A Mono-Modal Fiber-Optics Velocimeter for Electrochemically Generated Bubbles

Authors: Wedin, R., Davoust, L., Cartellier, A., Dahlkild, A. A.
Document Type: Conference
Pubstate: Published
Journal: Proceedings of Tenth International Symposium on Applications of Laser Techniques to Fluid Dynamics, Lisbon, 2000
Year: 2000


The aim of this paper is to assess the performance of a velocity measurement technique based on interferometry and using a cleaved mono-modal optical fiber. As this optical fiber is dived in a two-phase medium, interferometric signals are provided from the difference in optical paths between two light waves E1 and E2: a laser beam, injected inside the optical fiber from an external IR source, is separated into two waves because of the refraction at the end of the optical fiber; the first wave E1 is merely reflected at the end tip of the fiber whereas the second one E2 is transmitted and afterwards reflected by any incoming gas/liquid interface such as bubbles. The subsequent frequency-modulated signals (or Doppler bursts) lend themselves to a spectral processing allowing to assess the velocity of the bubbles. The hydrogen bubbles involved in this study are generated electrochemically so as to promote a mono-dispersed bubbly Poiseuille flow inside a straight vertical channel. Such an electrochemical generation allows to provide a typical diameter of the bubbles small enough to prevent them from any puncture by the optical fiber. These small bubbles, here referred to as micro-bubbles, behave as rigid spheres since 1.) the Weber number, the Bond number and the capillary number are all very small (of the order of 10-3 or less) and 2.) the surrounding salty liquid medium is contaminated by natural surfactants (coalescence of the bubbles is hindered). The optical signals suggest a passive behavior of the micro-bubbles with respect to the continuous phase streamlines around the optical fiber. If the bubble-Reynolds number increases up to Re~20, some of the micro-bubbles bounce upon the end tip of the optical fiber. In the case of bouncing, the subsequent signals exhibit two Doppler bursts: the first one is related to the incoming bubble interface whereas the second one is related to the backward motion of the bubble after bouncing. Whatever the shape of the optical signals (by-pass or bouncing signals), the numerical and analytical developments led by Dagan et al (1982) and O’Neill (1995) allow to develop a data processing relevant to our optical signals. In particular, it is possible to compute the bubble velocity far upstream the fibre.