On the Optimisation of a Geared Fan Intercooled Core Engine Design

Abstract

Reduction of CO2 emissions is strongly linked with the improvement of engine specific fuel consumption, as well as the reduction of engine nacelle drag and weight. One alternative design approach to improving specific fuel consumption is to consider a geared fan combined with an increased overall pressure ratio intercooled core performance cycle. Thermal benefits from intercooling have been well documented in the literature. Nevertheless, there is little information available in the public domain with respect to design space exploration of such an engine concept when combined with a geared fan. The present work uses a multidisciplinary conceptual design tool to further analyse the option of an intercooled core geared fan aero engine for long haul applications with a 2020 entry into service technology level assumption. The proposed design methodology is capable, with the utilised tool, of exploring the interaction of design criteria and providing critical design insight at engine-aircraft system level. Previous work by the authors focused on understanding the design space for this particular configuration with minimum specific fuel consumption, engine weight and mission fuel in mind. This was achieved by means of a parametric analysis, varying several engine design parameters — but only one at a time. The present work attempts to identify “globally” fuel burn optimal values for a set of engine design parameters by varying them all simultaneously. This permits the non-linear interactions between the parameters to be accounted. Special attention has been given to the fuel burn impact of the reduced HPC efficiency levels associated with low last stage blade heights. Three fuel optimal designs are considered, based on different assumptions. The results indicate that it is preferable to trade overall pressure ratio and pressure ratio split exponent, rather than specific thrust, as means of increasing blade height and hence reducing the associated fuel consumption penalties. It is interesting to note that even when considering the effect of HPC last stage blade height on efficiency there is still an equivalently good design at a reduced overall pressure ratio. This provides evidence that the overall economic optimum could be for a lower overall pressure ratio cycle. Customer requirements such as take-off distance and time to height play a very important role in determining a fuel optimal engine design. Tougher customer requirements result in bigger and heavier engines that burn more fuel. Higher overall pressure ratio intercooled engine cycles clearly become more attractive in aircraft applications that require larger engine sizes.