To meet the increasing demand for advanced portable power units, for example for use in personal electronics and robotics, a number of studies have recently focused on small gas turbine units in the 500 W to 1 kW range. The majority of the work to date is concerned with the design of efficient high-speed rotating machinery and electric components. An important aspect, especially critical for portable operation, is the cooling of the gas turbine and the exhaust gas. This is the focus of the present paper. The compact and small-scale architecture of such gas turbine engines poses major challenges in the thermal management as the required cooling mass flow for portable operation is relatively large and the flow mixing length is short and constrained by package size considerations. Previously, a mixer/ejector based cooling scheme was proposed and vortex generator rings and multi-walled ejector configurations were experimentally investigated with the goal to enhance the mixing of the exhaust gas with cooling flow [1]. Although the augmentations achieved a satisfactory cooling mass flow ratio of 16.8:1, hot spots still existed at the exit of the relatively long mixer duct due to the high area-ratio of the ejector configuration. To overcome this mixing challenge, an alternative cooling scheme was conceived. In this scheme, the hot exhaust gas flow is forced radially outward through a perforated cylindrical liner into the cooling air flow surrounding the exhaust duct. The concept resembles that of an inverted dilution liner where the hot exhaust gas is injected into the much larger cooling mass flow. The hypothesis is that the array of streamwise vortices formed by the hot jets reduces the mixing length and significantly mitigates the temperature non-uniformity. The design space was first explored using a control volume (CV) analysis and the performance of the proposed device and the detailed flow features were investigated using three-dimensional Computational Fluid Dynamics (CFD) simulations. The computations demonstrate enhanced mixing which reduces the turbine exhaust gas temperature of 630°C to a temperature distribution below 75°C at the mixer exit, comparable to the temperature levels and non-uniformity of a commercial hand dryer. The cooling mass flow ratio and required cooling fan power were 15.4 and 1.9% of engine power output respectively. Flow mixing guidelines were established together with a concept mixer configuration, generally applicable to small scale gas turbine devices.

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