Error in bubble velocity measurement using 4-sensor void probe due to interface deformation

Highlights: • The error in bubble velocity measurement caused by the meniscus was investigated. • The size and surface property of the sensor have a strong influence on the error. • The use of fluororesin-coated sensors can eliminate errors in air-to-water penetration. Multi-sensor void probe is efficient to measure the local parameters of two-phase flow such as bubble interface velocity. In general, meniscus due to surface tension is formed and shape of the gas–liquid interface deforms when an object contacts the gas–liquid interface. The deformation of the interface by penetration of the front sensor affects the penetration time of the rear sensor, and as a result, an error can occur in the measurement of the bubble interface velocity. The characteristics of the meniscus formation around the sensor was investigated and the error in the measurement of the interface velocity was evaluated. It was clarified that the size, shape of the sensor and the contact angle of the sensor surface affect the error in interface velocity measurement as well as the interface velocity itself, and no measurement error would occur in air-to-water penetration by using a sensor with a large surface contact angle. A double sensor void probe was applied to bubbly flow to measure the bubble interface velocity. The measurement error due to the meniscus also occurred in the actual experimental measurement. It was shown, that it is necessary to use only the velocity of the lower surface of the bubble or to make certain appropriate error correction for the velocity of the upper interface of the bubble.