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

Abstract

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.

Nonisothermal Droplet Growth in the Free Molecular Regime

“…Experiments are conducted in two separate nozzles for each experimental condition, where the nozzles differ in the material used for the sidewall windows. For SAXS experiments, the nozzle has 25 μm thick mica windows; and for PTM, we use a nozzle with 2 mm thick CaF 2 windows (Paci et al 2004;Tanimura et al 2005). Since the nozzles throat areas differ slightly, we adjust the flow rates to maintain both the desired stagnation pressure and the partial pressure of the condensible at the nozzle inlet for the corresponding PTM and SAXS experiments.…”

“…Beyond this point, the pressure increase associated with heat release can change the boundary layers and the mass fraction of condensate can be significantly underestimated near the nozzle exit (Tanimura et al, 2005). In the current experiments, analysis based on pressure measurements alone underestimate the temperatures by up to 10 K and overestimate densities by ∼4%.…”

“…The scattering from this size distribution is well-defined and the APS data analysis program is used to fit the experimental spectra to extract the average radius r , the spread σ , and the intensity as q goes to 0, I 0 . The intensity is converted to an absolute scale by matching the D 2 O volume fractions observed during the SAXS experiments to those obtained from tunable diode laser absorption spectroscopy (TDLAS) experiments conducted under the same conditions (Paci et al 2004;Tanimura et al 2005). The aerosol number density, N is calculated from these fitting parameters using the following formula…”

Modeling of H2O/D2O Condensation in Supersonic Nozzles

“…Supersonic nozzles have been used extensively to investigate particle formation and growth (Stein and Wegener 1967;Wegener 1969;Wegener et al 1972;Moses and Stein 1978;Wyslouzil et al 2000;Heath et al 2002;Khan et al 2003;Kim et al 2004;Tanimura et al 2005;Wyslouzil et al 2007), and, to a limited extent, to investigate nanodroplet structure (Wyslouzil et al 2006). Complementary modeling studies have been conducted for many years in order to better understand particle formation in these devices (Ostwatitsch 1942), to test existing nucleation or droplet growth models (Wegener et al 1972;Moses and Stein 1978;Young 1993;Wyslouzil et al 1994;Lamanna 2000), or to predict appropriate operating conditions for novel nozzle designs .…”

“…The spatial variation of these state variables has been followed using static pressure probes (Stein and Wegener 1967;Wyslouzil et al 2000;Heath et al 2002;Streletzky et al 2002;Khan et al 2003;Kim et al 2004), interferometry (Wyslouzil et al 1994;Lamanna 2000), or spectroscopy (Tanimura et al 1995;Tanimura et al 1996;Tanimura et al 1997;Paci et al 2004;Tanimura et al 2005, Tanimura et al 2007), respectively. Alternatively, the depletion of vapor from the gas phase, or the appearance of the condensate has been directly measured using spectroscopy (Tanimura et al 1995;Paci et al 2004;Tanimura et al 2005;Tanimura et al 2007). Finally, visible light scattering has been used to detect the appearance of the aerosol, and follow, at least qualitatively, its continued growth (Stein and Wegener 1967;Streletzky et al 2002;Karlsson et al 2007).…”