Stagnation Plane Research Articles

Frontal impingement of nanojets: formation, disintegration and mixing of nano liquid sheets

We investigate nano liquid sheets formed by frontal impingement of two cylindrical nanojets using the molecular dynamics method. The results show that only with a high enough velocity can a stable liquid sheet be formed because of the strong surface tension effect in nanoscale. In relatively low jet velocity range, the relationship between the intact sheet radius and the jet velocity takes on the power function form with the power being − 0.502. This relationship is explained by considering the thermal fluctuation effect, thus confirming the dominating role of the thermal fluctuation effect in the disintegration process. The influence of the jet velocity on the time-domain evolution of mixing of the system and the spatial mixing distribution of the liquid sheet are also investigated. Our results suggest that nanojets do not coalesce at the impingement point, the mixing occurs mainly through diffusion. And there is recoil that happens at the stagnation plane.

Rate-ratio asymptotic analysis of the structure and mechanisms of extinction of nonpremixed CH4/N2–O2/N2O/N2 flames

Rate-ratio asymptotic analysis is carried out to elucidate the influence of nitrous oxide on the structure and critical conditions for extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream is made up of a mixture of methane and nitrogen. The other stream is a mixture of oxygen, nitrous oxide, and nitrogen. A reduced mechanism of five global steps is employed in the analysis. Chemical reactions are presumed to take place in a thin reaction zone that is established in the vicinity of the stagnation plane. On either side of this thin reaction zone, the flow field is inert. These inert regions are called outer zones. Methane and nitrous oxide are completely consumed in the reaction zone, while oxygen is presumed to leak through the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers—the inner layer and the oxidation layer. In the inner layer fuel (methane) is consumed and the intermediate species hydrogen and carbon monoxide are formed. These intermediate species are oxidized in the oxidation layer to water vapor and carbon dioxide. Radicals are produced in the oxidation layer from chain-branching reactions that consume hydrogen. Asymptotic analysis gives the scalar dissipation rate at extinction. Critical conditions for extinction predicted by the analysis agree well with previous experimental data. Nitrous oxide is found to have an inhibiting effect on the flame, promoting extinction. The inhibiting effect is attributed to the competition between the net reaction of nitrous oxide with hydrogen to form water vapor and nitrogen and the chain-branching reaction between oxygen and hydrogen that produces radicals.

Droplet characteristics and local equivalence ratio of reacting mixture in spray counterflow flames

Spray combustion was studied by injecting monodisperse 125 or 200μm ethanol droplets in a premixed natural gas fuel flame flowing against an opposed heated air jet. Phase Doppler anemometry and chemiluminescence measurements allowed to characterise both droplet parameters and the local reacting mixture. Both types of droplets crossed the flow stagnation plane and entered the opposite jet. However, 125μm droplets reversed their motion and oscillated around the stagnation plane, leading to increased droplet residence time in hot regions. The fuel vapour released by droplets close to the stagnation plane, mixed with the surrounding air and led to the ignition of a second fuel vapour flame below the natural gas flame. For 125μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125μm and 200μm droplets burned in different “Group Combustion” regimes.

An experimental investigation of the turbulent mixing transition in the Richtmyer–Meshkov instability

AbstractThe Richtmyer–Meshkov instability (RMI) is experimentally investigated in a vertical shock tube using a broadband initial condition imposed on an interface between a helium–acetone mixture and argon ($A\approx 0.7$). The interface is created without the use of a membrane by first setting up a flat, gravitationally stable stagnation plane, where the gases are injected from the ends of the shock tube and exit through horizontal slots at the interface location. Following this, the interface is perturbed by injecting gas within the plane of the interface. Perturbations form in the lower portion of this layer due to the shear between this injected stream and the surrounding gas. This shear layer serves as a statistically repeatable broadband initial condition to the RMI. The interface is accelerated by either a$M= 1.6 $or$M= 2.2 $planar shock wave, and the development of the ensuing mixing layer is investigated using planar laser-induced fluorescence (PLIF). The PLIF images are processed to reveal the light-gas mole fraction by accounting for laser absorption and laser-steering effects. The images suggest a transition to turbulent mixing occurring during the experiment. An analysis of the mole-fraction distribution confirms this transition, showing the gases begin to homogenize at later times. The scalar variance energy spectra exhibits a near$k^{-5/3}$inertial range, providing further evidence for turbulent mixing. Measurements of the Batchelor and Taylor microscales are made from the mole-fraction images, giving${\sim }150\ \mu \mathrm{m}$and 4 mm, respectively, by the latest times. The ratio of these scales implies an outer-scale Reynolds number of$6\text {--}7\times 10^4$.

