High Vapor Concentrations Research Articles

Examining the role of density-driven transport on chlorinated vapor intrusion

This study investigates the influence of density-driven transport on chlorinated vapor intrusion through modeling and experiments. Density-driven transport involves downward vapor advection, potentially reducing vapor intrusion into buildings. A 1-D steady-state numerical model was developed using COMSOL Multiphysics, considering upward diffusion and downward density-driven advection in the subsoil and in the granular fill layer beneath building's foundations. Source vapor concentration and granular fill layer permeability emerged as the key factors affecting density-driven transport. Regardless of building characteristics, for permeabilities in the granular fill layer exceeding 10−7 m2, density-driven transport is expected to become relevant at vapor concentrations of 1 mg m−3, while for lower soil permeabilities (10−8-10−10 m2), density-driven transport impact is expected for vapor concentrations exceeding 1 g m−3. The results of laboratory column trichloroethylene (TCE) diffusion tests through sand and gravel supported these findings, showing a vertical stratification of TCE vapor concentrations consistent with the model. The trends expected by modeling also align with the findings of different field studies, where the source to building attenuation factors (AF) were found to decrease with increasing source vapor concentration. These outcomes highlight that the common approach adopted for vapor intrusion screening from soil gas data based on default AF values independent of source vapor concentration, may potentially lead to an overestimation of indoor concentrations in the presence of high vapor concentrations and highly permeable soils. Given the use of permeable granular fill layers in building construction, this study underscores the importance of accounting for density-driven transport to improve the accuracy of vapor intrusion risk assessment.

Study on Intermittent Microwave Convective Drying Characteristics and Flow Field of Porous Media Food

Numerical simulations were carried out for moist, porous media, intermittent microwave convective drying (IMCD) using a multiphase flow model in porous media subdomains coupled with a forced-convection heat-transfer model in an external hot air subdomain. The models were solved by using COMSOL Multiphysics was applied at the pulse ratio (PR) of 3. Based on drying characteristics of porous media and the distribution of the evaporation interface, IMCD was compared with convection drying (CD). Drying uniformity K, velocity difference, temperature difference, and humidity difference were introduced to evaluate the performance of three models with different inlets and outlet wall curvature. The numerical results show that as the moisture content of slices was reduced to 3 kg/kg, the drying rate in IMCD was 0.0166–0.02 m/s higher than that in CD, and the total drying time of the former was 81.35% shorter than that of the latter. In the late drying stage of IMCD, the core of the sample still had a high vapor concentration and temperature, which led to the evaporation interface remaining on the surface. The vapor evaporated from the slices can diffuse rapidly to the outside, which is why IMCD is superior to traditional convection drying. Through the comprehensive analysis of the models with different inlet and outlet wall curvatures, the drying uniformity K of the type III was the highest, reaching 89.28%. Optimizing flow-field distribution can improve uniform of airflow distribution.

Quantification of Interdependent Dynamics during Laser Additive Manufacturing Using X-Ray Imaging Informed Multi-Physics and Multiphase Simulation.

Laser powder bed fusion (LPBF) can produce high-value metallic components for many industries; however, its adoption for safety-critical applications is hampered by the presence of imperfections. The interdependency between imperfections and processing parameters remains unclear. Here, the evolution of porosity and humps during LPBF using X-ray and electron imaging, and a high-fidelity multiphase process simulation, is quantified. The pore and keyhole formation mechanisms are driven by the mixing of high temperatures and high metal vapor concentrations in the keyhole is revealed. The irregular pores are formed via keyhole collapse, pore coalescence, and then pore entrapment by the solidification front. The mixing of the fast-moving vapor plume and molten pool induces a Kelvin-Helmholtz instability at the melt track surface, forming humps. X-ray imaging and a high-fidelity model are used to quantify the pore evolution kinetics, pore size distribution, waviness, surface roughness, and melt volume under single layer conditions. This work provides insights on key criteria that govern the formation of imperfections in LPBF and suggest ways to improve process reliability.

