| Gas-liquid two-phase flow widely exists in many industrial processes, such as petroleum, chemical, energy, power engineering, etc.. Accurate void fraction measurement is very important for system monitoring, process controlling and safety assurance of gas-liquid two-phase flow system. However, due to the complexity of the characteristic of gas-liquid two-phase flow, void fraction measurement is very difficult and is a problem that has not been solved well in both scientific research and industrial fields.Capacitively Coupled Contactless Conductivity Detection (C4D) is a new contactless electrical measurement technique, which provides a new approach for solving the void fraction measurement of gas-liquid two-phase flow. Based on C4D technique, this dissertation aims to propose new methods for the void fraction measurement of gas-liquid two-phase flow in two different ways, obtaining and using the equivalent conductance or the total impedance information of gas-liquid two-phase flow.The main innovation points and contributions are listed as follows:1. A six-electrode C4D sensor, which is suitable for the void fraction measurement of gas-liquid two-phase flow in conventional pipes, is developed. The developed six-electrode C4D sensor aims to obtain the equivalent conductance of gas-liquid two-phase flow. The sensor measures equivalent conductance of gas-liquid two-phase flow by phase sensitive demodulation (PSD) technique and can obtain 15 conductance signals, which reflect void fraction distribution of gas-liquid two-phase flow. The results of conductance measurement experiments show that the developed six-electrode C4D sensor is successful and the conductivity detection accuracies of the sensor are satisfactory. In four pipes (the inner diameters are 22.0 mm,29.0 mm,36.5 mm and 47.0 mm, respectively), the maximum relative errors of conductivity detection are less than 5.0%(the conductivity range is 0.1 mS/cm-10 mS/cm).2. Based on the equivalent conductance signals obtained by the six-electrode C4D sensor, a new method for void fraction measurement of gas-liquid two-phase flow is proposed by combining the data mining technique. This method develops a corresponding void fraction measurement model for each typical flow pattern. In practical measurement, the six-electrode C4D sensor is firstly used to obtain the conductance signals of gas-liquid two-phase flow, then flow pattern is identified by flow pattern classifiers, finally, according to the result of flow pattern identification, the corresponding void fraction measurement model is selected to calculate the void fraction value. The flow pattern classifiers are developed by least squares support vector machine (LS-SVM) technique. The void fraction measurement models are developed by partial least squares (PLS) technique and LS-SVM technique.3. Experimental results show that the proposed void fraction measurement method is effective. Static void fraction measurement experiments in four pipes (the inner diameters are 22.0 mm,29.0 mm,36.5 mm and 47.0 mm, respectively), the maximum absolute errors for typical bubble flow, stratified flow and annular flow are all less than 3.76%. In dynamic void fraction measurement experiments, the maximum absolute errors in three pipes (the inner diameters are 17.0 mm,22.0 mm and 36.5 mm, respectively) for typical bubble flow, stratified flow, slug flow and annular flow can be less than 9.47%.4. With series resonance principle and PSD technique, a new radial two-electrode contactless impedance sensor suitable for the void fraction measurement of gas-liquid two-phase flow is developed. Series resonance principle is used to overcome the unfavorable influences of coupling capacitances (the capacitive reactances of the coupling capacitances are eliminated by the inductive reactance of an introduced inductor). PSD technique is used to obtain the total impedance information of gas-liquid two-phase flow (including the amplitude, the real part and the imaginary part of the impedance). Experimental results show that development of the radial two-electrode contactless impedance sensor is successful. Compared with the conventional conductance sensors, the radial two-electrode contactless impedance sensor can avoid polarization effect and electrochemical erosion effect, and can obtain the total impedance information of two-phase flow successfully.5. Feasibility of applying the total impedance information obtained by the radial two-electrode contactless impedance sensor to the void fraction measurement of gas-liquid two-phase flow is studied. Research results show that applying the radial two-electrode contactless impedance sensor to the void fraction measurement of gas-liquid two-phase flow is feasible. For each flow pattern (bubble flow and stratified flow), there are different relationships between the amplitude, the real part and the imaginary part of the impedance and the void fraction. Different part of the impedance of the two-phase flow reflects the void fraction information from different aspects. The research results also indicate that the imaginary part of the impedance of gas-liquid two-phase flow is very useful part for void fraction measurement. Making full use of the total impedance information can effectively improve the void fraction measurement accuracy. For the two developed radial two-electrode contactless impedance sensors (the inner diameters are 17.0 mm and 22.0 mm, respectively), the maximum absolute errors of static void fraction measurement are all less than 4.8% for typical bubble flow and stratified flow. |
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