Two-dimensional and three-dimensional self-sustained flow oscillations in interrupted fin heat exchangers

A numerical analysis of three-dimensional flow structures in a nominally two-dimensional fin geometry is presented. A sinusoidal louvred fin is considered. The heat transfer enhancement is achieved by combining boundary layer interruptions and vortical structures induced by the corrugation of the base fin. The fin shape and pitch, as well as flow conditions, are representative of typical automotive application. A wide ranging values of Reynolds number are investigated, spanning the steady laminar regime, the unsteady periodic laminar flow, and the chaotic transitional flow. Two- and three-dimensional numerical solutions are compared, looking for the onset of three-dimensional instabilities. At low values of the Reynolds number, up to the steady-unsteady flow transition, the flow is two-dimensional. As soon as unsteady oscillation appears, the simulation results show three-dimensional flow structures, even in a nominally two-dimensional geometry. The typical longitudinal vortex size is evaluated. In the periodic unsteady regime, fully three-dimensional computations yield time-averaged Nusselt number and friction factor significantly higher than those predicted by two-dimensional models. Furthermore, these flow structures induce an early transition from the periodic regime to the chaotic regime. In the chaotic regime, however, the heat transfer enhancement due to the three-dimensional flow structures is much lower.