Experimental visualization of temperature fields and study of heat transfer enhancement in oscillatory flow in a grooved channel

An experimental study was conducted of incompressible, moderate Reynolds nu mber flow of air over heated rectangular blocks in a two-dimensional, horiz ontal channel. Holographic interferometry combined with high-speed cinemato graphy was used to visualize the unsteady temperature fields in self-sustai ned oscillatory flow. Experiments were conducted in the laminar, transition al and turbulent flow regimes for Reynolds numbers in the range from Re = 5 20 to Re = 6600. Interferometric measurements were obtained in the thermall y and fluiddynamically periodically fully developed flow region on the nint h heated block. Flow oscillations were first observed between Re = 1054 and Re = 1318. The period of oscillations, wavelength and propagation speed of the Tollmien-Schlichting waves in the main channel were measured at two ch aracteristic flow velocities, Re = 1580 and Re = 2370. For these Reynolds n umbers it was observed that two to three waves span one geometric periodici ty length. At Re = 1580 the dominant oscillation frequency was found to be around 26 Hz and at Re = 2370 the frequency distribution formed a band arou nd 125 Hz. Results regarding heat transfer and pressure drop are presented as a function of the Reynolds number, in terms of the block-average Nusselt number and the local Nusselt number as well as the friction factor. Measur ements of the local Nusselt number together with visual observations indica te that the lateral mixing caused by flow instabilities is most pronounced along the upstream vertical wall of the heated block in the groove region, and it is accompanied by high heat transfer coefficients, At Reynolds numbe rs beyond the onset of oscillations the heat transfer in the grooved channe l exceeds the performance of the reference geometry, the asymmetrically hea ted parallel plate channel.