An investigation of measurement condition for non-intrusive velocity determination based on thermal tracing by Raman imaging

The present study proposes a velocity measurement based on thermal tracing by Raman imaging and investigates its applicability focusing on the error in temperature measurement, towards the establishment of a non-intrusive and micro-scale velocimetry. In order to realize fluorescence-free measurement, two-wavelength Raman imaging was employed to measure the temperature field in a channel flow. This technique exploits the contrasting temperature dependencies of hydrogen-bonded (HB) and non-hydrogen-bonded (NHB) OH stretching Raman bands of liquid water, and enables the determination of planar temperature distributions from the intensity ratio of the HB to NHB images. A calibration experiment showed a linear relationship between the temperature and the Raman intensity ratio in the range 293−333 K with temperature sensitivity of −0.56% K^-1. It was also confirmed that the spatial variation of the intensity ratio led to a large measurement error (approximately 9.1 K). Afterwards Raman images were acquired with various measurement conditions, and the influence of each parameter on the measurement error was quantitatively investigated. The apparent temperature variance was considerably reduced by increasing electron-multiplying gain and binning factor for spatial averaging, whereas an increase in the size of the measurement area resulted in a quadratic increase in the temperature variance due to the inhomogeneous excitation intensity. Finally, the requirements of the thermal flow conditions for the present methodology to be applied were quantitatively examined according to the measurement results.