Extinction studies of near-limit lean premixed syngas/air flames

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H2 mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O2 ⇔ O + OH and the main termination reaction H + O2 + M ⇔ HO2 + M in the H2/O2 chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO2 to OH + H2 ⇔ H + H2O with the increasing H2 mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H2 mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.

Effect of Fuel Molecular Structure and Premixing on Soot Emissions from n-Heptane and 1-Heptene Flames

Most liquid fuels contain compounds with one or more unsaturated C═C bonds. Previous studies have observed that the fuel reactivity and ignition behavior are strongly influenced by the presence and number of double bonds in the fuel molecular structure. Here, we report a numerical investigation on the effect of fuel unsaturation on PAHs and soot emissions in partially premixed flames (PPFs) burning n-heptane and 1-heptene fuels. A detailed soot and fuel oxidation model is validated against gaseous species measurements in n-heptane PPF and soot measurements in ethylene diffusion flames. Simulations are performed to examine the effects of double bonds on PAHs and soot emissions at different strain rates and levels of premixing. For both fuels, the global flame structure is characterized by a rich premixed reaction zone (RPZ) on the fuel side and a nonpremixed reaction zone (NPZ) on the oxidizer side. PAHs and soot are mainly formed in the region between the RPZ and stagnation plane. The presence of double b...

Impact of the Combustor-Turbine Interface Slot Orientation on the Durability of a Nozzle Guide Vane Endwall

The combustor-turbine interface is an essential component in a gas turbine engine as it allows for thermal expansion between the first stage turbine vanes and combustor section. Although not considered as part of the external cooling scheme, leakage flow from the combustor-turbine interface can be utilized as coolant. This paper reports on the effects of orientation of a two-dimensional leakage slot, simulating the combustor-turbine interface, on the net heat flux reduction to a nozzle guide vane endwall. In addition to adiabatic effectiveness and heat transfer measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed in the vane stagnation plane. Four interface slot orientations of 90 deg, 65 deg, 45 deg, and 30 deg located at 17% axial chord upstream of a first vane in a linear cascade were studied. Results indicate that reducing the slot angle to 45 deg can provide as much as a 137% reduction to the average heat load experienced by the endwall. Velocity measurements indicate the formation of a large leading edge vortex for coolant injected at 90 deg and 65 deg while coolant injected at 45 deg and 30 deg flows along the endwall and washes up the vane surface at the endwall junction.

A simple spray–flamelet model: Influence of ambient temperature and fuel concentration, vaporisation source and fuel injection position

This work presents an analysis of the external structure of diffusion flames established with liquid fuel vaporisation (ethanol) in a counterflow configuration. The model assumes that part of the fuel is in the liquid phase (droplets) dispersed in the gas phase, which presents an initial fuel mass fraction. The spray–flamelet model is distinguished from the flamelet model by the presence of droplets in the reaction zone. There is no relative velocity between the droplets and the gas phase and the droplets vaporise completely before the stagnation plane. The mathematical description of the dilute spray–flamelet is made by the conservation equations with source terms corresponding to the fuel droplets. In the fuel species conservation equation, the source term has a strength proportional to the droplets vaporisation rate. In the energy conservation equation, the source term (heat sink) has a strength proportional to the product of the droplet vaporisation rate and the latent heat. The model exhibits a parameter defined as the spray combustion parameter M that combines the chemical reaction, flow field and spray properties involved in the problem. An analysis is performed to determine how the dilute spray–flamelet is affected by the liquid fuel injection position, the spray combustion parameter M and the temperature and fuel mass fraction Φ in the incoming fuel stream. A uniform distribution is considered to describe the droplets. The results confirm the expected influence of the spray combustion parameter M and the ambient temperature on the spray–flamelet. Effects on the temperature profile can be observed on the spray–flamelet by varying the liquid fuel injection position and the fuel mass fraction Φ in the incoming fuel stream. The results also indicate that, depending on the value of M, there is a value of Φ above which a burning regime is imposed that is almost totally controlled by the incoming fuel stream conditions.

