Gas Transfer Rates Research Articles

Mathematical Modeling of Gas-phase Mass Transfer in Hydrous Materials for a Total Heat Exchange Ventilator

Total heat exchange ventilation systems are effective in achieving energy savings by reducing the ventilation load in buildings, while maintaining a certain amount of fresh outdoor air intake. As the system's elemental materials exchange both latent and sensible heat, hydrophilic chemical compounds may be exchanged simultaneously. Proper control of the exchange of hazardous chemicals and pollutants via these heat exchange elements is an important issue in the development of total heat exchange ventilation systems. In this respect, the development of a numerical model that facilitates repeated sensitivity analysis is important in the development of a new total heat exchanger that has high heat exchange efficiency and suppresses the exchange of polluting chemicals. This study proposes new hygrothermal and chemical compound transfer models for paper-based hydrous materials, which are the main components of total heat exchangers in indoor ventilation systems. Through a series of numerical analyses and experimental measurements, the prediction accuracy of the mathematical model was compared with experimental results for the gas transfer rate in hydrous materials and a building-sized total heat exchanger. The results demonstrated that an increase in water content in hydrous material has a significant impact on the permeability of water-soluble gases, with NH3 and HCHO permeability coefficients increasing by factors of 250 and 20 respectively. Conversely, for low-solubility gases such as CO2, the permeability coefficient only slightly increased at low humidity and remained largely unaffected thereafter. These findings contribute to the advancement of more efficient and safer total heat exchange ventilation systems.

Measurement and modeling of xenon gas transfer in the human brain with 1H and hyperpolarized 129Xe MRI

BackgroundThe feasibility of imaging hyperpolarized 129Xe dissolved in brain tissue following inhalation of xenon gas in the lungs has recently been demonstrated in humans. The image contrast in 129Xe brain MRI represents a combination of factors, including regional perfusion, polarization decay and gas transfer rate across the blood-brain barrier. PurposeTo investigate the repeatability of hyperpolarized 129Xe brain MRI in healthy normal individuals and to identify the dominant mechanisms of image contrast by assessing voxel-wise correlation between HP 129Xe brain MRI and models of 129Xe brain uptake derived from 1H arterial spin labeling (ASL) perfusion mapping. Materials and MethodsTo assess repeatability, 3 sets of hyperpolarized 129Xe brain images were acquired from 5 healthy volunteers. Quantitative maps of the human brain, including cerebral blood flow, volume and predicted xenon uptake, were derived from 1H arterial spin labeling and T2-weighted MRI. These maps were then spatially cross-correlated with hyperpolarized 129Xe brain MRI. ResultsSignal to noise ratios of 8.7–17.7 were observed across volunteers for a voxel size of 8 × 8 × 50 mm3 with intra-subject repeatability of between 6 and 29 %. Hyperpolarized 129Xe brain images showed voxel-wise correlations with cerebral blood flow (R = 0.32 to 0.62), volume (R = 0.33 to 0.63) and predicted xenon uptake (R = 0.34 to 0.63), but did not correlate with arterial transit time (R = 0.05 to 0.26). ConclusionVoxel-wise cross correlation between 129Xe and 1H ASL suggests that the regional quantity of dissolved xenon delivered by cerebral blood flow is the dominant mechanism of image contrast in HP 129Xe brain MRI, assuming normal blood-brain barrier function. Combining 1H and 129Xe brain MRI provides new opportunities to quantitatively investigate brain pathophysiology and function.

