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Method for measuring the void fraction of gas-liquid two-phase flow

Author: GongYiMing
Tutor: KongDeRen; ShangFei
School: Nanjing University of Technology and Engineering
Course: Precision instruments and machinery
Keywords: ERT ramjet porosity measurements FPGA current source
CLC: TP274
Type: Master's thesis
Year: 2014
Downloads: 17
Quote: 0
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The ramjet of gas-liquid two-phase flow underwater generates a lot of bubbles, the mixture of gas and liquid will generate the gas-liquid two-phase flow. The porosity reflects the area of the gas-liquid two-phase flow in the tube of one section at a certain time, the value of the porosity reflects the working performance of the ramjet. The accurate measurement of the porosity has important significance on development and performance of the ramjet.Porosity has various measurement methods, the new measurement technology based on the the ERT measurement principle has non-ablative, high security, simple structure, fast response speed and real-time visualization. This paper developed a set of real-time measurement system with12electrodes, aiming at continuous phase conducting medium, based on the analysis of the existing ERT measurement method.Traditional ERT measurement system only has1.4kΩ load capacity, which cannot satisfy the working environment requirement of the gas-liquid two-phase ramjet. Based on the improved constant current source design method, this paper used bipolar pulse current as excitation signal, realized the system load capacity reaching10kΩ, expanded the measuring range of ERT system.As the core controller of the system, the FPGA realized the system’s logic control, excitation signal generation, data acquisition, data reading and PC communications. In hardware circuit, through the control of the excitation signal generating circuit of bipolar current, electrode array gating circuit, programmable amplifier circuit, data acquisition circuit1, it realized conductivity measurement of the water ramjet multiphase flow.Based on quartus Ⅱ software development platform, we use VHDL language programming, achieving module design such as key debounce, direct exchange switch, electrode switching, programmable amplifier, AD acquisition and so on. Through programmable amplifier module, gain can be adjusted from0.08to10000by itself. Meanwhile, we adopt multiple sampling at separated time, eliminating DC component of signal and improving real-time performance of system.Through system calibration experiments, channel conformance test experiments, verification experiment simulated environment,we analyze and verify the performance of Digital ERT system. Experimental results show that every index of the system can content two-phase flow measurement which underwater ramjet produced.

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