Augmented Heat Transfer in a Triangular Duct by Using Multiple Swirling Jets

Measurements of detailed heat transfer coefficients on two principal walls of a triangular duct with a swirling flow are undertaken by using a transient liquid crystal technique. The vertex corners of the triangular duct are 45, 45, and 90 deg. The swirl-motioned airflow is induced by an array of tangential jets on the side entries. The effects of flow Reynolds number (8600 ≦ Re ≦ 21000) and the jet inlet angle (α = 75, 45, and 30 deg) are examined. Flow visualization by using smoke injection is conducted for better understanding the complicated flow phenomena in the swirling-flow channel. Results show that the heat transfer for α = 75 deg is enhanced mainly by the wall jets as well as the impinging jets; while the mechanisms of heat transfer enhancement for α = 45 and 30 deg could be characterized as the swirling-flow cooling. On the bottom wall, jets at α = 75 deg produce the best wall-averaged heat transfer due to the strongest wall-jet effect among the three angles (α) investigated. On the target wall, however, the heat transfer enhancements by swirling flow (α = 45 and 30 deg) are slightly higher than those by impinging jets (α = 75 deg). Correlations for wall-averaged Nusselt number for the bottom and target walls of the triangular duct are developed in terms of the flow Reynolds number for different jet inlet angles.