Time-resolved Concentration Measurements Research Articles

Planar Concentration Field Measurement In A Transient Jet Using Mie Scattering

An experimental method for time resolved concentration measurements in the near field of a transient starting turbulent air jet is presented. Planar Laser Mie Scattering (PLMS) using DEHS droplets produced by a Laskin nozzle as tracers of concentration is applied on an air jet with Reynolds number of 62.000 and is recorded with 10.000 frames per second. A jet penetration of about 20 nozzle diameters into the ambient environment is resolved with more than 100 frames. A linear response between grey scale values and concentration is found for mixtures up to 13% of the air-particle flow from the Laskin nozzle mixed with clean air. The calibration is based on images with no particles and images with a homogeneous particle concentration. A correction for light scattered by particles outside the laser sheet is proposed. The experimental results agree well with mean concentration measured with other techniques.

Experimental Determination of Drug Diffusion Coefficients in Unstirred Aqueous Environments by Temporally Resolved Concentration Measurements.

The diffusion coefficient (also known as diffusivity) of an active pharmaceutical ingredient (API) is a fundamental physicochemical parameter that affects passive diffusion through biological barriers and, as a consequence, bioavailability and biodistribution. However, this parameter is often neglected, and it is quite difficult to find diffusion coefficients of small molecules of pharmaceutical relevance in the literature. The available methods to measure diffusion coefficients of drugs all suffer from limitations that range from poor sensitivity to high selectivity of the measurements or the need for dedicated instrumentation. In this work, a simple but reliable method based on time-resolved concentration measurements by UV-visible spectroscopy in an unstirred aqueous environment was developed. This method is based on spectroscopic measurement of the variation of the local concentration of a substance during spontaneous migration of molecules, followed by standard mathematical treatment of the data in order to solve Fick's law of diffusion. This method is extremely sensitive and results in highly reproducible data. The technique was also employed to verify the influence of the environmental characteristics (i.e., ionic strength and presence of complexing agents) on the diffusivity. The method can be employed in any research laboratory equipped with a standard UV-visible spectrophotometer and could become a useful and straightforward tool in order to characterize diffusion coefficients in physiological conditions and help to better understand the drug permeability process.

A hierarchical method for Bayesian inference of rate parameters from shock tube data: Application to the study of the reaction of hydroxyl with 2-methylfuran

We developed a novel two-step hierarchical method for the Bayesian inference of the rate parameters of a target reaction from time-resolved concentration measurements in shock tubes. The method was applied to the calibration of the parameters of the reaction of hydroxyl with 2-methylfuran, which is studied experimentally via absorption measurements of the OH radical’s concentration following shock-heating. In the first step of the approach, each shock tube experiment is treated independently to infer the posterior distribution of the rate constant and error hyper-parameter that best explains the OH signal. In the second step, these posterior distributions are sampled to calibrate the parameters appearing in the Arrhenius reaction model for the rate constant. Furthermore, the second step is modified and repeated in order to explore alternative rate constant models and to assess the effect of uncertainties in the reflected shock’s temperature. Comparisons of the estimates obtained via the proposed methodology against the common least squares approach are presented. The relative merits of the novel Bayesian framework are highlighted, especially with respect to the opportunity to utilize the posterior distributions of the parameters in future uncertainty quantification studies.

Quantification of greenhouse gas emissions from a biological waste treatment facility

Whole-site emissions of methane and nitrous oxide, from a combined dry anaerobic digestion and composting facility treating biowaste, were quantified using a tracer dispersion technique that combines a controlled tracer gas release from the treatment facility with time-resolved concentration measurements downwind of the facility. Emission measurements were conducted over a period of three days, and in total, 80 plume traverses were obtained. On-site screening showed that important processes resulting in methane emissions were aerobic composting reactors, anaerobic digester reactors, composting windrows and the site’s biofilter. Average whole-site methane emissions measured during the three days were 27.5±7.4, 28.5±6.1 and 30.1±11.4kg CH4 h−1, respectively. Turning the windrows resulted in an increase in methane emission from about 26.3–35.9kg CH4 h−1. Lower emissions (21.5kg CH4 h−1) were measured after work hours ended, in comparison to emissions measured during the facility’s opening hours (30.2kg CH4 h−1). Nitrous oxide emission was too small for a downwind quantification. Direct on-site measurements, however, suggested that the main part of the emitted nitrous oxide came from the biofilter (about 1.4kg N2O h−1). Whole-site emissions were compared to emissions previously measured at different point sources on-site. Whole-site fugitive emissions were three to eight times higher than the sum of emissions measured at on-site sources. The magnitude of the emissions had a significant influence on the overall environmental impact of the treatment facility, assessed by consequential life cycle assessment. Including the higher whole-site fugitive emissions led to an increase in global warming potential, from a saving of 97kgCO2-eq.tonne−1 of treated waste (wet weight) to a loading of 71kg CO2-eq. tonne−1, ultimately flipping the environmental profile of the treatment facility.