The influence of water turbulence on surface deformations and the gas transfer rate across an air–water interface

We present experimental results of a study on oxygen transfer rates in a water channel facility with varying turbulence inflow conditions set by an active grid. We compare the change in gas transfer rate with different turbulence characteristics of the flow set by four different water channel and grid configurations. It was found that the change in gas transfer rate correlates best with the turbulence intensity in the vertical direction. The most turbulent cases increased the gas transfer rate by 30% compared to the low turbulence reference case. Between the two most turbulent cases studied here, the streamwise turbulence and largest length scales in the flow change, while the gas transfer rate is relatively unchanged. In contrast, for the two less turbulent cases where the magnitude of the fluctuations normal to the free surface are also smaller, the gas transfer rate is significantly reduced. Since the air–water interface plays an important role in the gas transfer process, special attention is given to the free-surface deformations. Despite taking measures to minimise it, the active grid also leaves a direct imprint on the free surface, and the majority of the waves on the surface originate from the grid itself. Surface deformations were, however, ruled out as a main driver for the increase in gas transfer because the increase in surface area is < 0.25%, which is two orders of magnitude smaller than the measured change in the gas transfer rate.

Central Arctic Ocean surface–atmosphere exchange of CO2 and CH4 constrained by direct measurements

Abstract. The central Arctic Ocean (CAO) plays an important role in the global carbon cycle, but the current and future exchange of the climate-forcing trace gases methane (CH4) and carbon dioxide (CO2) between the CAO and the atmosphere is highly uncertain. In particular, there are very few observations of near-surface gas concentrations or direct air–sea CO2 flux estimates and no previously reported direct air–sea CH4 flux estimates from the CAO. Furthermore, the effect of sea ice on the exchange is not well understood. We present direct measurements of the air–sea flux of CH4 and CO2, as well as air–snow fluxes of CO2 in the summertime CAO north of 82.5∘ N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021. Measurements of air–sea CH4 and CO2 flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden, and air–snow fluxes were determined from chambers deployed on sea ice. Gas transfer velocities determined from fluxes and surface-water-dissolved gas concentrations exhibited a weaker wind speed dependence than existing parameterisations, with a median sea-ice lead gas transfer rate of 2.5 cm h−1 applicable over the observed 10 m wind speed range (1–11 m s−1). The average observed air–sea CO2 flux was −7.6 mmolm-2d-1, and the average air–snow CO2 flux was −1.1 mmolm-2d-1. Extrapolating these fluxes and the corresponding sea-ice concentrations gives an August and September flux for the CAO of −1.75 mmolm-2d-1, within the range of previous indirect estimates. The average observed air–sea CH4 flux of 3.5 µmolm-2d-1, accounting for sea-ice concentration, equates to an August and September CAO flux of 0.35 µmolm-2d-1, lower than previous estimates and implying that the CAO is a very small (≪ 1 %) contributor to the Arctic flux of CH4 to the atmosphere.

Probing efficient microbial CO2 utilisation through metabolic and process modelling.

Acetogenic gas fermentation is increasingly studied as a promising technology to upcycle carbon-rich waste gasses. Currently the product range is limited, and production yields, rates and titres for a number of interesting products do not allow for economically viable processes. By pairing process modelling and host-agnostic metabolic modelling, we compare fermentation conditions and various products to optimise the processes. The models were then used in a simulation of an industrial-scale bubble column reactor. We find that increased temperatures favour gas transfer rates, particularly for the valuable and limiting H2 , while furthermore predicting an optimal feed composition of 9:1 mol H2 to mol CO2 . Metabolically, the increased non-growth associated maintenance requirements of thermophiles favours the formation of catabolic products. To assess the expansion of the product portfolio beyond acetate, both a product volatility analysis and a metabolic pathway model were implemented. In-situ recovery of volatile products is shown to be within range for acetone but challenging due to the extensive evaporation of water, while the direct production of more valuable compounds by acetogens is metabolically unfavourable compared to acetate and ethanol. We discuss alternative approaches to overcome these challenges to utilise acetogenic CO2 fixation to produce a wider range of carbon negative chemicals.