Ultrathin MgO Nanosheets Fabricated by Thermal Evaporation Method in Air at Atmospheric Pressure

Ultrathin MgO nanosheets were successfully synthesized by thermal evaporation of a mixture of Mg and graphite powders as the source material. The synthesis was performed at 1000 oC in air. Scanning electron microscopy showed that the two-dimensional MgO nanosheets had widths of several micrometers and the thickness of less than 20 nm. X-ray diffraction analysis revealed that the MgO nanosheets had a cubic crystal structure and high purity. Zero-dimensional MgO nanocubes were formed at temperatures below 1000 oC and one-dimensional MgO nanowires were grown at a temperature higher than 1000 oC. As the synthesis temperature increased, the morphology of the Mg nanocrystals changed from cube to sheet and then wire. The experimental results suggested that the difference in Mg vapor concentration could be responsible for the morphological change in the MgO nanocrystals. When Mg vapor concentration was low, MgO nanocrystals were grown with a cubic shape. A relatively high concentration of Mg vapor led to the growth of sheet-like MgO nanocrystals. A very high Mg vapor concentration favored the growth of MgO nanowires. The growth mechanism is discussed based on the Mg vapor concentration and the crystal structures of Mg and MgO. Visible emissions, which were attributed to lattice defects such as oxygen vacancies, were observed in the cathodoluminescence spectra of the MgO nanocrystals.

Interpenetrating network based polymeric sensors with enhanced specificity, sensitivity, and reusability

Cholesteric liquid crystals (CLCs) are widely used as optical sensors against volatile organic compounds (VOCs), whose sensitivity can be increased with the help of strain. However, the specificity and reusability of these structures should be increased to achieve their successful integration into useful applications. For this purpose, we developed an interpenetrating polymeric network (IPN) based on poly(RM257) structure of CLC-templated polymeric films and polydimethylsiloxane (PDMS). Contrary to the generic CLC-hosted IPN sensors that use CLC structures to optically report the changes in the polymeric matrix, we employed the collaborative responsiveness of the constituents of the IPN (both CLC-templated and penetrated polymeric structures) to improve the sensor performance. The elastic swelling of PDMS structure increased the durability and reusability of the poly(RM257) polymeric sensor upon exposure to even high VOC vapor concentrations up to their saturation. The sensor tests of both poly(RM257) and IPN films were carried out against four different VOC vapors namely toluene, hexane, acetone, ethanol and also water vapor. The results showed that the synergistic swelling of PDMS and poly(RM257) polymeric structures against vapors directly affected the sensor performance in terms of earned specificity and enhanced sensitivity. Moreover, the real-time experiments showed that the measurement accuracy, response, and recovery time of the polymeric sensors were also improved significantly by the development of IPN as compared to their non-IPN counterparts. This study shows a promise towards further engineering of sensors against various applications through detailed design of the constituent polymers against targeted species.

Investigations on the impact of phase change on single plume flash boiling radial expansion and drop-sizing characteristics

• Flash boiling plume expansion is primarily caused by the expansion of the vaporized fuel. • The volume fraction of the flash-boiling-vaporized fuel dominates the spray morphology. • The mass fraction of the flash-boiling-vaporized fuel shows a major impact on the spray SMD. • Flash boiling spray radial properties are firstly governed by near-field expansion, then far-field evaporation. • The measured SMD of flash boiling spray increases with axial position due to the strong evaporation during spray penetrating. Flash boiling atomization has been considered a promising approach to enhance spray atomization. Increasing the fuel temperature or reducing the ambient pressure can promote the phase-change of the spray, and boiling can be achieved during this process, so that spray atomization can be enhanced. However, the strong evaporative feature of flash boiling atomization induced a high vapor concentration in the plume thus changes spray characteristics. To understand the impacts of the vapor phase on spray properties in the aspects of radial expansion and drop sizing, this investigation is carried out based on a customized single-hole fuel injector with practical fuel injection settings. High-speed backlit imaging and Phase Doppler Interferometry are used to quantify the characteristics of flash boiling sprays under various boundary conditions. Macroscopic and microscopic spray morphologies were captured and it was shown that the geometry of the flash boiling spray plume was significantly impacted by the expansion of the compressed vapor phase. The evaporation of the droplets was studied by the single droplet evaporation model and the results were compared against drop sizing measurements. It was found that the evaporation and elimination of liquid droplets might affect the statistical interpretation of the drop sizing results, which is a notable feature for flash boiling sprays.

