An experimental investigation of the flow and heat transfer of an impinging sweeping jet at various nozzle-to-wall spacings

This study investigates how the nozzle-to-wall spacing ( H / D ) affects the heat transfer of an impinging sweeping jet, using time-resolved particle image velocimetry and lifetime-based thermographic phosphor thermometry. Experiments are conducted at a constant Reynolds number ( Re _swj = 2135), volume flow rate (20 L /min), and initial wall temperature ( T _{w 0} = 393 K), with the H / D is set to 1.5, 3.0, and 5.0 for comparison. For all H / D cases, the sweeping jets exhibit nearly identical jet spreading angles and sweeping frequencies. The flow fields also show self-similarity, with the radial profiles of both the mean velocity and root-mean-square velocity fluctuations well described by a bimodal fitting function using the same parameters. The influence of H / D is primarily observed in the near-wall flow characteristics and the resulting heat transfer performance. As H / D decreases, key near-wall flow parameters, including the radial velocity, root-mean-square radial velocity, vorticity, and Reynolds shear stress, are systematically increased. Moreover, a decrease in H / D leads to the formation of a more intense primary vortex closer to the centerline and promotes wall-jet development. This process enhances both the turbulent kinetic energy near the centerline and the periodic kinetic energy in the far field. These flow modifications explain the observed heat transfer enhancement with decreasing H / D , evidenced by both increased cooling effectiveness and a larger effective cooling area. Specifically, compared to the H / D = 5.0 case, the maximum time-averaged Nusselt number increases by 23% and 14% for H / D = 1.5 and 3.0, respectively. The correlation established between H / D -modulated flow characteristics and enhanced heat transfer provides mechanistic insight for optimizing sweeping jet impingement cooling.