Drag and Mixing from Forced Mixing Nozzles on Turbofan Engines

Abstract

An opportunity was identified for the suppression of unmixed high bypass ratio turbofan engine thermal signatures via forced mixing nozzles to increase mission safety. A feasibility study was proposed quantifying potential temperature and pressure uniformity factor enhancement at the expense of net thrust and added weight. An in-depth RANS-CFD parametric study of forced mixers on a full-scale target engine was warranted by gaps in the literature. First, a low-Mach experiment was designed to study the impact of fan generated swirl on mixing nozzle performance. Successful mixer design through reproducible methods observed no flow separation and converted angular momentum into enhanced thrust through the lobe passages. Normal and streamwise vortex features were quantified. Increased entrainment of mass flow accompanying greater than freestream levels of normal vorticity correlated best with total pressure uniformity index. Second, lack of full-scale engine geometry and operating conditions forced the creation of a quasi-2D design-point engine model of the PW2040 (F117-PW-100). An axisymmetrical geometry of the low-pressure system, derived from the model, was simulated at take-off and top-of-climb conditions using core intake and outlet modeled properties and a fan-stator source term user defined function. Taking the RANS-CFD results as baseline, mixer performance metrics could be readily compared. Finally, a full-scale lobed mixer parametric study, featuring 12 well-defined geometries, was performed by matching the core choke point. The effect of lobe number, length, height, crest/keel ratio, scalloping, and swirl was investigated on vorticity, drag composition, thrust, and mixing indices at top-of-climb. A direct tradeoff between net thrust and vorticity levels was observed with changes in mixer length for constant lobe height. Designs could enhance wake uniformity up to 25% at a cost of 5% in net thrust. Optimization would improve this result. Detailed engine maps could allow for the reduction in back pressure associated with enhanced mixing and jet perimeter to be propagated through the core engine. A vast quantity of literature, experimental methods, numerical setup, practical codes/scripts, and standard operating procedures were included to support the work and provide guidance and inspiration for future researchers.