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1.
Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot 总被引:1,自引:0,他引:1
H. A. Michelsen 《Applied physics. B, Lasers and optics》2009,94(1):103-117
This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite
surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules
and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important
in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and
are based on a small subset of information available in the literature. The expression derived in this study is based on published
data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence
of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite
surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees
of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all
degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive
cooling rates following laser heating of soot particles. 相似文献
2.
S. De Iuliis F. Migliorini F. Cignoli G. Zizak 《Applied physics. B, Lasers and optics》2006,83(3):397-402
In order to understand the processes involved in the laser-induced incandescence (LII) technique, the value of soot temperature at the peak of the incandescence signal has been studied. To this purpose, an absolute two-color LII technique has been applied on ethylene and methane diffusion flames, based on the comparison with a calibrated tungsten ribbon lamp. The dependence of peak temperature on the fluence has been investigated by using a sharply edged probe beam. Above a certain fluence threshold a value close to 4000 K was obtained for both flames at all locations, that means in largely different soot conditions. At a suitably selected laser fluence, radial and axial profiles of peak soot temperature and volume fraction were performed. Soot volume fraction data have been validated with results from laser extinction technique measurements. The quite low values observed for methane prove the sensitivity of the LII technique. Moreover, a discussion about soot refractive index is presented. In the visible region a test of its influence on both soot volume fraction and soot peak temperature was carried out, while in the infrared the heating process was analyzed. PACS 42.62.b; 42.87-d; 44.40+a 相似文献
3.
The combustion generated soot contamination effect on a single diesel droplet ignition and burning was investigated experimentally for the first time. Diesel droplet flame was used to contaminate the droplet to be investigated prior to ignition. Distinct differences in lifetime and stability of the burning of the neat and contaminated droplet samples were observed in their heating, boiling and disruptive phases. For a soot-contaminated droplet surface, the evaporation rate became weaker as a result of slower mass transfer thus contracted the flame formation. Contrary to the burning rate enhancement of droplet with stable and uniform suspension of particles observed by other researchers, the slightest contamination of soot particles in a fuel droplet surface can significantly reduce the burning rate. Denser agglomeration of soot can form a shell on the droplet surface which blocks the flow of gas escaping through the surface thus distort the droplet even further. At late combustion stage, bubbles are observed to rapture on the surface of the soot-contaminated droplet. Strong ejections of volatile liquid and vapour that would explode shortly after parting from the droplet are observed. It seems that the explosion and burning of ejected mixture have little interactions with the enveloped flame surrounding the primary droplet. Enhanced visualisation of droplet liquid-phase has clearly indicated the cause of declining trend in the burning rate and flame stand-off ratio of soot-contaminated diesel droplet. These insights are of significance for understanding the effect of fuel droplet contamination by combustion generated soot particles. 相似文献
4.
2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed
using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences,
the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse
experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization
threshold.
Received: 20 October 1997/Revised version: 16 February 1998 相似文献
5.
C. Schoemaecker Moreau E. Therssen X. Mercier J.F. Pauwels P. Desgroux 《Applied physics. B, Lasers and optics》2004,78(3-4):485-492
Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10-4 cm-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio. PACS 33.20.Lg; 42.62.Fi; 44.40.+a 相似文献
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A method based on pre-concentration, using electrolysis, has been proposed and used for the improvement of detection sensitivity
of mercury ions at ultra-trace level of concentration in an aqueous matrix by laser-induced breakdown spectroscopy (LIBS).
The experimental evaluation of this method was carried out on mercury chloride (HgCl2) in triple-distilled water for a concentration range of the element of 0.01–10 mg l−1. A pre-concentration factor of ∼180 was obtained for LIBS detection of mercury emission line at 253.65 nm. The limit of detection
(LoD) of ∼0.011 mg l−1 (or ∼11 ppb) for mercury in the water sample was achieved. 相似文献
9.
