Category: Uncategorized

D. Michelle Bailey and J. Houston Miller

Geophysical Research Abstracts Vol. 20, EGU2018-7401, 2018

Due to its ability to influence global climate change, it is imperative to continually monitor carbon dioxide emission levels. Although high-precision sensors are commercially available, these are not cost effective for mapping a large spatial area. A goal of this research is to build out a network of sensors that are accurate and precise enough to provide a valuable data tool for accessing carbon emissions from large, urban or remote areas. This greenhouse gas dataset can be used in numerous environmental assessments and as validation for remote sensing products. Each of our sensors (referred to as “Luftsinn” sensors) utilizes a non-dispersive infrared (NDIR) sensor for the detection of carbon dioxide along with a combination pressure/temperature/humidity sensor and a real-time clock. The sensors communicate using serial and I2C interfaces with a Raspberry Pi ARM-based, microcontroller. Laboratory characterization of the Luftsinn sensors shows ∼1% accuracy when evaluated with gas standards. For field deployment, solar-powered sensors, supporting a 5 W power burden, have been developed.

Two applications are discussed here. First, Luftsinn sensors are being deployed as a part of an innovative opennetworking platform being installed on LED street lights in Washington, DC. In this application, where internet access via WiFi or LoRa is readily available, each Luftsinn unit is connected to a website that leverages recent developments in open source GIS tools. In this way, data from individual sensors can be followed individually or aggregated to provide real-time, spatially-resolved data of CO₂ trends across a broad area. Alternatively, these sensors have been deployed in remote Alaska to evaluate carbon dioxide emissions above thawing permafrost. Data from these deployments are manually retrieved and posted onto the same webpage retroactively. A developing project will combine data collected from these unique environments and incorporate K-12 students residing above and below the 60th parallel to allow them to engage directly with the data and disseminate their findings to peers across the country.

Evaluating Foam Degradation and Fuel Transport Rates through Novel Surfactant Firefighting Foams for the Purpose of AFFF Perfluorocarbon Replacement
K. Hinnant et al., Eastern States Meeting of the Combustion Institute, March 2018

 

Abstract: Perfluorocarbon surfactants are used in aqueous film forming foams (AFFF) world-wide to suppress Class B pool fires. We believe the rapid fire suppression capabilities of perfluorocarbon surfactants are related to their ability to generate stable foams and foams that are resistive to fuel transport. Through a research program aimed at replacing perfluorocarbon surfactants in AFFF, we have evaluated foam degradation and fuel transport through foams at elevated temperatures for various classes of surfactants. The surfactants were evaluated individually and in a mixture with a hydrocarbon surfactant (Glucopon) to emulate a previously designed reference AFFF. This data was compared to the performance of a perfluorocarbon surfactant reference AFFF and a commercial AFFF.

Fast fire suppression is dependent on a foams ability to block fuel vapors traveling between the burning fuel pool below and the flame above, and maintain physical coverage over the fuel pool. Foam degradation is effected by heat from the fuel pool, heat from the fire above, and physicochemical interactions between the foam and fuel. Fuel transport through the foam is influenced by diffusion properties between surfactants in the foam and fuel as well as the foam layer thickness which is significantly impacted by increased foam degradation. By measuring foam degradation and fuel transport through the foam at elevated temperatures, we are better able to understand how fuel transport changes with increased foam degradation which mimics some characteristics of a flame environment.

Foam solutions were made by measuring the critical micelle concentration for individual surfactants and surfactants mixed in a 3:2 volumetric mixture with the surfactant and Glucopon, respectively. The foam solutions had surfactant concentrations 6 times greater than the critical micelle concentration. Foams were then generated from the foam solutions to measure foam degradation and fuel transport through the foam. Foam degradation was measured by placing 4 cm of foam above a heated n-heptane pool and monitoring the change in foam height over time. Fuel transport through the foam was measured by placing 4 cm of foam above a heated fuel pool in a specially designed fuel flux apparatus that monitored the concentration of fuel vapors above the foam over time using a nitrogen sparger and an FTIR. Surfactants analyzed included hydrocarbon surfactants, silicone surfactants, and sulfonated surfactants. From the currently evaluated surfactants, none have matched the foam degradation or fuel transport performance of the reference or commercial AFFF. However, certain surfactants performed better than others indicating potential directions for future AFFF surfactant replacement.

