Measuring local heat transfer by application of Rainbow Schlieren Deflectometry in case of different symmetric conditions

An experimental setup applying Rainbow Schlieren Deflectometry (RSD) is developed and used successfully for non-invasive measurement of thermal free convection. Tempered, solid test specimens in planar and cylindrical shapes are designed to exhibit a two-dimensional or axisymmetric temperature field. Experiments have been conducted with variation of the continuous phase and the surface temperature of the respective test specimen between 50 and 90 degrees C for air and 24-30 degrees C for water as continuous phase. To evaluate performance, surface temperature and heat transfer coefficient or Nusselt number respectively are extracted from the obtained experimental data. Quantitative validation is done by comparing RSD results with either experimental surface temperatures obtained with a PT100 sensor attached to the surface of the test specimen or with results from literature in case of the heat transfer coefficient Numerical simulations in COMSOL Multiphysics have been conducted to validate assumptions and are referred to where needed. The experimental setup is validated successfully for two-dimensional and axisymmetric refractive index fields and yields good results in case the signal-to-noise ratio is sufficient. Especially for measurements in water good accuracy is achieved due to the refractive index gradients being higher in water as continuous phase.