Spatio-temporal scale matching for grooved cooling channel under pulsating flow

The present study numerically and experimentally characterizes the heat transfer performance of a grooved channel under pulsating flow, with Strouhal number ranging from 0.04 to 0.8 and groove depth ratio between 1/4 and 1. The time-averaged Reynolds number of the trapezoidal pulsation profile is maintained at 200 and the temporal maximum is limited at 1000. The augmentation of heat transfer is attributed to both enhancement in flow convection intensity and improvement of thermal mixing efficiency. The former is illustrated by the asymmetry of kinetic energy change that arises from the loss of complementarity between wall shear stress and viscous entropy generation. The latter is explained by the reboost of thermal entropy generation that results from the transverse mixing between near-wall hot fluids and mainstream coolant. There exist significant phase lags and waveform distortion between the channel Nusselt number and the inlet velocity profile, indicating that the dominant mechanism for heat transfer enhancement transitions from mass convection at the pulse-on stage to thermal mixing at the pulse-off stage. Further scale-matching analysis reveals that the greatest Nusselt number improvement of 317% is obtained at the Strouhal number around 0.2 and the groove depth ratio of 1/2. With a penalty of 324% friction factor ratio, the thermal enhancement factor is still improved to 2.14. The optimal pulsation frequency that matches the characteristic time of cavity vortex growth and the optimal groove dimension that matches the characteristic size of cavity vortex expansion lead to the maximum kinetic energy residual and the minimum thermal entropy generation.