Presentation available here: Eastern States CI Presentation 3_2_18-250wawi

 

D. Michelle Bailey, Erin M. Adkins, and  J. Houston Miller

Appl. Phys. B (2017) 123:245

We have developed a low-power, open-path, near-infrared (NIR) tunable diode laser sensor for the measurement of near ground-level concentrations of greenhouse gases. Here, we report on instrument design, characterization, and initial measurements of carbon dioxide concentrations during deployment to a thermokarst collapse scar bog near Fairbanks, AK (USA). The optics “launch-box” portion of the instrument couples radiation from an NIR, distributed feedback diode laser operating near 1572 nm with a visible laser for alignment purposes. The outgoing beam is directed through a 3.2-mm hole in a parabolic mirror and the launchbox is oriented using a two axis, altitude-azimuth telescope mount such that the beam strikes a retroreflector target at a set distance downfield. The beam then retraces the path back to the launch-box where the light is collected on the surface of the parabolic mirror and focused onto a multimode fiber that transfers the radiation to an InGaAs detector. Sweeps over a ~1.6 cm^-1 spectral region were collected at a rate of 500 scans per second and were typically stored as 10 s sweep averages. These averaged sweeps could be individually spectrally fit for CO₂ concentration or averaged  into a single spectrum for fitting (after correction for slight frequency drift). Field data reported here was averaged for 2.5 min and was found to follow trends in diurnal cycles of CO₂ concentration cycles reported by sensors located nearby in the field site.

Erin M. Adkins  and  J. Houston Miller

Phys. Chem. Chem. Phys., 2017,19, 28458-28469

Trends linking the topological characteristics of polynuclear aromatic hydrocarbons (PAH) to their electronic properties are reported. TD-DFT electronic spectra computations, using the 6-31G* basis set and B3LYP exchange correlation functional, were calculated for a series of PAH, allowing for the HOMO–LUMO gaps to be reported. Clar structures provide an avenue to link the physical structure and the aromaticity of the molecule; which, when extended by bond length and harmonic oscillator model of aromaticity analysis, provide powerful tools to understand the link between electronic and physical structure. These results lead to the conclusion that all PAH structures show a decrease in HOMO–LUMO gap as a function of size, but the rate of that decrease is directly related to the topology of the molecules. A PAH taxonomy was developed that categorizes PAH into categories with similar topological properties, which allows for modelling of changes in the HOMO–LUMO gap with PAH size. An atom-pair minimization algorithm was used to calculate the binding energy (BE) of homogeneous dimers of the studied PAH. The BE per carbon atom increases with the overall size of the structure to an asymptotic limit, but as with the HOMO–LUMO gap, topology plays a critical secondary factor. Previously published, experimentally determined optical band gaps (OBG) from Tauc/Davis–Mott analysis of extinction spectra in various laminar, non-premixed flames produced a correlation between the HOMO–LUMO gaps of high-symmetry, nearly circular D_2h symmetry molecules to molecular size. The work presented here provides a much more nuanced and predictive evaluation of how OBG depends on structure and size.

Erin M.Adkins, Jennifer A.Giaccai, and  J. HoustonMiller

Proceedings of the Combustion Institute

Measuring Fuel Transport through Fluorocarbon and Fluorine-free Firefighting Foams
K. Hinnant et al., presented at the 12th International Symposium for Fire Safety Science, Fire Safety Journal, 2017, 91, 653-661

Abstract: A flux chamber was designed to measure the transient fuel transport through a foam layer before significant degradation of foam occurred. The fuel transport rate through AFFF (fluorinated foam) was much slower than through RF6 (fluorine-free foam) with break-through times being 820 s and 276 s respectively over n-heptane. The fuel flux through AFFF covering three fuel pools (n-heptane, iso-octane, and methyl-cyclohexane) was also measured. AFFF had the smallest flux over iso-octane with a break-through time over 1900 s and the highest flux over methyl-cyclohexane with a break-through time under 80 s even though the fuels have similar vapor pressures at room temperature. Despite the lack of aqueous film formation on an iso-octane fuel pool, the fuel vapor flux through AFFF was much smaller relative to the methyl-cyclohexane pool, which enables film formation due to its higher surface tension than iso-octane. Our measurements of transient fuel flux show that the foam layer is a significant barrier to fuel vapor transport. The data suggest a transient mechanism based on the suppression of fuel adsorption onto bubble lamellae surfaces due to the oleophobicity of fluorocarbon surfactants, which is consistent with fuel solubility data. This suggests that surfactants that suppress fuel adsorption and solubility into bubble lamellae surfaces may reduce fuel transport through foams.

