Experimental and Numerical Analysis of Multi-phase Aqueous Solutions at Steady and Unsteady Conditions Using Electro-optical Techniques

In homogeneous media light propagates uniformly, whereas in fluid flow, light bends due to phase distortions. Additionally, due to fluid forces and density variation, the refractive indices of fluids change. These principles are the foundational pillars of optical techniques in flow visualization. Schlieren and Shadowgraph techniques are a few of the methods used in transparent media to capture flow disturbances by detecting changes in the refractive index. Furthermore, particle image velocimetry (PIV), particle image thermometry (PIT) and molecular tagging velocimetry (MTV) use the reflection and absorption of tracking particles to characterize the flow dynamics and temperature gradients. However, very few experimental methods have been performed in agitators and scraped surface crystallizers using optical techniques. Unfortunately, the rotation frequencies of the propeller and material phase-transitions in the flow introduce challenging difficulties in order to retrieve clear signals for post-processing. On the other hand, the physical understanding of multi-phase flows is crucial in science and engineering because they are a medium to transform constituents into final products due to their favorable thermodynamic state. However, measuring the material properties of such systems is often challenging due to the different time and length scales present in the flow. In scraped surface heat-exchangers (SSHEs), scraped surface crystallizers (SSCs), mixers, and agitators, it is difficult to perform optical visualization measurements, because in order to freeze the liquid inside the stator, a cooling jacket with refrigerant is placed around the mixing chamber. This physical requirement alone calls for new methods of studying unsteady flows in agitators where a temperature gradient is applied through the wall. The main goal of this research was to investigate the electro-optic characteristics of an aqueous solution through the glass-transition temperature at steady and unsteady conditions. The latter objective was explored using optical spectroscopy in-situ to capture the phase-transition details of a multi-phase flow in an agitator under a non-isothermal process. The former objective was achieved using the complex-permittivity by means of the cavity perturbation method in the microwave S-band and C-band in a temperature controlled environment. Optical absorbance measurements of the unsteady flow show significant variations in the ultra-violet (UV) and visible (VIS) spectrum at the phase-transition temperature. Volume fraction calculations were performed based on the complex permittivity obtained experimentally as a function of temperature. A 3D frequency dependent finite element simulation was developed to study the dielectric properties of the multi-phase sample in a resonant cavity. The numerical solution and experimental data show good quantitative agreement at the resonant frequencies. In order to find the optimal location for the optical probe, a 3D transient numerical fluid flow simulation was developed using the finite volume method. The mathematical predictions were validated with temperature and pressure readings obtained in-situ.