No abstract

No abstract

Very few investigations of bubbly two-phase flows have used X-hot-film probes for the measurement of the turbulence field in the continuous phase. Therefore the interactions between the sensors of the X probe and the bubbles are not well known and thus can lead to errors in the determination of turbulence parameters. To gain understanding of the interaction, investigations of static and dynamic bubble-sensor interactions were carried out. In the static investigations a probe that is moved through stagnant water interacts with a bubble that is fixed to a small nozzle. These experiments help us to understand signals for situations in which the liquid and the bubble move with the same speed. In this case, the so-called post-signal occurring at the exit of the sensor penetration through the bubble is caused by the deformation of the bubble surface by the sensor and its prongs rather than by the flow field. In the dynamic investigations the probe is fixed in a vertical flow and it interacts with single bubbles of a bubble stream, as is the case in any two-phase flow situation. A bubble-probe interaction map is proposed that classifies the behaviour into four different modes of interaction. In both series of measurements, various parameters characterizing bubble signals were determined. A synchronized video system is used for visual investigations of deformations of the bubble surface. The results will be used for the future development of a so-called bubble elimination algorithm that is used for filtering the bubble signals out of the two-phase signal.

An innovative concept for decentralized processing of the signals of multiple sensors is presented. The basic idea is to connect every sensor via an analog-to-digital converter with a high-performance microprocessor, a transputer, that exclusively collects and processes its signal. The transputers are linked to a powerful network. Exemplarily, a measuring system based on this concept was used for the determination of local parameters of liquid–gas two-phase flows (e.g., local void fraction and bubble frequency). Up to six fiber-optical probes were utilized simultaneously to demonstrate the efficiency of the system. The signal processing under that special application may include on-line evaluation of flow parameters and on-line correction of errors of measurement due to sensor-induced effects. The concept of the new system, however, is basically independent of the number and type of sensors, as well as of the method of signal processing. The transputer-based measuring system thus provides a flexible, efficient tool for applications in which great amounts of data have to be evaluated or manipulated quickly.

An experimental procedure for investigating transient bubble flow for an adiabatic air/water system with vertical upward flow in a pipe is presented. The results of the measured local transient two-phase flow parameters are shown along a pipe length of approx. four meters. From the measured radial phase distributions under steady and under transient conditions one can draw conclusions about the interfacial forces. Here, the effects indicate the action of forces such as a transverse lift force and a time dependent force like the virtual mass force during the transient. For modelling the transverse lift force which seems to play a dominant role for that flow regime the formulation of Zun was chosen and it was implemented into the commercial CFD-Code Fluent Release 4.4.4 via user-defined subroutines. Finally, results from the simulation of the steady states of start and end conditions of an experimental measured transient are shown.