Study on the Thermal Transport Phenomenon in the Transition Region of Natural Convection along a Vertical Smooth Flat Plate Uniformly Heated

Initially, a boundary-layer development around a heated solid surface is laminar in natural convection, but at some critical distance from the leading edge, depending on the flow field and the thermo-physical properties, small disturbances in the flow begin to be amplified, and a transition process takes place until the flow becomes turbulent. In this transition region, the heat transfer and fluid flow of natural convection have not necessarily been elucidated. The present study therefore focused on natural convection along a vertical smooth flat plate uniformly heated under steady-state conditions to investigate the characteristic phenomena of the transition region. The two-dimensional temperature field was visualized by infrared tomography and analyzed using an image processing technique to acquire time series data for statistical analyses applying fast Fourier Transfer and Wavelet transfer. A series of visualizations and signal analyses revealed the possibility of characteristic temperature patterns (pseudo-horseshoe-shaped temperature patterns) in the transition region. It was also clarified that fluctuation components with comparatively short and long cycles are contained simultaneously in the fluctuating temperature that appears in the transition region. Periodic fluctuation that can be explained by a linear stable theory takes place until the flow becomes turbulent, and disappears with development of the turbulent flow.