CO2 Mixing Research Articles

Mixing of endogenous CO2 and meteoric H2O causes extremely efficient carbonate dissolution

Karst corrosion of carbonate rocks by water with dissolved gases proceeds in most cases along two major scenarios: (i) meteoric water absorbs CO2 from soil and atmosphere, or (ii) ascending water of deep circulation carries with it dissolved endogenous gases, mainly CO2 and H2S. We have observed a peculiar variant where meteoric water absorbs ascending endogenous gases at a natural gas vent on a travertine mound in Slovakia. Carbonate dissolution's extreme effectiveness is demonstrated by mineralization of rainwater ponded at a gas vent, rising to 3.2 g/L of dissolved solids shortly after the rainfall. One liter of water ponded at the vent and mixing with the venting gas, dissolved up to 800 mg of calcium at a rate exceeding 5.8 mg/L·min. Limestone tablets placed at the vent show signs of significant corrosion, at rates up to 126 mm/ka. The rate is comparable to those in coastal karst, where freshwater is mixing with seawater and to those in sulfuric acid speleogenesis (SAS), both the highest hitherto known rates of karst corrosion in carbonates. The geomorphic effects of the process described are depressions on the surface of travertine near the vents of endogenous CO2. This type of corrosion seems to be universal and probably occurs everywhere where endogenous CO2 is exhaled to the surface from carbonate rocks.

Distribution of ozone, carbon monoxide and oxides of nitrogen over an urban location in the foothills of the North-Western Himalayas

Increasing anthropogenic activities and biomass burning events over the Northern Indian region severely affect the pristine environment of North-Western Himalayas. To investigate their influence in this data sparse region, continuous monitoring of O3, CO and NOx are made during January 2018 to December 2019 over Dehradun, an urban location in the foothills of North-western Himalayas. Ozone shows daytime broad peak and CO and NOx show two peaks during morning and evening hours. The maximum daytime ozone is observed during spring with 58.2 ± 2.0 ppbv (56.2 ± 2.7 ppbv) during 2018 (2019). CO and NOx show maximum value during winter and minimum during summer-monsoon. Their concentrations are found to be influenced from stubble burning and forest fire emissions during late spring. WRF-Chem simulations are made with and without biomass burning emissions to quantify the contribution of these emissions on the variability of these gases over Dehradun. Sensitivity simulations reveal that biomass burning over northern Indian region contributed 43 ppbv, 79 ppbv and 1.2 ppbv increase in O3, CO and NOx mixing ratios over Dehradun. In addition, dust storm event in June 2018 contributed for lower ozone levels in 2018 (16.1 ± 9.3 ppbv) and stratosphere troposphere exchange process enhanced surface ozone in June 2019 (53.6 ± 22.3 ppbv).

Multiscale effects of radial mixing on mixotrophic microalgal growth and macromolecular synthesis in tubular bubble-column photobioreactors

Extremophillic microalga Chlorella sorokiniana is grown mixotrophically in a 5 L externally-illuminated tubular bubble-column photobioreactor with an external light intensity of 10,500 lx (≈195 μmol-photons.m−2.s−1) at atmospheric (0.04 %), 1 %, 2 %, 3 %, 4 %, and 5 % CO2 concentrations, with acetic acid and urea as the organic carbon and nitrogen sources, respectively, for 6–8 days. Two different setups are employed: one permits axial air flow, and another promotes radial mixing of algal particles, dissolved CO2 and nutrients along with axial sparging. We show that radial mixing increases biomass yields by reducing both photo-limitation and photo-inhibition up to CO2 concentrations <3 %, above which substrate (CO2) inhibition necessitates lower reactor mixing. Radial mixing increases the lipid yield from 1.06 and 1.61 g/L to 1.27 and 1.78 g/L at 2 % and 3 % CO2, respectively, in nitrogen-limited cases, resulting in a 10–20 % increase. We show that complete reactor mixing, facilitated by the dual effects of branched (radial) and axial sparger, is preferred only in the absence of substrate (CO2) inhibition. Subsequently, we stratify the tubular photobioreactor design strategy into two regimes based on CO2 inlet concentrations. In one regime (for inlet CO2 < 3 %), we recommend the deployment of branched sparger to facilitate both radial and axial mixing simultaneously, thereby creating a well-mixed reactor at lower CO2 concentrations. In the other regime (for inlet CO2 > 3 %), axial sparging alone is recommended since complete mixing of CO2 increases substrate inhibition by reducing the reactor pH below 7.5.

