円管内オリフィス下流における流れの再付着位置と熱伝達極大位置の関係

Abstract

The aim of the present study is to investigate the relation between the positions of flow reattachment and maximum heat transfer at flow reattachment region. Flow separation and reattachment at downstream of an orifice (bore ratio d/D ≈ 0.5, ReD ≈ 13000) in a pipe was chosen as a flow field. A technique using high-speed infrared thermography was used to measure the spatio-temporal heat transfer to a turbulent water pipe flow around an orifice. In this work, we attempted to evaluate the convection velocity of the heat transfer structure on the heated surface, which is considered to be associated with the convection of the vortical structure in the near-wall region. As a result, it was confirmed that the maximum position of the time-averaged heat transfer was not coincide with the time-averaged flow reattachment position, as it has been reported in the literature. In order to investigate the mechanism of this, ensemble-averaged heat transfer coefficients were calculated on the condition that instantaneous reattachment appeared at specific streamwise positions. The ensemble-averaged heat transfer coefficient shows a noticeable peak at the flow reattachment position. Also, the peak value of the ensemble-averaged heat transfer coefficient tended to decrease toward downstream. This trend indicates that the flow reattachment at upstream contributes to the heat transfer enhancement more significant than that at downstream. This is the reason why the maximum position of the time-averaged heat transfer exists upstream of the time-averaged flow reattachment position.