System-Level Performance of Microturbines With an Inside-Out Ceramic Turbine

Abstract

Ceramic turbines can reduce fuel consumption by increasing turbine inlet temperatures (TIT). The need for heat-resistant materials like ceramics is particularly acute for small turbomachines for which efficiencies are limited by the use of uncooled metal turbine as complex cooling schemes are impractical and costly. Efforts to introduce ceramics in the turbine rotor were made between the 1960s and the 1990s by gas turbines and automotive manufacturers in the U.S., Europe, and Japan. While significant progress was made, a suitable level of reliability still cannot be achieved as the brittleness of ceramics leads to crack propagation in the blades loaded in tension and catastrophic failure. The inside-out ceramic turbine (ICT) is a design alternative specific to ceramics that loads the blades in compression by using an outer, air-cooled composite rim that sustains the centrifugal loads. This paper provides an analytical model based on the Brayton cycle to compute the system-level performance of microturbines using an ICT. Loss submodels specific to ICT architectures are developed to account for: (1) composite rim drag, (2) composite rim cooling, (3) leakage through rotating seals, and (4) expansion heat losses. The thermodynamic core model is validated against three state-of-the-art, non-inside-out, microturbines. Based on a Monte Carlo simulation that takes into account the modeling uncertainties, the model predicts a cycle efficiency of 45±1% for a 240 kW ICT-based microturbine, leading to a predicted reduction in fuel consumption of 20% over current all-metal microturbines.