Effect of permeability and density difference on convective mixing and storage of CO2 in saline aquifers

The density-driven natural convection is believed to have a positive impact on CO2 storage in saline aquifers. In this regard, studying the parameters that influence the onset of convection is crucial for determining the optimal storage location. Here, we conducted a numerical analysis to investigate how the permeability of the porous medium and the density difference between the brine and CO2-brine mixture affect the mixing efficiency, speed, and development of convective fingers. Results showed that greater permeability and density difference result in a faster onset of convection; hence, fluid velocity and mixing efficiency will increase. Moreover, considering CO2 dissolution based solely on Rayleigh number is not entirely accurate. In addition, if there are different permeability and density difference values, even with a constant Rayleigh number, the mixing of CO2 and brine would not be the same.

Mixing and transport of CO2 across a monolayer-covered surface in an open cylinder driven by a rotating knife edge

The dissolution of CO2 into water has attracted much attention for the modeling of air-sea gas exchange, CO2 sequestration into porous aquifers, and gas transport in bioreactors. In nearly all circumstances, the surface of water is covered by surfactants that make the gas-liquid interface behave elastically and exhibit surface (excess) shear viscosity. Here, transport and mixing of CO2 across the interface is studied in an axisymmetric system with inertia. A rotating knife edge at the interface with negligible contact surface area takes advantage of surface shear viscosity to drive a strong overturning flow in the bulk via viscous coupling. When CO2 is dissolved into water, this system has a forced convection nature that enhances the transport of CO2 into water. This transport leads to intense radial gradients in the dissolved CO2 resulting in baroclinic production of azimuthal vorticity which alters the underlying meridional flow and further enhances mixing by breaking down transport barriers. Flow visualization experiments reveal baroclinic instability and numerical simulations detail the hydrodynamics, including the effects of varying the CO2 partial pressure above the water.

Thermal Fracture Simulation in Depleted Gas Field Carbon Capture and Storage: Implications for Injectivity and Flow Assurance

Summary CO2 injection into depleted gas fields causes long-term cooling of the reservoir. Therefore, even if injection pressure stays below the fracture initiation pressure, the cooled volume still creates an extensive stress disturbance that can induce the propagation of large fractures over time. The enhanced injectivity after the onset of this thermal fracturing might jeopardize injection operations due to the risk of hydrate plugging in the injection well caused by the combination of low pressure and low temperature, and large fractures may also increase the risk of loss of containment. Modeling the fracture evolution provides an estimate of the magnitude and timing of these effects. In this study, commercial compositional reservoir simulation software capable of modeling the physical phenomena associated with CO2 injection into depleted natural gas reservoirs has been used. These encompass CO2 mixing with natural gas, water vaporization, thermal effects, and geomechanics. The finite-element geomechanics module used “two-way” coupling, which computes pressure and temperature in the flow simulation module, transmits this information to the geomechanics module to update stress and strain parameters, and uses these parameters to adjust porosity and permeability, thereby enhancing the accuracy and reliability of the overall simulation results. The thermal fracture opening is simulated as increased permeability in the fracture domain by using a fracturing criterion based on the effective stress. The fracture simulations were developed in close relation with flow assurance modeling to determine the operational windows that avoid hydrate formation while maintaining the required injection target. Unlike matrix injection, thermal fracturing shows a substantial reduction in injection bottomhole pressure (BHP), reaching 26 000 kPa (260 bar) in a specific scenario. These findings underscore the crucial consideration of cooling effects and thermal fracturing in carbon capture and storage (CCS) operations, particularly in flow assurance studies where well injectivity significantly influences overall outcomes. Due to the intense cooling-induced stress reduction, thermal fractures may propagate uncontrollably, potentially reaching faults within the reservoir. Temperature distributions along boundary faults may differ markedly from matrix flow conditions, highlighting the need to incorporate these effects into geomechanical studies to mitigate risks associated with fault stability during cooling processes.

