Experimental Study On The Dynamic Characteristics Of Void Fraction For Cryogenic Two-Phase Flow

Void fraction is an important parameter in the research field of two-phase flow. The measurement accuracy of void fraction will direct affect the accuracy of other parameters, such as heat transfer coefficient. Based on different dielectric constant of the gas and liquid, the capacitance sensor for the measurement of void fraction has the advantages of simple structure, good dynamic response and none interference to the flow field. The capacitance has been extensively used in real-time measurement of two-phase flow. Cryogenic fluids as the special fluids in two-phase flow field play an important role in the aerospace, space division, superconducting and other areas. There are big differences in the physical properties between liquid nitrogen and other cryogenic fluids. Measurement of void fraction of cryogenic fluids is more difficult due to the almost equivalent dielectric constant, and has less related research.To study on the characteristics of the flow patterns and the void fraction of cryogenic fluids, the present work made the following contributions:1. A capacitive void fraction sensor is designed and developed to study the liquid nitrogen/nitrogen vapor (LN2/VN2) two-phase flow. The capacitance sensor is two half-cylindrical electrodes configurations with the key parameters being optimized by the finite element model. The capacitive digital converter chip as the core is used to establish the capacitance value acquisition circuit, forming a void fraction dynamic measurement system.2. The void fraction measurement system is applied to measure the LN2/VN2 flow in the quartz glass tube. The experimental results are calibrated by comparing with the image data from the high speed camera, which show that the measurement precision of the system can meet the demands.3. On the basis of void fraction measurements, study on the two-phase flow characteristics of LN2/VN2 in pipes with different angles was performed. Using Fourier transform to analyze the spectral characteristics of the void fraction wave, the relationship between void fraction and the two-phase pattern is drawn preliminary.