A non-intrusive measurement technique applying CARS for concentration measurement in a gas mixing flow

TL;DR

This paper demonstrates a non-intrusive method using CARS microscopy to measure gas concentrations in a mixing flow, confirming its effectiveness through calibration and spatial concentration profiling.

Contribution

It introduces a novel application of CARS microscopy for direct, non-intrusive gas concentration measurement in micro-scale mixing flows, validated by experimental results.

Findings

CARS signal intensity is proportional to the square of concentration.

The method successfully measured concentration profiles along the flow.

The technique is applicable without complex preprocessing like gas absorption.

Abstract

Coherent anti-Stokes Raman scattering (CARS) microscope system was built and applied to a non-intrusive gas concentration measurement of a mixing flow in a millimeter-scale channel. Carbon dioxide and nitrogen were chosen as test fluids and CARS signals from the fluids were generated by adjusting the wavelengths of the Pump and the Stokes beams. The generated CARS signals, whose wavelengths are different from those of the Pump and the Stokes beams, were captured by an EM-CCD camera after filtering out the excitation beams. A calibration experiment was performed in order to confirm the applicability of the built-up CARS system by measuring the intensity of the CARS signal from known concentrations of the samples. After confirming that the measured CARS intensity was proportional to the second power of the concentrations as was theoretically predicted, the CARS intensities in the gas mixing flow channel were measured. Ten different measurement points were set and concentrations of both carbon dioxide and nitrogen at each point were obtained. Consequently, it was observed that the mixing of two fluids progressed as the measurement point moved downstream. The results show the applicability of CARS to the non-intrusive concentration measurement of gas flows without any preprocess such as gas absorption into liquid or solid.