The NexGen Burner: Analysis of Turbulent Fuel-Air Mixing

Improving aircraft survivability in case of a fire is the key to improving safety. To achieve this goal, the Federal Aviation Administration (FAA) requires fire certification to determine the survivability of aircraft components in case of a fire. Any test environment used for certification must meet the temperature and heat flux requirements defined by federal regulations. To this end, the FAA is developing the NexGen burner in an attempt to standardize fire tests. In support of this effort, this paper describes a comprehensive computational investigation to identify the interactions between high-temperature air and (vaporizing) Jet A and its subsequent mixing with air in the NexGen burner. This is a sister paper to our previous article [1], where we discussed the dynamics of spray and swirling air under room temperature conditions. The burner consists of a draft tube, a stator that introduces swirl to the incoming air, a turbulator downstream of the stator to intensify turbulence, and a burner cone where Jet A fuel, injected through a Delavan type W nozzle at 2.5 Gph under room temperature conditions, vaporizes and subsequently burns. Air enters the burner at a velocity of 3.5 m/s at a temperature of 600 K; these conditions correspond to a Reynolds number of 26,872. The liquid jet is injected as a semi-hollow cone characterized by a sauter mean diameter of 38.0 microns. The gas phase is described in an Eulerian frame using a compressible large eddy simulation framework, and the discrete phase is modeled in the Lagrangian frame to simulate droplet kinematics, heating, and vaporization. Observations indicate that maximum vaporization occurs in the conical zone created by the semi-hollow spray cone. However, the vaporized fuel extends outward as a result of the expansion of the swirling core. A detailed turbulence budget analysis is conducted to discern energy transfers and interactions within the turbulent flow field. Turbulence production is observed to lead primarily to the generation of axial Reynolds normal stress Rxx. Furthermore, the correlation between pressure and strain rate plays an important role in coupling the axial Reynolds stress Rxx with the normal components Rrr and Rθθ, leading to a transfer of turbulent kinetic energy from the axial component to the azimuthal and radial components.