The efficiency of compression processes tends to decrease dramatically when the overall dimensions are scaled down to micron level. Then it can be assumed that the compression is more efficient if no moving parts are involved in the process. This is achieved by replacing, at least partially, turbo-compressor stages with a wave rotor. The work starts with estimating wave rotor efficiency at microscale by extrapolation. Then, an analytical model is introduced from which the theoretical efficiency of compression process in a microchannel is deduced. Knowing the inlet conditions and the pressure gain across the shock, the overall efficiency of the compression can be calculated. The model assumes constant friction along the walls and no heat exchange with the surroundings. The results suggest that an efficiency of 70-80% can be achieved in the channels of an ultra-micro wave rotor. It is shown that if the inlet temperature is high enough (about 1500 K), the efficiency is even higher, up to 90%.Keywords Wave rotor AE Shock wave AE Microchannel AE Efficiency AE CFD Nomenclature aSpeed of sound (m/s) CpSpecific heat (J/kg K)

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Ultra-micro gas turbines (UµGT) have shown difficulties in obtaining high overall thermal efficiency and output power, resulting from miniaturization. Particularly, obtained compressor efficiencies have been as low as 40-50%, reducing optimum pressure ratios down to about 2. This work presents investigations of the feasibility and potential of integrating four-port wave rotors in microfabricated gas turbines to increase compression efficiency and optimum overall pressure ratio, hence increase overall cycle efficiency and power output. Practical implementation schemes and results of efficiency estimates are shown. The wave rotor efficiency is estimated first by simple extrapolation and then verified by a mathematical model. The model is based on gas dynamic equations for a moving normal shock wave in a channel and considers wall friction of the gas flowing through the channel. Knowing the inlet conditions and the pressure gain across the shock, the overall efficiency of the compression process in a wave rotor channel can be predicted. The results suggest that a compression efficiency in the range of 70-80% can be achieved in ultra-micro wave rotors. Based on thermodynamic cycle analyses a performance map was created that also gives optimum pressure ratios for a typical UµGT application.

The wave rotor (pressure exchanger) is a device working based on a relatively simple idea of operation, but is challenging in its technical realization and difficult to simulate numerically. It has been common practice to create and use specialized codes for simulating the wave rotor operation. The current work presents an attempt of developing 2D and 3D models of radial and axial wave rotors using the commercial software package FLUENT. In this study geometrical models are used for the device casing and rotor cells. The application of carefully chosen initial and boundary conditions enabled the realization of relative motion of the rotor model. The vast information about the unsteady processes occurring during simulation are visualized. It occurs that such type of models are useful for the final test of devices, after the geometry was optimized by the use of specialized but much simpler 1D codes.