The impact of combustion characteristics and flame structure on soot formation in oxy-enhanced and oxy-fuel diffusion flames

Based on a detailed chemical mechanism, impacts of combustion characteristics and flame structure on soot formation in opposed-flow diffusion ethylene flames was studied with different stoichiometric mixture fractions in O2/N2 and O2/CO2 atmospheres. The results showed the followings. 1) In both atmospheres, with the increase of stoichiometric mixture fraction, the flame structure changed significantly. The stagnation plane shifted toward the oxidizer side. Furthermore, there were less C2H2 but more O and OH to occur in the soot inception zone, therefore the amount of soot in the flame decreased. 2) Compared with N2, CO2 had a suppression effect on soot formation, which was mainly due to thermal and direct chemical interaction effects of CO2. This is because the specific heat capacity of CO2 is higher than that of N2, which will cause the flame temperature to drop, and mole fractions of C2H2, H, O, OH and main PAHs to decrease. Soot oxidation played a dominant role, while soot surface growth was attributed to the secondary position. Meanwhile, when CO2 abounded in the flame, OH concentration was increased through the backward reaction of CO+OH=CO2+H, and this would be conducive to the oxidation of soot precursor and incipient soot particles. In addition, the results of maximum particle density indicated the thermal effect of CO2 on soot formation is more important than the direct chemical interaction effect.

An experimental investigation of catalytic oxidation of propane using temperature controlled Pt, Pd, SnO2, and 90% SnO2–10% Pt catalysts

A stagnation-point flow reactor was used to study the catalytic activity of platinum (Pt), palladium (Pd), tin dioxide (SnO2), and 90% SnO2–10% Pt (by mass) to premixed propane and air reactants at atmospheric pressure. The Pt and Pd catalysts served as the baseline by which to compare catalyst performance. The stagnation surface temperature was controlled to maintain constant temperatures using an electric heater in order to eliminate heat transfer effects on catalyst performance. The activity of each catalyst was evaluated for catalyst stagnation surface temperatures ranging from 300 °C to 800 °C. Three fuel–air equivalence-ratio conditions were studied (ϕ = 0.67, 1.0 and 1.5). The CO2, CO, O2, and C3H8 in the exhaust gases were measured to quantify the activity of each catalyst. The effect of heat released by surface reaction was quantified by the heater power required to stabilize the stagnation plane at the prescribed temperature. The catalysts, excluding 100% SnO2, exhibited comparable trends with respect to propane oxidation as a function of stagnation-plane temperature. While 100% SnO2 showed no catalytic activity at any conditions, the 90% SnO2–10% Pt catalyst showed significant activity to propane oxidation. The lowest catalyst activation temperature of the experiments (Tsurface = 250 °C) was observed for the 90% SnO2–10% Pt catalyst for ϕ = 1.5. The results demonstrate potential of Pt/SnO2 as a fuel oxidation catalyst.

Effects of Orientation and Position of the Combustor-Turbine Interface on the Cooling of a Vane Endwall

First stage, nozzle guide vanes and accompanying endwalls are extensively cooled by the use of film cooling through discrete holes and leakage flow from the combustor-turbine interface gap. While there are cooling benefits from the interface gap, it is generally not considered as part of the cooling scheme. This paper reports on the effects of the position and orientation of a two-dimensional slot on the cooling performance of a nozzle guide vane endwall. In addition to surface thermal measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed at the vane stagnation plane. Two slot orientations, 90 deg and 45 deg, and three streamwise positions were studied. Effectiveness results indicate a significant increase in area averaged effectiveness for the 45 deg slot relative to the 90 deg orientation. Flowfield measurements show dramatic differences in the horseshoe vortex formation.

