Evaluation of transducer configurations for ultrasound cross-correlation flowmeters

Ultrasound cross-correlation flowmeters are utilized to measure the flow velocity of liquids or gases in pipes. The basic concept is to measure the time interval needed by the flow for travelling a given distance between two measurement channels, each consisting of a pair of ultrasound transducers. At each channel, ultrasound waves are emitted in a direction perpendicularly to the flow, and phase and amplitude modulations, which are caused by turbulences and naturally occurring speed of sound variations inside the flowing medium, are assessed. In this paper, different configurations of circular and linear ultrasound transducers have been evaluated for flow velocity measurement in small pipes. The specific problem is that the apertures of the utilized ultrasound transducers are relatively large compared to the diameter of the pipe. The crosstalk between the two channels is another problem, especially for ultrasound beams with large opening angles. Experiments have been performed using different setups consisting of an acrylic glass pipe with 8 mm inner diameter and two measurement channels at a distance of 12 mm from each other. Measurements have been performed using circular (8 mm diameter, 2 MHz frequency) and, alternatively, linear transducers (8 mm length, 1 mm width, 1.4 MHz frequency), and a turbine flowmeter has been used as a reference. It will be shown that the normalized covariance function can be used as a measure to assess the performance of the cross-correlation flowmeter and the reliability of flow velocity estimates. Results of measurements obtained with degassed water have shown that in the case of highly homogeneous liquids the best performance of the flowmeter is achieved by using the phase modulation signals for the cross-correlation.