Theoretical and Experimental Investigation of a 34 Watt Radial-Inflow Steam Turbine With Partial-Admission

The Laboratory for Applied Mechanical Design (LAMD) designed and experimentally realized a micro steam turbine with a tip diameter of 15 mm. The nominal mass flow rate and the total-to-total pressure ratio is 2.1 kgh/h and 1.9, respectively, yielding a power of 36 W and a total-to-static isentropic efficiency of 38 %. The power and efficiency could be measured with two extremely thin thermocouples (Ø 0.15 mm), that were placed upstream of the turbine in the volute and downstream of the turbine behind the trailing edge.

The steam turbine is conceived as a radial-inflow, low-reaction (15 %), and partial admission (21 %) one. Since the steam mass flow rate is a limiting factor, a low-reaction and high-power-density design is preferred. The partial-admission design allows for reduced losses: The turbine rotor and stator blades are prismatic and have a radial chord length of 1 mm and a height of 0.7 mm. Since the relative blade tip clearance (0.160) is relatively high, the blade tip leakage losses are significant. Considering a fixed steam supply, this design allows to increase the blade height, and thus reducing the losses.

The steam turbine drives a dynamic steam-lubricated shaft and a radial fan. The fan operates at low Mach numbers, the design of the turbine is therefore challenging, since it operates at choked conditions, and thus at Mach numbers > 1. A numerical simulation correlates reasonable to the experimental results. Conventional zero- and one-dimensional analytical pre-design models correlate well to the experimental results, despite the extremely small turbine geometry.