Numerical Analysis of Powered Turbofan Engine Flow Suppression Effects

Abstract

Future ultra high bypass ratio turbofan engines will improve efficiency by decreasing fan pressure ratios and nozzle pressure ratios. With this shift of the engine’s operating point, the fan nozzle is expected to be unchoked even in cruise flight and the fan nozzle flow is affected by the flow conditions behind the nozzle exit. Nacelle external flow and installation effects influence the static pressure behind the engine, leading to a static pressure different from far-field ambient conditions. The interaction between the wing, the nacelle external flow and the engine efflux lead to a variation in the nozzle pressure ratio in unchoked conditions and, therefore, to a change in mass flow rate and gross thrust. The actual pressure at the nozzle is partially disregarded for gas-path measurements based thrust determination in wind-tunnel and flight test data reduction according to industry legacy thrust drag bookkeeping approaches. The consequence is that for a nozzle cp of 0.2, the net thrust may be overestimated by more than 1% for aircraft equipped with UHBR turbofan engines and fan pressure ratios of 1.2. Berens et al. [1] performed a parametric study using RANS simulations of NASA’s Common Research Model (CRM) and a Through-Flow Nacelle (TFN) to investigate how different flight conditions impact pressure coefficients at the engine’s nozzle. The cp results for the NASA HS CRM TFN are particularly high, both in isolated as well as installed conditions, which may have masked some of the airframe induced effects. Nozzle cp’s for the isolated powered nacelles in [2] are substantially lower compared to the CRM TFN. In this paper, a turbofan propulsion simulator model will be implemented into RANS-simulations with the goal of reducing uncertainties and drawing a more realistic picture of the flow field around the engine both in the isolated as well as the installed configuration. Suppression effects are quantified in terms of the fan nozzle exit pressure coefficient, the applied nozzle pressure ratio and the fan nozzle mass flow. The impact of flow suppression on thrust determination is investigated for a range of cruise flight Mach numbers, various thrust ratings and various angles of attack.