AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Effects of Laser Fluence Non-Uniformity on Black Carbon Measurements Using the Auto-Compensating Laser-Induce Incandescence Technique
FENGSHAN LIU, Steven Rogak, David Snelling, Kevin Thomson, Gregory Smallwood, National Research Council Canada
Abstract Number: 406 Working Group: Instrumentation and Methods
Abstract Laser-induced incandescence (LII) has been utilized to measure soot or black carbon (BC) concentrations in many applications, such as in flames, in the exhausts of automobile and aero engines, and in ambient environments. In LII, a pulsed laser of nanoseconds duration is used to rapidly heat the BC particles to temperatures much higher than the initial ambient temperature, typically to 3000 to 4000 K. Detection and analysis of the incandescence signals can reveal the BC particle concentration at the measurement location. In LII measurements, it is preferred to use a uniform laser fluence since it greatly simplifies the interpretation of the LII signals to arrive at the BC particle concentration. In practice, however, there is always some degree of laser fluence non-uniformity, depending on the laser and the optical setup, and such the non-uniformity must be accounted for in establishing the quantitative relationship between the LII signal and the BC concentration.
The traditional approach was to consider the spatial averaged laser fluence profile based on the multi-pulse averaging of individual laser pulse distributions. Due to the randomness of hot spots, such averaging tends to result in a fairly good top-hat laser fluence distribution, even though each individual profile displays the presence of some hot spots. Because LII signals are highly sensitive to particle temperatures, the detected LII signals are biased towards the highest particle temperatures associated with the hot spots. As a result, the traditional approach of multi-pulse averaging significantly underestimates the effects of non-uniform laser fluence in LII measurements.
This study proposes to analyze the laser fluence non-unifomity effects using the histogram or probability density function of laser fluence. It is shown through LII measurements and modelling that the soot volume fraction anomaly can be explained to a large extent by the laser fluence non-uniformity.