Experimental study on evolution of longitudinal vortex rolls subject to the duct tilted in the spanwise direction.

An experiment was performed in this study to investigate the effect of duct spanwise inclinations on the longitudinal vortex pattern in an air mixed convective flow. Key parameters, including Reynolds (Re) numbers from 20 to 30 and Grashof (Gr) numbers from 4071 to 10,855, are investigated. The spatial and temporal attributes of longitudinal vortex rolls (LVRs) are examined via flow field visualization and transient temperature measurement in details. Compared with the corresponding non-sloping case, the duct inclination significantly influences the LVR evolution. Due to the non-zero spanwise buoyancy, the vortex roll tends to be asymmetrical across the span and causes the delayed onset of lower LVRs. The temperature fluctuates significantly with time adjacent to the upper sidewall at the larger tilt angle. In an inclined duct, lower LVRs will stretch spanwise to squeeze upper vortex rolls with a decrease in Re number. Increasing the Gr number is beneficial to LVR formation for a smaller spanwise-inclined angle. However, the increase in either Gr number or tilt angle makes the vortex flow relatively unstable. We also provide an empirical expression to determine the vortex onset as a function of Re , Pr , Gr numbers, and the tilted angle. [ABSTRACT FROM AUTHOR]