Cross-corrugated triangular ducts provide high mass transfer capabilities in membrane related gas separations. The mixing effect would intensify the convective mass transfer coefficients on membrane surfaces. By modeling transport of water vapor in dry air, in this study, periodic fully developed fluid flow and mass transfer in a cross-corrugated triangular duct is numerically studied. To model the transitional flow in the topology, a validated low Reynolds number k-omega (LKW) turbulence model is employed to account for the turbulence in the flow. The vapor mass fractions, velocity, and turbulent kinetic energy and specific dissipation rate contours are obtained in the three-dimensional complex domain. The friction factors and the segment mean Sherwood numbers are calculated and correlated with Reynolds numbers, for uniform mass fraction boundary conditions. It is found that the transitional flow is represented by a turbulence center, which intensifies and migrates from the upper wall corrugation to the lower wall corrugation with increasing Reynolds numbers. (c) 2005 Elsevier B.V. All rights reserved.