Characterization and design of non-adiabatic micro-compressor impeller and preliminary design of self-sustained micro engine system

Abstract

As part of the MIT research program on micro-engines (of size [approximately] 1 cm), this thesis defines concepts and designs to improve micro-turbomachinery and overall system performance. Three-dimensional Reynolds-averaged Navier-Stokes computations (FLUENT) have been carried out to quantify the performance limiting processes in micro-impellers. These processes include (i) heat transfer to the compressor flow responsible for up to 25 points efficiency penalty, (ii) impeller casing drag (17 points penalty) and (iii) passage boundary layer loss (10 points penalty). The magnitude of the first effect is a result of the engine small length scale selection and is characterized by the total heat to impeller flow as fraction of inlet flow enthalpy. The magnitudes of the last two effects can be attributed to low Reynolds number. Scaling laws for elucidating the parametric controlling trend in these effects have been formulated. A mean-line analysis and design tool based on the above micro-impeller characterization is developed to formulate design guidelines. The guidelines show that the optimal micro-impeller geometry changes with impeller wall temperature, an effect, not present for large turbomachinery. In particular, impeller inlet angle, back-sweep angle, solidity and radial size for peak efficiency decrease with increasing impeller wall temperature. This behavior is a result of the competing effects of geometry on (i) aerodynamic loss and (ii) on heat transfer to impeller flow. In accord with these findings, CFD calculations show that configuring a micro-impeller excluding the heat addition as a design variable can incur a penalty of more than 10 efficiency points. An aero-thermal system model is developed to enable micro-engine system analysis and

(cont.) selection of system design parameters. It is shown that, in contrast to large engine design, an optimal turbine inlet temperature, associated with peak system efficiency, exists for the micro-engine thermodynamic cycle. This condition is related to the competition between the benefit in cycle performance associated with increasing turbine inlet temperature, and the degradation in compressor performance associated with increasing heat transfer. Furthermore, system efficiency approximately doubles as turbomachinery size is scaled up two times. This is related to the different scaling of heat transfer, parasitic power loss, and Reynolds number in micro-engines. Minimum requirements on advanced technology levels are established for a self-sustaining micro-engine. Two designs, based on different advanced technologies, producing 10-20 Watts of net shaft power with chemical-to-shaft mechanical conversion efficiency of 1.5-2.0% are proposed for micro-engine development.