The contribution of small leaks in a baghouse filter to dust emission in the PM2.5 range—A system approach

The contribution of leakage in a baghouse filter (defined as a short circuit between the upstream and downstream sides of the filter) to the emission of fine particles is quantified in comparison to other dust emission sources, and the influence of key operating variables on overall system response is analyzed. The study was conducted on a well-maintained pilot-scale filter unit (9 bags of 500g/m2 calendered polyester needle felt; total surface area 4.2m2) operated in Δp-controlled mode over a range of pulsing intensities, with two types of test dust (one free-flowing and the other cohesive) at inlet concentrations of 10 and 30g/m3. Leaks included single holes between 0.5 and 4mm diameter, intentionally placed in either the plenum plate or one of the filter bags, as well as seamlines from bag confectioning. Emissions were separated by source into a transient contribution due to dust penetration through the filter bags after each cleaning pulse, and a continuous contribution from leaks. This separation was based on a novel method of data processing that relies on time-resolved concentration measurements with a specially calibrated optical particle counter. Tiny leaks on the order of 1mm generated the same emission level as all the bags combined, and dominated continuous emissions. The equivalent leak cross section (leakage=media emission) was about 1ppm of the total installed filter surface, independent of upstream dust concentration. Leakage through open seamlines amounted to 75% of media emissions in case of free-flowing test dust. Leakage was restricted to aerodynamic diameters less than ∼5μm (roughly the PM2.5 mass fraction). For comparison, time-averaged mass penetration through conventional needle-felt media ranged from about 10−5 to 10−6, depending on cohesiveness of the particle material and pulse cleaning intensity, giving emission levels between about 0.02 and 0.2mg/m3 at the reference concentration of 10g/m2.

Modelling Short-Term Maximum Individual Exposure from Airborne Hazardous Releases in Urban Environments. Part ΙI: Validation of a Deterministic Model with Wind Tunnel Experimental Data.

The capability to predict short-term maximum individual exposure is very important for several applications including, for example, deliberate/accidental release of hazardous substances, odour fluctuations or material flammability level exceedance. Recently, authors have proposed a simple approach relating maximum individual exposure to parameters such as the fluctuation intensity and the concentration integral time scale. In the first part of this study (Part I), the methodology was validated against field measurements, which are governed by the natural variability of atmospheric boundary conditions. In Part II of this study, an in-depth validation of the approach is performed using reference data recorded under truly stationary and well documented flow conditions. For this reason, a boundary-layer wind-tunnel experiment was used. The experimental dataset includes 196 time-resolved concentration measurements which detect the dispersion from a continuous point source within an urban model of semi-idealized complexity. The data analysis allowed the improvement of an important model parameter. The model performed very well in predicting the maximum individual exposure, presenting a factor of two of observations equal to 95%. For large time intervals, an exponential correction term has been introduced in the model based on the experimental observations. The new model is capable of predicting all time intervals giving an overall factor of two of observations equal to 100%.

Applications of midinfrared quantum cascade lasers to spectroscopy

We review the use of both pulsed and continuous wave quantum cascade lasers in high-resolution spectroscopic studies of gas phase species. In particular, the application of pulsed systems for probing kinetic processes and the inherent rapid passage structure that accompanies observations of low-pressure samples using these rapidly chirped devices are highlighted. Broadband absorber spectroscopy and time-resolved concentration measurements of short-lived species, respectively exploiting the wide intrapulse tuning range and the pulse temporal resolution, are also mentioned. For comparison, we also present recent sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using a continuous wave external cavity quantum cascade laser system. Using this methodology the stability and resolution of this source is quantified. We find that the laser linewidth as measured via the Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates, but decreases to 1.3 MHz as the laser scan rate is increased such that the transition is observed at 30 kHz. Using this source, wavelength modulation spectroscopy of NO is presented.

Two-wavelength mid-IR diagnostic for temperature and n-dodecane concentration in an aerosol shock tube

A two-wavelength, mid-IR optical absorption diagnostic is developed for simultaneous temperature and n-dodecane vapor concentration measurements in an aerosol-laden shock tube. FTIR absorption spectra for the temperature range 323 to 773 K are used to select the two wavelengths (3409.0 and 3432.4 nm). Shock-heated mixtures of n-dodecane vapor in argon are then used to extend absorption cross section data at these wavelengths to 1322 K. The sensor is used to validate a model of the post-evaporation temperature and pressure of shock-heated fuel aerosol, which can ultimately be used for the study of the chemistry of low-vapor-pressure compounds and fuel blends. The signal-to-noise ratio of the temperature and concentration are ∼20 and ∼30, respectively, illustrating the sensitivity of this diagnostic. The good agreement between model and measurement provide confidence in the use of this aerosol shock tube to provide well-known thermodynamic conditions. At high temperatures, pseudo-first-order decomposition rates are extracted from time-resolved concentration measurements, and data from vapor and aerosol shocks are found to be in good agreement. Notably, the n-dodecane concentration measurements exhibit slower decomposition than predicted by models using two published reaction mechanisms, illustrating the need for further kinetic studies of this hydrocarbon. These results demonstrate the potential of multi-wavelength mid-IR laser sensors for hydrocarbon measurements in environments with time-varying temperature and concentration.

