| Popis: |
The increasing need for real-time monitoring and control of fluid properties such as viscosity and density in industry has led to a growing demand for a low-cost in-situ and on-line measurement system. This thesis is concerned with the design of two different low-cost systems for measuring viscosity of fluids. The first system, which is called torsional guided wave viscosity probe,· is based on the ultrasonic torsional guided waves and consists of a cylindrical waveguide immersed in a fluid whose viscosity is to be determined. The amplitude and the speed of the propagating torsional stress waves change due to the leakage of bulk shear waves into surrounding fluid. This change in attenuation and speed is proportional to the viscosity of the surrounding fluid. The advantages of using torsional guided wave attenuation instead of speed for viscosity estimation are established. The effects of probe material, dimensions and operating frequency on viscosity measurement are discussed in the context of a requirement to match the measured attenuation to the range of viscosity values that are required to be measured, given the constraints on measurability imposed by the overall signal and noise conditions. A prototype probe is shown to work well with Newtonian liquids but to appreciably underestimate the viscosities of polymeric oils; these anomalies are explained quantitatively on the basis of a model of intramolecular relaxation. The probe was unsuccessful when applied to slurries, and a basic explanation is given. A simple and inexpensive electronic platform based on generation of square-wave sequences is designed and its operation is experimentally verified. ---_._ --- The second system is a paddle probe based on the principle of a simple mixer. It is based on the properties of a permanent magnet DC motor (PMDC) and consists of an impeller immersed in a fluid whose viscosity is to be determined. The motor current increases with increase in the torque on the impeller due to the fluid drag forces. An analytical model is developed to predict the viscosity of the surrounding fluid from the motor current independent of its rotational speed. The effects of shape and size of the impeller on the model prediction of viscosity are experimentally examined. Successful experimental results for a range of fluids are achieved using impellers with certain shapes and sizes. |
