Homogeneous Air Research Articles

Electrical impedance tomography to evaluate air distribution prior to extubation in very-low-birth-weight infants: a feasibility study

OBJECTIVES:Nasal continuous positive airway pressure is used as a standard of care after extubation in very-low-birth-weight infants. A pressure of 5 cmH2O is usually applied regardless of individual differences in lung compliance. Current methods for evaluation of lung compliance and air distribution in the lungs are thus imprecise for preterm infants. This study used electrical impedance tomography to determine the feasibility of evaluating the positive end-expiratory pressure level associated with a more homogeneous air distribution within the lungs before extubation.METHODS:Ventilation homogeneity was defined by electrical impedance tomography as the ratio of ventilation between dependent and non-dependent lung areas. The best ventilation homogeneity was achieved when this ratio was equal to 1. Just before extubation, decremental expiratory pressure levels were applied (8, 7, 6 and 5 cmH20; 3 minutes each step), and the pressure that determined the best ventilation homogeneity was defined as the best positive end-expiratory pressure.RESULTS:The best positive end-expiratory pressure value was 6.3±1.1 cmH20, and the mean continuous positive airway pressure applied after extubation was 5.2±0.4 cmH20 (p = 0.002). The extubation failure rate was 21.4%. X-Ray and blood gases after extubation were also checked.CONCLUSION:This study demonstrates that electrical impedance tomography can be safely and successfully used in patients ready for extubation to suggest the best ventilation homogeneity, which is influenced by the level of expiratory pressure applied. In this feasibility study, the best lung compliance was found with pressure levels higher than the continuous positive airway pressure levels that are usually applied for routine extubation.

Assessing air quality with regards to its effect on human health in the European Union through air quality indices

This paper focuses on the assessment of the air quality with regards to its effects on human health at 14 monitoring stations in 8 EU Countries through two different Air Quality Index methodologies. Specifically, hourly values of SO2, CO, NO2, O3 and PM10 are used to calculate the PI and API indices. The analysis reveals that, although CO and SO2 do not form a public health hazard at the sites studied, in terms of NO2 the percentages of “low pollution” days are more than 65%, while a number of stations present significant percentages of “moderate pollution” (up to 33%) and “unhealthy conditions for sensitive groups” (up to 1.5%). Regarding PM10, “moderate pollution” days reach 36.5%, whereas another 0.5% of the days are found to be “unhealthy for sensitive groups” in some areas. It is also found that when examining the combined effect of all 5 above-mentioned pollutants, roughly 50–75% of the days present moderate risk of increased mortality, whereas in some areas high risk is observed in more than 35% of the days. Furthermore, some areas present significant percentages (up to 9%) of days with very high risk of increased mortality (i.e. more than 15.3%). Finally, it is found that the CO and SO2 contribution to the risk of increased total mortality is relatively low at all stations, while the O3 and NO2 contributions are quite variable (13–52% and 11–50%, respectively) depending mainly on the type of station. On the other hand, the PM10 contribution is rather uniform at all stations (approximately 30%), suggesting that PM10 forms a homogeneous air quality stressor in the EU.

A Data Mining Approach for Sharpening Thermal Satellite Imagery over Land

Thermal infrared (TIR) imagery is normally acquired at coarser pixel resolution than that of shortwave sensors on the same satellite platform and often the TIR resolution is not suitable for monitoring crop conditions of individual fields or the impacts of land cover changes that are at significantly finer spatial scales. Consequently, thermal sharpening techniques have been developed to sharpen TIR imagery to shortwave band pixel resolutions, which are often fine enough for field-scale applications. A classic thermal sharpening technique, TsHARP, uses a relationship between land surface temperature (LST) and Normalized Difference Vegetation Index (NDVI) developed empirically at the TIR pixel resolution and applied at the NDVI pixel resolution. However, recent studies show that unique relationships between temperature and NDVI may only exist for a limited class of landscapes, with mostly green vegetation and homogeneous air and soil conditions. To extend application of thermal sharpening to more complex conditions, a new data mining sharpener (DMS) technique is developed. The DMS approach builds regression trees between TIR band brightness temperatures and shortwave spectral reflectances based on intrinsic sample characteristics. A comparison of sharpening techniques applied over a rainfed agricultural area in central Iowa, an irrigated agricultural region in the Texas High Plains, and a heterogeneous naturally vegetated landscape in Alaska indicates that the DMS outperformed TsHARP in all cases. The artificial box-like patterns in LST generated by the TsHARP approach are greatly reduced using the DMS scheme, especially for areas containing irrigated crops, water bodies, thin clouds or terrain. While the DMS technique can provide fine resolution TIR imagery, there are limits to the sharpening ratios that can be reasonably implemented. Consequently, sharpening techniques cannot replace actual thermal band imagery at fine resolutions or missions that provide high quality thermal band imagery at high temporal and spatial resolution critical for many agricultural, land use and water resource management applications.

