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-Be´nard 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-Be´nard 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.

Investigations on gas-liquid flows in horizontal pipes are of immanent importance for Reactor Safety Research. In case of a breakage of the main cooling circuit of a Pressurized Water Reactor (PWR), the pressure losses of the gas-liquid flow significantly govern the loss of coolant rate. The flow regime is largely determined by liquid and gas superficial velocities and contains slug flow that causes high-pressure pulsations to the infrastructure of the main cooling circuit. Experimental and numerical investigations on adiabatic slug flow of a water-air system were carried out in a horizontal pipe of about 10 m length and 54 mm diameter at atmospheric pressure and room temperature. Stereoscopic high-speed Particle Image Velocimetry in combination with Laser Induced Fluorescence was successfully applied on round pipe geometry to determine instantaneous three-dimensional water velocity fields of slug flows. After grid independence studies, numerical simulations were run with the open-source CFD program OpenFOAM. The solver uses the VOF method (Volume of Fluid) with phase-fraction interface capturing approach based on interface compression. It provides mesh refinement at the interfacial area to improve resolution of the interface between the two phases. Furthermore, standard k-ε turbulence model was applied in an unsteady Reynolds averaged Navier Stokes (URANS) model to resolve self-induced slug formation. The aim of this work is to present the feasibility of both relatively novel possibilities of determining two-phase slug flows in pipes. Experimental and numerical results allow the comparison of the slug initiation and expansion process with respect to their axial velocities and cross-sectional void fractions.

Numerical as well as experimental investigations of the highly intermittent slug flow regime of a gas-liquid mixture in horizontal pipes are of particular interest for nuclear reactor safety in post loss-of-coolant accident (LOCA) situations. The strong variation of governing interfacial length scales, as they are characterizing the slug flow regime, pushes common numerical multi-phase approaches to their limits, since they are designed either for interface capturing or for modeling the sub-grid behavior of the dispersed mixture. In this work an enhanced hybrid two-phase flow solver is employed to investigate the global and local characteristics of adiabatic, horizontal slug flows in a water-air system. A dynamic switching algorithm for an interface capturing procedure is introduced to examine segregated and dispersed parts in the same flow domain. The inter-facial area transport equation (IATE) is used to detect dispersed flow regions as well as to determine variable bubble sizes and their distribution within the slug body. Experimental results of videometry measurements on a horizontal, 10 m long pipe with an inner diameter of 54 mm at atmospheric pressure and room temperature are compared with numerical results of the same geometry in terms of global characteristics such as slug frequency and onset position. Local properties, such as the interfacial area density in the slug body, are also examined. This study demonstrates the capability of a coupled multiscale approach based on the Euler-Euler two-fluid model (TFM) for the simulation of slug flow in horizontal pipes with a high amount of entrainment.