Shinobu Tanimura

Temperature and gas-phase composition measurements in supersonic flows using tunable diode laser absorption spectroscopy: The effect of condensation on the boundary-layer thickness

We used a tunable diode laser absorption spectrometer and a static-pressure probe to follow changes in temperature, vapor-phase concentration of D2O, and static pressure during condensation in a supersonic nozzle. Using the measured static-pressure ratio p/p0 and the mass fraction of the condensate g as inputs to the diabatic flow equations, we determined the area ratio (A/A*)wet and the corresponding centerline temperature of the flow during condensation. From (A/A*)wet we determined the boundary-layer displacement thickness during condensation (delta#)wet. We found that (delta#)wet first increases relative to the value of delta# in a dry expansion (delta#)Dry before becoming distinctly smaller than (delta#)Dry downstream of the condensation region. After correcting the temperature gradient across the boundary layers, the temperature determined from p/p0 and g agreed with the temperature determined by the laser-absorption measurements within our experimental error (+/-2 K), except when condensation occurred too close to the throat. The agreement between the two temperature measurements let us draw the following two conclusions. First, the differences in the temperature and mole fraction of D2O determined by the two experimental techniques, first observed in our previous study [P. Paci, Y. Zvinevich, S. Tanimura, B. E. Wyslouzil, M. Zahniser, J. Shorter, D. Nelson, and B. McManus, J. Chem. Phys. 121, 9964 (2004)], can be explained sufficiently by changes in delta# caused by the condensation of D2O, except when the phase transition occurs too close to the throat. Second, the extrapolation of the equation, which expresses the temperature dependence of the heat of vaporization of bulk D2O liquid, is a good estimate of the heat of condensation of supercooled D2O down to 210 K.

Spatially resolved gas phase composition measurements in supersonic flows using tunable diode laser absorption spectroscopy

We used a tunable diode laser absorption spectrometer to follow the condensation of D(2)O in a supersonic Laval nozzle. We measured both the concentration of the condensible vapor and the spectroscopic temperature as a function of position and compared the results to those inferred from static pressure measurements. Upstream and in the early stages of condensation, the quantitative agreement between the different experimental techniques is good. Far downstream, the spectroscopic results predict a lower gas phase concentration, a higher condensate mass fraction, and a higher temperature than the pressure measurements. The difference between the two measurement techniques is consistent with a slight compression of the boundary layers along the nozzle walls during condensation.

Homogeneous nucleation of a homologous series of n-alkanes (CiH2i+2, i=7–10) in a supersonic nozzle

Homogeneous nucleation rates of the n-alkanes (C(i)H(2i+2); i=7-10) were determined by combining information from pressure trace measurements and small angle x-ray scattering (SAXS) experiments in a supersonic Laval nozzle. The condensible vapor pressure p(J max), the temperature T(J max), the characteristic time Deltat(J max), and supersaturation S(J max) corresponding to the peak nucleation rate J(max) were determined during the pressure trace measurements. These measurements also served as the basis for the subsequent SAXS experiments. Fitting the radially averaged SAXS spectrum yielded the mean droplet radius r, 5<r/nm<31, the width of the aerosol size distribution sigma, 2<sigma/nm<6, and the particle number density N, 7x10(10)<N/cm(-3)<2.2x10(12). The nucleation rates for the n-alkanes J(max), 4x10(15)<J(max)/cm(-3) s(-1)<2x10(18), vary by almost three orders of magnitude as the temperature T(Jmax) decreases from approximately 200 K to as low as 150 K. At the lowest temperatures, the supersaturations S(Jmax) are on the order of 10(5). In spite of these extreme operating conditions, we find good agreement between the current experimental results and those available in the literature using Hales scaling formalism [Phys. Rev. A 33, 4156 (1986); Metall. Trans. A 23, 1863 (1992)] and the scaling parameters reported by Rusyniak and El-Shall [J. Phys. Chem. B 105, 11873 (2001)]. Comparing the experimental nucleation rates with the predictions of classical nucleation theory, we find that our experimental nucleation rates are 4.5-8 orders of magnitude higher than the predictions.

Tunable diode laser absorption spectroscopy study of CH3CH2OD∕D2O binary condensation in a supersonic Laval nozzle

We have developed a dual-beam tunable diode laser absorption spectroscopy system to follow the cocondensation of water and ethanol in a supersonic Laval nozzle. We determine the D(2)O monomer concentration in the vapor phase by fitting a Voigt profile to the measured line shape but had to develop a calibration scheme to evaluate the C(2)H(5)OD monomer concentration. To measure the temperature of the gas, we seed the flow with CH(4) and measure two absorption lines with different lower state energies. These data give a far more detailed picture of binary condensation than axially resolved pressure measurements. In particular, we observe that the C(2)H(5)OD monomer starts to be depleted from the gas phase well before D(2)O begins to condense.

