Combination of planar laser optical measurement techniques for the investigation of pre-mixed lean combustion

Abstract

This paper summarizes recent efforts of applying combinatory or simultaneous multifaceted planar laser optical measurement techniques in reactive flow. The homogeneous (partially) premixed lean combustion in aircraft engines is the most promising combustion concept for the sustainable reduction of fuel consumption and emissions. The concept requires a 30-40% reduction in cooling air flow. These changes require an efficient cooling concept to deal with the increased demands like higher cooling air temperature, higher pressure and reduced cooling air flow. Overall the demands on the homogeneity of the temperature distribution in the reactive flow increase to take into account the increased heat load on the walls at lean combustion. Planar optical measurement techniques capable of providing different measurands were successfully combined in a common set up to enable rapid data acquisition consecutively or even simultaneously. Within a high pressure single sector combustor (SSC) the interaction between a pre-mixed lean flame and the near wall cooling film is observed at realistic pressure (here 5 bars) and preheat temperature (450 K). One of the main advantages of the SSC facility is its optical accessibility from three sides. In the present application an effusion-cooled wall with a mesh of cooling air holes represents the fourth side in the SSC and serves for the qualification of the laser optical measurement techniques for near wall investigations. A first assessment of the combustor design is achieved by the acquisition of the global velocity field, the spatial release of heat, the temperature field and the pre-mixing behavior of the cooling air concept within a central (symmetry) plane. For this purpose previously established spectroscopic measurement techniques are used such as the chemiluminescence of OH radical for the qualitative visualization of the heat release, the planar laser-induced fluorescence (PLIF) of the OH radical to study the mixing properties of combustor flow and film cooling as well as planar temperature distributions. Particle image velocimetry (PIV) visualized and quantified the swirling, recirculating flow field. The PIV measurements were acquired synchronized with the OH PLIF measurements on a single-pulse basis to obtain correlated information of the velocity field and the [OH] distribution with high spatial and temporal resolution. After global characterization of the combustion flow field near-wall PIV/LIF measurements were carried out with spatial high resolution in close proximity of the effusion cooled wall in an effort to visualize the structure of the cooling air jets in detail.