Inverted Gas Turbine Design and Analysis

Abstract

Typical turbine engine technology has followed what is known as the Brayton Cycle (BC) since its conception in the 1920s. These engines have grown to be more powerful and more efficient, as well as smaller and more readily accessible for multiple applications. Currently, there is a demand for turbine engines for micro turbine technology for unmanned aerial vehicles and micro air vehicles, as both a source of propulsion and a source of power. Efficiencies of these micro components at high pressures is reduced from larger engine components, creating wasted energy, due mainly to tip clearances and tolerances on components such as the compressor and turbine. One proposed solution to this is the use of the Inverted Brayton Cycle (IBC). Instead of the typical compressor-combustor-turbine setup, the incoming air is mixed with fuel at atmospheric conditions upstream of the turbine. The heated flow is expanded and cooled through the turbine to sub atmospheric pressures. The flow is then compressed by the compressor to atmospheric pressure before exiting the engine. The advantage to this cycle comes from the increased component size as compared to the mass flow though the engine. This enables lower losses within the rotating components such as the compressor and turbine, and thus reduces the inefficiencies inherent in very small turbine engines. Experimental test data on a small turbine engine was taken in a standard BC configuration as a baseline and compared to published data to determine accuracy. The combustor, compressor, and turbine from the same engine were then used to create an IBC engine. An analytical model of the BC and IBC were done to predict expected performance of the IBC engine. Based on these results, the IBC configuration shows potential for increasing efficiency in micro turbines by operating at low pressures in the turbine. This allows for reduced losses throughout the hot section, especially in the turbine, where large tip clearances (with respect to overall vane height) are undesirable.