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Estimation of fluid viscosity from stirring motor current

Bunting, Kit (2014). Estimation of fluid viscosity from stirring motor current. Bachelor of Engineering (4th Year Project) Thesis, Charles Darwin University.

Document type: Thesis
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Author Bunting, Kit
Title Estimation of fluid viscosity from stirring motor current
Institution Charles Darwin University
Publication Date 2014
Thesis Type Bachelor of Engineering (4th Year Project)
Subjects 0999 - Other Engineering
Abstract The formation of gas hydrates in pipelines poses a serious problem for oil and gas industries. As gas hydrates form they retard the flow of the fluid, this is a behaviour of fluid viscosity. By measuring viscosity changes the nucleation rate of gas hydrates can be studied.
Commonly viscosity changes are measured by rheometers but due to the high pressure of nucleation, common rheometers cannot be used. A means of measuring changes in the viscosity is proposed by using a stirrer inside a PVT machine. The stirrer is attached to a dc motor and by keeping its speed constant it is theorised the torque will have to increase to overcome loading on the stirrer. As the motor will be constant the only variable left is the current, so by measuring the current to the motor changes in viscosity can be estimated.
It was decided to use a graphical program called LabVIEW from national instruments to set up a LabVIEW VI to measure current and speed changes due to its low cost, portable nature and it is a favourable programme for data acquisition learning among students and teachers. A Labjack U3 data acquisition device is used to send and receive analogue and digital signals from the LabVIEW VI. And a STK681 – 332 – E controller is used to relay PI and PWM control from the labjack to the motor.
Using a 32bit rising edge to rising edge frequency measurement in the LabVIEW VI and a two-channel Hall Effect encoder a digital PI controller is set up to measure the error between the RPM set point and the RPM from the encoder. The result is a working PI control that maintains a constant speed under varying load.
The current is measured by using a LTSR-6-NP. It was found that during initial experiments that the current being drawn by the DC motor could be improved by implementation of PWM. The use of PWM would mean the current being drawn by the motor was due to the force imparted on the stirrer. An 8 bit PWM was set up using the LabVIEW VI at 15kHz. Its duty cycle was set up so that it would adjust in reference to the output of the digital PI controller.
In order to test this LabVIEW VI an experiment was set up with two Newtonian fluids, honey and glycerol. At atmospheric pressure the honey and glycerol are heated to 60°C and then allowed to cool to room temperature. The stirrer is kept at a constant speed of 50 RPM for all tests. The results showed a linear relationship between known viscosities of the two fluids and the current drawn by the motor existed. This confirms the LabVIEW VI can successfully measure viscosity changes due to current changes.
Keyword PI controller
current
viscosity
PWM
Additional Notes Full text restricted.
 
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Created: Mon, 13 Oct 2014, 13:27:52 CST by Jessie Ng