Experimental Acquisition and Characterisation of Large-Scale Flow Structures in Turbulent Mixed Convection

Abstract

In this cumulative thesis, the superposition of shear flow and thermal convection, namely mixed convection, is investigated experimentally. The experiments were performed in a cuboidal sample with aspect ratios Gamma_xz = length=height = 5 and Gamma_yz = width=height = 1. The sample was composed of a heatable bottom and coolable ceiling as well as air inlet and outlet channels, which are located on the same side and both span the whole length of the sample. Different measurement techniques were applied and four main results were obtained. The first main result is the finding of large-scale flow structures, which arrange in different configurations within the sample depending on the characteristic numbers. In specific, the breakdown of a two-dimensional forced convective roll structure at low and vanishing Ar into three-dimensional configurations at higher Ar was found. Thereby, stable arrangements of three and four thermally induced convection rolls were also found. Their axis of rotation was found to be zigzag shaped and tilted with respect to the inflow velocity. This is ascribed to the superposition of the thermally induced convection rolls with shear forces. The second main result is the observation of a maximum in the enthalpy flux carried by the fluid between in- and outflowing air at Ar approx. 0.6. This was caused by the existence of an upper boundary of the temperature difference between in- and outflowing air with changing Ar. As a consequence, there are flow conditions, which maximise the enthalpy flux of the flow through the cell. The third main result is the finding of different dynamics of the large-scale flow structures, namely stable configurations, erratic changes and periodic oscillations. These results are based on long duration temperature measurements and smoke visualisations. A dependency on the underlying characteristic numbers, namely Re, Ra and Ar, was found: first the strength of the thermal convection (Ra) determines if oscillations can occur. Second, mixed convection (Ar) determines the dynamics (steady, erratic changes or periodic oscillations) if Ra allows oscillations. Third, forced convection (Re) determines the oscillation frequency if Ar allows periodic oscillations. The fourth main result is the experimental realisation of simultaneous measurement of instantaneous temperature and velocity fields in a system with continuous fluid exchange and air as working fluid. This is more a result of the experimental accessibility than of the fluid dynamical processes in mixed convection. However, for the challenging task of measuring both quantities non-intrusively and extensively, a solution is presented, which was developed and successfully applied. The main idea of this technique is the simultaneous usage of thermochromic liquid crystals as tiny thermometers and as tracer particles for particle image velocimetry. Experimental cornerstones are the particle generation, their illumination, image filtering and calibration. They are addressed in this thesis. Additional accomplishments of the thesis are the following findings: The large-scale flow structures couple the momentum transport from one wall to the opposing side wall. Even more, a back-coupling is found and a concept describing the mechanisms acting to trigger the periodic oscillations is presented. Furthermore, it showed that a proper adjustment of Re, i.e. the inflow velocity, can destroy stable arrangements of large-scale structures or oscillating configurations of them. Finally, probability density functions of the temperature distribution in a horizontal layer, slightly above the bottom thermal boundary layer, allowed for analysis of fingerprints of the sheet-like thermal plumes. For the given location of the measurement plane a change of the plume fraction P2=P1 occurred at Ra approx. 2.3x10^8. Here P1 denotes the abundance of fluid temperatures imprinted by the bulk flow, while P2 inhibits the abundance of temperatures ascribed to warm thermal plumes.