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Measurements of liquid film thickness, concentration, and temperature of aqueous urea solution by NIR absorption spectroscopy

Authors
  • Pan, R.1
  • Jeffries, J. B.2
  • Dreier, T.1
  • Schulz, C.1
  • 1 University of Duisburg-Essen, Institute for Combustion and Gas Dynamics – Reactive Fluids, Carl-Benz-Str. 199, Duisburg, 47057, Germany , Duisburg (Germany)
  • 2 Stanford University, High Temperature Gasdynamics Laboratory, Stanford, CA, USA , Stanford (United States)
Type
Published Article
Journal
Applied Physics B
Publisher
Springer Berlin Heidelberg
Publication Date
Jan 19, 2016
Volume
122
Issue
1
Identifiers
DOI: 10.1007/s00340-015-6290-y
Source
Springer Nature
Keywords
License
Yellow

Abstract

A multi-wavelength near-infrared (NIR) diode laser absorption sensor has been developed and demonstrated for real-time monitoring of the thickness, solute concentration, and temperature of thin films of urea–water solutions. The sensor monitors the transmittance of three near-infrared diode lasers through the thin liquid film. Film thickness, urea mass fraction, and liquid temperature were determined from measured transmittance ratios of suitable combinations of lasers. Available laser wavelengths were selected depending on the variation of the NIR absorption spectrum of the solution with temperature and solute concentration. The spectral database was measured by a Fourier transform infrared spectrometer in the range 5500–8000 cm−1 for urea solutions between 5 and 40 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was first validated with measurements using a calibration cell providing liquid layers of variable thickness (200–1500 µm), urea mass fraction (5–40 wt%) and temperature (298–318 K). Temporal variations of film thickness and urea concentration were captured during the constant-temperature evaporation of a liquid film deposited on an optically polished heated quartz flat.

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