LOW-REYNOLDS-NUMBER INSTABILITY OF THE LAMINAR FLOW BETWEEN WAVY WALLS
Instability of a viscous incompressible flow in a channel with wavy walls is investigated theoretically, numerically and experimentally. Linear stability analysis shows that appropriately chosen wall waviness leads to flow destabilization at surprisingly low Reynolds numbers. The unstable mode of disturbances forms a vortex array, which travels downstream. The remarkable feature is that the most destabilizing waviness does not introduce any additional flow resistance. The outcome of the stability analysis are consistent with the result of direct numerical simulation obtained using CFD finite volume package FLUENT (Ansys Inc.). Preliminary experimental data gained for a channel with appropriately corrugated wall seem to confirm these predictions.
Thermal performance of microchannels with wavy walls for electronics cooling
Wavy walls are investigated in this paper as a passive scheme to improve the heat transfer performance of low Reynolds number laminar flows in microchannel heat sinks for electronics cooling applications. Three-dimensional laminar fluid flow and heat transfer characteristics in micro-wavy channels are numerically studied for a 500 µm hydraulic diameter channel by varying the wavy feature amplitude at different Reynolds numbers (10, 20, 50 and 100). In addition, flow measurements are made using micro-PIV technique for understanding the fundamentals of fluid flow in the wavy microchannels for the considered Reynolds numbers. Based on the comparison with straight channels, it was found that wavy channels can provide improved heat transfer performance, while keeping the pressure drop within acceptable limits. Accordingly, wavy channels are to found to provide an improvement of up to 26% in the overall performance compared to microchannels with straight walls and hence are attractive candidates for cooling of future electronics.
Effect of Tollmien Schlichting wave on convective heat transfer in a wavy channel. Part I: Linear analysis
Abstract Hydrodynamic instabilities in wavy channels and their effect on the convective heat transfer are investigated using both linear stability analysis and integration of the time dependent Navier–Stokes and energy equations. The linear stability of the fully developed flow is determined from a generalized eigenvalue problem resulting from a Galerkin approach using divergence free Chebychev basis functions and trigonometric polynomials. Several axial periodicity lengths to geometry length ratios have been considered. For our geometry, the instability is found to set in as a Tollmien Schlichting wave, at a Reynolds number approximately equal to 90. The dynamics of the detachment and reattachment points and of temperature field for constant wall temperature are examined under the assumption of small amplitude fluctuations. It is shown that, although the average heat transfer remains almost constant, large amplitude variations of the local heat transfer coefficient can be observed, this effect is increasing with increasing Prandtl number.