Design and application of capacitive sensors for characterization of two‐phase flows

Abstract

Multiphase flow can usually be found in many branches of industry, both in processes and applications, as well as in many natural phenomena. In energy engineering, two‐phase systems (liquid‐gas systems, basically) represent a wide portion of cases of industrial interest, which are present in chemical and nuclear industry and in thermal generation and oil extraction plants. The comprehension of the behaviour of a liquid‐gas mixture has become critical in recent years due to necessary efficiency improvements and the safety levels required in all those systems. The identification of the flow regimes and their transitions is important for industrial application as well as for the theoretical analysis point of view. On one hand, almost all constitutive relations in a two‐fluid model depend on the flow regime since physical mechanisms vary with flow regime transitions. On the other, industrial equipment offers durability and safety only when the facility operates according to the flow regimes that it was designed. Other important parameters, such as heat and mass transfer rates, pressure drop, etc, are also directly connected with the operating flow regime. Focusing on the specific case of the petrochemical industry and wastewater management, the working fluids requires specific instrumentation in order to study and characterize the flow. In hydrocarbon distribution channels mainly coexist two non‐conductive phases: gas and liquid phases. In distribution networks of sewage, non‐intrusive measurement techniques are required because of the nature of the medium (aggressiveness, heterogeneity, fouling processes, etc). This leads to the appearance of several difficulties when the most common measurement instruments are applied, which are based solely on the different phases conductivity (electrical impedance tomography and local conductivity sensors). The aim of this paper is to provide different measuring instruments ‐ both intrusive and nonintrusive ‐ based on capacitive measurements (permittivity) for the identification of the different non‐conductive phases, with the addition of an electronic design that allows data collection with a high spatial and temporal resolution (> 10 kHz). Three different scenarios have been studied in this work: oil‐water two‐phase flow in vertical and horizontal configuration and dynamic level measurements. In vertical configuration, leading local flow parameters have been measured (void fraction, interfacial velocity, bubble frequency, interfacial area concentration, etc.) by using a multisensor capacitive sensor. In the case of horizontal flow, a nonintrusive two‐plate sensor has been used in order to detect and calculate speed and frequency of bubbles in slug regime because of its importance in blockages occurring in hydrocarbons distribution lines and solar thermal centrals. In the last section, a non‐intrusive three‐electrode sensor is been designed for the measurement of the dynamic level in conducts in order to offer the possibility of its implementation in sewage distribution channels.