Process analysis of the anaerobe Phocaeicola vulgatus in a shake flasks and fermenter reveals pH and product inhibition

PurposeThe anaerobic gut bacterium Phocaeicola vulgatus (formerly Bacteroides vulgatus) has a significant role in the human gut microbiome. It can produce bioactive compounds with antimicrobial properties and industrially relevant organic acids like succinate. However, there is a knowledge gap in understanding the metabolism of P. vulgatus, as cultivation of anaerobic gut bacteria is challenging and usually conducted with enriched microbiota cultures. We aim to close this gap by characterizing this anaerobe bacterium in different cultivation conditions and scales.MethodsIn this work, axenic cultures were studied in a shake flask and 2 L fermenter scale to characterize the influence of initial pH, buffer concentration, osmolality, and product inhibition on growth and organic acid production by P. vulgatus. Both cultivation systems had online gas measurements for total gas and CO2 production. HPLC analysis generated closed carbon balances, accounting for all produced acids.ResultsTotal gas transfer rates and CO2 transfer rates revealed that 65% of produced gas was attributed to H2, while just 35% was connected to CO2 production. A minimum buffer concentration of 50 mM MOPS and an initial pH of 7.3 were identified to mitigate pH inhibition in P. vulgatus cultivations with a defined minimal medium and glucose as substrate. The initial addition of lactate showed an inhibitory effect, starting at a concentration of 1 g L−1. On the contrary, initial acetate addition was beneficial for organic acid production. A comparison of a pH-buffered and a pH-controlled 2 L fermentation demonstrated a switch in acid production toward succinate under pH control.ConclusionThe study provides insight into improved cultivation conditions for the gut bacterium P. vulgatus and demonstrates a successful scale-up from the shake flask to the 2 L bioreactor. By applying pH control in the bioreactor, growth was increased, and the organic acid production was switched from lactate to succinate. Even though P. vulgatus could serve as a production organism for interesting bioactive compounds and organic acids, further characterization and improvement are necessary to improve titers.

Flow and mass transfer prediction in anisotropic TPMS-structures as extracorporeal oxygenator membranes using reduced order modeling

Currently, hollow fiber membranes are the standard technology for extracorporeal membrane oxygenators. Apart from the inevitable contact of the circulating blood with the artificial material, a suboptimal flow distribution within the oxygenator favors thrombus formation which leads to a rapid loss of gas exchange capacity. The current advancement in additive manufacturing allows the design of three-dimensional membrane-structures, based on triply periodic minimal surfaces (TPMS). One of their unique advantages is local geometry variation to manipulate the flow distribution. But how this anisotropy influences the overall device performance is non-trivial and requires numerical simulation. The aim of this study was to develop a reduced order model (ROM) that is able to efficiently predict three-dimensional flow distribution and gas transfer inside TPMS-structures. We performed a parametric study using a validated micro scale computational fluid dynamics (CFD) model. Afterwards, two different modeling approaches of Sherwood-correlations and artificial neural networks (NN) were compared to characterize flow and mass transfer from the simulated data. To create the ROM, the NN modeling strategy was then implemented into a porous medium CFD model. The developed ROM was also validated. Finally, an anisotropic TPMS-membrane-structure was compared numerically with an isotropic predicate. The NN fitting strategy showed superior accuracy over the Sherwood-correlations for characterizing mass transfer in TPMS-structures. With the ROM, the gas transfer rates of oxygen and carbon dioxide from the micro CFD model could be predicted within relative root mean squared errors (RRMSE) of 2.7% and 5.1%. The pressure drop in the experiments was predicted with an RRMSE accuracy of 7.6%. In comparison with the isotropic TPMS-structure, the anisotropic structure showed a homogenized flow distribution and increased gas transfer rates of 4% to 5% for oxygen and 2.3% to 7.4% for carbon dioxide at the simulated flow rates.

Flower-like In2O3-ZnO nanosensor with improved acetone performance at low temperature

In2O3-ZnO nanoflower-like acetone gas sensors with varied reaction time were created using a simple and green synthesis idea. Morphological and structural analysis indicated that the In2O3-ZnO nanoflowers were made up of nanoplates with a thickness of around 35–60 nm and some small nanoparticles with the particle size of about 11 nm adhered to the surface. With a sensitivity of 238 for 100 ppm acetone at 140 °C, a lower detection limit as low as 0.64 ppm, good selectivity and excellent stability, the In-Zn(5) sensor with a hydrothermal time of 5 h has the most excellent performance. The enhanced gas-sensitive performance of In-Zn(5) can be attributed to the heterogeneous structure between the two and the three-dimensional flower-like structure with a large specific surface area and fast gas transfer rate. This work brings a better prospect for the future detection of acetone.

