Laminar flow and heat transfer in a periodic trapezoidal channel with semi-circular cross-section

Computational fluid dynamics (CFD) has been used to study fully developed laminar flow and heat transfer behaviour in periodic trapezoidal channels with a semi-circular cross-section. The trapezoidal elements are characterised by their wavelength (2L), channel diameter (d), radius of curvature of bends (Rc), the amplitude (2A) and the length of the straight section (B) with results reported for Reynolds numbers (Re) up to 400, as well as for a range of geometric configurations image, image, image, image at Re = 200. This generic geometry takes a variety of shapes with limiting forms of a regular square serpentine (B = 2A = L) and a zig-zag or saw-tooth (B → 0). The flow in these channels is characterised by the formation of Dean vortices following each bend. As the Reynolds number is increased, stronger vortical flow patterns emerge and these vortices lead to efficient fluid mixing and high rates of heat transfer. Constant wall heat flux (H2), constant axial heat flux with peripherally constant temperature (H1) and constant wall temperature (T) boundary conditions are examined for a fluid with a Prandtl number of 6.13. Higher rates of heat transfer with relatively small pressure loss penalty are found relative to fully developed flow in a straight pipe, with heat transfer enhancements of up to four at the highest Reynolds number. In addition to presenting channel enhancements the stackability of channels on a plate is considered. The concepts of area enhancement (based solely on geometric factors) and heat transfer intensification, the product of the heat transfer enhancement and the area enhancement, are introduced and used to compare different geometrical configurations. The swept zig-zag pathway provided the greatest intensification of heat transfer in a multi-channel plate structure.