Simulation of a local four-sensor conductance probe using a rotating dual-sensor probe

This paper describes a rotating dual-sensor conductance probe for measuring the velocity vectors of dispersed gas bubbles in time invariant gas–liquid flows. The probe can be rotated about its principal axis, thereby altering the position of the rear sensor with respect to the lead sensor, and so can be used to simulate a more conventional four-sensor probe. The advantage of the rotating dual-sensor probe is that it has a much smaller frontal area than the equivalent four-sensor probe, thereby presenting a much smaller obstruction to the oncoming bubbles. The rotating dual-sensor probe can therefore be used to investigate the effects of intrusiveness associated with four-sensor probes. It must be stressed, however, that the rotating dual-sensor probe can only be used in bubbly flows in which, at a given spatial location, all bubbles have the same trajectory, size and shape and the same vector velocity and orientation when they strike the probe. This limits the application of the rotating dual-sensor probe to flows consisting of a well-controlled train of bubbles. For a general steady-state bubbly flow, in which the above flow conditions are unlikely to apply, the rotating dual-sensor probe may well operate with lower accuracy than suggested by the results presented herein. A series of experiments was undertaken in which the direction of motion of the gas bubbles relative to the probe was varied. It was found that (i) the mean absolute error in the measured polar angle of the bubble velocity vector relative to the probe was equal to −0.1875°, (ii) the mean absolute error in the measured azimuthal angle of the bubble velocity vector relative to the probe was equal to −1.388° and (iii) the mean percentage error in the measured bubble velocity magnitude was equal to +6.304%. This last result indicates that the rotating dual-sensor probe has a much smaller retarding effect on the gas bubbles than the equivalent, conventional four-sensor probe