Our recent work on the detection of explosives by laser-induced breakdown spectroscopy (LIBS) is reviewed in this paper. We have studied the physical mechanism of laser-induced plasma of an organic explosive, TNT. The LIBS spectra of TNT under single-photon excitation are simulated using MATLAB. The variations of the atomic emission lines intensities of carbon, hydrogen, oxygen, and nitrogen versus the plasma temperature are simulated too. We also investigate the time-resolved LIBS spectra of a common inorganic explosive, black powder, in two kinds of surrounding atmospheres, air and argon, and find that the maximum value of the O atomic emission line SBR of black powder occurs at a gate delay of 596 ns. Another focus of our work is on using chemometic methods such as principle component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) to distinguish the organic explosives from organic materials such as plastics. A PLS-DA model for classification is built. TNT and seven types of plastics are chosen as samples to test the model. The experimental results demonstrate that LIBS coupled with the chemometric techniques has the capacity to discriminate organic explosive from plastics. 相似文献
10.
利用激光击穿光谱的方法对水溶液样品中的金属元素进行定性和定量分析。分别采用竖直喷流(口径0.5 mm)和静止液面两种样品采样模式,针对不同质量浓度的Cd,Fe,Al和Pb元素进行了检测分析并得到了其定标曲线(线性拟合相关度基本在0.99以上),初步确定了4种元素在喷流模式下的检测限为0.206 5(Cd),0.147 6(Fe),0.061 9(Al)和0.200 9(Pb) g/L;静止液面模式下的检测限为0.050 1(Cd),0.023 9(Fe),0.014 8(Al)和0.006 9(Pb) g/L。实验所得的结果为检测工业废水中金属元素的含量提供了依据。 相似文献
11.
A method for the measurement of transition probabilities using laser-induced plasmas as spectroscopic sources is presented. To check its validity, the method is applied to 19 Ni II lines, most of them with known Aki values. The method is based on the measurement of curves of growth using samples with varying concentrations of the element of interest (here, nickel-copper alloys with nickel contents in the range 0.11-27 at%). The plasma is characterized by a reduced set of parameters, obtained by usual techniques (Boltzmann plot and Stark shift) combined with the curve-of-growth methodology. The transition probabilities are determined by fitting experimental curves of growth to calculations based on the plasma parameters. The use of curves of growth allows eliminating the systematic error due to the self-absorption in the measurement of transition probabilities, and is applicable to intense as well as very weak lines. 相似文献
12.
Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-injection (SIDI) engines. In this study, a direct-injection model experiment was developed to visualize soot formation in the vicinity of evaporating fuel films. Isooctane is injected by a multi-hole injector into the optically accessible part of a constant-flow facility at atmospheric pressure. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber, and subsequently sooting combustion is detected near the fuel films. Overlapping laser light sheets at 532 and 1064 nm excite laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. The 532 nm light sheet has low fluence to avoid the excitation of LII. The LII and LIF signals are detected simultaneously and spectrally separated on two cameras. In complementary line-of-sight imaging, the fuel spray, chemiluminescence, and soot incandescence are captured with a high-speed color camera. In separate experiments, toluene is added to the isooctane as a fluorescent tracer and excited by pulsed 266 nm flood illumination. From images of the LIF signal, the fuel-films’ thickness and mass evolutions are evaluated. The films survive the entire combustion event. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially separated from, but adjacent to the PAH, both with high spatial intermittency. Average images additionally indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and PAH. 相似文献
13.
