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

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 \(Ra=1.0{-}3.2\times 10^8\), \(Re=0.5{-}1.7\times 10^4\) and \(Ar=1.1{-}7.6\). Combined particle image thermography and particle image velocimetry is conducted in a horizontal layer close above the bottom thermal boundary layer. This combination of measurement techniques, using thermochromic liquid crystals 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. (Meas Sci Technol 25:035302, 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 \(P_2/P_1\) reveals a sudden change at a critical Ra _c  ≈ 2.3 × 10⁸. 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. The change occurs in the \(Ar\) regime \(2.7\,\lesssim\, Ar\,\lesssim\, 3.3\), in which the interaction of buoyancy-induced large-scale circulations with the wall jet of the incoming air results in an instability reported already by Schmeling et al. (Exp Fluids 54:1517, 2013). A combined evaluation of the temperature and velocity fields reveals a change in the horizontal heat fluxes at \(Ar\approx 2.7{-}3\). Furthermore, the total amount of heat transported in x direction within the measurement layer increases with \(Ra\) in bulk-dominated regions, while it stays almost constant for plume-dominated ones.