Geometry effect on flow fluctuation and heat transfer in unsteady forced convection over backward and forward facing steps

This study presents a direct numerical simulation of the geometry effect on fluid flow and heat transfer characteristics in the transitional flow over backward and forward facing steps. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the bottom wall length and the step height are investigated. With the increase of bottom wall length, a fundamental frequency of flow fluctuation appears at Lb = 10 h and then disappears at Lb = 14 h. This fundamental fluctuating frequency decreases with the increase of step height. The fact that the heat transfer performance is better in the reattachment region at Lb = 12 h and S = h indicates that the periodic flow fluctuation with a larger fluctuating velocity has positive effects on heat transfer enhancement. The results of the relationship between flow fluctuation and heat transfer enhancement are significant for certain practical applications of energy conversion. •The periodic flow fluctuation appears in the cases of certain bottom wall lengths.•The periodic flow fluctuation has positive effects on heat transfer enhancement.•The fundamental fluctuating frequency decreases with the increase of step height.•The results of heat transfer enhancement are significant for practical applications.