Optical void measurement method for stratified wavy two phase flows
Introduction
Two-phase flows, such as horizontal stratified flows, are relevant in several technical applications including thermal power plants. A promising approach to understand and predict these complex flows in detail are Computational Fluid Dynamics codes (CFD-Codes). For the development of phase exchange and turbulence models which are part of these codes experimental high resolution data for the phase distribution is needed.
Many different measurement methods are available for this flow value. For CFD-validation data an accurate, high-resolution, cost-effective measuring method is needed which should be easy to use. Most methods only partially fulfill these criteria. Fast closing cut-off valves [19], [20] are precise in measuring the absolute void, but are not able to determine its local distribution and have a huge impact on the flow. Also, hot wire anemometry is used and its reliability for air-water flows was shown by Utiger et al. [21]. But it also has disadvantages concerning highly turbulent flows. On the one hand, the fragile wire of the sensor might be damaged. On the other hand, the measured voltage is not only a function of the phase distribution but also on flow velocity. The capacitance sensor of Jaworek et al. [11] is mounted on the outside of pipe walls, detecting the void by changes of the equivalent permittivity of the dielectric between two electrodes. Due to the non-linear characteristics, much effort has to be put in the calibration. Unfortunately, no information about the spatial resolution of the void distribution is achieved.
Several devices have been developed in the past years possessing a better spatial resolution. For example the fast electron beam X-ray tomography [7], [9] and the wire-mesh sensor [16], [6]. These devices allow 2D or 3D information in high temporal and spatial resolution. The advantage of the tomographic method is its non-intrusiveness and its application even on nontransparent fluids. But the handling of such an apparatus is costly and involves a high demand for safety precautions. The mesh sensor possesses besides its high resolution the disadvantage of being invasive [17]. The measurement principle of the wire mesh sensors can either base on the conductivity or the capacitance.
Some other commonly used sensors are local probes, such as the electrical resistivity probe [18], [15] or fiber optical probes [3], [4], [5], [12] that possess a high temporal resolution. The probe is installed in a way that it faces the flow direction. The void is detected by being pierced by the sensor tip. Efforts in estimating the bubble size out of the chord length distribution of the signal were done by Bankoff [1], but require spherical and non-deformable bubbles. A further development of these probes lead to four-tip probes that allow additionality the measurement of the bubble diameter and the velocity of the bubbles without assumptions on the bubble form and behavior [14].
A videometric procedure was also developed that detects the interface of a stratified flow by image processing from shadow images [22]. At Karlsruhe Institute of Technology (KIT) further development based on the idea of this last videometric method was done at the Water Entrainment Channel Karlsruhe (WENKA-channel). It allows a high-resolution void detection in two-dimensions in the whole relevant measurement volume. In the present paper, the principles of this optical void measurement (OVM) are presented as well as a validation by a comparison of the new method with a local electrical resistivity probe.
Section snippets
Experimental setup
The WENKA-channel (Fig. 1, Fig. 2) is a two-phase flow channel for horizontal counter-current flows under ambient conditions, possessing a rectangular cross-section of 90 × 110 mm2. The fluids used are water and air. Maximum local velocities for water in main flow direction about 0.9 m/s were observed. For continuous air flow local velocities up to 22 m/s were observed. The maximum velocity in main flow direction for droplets is equal to the maximum local velocity of the air flow. Similarly,
Methodology
In a stratified flow, it can be assumed that the fluid with higher density is at the bottom and the fluid with lower density is on top. In-between both layers, a huge interfacial area appears (Fig. 3). Based on this behavior, an image processing algorithm can be elaborated to get a complete void fraction distribution for the observed channel section. The main principle of the presented measurement method is to detect and determine the interfacial area between the continuous water flow at the
Validation and discussion
The validation of the OVM was done by a comparison with the determined values achieved by an electrical resistivity probe [18], [8].
Conclusion
In the present paper a new optical void measurement (OVM) was presented which is applicable to stratified two-phase flows of transparent fluids. In contrast to many other non-intrusive measurement techniques, it allows not only the determination of the void content, but also the local distribution in a horizontal, rectangular channel, without the need of high safety precautions or costs. The only physical requirement is a channel which is optically accessible, illuminated from behind and with a
Acknowledgements
This work was supported by the German Bundesministerium für Wirtschaft und Technologie (BMWi), GRS 1501375.
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