MEASURING MIXING AND LOCAL PH THROUGH LASER INDUCED FLUORESCENCE ABSTRACT In the following work we demonstrate the feasibility of using the pH sensitivity of fluorescein as a measure of local molecular mixing. By introducing a base into an acidic environment, we were able to calculate local pH based on pixel intensity. After accounting for the Gaussian distribution, this relationship between pH and intensity is essentially independent of spatial position in the laser sheet. Finally, we demonstrated the method on a simple free jet (Re=10 3 ), showing time-averaged local pH. INTRODUCTION Many current techniques for assessing mixing capability fail to take into account the difference between unsteadiness and molecular mixing (Parekh 1994). In order to differentiate the two we have developed a method based on laser-induced fluorescence (LIF). By measuring the local instantaneous pH field, we are able to examine molecular-level mixing. The local pH was measured by the ability of the fluorescein to fluoresce. LIF has been predominately used in modeling the combustion field - measuring the local concentrations of reactant and product. Dimotakis (1984, 1986, 1990, 1991 a&b) isolated the areas in the jet that have not “reacted” by injecting a basic fluorescein solution into an acidic ambient with no fluorescence - thus areas which fluoresced had not yet mixed with the ambient. They also examined the areas of “reacting” fluid by injecting an acidic fluorescein solution into a basic environment - thus the areas which have “reacted” will fluoresce. In these cases, the acid and base levels of the ambient and jet where kept sufficiently different so that the system was examined as a binary solution - either the flow was fluorescing (a “product”) or not (a “reactant”). This technique has been applied to both mixing layers (Dimo 1986) and jets (Miller 1991). Lee and Reynolds (1985) also used it for visualizing approximate flame fronts for bifurcating and blooming jets. In the present work, we use the local amount of fluorescence to estimate the local pH. From that one can estimate the extent of molecular-level mixing. The major difference being that both the acidic and the basic flows have the same concentration of fluorescein. Further, instead of having a large difference between the pH of the ambient and jet flow and regarding the fluorescein as either on or off, we use a smaller pH difference and use the local intensity of light to measure pH. Although this method does not work well for combustion - where the fluid is either a reactant or a product, it works well for measuring mixing where the fluid is partially or fully mixed. This method has an advantage in that the camera resolution does not need to be on the order of the smallest scales in the flow to accurately resolve the large-scale mixing. In this case, the pixel value will reflect an average of the pH. Thus, by mapping the pH of the flow field, one has a 2-d map of the mixing - more accurately the mixing history - of any fluid point. One must ensure that the convective time scales are much faster than the diffusion time scales so that diffusion of the acid into the base does not affect the results - that is large Schmidt numbers. Using this technique, then, we can isolate mixing on a molecular level from large-scale unsteadiness. EXPERIMENTAL SETUP The data was taken in the 30 x 30 x 60 cm, rectangular Plexiglas ® tank in figure 1. As a test case, we examined a 1.2 cm diameter jet - implying an ambient tank size of 25 by 50 jet diameters - thereby reducing wall effects. The actual interrogation region was only about 5 jet diameters square. The flow field was imaged by illuminating the fluorescein with a laser sheet. Argon-ion laser light entered the tank through a cylindrical lens and a long focal length spherical lens that produced a two-dimensional laser sheet (its thickness was focused to about 0.5 mm). A Sony Video Hi8 video recording MEASURING MIXING AND LOCAL PH THROUGH LASER INDUCED FLUORESCENCE Jeffrey A. Bellerose and Chris B. Rogers Department of Mechanical Engineering Tufts University Medford, Massachusetts