Pressure Trace Measurements Research Articles

Laser-induced plasma-ignited hydrogen jet combustion in engine-relevant conditions

This work aims to investigate the mechanisms governing H2 jet flame evolution and stabilisation by precisely controlling the jet ignition location using a laser-induced plasma in compression-ignition engine-relevant conditions. The experiments examine the flame evolution with high-speed schlieren imaging and pressure trace measurements in an optically-accessible constant-volume combustion chamber over a range of ambient O2 concentration (10–21 vol%) and temperature (600–800 K) conditions. Optical results reveal that in most cases, a localised flame kernel forms at the time and location of the laser-induced plasma and grows in connected regions to engulf the entire upstream and downstream jet volume of the ignition spot. The images also reveal that the flame lift-off is sensitive to ambient O2 and temperature changes. The flame appears attached to the nozzle at 21 vol% O2, but becomes lifted at a lower ambient O2 concentration and a colder temperature. A simplified numerical analysis suggests that edge-flame deflagration into stratified premixed fuel-ambient reactant streams explains the lift-off response to ambient O2 and temperature changes. Furthermore, at the lowest tested O2 concentration, the flame stabilisation occurs in leaner mixtures at the jet peripheral where it is likely exposed to stronger turbulence-chemistry interactions.

The freezing behavior of aqueous n-alcohol nanodroplets.

We generate water-rich aerosols containing 1-propanol and 1-pentanol in a supersonic nozzle to study the effects of these solutes on the freezing behavior of water. Condensation and freezing are characterized by two complementary techniques, pressure trace measurements and Fourier Transform Infrared spectroscopy. When 1-pentanol and 1-propanol are present, condensation occurs at higher temperatures because particle formation from the vapor phase is enhanced by the decrease in interfacial free energy of mixed aqueous-alcohol critical clusters relative to those of pure water. FTIR results suggest that when ∼6 nm radius droplets freeze, the tetrahedral structure of the ice is well preserved up to an overall alcohol mole fraction of 0.031 for 1-propanol and 0.043 for 1-pentanol. In this concentration range, the ice nucleation temperature decreases continuously with increasing 1-propanol concentration, whereas the onset of freezing is not significantly perturbed by 1-pentanol up to a mole fraction of 0.03. Furthermore, once freezing starts the ice nucleation rates in the aqueous-alcohol droplets are very close to those for pure water. In contrast, at the highest mole fractions of either alcohol it is not clear whether droplets freeze to form crystalline ice since the final state of the particles cannot be adequately characterized with the available experimental techniques.

Visualization of hydrogen jet evolution and combustion under simulated direct-injection compression-ignition engine conditions

The evolution and combustion of H2 jets were investigated in an optically-accessible constant-volume chamber under simulated direct-injection (DI) compression-ignition (CI) engine conditions. The parameters varied include injection pressure (84–140 bar) and ambient temperature (1000–1140 K). A detailed characterization of the injector system and the ensuing jet penetration process is reported first. High-speed schlieren imaging, OH∗ chemiluminescence imaging and pressure trace measurements were subsequently used to investigate the auto-ignition and combustion of the H2 jets. The results show that the ignition delay of H2 jets under such conditions is sensitive to ambient temperature variations, but not to injection pressure. Optical imaging reveals that the combustion of H2 jets mostly initiated from a localized kernel, before spreading to engulf the whole jet volume downstream of ignition location. The imaging also indicates that after ignition, the flame recesses back towards the nozzle and appears to attach to the nozzle to form a diffusion flame structure.

Homogeneous nucleation of carbon dioxide in supersonic nozzles I: experiments and classical theories.

We studied the homogeneous nucleation of carbon dioxide in the carrier gas argon for concentrations of CO2 ranging from 2 to 39 mole percent using three experimental methods. Position-resolved pressure trace measurements (PTM) determined that the onset of nucleation occurred at temperatures between 75 and 92 K with corresponding CO2 partial pressures of 39 to 793 Pa. Small angle X-ray scattering (SAXS) measurements provided particle size distributions and aerosol number densities. Number densities of approximately 1012 cm-3, and characteristic times ranging from 6 to 13 μs, resulted in measured nucleation rates on the order of 5 × 1017 cm-3 s-1, values that are consistent with other nucleation rate measurements in supersonic nozzles. Finally, we used Fourier transform infrared (FTIR) spectroscopy to identify that the condensed CO2 particles were crystalline cubic solids with either sharp or rounded corners. Molecular dynamics simulations, however, suggest that CO2 forms liquid-like critical clusters before transitioning to the solid phase. Furthermore, the critical clusters are not in thermal equilibrium with the carrier gas. Comparisons with nucleation theories were therefore made assuming liquid-like critical clusters and incorporating non-isothermal correction factors.

