Convective heat transfer in the channel entrance with a square leading edge under forced flow pulsations

Abstract

Flow characteristics are generally affected by both the regime parameters and channel geometry (cross sectional area, length, converging or diverging parts, bends, obstacles, etc.). Relevant geometric parameters also include the geometry of the channel inlet and entrance length. The present work is an experimental study of convective heat transfer in the entrance region of a plane channel. Steady and pulsating air flow regimes were considered. Heat transfer coefficient was estimated using the method in which the wall was heated and simultaneously its temperature was measured by one and the same electrical circuit. The distributions of heat transfer coefficient were obtained downstream of the square leading edge of the channel. Forced flow pulsations were shown to result in ±20% variation of the heat transfer coefficient compared to the corresponding steady flow values, which was observed at the distance of one equivalent diameter from the channel inlet and further downstream. Near the channel inlet (less than one equivalent diameter), deviation of heat transfer from the steady flow values was far more pronounced and non-monotonic in streamwise direction. Local heat transfer enhancement (up to 2.5 times) due to flow pulsations was observed immediately downstream of the channel leading edge. Flow visualization was performed to reveal the mechanisms behind the enhancement. Regular vortices were observed in pulsating flow behind the channel leading edge. At low frequencies of pulsations they promoted mass transfer and hence heat transfer between the core flow and the near-wall region in the channel entrance. The opposite effect was observed at high frequencies.