Analysis of thermal plumes by means of combined velocity and temperature field measurements

Abstract

Combined particle image thermography and particle image velocimetry (PIT / PIV) using thermochromic liquid crystals (TLCs) is presented as a measurement technique for the simultaneous acquisition of instantaneous temperature and velocity fields in air flows. Details of the particle generation, their illumination, a noise-filtering routine and the color-temperature calibration are presented. This technique is applied to mixed convection in a cuboidal sample. Therein, the dynamics of thermal plumes and their abundance is investigated. The working fluid is air at ambient pressure. Combined PIT and PIV is conducted in a horizontal layer close above the bottom thermal boundary layer and in a vertical layer. Within the vertical layer the formation of thermal plumes, which transport the heat from the warm bottom plate towards the cold top plate of the sample, are visible. The temperature and the velocity fields reveal the mean wind and the detachment of the plumes. The fingerprints of sheet-like plumes and those of the stems of mushroom like plumes are visible in the instantaneous temperature fields of the horizontal measurement plane. A study of temperature PDFs reveals, that the distributions can be described by a sum of two Gaussian distributions. Analyzing the ratio of the probabilities P_2/P_1 reveals a sudden change at a critical Ra_c. Here P_1 denotes the abundance of fluid temperatures imprinted by the bulk flow, while P_2 represses the abundance of temperatures ascribed to warm thermal plumes. Accordingly, P_2/P_1 is a measure for the plume fraction in the measurement plane.