Characterization of inverse diffusion flames in vitiated cross flows via two-photon planar laser-induced fluorescence of CO and 2-D thermometry

Abstract

Two-photon, planar laser-induced fluorescence (PLIF) of carbon-monoxide (CO) and two-dimensional thermometry employing two-color, hydroxyl radical (OH) PLIF are used to characterize atmospheric-pressure inverse diffusion flames. These flames are important tools to aid the understanding of secondary reaction zones that may form in gas turbine engines when film-cooling air reacts with fuel-rich packets from the combustor. For the experiments performed in the present study, exhaust from a propane–air well-stirred reactor is channeled to a test section where three different film-cooling geometries are used to create inverse diffusion flames: (1) a single row of normal cooling holes, (2) a slot cut at an angle of 30° with respect to the wall, and (3) an 5×11 array of cooling holes. It is found that CO and H2 concentrations of a few percent can lead to secondary reaction zones and that different cooling-hole geometries can produce dramatically different secondary reaction-zone shapes. These secondary reaction zone flames have Damköhler numbers greater than unity and are diffusion limited. The PLIF measurements show regions where CO is consumed, OH produced, and the temperature perturbed. For film-cooling flows that remain attached to the wall, the secondary reaction zone is also close to the wall and can cover a relatively long axial length. For film-cooling flows that separate from the wall, the secondary reaction zones protrude farther into the cross flow then quickly mix with the cross flow. By comparing the CO, OH, and temperature fields, three characteristic regions of flows with secondary reaction zones are identified: the injection region where cooling air displaces the vitiated cross flow, the secondary reaction zone region, and the mix-out region where all of the oxygen has been consumed and mixing with the vitiated cross flow controls the local composition and temperature.