Spatially resolved measurement of transient concentration and temperature fields using Schlieren and LIF technique

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The proposed project deals with the quantitative determination of local and transient concentration and temperature fields near interfaces. Those fields determine the occurring mass or energy transport and will be quantified optically using Schlieren technique. This measurement technique allows a nearly instantaneous, two-dimensional examination of the measurement volume. From the obtained images, the transfer coefficients can be determined non-invasively as well as space- and time-resolved. As a result, it is possible to elucidate transient behavior during mass transfer pro-cesses. This behavior has not yet been sufficiently taken into account for the design of contact apparatuses, which currently depends on integral empirical equations. An example of transient interfacial phenomena is Marangoni convection, which influences the mass transfer at quiescent single droplets. The consideration of these influences opens up great potential for optimizing appa-ratus design and helps to gain a deeper understanding of the basics of mass transfer processes. In addition, the measurements provide accurate experimental data to validate numerical approaches and improve predictions.As part of the measurements, transport coefficients at different geometries will be determined. At first, energy transport on self-constructed test specimen is measured since the use of temperature fields allow better reproducibility and enable a reliable validation of the measuring procedure. In the context of this validation process, an algorithm for automated analysis of spherical fields is imple-mented. Based on this, concentration fields near planar surfaces and single droplets are examined. For single droplets, liquid/liquid systems without interfacial instabilities are investigated followed by systems with interfacial instabilities.An important part of the project is the development of the measurement setup with regard to measurement accuracy and reliability. Here, improvements of hardware and software, for example, the integration of a linear guide for a more precise filter positioning or the use of advanced image processing methods are implemented.Due to the first-time use of Schlieren technique for quantitative determination of liquid/liquid mass transport, several validation steps are carried out. Laser Induced Fluorescence (LIF) is used to validate the Schlieren data experimentally. In addition, measurement data will be validated numerically using computational fluid dynamics (CFD) and ray tracing calculations. Additionally, ray trac-ing calculations offer the possibility to check accuracy as well as sensitivity of the Schlieren meas-urements. Finally, the construction of a second optical axis enables the validation of assumptions made about the refractive index field. Furthermore, the use of a second optical axis acts as starting point for the analysis of irregularly shaped concentration fields as they occur during interfacial instabilities.