Mid-IR Hyperspectral Imaging of Laminar Flames for 2-D Scalar Values

This work presents a new emission-based measurement which permits quantification of two-dimensional scalar distributions in laminar flames. A Michelson-based Fourier-transform spectrometer coupled to a mid-infrared camera (1.5 μm to 5.5 μm) obtained 256 × 128pixel hyperspectral flame images at high spectral (δν̃ = 0.75cm−1) and spatial (0.52 mm) resolutions. The measurements revealed line and band emission from H2O, CO2, and CO. Measurements were collected from a well-characterized partially-premixed ethylene (C2H4) flame produced on a Hencken burner at equivalence ratios, Φ, of 0.8, 0.9, 1.1, and 1.3. After describing the instrument and novel calibration methodology, analysis of the flames is presented. A single-layer, line-by-line radiative transfer model is used to retrieve path-averaged temperature, H2O, CO2 and CO column densities from emission spectra between 2.3 μm to 5.1 μm. The radiative transfer model uses line intensities from the latest HITEMP and CDSD-4000 spectroscopic databases. For the Φ = 1.1 flame, the spectrally estimated temperature for a single pixel 10 mm above burner center was T = (2318 ± 19)K, and agrees favorably with recently reported laser absorption measurements, T = (2348 ± 115)K, and a NASA CEA equilibrium calculation, T = 2389K. Near the base of the flame, absolute concentrations can be estimated, and H2O, CO2, and CO concentrations of (12.5 ± 1.7) %, (10.1 ± 1.0) %, and (3.8 ± 0.3) %, respectively, compared favorably with the corresponding CEA values of 12.8%, 9.9% and 4.1%. Spectrally-estimated temperatures and concentrations at the other equivalence ratios were in similar agreement with measurements and equilibrium calculations. 2-D temperature and species column density maps underscore the Φ-dependent chemical composition of the flames. The reported uncertainties are 95% confidence intervals and include both statistical fit errors and the propagation of systematic calibration errors using a Monte Carlo approach. Systematic errors could warrant a factor of two increase in reported uncertainties. This work helps to establish IFTS as a valuable combustion diagnostic tool

Comparison of the Accuracy and Applicability of Forebody Wake Effect Models for Parachute System Design

The forebody wake effect (FWE) is important to consider when designing parachute systems because it can affect parachute performance. Parachutes work by altering the aerodynamic properties of an attached forebody to control a descent. The FWE can reduce parachute drag, causing the system to descend faster than desired. This drag reduction coupled with wind or other factors can change the descent trajectory and landing impact speed. Modeling the FWE is important for ensuring that the parachute system descends and lands safely at the desired location. In the available literature there are three prominent FWE models for parachute system design. These models fall under two general modelling methods. The first method is to generate a statistical or empirical model based on a high number of full-scale experimental flights or tests. The second method is to create a case specific model with computational fluid dynamics(CFD). The goal of this paper is to explore the limitations and appropriate uses of the existing FWE models. Their applications and limitations were evaluated and compared in terms of modelling method, accuracy in determining drag reduction and breadth of situational applicability. The investigation showed that the models are applicable in specific design cases and vary in accuracy. The three models presented have different strengths and limitations, as expected. This review lays the foundation for developing a more comprehensive FWE model for parachute system design

On flames established with air jet in cross flow of fuel-rich combustion products

Advances in combustor technologies are driving aircraft gas turbine engines to operate at higher pressures, temperatures and equivalence ratios. A viable approach for protecting the combustor from the high-temperature environment is to inject air through the holes drilled on the surfaces. However, it is possible that the air intended for cooling purposes may react with fuel-rich combustion products and may increase heat flux. Air Force Research Laboratory (AFRL) has developed an experimental rig for studying the flames formed between the injected cold air and the cross flow of combustion products. Laser-based OH measurements revealed an upstream shift for the flames when the air injection velocity was increased and downstream shift when the fuel content in the cross flow was increased. As conventional understanding of the flame stability does not explain such shifts in flame anchoring location, a time-dependent, detailed-chemistry computational-fluid-dynamics model is used for identifying the mechanisms that are responsible. Combustion of propane fuel with air is modeled using a chemical-kinetics mechanism involving 52 species and 544 reactions. Calculations reveled that the flames in the film-cooling experiment are formed through autoignition process. Simulations have reproduced the various flame characteristics observed in the experiments. Numerical results are used for explaining the non-intuitive shifts in flame anchoring location to the changes in blowing ratio and equivalence ratio. The higher diffusive mass transfer rate of hydrogen in comparison to the local heat transport enhances H₂–O₂ mixing compared to thermal dissipation rate, which, in turn, affects the autoignition process. While increasing the blowing ratio abates the differences resulting from non-equal mass and heat transport rates, higher concentrations of hydrogen in the fuel-rich cross flows accelerate those differences.KEYWORDS: Autoignition, Diffusion flame, Film-cooling, Preferential diffusion, Jet-in-cross-flowThis is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/fue