Open-air direct current plasma jet: Scaling up, uniformity, and cellular control

Atmospheric-pressure plasma jets are commonly used in many fields from medicine to nanotechnology, yet the issue of scaling the discharges up to larger areas without compromising the plasma uniformity remains a major challenge. In this paper, we demonstrate a homogenous cold air plasma glow with a large cross-section generated by a direct current power supply. There is no risk of glow-to-arc transitions, and the plasma glow appears uniform regardless of the gap between the nozzle and the surface being processed. Detailed studies show that both the position of the quartz tube and the gas flow rate can be used to control the plasma properties. Further investigation indicates that the residual charges trapped on the inner surface of the quartz tube may be responsible for the generation of the air plasma plume with a large cross-section. The spatially resolved optical emission spectroscopy reveals that the air plasma plume is uniform as it propagates out of the nozzle. The remarkable improvement of the plasma uniformity is used to improve the bio-compatibility of a glass coverslip over a reasonably large area. This improvement is demonstrated by a much more uniform and effective attachment and proliferation of human embryonic kidney 293 (HEK 293) cells on the plasma-treated surface.

Improvement of air distribution in a fixed-bed dryer using computational fluid dynamics

Uneven air distribution is a major problem in the performance of batch dryers. Zones receiving a higher airflow rate dry faster, and this heterogeneity reduces efficiency by increasing energy consumption and drying time. A simple box dryer with 36 boxes placed over the plenum chamber was built and tested and computational fluid dynamics was used to simulate its air distribution. Simulations showed that this configuration produced a poor air distribution. Trying to overcome this problem, simulations were conducted with a modified design consisting of a wide inlet into the plenum. Results showed an almost perfectly uniform air distribution. This was considered satisfactory for further study, which consisted of finding a suitable transition between the small cross section of the air ducts and the wide entrance to the plenum. It was seen, from theory and flow simulations, that diffusers need to be prohibitively long to function properly. However, short, wide-angle diffusers can be equipped with air guides and perforated plates to remain effective. Drying trials with woodchips were conducted for the original and modified dryer configurations, during which the drying course and airflow of each box were measured. Results for the original configuration showed, like the simulations, that there was a wide variation in airflow among the boxes, and also the expected wide differences in drying rate. A very significant correlation between these two variables was found. The modified version resulted in much more homogenous air distribution and drying rates and therefore represents a viable approach to improve dryer performance.

Lack of Association between Air Pollutant Exposure and Short-Term Risk of Ischaemic Stroke in Lyon, France

Some observational and experimental studies have suggested a short-term relationship between air pollutants and ischaemic stroke; however, the results conflict. The objective of this study was to investigate the association between particulate matter less than 2·5 and 10 microns in aerodynamic diameter, nitrogen dioxide, sulphur dioxide and ozone, and short-term risk of ischaemic stroke in Lyon, France. The AVC69 study was a multicenter cohort study in which all consecutive adult patients admitted to one of the emergency or neurological departments of the Rhône area for suspicion of stroke were included during a seven-month period. Only patients with ischaemic stroke living within the study area, composed of Lyon and 18 neighbouring communities with homogenous air pollutants exposure, formed the basis of our study. We adopted a time-stratified case-crossover design to analyse the short-term effect (up to two-days) of air pollutants on ischaemic stroke incidence. Models were adjusted for temperature, variation of atmospheric pressure, minimal relative humidity, influenza epidemics, pollen count, and holidays. Stratified analyses by gender and class age were performed. Different lag times were analysed. 376 patients were included. Mean age was 76·6 years (± 13·7). 53·7% were women. No association was observed between air pollutants and short-term risk of ischaemic stroke after adjustment for main confounding factors. Results remained unchanged whatever the gender or age. These results suggest a lack of association between air pollutant exposure and short-term risk of ischaemic stroke in a French urban area.

