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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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