AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Effects of Detection Wavelengths on Black Carbon Measurements Using the Auto-Compensating Laser-Induce Incandescence Technique
FENGSHAN LIU, David Snelling, Kevin Thomson, Gregory Smallwood, National Research Council Canada
Abstract Number: 175 Working Group: Instrumentation and Methods
Abstract Autocompensating laser-induced incandescence (AC-LII) has been utilized to measure black carbon (BC) concentrations in many applications. In LII, a pulsed laser of nanoseconds duration is used to rapidly heat the BC particles to temperatures much higher than the ambient temperature, typically to 3000 to 4000 K. Detection and analysis of the incandescence signals reveal the BC concentration. It is preferred to use a uniform laser fluence in LII measurements; however, there is always certain laser fluence non-uniformity in practice. The detected LII signals are biased towards the highest particle temperatures associated with the hot spots in the laser beam. It has been demonstrated that the laser fluence non-uniformity is partially responsible for the so-called soot volume fraction anomaly – the measured soot volume fraction increases with increasing the mean laser fluence in the low-fluence regime.
In AC-LII, soot volume fraction is inferred from the absolute LII signal intensity, the equivalent laser beam width, and the effective soot temperature based on the ratio of LII signals detected at two wavelength bands in the visible spectrum, i.e., the two-color pyrometry principle. When soot particles are non-uniform in temperature due to the laser fluence non-uniformity, the derived effective soot temperature is not unique but dependent on the detection wavelengths. Consequently, the AC-LII inferred soot volume fraction is likely influenced by the pair of detection wavelengths. It is noticed that the effect of detection wavelengths on AC-LII inferred soot volume fraction or the magnitude of soot volume fraction anomaly has not been investigated thus far.
The effect of detection wavelength on soot volume fraction measurement using AC-LII was experimentally and numerically investigated. Soot was produced from a MiniCAST. AC-LII measurements were conducted using a pulsed Nd:YAG laser at 1064 nm and the resultant LII signals were detected at 445, 692, and 797 nm. It was found that the detection wavelength pair affects the effective soot temperature and consequently the inferred soot volume fraction. Strategy to alleviate the detection wavelength effect and soot volume fraction anomaly in AC-LII is also provided.