The Development and Modelling of a Novel Clamp-on Ultrasonic-Thermal and Ultrasonic Multiple Reflection Flowmeter for Liquid Applications.

摘要

The development of a novel combined "ultrasonic/thermal" with "ultrasonic multiplereflections" clamp-on meter for measuring a wide flowrate range of clean liquids insmall diameter pipes is presented. Current existing flowmeters based on ultrasoundcannot measure very low flowrates for single phase liquids. The ultrasonic/thermaltechnique can measure single phase flows in the range 0 to 0.6 m s' in pipes withdiameters as small as 15 mm. It can also detect and measure reverse flows. Theminimum flowrate for the ultrasonic multiple reflection technique is about 0.55 m s',and theoretically, the measurement accuracy increases with increased flow velocity.The ultrasonic/thermal technique is based on a heating element and transducer pair(s)which can be clamped to the outside of a pipe. With the heaters switched on, thechanges in the temperature of the pipe and the liquid inside it result in changes intransit time. The flowrate can be therefore estimated by either the transit time differenceacross the pipe at the two symmetric locations with respect to the heater centre, or atone location with a heater off/on comparison. The latter approach was felt to be thepromising for low flowrate measurements and therefore selected for the numerical andthe experimental investigations. The multiple reflection technique was developed basedon the conventional transit time flowmeter. This technique extended the measuringrange of the flowmeter and provided cross calibration for the ultrasonic/thermaltechnique. A computer model was developed for the ultrasonic multiple reflectiontechnique. However, there was insufficient experimental data to confirm the computerprediction.Results from computational fluid dynamics (CFD) analysis of the meter are presented.For vertical pipes an axisymmetric model was used, but the presence of buoyancyforces required the use of a 3-D model for horizontal pipes. Temperature and velocitydistributions and ultrasonic transit times have been computed and are presented.In order to overcome the problem of mode conversion and refraction at the pipewall/transducer mounting interface, novel transducers and mounting blocks arepresented. A prototype heater and ultrasonic transducer system together with electronicsfor signal generation and transit time measurement have been designed and constructed.A hydraulic system has also been designed and constructed for testing the developedclamp-on flowmeter. Experimental results from this apparatus are presented and comparedwith the CFD predictions, and a technique for compensating for variations in inlettemperature is described. The full scale difference between the computed values andexperimental results of the meter for low flowrate measurement was about 3.5%.