Research on the influence of aeration manners on the mass transfer characteristics of bubbles in multi‐column airlift photobioreactor

AbstractMass transfer of bubbles is important in microalgae cultivation. In this study, aiming to improve the microalgae culturing efficiency, computational fluid dynamics (CFD) was adopted to study the influence of the aeration manners on mass transfer of bubbles within ascending columns in multi‐column airlift photobioreactor (PBR) and the hydrodynamic conditions within the PBR under different aeration manners. In addition, the bubble generation time, the detachment diameter, and the average volumetric mass transfer coefficient of bubbles in ascending columns were analyzed. Furthermore, the experimental results were compared with the simulation results obtained from microalgae cultivation. The results showed that the whole aeration manner yields the lowest mixing strength and mass transfer efficiency of bubbles at an aeration rate of 0.2 vvm. Conversely, both the mixing strength of the liquid and mass transfer coefficient of bubbles were enhanced under the half and alternate aeration manners. However, the results demonstrated that the distribution of the flow field was not uniform under the half aeration manner and there were obvious high‐speed and low‐speed zones. In contrast, the flow field distribution in the PBR was more uniform under the alternate aeration manner, which was suitable for microalgae cultivation at high concentration. This study effectively enhanced the mixing strength and CO2 transfer rate in the PBR.

Flow Characteristic of Supersonic Jet Related to O2–CO2 Mixing Injection Method

Flow Characteristic of Supersonic Jet Related to O2–CO2 Mixing Injection Method

Calculation of hydrogen adsorption isotherms and Henry coefficients with mixed CO2 and CH4 gases on hydroxylated quartz surface: Implications to hydrogen geo-storage

Hydrogen is a clean fuel that is becoming increasingly popular in addressing future energy challenges, however, its viable storage requires an understanding of its interaction with underground rock minerals. For this purpose, this study focuses on developing adsorption isotherms and Henry coefficients for hydrogen in hydroxylated quartz surfaces, which are predominant in sandstone and shale rocks. In this study, a molecular-scale hydroxylated quartz structure with a pore space is generated and the Monte Carlo (MC) simulation is utilized to assess the adsorption isotherms and Henry coefficients. Initially, the CO2 adsorption with the hydroxylated quartz is matched with the published data to determine the accuracy of MC simulation and the mineral-fluid interaction. After that, the concentration of H2 within the pore is progressively increased and the competitive adsorption is calculated under various subsurface temperature and pressure conditions. Additionally, CH4 is introduced to understand the competitive adsorption dynamics between H2 and CH4. The findings indicate that increasing pressure results in increasing molecules of hydrogen that monotonically increase H2 adsorption. On the contrary, increasing temperature reduces the affinity of hydrogen with the quartz surface causing a decrease in adsorption capacity and Henry coefficients. Furthermore, a weak single hydrogen adsorption layer is developed at all pressure and temperature conditions, while most of the hydrogen remains as a free gas in the pore. In addition, the hydroxyl group controls most of the adsorption of hydrogen in the hydroxylated quartz. Furthermore, the addition of water film (approximately 1 wt%) reduces the hydrogen adsorption by around 3 %. The H2 adsorption values further experience a notable decrease with the introduction of CO2 and CH4, primarily due to the establishment of a strong adsorption layer caused by the high electrostatic attraction between these gases and the quartz mineral. The study offers crucial microscopic details regarding H2 adsorption alongside CO2 and CH4 gases within hydroxylated quartz pores. These insights support efficient hydrogen geo-storage and larger-scale simulation research.

