Optical measurements of mixing processes in turbulent fluid flows

Many processes in chemical engineering and oceanography rely on fluid mixing. This thesis focuses on optical measurement techniques to experimentally investigate miscible liquid-liquid mixing and gas-liquid mixing. Previous numerical studies have examined the turbulent small-scale mixing, which is experimentally challenging because of the limited spatial resolution of the measurement techniques. A further difficulty emerges for active fluid mixing. When fluids of different densities, such as water and ethanol are mixed, experimental measurements for the small-scale fluctuations in turbulence are impeded by the optical distortion due to the spatio-temporally varying refractive index field.
In this thesis, in order to investigate small-scale liquid-liquid mixing, an upscaled T-mixer was built with a height of 40 millimetres. A planar laser-induced fluorescence (PLIF) with high resolution was employed in proof-of-concept experiments concerning the T-junction. The results indicate that measuring the small-scale mixing in the viscous-convective range where the velocity does not fluctuate, but the scalar does, is possible. Subsequently, long inlet channels were added to the T-junction and the particle image velocimetry (PIV) technique was used to verify the fully developed laminar flows and to investigate the flow regimes for different Reynolds numbers. It was found that the present setup can achieve inlet conditions with fully developed laminar flows when the Reynolds number is smaller than 1100. The setup was validated by repeating the flow regimes in previous studies. However, small temperature differences had a weak effect on flow regimes, which should be avoided in the next measurements. Overall, high-quality measuring of the small-scale mixing dynamics in the T-mixer is now within reach.
For active mixing with fluids of different densities, mixing usually causes optical measurement errors. In this thesis, a ray tracing simulation method is used in a three-dimensional flow to quantify the measurement errors of the flow velocity and flow acceleration for tracer-based velocimetry, i.e., particle tracking velocimetry (PTV). The flow field is from a direct numerical simulation of single-phase turbulent mixing of two miscible fluids. The measurement errors increase with increasing the refractive index difference. The errors of both velocity and acceleration are attributed to the spatial and the spatio-temporal gradients of the refractive indices. Since PTV, PIV and PLIF share the same working principles based on geometric optics, the findings are also assumed to hold for the measurements of other techniques. The thermal effect in the T-mixer on the measurement errors is subsequently estimated and found to be negligible.
Finally, gas-liquid mixing is examined, in which a weakly soluble gas is transferred across the gas-liquid interface. This process is related to the climate balance and depends on surface waves and turbulence underneath the water in the ocean. Utilizing the refraction at the interface and optical displacements, a synthetic Schlieren method is developed to measure the surface topography. This method is implemented in experiments and allows high-accuracy measurements of free surface waves. This makes it feasible to implement simultaneous measurements of surface waves, mass transportation and turbulence under the water.