Uncertainty assessment of species measurements in acetone counterflow diffusion flames

To quantitatively understand the uncertainty of intrusive species sampling measurements using a microprobe, velocity and speciation profiles of acetone counterflow diffusion flames have been experimentally investigated with cross validations using non-intrusive particle image velocimetry (PIV) and laser induced fluorescence (LIF) measurements. It is shown that the separation distance between the fuel and oxidizer nozzles needs to be sufficiently large to achieve uniform radial velocity profiles at the nozzle exit and accurate measurements of fuel concentration distributions in flames. The impacts of the diffusion flame location relative to the stagnation plane and the diffusion flame thickness on quantitative species sampling are investigated by varying the fuel to oxygen ratio as well as nitrogen and helium as fuel diluents. The results show that the diffusion flame needs to be located on the fuel side far from the stagnation plane in order to obtain reliable speciation measurements of fuel oxidation-related species. For helium dilution in the fuel side, a significant deviation from the model prediction is found due to the excessively fast diffusion velocity of helium. The impact of the intrusive probe on the flow field and the structure of the counterflow diffusion flame are identified by acetone and OH LIF measurements. The uncertainty in the speciation measurement associated with flow perturbations by the probe is quantified and found to be comparable to the outer diameter of the probe, ±0.3mm. A simple Reynolds number analysis shows that the flow near the probe is just on the outskirts of the Stokes regime. Finally, the structure of the acetone diffusion flame is measured quantitatively with species measurements of ethane, ethylene, and acetylene. The comparison between predictions and measurements indicate that the current C2 kinetic mechanism needs to be improved for quantitative prediction of the acetone flame structures.

Sooting limit in counterflow diffusion flames of ethylene/propane fuels and implication to threshold soot index

Sooting limits in counterflow diffusion flames of propane/ethylene fuels have been studied experimentally using a light scattering technique, including the effects of dilution, fuel mixing, and strain rate. The results are discussed in view of the threshold soot index (TSI). In soot-formation (SF) flames, where the flame is located on the oxidizer side of the stagnation plane, the sooting limit depends critically on fuel type and subsequently on flame temperature. The sooting limit has a non-linear dependence on the fuel-mixing ratio, which is similar to the non-linear mixing rule for TSI observed experimentally in rich premixed flames, where soot oxidation is absent for both SF and rich premixed flames. In soot-formation-oxidation (SFO) flames, where the flame is located on the fuel side, the sooting limit depends critically on flame temperature, while it is relatively independent on fuel type. This result suggests a linear mixing rule for sooting limits in SFO flames, which is similar to the TSI behavior for coflow diffusion flames. Soot oxidation takes place for both types of flames. The aerodynamic strain effect on the sooting limits has also been studied and an appreciable influence has been observed. Under sooting conditions, soot volume fraction was measured using a light extinction technique. The soot loadings in SF flames of the mixture fuels demonstrated a synergistic effect, i.e., soot production increased for certain mixture fuels as compared to the respective single-component fuels.

Turbulent mixing measurements in the Richtmyer-Meshkov instability

The Richtmyer-Meshkov instability is experimentally investigated in a vertical shock tube using a new type of broadband initial condition imposed on an interface between a helium-acetone mixture and argon (A = 0.7). The initial condition is created by first setting up a gravitationally stable stagnation plane between the gases and then injecting the same two gases horizontally at the interface to create a shear layer. The perturbations along the shear layer create a statistically repeatable broadband initial condition. The interface is accelerated by a M = 1.6 planar shock wave, and the development of the ensuing turbulent mixing layer is investigated using planar laser induced fluorescence. By the latest experimental time, 2.1 ms after shock acceleration, the layer is shown to be fully turbulent, surpassing both turbulent transition criteria based on the Reynolds number and shear layer scale. Mixing structures are nearly isotropic by the latest time, as seen by the probability density function of gradient angles within the mixing layer. The scalar variance energy spectrum suggests a k−5/3 inertial range by the latest time and an exponential region at higher wavenumbers.