Electrocatalytic reduction of simulated industrial CO2 and CO mixtures: Revising chronoamperometry to enable selective gas mixture reduction via cyclic voltammetry

While substantial advancements have been achieved in the electrocatalytic reduction of pure CO2 or CO, the reduction of carbon oxide mixtures remains underexplored, a key limitation in scaling the technology for industrial applications. This underexplored domain gains practical importance by considering carbon oxide mixtures, given that CO and CO2 together constitute a large proportion of diverse industrial gas streams. However, the simultaneous reduction of mixed CO and CO2 poses a potential challenge due to their differing optimal reduction potentials, which complicates an efficient reduction process. While cyclic voltammetry (CV) is typically used for characterization, which involves sweeping between positive and negative potentials, we utilized it to conduct reduction reactions within a defined negative potential window. This approach, serving as an alternative to conventional chronoamperometry that maintains a constant potential, ensures a continuous reduction reaction throughout the entire cycling process. By employing cycling between the optimized reduction potentials of CO2 (−1.4 V) and CO (−1.6 V), CV achieved an increase in production efficiency, ranging from 25 % to 40 % for predominant products (HCOOH and C2H4), in comparison to chronoamperometry. Meanwhile, the results underscored the dependency of performance enhancement on the scan rate of cycling, revealing plausible correlations with mass transfer and electrode surface dynamics. Furthermore, the findings indicated that the selectivity towards HCOOH and C2H4 is closely related to the CO2/CO ratios of gas sources. This provides insight into the potential for modulating selectivity of industrial gas reduction for improved product quality and customization.

Potential of wind turbines on the alteration of carbon dioxide concentration

Anthropogenic carbondioxide (CO2) emissions are a major factor in global warming, requiring significant cuts to combat climate change. A crucial technology to reduce global CO2 concentration is direct air capture (DAC) of CO2. However, existing DAC techniques are expensive because of low CO2 concentrations, and they frequently rely on fossil fuel-based energy. In this article, we investigate how wind turbines can influence local CO2 levels and potentially collaborate with DAC and other technologies. To explore this idea, we performed large-eddy simulations using two 5 MW commercial-scale wind turbines. We incorporated realistic CO2 profiles collected from 13 different global locations across different seasons. The simulations were performed under neutral atmospheric boundary layer conditions. The results demonstrate that the wake recovery mechanism of a wind turbine promotes rapid mixing of CO2 both above and below the turbine blade tips in the wind turbine wake. In cases where the initial concentrations of CO2 were elevated above the turbine, downward entrainment of CO2 occurred. Conversely, when high concentrations of CO2 were present in the lower atmosphere, wind turbines facilitated a decrease in concentration at that layer by up to 138 kg/m within the intermediate wake (within 7 diameters) of the second turbine, T2. These discoveries inspire further investigation into the potential synergies between wind turbines and DAC devices or local CO2 pollutant diverters, depending on the prevailing CO2 profile. Consequently, this article marks the initial showcase of wind turbines' capability to influence CO2 levels by creating an entrainment and removal effect.

The Lagrangian Atmospheric Radionuclide Transport Model (ARTM) – sensitivity studies and evaluation using airborne measurements of power plant emissions

Abstract. The Atmospheric Radionuclide Transport Model (ARTM) operates at the meso-γ scale and simulates the dispersion of radionuclides originating from nuclear facilities under routine operation within the planetary boundary layer. This study presents the extension and validation of this Lagrangian particle dispersion model and consists of three parts: (i) a sensitivity study that aims to assess the impact of key input parameters on the simulation results, (ii) the evaluation of the mixing properties of five different turbulence models using the well-mixed criterion, and (iii) a comparison of model results to airborne observations of carbon dioxide (CO2) emissions from a power plant and the evaluation of related uncertainties. In the sensitivity study, we analyse the effects of the stability class, roughness length, zero-plane displacement factor, and source height on the three-dimensional plume extent as well as the distance between the source and maximum concentration at the ground. The results show that the stability class is the most sensitive input parameter as expected. The five turbulence models are the default turbulence models of ARTM 2.8.0 and ARTM 3.0.0, one alternative built-in turbulence model of ARTM, and two further turbulence models implemented for this study. The well-mixed condition tests showed that all five turbulence models are able to preserve an initially well-mixed atmospheric boundary layer reasonably well. The models deviate only 6 % from the expected uniform concentration below 80 % of the mixing layer height, except for the default turbulence model of ARTM 3.0.0 with deviations of up to 18 %. CO2 observations along a flight path in the vicinity of the lignite power plant Bełchatów, Poland, measured by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Cessna aircraft during the Carbon Dioxide and Methane Mission (CoMet) campaign in 2018 allowed for evaluation of model performance for the different turbulence models under unstable boundary layer conditions. All simulated mixing ratios are of the same order of magnitude as the airborne in situ data. An extensive uncertainty analysis using probability distribution functions, statistical tests, and direct spatio-temporal comparisons of measurements and model results help to quantify the model uncertainties. With the default turbulence setups of ARTM versions 2.8.0 and 3.0.0, the plume widths are underestimated by up to 50 %, resulting in a strong overestimation of the maximum plume CO2 mixing ratios. The comparison of the three alternative turbulence models shows good agreement of the peak plume CO2 concentrations, the CO2 distribution within the plumes, and the plume width, with a 30 % deviation in the peak CO2 concentration and a less than 25 % deviation in the measured CO2 plume width. Uncertainties in the simulations may arise from the different spatial and temporal resolutions of simulations and measurements in addition to the turbulence parametrisation and boundary conditions. The results of this work may help to improve the accurate representation of real plumes in very unstable atmospheric conditions through the selection of distinct turbulence models. Further comparisons at different stability regimes are required for a final assessment of model uncertainties.