Evolutions of extracorporeal membrane oxygenator (ECMO): perspectives for advanced hollow fiber membrane.

Hollow fiber membrane is incorporated into an extracorporeal membrane oxygenator (ECMO), and the function of the membrane determines the ECMO's functions, such as gas transfer rate, biocompatibility, and durability. In Japan, the membrane oxygenator to assist circulation and ventilation is approved for ECMO support. However, in all cases, the maximum use period has been only 6h, and so-called 'off-label use' is common for ECMO support of severely ill COVID-19 patients. Under these circumstances, the HLS SET Advanced (Getinge Group Japan K.K.) was approved in 2020 for the first time in Japan as a membrane oxygenator with a two-week period of use. Following this membrane oxygenator, it is necessary to establish a domestic ECMO system that is approved for long-term use and suitable for supporting patients. Looking back on the evolution of ECMO so far, Japanese researchers and manufacturers have also contributed to the developments of ECMO globally. Currently, excellent membrane oxygenators and systems have been marketed by Japanese manufacturers and some of them are globally acclaimed, but in fact, most of the ECMO membranes are not made in Japan. Fortunately, Japan has led the world in the fields of membrane separation technology and hollow fiber membrane production. In the wake of this pandemic, from the perspective of medical and economic security, the practical use of purely domestic hollow fiber membranes and membrane oxygenators for long-term ECMO is imperative in anticipation of the next pandemic.

Alternating Flow Field Design Improves the Performance of Proton Exchange Membrane Fuel Cells.

The flow field structure of a proton exchange membrane fuel cell (PEMFC) is a determining factor for improving the cell power density. In this study, a universal alternating flow field design for the first time is proposed, which arranges structural units with different flow resistances in an alternating way to significantly improve the gas transfer rate into the electrode, with the advantages of easy machining and low pumping loss. Based on the design, it is proposed and tested large-scale fuel cells with three novel flow fields by combining a parallel channel, baffled channel, serpentine channel, and narrowed channel. The results show that the design can significantly enhance the gas supply efficiency and that the novel baffled flow field improves the PEMFC performance by 23% with low pumping loss. The design employed in the study offers additional options for flow field optimization and contributes to the early achievement of next-generation ultrahigh power density fuel cells.

Active Carboxymethyl Cellulose-Based Edible Coatings for the Extension of Fresh Goldenberries Shelf-Life

Edible coatings based on natural sources are a good alternative to protect and increase the shelf-life of the fruits. In this work, active carboxymethyl cellulose (CMC)-based edible coatings enriched with functional ingredients, extracted from spent coffee grounds (SCG), were produced and used to increase the shelf-life of fresh goldenberries. Thus, three different treatments were tested on fresh goldenberries: (i) coating CMC (CMC-based edible coating); (ii) coating CMC-A (CMC-based edible coating with 0.2% (w/v) of polysaccharide-rich extract from SCG); and (iii) coating CMC-B (CMC-based edible coating with 0.2% (w/v) of polysaccharide-rich extract, and 0.2% (w/v) of phenolic compounds encapsulated from SCG), being compared with uncoated goldenberries. Physicochemical and microbiological properties of the fruits were evaluated throughout 12 and 28 days of storing at 20 °C and 65% relative humidity and 4 °C and 95% RH, respectively. The results showed that the temperature and relative humidity significantly influenced the quality of goldenberries, increasing their shelf-life almost two-fold when stored at 4 °C and 95% RH. Both CMC-A and CMC-B coatings controlled weight loss and decreased the gas transfer rates (O2, CO2, and C2H4), mainly when goldenberries were stored at 20 °C and 65% RH. Moreover, coating CMC-A was better for delaying microbial growth, while the edible coating CMC-B gave an extra content of phenolic compounds to goldenberries, increasing thus their antioxidant potential.

