Experimental study on forced convection hydraulic and thermal characteristics in a dimpled flat duct

This paper presents friction factor and Nusselt number for a dimpled flat duct in single phase (liquid and gas flows) of R134a. It compares experimentally obtained data for the dimpled flat duct to correlation predictions for a rectangular duct with the same aspect ratio. Data show that the friction factor for liquid flow decreases remarkably with an increase in Reynolds number from 580 to 2700, while for vapor flow, the friction factor was barely affected in the range of Reynolds number from 1 × 10⁴ through 5 × 10⁴. The Nusselt numbers for both liquid flow and vapor flow increase gradually with increasing Reynolds number. The dimpled flat duct has 5.5–15.7 times friction factors and 3.2–8.6 times Nusselt numbers compared to the predicted values of correlations for the rectangular duct. The reasons for the higher friction factor and Nusselt number are attributed to the dimples which can generate secondary flows and destroy the boundary layer of flow.