Electromagnetic Resonances of a Straight Wire on an Earth-Air Interface

Using a variational method, we recently determined an electromagnetic “signature” for characterizing a straight wire in free space. The signature consists of the first five resonant frequencies and their widths, more compactly expressed as the first five complex-valued resonant frequencies. Here we apply the variational method to the much more complicated case of determining the same signature for a straight wire or wire pair on a flat interface between a homogeneous earth and air. To calculate the resonances we obtain an integral equation for the current on a wire on the interface between two dielectric media. Complex-valued resonant frequencies are defined as those for which the homogeneous integral equation for the current in an equivalent thin strip on the interface has non-zero solutions. The variational method extracts good approximations to these complex-valued resonant frequencies, without having to solve the integral equation. A table of resonances is given for the case of a relative dielectric constant of the earth equal to 4 and for three values of the ratio of wire radius to wire half-length.

Hole-assisted dual concentric core fiber with ultralarge negative dispersion coefficient of −13,200 ps/nm/km

We propose a dual concentric core fiber (DCCF) with six homogeneous air holes, designed to realize a large negative dispersion coefficient. We clarify numerically that the dispersion property of the proposed DCCF can be controlled flexibly by adjusting the air-hole structure, and we realize the largest reported negative dispersion of -13,200 ps/nm/km experimentally.

Is Stokes number an appropriate indicator for turbulence modulation by particles of Taylor-length-scale size?

It has been established both numerically and experimentally [see, e.g., A. Ferrante and S. E. Elghobashi, “On the physical mechanisms of two-way coupling in particle-laden isotropic turbulence,” Phys. Fluids 15, 315 (2003); T. S. Yang and S. S. Shy, “Two-way interaction between solid particles and homogeneous air turbulence: Particle settling rate and turbulence modification measurements,” J. Fluid Mech. 526, 171 (2005)] that the Stokes number, τp/τk, can be used as an indicator to determine the extent to which small particles (dp<η) modify the turbulence structure for fixed values of their volume fraction and mass fraction, where dp, τp, η, and τk are the particle diameter, the response time, the Kolmogorov length, and time scales, respectively. The objective of the present study is to investigate whether the Stokes number can also be used as an indicator for the modulation of turbulence by particles of the Taylor-length-scale size, i.e., dp∼λ⪢η. We employ direct numerical simulation with an immersed boundary method to fully resolve the flow around 4800 freely moving particles of Taylor-length-scale size (dp∼λ) in decaying isotropic turbulence with initial Taylor-microscale Reynolds number, Reλ=110, using a computational mesh of 5123 grid points. Our results show that particles whose diameter dp∼λ⪢η and have identical τp (but different dp and/or ρp) can have different effects on isotropic turbulence. Thus, τp/τk should not be used as an indicator of turbulence modulation by these “large” particles.

Principal Limitations in Homogeneous Gas Phase Chemistry in Non-Thermal Plasmas

This study highlights the oxidation of H2, CH4, and HCl present in the range of some volume percent in a homogeneous O2 or air phase in a flow through glass barrier discharge reactor. The oxidation of all three compounds is highly exothermic and exergonic at ambient temperature and proceeds at sufficiently high temperatures as radical chain reaction. The conversion of each compound was below 10% in a non-thermal oxygen plasma under various reaction conditions. Increasing concentrations of H2 and CH4 above the lower explosion limit did not lead to higher conversion degrees. It is assumed that only initial radical formation by electron impact dissociation and exothermic steps within the chain process run in a sufficiently fast manner at ambient temperature. For endothermic steps within the radical chain, the necessary activation energy is not available and the chain reaction aborts, most likely, after formation of peroxyl (hydro- or methyl-peroxyl) radicals.

Nonlinear propagation of spark-generated N-waves in air: Modeling and measurements using acoustical and optical methods

The propagation of nonlinear spherically diverging N-waves in homogeneous air is studied experimentally and theoretically. A spark source is used to generate high amplitude (1.4 kPa) short duration (40 μs) N-waves; acoustic measurements are performed using microphones (3 mm diameter, 150 kHz bandwidth). Numerical modeling with the generalized Burgers equation is used to reveal the relative effects of acoustic nonlinearity, thermoviscous absorption, and oxygen and nitrogen relaxation on the wave propagation. The results of modeling are in a good agreement with the measurements in respect to the wave amplitude and duration. However, the measured rise time of the front shock is ten times longer than the calculated one, which is attributed to the limited bandwidth of the microphone. To better resolve the shock thickness, a focused shadowgraphy technique is used. The recorded optical shadowgrams are compared with shadow patterns predicted by geometrical optics and scalar diffraction model of light propagation. It is shown that the geometrical optics approximation results in overestimation of the shock rise time, while the diffraction model allows to correctly resolve the shock width. A combination of microphone measurements and focused optical shadowgraphy is therefore a reliable way of studying evolution of spark-generated shock waves in air.

