Research On The Application Of C~4D Technique To The Parameter Measurement Of Gas-liquid Two-phase Flow

Gas-Liquid two-phase flow widely exists in the nature and many industrial processes, such as petroleum, energy, power, chemical, pharmaceutical engineering, etc.. Accurate parameter measurement is very important for mechanism study and operation monitoring, process controlling, energy conservation of related production process. However, due to the complexity and randomicity of gas-liquid two-phase flow system, its parameter measurement is very difficult and is a problem that has not been solved well in both secientific research and industry fields.Capacitively Coupled Contactless Conductivity Detection (C4D) technique is a new conductivity detection technique and has the characteristic of non-contact. It avoids the problems of electrode polarization, electrochemical erosion and electrode contamination in conventional contact conductivity detection and provides a new approach for solving the problem of gas-liquid two-phase flow parameter measurement. Our research group has made a preliminary attempt at applying C4D technique to the gas-liquid two-phase flow field. This dissertation aims to study the feasibility and potential of applying C4D technique to the parameter measurement of gas-liquid two-phase flow in millimeter-scale pipes based on the existing researches of our research group. The research work focuses on the velocity measurement and void fraction measurement in millimeter-scale pipes.The main innovation points and contributions in this dissertation are listed as follows:1. Two new C4D sensors suitable for parameter measurement of gas-liquid two-phase flow in millimeter-scale pipe, the new five-electrode C4D sensor and the new three-electrode C4D sensor, are developed. The two sensors respecitively adopt the method based on voltage difference detection, which was proposed by Laugere et al. and the method based on series resonance, which was proposed by our research group to avoid or eliminate the unfavourable influence of coupling capacitances. The results of conductance measurement experiments illustrate that the two new developed sensors are successful. In millimeter-scale pipes (inner diameters are 1.8 mm,2.8 mm,3.9 mm,5.3 mm and 6.1 mm, respectively), relative errors of conductivity detection can be less than 6% and 5%, respectively. Compared with the conventional C4D sensor, detection range and resolution of the two new developed C4D sensors are improved. For the two new developed C4D sensors, performance of the new three-electrode C4D sensor (detection range, sensitivity, stability and interference immunity) is better.2. Feasibility and potential of applying C4D technique to the velocity measurement in millimeter-scale pipe is studied. Combining cross-correlation technique, a new method for velocity measurement of gas-liquid two-phase flow based on C4D technique is proposed. This method adopts the new developed C4D sensors to obtain upstream and downstream conductance signals of gas-liquid two-phase flow. Cross-correlating the two conductance signals, transit time between two signals is obtained. With the calibration coefficient previously determined, velocity will be calculated. To verify the effectiveness of the proposed velocity measurement method, velocity measurement experiments are carried out for single bubble, typical bubble flow and slug flow. Velocity measurement performance of the new five-electrode sensor and the new three-electrode sensor are studied respectively. Experimental results show that C4D technique can be successfully applied to the velocity measurement of gas-liquid two-phase flow and has a good potential. The proposed velocity measurement method which combines C4D technique and cross-correlation technique is effective. Velocity measurement accuracies in five pipes (the inner diameters are 1.8mm, 2.8mm, 3.9mm, 5.3mm and 6.1mm, respectively) are satisfactory. Maximum relative differences of velocity meaurement with the new five-electrode C4D sensor can be less than 10%, relative differences of velocity measurement with the new three-electrode C4D sensor can be less than 6%.3. Feasibility of applying C4D technique to the void fraction measurement of gas-liquid two-phase flow in millimeter-scale pipe is studied and a new method for void fraction measurement of gas-liquid two-phase flow is proposed. This method developes different void fraction measurement models for different typical flow patterns. In practical measurement, the new three-electrode C4D sensor is firstly used to obtain the conductance signal of gas-liquid two-phase flow, then flow pattern is identified with the obtained conductance signals, finally the corresponding void fraction measurement model is selected according to the flow pattern and calculates the void fraction value. For the flow pattern identification, statistical analysis and Empirical Mode Decomposition (EMD) methods are used to extract the features of conductance signals, Support Vector Machine (SVM) technique is adopted to develop flow pattern classifier. By inputting the extracted feature vector to the flow pattern classifier, flow pattern identification is implemented. The void fraction measurement models are developed by Least Squares method. Research results show that applying C4D technique to the void fraction measurement of gas-liquid two-phase flow in millimeter-scale pipe is feasible and the new proposed void fraction measurement method based on C4D technique is effective. In four millimeter-scale pipes, maximum absolute errors for typical bubble flow, slug flow, stratified (wavy) flow and annular flow are all less than 7.0%.This dissertation verifies the feasibility and potential of applying C4D technique to the parameter measurement of gas-liquid two-phase flow and provides a new approach for two-phase flow parameter measurement. Meanwhile, it also provides a useful reference for application of C4D technique in other fields.