Oxy-steam combustion of torrefied biomass under high water vapour concentrations

Oxy-steam combustion of torrefied biomass under high water vapour concentrations

Studies of critical heat fluxes in small diameter channels

The paper presents the results of experimental studies of critical heat flows in vertical small diameter channels, when the coolant moves from bottom to top, which were carried out in the Obninsk branch of MEPhI in the 1970s of the last century but have not become widespread due to the lack of demand for their practical use. Nowadays, the interest in such works is manifested, first of all, in the development of compact plants and devices, particularly in nuclear power engineering. As a coolant, water, Freon-12 and 96% ethyl alcohol were used. High velocities of underheated liquid at high heat fluxes on the channel wall lead to the so-called “fast crisis” of heat transfer. In this case, the magnitude of the heat flux depends mainly on the parameters of the coolant flow in the wall zone rather than the flow core. The “slow crisis” is mainly observed at high vapor concentrations, relatively low mass flow rates and in an annular-dispersed flow. The value of the critical heat flow in this case depends mainly on the flow parameters in the core, which are probably close to the average coolant flow parameters. The conditions in the near-wall region are also largely determined by the flow in the core. High heat transfer coefficients in a flow moving at high speed usually result in a much smaller and slower rise in the wall temperature. Sometimes a DNB heat flux can occur bypassing the boiling process. In the core of a VVER-type reactor operating in its nominal mode, surface boiling is present in a number of fuel rods. Probably, surface boiling will also be present in transportable and small-size nuclear power plants. Therefore, an important task is to conduct relevant research in this area.

Characteristics of Aerosol Formation and Emissions During Corn Stalk Pyrolysis

The inevitable emission of aerosols during pyrolysis can negatively affect the downstream process and even pollute the environment. In this work, the characteristics of aerosols were investigated during corn stalk pyrolysis at 400–900 °C. The effects of other operation conditions on the aerosol emissions were also probed with online and offline instruments. Results show the yield of aerosol presents a regular change with temperature in a wide range ratio of 3.4–8.7 wt.%. The aerosol size distribution reveals a unimodal form mainly in the 1.1–2.1 μm accumulation range and the maximum emission achieved is about 35 mg/g for SR and SP at 500 °C. Nevertheless, SL gives about 34 mg/g at 600 °C. High temperature promotes the decomposition of polymers into partciles with small diameter (less than PM1.0). The microtopography of aerosol presents spherical droplets, elongated-like liquid and solid particles that form heterogenous or homogeneous aggregations, that also happen on account of collisions. Aerosols contain mostly organic matter, a small amount of salt and over 50% of volatile organic carbon molecules (VOCs) in the total organic carbon (OC). Proper gas flow, high vapor concentration and longer path way boost the yield of bio-oil and reduce the emission of aerosols. The direct contact is beneficial for adequate extraction, but also causes additional solvent emissions.

Collisions of water drops in a gas-vapor environment at high temperatures and vapor concentrations

The study of the characteristics of secondary droplet atomization, leading to formation of an aerosol cloud of polydisperse child droplets appears to be promising. It is topical to assess the influence of properties of liquid and gas medium on the position of transition boundaries between the regimes of drop collisions and characteristics of the formed child droplets. This article presents the experimental results for the characteristics of drop collisions at various temperatures of the liquid and gas-vapor mixture and water vapor concentration in the latter with the aim of developing the prospective heat and mass transfer gas-vapor technologies. For this purpose, we have created the experimental set-up that allows varying the relative humidity of gas-vapor mixture in the area of drop collisions from 20-100%, its temperature from 20-100?C and the temperature of the liquid from 20-90?C. The test fluid is water. The collisions are recorded by a high speed video camera. The consequences of collision and the boundaries between them on the regime maps are determined in accordance using the approach, distinguishing: bounce, coalescence, separation, and disruption.

