Generalized Optimization of Counter-Rotating and Single-Rotating Fans

Abstract

On possible fan concept for future high and ultra-high bypass ratio turbofan engines is the counter-rotating (CR) fan. Several studies [1][2][3][4] dealt already with the optimization of CR fans, however the mass flow and the total pressure ratio were typically given and fixed for a specified application. The results of these studies showed a benefit of the CR fan compared to the conventional single-rotating (SR) fan, which strongly depended on the engine cycle. Following this experience, it was necessary to further specify the efficiency benefits more precisely in association with fan total pressure ratio and fan inlet axial Mach number. The results are discussed in this present paper. A special emphasis was given on determining the optimal pressure ratio, for which the CR-fan expectably achieves the maximal efficiency benefit. The idea was to perform a global optimization study without any constraints for the operating point inside of a broad (ΠFan, Max) –range, for the rotational speeds and with only a few constraints for the geometry of the blades to avoid infeasible geometries. An adequate range for the fan pressure ratio (ΠFan) and for the axial Mach number (Max) was chosen for the global optimization covering the entire range from current to potential future ultra-high bypass ratio engine applications, also taking into account a reduced nacelle diameter and thus high axial fan inflow Mach numbers. The focus of the present study was to develop a method for the global optimization of a fan stage. As a result of this study, the maximal achievable efficiency is shown as a function of the fan pressure ratio and the axial Mach number. Thus the efficiency differences between the CR and SR fan can be calculated through the differences between the surfaces for any given set of parameters defining a potential engine. This allows for a generalized assessment of this particular fan concept over the entire range of relevant applications.