Unveiling the synergistic effect of in-situ generated calcium carbonate by CO2 mixing on the reaction of calcined clay

This study investigates the effect of in-situ generated calcium carbonate by CO2 mixing on the reaction of calcined clay and demonstrates that the in-situ generated calcium carbonate remarkably accelerates the reaction of calcined clay. The mechanism is revealed through the application of multiple techniques and thermodynamical modeling. The in-situ generated calcium carbonates are generally nano/micro-particles and provide the seeding effect for promoting the formation of reaction product. More importantly, it is found that in-situ generated calcium carbonates are highly reactive and could react with metakaolin from calcined clay to form carboaluminate, while prohibits the formation of stratlingite. Therefore, this study unveils the synergistic effect of in-situ generated calcium carbonate in promoting the reaction of calcined clay, indicating that CO2 mixing is a promising technique in promoting the performance of calcined clay based cementitious materials.

Cenozoic stable isotope constraints on the Eurasian continental interior hydroclimate response to high CO2

The response of the terrestrial hydrologic cycle to higher atmospheric CO2 remains poorly constrained, largely due to difficulty in predicting how land surface processes may modify individual hydroclimate parameters such as precipitation (P), evapotranspiration (ET), and runoff (q). To interrogate how the terrestrial hydrologic cycle may change with warming and higher CO2, we utilize the Cenozoic geologic record of terrestrial stable oxygen isotopes (δ18O) as recorded in authigenic minerals, which reflect precipitation δ18O (δ18Op). Values of δ18Op are sensitive to changes in terrestrial hydroclimate, including to the ratio of P/ET and to precipitable water. In short, decreasing P/ET or higher precipitable water produces shallower δ18Op gradients inland, whereas increasing P/ET or lower precipitable water results in steeper δ18Op continental gradients. We compile nearly 15,000 samples of authigenic carbonate and tooth enamel across Eurasia—the largest continental landmass—that span the Cenozoic Era to reconstruct the past spatial distribution and zonal (i.e. east-to-west) gradients of δ18Op. We find that, in epochs with higher CO2, zonal δ18Op gradients increase, suggesting an increase in P/ET across Eurasia despite the expected increase in precipitable water in greenhouse climates. We compare these results to δ18Op gradients simulated by an isotope-enabled climate model (iCESM) forced with both pre-industrial (PI) and 4xPI CO2 mixing ratios. Simulated δ18Op gradients shallow as CO2 rises, in contrast to the reconstructed δ18Op gradients. This data-model mismatch suggests either that Earth system feedbacks not represented in our iCESM simulations, such as changes in Northern Hemisphere ice extent and vegetation, may have fundamentally changed Northern Hemisphere hydroclimate or that land-surface processes under high CO2 climates may be mis-represented in iCESM.

