Development of a compact radial recuperator prototype for a previously demonstrated 8 kW turboalternator has been completed. Its novel geometry has resulted in measured heat transfer effectiveness that surpasses 90% at the operating conditions of the engine, with acceptable pressure penalty (< 35 kPa) that can be easily accommodated by the compressor. Unrecuperated, the oil-free, high-speed micro-turboalternator, operating at 180,000 rpm and featuring compliant-foil bearings, presented a thermal (or cycle) efficiency of 12%. With the recuperator, thermal efficiency of approximately 30% is possible. The recuperator configuration is unique from both a geometric and a heat transfer point of view. Its radial (axisymmetric) configuration allows for compact system integration, concentric to the existing engine/combustor hardware assembly. While the addition of the recuperator prototype has increased the overall weight of the system, and hence reduced its specific power from the previously reported 1.6 kW/kg (1 hp/lbm) to approximately 0.9 kW/kg, the unprecedented gain in efficiency by such a compact device justifies its implementation. Furthermore, it is anticipated that continuing prototype refinement, along with the use of novel manufacturing technologies and materials (e.g. 3D printing and ceramics) will result in a significant increase in power density. Performance characterization has been performed for mass flow rates up to 0.08 kg/s and gas inlet temperatures up to 925 K, which are representative of meso-scale turbine engines. Scalability of the device has also been evaluated, down to a 1 kW engine, and up to MW order.

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