Research Summary: The diffusion process occurring at small scales of turbulence is probed by forcing instantaneous diffusion controlled chemical reactions to drive indicator reactions. The resulting reactions, whose rates are controlled by turbulent micromixing, cause color changes which are measured with a fiber optic light probe. The theoretical relationship between the light absorption and micromixing is validated by varying the chemical species and their concentration levels. Measurement at one reactant concentration ratio yields the intensity of segregation, a measure of the micromixing, while measurements at various reactant ratios gives a more detailed measure of micromixing in terms of a tracer probability density function. Additionally, the position of a visible color border itself contains micromixing information because the visible cut-off point of the eye corresponds to a certain color intensity. Consequently, the exciting possibility is explored that visual observation alone can be used to determine such an elusive quality as intensity of segregation. Uniqueness/Innovation: The theoretical underpinning for this work, the use of a fiber optic light probe with an indicator and the technique of using the location of a visual border to obtain micromixing information are all unique and novel. Significance: Micromixing is vitally important in processes as diverse as reaction injection molding, combustion, chemical lasers and reactor design for fast chemical reactions. The common characteristics of these technologies which depend upon reactive mixing is that the rate at which chemical reactants are brought together at the molecular level controls or strongly affects the process. Measurement of micromixing in the conventional manner with an inert tracer is not only difficult because of the need to measure fluctuations around the mean, but unreliable in principle because a tracer probe measures an average concentration over a finite probe volume. Hence part of the signal is always lost. An important characteristic of this novel indicator probe method is that it is not limited by probe resolution; it is capable of capturing the entire signal. This ability to measure micromixing in liquids can have an important impact on the understanding of turbulent micromixing. Furthermore, rational design with reactive mixing is now severely limited by lack of knowledge of the micromixing in the industrial situations. In most cases, knowledge of the intensity of segregation would allow such design and the ability to measure this quantity in industry with a simple, inexpensive, visual experiment can significantly improve design capability in the diverse processes which depend upon reactive mixing.
