Florin Iancu

Feasibility Study of Integrating Four-Port Wave Rotors into Ultra-Micro Gas Turbines (UµGT)

§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.

NUMERICAL SIMULATION OF UNSTEADY- FLOW PROCESSES IN WAVE ROTORS

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.

Three-dimensional investigation of thick single-lap bolted joints

Stresses in single-lap bolted joints of thick plates are complex and difficult to analyze. Previous studies involving stresses through the thickness of bolted joints have been limited to finite element method (FEM) simulations and have been implemented only for the joining of relatively thin plates. In this paper we report on several experimental and numerical analyses that were conducted to evaluate the stress distribution inside thick bolted plates along the bearing plane normal to the plate surface. Experimental analysis was conducted via embedded-polariscope photoelasticity and embedded resistance strain gages. The FEM analysis was performed with the ABAQUS commercial code using material properties and other data obtained experimentally as input. Experimental and numerical results agreed reasonably well, and are believed to depict the behavior of the joint under load well enough to assist in development of improved joint design.

Numerical Solutions for Ultra-Micro Wave Rotors (UµWR)

‡Starting in 1995, with the MIT “Micro Gas Turbine” project, the mechanical engineering research world has explored more and more the idea of “Power MEMS” 1 . Microfabricated turbomachinery like turbines, compressors, pumps, but also electric generators, heat exchangers, internal combustion engines and rocket engines have been on the focus list of researchers for the past 10 years. The reason is simple: the output power is proportional to the mass flow rate of working fluid through the engine, or the cross-sectional area and the mass or volume of the engine is proportional to the cube of the characteristic length, thus the power density (Power/Mass=L -1 ). This is the so-called “cube square law”. Although in theory everything is perfect, the following investigations showed that there are many engineering challenges at microscale and the solutions found in the past half of century for large scale mechanical devices do not necessarily apply to the new design space. This paper studies the possibilities of incorporating a wave rotor to an ultra-micro gas turbine. It discusses the advantages of wave rotor as topping units for gas turbines, especially at microscale and proposes some designs of ultra-micro wave rotors. The numerical simulations of these wave rotors are presented, results obtained using FLUENT, a Computational Fluid Dynamics (CFD) commercial code.