Influence of fuel on foam degradation for fluorinated and fluorine-free foams
K. Hinnant et al.
Colloids and Surfaces A, 2017, 522, 1-17

Abstract: We performed experiments to quantify fuel-induced foam degradation by applying foams onto liquid fuels and water (for comparison) and measuring foam thickness over time. Our investigation included two firefighting foams, one fluorine-free (RF6-ICAO) and the other fluorinated (AFFF), and a foam made with a common surfactant, SDS. We applied a roughly 2 cm thick foam layer onto three liquid fuels (n-heptane, methylcyclohexane, and isooctane) at room and elevated temperatures. Foam lifetime was reduced by 50 and 75% for AFFF and RF6 respectively for foams on fuels compared to foams on water at room temperature. For all experiments, the fluorine-free foams (RF6 and SDS) degraded much faster than AFFF. Further, the effect of fuel temperature was significant when the foams were placed over hot fuel: the lifetime of the firefighting foams decreased by 1–2 orders of magnitude between experiments conducted with fuel at room temperature and 50 °C. Prior to the onset of foam degradation over fuels, the firefighting foams experienced a preliminary expansion (by up to 50% in volume). Video recordings of degradation show that expansion results primarily from bubbles near the interface increasing in size with accelerated coarsening by coalescence. We propose and discuss a mechanism for fuel-induced foam degradation based on our observations. Our results show that fluorine-free RF6 degrades faster than AFFF (by a factor of 3 at room temperature and 12 at elevated temperatures over fuel), which may contribute to differences in their firefighting performance.

 

Development of an analytical AFFF formulation for the evaluation of alternative surfactants
K. Hinnant et al., presented at the 10th U.S. National Meeting of the Combustion Institute
Full paper available for download here: 10thCombustionMeetingPaper_Final-1spqpee

Abstract: Following criteria set by MIL-F-24385F for firefighting foams, we are developing an analytical aqueous firefighting foam (AFFF) formulation to be used for laboratory testing in place of proprietary commercial AFFF formulation. An aqueous reference foam solution containing 0.15% Capstone (fluorocarbon surfactant), 0.05% Triton X-100 (hydrocarbon surfactant), and 0.95% diethylene glycol butyl ether, by weight and was found to meet MilSpec criteria for aqueous film formation, which was inferred to be essential for improved fire extinction. However, the reference AFFF extinguished the 28 ft² gasoline pool fire in 54 seconds, exceeding the maximum 30 second qualification standard. Despite similar surface tensions and spreading coefficients between the reference and commercial AFFF indicating adequate film formation, the fire extinction time criterion could not be met. However, we found that the reference AFFF and a commercial AFFF degraded in 30 and 45 minutes respectively when the foams were placed over 50°C n-heptane fuel at bench-scale consistent with burn-back Milspec test results. Differences in foam degradation may prove to be more valuable to fire extinction than the role of film formation.

Maria Botero, Erin M. Adkins, Silvia Gonzalez Calera, Houston Miller, and Markus Kraft

Combustion and Flame 164, 250-258, (2016)

Soot particles formed in a system of non-premixed liquid fuel flames supported on a wick-fed, smoke point test burner (ASTM D1322-08) were characterised by in-situ visible light extinction and thermophoretically-sampled high-resolution transmission electron microscopy measurements, HRTEM. The fuels studied were heptane, toluene and their iso-volumetric mixture (H50T50), given their relevance as surrogate fuels. Extinction measurements were used to calculate the soot volume fraction, Fv, and determine the optical band gap (OBG) as a function of flame position. The OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. For the HRTEM analysis, soot samples were collected at different locations in the flame using thermophoretic sampling and a fast-insertion technique. The images were then analysed using a `lattice-fringe' algorithm, to determine important parameters such as the fringe length. Polycyclic aromatic hydrocarbon (PAH) sizes were estimated from conjugation length obtained from OBG measurements and fringe lengths from HRTEM measurements. Across all studied flames, the peak Fv ranged from 3.8 ppm in the heptane flame to 18.0 ppm in the toluene flame. Despite this wide range, the average OBG across the different flames only varied from 1.99 eV in the H50T50 to 2.06 eV in the heptane flames, which is consistent with molecule lengths of between 0.98 nm and 1.02 nm. Lattice fringe analysis yielded slightly lower average fringe lengths between 0.85 - 0.96 nm throughout the different flames. This work provides experimental support to the model of soot formation where the transition from chemical to physical growth starts at a modest molecular size; about the size of circumpyrene.

GB Clarke, EL Wilson, JH Miller, HR Melroy

Presented here is a sensitivity analysis for the miniaturized laser heterodyne radiometer. This passive, ground-based instrument measures carbon dioxide (CO₂) in the atmospheric column and has been under development at NASA/GSFC since 2009. The goal of this development is to produce a low-cost, easily-deployable instrument that can extend current ground measurement networks in order to (1) validate column satellite observations, (2) provide coverage in regions of limited satellite observations, (3) target regions of interest such as thawing permafrost, and (4) support the continuity of a long-term climate record. In this paper an uncertainty analysis of the instrument performance is presented and compared with results from three sets of field measurements. The signal-to-noise ratio (SNR) and corresponding maximum uncertainty for a single scan are calculated to be 329.4 ± 1.3 by deploying error propagation through the equation governing the SNR. Reported is an absorbance noise of 0.0024 for six averaged scans of field data, for an instrument precision of 0.14 ppmv for CO₂.