Effect of carbon sequestration methods on uptake potential and characteristics of ordinary portland cement-based concrete

Cementitious concrete is considered potentially the largest avenue to utilize the captured carbon dioxide (CO2). Its capacity is significantly influenced by various parameters, including the CO2 introduction method for sequestration. This paper attempted to study the variation in carbon sequestration potential of ordinary Portland cement (OPC) based concrete and paste when the CO2 is introduced to the samples during mixing, curing and both. The carbonated samples obtained using the abovementioned methods have been compared in terms of CO2 uptake capacity, microstructural densification, porosity reduction, and strength variations. A maximum CO2 Uptake Capacity of about 25% was achieved under Standard Ambient Temperature and Pressure (SATP) conditions, with a maximum CO2 Conversion of 70.35% and 93.80% obtained at the surface for CO2 Mixing and CO2 Curing, respectively. Further, carbon-sequestered concrete (CSC) resulted in enhanced compressive strength, and reduced the size of pores in addition to making them uniform in size.

Smart predictive viscosity mixing of CO2–N2 using optimized dendritic neural networks to implicate for carbon capture utilization and storage

Crucial for carbon capture, utilization, and storage (CCUS) initiatives and diverse industries, heat transfer underscores the need for a precise assessment of carbon dioxide (CO2) and nitrogen (N2) viscosities in gaseous blends across various temperatures. This research pioneers an intelligent model by enhancing the dendritic neural regression (DNR) framework, employing the Seagull Optimization Algorithm with Marine Predator Algorithm (SOAMPA) for optimal predictions. Leveraging recent advancements in metaheuristic optimization techniques, the study reveals the superior performance of the novel SOAMPA approach in predictive accuracy, marking a significant breakthrough in predicting CO2-N2 mixture viscosities with implications for advancing CCUS projects and diverse industries. The optimized DNR model, empowered by the modified SOAMPA optimization technique, contributes to estimating the viscosity of N2-CO2 mixture gases. Utilizing inputs like pressure, temperature, mole fraction of N2, and model fraction of CO2, the models are trained and tested on a dataset comprising over 3030 data samples from public literature. Key contributions encompass proposing an optimized DNR approach, introducing the modified SOAMPA technique, and demonstrating its superiority over established optimization methods in conjunction with the traditional DNR model for predicting viscosity based on real experimental datasets.

Design of a 130 MW Axial Turbine Operating with a Supercritical Carbon Dioxide Mixture for the SCARABEUS Project

Supercritical carbon dioxide (sCO2) can be mixed with dopants such as titanium tetrachloride (TiCl4), hexafluoro-benzene (C6F6), and sulphur dioxide (SO2) to raise the critical temperature of the working fluid, allowing it to condense at ambient temperatures in dry solar field locations. The resulting transcritical power cycles have lower compression work and higher thermal efficiency. This paper presents the aerodynamic flow path design of a utility-scale axial turbine operating with an 80–20% molar mix of CO2 and SO2. The preliminary design is obtained using a mean line turbine design method based on the Aungier loss model, which considers both mechanical and rotor dynamic criteria. Furthermore, steady-state 3D computational fluid dynamic (CFD) simulations are set up using the k-ω SST turbulence model, and blade shape optimisation is carried out to improve the preliminary design while maintaining acceptable stress levels. It was found that increasing the number of stages from 4 to 14 increased the total-to-total efficiency by 6.3% due to the higher blade aspect ratio, which reduced the influence of secondary flow losses, as well as the smaller tip diameter, which minimised the tip clearance losses. The final turbine design had a total-to-total efficiency of 92.9%, as predicted by the CFD results, with a maximum stress of less than 260 MPa and a mass flow rate within 1% of the intended cycle’s mass flow rate. Optimum aerodynamic performance was achieved with a 14-stage design where the hub radius and the flow path length are 310 mm and 1800 mm, respectively. Off-design analysis showed that the turbine could operate down to 88% of the design reduced mass flow rate with a total-to-total efficiency of 80%.

