Distribution of the heat transfer coefficient in a channel with periodic transverse grooves

Distributions of the heat transfer coefficient and of the pressure drop along the wall inside an asymmetrically ribbed channel are measured for thermally developing and periodic turbulent flow at 104 ≤ Re ≤ 105. The thermal boundary condition of constant heat flux is provided by an electrically heated metal foil. The temperature distribution at the ribbed wall including both flanks of the ribs is measured by infrared thermography (IRT). The IR camera views the ribbed surface from four different positions with two angles of orientation relative to the main flow direction to image the surface completely. The complete temperature distribution along one geometric period is obtained by combining the resulting four images. Local convective heat transfer coefficients are evaluated from local temperatures, taking into account conduction in the heated foil and radiation. The global Nusselt number at the grooved wall is augmented by 1.52 ≤ Nu/Nupl ≤ 1.75 compared to a plane channel. Thermal periodicity is reached downstream of the fifth heated period. Thermal boundary layers develop locally, starting from the stagnation point near the front edge of a rib along the top of the rib and the front flank, respectively; high average heat transfer coefficients result compared to a plane channel. A small region with low heat transfer indicates that a secondary vortex exists in the corner between the front flank of the rib and the bottom of the groove. No similar effect is observed at the rear corner. Tangential heat conduction in the heating foil has effects mainly at the rear flank of the rib because of a point of inflection in the Nusselt distribution at this flank and at the edges of the rib.