基于电容法的管内低温流体液膜厚度测量方法

Permittivity-based liquid film sensor for cryogenic fluid

研制基于电容方法测量液氮/氮蒸汽分层两相流液膜厚度的装置.电容传感器采用双曲面板大包围对称结构,通过有限元模型对环向角、曲面轴向宽度等主要结构参数进行优化;以ADI公司的24位电容数字转换芯片AD7746为核心,建立电容采集电路,有效地减小了导线带来的杂散电容,构成了一个实时的高精度液膜厚度动态检测系统.通过实验测量液氮/氮蒸汽在倾斜透明石英玻璃管内流动过程的液膜厚度,与根据流量计算的理论值进行对比.结果显示,当采集频率达到16Hz时,液面高度采集误差小于0.5mm.

A new permittivity-based liquid film sensor for the liquid nitrogen and the vapor(LN2/VN2)two-phase flow was designed. The sensor was made by two half-cylindrical panels in symmetric arrangement.The main parameters of the sensor,such as circumferential angel and axial surface width,were optimized with the finite element method.The read-out circuit was designed with ADI's 24-bits capacitance-digital converter,AD7746,which can effectively reduce stray capacitance.The measured liquid film thicknesses of LN2 film were compared with the theoretical values calculated from the flow rates.Results showed that the liquid film thickness acquisition error was less than 0.5mm when the sampling frequency was 16 Hz.