Characterization of the temporal and spatial homogeneity of the fuel placement in a swirl cup: Non-reacting flow investigation by light sheet imaging, LDA and PDA

Kurzfassung

Spatial and temporal aspects of the liquid fuel placement in a swirl cup were investigated experimentally at elevated pressure. The operating parameter whose effect was investigated was air pressure. Air-to-fuel ratio (AFR) and relative pressure drop were held constant during variations of air pressure. Geometry parameters investigated included swirl angle, fuel nozzle axial position and fuel nozzle flow number. Measurement techniques included Mie-scattering laser light sheets for spray visualization, PDA for a detailed spay analysis and LDA for investigation of the air flow field in the absence of the spray. The test swirl cup geometry consisted of axial segments, so that cross-sectional planes in the interior of the swirl cup were accessible for light sheet imaging and LDA measurements. PDA measurements were conducted downstream of the swirl cup exit plane. Operating conditions realized for PDA measurements were 6, 12 and 18 bar at 700 K. Light sheet imaging and LDA measurements were performed at scaled operating conditions. It was found that the fuel placement is governed overwhelmingly by the airflow swirl angle. The two-phase flow in the swirl cup depends heavily on whether or not there is a recirculation zone within the swirl cup or not. In the absence of a recirculation zone, the radially inward airflow at the upstream end of the swirl cup causes the spray cone generated by the fuel nozzle to collapse. Subsequent transfer of tangential momentum to the fuel droplets then leads to a re-opening of the spray cone by swirl-induced centrifugal forces. If there is a recirculation zone present within the swirl cup, then the spray generated by the fuel nozzle is injected into a region of upstream and radially outward flow. This causes some fuel to be present outside of the nominal cone angle of the pressure swirl fuel nozzle and ensures that virtually all of the liquid fuel is placed onto the filmer.