LaminarFlowandHeatTransferinaPeriodicSerpentine Channel By Nathan R. Rosaguti, David F. Fletcher*, and Brian S. Haynes The fully developed laminar flow and heat transfer behavior in periodic serpentine circular-section channels has been studied using computational fluid dynamics (CFD). The serpentine elements are characterised by their wavelength (2 L), channel diameter (d), and radius of curvature of bends (R c ) and results are reported for (3 < L/d <12.5, 0.525 < R c /d < 2.25) and Rey- nolds numbers up to 200. The flow is characterised by the formation of a single pair of Dean vortices after each bend, these being stronger if the bend is in the same direction as the previous bend. The vortices decay downstream of the bends, but, as the Reynolds number is increased, the flow space is increasingly dominated by these vortices. At higher values of Reynolds number (corresponding to a Dean number of about 150), a second pair of vortices develops. For Re > ~ 200, the flow be- comes unsteady. Constant wall heat flux (H2) and constant wall temperature (T) boundary conditions have been examined for a range of fluid Prandtl number (0.7 < Pr < 100). The formation of Dean vortices produces significant heat transfer en- hancement relative to flow in a straight pipe, with the effect being greater at higher values of Pr. Pressure drop is also in- creased but to a lesser extent. The alignment of the flow with the vorticity in these structures allows significant mixing of the fluid without creating large pressure-drop penalties. Dean vortices are also found to inhibit flow separation. These results suggest an effective method for enhancement of heat transfer in deep laminar flows. 1Introduction The augmentation of convective heat transfer assists in reducing the size, weight and ultimately the operating and capital cost of compact heat exchangers. A common ap- proach to heat transfer augmentation is to disrupt the ther- mal boundary layer which results in an increase in the con- vection heat transfer coefficient. Compact heat exchangers achieve this by introducing channel bends, ribs or interrup- tions. A method to study fully developed flow and heat transfer in geometries analogous to micro- or mini-channels within compact heat exchangers is presented in this paper. The objectives of this work are to establish the effect of geo- metrical parameters, as well as the flow and fluid parameters of Reynolds and Prandtl number on flow and heat transfer performance. The geometry under examination is a serpentine duct of circular cross-section. A serpentine channel geometry, as shown in Fig. 1, has been studied as it is representative of a channel geometry used in complex compact heat exchangers. The definition of serpentine channels used in this work fol- lows the work of Choi and Anand [1] and Liu et al. [2], both of whom also studied channels with a series of right-angle bends. Typically, flow channels found in compact heat ex- changers would consist of many modules or units (e.g., Fig. 1) in series. Flow and non-dimensional temperature patterns become invariant from module to module after a sufficient entrance length, and are said to be fully-developed. In this work we present results for the thermal boundary conditions of constant heat flux without the requirement of a uniform peripheral wall temperature at a section (the ªH2º condition of Shah and London [3]) and constant wall temperature (the ªTº boundary condition [3]). A method to study fully developed flow and heat transfer in channels with periodically varying shape was first devel- oped by Patankar et al. [4]. Their method takes advantage of the repeating nature of the flow field to limit the extent of the computational domain to a single repeating unit. A pre- scribed linear pressure gradient is applied to this repeating unit to drive the flow, and the outlet velocity field and its gradient are ªwrappedº to the inlet to produce periodic boundaries. Flow velocities within the geometry are then calculated using momentum and mass conservation equa- tions, assuming constant fluid properties. In this manner, the fully developed flow field is determined. Fully developed temperature fields are calculated using the energy equation, with appropriate scaling applied to the outlet temperatures before wrapping to the inlet. Full details can be found in [4]. Chem. Eng. Technol. 2005, 28, No. 3 DOI: 10.1002/ceat.200407148  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 353 Figure1. Schematic of a repeating module of the serpentine geometry. Non- dimensional geometrical parameters of interest are L/d and R c /d. ± [*] N. Rosaguti, D. F. Fletcher (author to whom correspondence should be addressed, davidf@chem.eng.usyd.edu.au), B. S. Haynes, Department of Chemical Engineering, University of Sydney, Sydney, NSW 2006, Australia Full Paper