Evaluation of Turbulence Kinetic Energy Budget in Turbulent Flows by using Photobleaching Molecular Tagging Velocimetry
Shigeo HOSOKAWA and Akio TOMIYAMA*1
*1 Kobe University, Department of Mechanical Engineering
Rokkodai 1-1, Nada, Kobe, Hyogo, 657-8501 Japan
Abstract
Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed molecular tagging velocimetry based on photobleaching reaction (PB-MTV)(1),(2) and applied it to turbulent flows in a square duct to demonstrate the feasibility of evaluation of TKE budget(3)-(5).
This report reviews the principle of PB-MTV and the application to turbulent flows in a square duct. Figure 1 shows a schematic of PB-MTV. The photobleaching is irreversible photodegradation of fluorescent dye induced by intense illumination, and it changes the fluorescent dye to non-fluorescent dye. Uranin (fluoresceine sodium salt) was solved in water as the fluorescent dye. The dye was photobleached by an intense laser beam, which was emitted from an Ar-ion laser and passed through the focusing lens. By illuminating the fluorescent dye using the low intensity laser sheet, the tagged region formed by the intense beam appeared as dark regions, i.e., non-fluorescent regions, as shown in the photographs in Fig. 1. A set of images of the tagged region with a time interval was recorded by the CCD camera. Liquid velocity and its gradient were evaluated from the displacement of the center and deformation of the tagged region between two consecutive images. TKE budgets in the single-phase and bubbly flows are shown in Fig. 2. Although the difference in the dimensionless dissipation rate ε+ between single-phase and bubbly flows is small, the dimensionless production rate P+ and dissipation rate D+ are modulated due to the presence of bubbles. This results in non-zero residuals in the TKE budget (P++D+-ε+) in the bubbly flow, whereas the residuals are almost zero in the single-phase flow. Since the flow was confirmed to be steady and developed in the preliminary experiment, this result indicates that modeling bubble-induced pseudo turbulence is indispensable for bubbly flows. It is demonstrated that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) the sum of the production rate due to shear, diffusion rate and dissipation rate is not to be zero due to the production of TKE caused by momentum transfer between the phases, and (3) PB-MTV opens a new way for experimental validation of turbulence models. Details on the method and discussion on turbulence in bubbly flows can be found in the following references.
Keywords
Turbulent Flow, Gas-Liquid Two-Phase Flow, Bubble, Turbulence Kinetic Energy, Photobleaching Molecular Tagging Velocimetry
References
Fig. 1 Schematic of measurement system
Fig. 2 TKE budgets in single-phase and bubbly flows