Development of an inline multiphase flow metering sensor

Summary
Measurement of multiphase flow rates, for example in gas-liquid two-phase flow, is a challenging issue in many industrial applications. While current solutions are based on upstream phase separation or combination of different methods (e.g. densitometry and Venturi tube flow meter), the target of our new sensor is to allow an inline measurement of multiphase flow rates at lower cost and space requirements. Starting with flow rate measurements in gas-liquid systems, the sensor principle is to subdivide the gas-liquid flow into partial streams of alternating gas and liquid fractions (also called Taylor flow) in a grid of small tubes. This allows contactless instantaneous phase detection and time-resolved detection of subsequent phase transitions using a capacitance sensing principle based only on the dielectric properties of the flu-ids. The sensor design will be discussed in detail, a prototype introduced and simulation and experimental results presented. Further, application requirements and limitations of the measurement principle will be discussed. The performance of the multiphase flow meter was evaluated from experiments in a gas-liquid flow loop.
Motivation
Multiphase flows, whether wanted or not, are encountered in very different industrial areas, e.g. oil explo-ration, fueling of petrol or in milk floats, chemical production units, etc. However, they have in common, that they often require quantitative measurement or at least qualitative detection of multiple phases. Con-ventional measurement systems typically require expensive phase separation units to allow the applica-tion of well-established single phase measurement equipment. Alternatively, transmission-based systems that are costly and require stringent safety regulations may be combined with measurement devices that reveal mixture velocity by cross-correlation methods. However, measurement accuracy decreases clearly if the flow deviates from a-priori assumed well-defined regimes and if phases separate locally. To summarize, economic and space saving sensor concepts would be highly appreciated.
Results
The development of the new sensor concept makes use of preferable flow conditions (superficial gas and liquid flow rates), such as the Taylor flow regime, which allows sharp discrimination of phase transitions at intermitted liquid and gas fractions. As a preliminary study such gas-liquid flow scenarios were ob-served in a transparent sensor model using a high speed camera (see Fig. 1). Important design specifications for a high functionality of the sensor concept (prevention of micro bubbles or single-phase formations) were derived. Furthermore, characteristic flow behavior was studied and findings of earlier studies confirmed.
At Taylor flow conditions, phase transition can be detected at two positions with an axial offset (see Fig. 2) and velocity of the corresponding fraction can be determined by cross-correlation. As a detection principle we use local capacitance measurement due to its conceptual simplicity and capability of discriminating many different fluids. An optimized electrode configuration was chosen by performing field simulations on the fluid layers. (see Fig. 3)