NUMERICAL STUDY OF TURBULENT FORCED CONVECTION IN A PERIODICALLY RIBBED CHANNEL WITH OSCILLATORY THROUGHFLOW

Abstract

Engineering Systems International/Uihon ESI K.K., 2-47-18 Uehara, Shibuya. Tobo 151, Japan SUMMARY Numerical computations are performed on the fully developed flow and heat transfer in a periodically ribbed channel with oscillatory throughflow. A uniform heat flux is imposed at the lower plate of the channel. An externally sustained pressure gradient vanes sinusoidally in time. A low-turbulent-Reynolds-number version of the k+ two-equation model of turbulence is invoked, together with a preferential dissipation modification, to predict the complex turbulent flow field. Computed results indicate that much heat transfer enhancement is expected by increasing the Womersley number, which measures the relative strength of the oscillatory motion to the viscous effects. Turbulent heat transfer in a channel with rib-roughened walls is of much engineering relevance. It is known to be an effective means of heat transfer enhancement in compact heat exchangers, cooling of electronic devices and other types of thermal engineering equipment. A higher heat transfer rate may be expected by imposing on the channel an externally sustained oscillatory pressure gradient, by which better mixing of the medium can be accomplished. It is thought to be a promising technique for cooling high-density electronic circuit boards, for which an excessively high flow rate of the cooling agent is not desirable from a mechanical design point of view. Despite its potential for innovative engineering applications, the available information on turbulent heat transfer in oscillatory flow is limited. Ohmi et d.' measured the velocity profiles of oscillatory througMow in a rectangular duct using hot-wire anemometry. Instantaneous velocity profiles for turbulent oscillatory flow were found to be similar to those of non-oscillatory flow. Consequently, the profiles contained little phase shift, contrasting with their laminar flow counterparts. The same authors have also constructed a flow regime diagram, thereby identifying the role of the mechanically imposed frequency (characterized by the Womersley number) in the flow transition. Their experiments were conducted under an isothermal condition. Features of the flow field and mass transfer rate were investigated for a wavy walled channel by Nishimura et aL2 With a smoothly contoured wall, flow separation and subsequent growth of a vortex, which eventually occupied the entire flow passage, were clearly observable in the acceleration stage. In the deceleration stage accompanying the reversal in the flow direction, these vortices were