Mechanism of counterflows and circulation cells in swirling flows

Abstract

Abstract This paper reviews counterflows, double counterflows, and circulation cells in swirling flows and argues that all these seeming paradoxical phenomena can be caused by a common swirl- decay mechanism (SDM). It is shown that SDM explains (a) the counterflow of water and oil in hydrocyclones, (b) the elongated counterflow of hot and cold air in vortex tubes, and (c) the double counterflow occurring in vortex combustion chambers. SDM also explains the development and disappearance of circulation cells, often referred to as vortex breakdown bubbles, in sealed cylindrical containers where the flow is driven by rotation of one end disk. In a few words, SDM is the following. The balance of the centrifugal force and the radial gradient of pressure in a fast- swirling flow results in that pressure at the rotation axis is smaller than pressure at the periphery. If swirl decays downstream, pressure grows along the axis. This axial gradient of pressure decelerates the near-axis flow and can reverse it thus developing a local or global counterflow. It is shown that SDM also works in two-fluid flows modeling vortex bioreactors.