Abstract The global custody transfer market for liquefied natural gas (LNG) has grown at a strong pace in the last decade and use of LNG as transport fuel has considerable environmental benefits. The quantity of LNG is traded on the basis of energy transferred, calculated from volume, density and gross calorific value. High-speed, accurate density measurement is therefore of significant commercial value. The electrical capacitance tomography (ECT) device described in this paper has the potential to measure the LNG density rapidly, on-line at a moderate cost. Continuous monitoring of variation in LNG density during dynamic LNG flow measurement also gives a good indication of change in fluid quality and thus onset of boiling which is known to affect measurement accuracy. ECT is a leading candidate to be explored for online density measurements through measurement of electrical permittivity, as in addition to average value, it offers the image of permittivity across the whole flow conduit, allowing localised bubbles, boiling or other variations to be identified and measured. We report here experiments to explore the use of ECT in cryogenic applications. An 8-electrode test ECT sensor was designed, built and tested in laboratory conditions and then in liquid nitrogen. The resolution and imaging capability in cryogenic conditions are shown to be comparable to that under laboratory conditions. The experiments reported here use liquid nitrogen as an analogue fluid, but the results presented are believed to be representative of many cryogenic fluids. Although the use of ECT has been widely reported in the literature for multiphase flows in general, its use has not previously been reported for cryogenic flows. This paper offers proof of principle for ECT cryogenic multi-phase density and flow measurement. Dielectric constant is strongly linked to fluid density, and the ECT sensor design tested here shows an estimated measurement of the relative permittivity of liquid nitrogen of 1.45 with a standard measurement error of 0.034. Measurement stability at cryogenic conditions gave an rms variation of output under static conditions of better than 0.001 relative permittivity units even though it was unguarded and only a single electrode ring. The primary errors are associated with the unguarded nature of the test sensor, which was primarily designed as a proof of concept and material demonstrator. In addition, such an ECT sensor would provide clear images of any gas in the liquid and give a good estimation of the concentration and velocity of the gas bubbles. The scope of this work is to provide a proof of concept of the cryogenic ECT sensor.

中文翻译：

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使用电容断层扫描对 LNG 和其他低温流体进行高速密度测量

摘要 液化天然气 (LNG) 的全球贸易交接市场在过去十年中以强劲的速度增长，使用 LNG 作为运输燃料具有相当大的环境效益。LNG 的交易量是根据转移的能量，根据体积、密度和总热值计算得出的。因此，高速、准确的密度测量具有重要的商业价值。本文中描述的电容断层扫描 (ECT) 设备具有以适中的成本快速、在线测量 LNG 密度的潜力。在动态 LNG 流量测量过程中对 LNG 密度变化的连续监测也提供了流体质量变化的良好指示，因此沸腾的开始已知会影响测量精度。ECT 是通过测量介电常数进行在线密度测量探索的领先候选者，因为除了平均值之外，它还提供整个流道的介电常数图像，允许识别和测量局部气泡、沸腾或其他变化. 我们在这里报告了探索 ECT 在低温应用中的使用的实验。一个 8 电极测试 ECT 传感器是在实验室条件下设计、制造和测试的，然后在液氮中进行测试。低温条件下的分辨率和成像能力与实验室条件下的分辨率和成像能力相当。这里报告的实验使用液氮作为模拟流体，但所呈现的结果被认为是许多低温流体的代表。虽然 ECT 的使用已在一般多相流的文献中得到广泛报道，但以前没有报道过它在低温流中的使用。本文提供了 ECT 低温多相密度和流量测量的原理证明。介电常数与流体密度密切相关，此处测试的 ECT 传感器设计显示液氮相对介电常数的估计测量值为 1.45，标准测量误差为 0.034。低温条件下的测量稳定性给出了静态条件下输出的 rms 变化，优于 0.001 相对介电常数单位，即使它没有防护并且只有一个电极环。主要错误与测试传感器的无防护性质有关，其主要设计为概念验证和材料演示器。此外，这种 ECT 传感器将提供液体中任何气体的清晰图像，并能很好地估计气泡的浓度和速度。这项工作的范围是提供低温 ECT 传感器的概念证明。