The influence of aspect ratio on the iso-thermal flow characteristics of multiple confined jets

We report a systematic study of the interaction between four rotationally symmetric jets within a cylindrical chamber obtained with particle image velocimetry, under conditions relevant to a wide range of practical applications including the hybrid solar receiver combustor. The geometry consists of a cylindrical cavity with four inlet jets (representing four burners), which are configured in an annular arrangement and aligned at an inclination angle (αj) to the axis with a tangential component (azimuthal angle θj) to generate a swirl in the chamber. The configurations of αj = 25° were assessed with two azimuthal angles θj = 5° and 15°, a range of chamber aspect ratios (Lc/Dc), and a fixed nozzle Reynolds number of ReD = 10 500. The experimental results reveal a significant dependence of the mean and turbulent flow-fields on the aspect ratio Lc/Dc for the values of αj and θj considered here. Three different flow regimes and their controlling parameters were identified within the range 1 ≤ Lc/Dc ≤ 3. The dependence of the flow characteristics on the chamber length Lc was weak within 1.5 < Lc/Dc ≤ 3, but significant for 1 ≤ Lc/Dc ≤ 1.5. It was also found that the value of Lc/Dc has a controlling influence on the position and strength of large-scale recirculation regions, together with the extent of flow unsteadiness, although this influence is reduced as θj is increased.