INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids 2010; 64:163–186 Published online 18 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/fld.2148 Effect of thermo-solutal stratification on recirculation flow patterns in a backward-facing step channel flow D. Senthil Kumar ‡ , K. Murugesan ∗, †, § and Akhilesh Gupta ¶ Mechanical and Industrial Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247 667, India SUMMARY This paper presents results on the combined effect of thermo-solutal buoyancy forces on the recirculatory flow behavior in a horizontal channel with backward-facing step and the ensuing impact on heat and mass transfer phenomena. The governing equations for double diffusive mixed convection are represented in velocity–vorticity form of momentum equations, velocity Poisson equations, energy and concentration equations. Galerkin’s finite-element method has been employed to solve the governing equations. Recir- culatory flow fields with heat and mass transfer are simulated for opposing and aiding thermo-solutal buoyancy forces by assuming suitable boundary conditions for energy and concentration equations. The effect of Richardson number (0.1Ri 10) and buoyancy ratio (-10N 10) on the recirculation bubble and Nusselt and Sherwood numbers are studied in detail. For Richardson number greater than unity, distinct variations in the gradients of Nusselt number and Sherwood number with buoyancy ratio are observed for flow regimes with opposing and aiding buoyancy forces. Copyright 2009 John Wiley & Sons, Ltd. Received 16 January 2009; Revised 2 July 2009; Accepted 11 July 2009 KEY WORDS: velocity–vorticity formulation; thermo-solutal buoyancy forces; recirculation; Richardson number; buoyancy ratio; wall shear stress 1. INTRODUCTION Flow through channels with backward-facing step (BFS) resembles many engineering applica- tions such as electronic cooling, turbine blade flow passage, combustors, high-performance heat exchangers, etc. wherein flow separation, recirculation and subsequent reattachment are observed in the vicinity of the step. The recirculation zone thus formed assumes importance in forced ∗ Correspondence to: K. Murugesan, Mechanical and Industrial Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247 667, India. † E-mail: krimufme@iitr.ernet.in ‡ Research Scholar. § Assistant Professor. ¶ Professor. Copyright 2009 John Wiley & Sons, Ltd.