Transient free convection flow on a vertical surface with an array of large-scale roughness elements

Abstract

Transient natural convection on a vertical ribbed wall is studied experimentally with a wall-boundary condition of uniform heat flux. To get an idea about the roughness geometry influence on the heat transfer, several distinctive ribbed geometries were tested. The experimental analysis is deduced from both flow field visualizations and thermal measurements. It is shown that instantaneous flow patterns result in complex eddy structures in the vicinity of the ribs. As well vortex birth as vortex shedding process are evidenced during the transient in the open cavities between the ribs to evolve increasing time towards 3D turbulent structures. Whatever the arrangements and the time, one observes a degradation in the convective heat transfer below the first rib and an enhancement past the last one compared to the smooth case. In the open cavities, conclusions are contrasted: during the early transient an important heat transfer enhancement occurs in the upstream part of the cavity while increasing time reduces the heat transfer performance in the whole cavity.