Enhanced impingement heat transfer in coaxial synthetic jets through frequency ratio and mass flux control

This study investigates the influence of the oscillation frequency ratio (F_r) and mass flux ratio (M_r) between actuators on the flow and heat transfer characteristics of a novel coaxial synthetic jet (CSJ) system. The CSJ employed features independently controlled inner and annular actuators, allowing precise manipulation of jet dynamics through adjustment of each actuator's operating parameters. Flow behavior was characterized using hot-wire anemometry, while impingement heat transfer was measured using the thin foil technique with infrared thermometry. Experiments were conducted at a Reynolds number of 1000 for multiple frequency ratios (F_r= 0.56,1, and 1.22) and mass flux ratios (M_r= 0.72,1, and 2), where F_r is defined as the ratio of inner to annular diaphragm frequency, and M_r as the ratio of inner to annular jet mass flux. The results indicate that the influence of F_r is particularly significant at higher overall jet strengths. Results indicate that operating the CSJ at the lower F_r (0.56) and lower M_r (0.72) leads to significant enhancement in both flow and thermal performance. Specifically, for F_r= 0.56 and M_r= 0.72, the peak centerline velocity (V_cl/V_o) increased by approximately 27 %, and the peak time-and area-average Nusselt number (Nu¯_) improved by about 135.18 % relative to the baseline case (F_r= 1, M_r= 1). These findings demonstrate that by adjusting the frequency and mass flux ratios, the CSJ can be effectively used to improve heat transfer, especially for hot spot mitigation in real applications.