Design and experimental study of electrical resistance tomography system with wide range and high speed
[Objective]The operation of underwater two-phase flow ramjet engines generates several bubbles,and the combined flow of gas and water produces a gas-liquid two-phase flow.If bubbles in a gas-liquid two-phase flow can be uniformly distributed on the inner side of a ramjet engine pipeline,it is beneficial for bubbles to work on water,which can effectively increase the engine thrust.When high-pressure gas is introduced into the engine pipeline,it is necessary to obtain real-time cross-sectional images to ensure that the generated gas-liquid two-phase flow pattern is a bubbly flow.Electrical resistance tomography(ERT)is an important method used for multiphase flow detection.The excitation source circuit of traditional imaging systems designed based on the ERT principle mostly uses digital synthesis chips to directly generate sinusoidal currents.The maximum load capacity of the constant-current source circuit can only reach 2 kΩ.This scheme requires the addition of filtering circuits and phase-sensitive demodulation circuits in subsequent data processing circuits,which significantly improves the signal delay and slows down the measurement speed of the system,usually only reaching 27 frame/s.Due to the special working environment of an underwater two-phase ramjet engine,it generates a load of up to 6 kΩ.To improve the load capacity and measurement speed of the imaging system,it is necessary to design a wide-range and high-speed resistance tomography imaging system.[Methods]First,based on the working principle of the ERT system,the design circuit is divided into four basic modules:an excitation signal generation module(DC/AC switching unit,excitation constant-current source unit),a 12-electrode array module,a data acquisition module,and a main control module with a data processing function.The functional units of the measurement system are implemented within each module.Second,the system is calibrated,and reliability tests are conducted to ensure that the measurement range reaches the expected 10 kΩ and can operate stably and reliably.Finally,various imaging algorithms were analyzed,and based on the requirements of the engine working environment,a linear backprojection algorithm was developed for imaging experiments of the boundary measurement voltage.This confirms that the system can recognize the gas-liquid two-phase flow pattern in the working pipelines of underwater ramjet engines.[Results]The experimental results of the ERT imaging system are summarized as follows.① The range of the system can reach 10 kΩ.When the excitation frequency is within the range of 20 kHz,the system is relatively stable,with a relative error between the channels of less than 5%.② The measurement consistency of each channel in the system is good.When the glass rod is located at different positions inside the barrel,the boundary voltage measured by the system conforms to its changing trend,and the data is reliable.③ Using a bipolar current source as the excitation signal,the imaging speed of the system can reach 51 frame/s,which is two times faster than that of traditional systems.④ Image reconstruction of the working pipeline section of an underwater ramjet engine can identify the gas-liquid two-phase flow pattern.[Conclusions]This study explores the foundational imaging theory of the ERT system and presents a high-speed ERT imaging system with 12 electrodes.This system was designed using an FPGA controller to identify the gas-liquid two-phase flow pattern in the working pipeline of an underwater ramjet engine,thereby offering critical equipment support for analyzing the engine performance.The developed system has achieved the expected goals and significantly improved its measurement range and speed.
underwater ramjetpipe sectionelectrical resistance tomographyFPGAlinear back projection algorithmimage reconstruction
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