Heat Transfer and Fluid Flow Characteristics of a Backward-Facing Step Flow in a Duct

Abstract

Spatial distributions of local heat transfer coefficients on the bottom wall downstream of a backward-facing step in a duct flow have been measured by making use of a thermo-sensitive liquid crystal sheet. Heat transfer experiments and three components of the time averaged velocity measurements by 2D PIV have been done for wide variety of the flow Reynolds number ranging from laminar (1000) to turbulent (10000) flow regimes for the stepped duct having an aspect ratio of 16 and expansion ratio of 2. Obtained local Nusselt number changes not only in the streamwise direction but also spanwise direction, that is, the maximum Nusselt number always appears near the side wall region even if the flow Reynolds number is varied. Strong downward flow from the step induced along the side wall produces the secondary flow toward the central part of the duct and the area where this secondary flow exists agrees well to the area of the high heat transfer coefficient. The streamwise location of the maximum Nusselt number quickly moves closer to the step as the Reynolds number increases from 1000 to 2300, then it gradually moves to the downstream from 2300 to 10000. This streamwise location corresponds quite well to that of the flow reattachment for each Reynolds number. A pair of the vertical vortices having axes in the wall normal direction is found to be generated at the corner just behind the step near the side wall and this causes the minimum heat transfer there.