The structure of toluene-doped counterflow gaseous diffusion flames

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized toluene is studied in two distinct flame environments: a blue methane flame stabilized on the fuel side of the gas stagnation plane and an incipiently sooting ethylene flame stabilized on the oxidizer side. The goal is to provide a well-defined testbed in terms of temperature–time history, major species and part of the radical pool, for the examination of reference fuels that are critical components of practical fuel blends. Gas samples are extracted from the flame with fused silica microprobes for subsequent GC/MS analysis and thermocouples and thin filament pyrometry are used to characterize the temperature field. Profiles of critical toluene pyrolysis products and stable soot precursors are compared with computational models using two semi-detailed chemical mechanisms. Results show that in the methane flame some oxygen containing radicals like O and OH are contributing early on to the toluene destruction path. In the incipiently sooting ethylene flame, the primary attack is from H alone. This finding confirms the different challenges that such flames pose to the validation of a chemical kinetic mechanism. The onset of toluene decay in these flames begins at relatively modest temperatures, on the order of 800K. This reactivity is captured reasonably well by both chemical mechanisms in the methane flame, in the absence of reactants larger than C2, but not so in the ethylene flame, in the presence of a richer, more complex mixture. The aromatic ring opening mechanisms are not adequately modeled in either case. This discrepancy has implications for the modeling of practically relevant fuel blends with both aliphatic and aromatic compounds. The dominant species larger than toluene in the doped methane flame is ethylbenzene, which at least one of the mechanisms reproduces quite well. The largest measured species in the incipiently sooting flame is indene, whose concentration increase due to toluene addition is properly captured by one of the models. The experimental dataset reported here may help identifying future improvements to chemical kinetic mechanisms and complement other reactor datasets lacking the coupling of kinetics and transport of flame environments.

Influence of flow injection angle on a leading-edge horseshoe vortex

Junction flows that develop at the base of protruding obstructions occur in many applications. An unsteady horseshoe vortex is formed as a component of these junction flows, which increases the local heat transfer on the associated endwall. Augmenting this junction flow can be achieved through the injection of fluid upstream of the obstruction. This experimental study evaluated the effects of injection angle for a two-dimensional slot placed upstream of a vane leading-edge with four injection angles of 90°, 65°, 45°, and 30°. Results showed that high momentum injection increased the endwall heat transfer at each slot angle while low momentum injection resulted in a relatively lower augmentation of endwall heat transfer. A leading-edge vortex turning into the endwall was formed at the junction in the stagnation plane for high momentum injection at 90° and 65° while a leading-edge vortex turning away from the wall was formed for 45° and 30° injection. For low momentum injection, a vortex turning into the endwall was formed at all injection angles.

Flame-flow interactions and flow reversal

The interaction of a premixed methane/air flame with flow unsteadiness is studied computationally using a stagnation-point flow configuration. The problem is of fundamental interest and also relevant for turbulent combustion in the laminar flamelet regime. In the present study, of particular interest is the flame residing in a weakly strained flow field such that the flame is stabilized away from the viscous boundary layer adjacent to the stagnation plane and is free to move in response to flow perturbations. An unsteady sinusoidal strain rate field is imposed on the flame, and an extensive parametric study is conducted by varying the frequency and amplitude of strain rate fluctuation. It is found that for high frequencies and large amplitudes, flow direction reverses upstream of the flame, thereby establishing a new stagnation plane in the preheat zone ahead of the flame. This observation indicates that the flame strongly affects the upstream flow field and could also possibly explain the reported occurence of flow reversal in experimental studies of turbulent jet flames. Effects of other key parameters such as the Lewis number, mean flame speed, and gas expansion ratio on flame-flow interaction and flow reversal are studied by investigating highly lean (Le<1) and rich (Le>1) hydrogen/air flames. The physical mechanism responsible for flow reversal phenomenon is explained.

Detection of aromatic hydrocarbons and incipient particles in an opposed-flow flame of ethylene by spectral and time-resolved laser induced emission spectroscopy

Spectral and time-resolved fluorescence and incandescence measurements are used to identify aromatic compounds and particles in an atmospheric-pressure opposed-flow flame of ethylene. The fourth harmonic radiation, at 266nm, of a Nd–YAG laser is used to excite electronic transitions in aromatic intermediates resulting in different laser induced emission signals. Time-resolved analysis of the emission spectra is performed to obtain information about the structure of the compounds responsible for emission signals. Four fluorescence bands in the UV and visible range and a continuum with a maximum at larger wavelengths, attributed to incandescence radiation by solid particle, have been identified in the collected spectra. On the basis of fluorescence emission spectra and fluorescence lifetimes, two different types of high-molecular mass aromatic compounds are individuated in the flame: highly-packed, sandwich-like structures (cluster of PAHs held together by van der Waals forces) and more loose aromatic moieties linked together by σ-bonds, such as the aromatic–aliphatic linked hydrocarbons. The first class of particles has been found in the fuel zone and across the stagnation plane of the opposed-flow flame. In this region (<1500K) coagulation of aromatics is the leading mechanism forming high-molecular mass structures. The second class of particles is preferentially formed in the region close to the oxidizer side characterized by high temperatures (>1500K) and growth of aromatics enhanced by the high concentration of small radicals and gas-phase polycyclic aromatic hydrocarbons.

