Research On The Application Of Electrical Capacitance Tomography To The Parameters Measurement Of The Two-Phase Flow

Two-phase flow widely exists in many industries such as chemical industry, metallurgical industry, energy industry and so on. The research on two-phase flow, which has been one of the frontier subjects concerned deeply in the world, is developing rapidly. It is significant to correctly measure parameters of two-phase flow for process control, energy conservation, quality assurance and safety. However, the inherent complexity of the two-phase mixture causes many difficulties to measure the two-phase flow. In many cases, the measurement technology for two-phase flow has been one of the bottlenecks in the research field of the two-phase flow.Flowrate and voidage are two important parameters of gas-liquid two-phase flow. It is difficult to measure these parameters accurately because of the complicated interaction between the two phases. Electrical capacitance tomography (ECT) has become one of the important techniques for two-phase flow parameter measurement due to its non-invasion, reliability, simplicity, low cost and high-speed capabilities. In recent years this technique has been used to the measurement of the quantitative parameters of gas-liquid two-phase flow. However, some problems and difficulties still exist in ECT system and its applications.The topic of this dissertation is the research on the application of ECT to the parameters measurement of two-phase flow. The main work of the dissertation includes:1) The hybrid algorithm, which combines Algebraic Reconstruction Technique (ART) and Tikhonov regularization, is applied to reconstruct the images. Tikhonov regularization is used to overcome the instability and ill-posed problem of image reconstruction process, and obtain the initial grey level values for ART. The optimal Tikhonov regularization factor for different flow patterns is analyzed. The relationship between the ART iteration steps and the quality of reconstructed images is studied. The ART iteration steps are set to 10 by experiments. The quality of images reconstructed by the hybrid algorithm is satisfactory. Also, the ECT system has the considerable real-time capability.2) A new voidage measurement method for gas-liquid two-phase flow based on images reconstructed by ECT and the Otsu algorithm is proposed. The threshold for the filtration of the image can be obtained adaptively by theOtsu algorithm. The voidage measurement model is established according to the grey values of the images and the threshold. The online voidage measurement system for gas-liquid two-phase flow is established. The experimental results show that the voidage values obtained by the new method are consistent with the voidage values obtained by quick-valve method.3) The primary study on voidage measurement by the raw capacitance values is carried out. A multi-model method for voidage measurement of gas-liquid two-phase is proposed. The flow pattern identification method based on fuzzy pattern recongnition technique is applied. The raw capacitance values captured by ECT system are used directly to build the voidage measurement models by principal component regression. Combining the models and flow pattern identification system, the multi-model voidage measurement system is established. The voidage can be achieved by ECT system without the complex image reconstruction.4) A new flowrate measurement method for gas-liquid two-phase flow based on ECT system and Venturi meter is proposed. The voidage is obtained by ECT system. The model, which indicates the relationship between the voidage and the mass quality of the two-phase flow, is obtained empirically by experiments. Several flowrate measurement models of Venturi meter for gas-liquid two-phase flow are analyzed. Primary studies have been taken to measure the total mass flowrate and liquid-phase mass flowrate. The experiments show that the measurement results calculated by some measurement models are satisfactory under several flow patterns.