Effects of Mo vapor concentration on the morphology of vertically standing MoS2 nanoflakes

Vertically standing MoS2 nanoflakes are favourable in applications such as energy storage devices, hydrogen evolution reactions, and gas sensors due to their large surface area and high density of exposed edges. In this work, we report the effect of Mo vapor concentration on the morphology of vertical MoS2 nanoflakes prepared by chemical vapor deposition at atmospheric pressure. A series of MoS2 samples were grown under different Mo vapor concentrations by varying the separation distance (x) between the MoO3 source and the substrate. Field emission scanning electron microscopy showed the sample grown at x = 1 cm had a high density of vertical flakes (7 vertical flakes µm−2) with an average flake length of ~770 nm and thickness of ~10 nm. As x increased to 4 cm, the average flake length was reduced to ~150 nm while the flake orientation changed from vertical to lateral. That is, high Mo vapor concentration favours the formation of large and vertical MoS2 nanoflakes. However, oversupply of Mo vapor results in significantly thicker flakes. Raman spectra of all samples showed two main peaks at 380 and 407 cm−1 that correspond to the E12g and A1g vibrational peaks of MoS2. As x decreased from 4 to 1, the peak intensity ratio (E12g/A1g) reduced from 0.58 to 0.42, suggesting greater dominance of vertical flakes at low x. X-ray diffraction data showed a prominent peak at 14.4°, which corresponded to the (002) diffraction peak of 2H MoS2. Transmission electron microscopy verified the flakes consist of eight layers with an interlayer spacing of 0.62 nm. Based on hydrogen evolution reaction measurements, samples with thin flakes have high catalytic activity. This work highlights the importance of optimizing Mo vapor concentration to obtain a high density of thin, large, and vertically standing MoS2 nanoflakes.

Stable compensation voltages in differential mobility spectra by separating neutral vapors from ions in sample flow

In small planar differential mobility spectrometers with ambient pressure ion sources, both ions and neutral vapors of sample commonly flow from the ion source directly into and through the analyzer region. Although simple and convenient both for fabrication and operation, product ions derived from sample in this design may form clusters with neutral unreacted sample or matrix constituents particularly at vapor levels of 50 ppm or greater. Uncontrolled and changing levels of such neutrals cause concentration dependent formation of ion clusters with variations in compensation voltages of DMS spectra. Stable compensation voltages were achieved for product ions by extracting ions into a counter flow of purified gas excluding neutral vapors from ppm to percent levels. An interface with electric fields and purified air was placed between ion source and analyzer region and operated with both on-axis and orthogonal extraction of ions. Effectiveness of removal of neutrals was determined using variations in peak position for protonated monomers and proton bound dimers of dimethylmethylphosphate, significantly avoiding complications of ionization chemistry with high vapor concentrations of methylene chloride, iso-propanol, water, and methanol. In-silico modeling using COMSOL and SIMION clarified the flow of ions and gases in two modes of operation. Findings anticipate the development of multiple stages of DMS analyzers where vapors may be intentionally added as modifiers in one stage and removed before entering a subsequent stage.

A calculation method for the numerical simulation of oil products evaporation and vapor diffusion in an internal floating-roof tank under the unsteady operating state

A calculation method was established for numerically simulating the evaporation of oil products and vapor diffusion from the annular rim gap of the floating deck in an internal floating-roof tank (IFRT) under the unsteady operating state. The proposed method is based on the “thin film” theory, Stefan diffusion, and the realizable k-ε turbulence model. The accuracy of the calculation method was verified by wind tunnel experiments. Considering various operating conditions, the oil evaporation and vapor diffusion were investigated, and the mass transfer mechanism in the gas space was revealed. The results show that the vapor concentration isosurface in the gas space exhibits a “wave-shaped” distribution, and the vapor concentration can easily accumulate in the windward side of the gas space under the influence of a wind speed. In addition, the evaporation rate is closely related to the type of oil product, and the evaporation rate increases with increasing wind speed or floating deck height. The evaporation rate was initially in a fluctuating state and stabilized soon. Furthermore, when the width of the annular leaking rim gap was 0.2 m in a gasoline IFRT with a volume of 1000 m3, the vapor concentration in the most of the gas space was higher than the lower explosive limit; the region with a high vapor concentration will further expand with increasing wind speed. The calculation method and the numerical simulation results in this study can provide important theoretical support for reducing oil evaporation loss, controlling air pollution, and ensuring safe operation of IFRTs.

