Centrifugal instability of the boundary layer is known to induce spiral vortices over a rotating slender cone that is facing an axial inflow. This paper shows how a deviation from the symmetry of such axial inflow affects the boundary layer instability over a rotating slender cone with half-angle $\psi =15^\circ$. The spiral vortices are experimentally detected using their thermal footprint on the cone surface for both axial and non-axial inflow conditions. In axial inflow, the onset and growth of the spiral vortices are governed by the local rotational speed ratio $S$ and Reynolds number $Re_l$ in agreement with the literature. During their growth, the spiral vortices significantly affect the mean velocity field as they entrain and bring high-momentum flow closer to the wall. It is found that the centrifugal instability induces these spiral vortices in non-axial inflow as well; however, the asymmetry of the non-axial inflow inhibits the initial growth of the spiral vortices, and they appear at higher local rotational speed ratio and Reynolds number, where the azimuthal variations in the instability characteristics (azimuthal number $n$ and vortex angle $\phi$) are low.

One of the main challenges of future aircraft engines is to achieve low pollutant emissions while maintaining high combustion efficiencies and operability. The Flameless Combustion (FC) regime is pointed as one of the promising solutions due to its well-distributed reaction zones that yield low NOx emissions and oscillations. A dual-combustor configuration potentially facilitates the attainment of FC in the Inter-Turbine Burner (ITB). The development of such burner is dependent on knowledge regarding NOx formation and the parameters affecting it. It is known from the literature that the NOx formation mechanisms are different in FC. Therefore, in an attempt to clarify some of the mechanisms involved in NOx formation at relevant conditions, a chemical reactor network model developed to represent the ITB is explored. The role of prompt NOx was previously shown to be dominant at relatively low inlet temperatures and atmospheric pressure. In order to check these findings, five chemical reaction mechanisms were employed. All of them overpredicted NOx emissions and the overprediction is likely to be caused by the prompt NOx subset implemented in these mechanisms. Higher reactants temperatures and operational pressures were also investigated. Overall NOx emissions increased with temperature and the NOx peak moved to lower equivalence ratios. Operational pressure changed the emissions trend with global equivalence ratio. Leaner conditions had behaviour similar to that of conventional combustors (increase in NOx), while NOx dropped with further increase in equivalence ratio due to suppression of the prompt NOx production, as well as an increase in NO reburning. These trends highlight the differences between the emission behaviour of the ITB with those of a conventional combustion system.

The historical trends of reduction in fuel consumption and emissions from aero engines have been mainly due to the improvement in the thermal efficiency, propulsive efficiency and combustion technology. The engine Overall Pressure Ratio (OPR) and Turbine Inlet Temperature (TIT) are being increased in the pursuit of increasing the engine thermal efficiency. However, this has an adverse effect on engine NOx emission. The current paper investigates a possible solution to overcome this problem for future generation Very High Bypass Ratio (VHBR)/Ultra High Bypass Ratio (UHBR) aero-engines in the form of an Inter-stage Turbine Burner (ITB). The ITB concept is investigated on a next generation baseline VHBR aero engine to evaluate its effect on the engine performance and emission characteristics for different ITB energy fractions. It is found that the ITB can reduce the bleed air required for cooling the HPT substantially (around 80%) and also reduce the NOx emission significantly (>30%) without penalising the engine specific fuel consumption.