled with airlines’ desire to reduce fuel expenses and other costs, drive engine makers to seek every possible advantage in producing more efficient aircraft engines.
One way to reduce an aircraft engine’s emissions and improve engine performance is through active clearance control (ACC). This is achieved by managing the clearance between the gas turbine casing and the tips of the rotating blades, referred to as turbine tip clearance. An engine’s turbine clearance control system (TCCS) relies on turbine clearance control valves (TCCVs) to control this tip clearance by managing the thermal expansion of the turbine case that surrounds the turbine stages of the engine.
The Fluid Systems Division of Parker Aerospace developed its line of TCCVs with a goal of exceeding customer requirements for reliability, safety, and performance. The product offers engine manufacturers a proven control mechanism that has not only undergone extensive testing but demonstrated improvements in engine fuel burn, which translate into measurable savings for its engine and airline customers.
Turbine tip clearance between the turbine blades and the turbine case is a key parameter that influences turbine efficiency and the propulsive efficiency. The tip clearance should be kept to a minimum value, considering the turbine blade and the case expansion resulting from temperature excursions during the entire operating envelope of the engine. These temperature excursions are a result of the extremely hot combusted gases that enter the turbine stage of the engine, downstream of the combustion chamber and provide the thrust required to power the engine.
The combusted air temperatures can be in excess of 2,000°F, resulting in the expansion of the turbine blades and the case, thereby increasing the tip clearance and loss of turbine efficiency. The net effect is that more fuel needs to be combusted to compensate for this loss of efficiency, in order to generate the required thrust, resulting in increased fuel burn and increase in specific fuel consumption.
By controlling the thermal expansion and contraction of the engine’s turbine casing over its operating envelope, engine manufacturers can better optimize the turbine tip clearances in the engine. A proven method of controlling this clearance is to either direct cooler air around the turbine case to cool and contract the casing ‒ or ‒ to restrict the cooler air, allowing the casing to expand when required to compensate for the turbine blade expansion. thereby maintaining the tip clearance.
This delicate balance is realized through temperature sensors in the engine that measure turbine air temperatures during the entire flight cycle. This information is relayed in real time to the engine’s full authority digital engine control (FADEC), an autonomous, system that monitors and controls all aspects of an engine’s operation, including its turbine tip clearance control system.
Depending on the flight status, the FADEC sends electrical commands to the engine’s turbine clearance control valves, signaling them to incrementally open or close (modulate the flow through the valve), to control the case thermal expansion. The opening and closing of these valves ultimately controls the amount cooling air taken from the engine’s bypass flow to manage engine casing temperatures, thereby facilitating optimum blade tip clearance control.
Parker’s TCCV consists of a butterfly valve actuated using an integral fuel-actuated actuator. The fuel actuator consists of a Parker electro-hydraulic servo valve (EHSV) integrated as part of the actuator. The EHSV receives an electrical command from the FADEC and directs the fuel flow appropriately for the actuator to either extend or retract the actuator rod. Actuator retraction or extension results in modulating the valve position to either fully open or fully closed or anywhere in between, depending on the stage of flight.
The actuator and the valve position are monitored by a linear variable displacement transducer (LVDT), which is integrated within the actuator rod. The LVDT provides the position feedback to the FADEC, which through its built-in software deduces the position of the valve (hence, the TCCV flow). Therefore, the TCCV valve system forms a closed loop sub-system with the FADEC; it receives a command, executes, and relays back the result of its action back to the FADEC for further instructions.
Turbine clearance control valves operate in a hostile environment, being exposed to aircraft engine surrounding air temperatures that can range from -65° to 350° Fahrenheit. The valves also handle the contaminated air flowing through them, as well as engine-induced vibration, and continue to function throughout the engine life.
To survive and perform in this environment, Parker’s butterfly-type valve incorporates several design features to enhance valve life, reliability, and performance. Features such as specially designed dynamic seals have been validated for long-term performance under extreme conditions, enabling superior sealing capability, low friction, and high wear resistance.
These seal designs are critical in ensuring that air flowing through the valve does not leak externally. This type of leakage is wasteful; not only does it rob the thrust-producing bypass air, it also results in less-than-optimum functionality of TCCV sub-system. Together the valve and actuator designs have a proven track record of meeting strict fire requirements during flight certification. The mechanical linkages between the actuator and the butterfly valve shaft are designed to withstand the vibration and endurance cycles required to ensure accurate position feedback and control of the TCCV system.
Parker’s Jet-Pipe® electrohydraulic servo valve (EHSV), designed and manufactured by the Parker Aerospace Control Systems Division. The EHSV is a proven, robust two-stage design that is contamination-resistant, providing the accuracy needed to precisely move the actuator to its commanded position, while providing the durability needed for long, trouble-free service life.
Parker Aerospace’s Fluid Systems Division in Irvine, California, has been providing TCCVs to engine manufacturers for nearly 40 years, continually improving the design and performance of its valves, making them extremely accurate and durable. Our longstanding engine customers include Rolls-Royce, GE Aviation, and Pratt & Whitney, among others.
Parker’s Fluid Systems Division offers its customer the benefit of extensive in-house testing capabilities for its TCCVs as well as its full line of products and systems. Parker TCCVs are designed and tested to meet and exceed vibration and endurance life requirements.
Complete endurance testing of the valves to multiple life cycles, which includes applying a full flight profile to simulate flight conditions and mimic valve performance in flight, helps ensure a TCCV design that has achieved maturity at entry into service. Our endurance test routines also include the introduction of contaminants to further prove the valves’ integrity. Additionally, we provide complete control system simulation models of the TCCV control system, utilizing either SIMULINK or Amesym for our engine customers, who in turn use this model within their larger engine control system model.
By working with our engine customers and aircraft operators, Parker FSD engineers have turned lessons learned into bankable savings for our end-use customers. The valves are designed for maintainability with the goal of lower removal and installation times on wing while achieving optimum repair and overhaul times. Put very simply, Parker valves offer lower total-lifecycle cost proposition for our customers.
The extensively tested and proven technology of Parker’s turbine clearance control valves allows aircraft engine manufacturers to achieve their desired engine performance, including extended service life while reducing fuel consumption (lower specific fuel consumption) and fuel emissions. By helping airlines meet more stringent international standards for CO2 emissions, Parker and its engine manufacturing partners become part of a global commitment to ensure an environmentally responsible future for aviation.
For additional information on Parker Aerospace systems and capabilities, please visit our website.
This post was contributed by Sanjay Bhat, new business development manager for Parker Aerospace’s Fluid Systems Division.