A Silicon Microturbopump for a Rankine-Cycle Power-Generation Microsystem—Part II: Fabrication and Characterization

Abstract

In Part I of this two-part paper, the design approach for a microturbopump was presented. This second part describes the fabrication and experimental characterization of the demo microturbopump device, which includes hydrostatic bearings, a spiral-groove viscous pump, and a multistage microturbine. The device is composed of five wafers: one glass wafer, one silicon-on-insulator (SOI) wafer, and three silicon wafers. The silicon and SOI wafers are patterned using shallow and deep reactive ion etching (total of 14 masks), while the Pyrex glass wafer was ultrasonically drilled. Anodic bonding, fusion bonding, and manual assembly with alignment structures were then used to complete the device and enclose the 4-mm-diameter rotor. The approach allowed the microfabrication of unique interdigitated blade rows in the microturbine and interchangeable parts for flexible testing. After completion of the device, bearings were first tested in static and dynamic conditions. Then, the turbine was characterized with compressed air only and spun up to 330 000 r/min, producing 0.38 W of mechanical power. The pump performance map was also completely characterized for speeds up to 120 000 r/min, showing a maximum pump flow rate of 9 mg/s and maximum pressure rise of 240 kPa. In a turbopump system performance test using compressed air to the turbine and water in the pump, the rotor was spun up to 116 000 r/min, which corresponds to 25 m/s in tip speed. At this condition, the turbine produced 0.073 W of mechanical power with 41 kPa of differential pressure and 24 mg/s of flow rate, and the pump pressurized water by 88 kPa with a flow rate of 4 mg/s, maintaining constant efficiency of 7.2% over the operating range. Out of the total power produced by the turbine, 10% was consumed by the viscous pump, while the rest was dissipated by other components through viscous drag. The system-level predictions by models introduced in Part I also match the measured performance, suggesting that a valid design basis has been established for this type of rotating micromachine.