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

摘要

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