Performance evaluation of conductivity wire-mesh sensors in vertical channels
Introduction
The properties of flows of fluid mixtures have been studied extensively, both experimentally and analytically, as such flows are encountered in many engineering applications. Flows of mixtures may be classified into two general classes: multiphase flows, which consist of immiscible fluids with generally distinct velocities and temperatures, and multicomponent ones, in which the constituent fluids are miscible and typically share a common velocity and temperature [9]. Multiphase flows are encountered in many heat exchangers and thermal power generation systems, whereas multicomponent mixtures are present in combustors and chemical reactors.
Available techniques for the measurement of flow properties in multiphase and multicomponent flows may provide either volume–, area– or line–averaged values of a property or local values at discrete locations. Spatially averaged measurements may usually be collected faster and more conveniently than local measurements, and they often include sufficient information for the needs of many applications. On the other hand, the availability of spatially distributed local measurements allows for a more detailed analysis of the flow structure and insight into phenomena and processes that cannot be described by global measurements alone. A number of measurement procedures, commonly referred to as tomographic methods, aim at reconstructing the cross–sectional or volumetric distribution of a flow property from a number of discrete measurements collected simultaneously at many points in the flow. Several tomographic methods have been applied to multiphase and multicomponent flows with the objective of distinguishing the constituents of the mixture.
Multiphase/multicomponent tomographs distinguish between the mixture constituents by detecting differences in electrical properties (impedance, permittivity and conductivity tomographs), radiation attenuation (X–ray and gamma ray tomographs), sound attenuation (ultrasonic tomographs) or light attenuation (optical tomography systems) [57]. Electric tomographs, in particular, comprise a number of electrodes, which are either mounted on the periphery of the flow channel or stretched across its cross–section. In these devices, an electric property is measured in the space between each pair of electrodes and its spatial distribution is reconstructed from these measurements with the use of analytical algorithms.
The subject of the present study is the wire–mesh tomographs (WMT), which measure either the conductivity or the permittivity of the fluid in the vicinity of electrodes stretched across the flow domain [13], [41], [36]. Past literature on WMT includes numerous experimental studies, most of which have appeared in the last 15 years. Although some previous publications include reviews of applications [32] and measurement uncertainty [4] of a specific type of WMT, an all–encompassing review of WMT applications and a critical evaluation of the performance and measurement uncertainty of various WMT devices operating under wide ranges of flow conditions have not yet become available. The present article is meant to fill this gap by addressing these issues. WMT are particularly suitable for measurements in gas–liquid flows, as gases and liquids that are commonly encountered in industrial applications have very different electrical properties. The main focus of the present review is the use of WMT in gas–liquid flows, but the applicability of WMT to the study of multicomponent flows will also be reviewed in a separate section.
In the following sections, we will first describe the different types of WMT, then identify the various flow parameters that may be extracted from WMT signals, overview successful applications of WMT and present an in–depth discussion of the measurement uncertainty of both types of these devices. This information will hopefully be of interest to readers conducting WMT measurements or considering the possible use of WMT in types of systems that are either similar to or different from those of past WMT application.
Section snippets
Types of WMT
Wire–mesh tomographs may be broadly classified into two categories, depending on the electrical property that is measured. Conductivity WMT measure a current (or a voltage proportional to this current) that is proportional to the local conductivity of the fluid. Permittivity WMT, sometimes referred to as capacitance WMT, measure the capacitance of a space near the electrodes, which is proportional to the permittivity of the fluid, i.e., its ability to transmit electric fields. For the WMT to
Extraction of flow properties from WMS signals
Wire–mesh sensors provide as output the cross–sectional distribution of conductivity or permittivity. In multiphase flows, one may post–process this output to obtain estimates of the cross–sectional phase distribution, the area–averaged phase fraction and the flow regime. Under certain flow conditions, one may also obtain estimates of the interfacial velocity, the individual bubble diameters and the interfacial area density (namely, the surface area of the interface between the two phases per
Design considerations
The main geometrical design parameters for the construction of a WMS are the wire diameter, the in–plane wire spacing (“mesh size”) and the distance between the wire planes; the main parameter for the construction of the data acquisition system is the sampling rate.
The spatial resolution of a WMS is intimately connected to the in–plane wire spacing [42]. Typical values of wire spacing for both types of WMS have been in the range from 2 mm to 5 mm, but WMS with a wire spacing of up to 15 mm have
Global wire–mesh tomographs in multiphase flows
The WMS and GWMT follow different procedures for the reconstruction of the phase distribution from discrete conductivity/permittivity measurements. The conductivity/permittivity distribution can be obtained from the output of a GWMT using the image reconstruction techniques described by Reinecke et al. [41]. As mentioned earlier, in the original design of the GWMT, using 29 wires in each wire plane, the flow cross–section was discretized into 1000 equilateral–triangle–shaped pixels. This meant
Application of wire–mesh tomography to multicomponent flows
Wire–mesh tomography has been used for studying properties of multicomponent flows, consisting of streams of a reference fluid and a tracer fluid with significantly different conductivities. A popular choice has been to use salt water as a tracer fluid and distilled water as the reference fluid. The cross–sectional component distribution in multicomponent flows may be obtained using similar techniques to those used in reconstructing the phase distribution in multiphase flows. This has been
Summary and concluding remarks
Wire–mesh tomography is a measurement technique that produces cross–sectional phase or component maps in multiphase and multicomponent flows with relatively high spatial and temporal resolutions. Several other important flow properties can be determined by post–processing of WMT output signals. This document has summarised the available literature on various aspects of wire–mesh tomography, and complemented some of this information with additional original analysis. The different types of WMT
Acknowledgements
Financial support for this study was provided by the University Network of Excellence in Nuclear Engineering (UNENE), Atomic Energy of Canada Limited (AECL) and the Natural Sciences and Engineering Research Council of Canada (NSERC).
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