On vortex combustion

Known and previously unknown experimental observations of transfer, exchange, and combustion in vortex flames in high-velocity flows are analyzed. The evolution of our concepts of these processes with discoveries of new facts is traced. It is demonstrated that four regimes of homogeneous combustion of the vortex ball in a high-velocity flow are possible: deterministic (coherent), resonant (vibrational), stochastic, and pseudodetonation regimes. Pseudo-detonation combustion is understood as simultaneous homogeneous microturbulent frontal combustion of all layers of the vortex ball with the radius expansion rate several times greater than the entraining flow velocity. The intensity of injection of a homogeneous mixture or air into the burning vortex layer is found to be identical to the notion of intensity of homogeneous or diffusion combustion. It is demonstrated that the classical notions and the Shchelkin-Shchetinkov relations for burning surfaces and volumes are applicable, but only for certain phases and local zones of the burning vortex ball.

Mechanistic and kinetic insights into the wet air oxidation of phenol with oxygen (CWAO) by homogeneous and heterogeneous transition-metal catalysts

The activity–selectivity pattern of homogeneous (Cu2+, Fe3+, Mn2+) and ceria-supported (CuCeOx, MnCeOx) transition-metal catalysts in the wet air oxidation of phenol (CWAO) has been probed using a stirred batch reactor with continuous oxygen feeding (T, 150°C; PO2, 0.9Mpa). Both non-catalytic and catalytic homogeneous wet air oxidations proceed via an unselective autocatalytic free-radical path leading mostly to refractory C1–C2 acids, while a Langmuir–Hinshelwood (L–H) mechanism accounts for the superior CWAO performance of the MnCeOx system. A thorough kinetic analysis of the studied systems on the basis of homogeneous autocatalytic free-radical and heterogeneous surface L–H reaction paths has been addressed. The kinetic constants of the various reaction steps show that the MnCeOx system prompts a fast adsorption of phenol with the consequent abatement of TOC, though a slow oxidation rate determines the buildup of carbonaceous deposits on the catalyst surface. Lower oxidation strength and extensive leaching definitively argue against Cu-based catalysts for the CWAO process.

Laminar Burning Velocities of 2,5-Dimethylfuran Compared with Ethanol and Gasoline

The 2,5-dimethylfuran (DMF) has attracted renewed global interest since its improved production methods were published in Nature and Science in 2007. Its high energy density makes it a promising biofuel and a possible alternative to gasoline. Consequently, a series of studies, led by the University of Birmingham, aims to assess the potential of DMF as an automotive energy carrier. These studies will include an analysis of the spray properties, the laminar flame characteristics, the engine performance, and the subsequent emissions. This paper examines the laminar flame characteristics from a quiescent homogeneous air−fuel mixture. The experiments were conducted using a constant volume vessel and were recorded by high speed schlieren visualization. By measurement of the flame growth following ignition, the laminar flame speed was determined. The calculation of flame stretch yielded the Markstein lengths and the laminar burning velocities. This paper presents the results of DMF combustion for a range of equi...

Climatology of winter transition days for the contiguous USA, 1951–2007

In middle and high latitudes, climate change could impact the frequency and characteristics of frontal passages. Although transitions between air masses are significant features of the general circulation that influence human activities and other surface processes, they are much more difficult to objectively identify than single variables like temperature or even extreme events like fires, droughts, and floods. The recently developed Spatial Synoptic Classification (SSC) provides a fairly objective means of identifying frontal passages. In this research, we determine the specific meteorological patterns represented by the SSC’s Transition category, a “catch-all” group that attempts to identify those days that cannot be characterized as a single, homogeneous air mass type. The result is a detailed transition climatology for the continental USA. We identify four subtypes of the Transition category based on intra-day sea level pressure change and dew point temperature change. Across the contiguous USA, most transition days are identified as cold fronts and warm fronts during the winter season. Among the two less common subtypes, transition days in which the dew point temperature and pressure both rise are more frequently observed across the western states, and days in which both variables fall are more frequently observed in coastal regions. The relative frequencies of wintertime warm and cold fronts have changed over the period 1951–2007. Relative cold front frequency has significantly increased in the Northeast and Midwest regions, and warm front frequencies have declined in the Midwest, Rocky Mountain, and Pacific Northwest regions. The overall shift toward cold fronts and away from warm fronts across the northern USA arises from a combination of an enhanced ridge over western North America and a northward shift of storm tracks throughout the mid-latitudes. These results are consistent with projections of climate change associated with elevated greenhouse gas concentrations.

