Effects of laser fluence non-uniformity on ambient-temperature soot measurements using the auto-compensating laser-induced incandescence technique

被引:11
作者
Liu, Fengshan [1 ]
Rogak, Steven [2 ]
Snelling, David R. [1 ]
Saffaripour, Meghdad [1 ]
Thomson, Kevin A. [1 ]
Smallwood, Gregory J. [1 ]
机构
[1] Natl Res Council Canada, Black Carbon Metrol Measurement Sci & Stand, Ottawa, ON K1A 0R6, Canada
[2] Dept Mech Engn, 2054-6250 Appl Sci Lane, Vancouver, BC V6T 1Z4, Canada
来源
APPLIED PHYSICS B-LASERS AND OPTICS | 2016年 / 122卷 / 11期
关键词
OPTICAL-PROPERTIES; PRIMARY PARTICLE; AGGREGATE SIZE; BLACK CARBON; MINI-CAST; COMBUSTION; FLAME; DIAGNOSTICS; LII;
D O I
10.1007/s00340-016-6553-2
中图分类号
O43 [光学];
学科分类号
070207 ; 0803 ;
摘要
Multimode pulsed Nd:YAG lasers are commonly used in auto-compensating laser-induced incandescence (AC-LII) measurements of soot in flames and engine exhaust as well as black carbon in the atmosphere. Such lasers possess a certain degree of fluence non-uniformity across the laser beam even with the use of beam shaping optics. Recent research showed that the measured volume fraction of ambient-temperature soot using AC-LII increases significantly, by about a factor of 5-8, with increasing the laser fluence in the low-fluence regime from a very low fluence to a relatively high fluence of near sublimation. The causes of this so-called soot volume fraction anomaly are currently not understood. The effects of laser fluence non-uniformity on the measured soot volume fraction using AC-LII were investigated. Three sets of LII experiments were conducted in the exhaust of a MiniCAST soot generator under conditions of high elemental carbon using Nd: YAG lasers operated at 1064 nm. The laser beams were shaped and relay imaged to achieve a relatively uniform fluence distribution in the measurement volume. To further homogenize the laser fluence, one set of LII experiments was conducted by using a diffractive optical element. The measured soot volume fractions in all three sets of LII experiments increase strongly with increasing the laser fluence before a peak value is reached and then start to decrease at higher fluences. Numerical calculations were conducted using the experimental laser fluence histograms. Laser fluence non-uniformity is found partially responsible for the soot volume fraction anomaly, but is insufficient to explain the degree of soot volume fraction anomaly observed experimentally. Representing the laser fluence variations by a histogram derived from high-resolution images of the laser beam energy profile gives a more accurate definition of inhomogeneity than a simple averaged linear profile across the laser beam.
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页数:13
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