Effects of fuel content and density on the smoldering characteristics of cellulose and hemicellulose

Smoldering combustion in wildland fires poses hazards for both ecosystems and humans by destroying biomass, transitioning to flaming combustion, and releasing significant quantities of pollution. Understanding the parameters that control smoldering is necessary to help predict and potentially mitigate these hazards. A challenge in identifying these parameters is the wide variety of biomasses which occur in nature. The objective of this study is to identify the effects of density and fuel concentration on the smoldering characteristics of cellulose and hemicellulose mixtures. These fuels were considered because they are some of the major organic constituents within biomass. To this end, downward smoldering propagation velocities were measured for 50%, 75%, and 100% cellulose content at densities varying from 170 to 400 kg/m(3). The horizontal smoldering propagation velocities and temperature distributions were also determined for loosely packed samples ranging from 100% to 0% cellulose (with residual hemicellulose). Additionally, horizontal smoldering propagation velocities were determined for systematically varied ratios of cellulose (50-100%) and densities (200-400 kg/m(3)). The fuel was burned in an insulated reactor box. An infrared camera measured the horizontal propagation velocity, and thermocouples measured the downward propagation. A one-dimensional reactive porous media model with reduced chemistry was used to identify key processes causing the observed sensitivities. At constant packing density, the propagation velocity increased as cellulose content decreased because of decreased heat release with increased cellulose content and the earlier onset of hemicellulose pyrolysis. The propagation velocity decreased with respect to packing density when the fuel content was constant because of reduced oxygen diffusion. The propagation velocity increased with cellulose content when the fuel was loosely packed because of the decreasing density. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Who Eats Whom in a Pool? A Comparative Study of Prey Selectivity by Predatory Aquatic Insects

Predatory aquatic insects are a diverse group comprising top predators in small fishless water bodies. Knowledge of their diet composition is fragmentary, which hinders the understanding of mechanisms maintaining their high local diversity and of their impacts on local food web structure and dynamics. We conducted multiple-choice predation experiments using nine common species of predatory aquatic insects, including adult and larval Coleoptera, adult Heteroptera and larval Odonata, and complemented them with literature survey of similar experiments. All predators in our experiments fed selectively on the seven prey species offered, and vulnerability to predation varied strongly between the prey. The predators most often preferred dipteran larvae; previous studies further reported preferences for cladocerans. Diet overlaps between all predator pairs and predator overlaps between all prey pairs were non-zero. Modularity analysis separated all primarily nectonic predator and prey species from two groups of large and small benthic predators and their prey. These results, together with limited evidence from the literature, suggest a highly interconnected food web with several modules, in which similarly sized predators from the same microhabitat are likely to compete strongly for resources in the field (observed Pianka’s diet overlap indices >0.85). Our experiments further imply that ontogenetic diet shifts are common in predatory aquatic insects, although we observed higher diet overlaps than previously reported. Hence, individuals may or may not shift between food web modules during ontogeny

Cladistical analysis of the Jovian and Saturnian satellite systems

Jupiter and Saturn each have complex systems of satellites and rings. These satellites can be classified into dynamical groups, implying similar formation scenarios. Recently, a larger number of additional irregular satellites have been discovered around both gas giants that have yet to be classified. The aim of this paper is to examine the relationships between the satellites and rings of the gas giants, using an analytical technique called cladistics. Cladistics is traditionally used to examine relationships between living organisms, the 'tree of life.' In this work, we perform the first cladistical study of objects in a planetary science context. Our method uses the orbital, physical, and compositional characteristics of satellites to classify the objects in the Jovian and Saturnian systems. We find that the major relationships between the satellites in the two systems, such as families, as presented in previous studies, are broadly preserved. In addition, based on our analysis of the Jovian system, we identify a new retrograde irregular family, the Iocaste family, and suggest that the Phoebe family of the Saturnian system can be further divided into two subfamilies. We also propose that the Saturnian irregular families be renamed, to be consistent with the convention used in Jovian families. Using cladistics, we are also able to assign the new unclassified irregular satellites into families. Taken together, the results of this study demonstrate the potential use of the cladistical technique in the investigation of relationships between orbital bodies