The heterogeneity and mechanical response of hot mix asphalt laboratory specimens

Many pavement distresses, including rutting, fatigue cracking and low-temperature cracking, are influenced by the air void distribution in hot mix asphalt (HMA). This study investigated the influence of the type of Superpave gyratory compactors (SGCs) and specimen preparation/geometry on air void distribution and its manifestation on mechanical response. X-ray computed tomography along with image analysis techniques was used to capture and characterise the air void distribution within HMA microstructure. The mechanical response was characterised using the dynamic modulus test and static creep test. An index called heterogeneity index was developed to quantify the air void distribution in vertical and lateral directions. The study revealed that laboratory compacted specimens are heterogeneous and there is a need to prepare laboratory specimens with homogeneous air void distribution for accurate interpretation of results. In addition, the study revealed that the type of the SGC used influenced the dynamic modulus of compacted HMA specimens.

Pulmonary congenital cystic disease in adults. Spiral computed tomography findings with pathologic correlation and management

The aim of this study was to assess the computed tomography (CT) features of intrapulmonary congenital cystic diseases in adults and to correlate the imaging features with the pathological findings, with emphasis on the oncogenic potential of the lesions. We retrospectively reviewed the CT scans in three institutions from August 1996 to December 2008, of nine patients (six men, three women; mean age 48.6 years; range 26-75 years) who had histological diagnosis of pulmonary cystic disease after surgery. Six patients had a diagnosis of intrapulmonary bronchogenic cyst (IBC), and three had a type-I cystic adenomatoid malformation (CAM). In one case, intralobar sequestration (ILS) was associated with type-I CAM. Three patients were symptomatic and six were asymptomatic. On CT scans, IBCs showed homogeneous fluid attenuation (n=2), air-fluid level (n=2), air attenuation (n=1) or soft-tissue attenuation (n=1). The surrounding lung tissue showed areas of band-like linear attenuation in three IBCs, atelectasia in two and mucocele-like areas in one. On CT, type-I CAM appeared as a unilocular cystic lesion with air-fluid level (n=1) or air content (n=1). Both cases had thin walls surrounded by normal lung parenchyma. ILS appeared as a fluid-filled cyst with afferent and efferent vessels. Of the six IBCs, one occurred in the upper right lobe, two in the middle lobe and three in the lower right lobe. Of the three type-I CAMs, one was in the upper left lobe and one in the middle lobe. The type-I CAM associated with ILS was located in the left lower lobe. The similar CT patterns preclude differentiation between IBC and type-I CAM. Surgical resection of all intrapulmonary cystic lesions detected in adults is mandatory because type-I CAM is a precursor of mucinous bronchioloalveolar carcinoma.

10.1007/s11454-008-3005-y

The microstructure of a current channel is experimentally found under the conditions when homogeneous air gaps are subjected to nanosecond voltage pulses in an electric field insufficient for streamer generation. As a possible mechanism of microstructure formation, instability of the ionization process at the avalanche stage leading to the formation of a self-similar spatial structure is considered. The fractal dimension of this structure is determined. In inhomogeneous gaps, the avalanche is shown to be unstable as well. The energy benefit of structuring is considered. It is demonstrated that the microstructure of streamer discharges in homogeneous gaps can also be treated in terms of the model suggested.

Optimization of large homogeneous air Cherenkov arrays and application to the design of a 1–100 TeV γ-ray observatory

At the time large air Cherenkov arrays are being discussed for future γ-ray observatories, we review the relationship between the targeted capabilities and the main design parameters taking into account construction costs. As an example application, we describe a telescope array optimized for observations between 1 TeV and a few 100 TeV and use detailed simulations to estimate its performances in comparison to science objectives.

Pressure rise generated by the expansion of a local gas volume in a closed vessel

Experiments were conducted to determine the pressure rise that results from either the combustion of a localized gas volume or the expansion of a pressurized gas volume adjacent to an inert gas in a closed vessel. The experiments consisted of either pressurized air or the combustion of stoichiometric and fuel-lean hydrogen–air mixtures compressing an inert gas. The pressure rise in the inert gas was measured as a function of either the volume fraction or the initial pressure of the expanding gas. Helium, nitrogen, air and carbon dioxide were tested to explore the effect of inert gas heat capacity on the pressure rise. The final pressure of the inert gas increased with the volume fraction and initial pressure of the expanding gas, and was influenced to a lesser extent by the heat capacity of the inert gas. A model was assessed using the experimental data, and the theoretical results were consistent with the observed trends. This model and other published models were assessed and compared using prior data for localized gas combustion surrounded by an inert gas and the partial combustion of homogeneous methane–air mixtures.