Turbulent Thermal Convection under Modified Boundary Conditions: An Experimental Study of Heat Transport and Boundary Layers.

摘要

In this thesis three sets of experiments of turbulent Rayleigh-B é nard convection with modified boundary conditions are presented.;The first set of measurements were made in cylindrical cells with aspect ratio one and with various combinations of smooth and rough plates in the form of regularly-arrayed pyramids. The experimental results suggest that the Nu – Ra relationship can be represented by the combination of two power laws, corresponding to the bulk dominant regime (exponent=1/2) and boundary layer dominant one (exponent=1/4) of the Grossmann-Lohse model. The behaviors of the coefficients of the two power laws suggest that the roughness of the plate can enhance the contribution of bulk and push the system to change from the boundary dominant state to bulk dominant state. A further examination of the individual plates reveal that the heat transport properties of smooth plates are insensitive to the surface and boundary conditions of the other plate of the same cell, whether smooth or rough, or whether under constant flux or constant temperature. The heat transport properties of the rough plates, on the other hand, appear to depend on surface and boundary conditions of the other plate of the same cell.;In the second set of experiments we study the effect of polymer additives in two Rayleigh-Bé nard convection cells, one with smooth top and bottom plates and the other with rough top and bottom plates. For the cell with smooth plates, a reduction of the measured Nusselt number (Nu) was observed. Furthermore, the amount of Nu reduction increases with increasing polymer concentration (c), reaching ~ 12% for c = 120 ppm and an apparent level-off thereafter. For the cell with rough plates, however, an enhancement (~ 4%) of Nu was observed when the polymer concentration is greater than 120 ppm.;The third set of experiments investigates the properties of the velocity boundary layer in turbulent Rayleigh-B&