Spatio-temporal heat transfer characteristics of an impinging pulsating coaxial round jet

Novel experiments on temporally and spatially changing temperature fields and associated heat transfer characteristics of pulsating coaxial air jets impinging on an ultra-thin (400 nm), Joule heated flat surface are reported. Measurements were performed at a constant total mass flow rate for three different outer and inner jet velocity ratios, r u = U o / U i = 0.5, 1.0, and 2.0, at a single stand-off distance of twice the outer jet diameter. Inner or outer jet were pulsed at frequencies up to 200 Hz, and instantaneous temperature fields were measured using high-speed infrared thermography. Findings indicated that while steady, non-pulsating coaxial jet operation yields the highest overall heat transfer at a velocity ratio of r u =2, further enhancement of the heat transfer coefficient was achieved across all velocity ratios by applying pulsation within a narrow frequency band to the weaker of the two coaxial jets. The fluctuating temperature fields were further analyzed by spectral proper orthogonal decomposition providing the fluctuating temperature amplitudes and wave lengths of the thermal disturbance induced by the pulsation. Based on simple scaling arguments, it was shown that the latter scaled by a power law dependence on the pulsation Strouhal number. • Spatio-temporal heat transfer of pulsating coaxial impinging jets was measured. • A 400 nm Joule-heated surface enabled high-speed IR thermography up to 200 Hz. • Heat transfer was assessed at three velocity ratios for steady and pulsating flows. • Pulsating the weaker jet in a narrow frequency range enhanced heat transfer rates. • POD and SPOD showed radially propagating thermal waves and Strouhal-based scaling.