Large-Scale Structures and Heat Transport of Turbulent Forced and Mixed Convection in a Closed Cavity

An experimental investigation of flow structure formation in turbulent forced and mixed convection in a closed rectangular cavity with an aspect ratio of 1:1:5 and air as working fluid is presented. Forced convection at 1.07x10^4 < Re < 3.5x10^4 and mixed convection at 1.01x10^4 < Re < 3.4x10^4 and Ra=2.4x10^8 are studied under well-defined conditions by means of Particle Image Velocimetry and local temperature measurements. For purely forced convection a 2D mean wind, which can be approximated by a solid body rotation (Rankine vortex), is found. With increasing Archimedes number (Ar), realised by a temperature gradient between bottom and ceiling of the convection cell, this structure becomes instable, leading to four convection rolls for Ar>1.7. These rolls are oriented in longitudinal direction of the cell and their rotation direction alternates for different Archimedes numbers, realising an updraught for Ar=2.2 and a downdraught for Ar=1.7 and Ar=3.3 in the central cross section. Temperature measurements of the outflowing air reveal a temporally and spatially constant fingerprint with two maxima over the length of the outlet. Investigations of the enthalpy transport of the fluid indicate the existence of a maximal enthalpy transport by the fluid for approximately Ar=0.6