Modelling the absorption of nitrogen oxides by ferrous chelate solution in rotating packed bed

ABSTRACT With mushrooming growth of coal-based industry, the discharged tail gas containing nitrogen oxides had given significant effect on air quality and people’s health in China. Therefore, more stringent NOx emission standards were proposed. Among many technologies, wet scrubbing technology may be an effective way to solve the above problem, but it was limited by absorption of nitrogen oxides. Therefore, rotating packed bed (RPB) was used to intensify the absorption process of nitrogen oxides by ferrous chelate solution. To speed up its industrialization, a theoretical model was proposed by considering chemical reactions, gas and liquid mass transfer rate, liquid holdup and gas–liquid effective interfacial area. The effects of high gravity factor, gas velocity, gas–liquid ratio, temperature and concentration of ferrous chelate solution on NOx removal efficiency were conducted by model and also compared with experimental results. With high absorption efficiency and small equipment size in RPB, it had a great potential to apply in the absorption other harmful matters.

Enhanced gas-liquid mass transfer coefficient by bulk nanobubbles in water

Liquid-gas operation explores the widespread applications in multiple chemical processes industries. For an instance, the inclusion of fermentation, dissolved air flotation, paper and pulp processing, sludge decomposition, aeration, ozonation, etc. are majorly covered. Liquid-gas contacting devices are primarily employed to perform their mixing, and the conventional contacting devices envelopes the fine bubble diffusers, jet aerators, surface aerators, static mixers, ejectors, etc. The gas transfer into the liquid is done by bubble formation, and the bubbles furthermore diffuses into the bulk liquid. The bubble size plays an important role in the transfer of gas, lower the size of bubbles, more is the rate of gas transfer, such as the mass transfer coefficient is remarkably enhanced by microbubbles when compared with milli-bubbles. In this work, we created O2 nanobubbles by sparging oxygen gas into nanopores, and during the production of nanobubbles, we examined the oxygen gas volumetric mass transfer coefficient (kLa) into the water. Nanoparticle tracking analysis was used to determine the population and size of nanobubbles, and we found that the population of nanobubbles is in the order of 107 bubbles/mL, with a mean diameter of 100–200 nm. The gassing out technique determines the volumetric mass transfer coefficient (kLa), and the influence of gas flow rate on the volumetric mass transfer coefficient (kLa) has been demonstrated to be positive. The volumetric mass transfer coefficient (kLa) in the case of nanobubbles is found significantly larger than that of microbubbles, ranging from 2.2 to 3.8 min−1.

From Substrate to Surface: A Turbulence‐Based Model for Gas Transfer Across Sediment‐Water‐Air Interfaces in Vegetated Streams

AbstractDissolved Oxygen (DO) fluxes across the air‐water and sediment‐water interface (AWI and SWI) are two major processes that govern the amount of oxygen available to living organisms in aquatic ecosystems. Aquatic vegetation generates different scales of turbulence that change the flow structure and affect gas transfer mechanisms at AWI and SWI. A series of laboratory experiments with rigid cylinder arrays to mimic vegetation was conducted in a recirculating race‐track flume with a lightweight sediment bed. 2D Planar Particle Image Velocimetry was used to characterize the flow field under different submergence ratios and array densities to access the effect of vegetation‐generated turbulence on gas transfer. Gas transfer rate across AWI was determined by DO re‐aeration curves. The effective diffusion coefficient for gas transfer flux across SWI was estimated by the difference between near‐bed and near‐surface DO concentrations. When sediment begins to mobilize, near‐bed suspended sediment provides a negative buoyancy term that increases the critical Reynolds number for the surface gas transfer process according to a modified Surface Renewal model for vegetated flows. A new Reynolds number dependence model using near‐bed turbulent kinetic energy as an indicator is proposed to provide a universal prediction for the interfacial flux across SWI in flows with aquatic vegetation. This study provides critical information and useful models for future studies on water quality management and ecosystem restoration in natural water environments such as lakes, rivers, and wetlands.

