Numerical Simulations of Steady Low-Reynolds-number Flows and Enhanced Heat Transfer in Wavy Plate-fin Passages

Book excerpt: Extended or finned surfaces are widely used in compact heat exchangers to reduce the thermal resistance of air- or gas-side flows. Besides increasing the effective heat transfer surface area, geometrically modified finned surfaces also improve the heat transfer coefficient by altering the flow field. Wavy plate-fin surfaces have such properties and promote relatively high thermal-hydraulic performance. They are also attractive for their simplicity of manufacture and ease of use in compact heat exchangers. The current study numerically investigates the fluid flow and enhanced convection heat transfer in two-dimensional and three-dimensional wavy plate-fin passages with sinusoidal wall corrugations in the low Reynolds number regime. Constant property, periodically fully developed, and laminar or low Reynolds number forced convection are considered. The governing equations of continuity, momentum, and energy are solved computationally using finite-volume techniques. The solution procedure is based on the SIMPLE algorithm and a non-orthogonal, non-uniform grid. The influences of fin geometry (fin spacing, fin height, fin amplitude and fin length) on the enhanced heat transfer and fluid flow behaviors are investigated. The simulation results for the velocity and temperature distributions, isothermal Fanning friction f, and Colburn factor j are presented and discussed. The complex flow patterns in the wavy-fin channel are characterized by re-circulating and/or helical swirl flows with periodic flow separation and reattachment. Two flow regimes can be classified based on these results, namely, (1) low-Re streamline-flow regime where viscous forces dominate, and (2) high-Re swirl-flow regime characterized by flow recirculation and/or helical vortices. Heat transfer enhancement is observed in the swirl flow regime along with an increased pressure drop penalty, as a consequence of the periodic thermal boundary-layer thinning, strong flow mixing, and periodic generation and dissipation of vortices or re-circulating cells. In the streamline-flow regime, the flow and heat transfer behavior are similar to that in straight flow channel, though an enhanced performance is obtained. Also, results of flow visualization experiment for a two-dimensional wavy flow channel are shown to agree well with the numerical results. Finally, the computational methodology is extended to illustrate the flow behaviors in out-of-phase wavy flow passages.