FLUG REVUE October 2004: MTU prepares CLEAN engine demonstrator

CLEAN DEMONSTRATOR POISED FOR TESTS

By Patrick Hoeveler

CLEAN could not have come at a better moment. After all, the recent hike in the price of oil is causing disquiet at many airlines. It is therefore even more imperative that fuel consumption is reduced. But today's engines offer only minor potential for improvement. Only new concepts, such as the intercooled recuperative engine can offer major advances. The key components of this technology – geared turbofan and heat exchanger – are to be demonstrated on the high-altitude test facility of Stuttgart University with the Component Validator for Environmentally Friendly Aero Engines (CLEAN) commencing in September. The project promises to achieve reductions of 80 percent in nitrogen oxide emissions and of up to 20 percent in carbon dioxide emissions. Fuel consumption could decline by a full 20 percent.

CLEAN by MTU

In a recuperative engine, first of all the air between the low and high pressure compressors is cooled in a circular process. This means that the high pressure compressor can operate at a lower overall pressure ratio (OPR) because the air does not expand as much as normal. The airflow is then conducted into the heat exchanger positioned in the exhaust flow, heated up and conveyed to the combustor, which thus requires less fuel to achieve the necessary temperatures.

On the other hand, this architecture requires a gearbox between the low pressure shaft and fan for the biggest possible bypass ratio. The fan has a large diameter and rotates correspondingly slowly, whereas the turbine is able to turn at high speed.

The CLEAN demonstrator is based on a medium-range engine producing 133 to 155 kN of power, but lacks a fan. At the heart of the demonstrator is a 70cm long heat exchanger developed by MTU, in which the cooler air is collected in a pipe and then conducted in 2478 individual pipes through the hot exhaust flow. The total exchange area measures 22 square meters. The air thus heated to around 200ºC is collected in a second pipe and led to the combustor. The individual, 6mm wide pipes have an elliptical cross section and have sufficient freedom of movement to compensate for temperature stress. They consist of steel on a nickel base. “Titanium would be lighter, but also expensive,” explaines Dr. Hermann Scheugenpflug, head of technology management at MTU, in an interview with FLUG REVUE.

When fully developed, a recuperative engine would possess eight such elements, four of them arranged in a rectangle around the diffuser cone, with the other four closing the system in a conical arrangement. On the CLEAN demonstrator, there is only one module, which will be tested in two different positions.

The axially staged combustor developed by Snecma and Avio has further emission reduction potential. It consists of a conventional pilot stage and a main stage for the full-load area. During injection, swirl generators in the latter produce a pre-mixed and pre-vaporised mixture of fuel and air (Lean Premixed Prevaporised, LPP) which is supposed to improve the effectiveness of the combustion process.

The high pressure compressor with active surge control developed by Snecma helps to improve efficiency. It is equipped with pressure sensors that are able to detect small amplitude flow precursors so that an actuator system with adjustable guide vanes can react before any instabilities can occur. The compressor is based on the DEM21 core, which also forms the basis for the SM146 from PowerJet (see FLUG REVUE 9/2004). The control system was developed under the 4th EU Framework Programme, in which MTU was also involved. The high pressure turbine is a single stage design.

The three-stage, high-speed low pressure turbine from MTU was developed under the Engine 3E research programme. As Dr. Scheugenpflug explained, “It is a continuation of the technology with regards to aerodynamic design and efficiency.” The high rotational speeds result in heavy loading of the discs and blades, which necessitate a greater profile thickness in the hub area of the blades. Additionally, the first stage has to be cooled because of the high temperatures. The first low pressure guide vane is integrated in the intermediate casing.

The heat exchanger concept has already been demonstrated, for example, in gas turbines for tanks. However, aviation poses different challenges, such as the need to maintain thermal and aerodynamic effectiveness over the entire cycle, plus the manufacturing costs and, of course, integration into the extremely challenging environment of an aero engine. Thus the EU's Advanced Exhaust Gas Recuperator Technology for Aero Engine Applications (AEROHEX) programme sets out the following objectives for the heat exchanger: it must not increase the engine weight by more than 15 percent, overall length by more than five percent and costs by more than 15 percent. Service life should be 36,000 hours or 5,500 long-distance flight cycles. The CLEAN heat exchanger is still a lot too heavy. The pressure losses over the long route (the tiny pipes together add up to 1593m in length) are another critical factor and must not exceed 10 percent. “Ultimately the efficiency of the heat exchanger will depend significantly on the extent of pressure loss,” explains Dr. Scheugenpflug.

To investigate precisely these characteristics, the research team has planned a total of 75 operating hours on the test facility by the end of October, during which complete flight cycles will be simulated. The main focus here will be the system as a whole, as the engineer explains. “The heat exchanger must be viewed in the overall concept, not just in relation to component efficiency.” The first two weeks of testing will concentrate on checking that the experimental set-up as a whole is working properly. Data will be evaluated online. Dr. Scheugenpflug expects the first results at the beginning of October.

There is still a long way to go before a specific application is developed, although some components, such as the combustor and low pressure turbine, should be available between 2010 and 2015. Here the designers have their eye on a possible successor to the V2500 of IAE. Whether some of these innovations could be used on the new engine for the Bombardier CSeries (see FLUG REVUE 9/2004), which is due to be certified in 2008, remains to be seen.

“The heat exchanger technology is longer-term. We are talking here of 2020 onwards,” says Dr. Scheugenpflug. “Moreover, in practice it will be necessary to develop the aircraft around the engine so as to achieve the full capacity. The 20 percent reduction in fuel consumption can only be achieved by optimising the system as a whole.” For example the shift in the centre of gravity of the engine alone prevents it from being mounted on the pylon of a current aircraft type. Moreover, every compromise reduces the savings still further. On the other hand, when one considers the possible framework conditions that could prevail in the year 2020, such as a high oil price, fuel taxes or emission-dependent charges, even relatively small reductions could make all the difference.

From page 96 of FLUG REVUE 10/2004

Last updated 13 September 2004

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