Effect of laminarization and retransition on heat transfer for low Reynolds number flow through a converging to constant area duct

The process of reversion from the laminarized flow to turbulence is investigated by means of systematic experiments using the k-kL model. A fully developed low Reynolds number turbulent air flow between two parallel plates with a spacing of 15 mm is laminarized through a linearly converging passage 200 mm in length. It then flows into a parallel-plate channel again, resulting in a decrease in the Nusselt number, at first, in the downstream parallel plate section together with the friction coefficient. Reversion to turbulence proceeds gradually and manifests itself as a sharp increase in the Nusselt number and friction coefficient. Flow and heat transfer in the converging and parallel sections can be accurately predicted by the k-kL model, provided that the extent of laminarization at the inlet of the parallel plate section is not very high.