Experimental study on different configurations of capacitive sensors for measuring the volumetric concentration in two-phase flows

• Helical, concave and double ring capacitive sensors with the same sensing area were built.
• They were mounted horizontally in a static test bench with a Plexiglas pipe section.
• They were rotated with different amounts of air and water, such as a stratified flow.
• Sensitivity, effect of spatial distribution of phases, and linearity of these were compared.
• Results show the double ring as the best configuration for measuring the volumetric concentration.

Two-phase flows occur in the presence of two immiscible components inside pipelines or process equipments. They are commonly found in the chemical, food, energy and oil industries. For them, among the instruments used to measure the volumetric concentration of one phase, there are capacitive probes consisting basically of two parts: a sensor with electrodes mounted around the tube as a capacitor with guards and shielding, and a capacitance transducer circuit. Capacitive probes are robust, simple to handle, no special safety and care is required as for gamma or X-rays based instruments, and they are relatively inexpensive. However, there are several challenges that still require investments in research for making them widely used in industry. One of them is related to effect of the spatial distribution of phases on the response of the sensor when measuring the volumetric concentration. Hence, there are different geometric configurations feasible for the electrodes, being the main ones the helical, the double ring and the concave configurations. Each one has its own characteristics in terms of sensitivity, immunity to the effect of spatial distribution of phases, the linearity of response, etc. In this work, capacitive sensors with these configurations were designed and assembled by keeping some proportion among them. Then, they were tested in a static test bench with a transducer circuit, which was done by mounting them in the horizontal position and rotating around the axial axis with different amounts of air and deionized water in the section, such as a stratified smooth air–water flow. The results showed the characteristics of each different geometric configuration and allowed to compare themselves, showing the double ring as the best configuration for measuring the volumetric concentration of two-phase air–water flows.