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Advances in the Development of a Microturbine EngineTo reduce the size and weight of power generation machines for portable devices, several systems to replace the currently used heavy batteries are being investigated worldwide. As micro gas turbines are expected to offer the highest power density, several research groups launched programs to develop ultra micro gas turbines: IHI firm (Japan), Power-MEMS Consortium (Belgium). At Onera, a research program called DecaWatt is under development in order to realize a demonstrator of a micro gas turbine engine in the 50 to 100 Watts electrical power range. A single-stage gas turbine is currently being studied. First of all, a calculation of the overall efficiency of the micro gas turbine engine has been carried out according to the pressure ratio, the turbine inlet temperature, and the compressor and turbine efficiencies. With realistic hypotheses, we could obtain an overall efficiency of about 5% to 10%, which leads to around 200 WI kg when taking into account the mass of the micro gas turbine engine, its electronics, fuel and packaging. Moreover, the specific energy could be in the range 300 to óOOWh/kg, which largely exceeds the performance of secondary batteries. To develop such a micro gas turbine engine, experimental and computational work focused on: (1) a 10-mm diameter centrifugal compressor, with the objective to obtain a pressure ratio of about 2.5: (2) a radial infiow turbine; (3) journal and thrust gas bearings (lobe bearings and spiral grooves) and their manufacturing: (4) a small combustor working with hydrogen or hydrocarbon gaseous fuel (propane): (5) a high rotation speed microgenerator: and (6) the choice of materials. Components of this tiny engine were tested prior to the test with all the parts assembled together. Tests of the generator at 700,000 rpm showed a very good efficiency of this component. In the same way, compressor testing was performed up to 500,000 rpm and showed that the nominal compression rate at the 840,000 rpm nominal speed should nearly be reached.

An optimization process is used to design bladings in turbomachinery. A gradient-based method is coupled to Navier-Stokes solvers and is applied to three different bladings. A new rotor blade of a transonic compressor is designed by using a quasi three-dimensional approach, with a significant efficiency improvement at the design point. The off-design behavior of this new compressor is also checked afterwards. The same quasi three-dimensional approach is used on a stator blade of a turbine, but the whole stage is computed in this case. The losses are locally reduced, proving the good sensitivity of the solver. Finally, a new three-dimensional rotor blade of a compressor is designed by applying deformation functions on the initial shape. The efficiency is improved over a wide range of mass flow. The whole results indicate that the optimization process can find improved design and can be integrated in a design procedure.

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This paper presents the validation of a design method for aspirated compressor blades, combining a passive separation control by blade shaping with an active flow control by aspiration. In a first part, a linear aspirated cascade designed according to this method was built and tested at low speed, without and with aspiration. The latter was only applied on the suction surfaces of the blades. Particle Image Velocimetry measurements performed at mid-span of the cascade, in the central passage, showed a complete reattachment of the separated boundary layer on the suction side of the blade. A flow deflection of approximately 65 degrees was achieved requiring an aspirated mass flow rate of 3.3%. However, boundary layer reattachment is effective in a zone centered at mid-span covering 30% of blade span. Flow visualization revealed large corner separation in the presence of aspiration. This is due to the re-establishment of strong pressure gradient on sidewalls of the cascade. No flow control was applied on these zones for optical access purpose. These secondary-flow regions reduced the diffusion occurring within the cascade by nearly 60% in comparison with the design intent. They also increased the expected level of total pressure losses measured by wake traverses downstream of the cascade. In a second part, numerical simulations of the aforementioned experiment were carried out to help the understanding of the experimental results. The simulations were able to reproduce correctly the characteristic flow features, without and with aspiration, observed and measured during the experiment. Thus, they confirmed the potential of this design method developed for aspirated compressor blades, as well as CFD capabilities to simulate the influence of technological effects like suction slots. A uniform and a non-uniform aspiration distribution along the blade span direction were considered during simulations. Suction distribution was found to have a significant impact on the control by aspiration. This design feature, in addition to flow control on endwalls, has to be taken into account in the three-dimensional design of highly loaded aspirated compressor blades.

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