Triple oxygen isotope constraints on atmospheric O2 and biological productivity during the mid-Proterozoic

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO2) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO2 and productivity during the Proterozoic. O-MIF, reported as Δ'17O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS2) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air-sea gas exchange. Previous analyses of these data concluded that pO2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO2 is essentially unconstrained by these data. Indeed, pO2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O2) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of pCO2 and/or improved proxies for Δ'17O of atmospheric O2 would allow tighter constraints to be placed on mid-Proterozoic pO2.

Fluid dynamics of a pilot‐scale OrbShake bioreactor under different operating conditions

AbstractBACKGROUNDFor several years, orbitally shaken cylindrical bioreactors (OSRs) have become a new approach for propagation of mammalian cells cultivated under suspension. With new commercial cell culture media available today, allowing exceptionally high cell densities, the identification of suitable operating conditions presents a special challenge. Balancing hydrodynamic stress and other factors in reactors, this is particularly true for gas transfer rates providing sufficient oxygen (O2) to cultures that can attain ten‐fold greater biomasses than just a decade ago. This challenge is more profound the larger the working volume of the reactor becomes.RESULTFor 10‐L working volume cylindrical OSRs, we established a computational fluid dynamics model to evaluate the impacts of different operational conditions. Both fluid velocity distributions and O2 transfer coefficients (kLa) were calculated for different filling volumes and shaking speeds. Also, hydrodynamic stress parameters were investigated considering the most critical working conditions.CONCLUSIONThe fluid velocity decreased with larger working volumes and, as expected, was found to increase with higher shaking frequency. Gas transfer rates between 10 and 54 h−1 were found in working volumes from 2.5 to 10 L and under shaking frequencies from 100–160 rpm. Critically assessing the obtained data, with no liquid‐submerged gas supply, we conclude that an air flow into the head‐space of a disposable bag appears to be satisfactory under certain operating conditions to support the culture of the most sensitive mammalian or insect cell cultures towards high cell density, presently in fed‐batch cultures achieving ≤30 × 106 cells mL–1. Shear stress parameters were found to be exceptionally low. © 2021 Society of Chemical Industry (SCI).

Online monitoring of gas transfer rates during CO and CO/H2 gas fermentation in quasi-continuously ventilated shake flasks.

Syngas fermentation is a potential player for future emission reduction. The first demonstration and commercial plants have been successfully established. However, due to its novelty, development of syngas fermentation processes is still in its infancy, and the need to systematically unravel and understand further phenomena, such as substrate toxicity as well as gas transfer and uptake rates, still persists. This study describes a new online monitoring device based on the respiration activity monitoring system for cultivation of syngas fermenting microorganisms with gaseous substrates. The new device is designed to online monitor the carbon dioxide transfer rate (CO2 TR) and the gross gas transfer rate during cultivation. Online measured data are used for the calculation of the carbon monoxide transfer rate (COTR) and hydrogen transfer rate (H2 TR). In cultivation on pure CO and CO + H2 , CO was continuously limiting, whereas hydrogen, when present, was sufficiently available. The maximum COTR measured was approximately 5 mmol/L/h for pure CO cultivation, and approximately 6 mmol/L/h for cultivation with additional H2 in the gas supply. Additionally, calculation of the ratio of evolved carbon dioxide to consumed monoxide, similar to the respiratory quotient for aerobic fermentation, allows the prediction of whether acetate or ethanol is predominantly produced. Clostridium ljungdahlii, a model acetogen for syngas fermentation, was cultivated using only CO, and CO in combination with H2 . Online monitoring of the mentioned parameters revealed a metabolic shift in fermentation with sole CO, depending on COTR. The device presented herein allows fast process development, because crucial parameters for scale-up can be measured online in small-scale gas fermentation.