Experimental investigation of structure vulnerabilities to firebrand showers

Attempting to experimentally quantify the vulnerabilities of structures to ignition from firebrand showers has remained elusive. The coupling of two facilities has begun to unravel this difficult problem. The NIST Firebrand Generator (NIST Dragon) is an experimental device that can generate a firebrand shower in a safe and repeatable fashion. Since wind plays a critical role in the spread of WUI fires in the USA and urban fires in Japan, NIST has established collaboration with the Building Research Institute (BRI) in Japan. BRI maintains one of the only full scale wind tunnel facilities in the world designed specifically for fire experimentation; the Fire Research Wind Tunnel Facility (FRWTF). The present investigation is aimed at extensively quantifying firebrand penetration through building vents using full scale tests. A structure was placed inside the FRWTF and firebrand showers were directed at the structure using the NIST Dragon. The structure was fitted with a generic building vent, consisting of only a frame fitted with a metal mesh. Six different mesh sizes openings were used for testing. Behind the mesh, four different materials were placed to ascertain whether the firebrands that were able to penetrate the building mesh assembly could ignite these materials. Reduced scale test methods afford the capability to test new vent technologies and may serve as the basis for new standard testing methodologies. As a result, a new experimental facility developed at NIST is presented and is known as the NIST Dragon's LAIR (Lofting and Ignition Research). The NIST Dragon's LAIR has been developed to simulate a wind driven firebrand attack at reduced scale. The facility consists of a reduced scale Firebrand Generator (Baby Dragon) coupled to a bench scale wind tunnel. Finally, a series of full scale experiments were conducted to visualize the flow of firebrands around obstacles placed downstream of the NIST Dragon. Firebrands were observed to accumulate in front of these obstacles at a stagnation plane, as was observed when the structure was used for firebrand penetration through building vent experiments, due to flow recirculation. The accumulation of firebrands at a stagnation plane presents a severe threat to ignitable materials placed near structures.

Analysis of cloud condensation nuclei composition and growth kinetics using a pumped counterflow virtual impactor and aerosol mass spectrometer

Abstract. We present a new method of determining the size and composition of CCN-active aerosol particles. Method utility is illustrated through a series of ambient measurements. A continuous-flow thermal-gradient diffusion chamber (TGDC), pumped counterflow virtual impactor (PCVI), and Aerodyne time-of-flight mass spectrometer (AMS) are operated in series. Ambient particles are sampled into the TGDC, where a constant supersaturation is maintained, and CCN-active particles grow to ~2.5 &amp;amp;pm; 0.5 μm. The output flow from the TGDC is directed into the PCVI, where a counterflow of dry N2 gas opposes the particle-laden flow, creating a region of zero axial velocity. This stagnation plane can only be traversed by particles with sufficient momentum, which depends on their size. Particles that have activated in the TGDC cross the stagnation plane and are entrained in the PCVI output flow, while the unactivated particles are diverted to a pump. Because the input gas is replaced by the counterflow gas with better than 99 % efficiency at the stagnation plane, the output flow consists almost entirely of dry N2 and water evaporates from the activated particles. In this way, the system yields an ensemble of CCN-active particles whose chemical composition and size are analyzed using the AMS. Measurements of urban aerosol in downtown Toronto identified an external mixture of CCN-active particles consisting almost entirely of ammonium nitrate and ammonium sulfate, with CCN-inactive particles of the same size consisting of a mixture of ammonium nitrate, ammonium sulfate, and organics. We also discuss results from the first field deployment of the TGDC-PCVI-AMS system, conducted from mid-May to mid-June 2007 in Egbert, Ontario, a semirural site ~80 km north of Toronto influenced both by clean air masses from the north and emissions from the city. Organic-dominated particles sampled during a major biogenic event exhibited higher CCN activity and/or faster growth kinetics than urban outflow from Toronto, despite the latter having a higher inorganic content and higher O:C ratio. During both events, particles were largely internally mixed.