Thermal annealing to influence the vapor sensing behavior of co-continuous poly(lactic acid)/polystyrene/multiwalled carbon nanotube composites

With the main purpose of being used as vapor leakage detector, the volatile organic compound (VOC) vapor sensing properties of conductive polymer blend composites were studied. Poly(lactic acid)/polystyrene/multi-walled carbon nanotube (PLA/PS/MWCNT) based conductive polymer composites (CPCs) in which the polymer components exhibit different interactions with the vapors, were prepared by melt mixing. CPCs with a blend composition of 50/50 wt% resulted in the finest co-continuous structure and selective MWCNT localization in PLA. Therefore, these composites were selected for sensor tests. Thermal annealing was applied aiming to maintain the blend structure but improving the sensing reversibility of CPC sensors towards high vapor concentrations. Different sensing protocols were applied using acetone (good solvent for PS and PLA) and cyclohexane (good solvent for PS but poor solvent for PLA) vapors. Increasing acetone vapor concentration resulted in increased relative resistance change (Rrel) of CPCs. Saturated cyclohexane vapor resulted in lower response than nearly saturated acetone vapor. The thermal annealing at 150 °C did not change the blend morphology but increased the PLA crystallinity, making the CPC sensors more resistant to vapor stimulation, resulting in lower Rrel but better reversibility after vapor exposure.

Lateral and Vertical MoSe2-MoS2 Heterostructures via Epitaxial Growth: Triggered by High-Temperature Annealing and Precursor Concentration.

Atomically thin transition-metal dichalcogenide (TMDC) heterostructures have attracted increasing attention because of their unprecedented potential in the fields of electronics and optoelectronics. However, selective growth of either lateral or vertical TMDC heterostructures remains challenging. Here, we report that lateral and vertical MoS2/MoSe2 epitaxial heterostructures can be successfully fabricated via a one-step growth strategy, which includes triggering by the concentration of sulfur precursor vapor and a high-temperature annealing process. Vertically stacked MoS2/MoSe2 heterostructures can be synthesized via control of the nucleation and growth kinetics, which is induced by high sulfur vapor concentration. The high-temperature annealing process results in the formation of fractured MoSe2 and in situ epitaxial growth of lateral MoSe2-MoS2 heterostructures. This study has revealed the importance of sulfur vapor concentration and high-temperature annealing processes in the controllable growth of MoSe2-MoS2 heterostructures, paving a new route for fabricating two-dimensional TMDC heterostructures.

Spray cooling system design and optimization for cooling performance enhancement of natural draft dry cooling tower in concentrated solar power plants

In concentrated solar power (CSP) plants built in dry and arid areas, natural draft dry cooling tower (NDDCT) are commonly employed to dissipate waste heat into the atmosphere. The cooling performance of NDDCT mainly depends on the induced air flow caused by the buoyancy effect. However, the high ambient temperature in summers reduce the cooling efficiency of dry cooling towers and cause significant power loss for CSP plants. To address this problem, spray cooling system utilizing water evaporation was developed to pre-cool the inlet hot air. Different designs of spray cooling systems were proposed and tested on a 20 m high experimental tower. Experimental data were collected to evaluate the performance of the spray cooling system. To our knowledge, this is the world's first attempt to practice spray cooling on a full-scale natural draft dry cooling tower. This study confirms the feasibility and effectiveness of employing spray cooling for cooling performance enhancement of NDDCT. With the goal of maximal cooling effect with least water consumption, the optimal design was proposed, which consists of 3 upward injections at the low level (Height = 2 m), 2 counterflow injections at the middle level (H = 3 m) and 3 counterflow injections at the high level (H = 4 m). The cooling capacity of NDDCT increases from 789 kW to 841.73 kW, as the result of an intensified natural convection. Moreover, in the spray zone, the presence of a low-temperature area is featured by high relative humidity (70%–80%). The intensified natural convection caused by pre-cooled air and the presence of high vapour concentration are attributed to spray evaporation, which confirms the necessity to introduce the spray cooling system.

