Single and Multiphase Flow Characterization by Means of an Optical Fiber Bragg Grating Grid

This study introduces a new approach to characterize single and multiphase flow of water and air/water blends, respectively, by means of the utilization of optical fiber Bragg gratings (FBGs) arranged in a grid pattern. Here, the FBGs act as transducers between the force applied on the optical fiber surface by the liquid or air/liquid flow and the strain-induced Bragg wavelength shift. Since the force is proportional to the square of the velocity, associated to the kinetic energy, it is possible to establish a relationship between the Bragg wavelength shift and flow speed for single-phase flow monitoring. When multiphase flows are taken in consideration, a sudden Bragg wavelength shift represents an abrupt change in the force applied onto the fiber, which means a transition between liquid and air. It is hard to localize turbulences in single phase flow or establish the bubble position for multiphase flow from the response of a single FBG. Therefore, the sensors in this study have been arranged forming an 8 x 8 grid, with a total of 16 different FBGs multiplexed in wavelength. FBG grid enables the detection of turbulences or air bubbles within the pipe by means of an adequate aggregation and processing of the response of the FBGs at each crossing point, with a total of 64 crossings (12 crossings are out of the cylindrical shape pipe). Different flow speeds and void conditions with distinct void fractions and flow rates have been studied. The optical fiber sensors performance agreed with that of a wire-mesh system, which is conventionally used as a reference high performance measurement tool for multiphase flow. Results showed the great potential of this technique that reduces in more than a half the costs, complexity and size of actual devices used for the same purpose.