Measurements of the dynamics of thermal plumes in turbulent mixed convection based on combined PIT and PIV

Abstract

The dynamics of thermal plumes and their abundance is investigated in mixed convection in a cuboidal sample with respect to the characteristic numbers. The parameter range spans Ra = 1.0 - 3.2 x10^8, Re = 0.5 - 1.7 x 10^4 and Ar = 1.1 - 7.6. Combined Particle Image Thermography (PIT) and Particle Image Velocimetry (PIV) is conducted in a horizontal layer close above the bottom thermal boundary layer. This combination of measurement techniques, using thermochromic liquid crystals (TLCs) as tracer particles, which is novel for air flows, allows for simultaneous measurement of temperature and velocity fields. Details of the measurement technique are published in Schmeling et al (2014). The fingerprints of sheet-like plumes and those of the stems of mushroom like plumes are visible in the instantaneous temperature fields. A study of temperature PDFs reveals, that the distributions can be well described by a sum of two Gaussian distributions. Analysing the ratio of the probabilities P2/P1 reveals a sudden change at a critical Ra_c approx. 2.3 x 10^8. Here P1 denotes the abundance of fluid temperatures imprinted by the bulk flow, while P2 represses the abundance of temperatures ascribed to warm thermal plumes. Accordingly, P2/P1 is a measure for the plume fraction in the measurement plane. The change occurs in the Ar regime 2.7 < Ar < 3.3, in which the interaction of buoyancy induced large-scale circulations (LSCs) with the wall jet of the incoming air results in an instability reported already by Schmeling et al (2013). A combined evaluation of the temperature and velocity fields reveals a change of the horizontal heat fluxes is found at Ar approx. 2.7-3. Furthermore, the total amount of heat transported horizontally within the measurement layer increases with Ra in bulk dominated regions, while it stays almost constant for plume dominated ones. A flow reversal in the measurement plane occurs as a function of Ar. At low Ar the angle between the mean flow direction and the x-axis amounts to approximately 120 degree. An intermediate Ar results in an angle of 18 degree, which we ascribe to the interactions of buoyancy with the wall jet. At higher Ar we find an angle of approximately -75 degree. Moreover, no unique dependency of the time averaged angle on Re or Ra could be observed.