Spectroscopic investigation of partial LTE assumption and plasma temperature field in pulsed MAG arcs

The study of gas–metal arc welding (GMAW) often relies on the hypothesis of local thermodynamical equilibrium (LTE). Few works are dedicated to its validation and none is focused on the case of pulsed metal active gas welding. This paper presents the result of a spectroscopic diagnostic performed on a pulsed welding arc with 82%Ar–18%CO2 shielding gas. High-speed imaging and electrical data recording are also performed. The excitation temperature is obtained with the Boltzmann plot method, while iron and argon line Stark broadening measurement is used to obtain the electron temperature and density. The iron mole fraction in the plasma is calculated by solving the conservation equations. Partial LTE hypothesis validity across the arc is discussed considering the agreement between the two temperatures, electron density and iron content. The results show supporting evidence for the main part of the plasma, along radial and axial directions. This can be associated with high iron vapour concentration, close to 100% near the consumable anode tip. Discrepancies occur only at the fringe of the arc, where the two temperatures differ by more than 2000 K. The temperature drop close to the arc axis observable for the upper part is weaker when compared with an inert gas welding operation.

Simulation of fine polydisperse particle condensational growth under an octadecane–nitrogen atmosphere

The evolution of particle size distribution (PSD) of fine polydisperse particles at high number concentrations (>105cm−3) was simulated through a combined model employing direct quadrature method of moments (DQMOM) with heat and mass transfer equations. The PSD was assumed to retain log-normal distribution during the heterogeneous condensation process. The model was first verified by exact solution and experimental data prior to investigating the influence of initial conditions on final PSD under an octadecane–nitrogen atmosphere. Low particle number concentrations and high vapor concentrations were beneficial to shift the PSD to larger particles having a narrower distribution. Additionally, vapor depletion has more influence on the final PSD than the heat release parameter for a number concentration of 106cm−3. This study may assist the design process of a gas–solid separating cyclone, to eliminate dust from high-temperature volatiles by pyrolysis of solid fuels.

Small Purge Method to Sample Vapor from Groundwater Monitoring Wells Screened Across the Water Table

AbstractGroundwater monitoring wells are present at most hydrocarbon release sites that are being assessed for cleanup. If screened across the vadose zone, these wells provide an opportunity to collect vapor samples that can be used in the evaluation of vapor movement and biodegradation processes occurring at such sites. This paper presents a low purge volume method (modified after that developed by the U.S. EPA) for sampling vapor from monitoring wells that is easy to implement and can provide an assessment of the soil gas total petroleum hydrocarbon (TPH) and O2 concentrations at the base of the vadose zone. As a result, the small purge method allows for sampling of vapor from monitoring wells to support petroleum vapor intrusion (PVI) risk assessment. The small purge volume method was field tested at the Hal's service station site in Green River, Utah. This site is well‐known for numerous soil gas measurements containing high O2 and high TPH vapor concentrations in the same samples which is inconsistent with well‐accepted biodegradation models for the vapor pathway. Using the low purge volume method, monitoring wells were sampled over, upgradient, and downgradient of the light nonaqueous phase liquid (LNAPL) footprint. Results from our testing at Hal's show that vapor from monitoring wells over LNAPL contained very low O2 and high TPH concentrations. In contrast, vapor from monitoring wells not over LNAPL contained high O2 and low TPH concentrations. The results of this study show that a low purge volume method is consistent with biodegradation models especially for sampling at sites where low permeability soils exist in and around a LNAPL source zone.

Fault detection and isolation of high temperature proton exchange membrane fuel cell stack under the influence of degradation

Abstract This study proposes a data-drive impedance-based methodology for fault detection and isolation of low and high cathode stoichiometry, high CO concentration in the anode gas, high methanol vapour concentrations in the anode gas and low anode stoichiometry, for high temperature PEM fuel cells. The fault detection and isolation algorithm is based on an artificial neural network classifier, which uses three extracted features as input. Two of the proposed features are based on angles in the impedance spectrum, and are therefore relative to specific points, and shown to be independent of degradation, contrary to other available feature extraction methods in the literature. The experimental data is based on a 35 day experiment, where 2010 unique electrochemical impedance spectroscopy measurements were recorded. The test of the algorithm resulted in a good detectability of the faults, except for high methanol vapour concentration in the anode gas fault, which was found to be difficult to distinguish from a normal operational data. The achieved accuracy for faults related to CO pollution, anode- and cathode stoichiometry is 100% success rate. Overall global accuracy on the test data is 94.6%.