Binary nucleation rates for ethanol/water mixtures in supersonic Laval nozzles

Although the conditions corresponding to the onset of condensation of aqueous-alcohol mixtures have been measured in supersonic nozzles [B. E. Wyslouzil et al., J. Chem. Phys. 113, 7317 (2000)], the true nucleation rates have not. Here, we propose a new analytical method to estimate the temperature, the concentrations of condensable species in both the vapor and the liquid phases, and the amount of the condensate using only the measured static pressure profiles in the nozzle. We applied the method to ethanol/water (CH(3)CH(2)OH/D(2)O or CH(3)CH(2)OD/D(2)O) mixtures and confirmed that the aerosol volume fractions derived from pressure measurements and small angle neutron scattering measurements are in very good agreement when this method is used. Combining the results from the pressure measurements with the number densities of the condensed droplets, measured either by small angle neutron or small angle x-ray scattering, we determined the first quantitative ethanol/water binary nucleation rates in the supersonic nozzle at a temperature of 229±1 K.

Nonisothermal Droplet Growth in the Free Molecular Regime

The growth rates of nonane and D2O nanodroplets produced in supersonic expansions are characterized using small angle X-ray scattering (SAXS) and pressure trace measurements (PTM). The experimental growth rates are compared to the predictions of a Hertz–Knudsen model that assumes either isothermal or nonisothermal droplet growth in the free molecular regime. For nonane, the predicted growth rates are insensitive to both droplet temperature and the evaporation coefficient, and agree well with the experimentally measured growth rates assuming a condensation coefficient of 1. For D2O, droplet growth rates are quite sensitive to droplet temperature, and the best agreement between experiments and theory are achieved for a condensation coefficient of 1 and an evaporation coefficient in the range from 0.5 to 1. Under our experimental conditions, incorporating coagulation is important to match the measured D2O growth rates but not those of nonane. Copyright 2013 American Association for Aerosol Research

Methanol nucleation in a supersonic nozzle

We determined the partial pressures p(Jmax), temperatures T(Jmax), monomer supersaturations S(Jmax), and characteristic times Δt(Jmax ) corresponding to the maximum nucleation rates of methanol in a supersonic nozzle. We found that T(Jmax) increased from 202.2 K to 223.7 K as p(Jmax) increased from 67.1 to 413.2 Pa, while the maximum nucleation rate J(max) changed by less than a factor of 4 over the measurement range. Our nucleation rates appear reasonably consistent with measurements in other devices and are within one order of magnitude of the nucleation rates predicted by classical nucleation theory.

Molecular dynamics simulation of the homogeneous nucleation of UF6 molecules: Configurations and infrared spectra of the excited hot clusters

The temperature, potential energy, and configurations of the clusters produced in the homogeneous nucleation of UF6 molecules from the supercooled (supersaturated) vapor phase were determined by classical molecular dynamics (MD) simulations. We observed two phenomena which demonstrate that the nucleation process occurs in the state far from thermal equilibrium. First, the excited hot clusters, the temperature of which is much higher than that of the monomer, were produced and continued to exist during the nucleation process. Second, the relationship between the potential energy and temperature of the clusters depends on the monomer temperature, that is, the potential energy at a temperature decreases with the increase in monomer temperature. In the simulations, various types of cluster configurations were observed: prolate, oblate, spherelike, and confeitolike. The confeitolike cluster is composed of one core and a few horns, and it was found predominantly in the hotter clusters. The infrared spectra of t...

THE HAMILTONIAN OF THE INDUCTION EFFECT IN INFRARED SPECTRA OF SF6 CLUSTERS

Abstract The Hamiltonian of the induction effect (dipole-induced dipole interaction) in the model proposed by Snels and Reuss is corrected. This model was proposed for the calculation of the frequency shift and transition strength of the IR spectra of clusters of highly symmetric molecules such as (SF 6 ) n (SiF 4 ) n and SiH 4 ) n Disagreement between the observed frequency shifts and the calculated results obtained using this model for SF 6 ) 3 and SF 6 ) 4 was found to be caused by the incorrect Hamiltonian of the dipole-induced dipole interaction. The calculated results for SF 6 ) 3 and SF 6 ) 4 with the corrected Hamiltonian agree with the observed spectra.

Molecular dynamics simulations of the homogeneous nucleation of UF6 and SF6 molecules: Effects of the intramolecular vibrational relaxations on the nucleation rates

The effects of flexibility in the homogeneous nucleation processes of UF6 and SF6 molecules from vapor phase were investigated by classical molecular dynamics (MD) simulations. We performed MD simulations using a flexible-molecule model and compared the results with those obtained from a rigid-molecule model. We took into account the flexibility of molecules in MD simulations by a harmonic intramolecular potential. We found that the nucleation rate in the flexible model of the UF6 molecule was about twice as large as that in the rigid model of UF6. This acceleration in nucleation rate was attributed to the flow of the condensation heat into the intramolecular vibrations. On the other hand, the nucleation rates in rigid and flexible models of SF6 were almost the same because the flow of the condensation heat into the intramolecular vibrations in the flexible model of SF6 was negligibly small. In order to confirm the reliability of the classical intramolecular vibrational model in the present work, we estim...