Numerical Analysis of Combustor Burn-Through Representative Jet

Aircraft safety has always been of paramount importance to the civil aviation industry. The continuous increase in the number of flying hours has caused the necessity to reassess those safety issues that have historically been rare, as such, though an event may be statically improbable, the total number of occurrences can increase with an increase in the total number of operational flying hours. One such aircraft safety issue is combustor burn-through, which is a recognised cause of fire originating from within the aircraft engine casing.

The nature of the jet associated with combustor burn-through is that of a highly under-expanded jet, due to the internal combustor-to-ambient pressure ratio. Nevertheless, the nature of the flow can be considerably changed in the presence of a solid boundary placed downstream of the nozzle. In such conditions, a strong shock is formed, and the nature of the jet is varied. Outside of the core flow, recompression to the ambient pressure is achieved through compression waves that coalesce to form a shock structure known as a barrel shock. This shock structure is of an oblique shock nature and is initially swept away from the nozzle centre-line axis by the radial component of velocity from the jet expansion. However, further downstream the ambient pressure is sufficiently above the flow in the region of this shock to push the barrel shock back towards the nozzle axis. There is sufficient pressure gradient across the shock, caused by the fluid’s acceleration, to force it to follow the barrel curvature [1]. It has been noted that a repeat of the above-described shock cell structure can occur up to pressure ratios of 7 [2]. Shear instabilities create mixing layers around the jet and with pressure loss occurring through the shock structures cause a damping effect on the pressure variation along the jet centreline. This eventually affects the dissipation of the potential core and pressure in the jet to reach equilibrium with the ambient condition. For certain conditions, the free jet can produce crossing oblique shocks formed in a classical shock diamond structure downstream of the shock cell/s. Within the jet boundary, it has been concluded that there exists the presence of streamwise flow features of a vortical nature [3]. These features exist in the supersonic region of the jet boundary and are observed to be subject to a process of vortex merging. The presence of these vortices originates from the concave curvature of streamlines at the jet boundary due to the jet core’s pressure variation. This results in the well-documented creation of Taylor-Görtler vortices [4]. The assumption is often made that the jet impingement is axisymmetric. However repeatable evidence has been obtained from impingement research on engine operating flow. Previous experimental research conducted by the authors [5] showed that the central area of the impingement region on the plate was at a lower temperature than the immediate surrounding region. The increase in temperature immediately adjacent to this region was associated with the formation of Taylor-Görtler vortices which produced high heat transfer coefficients. These results confirmed that the highest heat flux was not located in the central impingement region, but in an outside ring whose position was determined by the formation of the above-mentioned vortices. The objective of this study was to gain further insight into the characteristics of such flows, using a numerical approach. A geometry representing an under-expanded free jet and an impinging on a perpendicular planar was modelled using a density-based formulation of OpenFOAM CFD solver, adopting the IDDES turbulence model. Assuming the flow starting from a reservoir with a pressure ratio of 40, two geometries were simulated, having a nozzle-to-plate spacing of 3 and 5 nozzle diameters respectively. According to the distance of the nozzle to the plate, a different flow pattern was developed. Moreover, as already evidenced in previous experiments, it was confirmed that the highest heat transfer on the impingement plate is largely determined by Taylor-Görtler vortex flow and total temperature separation resulting from shearing flows (see Fig. 1 ). Finally, the result analysis indicated that the shock system was not stable but experienced an oscillatory behaviour locked at exact frequencies, which varied with plate distance.