研究将激光感生击穿光谱技术用于元素定量分析的可行性。利用Nd∶YAG激光器发出的激光与一个大气压下的空气相互作用产生激光等离子体,等离子体的光经单色仪分光后转化为电信号进入计算机,存储了600~800nm的等离子体光谱数据。利用自由定标模型对等离子体进行了分析。在延迟时间8μs、采样门宽0.4μs时,利用二维波尔兹曼平面得到了大气等离子体处于局部热平衡时的温度(1.62×10K)。在假设空气全部由N和O组成的条件下由自由定标模型得到了空气中氧的含量(20.75%)和氮的含量(79.25%)。试验结果与实际吻合得很好。实验证实了用激光感生击穿光谱对大气进行定量分析是可行的,为大气污染监测研究和试验工作奠定了基础。 相似文献
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Ray P. Bambha Mark A. Dansson Paul E. Schrader Hope A. Michelsen 《Applied physics. B, Lasers and optics》2013,112(3):343-358
We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer, centrifugal particle mass analyzer, and transmission electron microscope. The results demonstrate a strong influence of coatings on the magnitude and temporal evolution of the LII signal. For coated particles, higher laser fluences are required to reach signal levels comparable to those of uncoated particles. The peak LII curve is shifted to increasingly higher fluences with increasing coating thickness until this effect saturates at a coating thickness of ~75 % by mass. These effects are predominantly attributable to the additional energy needed to vaporize the coating while heating the particle. LII signals are higher and signal decay rates are significantly slower for thermally denuded particles relative to coated or uncoated particles, particularly at low and intermediate laser fluences. Our results suggest negligible coating enhancement in absorption cross-section for combustion-generated soot at the laser fluences used. Apparent enhancement in absorption with restructuring may be caused by less conductive cooling. 相似文献
16.
激光诱导击穿光谱(Laser-induced breakdown spectroscopy, LIBS)是一种分析多元素的光学技术,可用于新鲜蔬菜的快速检测.以洋葱为例,采用LIBS对其含有的元素进行了在线原位检测.用乙酸铅溶液污染洋葱以模拟大气湿沉降的重金属污染现象,并进行重金属Pb元素检测.使用主成分分析(principal components analysis, PCA)和反向传播人工神经网络(back-propagating artificial neutral network, BP-ANN),以洋葱、大蒜、小葱为样品进行区分检测.洋葱光谱中的特征谱线包括Si、Fe、C、Mg、Al、Ca、Ti、Sr、Ba、Na、Li和K等元素,以及N、H、O的谱线和CN分子谱带.不同浓度梯度乙酸铅溶液污染的洋葱样品中都能检测出Pb元素,其相对强度与溶液浓度成比例.此外,PCA的结果表明洋葱、小葱、大蒜的区分效果明显,BP-ANN交叉验证的识别率为89.47%.结果证明LIBS在对元素进行快速分析时具有较好的识别能力,是检测新鲜蔬菜的有效手段. 相似文献
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H. Bladh J. Johnsson J. Rissler H. Abdulhamid N.-E. Olofsson M. Sanati J. Pagels P.-E. Bengtsson 《Applied physics. B, Lasers and optics》2011,104(2):331-341
Laser-induced incandescence (LII) is a versatile technique for quantitative soot measurements in flames and exhausts. When
used for particle sizing, the time-resolved signals are analysed as these will show a decay rate dependent on the soot particle
size. Such an analysis has traditionally been based on the assumption of isolated primary particles. However, soot particles
in flames and exhausts are usually aggregated, which implies loss of surface area, less heat conduction and hence errors in
estimated particle sizes. In this work we present an experimental investigation aiming to quantify this effect. A soot generator,
based on a propane diffusion flame, was used to produce a stable soot stream and the soot was characterised by transmission
electron microscopy (TEM), a scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer coupled in series
after a differential mobility analyzer (DMA-APM). Despite nearly identical primary particle size distributions for three selected
operating conditions, LII measurements resulted in signal decays with significant differences in decay rate. However, the
three cases were found to have quite different levels of aggregation as shown both in TEM images and mobility size distributions,
and the results agree qualitatively with the expected effect of diminished heat conduction from aggregated particles resulting
in longer LII signal decays. In an attempt to explain the differences quantitatively, the LII signal dependence on aggregation
was modelled using a heat and mass transfer model for LII given the primary particle and aggregate size distribution data
as input. Quantitative agreement was not reached and reasons for this discrepancy are discussed. 相似文献
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