Daniel Schmeling

Simultaneous Measurement of Temperature and Velocity Fields in Convective Air Flows

Thermal convective air flows are of great relevance in fundamental studies and technical applications such as heat exchangers or indoor ventilation. Since these kinds of flow are driven by temperature gradients, simultaneous measurements of instantaneous velocity and temperature fields are highly desirable. A possible solution is the combination of particle image velocimetry (PIV) and particle image thermography (PIT) using thermochromic liquid crystals (TLCs) as tracer particles. While combined PIV and PIT is already state of the art for measurements in liquids, this is not yet the case for gas flows. In this study we address the adaptation of the measuring technique to gaseous fluids with respect to the generation of the tracer particles, the particle illumination and the image filtering process. Results of the simultaneous PIV/PIT stemming from application to a fluid system with continuous air exchange are presented. The measurements were conducted in a cuboidal convection sample with air in- and outlet at a Rayleigh number Ra ? 9.0 ? 107. They prove the feasibility of the method by providing absolute and relative temperature accuracies of ?T = 0.19?K and ??T = 0.06?K, respectively. Further open issues that have to be addressed in order to mature the technique are identified.

Flow Structure Formation of Turbulent Mixed Convection in a Closed Rectangular Cavity

An experimental investigation of flow structure formation in turbulent mixed convection in a closed rectangular cavity with an aspect ratio of 1:1:5 and air as working fluid is presented. Mixed convection at Re = 1.1·104 and Ra = 3.0·108 is studied under well-defined conditions by combination of forced and thermal convection. The resulting flow structures strongly depend on the ratio of inertia and buoyancy forces. A 2D mean wind, which can be approximated by a solid body rotation, is found at pure forced convection. With increasing Archimedes number (Ar), realized by a temperature gradient between bottom and ceiling of the convection cell, this structure becomes instable. Leading to four convection rolls for Ar = 3.4, which are oriented in longitudinal direction of the cell, are observed.

Development of combined particle image velocimetry and particle image thermography for air flows

Simultaneous measurements of instantaneous velocity and temperature fields of air flows by means of Particle Image Velocimetry (PIV) and Particle Image Thermography (PIT) enables highly demanded studies on thermal plumes, their dynamics and the resulting heat transfer for Pr ≈ 0.7. Thereby, small particles of thermochromic liquid crystals (TLCs), which reveal temperature depending reflection properties are used as tracer particles for combined PIT and PIV. The feasibility of the method is demonstrated in a Rayleigh-Benard convection experiment in a cubical enclosure. Furthermore, a new particle generator being able to produce continuously very small monodisperse droplets of TLCs has been designed. The improvement of the developmental process for mixed and Rayleig-Benard convection studies is discussed. Thereby, special focus is laid on the production process of small TLCs, the generation of monodisperse acetone-TLC droplets and the temperature depending colour play of the produced particles.Copyright

Oscillations of Large-Scale Structures in turbulent Mixed Convection in a rectangular enclosure

The term mixed convection (MC) is used to describe the process of heat transfer in fluids where forced convection (FC) and thermal convection (TC) coexist. Mixed convection is an often occurring flow condition e.g. in the oceans, atmosphere, indoor climatisation or industrial processes and applications [1]. In many flow situations convection is the prevalent transport mechanism of heat whereas the heat transfer strongly depends on the dynamics of the largescale structures. In this study we investigate the formation of large-scale circulation (LSC) in mixed convection and the influence of the dynamics of the LSC, also known as mean wind, on the heat transfer.

Temperature Oscillations in Turbulent Mixed Convective Air Flows

We report on measurements of temperature oscillations observed in turbulent mixed convection in a rectangular enclosure. They are correlated to fluctuations and spontaneous reorientations of the large-scale circulations in the flow. Stable temperature configurations, periodic reconfigurations as well as spontaneous reversals and reorientations were detected. We ascribe the observed dynamics of the large-scale flow to the interaction between the superimposed shear forces of the forced convective flow and the buoyancy driven flow. Comparison with smoke visualisations allowed to assign the observed temperature distributions to a certain number and configuration of convection rolls. A frequency analysis of the oscillations as a function of the characteristic numbers is conducted revealing a sophisticated dependency of the dynamics of the large-scale flow on Ar, Re or Ra.