With stricter regulations on engine altitude relight capability, the understanding of low-speed axial compressor performance is becoming increasingly important. At such far off-design conditions, compressors behave differently from design point, with large changes in the flow phenomena and reduced reliability on the established empirical equations and assumptions. This work focuses on the aerodynamics of a locked-rotor axial compressor at high inlet Mach number conditions, using a validated numerical simulation approach. In a locked-rotor compressor there is very little compression of the inflow. The air is forced to accelerate, with the rear stages seeing the highest velocities. Depending on the inlet Mach number, the velocity at the rear stages can be close to sonic, until choking conditions are reached. To predict accurately the zero-speed compressor performance close to the choking point, the corresponding blade aerodynamic coefficients are evaluated as a function of the blade’s physical parameters and the inlet Mach number. In addition, the blockage due to the separated flow as a result of the high negative incidences is investigated as a function of inlet Mach number, incidence, solidity and stagger angle. Models that predict the characteristics and choking mass flow of the compressor, require such data. This work offers a better insight into the low-speed and locked rotor characteristics of the compressor. The zero-speed line can be calculated through a stage-stacking technique using the aerodynamic coefficients and flow blockage derived from the numerical simulations. Low-speed lines between the zero and idle-speed line can subsequently be created through interpolation. Using this methodology, it is possible to generate a complete sub-idle map for a multi-stage axial compressor, enhancing the predictive capability of whole engine performance solvers.

Compared to partially premixed combustion (or combustion of non-homogeneous reactants in general), fully premixed and diffusion flames represent only two asymptotic limits of combustion modes. However, the deep knowledge accumulated over the years on these two elementary and archetypal flame prototypes is such that they remain the cornerstone and reference building blocks of most combustion modelling proposals. Therefore, from a general point of view, being able to distinguish between premixed and non-premixed modes of combustion thanks to a flame index appears as a quite appealing but challenging task that still concentrates many research efforts. Indeed, the availability of such an index is not only appealing to proceed with the analysis of either experimental or computational data issued from DNS (or highly resolved LES) databases. It is also an essential ingredient to elaborate advanced flamelet-based multiregime combustion models on the basis of single regime tabulated flamelet databases. In the present study, a new definition of the premixedness index ζ PF is proposed for partially premixed combustion. It is based on a weighted form of the cross-scalar dissipation rate of the mixture fraction Y ξ and progress variable Y c , i.e., quantities that have been previously identified as relevant parameters to describe partially premixed combustion regimes. The relevance of the corresponding index is assessed through a detailed computational procedure that includes three successive validation subsets: counterflow flames (including premixed, rich partially-premixed, and diffusion flames), (ii) stabilized triple flames for three distinct values of the inlet mixture fraction gradient, and finally (iii) unsteady flame kernel developments in nonhomogeneous mixtures of fresh reactants, which are characterized by various initial levels of the segregation rate between the fuel and oxidizer. The proposed premixedness index ζ PF and its counterpart ζ DF = 1 − ζ PF are used as the weighting coefficients between tabulated premixed flamelets (TPF) and tabulated diffusion flamelets (TDF) data, which have been parameterized as functions of Y ξ and Y c . It is noteworthy that, in contrast to some previous proposals of the literature, the present flame index does not require the consideration of any other quantities in addition to those already used to parameterize the flamelets databases, i.e., Y ξ and Y c . The validation procedure makes use of steady and unsteady processes with a priori and a posteriori analyses. In both cases, the comparisons between the results obtained with the proposed flame partitioning and detailed chemistry (DC) computations lead to a satisfactory level of agreement and, from a general viewpoint, the level of agreement is better than the one obtained with either premixed or diffusion flamelet-based models.

No abstract

No abstract