M. Greiner ASME Mem. Professor of Mechanical Engineering, University of Nevada, Reno, NV 89557 e-mail: greiner@unr.edu P. F. Fischer Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, IL 60439 e-mail: fischer@mcs.anl.gov H. M. Tufo Department of Computer Science, University of Chicago, Chicago, IL 60637 e-mail: hmt@cs.uchicago.edu Two-Dimensional Simulations of Enhanced Heat Transfer in an Intermittently Grooved Channel Two-dimensional Navier-Stokes simulations of heat and momentum transport in an inter- mittently grooved passage are performed using the spectral element technique for the Reynolds number range 600Re 1800. The computational domain has seven contigu- ous transverse grooves cut symmetrically into opposite walls, followed by a flat section with the same length. Periodic inflow/outflow boundary conditions are employed. The development and decay of unsteady flow is observed in the grooved and flat sections, respectively. The axial variation of the unsteady component of velocity is compared to the local heat transfer, shear stress and pressure gradient. The results suggest that intermit- tently grooved passages may offer even higher heat transfer for a given pumping power than the levels observed in fully grooved passages. DOI: 10.1115/1.1459730 Keywords: Channel Flow, Computational, Enhancement, Forced Convection, Instability Introduction Engineering devices frequently employ enhanced heat transfer surfaces 1. Fins are typically used to extend surface areas while offset strips are commonly used to promote thin boundary layers. In recent years, a number of configurations that increase fluid mixing by triggering flow instabilities have been considered. Transversely grooved channel 2–5 passages with eddy promot- ers 6,7 and communicating channels 8 all contain fairly large features whose sizes are roughly half the channel wall to wall spacing. These structures are designed to excite normally damped Tollmien-Schlichting waves at moderately low Reynolds num- bers. The current authors have presented a series of articles on heat transfer augmentation in rectangular cross section passages with contiguous grooves cut into the walls. Experimental flow visual- izations in a long grooved channel downstream of a laminar flat passage show that two-dimensional waves appear after an initial quiescent development length 9. Unsteadiness is first observed thirty-five hydraulic diameters downstream of the first groove at a Reynolds number of Re=350. As the Reynolds number is in- creased, the onset moves upstream and the flow behavior at a given location becomes increasingly three-dimensional. Experi- mental and numerical results in a passage with eddy promoters indicate that the instability that leads to unsteady flow is convec- tive rather than absolute in nature 10. Measurements using air show that fully developed heat transfer is enhanced relative to laminar flat channel flow by as much as a factor of 4.6 at equal Reynolds numbers and by a factor of 3.5 at equal pumping powers 11,12. Numerical simulations of fully developed convection in trans- versely grooved passages were performed using the spectral ele- ment technique for Re2000 13,14. Those simulations em- ployed three-dimensional computational domains that represented one periodicity cell of the contiguously grooved passage. The pressure gradient and heat transfer results were within 20 percent of the measured values. At Re=1000 two-dimensional simulations gave Nusselt number values that were 20 percent below three- dimensional results while friction factors were smaller by a factor of two. This suggests that three-dimensionality strongly affects the transport characteristics of these flows, especially drag. Experimental measurements in a flat passage downstream of a grooved channel were performed for Reynolds number range 1500Re5000 15,16,17. These measurements show that the heat transfer coefficient remained high for a substantial distance in the flat region. The pressure gradient dropped down to the flat passage value much more rapidly, especially for Re2500. As a result, the heat transfer for a given pumping power was even greater in the first five hydraulic diameters of the decay region than in the grooved passage itself. Three-dimensional Navier-Stokes simulations in a flat passage downstream of a fully developed grooved channel were performed for 405Re764 17. The grooved channel had transverse grooves cut symmetrically into both walls. Two different compu- tational sub-domains were employed. The first represented one periodicity cell of a continuously grooved passage. It had periodic inflow/outflow boundary conditions in order to simulate fully de- veloped flow. The second sub-domain consisted of a single groove cell coupled to a flat passage at the downstream end. The inflow conditions to the grooved/flat sub-domain were taken from the outflow of the fully developed domain. Unsteady flow from the grooved region persisted several groove-lengths into the flat pas- sage. This unsteadiness increased both local heat transfer and pressure gradient relative to steady flat passage flow. Moreover, the heat transfer for a given pumping power in the first three groove-lengths of the flat passage was even greater than the high levels observed in a fully developed grooved passage. However, the numerical Nusselt number decayed more rapidly in the flat passage than was expected from measurements. The favorable heat transfer versus pumping power performance of flat passages downstream from grooved channels suggests that intermittently grooved passages, in which flat regions separate grooved sections, may have significant advantages in engineering heat transfer devices. However, the development of unsteady flow in grooved regions as well as the decay of unsteady flow exiting from a short grooved section must be investigated before the de- sign of intermittently grooved passages can be optimized. The current work is a two-dimensional numerical investigation of heat transfer in an intermittently grooved passage for the Rey- nolds number range 600Re1800. The grooved portions of this passage have seven right-triangular slots cut symmetrically into opposite walls. The flat portion is also seven groove-lengths long Contributed by the Heat Transfer Division for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received by the Heat Transfer Division February 6, 2001; revision received October 17, 2001. Associate Editor: M. Faghri. 538 Õ Vol. 124, JUNE 2002 Copyright © 2002 by ASME Transactions of the ASME