Radiation Diagnostics of High Temperature High Speed Flows

Radiation emissions from exhaust plumes are of interest for security reasons and for studying the turbulent nature of high temperature flows. Temporal and spatial turbulent statistics of high temperature radiating flows can be obtained from planar radiation intensity measurements. Comparisons between measured and calculated mean and fluctuating radiation intensity values provide insights into the validity of scalar value calculations. Motivated by this, planar infrared images of a thin filament stretched across a buoyancy driven unsteady laminar hydrogen diffusion flame were obtained using an infrared camera. Transient line measurements of temperature and water vapor mole fractions were achieved using thin filament pyrometry and inverse radiation intensity calculations. The time-dependent spatial distributions of water vapor mole fractions and temperatures were affected by preferential diffusion and the altered velocity field of the flame. The experimentally determined scalar values are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. Experience gained studying this laboratory scale flame was used to enhance studies of exhaust plumes. Narrowband radiation intensity measurements are reported for exhaust plumes exiting from an axisymmetric converging nozzle with varying temperatures and species concentrations corresponding to low equivalence ratios (0.17 - 0.28), Mach numbers between 0.4 and 1.0, and Reynolds numbers between 2.4 and 6.1×105. The intensity was measured using an infrared camera fitted with a narrowband filter (4.34+/-0.1 µm). The intensity leaving a diametric path exhibited a power dependence of 2.8 on the equivalence ratio. This dependence results from changes in the scalar values in the flow. Smaller changes in the intensity were observed for plumes where the velocity was varied. The variations are attributed to changes in entrainment into the plume and the nozzle exit temperature. Calculations of the radiation intensity indicate that the outer portion of the shear layer attenuates the radiation emitted from the inner portion of the plume. Turbulent radiation statistics have been reported for reacting flows and used to assess turbulence models, provide insights into turbulence radiation interactions, and estimate integral time and length scales. Seeking similar insights, the mean, root mean square, probability density function, auto and spatial correlation coefficients, integral time and length scales, and power spectral density functions of the radiation intensity are reported for three exhaust plumes. Axial and radial variation in the normalized root mean square of the radiation intensity are similar to those reported for flames. Autocorrelation coefficients of the radiation intensity emitted downstream of the core region are approximated reasonably well by exponential curves. Integral time and length scales increase monotonically downstream of the core region. The break frequency and slope of the normalized power spectral density function are comparable to those reported for turbulent jet flames. Measured and computed mean and fluctuating radiation properties are reported for a subsonic exhaust plume. Unsteady three-dimensional calculations were used to estimate both time-dependent and mean temperature and partial pressure values of the exhaust species in the flow. From these scalar values the mean and root mean square of the radiation intensity were calculated using a narrowband radiation model. Axial distributions of the calculated intensities based on the time-dependent scalar values qualitatively show trends similar to measurements, and quantitatively over predict the intensity by 40 to 60%. Intensity distributions based on mean scalars were in better quantitative agreement, but decay more rapidly downstream than the measured values. The importance of turbulent fluctuations increased monotonically downstream and radially resulting in the mean intensity being larger by factors greater than two toward the edges of the plume. Comparisons of intensity values found from computed three- and two-dimensional scalar values emphasize the need for the former calculations. The techniques developed for acquiring radiation intensity measurements and estimating scalar values in flows were implemented to study the spatial development and temperature of spark kernels. Infrared images show that the kernels develop into a toroidal shape after exiting from the igniter. Regions of high and low radiation intensity are observed in the kernels, indicating temperature gradients within the gases. Average temperature values decrease by less than 30% over two centimeters of the spark trajectory. Over that same distance the internal energy of the kernel decreases by 80%

An analysis of spotting distances during the 2017 fire season in the Northern Rockies, USA

The wildfires that burned in the Northern Rockies region of the USA during the 2017 fire season provided an opportunity to evaluate the suitability of using broad-scale and temporally-limited infrared data on hotspot location to determine the influence of several environmental variables on spotting distance. Specifically, relationships between the maximum observed spot fire distance for each unique combination of fire and day and geo-referenced environmental data on wind speed, vegetation, and terrain, along with specific fire characteristics (size, fire perimeter shape, and growth) were assessed. The data were also utilized to evaluate a popular theoretical model developed by Albini (1979) for predicting the maximum spotting distance for single and group tree torching. The results indicated a significant positive relationship between the maximum observed spot fire distance and an interaction between fire growth and wind speed. Significant negative relationships between maximum spotting distance and fire perimeter shape, canopy height, and terrain steepness were also discovered. The evaluation of Albiniâ s (1979) model suggested that selecting a high estimate of potential wind speed was important in order to minimize the likelihood of under-predicting maximum spotting distance.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author