Determination Of Flow Velocities Using Fiber-Optic Temperature Measurements

A novel flow measuring technique is introduced to measure under harsh circumstances in environments with dirt, high pressures and elevated temperatures as in boreholes within the earth's crust. A glass fiber embedded in a cable with heating wires measures the temperature within the heated cable with distributed temperature sensing (DTS). Similar to Hot Wire Anemometry, the velocity dependence of convective heat transfer is exploited to measure the velocity of the cable as a cylinder in crossflow. A borehole-mimicking test rig and a realistic prototype of a borehole probe were built and the flow along the borehole axis was investigated. The expected Nusselt Reynolds characteristic of a cylinder in crossflow has been measured which proves the concept of this novel measurement technique.  Challenges arise with the insufficient spatial resolution of DTS measurements and in the heat transfer modeling because the temperature profile of the cables cross-section must be taken into account. More detailed investigations and developments are planned to elevate this measurement technique from the current proof-of-concept stage to a reliable flow measurement technique. As a first step, the DTS technique will be extended the application of fiber Bragg grating (FBG) temperature measurements at specific points along the glass fiber. Subsequently a straight segment of a new hybrid cable will be placed in a water channel perpendicular to the flow direction. The Flow will be precisely specified using Particle Image Velocimetry and multiple temperature sensors in the channel and on the cable's sheath will deliver the information for enhanced heat transfer modelling.