Localised Dynamics of Laminar Pulsatile Flow in a Rectangular Channel

The exploitation of flow pulsation in low-Reynolds number micro/minichannel flows is a potentially useful technique for enhancing cooling of high power photonics and electronics devices. Although the corresponding mechanical and thermal problems are inextricably linked, decoupling of the local instantaneous parameters provides insight into underlying mechanisms. Using two-dimensional, two-component particle image velocimetry, the current work intends to characterise the hydrodynamic problem in a generic liquid flow channel geometry with sinusoidally-pulsating flows of Womersley numbers 1.4 Wo 7.0 and a fixed ratio of oscillating flow rate amplitude to steady flow rate equal to 0.9. The results are compared to the analytical solution for flow in a rectangular channel, reorganised in terms of amplitude and phase values relative to a prescribed flow rate. The amplification of the wall shear stress, or the oscillating component normalised by the steady flow value, is found to increase with frequency owing to growing phase delays and augmented amplitudes in the near-wall region of the velocity profiles. Furthermore, the local instantaneous effect on the frictional stress varies depending on the regime of unsteadiness: (i) For quasi-steady flows, the alterations are of the order of the steady component and cause both an amplification and reduction during a single period. (ii) At intermediate frequencies, the magnitudes become relatively large, causing a habitual bias towards amplification. (iii) For plug-like flows, the steady component is negligible and local amplification is invariant in time. The overall fluid mechanical performance of pulsating flow, measured by the ratio of wall shear stress and pressure gradient amplifications, is found to reduce from an initial value of 1 at Wo = 1.4 to 0.27 at Wo = 7.0.