THERMAL INSTABILITIES FOR A HORIZONTAL FL OW IN MIXED CONVECTION - A LAGRANGIAN VIEWPOINT

As part of the study of mixed convection in flow in a horizontal cylin drical duct uniformly heated at the wall, we show a thermal instabilit y phenomenon. This manifests itself through large amplitude temporal f luctuations of the wall temperature which leads to an intermittent the rmal behaviour of the system. Temperature fluctuations are characteriz ed by variable stationary phases which succeed them. This paper provid es, at first, a brief description of the experimental loop; then some numerical results corresponding to the stable state, and some experime ntal results related to the thermal instabilities are reported. Finall y we propose a description of the properties of the phenomenon using a non-linear map. This map is defined by a characteristic function rela ted to the thermohydrodynamic state of the flow. This approach based o n a Lagrangian model allows us essentially to show the non-linear aspe ct of the system.