Cover Picture: Chem. Eng. Technol. 7/2019

Non‐invasive real‐time measurement of local heat and mass transfer by application of Rainbow Schlieren Deflectometry. Copyright: JM Schulz@ TU Berlin‐FGVT Rainbow Schlieren Deflectometry (RSD) is an optical, non‐invasive measuring technique based on the change of refractive index due to differences in temperature, pressure, or concentration. The refractive index gradient is resolved locally with a high spatiotemporal resolution by color‐coding the displacement in the filter plane. Integration of the gradient field with respect to the state variable of concern yields, e.g., a spatially resolved real‐time measurement of the temperature or concentration field. Due to the integrational character of RSD, the refractive index distribution has to exhibit specific symmetric conditions, e.g., standard rotational symmetry or a tomographic approach is needed. Quantitative validation can be done by measuring local heat transfer from solid test specimen in liquid or gaseous surroundings. Mass transfer measurements are done by visualization of local concentrations, e.g., for jets emitted into a stagnant fluid or liquid‐liquid mass transfer in disperse multiphase systems. RSD as well as other optical measurement techniques have seen a rise in popularity, application depth, and measuring precision due to the advances in digital photography in the last decades. Their non‐invasive, spatially resolved character in combination with real‐time acquisition poses a promising future with deeper insights into highly transient processes and the possibility to match and validate common CFD simulations, e.g., in multiphase systems.