Measurement of three-dimensional flow structure and transient heat transfer on curved surface impinged by round jet

摘要

In this study, the transient heat transfer characteristics of a circular cylinder surface cooled by an impinging air jet were measured through thermographic phosphor thermometry. Time-resolved surface temperature fields were visualized under constant heat flux conditions. After the jet impingement on the circular cylinder, the curved three-dimensional (3D) wall jet developed toward both spanwise and streamwise directions, followed by most of the fluid moving along the curved surface due to the Coanda effect. Therefore, the high heat transfer region was elongated toward the streamwise direction. The impinging angle was established to be a dominant factor in heat transfer characteristics. The heat transfer was improved with an increase in the impinging angle, as the cooling area expanded toward the streamwise direction. To discuss the transient heat transfer characteristics, time-resolved two-dimensional Nusselt number distributions were obtained according to the varied impinging angles and Reynolds numbers. The secondary peak of heat transfer was not clearly observed due to the stabilizing effect of curvature on the wall jet. To understand the heat transfer phenomenon, the 3D flow structures of the jet on the curved surface were obtained using time-resolved volumetric particle tracking velocimetry (PTV). The velocity vectors and streamwise vortices were visualized according to the impinging angle and analyzed in three-dimensions. The 3D curved wall jet possessed a big counter-rotating streamwise vortex structure in the upper region of the attached wall jet on the cylinder wall. With an increase in the impingement angle, the stream-wise vortex developed longer at the same Reynolds number. The 3D flow structure and heat transfer distribution were found to demonstrate similar tendencies.