Effect of Entrance Geometry on Heat Transfer in a Rib-Roughened Rectangular Channel

The effect of the entrance geometry conditions on the local and average heat transfer coefficient in a rectangular channel with 45 degrees angled ribs has been investigated. Ribs were deployed on one side only, the remaining walls of the channel being assumed smooth and unheated. The influence of entrance conditions was addressed by considering either a long ribbed unheated section or a smooth channel upstream of the ribbed heated section. As known, channels with 45 degrees angled ribs are characterized by spanwise variations of heat transfer coefficient due to secondary flows, with high heat transfer levels close to one side and low heat transfer levels on the opposite side. Numerical results, checked against experimental data, showed that heat transfer coefficients of first modules are affected by entrance conditions, especially in the most effective heat transfer region. Regardless of entrance condition, a quasi-periodically fully developed heat transfer condition is attained only in the low heat transfer region, while elsewhere heat transfer coefficient distributions are periodic in shape but values tend to increase with the streamwise coordinate. These findings indicate that heat transfer experiments on ribbed channels should be generally accompanied by a careful description of the entrance conditions assumed in the experiments.