Fluid flow measurement using electrical and optical fibre strain gauges

Abstract

The design, development and calibration of three flow sensors to measure the speed and direction of fluid flow is presented in this thesis. The force exerted by the fluid flow on the sensors are measured using strain gauges. Multidirectional fluid flow measurement has been made possible by vectorial addition of the orthogonal flow components. The fluid speed and direction are generated irrespective of each other. Electrical resistance strain gauges are used as the force measuring device for the first version of the flow meter. These strain gauges are bonded to the four longitudinal surfaces of a square-sectioned, elastic, rubber cantilever having a drag element attached to its free end. An attempt has been made to optimise the shape and dimensions of the elastic beam to obtain a constant drag co-efficient over a wide flow range. Calibration of the electrical strain gauge flow sensor has been performed in a wind tunnel to measure air flow. The sensor has a repeatability of 0.02%, linearity within 2% and a resolution of 0.43 m/s. The most noteworthy feature of the flow sensor is its quick response time of 50 milliseconds. The sensor is able to generate a measurement of flow direction in two dimensions with a resolution of 3.6. Preliminary measurements in a water tank enabled the speed of water to be measured with a resolution of 0.02 m/s over a range from 0 to 0.4 m/s. An optical fibre strain sensor has been designed and developed by inserting grooves into a multimode plastic optical fibre. As the fibre bends, the variation in the angle of the grooves causes an intensity modulation of the light transmitted through the fibre. A mathematical model has been developed which has been experimentally verified in the laboratory. The electrical strain gauge was replaced by the fibre optic strain gauge in the second version of the flow sensor. Two dimensional flow measurement was made possible by attaching two such optical fibre strain gauges on the adjacent sides of the square sectioned rubber beam. The optical fibre flow sensor was successfully calibrated in a wind tunnel to generate both the magnitude and direction of the velocity of air. The flow sensor had a repeatability of 0.3% and measured the wind velocity up to 30 M/s with a magnitude resolution of 1.3 m/s and a direction resolution of 5.9'. The third version of the flow sensor has used the grooved optical fibre strain sensor by itself without the rubber beam to measure the fluid flow. Wind tunnel calibration has been performed to measure two dimensional wind flow up to 35 m/s with a resolution of 0.96 m/s.