Freezing of supercooled n-decane nanodroplets: from surface driven to frustrated crystallization.

Whether crystallization starts at the liquid-vapor interface or randomly throughout the bulk has been the subject of intense debate. In our earlier work, we investigated the freezing of supercooled nanodroplets of short chain (C8, C9) n-alkanes formed by homogeneous condensation in a supersonic nozzle. The rate at which the solid appeared suggested freezing starts at the droplet surface well before the rest of the droplet freezes. Experiments were, however, limited to a single condition for each compound and it was not clear whether freezing of n-alkanes always occurs in this two step manner. Here, we expand our work to include freezing of a third n-alkane, n-decane, and, furthermore, we vary the temperatures at which droplets are formed and freeze. The phase transitions are again characterized using three experimental techniques - pressure trace measurements (PTM), Fourier Transform Infrared Spectroscopy (FTIR), and Small Angle X-ray Scattering (SAXS). We also use Wide Angle X-ray Scattering (WAXS) to confirm, for the first time, the crystalline nature of our frozen n-alkane nanodroplets. As the temperature at which the droplets form and freeze decreases, the kinetics of the phase transition changes. At higher temperatures, the phase transition occurs in two steps characterized by different rates, whereas at lower temperatures we observe only a single step. Finally, in the lowest temperature experiment, where droplets start to form and freeze ∼50 K below the bulk melting temperature, we found that the particles develop a fractal structure and appear locked in a "frustrated" crystalline state.

Isothermal Nucleation Rates of n-Propanol, n-Butanol, and n-Pentanol in Supersonic Nozzles: Critical Cluster Sizes and the Role of Coagulation.

We follow the nucleation of n-alcohols, n-propanol through n-pentanol, in two sets of supersonic nozzles having differing linear expansion rates. Combining the data from static pressure trace measurements with small-angle X-ray scattering we report the experimental nucleation rates and critical cluster sizes. For n-propanol, position resolved measurements clearly confirm that coagulation of the 2-10 nm size (radius) droplets occurs on the time scale of the experiment but that the effect of coagulation on the results is minimal. Under the conditions of the current experiments, our results suggest that alcohols have critical clusters that range from the dimer (n-pentanol) to the hexamer (n-propanol). We then compare the experimental results with classical nucleation theory (CNT), the Girshick-Chiu variant of CNT (GC), and the mean field kinetic nucleation theory (MKNT). Both CNT and MKNT underestimate the nucleation rates by up to 5 and 7 orders of magnitude, respectively, while GC theory predicts rates within 2 orders of magnitude. Correspondingly, the critical cluster size for all alcohols is overpredicted by factors of 2-9 with agreement improving with increasing chain length. An interesting byproduct of our experiments is that we find that the coagulation rate is enhanced by a factor of 3 over the value one would calculate for the free molecule regime.

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

Binary nucleation rates for ethanol/water mixtures in supersonic Laval nozzles: Analyses by the first and second nucleation theorems

We performed pressure trace measurements and small angle x-ray scattering measurements to determine the vapor-liquid nucleation rates of EtOH/H2O mixtures including pure EtOH and pure H2O in two supersonic Laval nozzles with different expansion rates. The nucleation rates varied from 0.9 × 10(17) to 16 × 10(17) cm(-3) s(-1) over the temperature range of 210 K to 230 K, EtOH activity range of 0 to 11.6, and H2O activity range of 0 to 124. The first and second nucleation theorems were applied to the nucleation rates to estimate the sizes, compositions, and excess energies of the critical clusters. The critical clusters contained from 4 to 15 molecules for pure H2O and EtOH/H2O clusters, and from 16 to 23 molecules for pure EtOH clusters. Comparing the excess energies of the pure H2O critical clusters with the results of a quantum-chemistry calculation suggested that the pre-factor of the theoretical nucleation rate is almost constant regardless of the monomer concentration. One possible explanation for this result is that cooling of the critical clusters limits the nucleation rate under the highly supersaturated conditions. The results of the analyses also yielded the relation between the surface energy and the composition of the critical clusters, where the latter are predicted to consist only of surface molecules. Applying this relationship to the EtOH/H2O bulk liquid mixtures, we estimated the EtOH mole fraction in the surface layer and found it is higher than that derived from the surface tension based on the Gibbs adsorption equation when the EtOH mole fraction in the liquid is higher than about 0.2 mol/mol. This discrepancy was attributed to the existence of the EtOH depletion layer just below the surface layer of the liquid.

