18 TH UIT NATIONAL HEAT TRANSFER CONFERENCE CERNOBBIO, COMO, ITALY, 26-28 JUNE, 2000 THREE-DIMENSIONAL FLOW AND TEMPERATURE DISTRIBUTION IN RAYLEIGH-BENARD CONVECTION USING THERMOCHROMIC LIQUID CRYSTALS AND DIGITAL IMAGE PROCESSING F. Palazzolo, F. Magnasco and M. Ciofalo Dipartimento di Ingegneria Nucleare (DIN), Università di Palermo Viale delle Scienze, I-90128 Palermo, Italy Abstract The application of Thermochromic Liquid Crystals (TLC) suspended in glycerol to the investigation of Rayleigh-Bénard convection is described. Multiple-exposure images of TLC particles in cross sections parallel to the long and short sides of an enclosure are recorded on transparency film using a flash equipped with a collimator, yielding a thin light sheet. Images are then digitized and split into hue-saturation-intensity components; hue is converted into temperature using a previously obtained calibration curve, while the intensity component (B/W image) is processed by the AEA software VISIFLOW using a correlation method to give in-plane velocities. Planar distributions can then be interpolated to reconstruct the three- dimensional flow and temperature fields, giving Tomographic Particle Image Velocimetry and Thermography (TPIVT). 1. INTRODUCTION The present work involves two independent fields of interest: the first is one of the most basic problems of fluid dynamics, Rayleigh-Bénard convection, while the second is the development of imaging techniques for the visualization and the quantitative characterization of flow and temperature fields in fluids. The fluid in a shallow cavity heated from below is a classic example of nonlinear systems exhibiting a sequence of transitions as a control parameter increases. Here, the control parameter is the Rayleigh number Ra=(gβ∆TH 3 )/(να), H being the cavity height, ∆T the temperature difference between the walls, g the acceleration due to gravity, and β, ν, α the fluid’s thermal expansion coefficient, kinematic viscosity, and thermal diffusivity. Despite the long time elapsed since the first contribution by Lord Rayleigh [1] and the many theoretical and experimental studies dedicated to the problem, it is still only partially understood. For shallow enclosures, the first instability (transition from conduction to convection) occurs at a Rayleigh number close to the theoretical value for infinite-aspect ratio cavities (~1708). According to the literature [2-3], flow patterns initially take the form of steady transverse rolls, parallel to the shorter side; as Ra increases, rolls orthogonal to the above appear near the short sides, where they are superimposed on the base transverse-roll pattern. A complex interface (“grain boundaries”) develops between the regions dominated by the two alternative flow patterns [4]. Further increases of Ra lead to a growth of the regions dominated by longitudinal rolls. Eventually, through different and complex mechanisms, steady-state flow becomes unstable and a fully three-dimensional and time-