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.
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.
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.
This paper presents measurements of spectrally and temporally resolved laser-induced incandescence (LII) from soot. The second harmonic (532 nm) from a nanosecond Nd:YAG laser was used to heat the soot over a wide range of fluences. The emission was spectrally resolved using a spectrograph attached to an intensified CCD camera with a gate width of 1.5 ns. At fluences below 0.2 J/cm2, corresponding to the sublimation threshold, spectra demonstrate broadband featureless emission characteristic of laser-induced incandescence, whereas at higher fluences spectra show sharp features attributable to C2 Swan band emission, C3 Swings band emission, and other species. These features perturb the LII signal at wavelengths between 380 and 680 nm, suggesting that this detection region should be avoided for LII measurements made using a 532-nm laser beam at fluences of 0.2 J/cm2 and above. The detection wavelength regions to be avoided are much more extensive than previously believed.
This paper presents measurements of spectrally and temporally resolved laser-induced incandescence (LII) from soot. The second harmonic (532 nm) from a nanosecond Nd:YAG laser was used to heat the soot over a wide range of fluences. The emission was spectrally resolved using a spectrograph attached to an intensified CCD camera with a gate width of 1.5 ns. At fluences below 0.2 J/cm2, corresponding to the sublimation threshold, spectra demonstrate broadband featureless emission characteristic of laser-induced incandescence, whereas at higher fluences spectra show sharp features attributable to C2 Swan band emission, C3 Swings band emission, and other species. These features perturb the LII signal at wavelengths between 380 and 680 nm, suggesting that this detection region should be avoided for LII measurements made using a 532-nm laser beam at fluences of 0.2 J/cm2 and above. The detection wavelength regions to be avoided are much more extensive than previously believed.
This paper presents measurements of spectrally and temporally resolved laser-induced incandescence (LII) from soot. The second harmonic (532 nm) from a nanosecond Nd:YAG laser was used to heat the soot over a wide range of fluences. The emission was spectrally resolved using a spectrograph attached to an intensified CCD camera with a gate width of 1.5 ns. At fluences below 0.2 J/cm2, corresponding to the sublimation threshold, spectra demonstrate broadband featureless emission characteristic of laser-induced incandescence, whereas at higher fluences spectra show sharp features attributable to C2 Swan band emission, C3 Swings band emission, and other species. These features perturb the LII signal at wavelengths between 380 and 680 nm, suggesting that this detection region should be avoided for LII measurements made using a 532-nm laser beam at fluences of 0.2 J/cm2 and above. The detection wavelength regions to be avoided are much more extensive than previously believed.
"The laser-induced incandescence (LII) signal is proportional to soot volume fraction" is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known.
"The laser-induced incandescence (LII) signal is proportional to soot volume fraction" is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known.