Time Resolved Concentration Measurements in an Axial Flow Mixer

Experimental results are reported providing information on the downstream mixing evolution in axial pipe flow mixers where a scalar is introduced into the pipe via a coaxial injection tube. Experiments were conducted in a 25.4 mm diameter water pipe flow loop 25,700>RD>28,500, in which a fluorescein dye was coaxially injected. The injection tube diameter was 1.5 mm. Three velocity ratios, VR=0.5, 1.0, and 2.0 were explored, where VR=Vjet/Vmain. The present results indicate that the effects of velocity ratio on the scalar concentration statistics are mainly evident in the first several outer pipe diameters downstream. In the far field, velocity ratio effects are shown to be insignificant on the concentration statistics. All cases showed a similar trend of an initial increase in variance at the centerline as the injected fluid begins mixing with the main pipe flow. This is followed by a region of rapid “exponential-like” decay, followed by a much slower decay rate after approximately 50 pipe diameters. Space-time correlations of the scalar concentration between far field locations verify the low wavenumber motions as predicted by the recent theory of Kerstein and McMurtry [A. Kerstein and P. McMurtry, “Low-wave-number statistics of randomly advected passive scalars,” Phys. Rev. E 50, 2057 (1994)], and are consistent with the slower than exponential downstream mixing rate.

Measurements of methane emissions from landfills using a time correlation tracer method based on FTIR absorption spectroscopy.

Methane is an important climate gas contributing significantly to global warming. A large part of the anthropogenic emissions of methane comes from landfills. Due to the biogenic origin of these emissions and the inhomogeneous characteristics of landfills and their soil cover, these emissions show large spatial variation. Thus, development of reliable and cost-effective methods for measurements of these emissions is an important task and a challenge to the scientific community. Traditionally, field chamber methods have been used but also different area integrating methods based on downwind plume measurements. These measurements have been supported by meteorological data either directly from local measurements or by controlled release of tracer gas from the landfill providing the dispersion characteristics of the plume. In this paperwe describe a method,the Time Correlation Tracer method, combining controlled tracer gas release from the landfill with time-resolved concentration measurements downwind the landfill using FTIR absorption spectroscopy. The method has been tested and used on measurements at a landfill in southern Sweden over the past 1.5 years. The method has proven to be a usable method for measurements of total methane emission from landfills, and under favorable meteorological conditions we estimate an achievable accuracy of 15-30%. The real time analysis capability of the FTIR makes it possible to judge the success of the measurement already on site and to decide whether more measurements are necessary. The measurement strategy is relatively simple and straightforward, and one person can make a measurement from a medium sized landfill (1-4 ha) within a few days to a week depending on the meteorological situation.

Simultaneous Laser Velocimeter and Concentration Measurement

Inability to predict the high Schmidt number mixling of a concentrated polymer solution injected into a channel flow of water motivated the development of a method for measuring the product of the velocity fluctuation normal to the wall and the fluctuation in the polymer concentration. Time-resolved concentration measurements were obtained using a laser-induced fluorescence technique. The injected fluid was marked with a fluorescent dye and the spatial distribution of the intensity of fluorescent light emitted from an argon-ion laser beam directed normal to the channel wall was measured with a line-scan camera. Concentration of the injected fluid was inferred from the dye concentration. A dual scatter, helium-neon laser Doppler velocimeter traversed along the line of the concentration measurements was used to measure the velocity normal to the wall. Successful implementation of this technique required very rapid sampling of the output from the line-array camera as well as a large addressable random access memory. Instantaneous and time-average results have been obtained for the injection of both water and a 700 ppm aqueous solution of SEPARAN AP-273 into fully developed channel flows of water at a Reynolds number of 42,800.

Improved direct sampling electron impact fluorimetry apparatus for combustion studies

This note describes an improved laboratory apparatus for performing direct sampling electron impact fluorimetry (DSEIF) experiments on combustion systems. In comparison with previous work, the new apparatus is a dedicated diagnostic tool having greater optical access to the fluorescence region and uses a fully controllable electron gun residing in a separate vacuum chamber. These features account for an estimated factor of 10 increase in S/N ratio over the previous setup. The new apparatus is expected to allow time-resolved concentration measurements of major species during transient combustion events.

Time-resolved two-dimensional concentration measurements in an acoustically driven flow

A nonintrusive optical technique for making simultaneous time-resolved concentration measurements at 10,000 points within a plane intersecting an acoustically forced axisymmetric jet is described. The technique, which involves the pulsing of a laser synchronously with an acoustic perturbation, is similar to a method developed for two-dimensional mapping of concentrations using a single laser pulse. Different scattering mechanisms, including Lorenz/Mie scattering and fluorescence, have been used for mapping the concentration distributions of large-scale structures in forced axisymmetric jets.