Effect of buoyant turbulence and water quality factors on the CO2 net atmospheric flux changes in a stratified reservoir

The temporal variations in CO2 net atmospheric flux (NAF) in stratified reservoirs are controlled by both physical and biological factors. However, research on the factors and processes affecting CO2 NAF variability over time is insufficient, and as a result, there is considerable uncertainty in present estimations of global reservoir CO2 emissions. In the present study, we analyzed the effects of hydrodynamic and water quality factors on CO2 NAF variability in a stratified reservoir based on field studies and data modeling. Three empirical and four surface renewal gas transfer models were used to characterize the effects of hydrodynamic factors on gas transfer rate and CO2 NAF at the air–water interface. Buoyant turbulence notably affected CO2 NAF when the stratification strength was reduced. As a result, the CO2 NAF (1485 mg-CO2 m−2 day−1) estimated using surface renewal models that considered the effects of buoyant turbulence were twice greater than the NAFs estimated using empirical models that only considered wind force (724 mg-CO2 m−2 day−1). The best linear regression model explained 81.6% of the temporal variation in CO2 NAF using water temperature (Tw), electrical conductivity (EC), pH, chlorophyll a, total organic C (TOC), and alkalinity. The nonlinear parsimonious random forest model explained 84.4% of the temporal change in CO2 NAF using only three independent variables (EC, dissolved oxygen, and TOC). Principal component analysis revealed that the CO2 NAF tended to be large under low Tw, weak stratification, and low pH. These results indicate that the temporal variability of CO2 NAF in the stratified reservoir can be predicted using data-driven modeling with minimal water quality variables and selection of an appropriate gas exchange model. The findings improve the accuracy of estimates of CO2 emissions and monitoring activities in stratified reservoirs.

Newly Developed Pediatric Membrane Oxygenator that Suppresses Excessive Pressure Drop in Cardiopulmonary Bypass and Extracorporeal Membrane Oxygenation (ECMO).

This article developes a pediatric membrane oxygenator that is compact, high performance, and highly safe. This novel experimental approach, which imaging the inside of a membrane oxygenator during fluid perfusion using high-power X-ray CT, identifies air and blood retention in the local part of a membrane oxygenator. The cause of excessive pressure drop in a membrane oxygenator, which has been the most serious dysfunction in cardiovascular surgery and extracorporeal membrane oxygenation (ECMO), is the local retention of blood and air inside the oxygenator. Our designed blood flow channel for a membrane oxygenator has a circular channel and minimizes the boundary between laminated parts. The pressure drop in the blood flow channel is reduced, and the maximum gas transfer rates are increased by using this pediatric membrane oxygenator, as compared with the conventional oxygenator. Furthermore, it would be possible to reduce the incidents, which have occurred clinically, due to excessive pressure drop in the blood flow channel of the membrane oxygenator. The membrane oxygenator is said to be the “last stronghold” for patients with COVID-19 receiving ECMO treatment. Accordingly, the specification of our prototype is promising for low weight and pediatric patients.

Local gas transfer rate through the free surface in spatially accelerated open-channel turbulence

It is important to determine the transport mechanisms of dissolved gases through free surfaces in open channels. Even though many physical and phenomenological models have been proposed, not much is known about the local distribution of the gas transfer velocity. Thus, this study formulated a theoretical equation for developing a concentration boundary layer in an open-channel flow. Additionally, the free-surface velocity component and the concentration of dissolved oxygen were measured in a spatially accelerated open-channel flow. In particular, the streamwise profile of concentration boundary layer thickness was measured using an extra-fine needle-type dissolved oxygen probe, and the local gas transfer rate was experimentally obtained. The present theory suggests that the local gas transfer is controlled by two significant terms: the streamwise gradient of the mean velocity and the relative intensity of turbulent diffusion. By focusing on accelerated open-channel flows with bottom-situated wedges, the formation mechanism of the concentration boundary layer was explained by a comparison with the presented theory. The comparison of the theoretical model and measurement data indicated that the contributions of both the mean velocity and turbulence diffusion are comparable and significant in the acceleration zone. The thickness of the oxygen concentration boundary layer began to decrease a bit downstream of the entrance, corresponding to the variation in the free-surface turbulence factors. The inflectional point of the concentration thickness appeared downstream of the acceleration zone and continued to decrease at the exit related to relaminarization. These interesting features are explained by the streamwise profiles of the terms in the presented theory.