Cylindrical microwave resonator sensors for measuring materials under flow

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Doctoral thesis (monograph)
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Helsinki University of Technology Radio Laboratory publications. Report S, 243
In this thesis the various possibilities of implementing microwave sensors for measureing materials flowing in pipes are studied, with special emphasis on full-bore resonator sensors. With such resonator sensors a method to confine the electro-magnetic energy in the sensor must be used. Two main principles are studied in detail, the principle of resonance below cut-off, and the design using end grids. The first principle is shown to allow sensors with fairly open, or even completely nonintrusive structures to be designed. The second often involves sectorial or semisectorial structures. The waveguide modes in sectorial and semisectorial waveguides are therefore analyzed. From the analysis of the sectorial waveguides came the idea for a new type of resonator sensor based on the principle of resonance below cut-off, the cylindrical fin resonator sensor (CFR). Various design aspects of this sensor are studied based on calculations, measurements, and simulations using the Hewlett-Packard HFSS software. A sensor suitable for measuring the composition of mixtures of hydro-carbons and water is developed based on the discussion. The sensor has a simple mechanical structure and is less expensive to manufacture than the end grid sensors. Various designs of end grids are compared based on theoretical considerations, simulations, and measurements, and recommendations for optimized designs are given. The ring grid with eight sectors and a ratio of radii of roughly 40% is shown to provide the best isolation for a given thickness of the grid. A new type of resonator sensor based on semisectorial resonance modes, is developed for measuring the mixture ratio of oil and water in an oil well deep in the ground, where the temperature and pressure are high. The sensor is designed for installation in the annulus (i.e. the annular space between the casing or liner and the production tubing in a well), where it can be used for measuring the inflow from a specific zone in a smart well. The emphasis is on matching the spatial constraints and achieving the desired frequency response, which is analyzed theoretically, simulated with the HFSS, and finally measured. A humidity sensor for harsh environments is developed. The emphasis is on the choice of resonance mode to achieve the best immunity to contamination, and on the design of the end grids. Both calculations and field tests are presented. Predicting the height of the resonance peak under changing measurement conditions is treated also on a general level in the thesis. This issue is important, when the coupling probes are designed based on results from simulations with HFSS. The accuracy of the results obtained with HFSS is studied based on observations of scatter in the results, the dependence of the results on the convergence parameter ΔS, and by comparing the calculated and measured results. It is shown that the cut-off frequency and the resonant frequency are predicted with a higher accuracy than the quality factor.
microwave sensors, fluid flow, pipes, materials flow, waveguides
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