Freezing of Heavy Water (D2O) Nanodroplets

We follow the freezing of heavy water (D2O) nanodroplets formed in a supersonic nozzle apparatus using position resolved pressure trace measurements, Fourier transform infrared spectroscopy, and small-angle X-ray scattering. For these 3-9 nm radii droplets, freezing starts between 223 and 225 K, at volume based ice nucleation rates Jice,V on the order of 10(23) cm(-3) s(-1) or surface based ice nucleation rates Jice,S on the order of 10(16) cm(-2) s(-1). The temperatures corresponding to the onset of D2O ice nucleation are higher than those reported for H2O by Manka et al. [Manka, A.; Pathak, H.; Tanimura, S.; Wölk, J.; Strey, R.; Wyslouzil, B. E. Phys. Chem. Chem. Phys.2012, 14, 4505]. Although the values of Jice,S scale somewhat better with droplet size than values of Jice,V, the data are not accurate enough to state that nucleation is surface initiated. Finally, using current estimates of the thermophysical properties of D2O and the theoretical framework presented by Murray et al. [Murray, B. J.; Broadley, S. L.; Wilson, T. W.; Bull, S. J.; Wills, R. H.; Christenson, H. K.; Murray, E. J. Phys. Chem. Chem. Phys.2010, 12, 10380], we find that the theoretical ice nucleation rates are within 3 orders of magnitude of the measured rates over an ∼15 K temperature range.

Freezing water in no-man's land

We report homogeneous ice nucleation rates between 202 K and 215 K, thereby reducing the measurement gap that previously existed between 203 K and 228 K. These temperatures are significantly below the homogenous freezing limit, T(H)≈ 235 K for bulk water, and well within no-man's land. The ice nucleation rates are determined by characterizing nanodroplets with radii between 3.2 and 5.8 nm produced in a supersonic nozzle using three techniques: (1) pressure trace measurements to determine the properties of the flow as well as the temperature and velocity of the droplets, (2) small angle X-ray scattering (SAXS) to measure the size and number density of the droplets, and (3) Fourier Transform Infrared (FTIR) spectroscopy to follow the liquid to solid phase transition. Assuming that nucleation occurs throughout the droplet volume, the measured ice nucleation rates J(ice,V) are on the order of 10(23) cm(-3) s(-1), and agree well with published values near 203 K.

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 Hale's 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.

Using small angle x-ray scattering to measure the homogeneous nucleation rates of n-propanol, n-butanol, and n-pentanol in supersonic nozzle expansions

In our earlier publication [M. Gharibeh et al., J. Chem. Phys. 122, 094512 (2005)] we determined the temperatures and partial pressures corresponding to the maximum nucleation rate for a series n-alcohols (C(i)H(2i+l)OH; i=3-5) during condensation in a supersonic nozzle. Although we were able to determine the characteristic time Deltat(Jmax) corresponding to the peak nucleation rate, we were unable to measure the number density of the aerosol and, thus, unable to directly quantify the nucleation rate J. In this paper we report the results of our pioneering small angle x-ray scattering (SAXS) experiments of n-alcohol droplets formed in a supersonic nozzle together with a new series of complementary pressure trace measurements. By combining the SAXS and pressure trace measurement data we determine the nucleation rates as a function of temperature and supersaturation.

Controlling nucleation and growth of nanodroplets in supersonic nozzles

We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3×1015⩽J/cm−3 s−⩽6.0×1015 at 230.1⩽T/K⩽230.4 and 29.2⩽S⩽32.4.