2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation changes

Abstract Coral reefs, the most sensitive ecosystem to high temperature, are on the precipice of mass extinction from global warming [1, 2]. 2023 was the hottest year in recorded history on land and in the sea, with dramatic and unexpected temperature increases [3, 4]. Coral Reef Bleaching HotSpot maps provide unique insight into global ocean circulation changes in response to greenhouse gas (GHG) forcing that caused dramatic global temperature rises [1, 2]. The highest excess daily air temperatures recorded in 175 countries, as well as the most prolonged excessive sea surface temperatures, were centered around Jamaica. 2023 marked the worst coral reef bleaching yet in the Northern Hemisphere, with the Southern Hemisphere poised to follow in early 2024. The HotSpot maps strongly suggest accelerated ocean poleward heat transport, slowdown in upwelling, and decreased deep water formation linked to sharply increased 2023 anomalous sea surface and air temperatures. The 2023 distribution of severe heat and bleaching follows both spatial patterns and temporal trends first shown from a baseline 1982–2001 global SST trend analysis [5]. Increased warming of both hot and cold ocean currents shows that horizontal mixing of tropical heat to the poles is accelerating, and that vertical mixing with cold deep water is slowing down, leading to increased ocean stratification, which will cause sea temperature to increase more rapidly and CO2 mixing with the deep ocean to decrease.

Evaluation of Car-Following Model for Mixed Autonomous and Human Driven Vehicles on Road Facilities.

Driverless cars are emerging slowly but bear the opportunity to improve the traffic system efficiency and user comfort. For the near future, a mix of human-driven and driver-less vehicle co- existence is inevitable. The quest to integrate autonomous and human-driven vehicles has created numerous critical questions in the road traffic system: How can autonomous and human- driven vehicles co-exist efficiently? What are the prospects of breaching the existing car-following model widely adopted to control the longitudinal movement of vehicles and control the driver’s maintenance of a safe distance? There is no doubt, that the emergence of autonomous vehicles and their integration is altering the existing traffic theories and the idea behind the car-following model. Currently, researchers are working hard to efficiently describe the physics behind the traffic flow theory of car-following models in mixed traffic. This work investigated a homogeneous car-following model sensitivity with varying human drivers’ reaction time in mixed traffic. The ability to predict the optimal vehicle response time in a platoon to the behaviour of its predecessor is vital in predicting the effect a change in the driving environment of mixed traffic has on traffic flow. The major complexities surrounding the car- following model in a mixed traffic flow environment are how the traffic participants share the road intersection space and the car-following behavior of human-driven vehicles. Investigation in microscopic traffic flow describes the way a single car be- haves and then introduces the impact another car has on a car’s behavior to address the mixed traffic flow. The traffic speed performance under the different traffic conditions suggests that the proposed mixed traffic car-following model outperforms the existing car-following model for human-driven vehicles with an average of 22.59mph. There is a significant throughput improvement in a mixed traffic environment when the distance headway of autonomous vehicles is adjusted to support the research hypothesis.

The synergistic effect of greenhouse gas CO2 and silica fume on the properties of 3D printed mortar

To fulfil the requirements of pumpability and buildability, the rheological properties of mortar/concrete for 3D printing should follow much stricter requirements than traditional-cast mortar/concrete. This study proposed to use a secondary CO2 mixing technique coupled with the addition of silica fume (SF) to achieve in-situ rheology control of 3D printed mortar. The influences of CO2 mixing on the properties of the cast mortar and the 3D printed mortar prepared with SF were systematically studied. The pH, conductivity and ion concentration were tested to revel the essential chemical reactions and phase evolutions during CO2 mixing. It was found that the CO2 mixing effectively enhanced the early-age penetration resistance and yield stress of mortar incorporated with SF, and thus significantly improved the buildability of 3D printed mortar, while the maximum printing layer increased higher than 33 layers. Meanwhile, the mechanical properties of the SF-incorporated mortar were improved by this process. It was revealed that during the CO2 mixing process, the injected CO2 rapidly reacted with calcium ions to form CaCO3, which further reacted with C3A to form monocarboaluminate (Mc). The fast precipitation of calcium ions during CO2 mixing accelerated the hydration of C3A and C3S, contributing to the rapid development of strength at early ages and the improved performance of hardened mortar. The addition of SF in mortar greatly amplified the beneficial effect of CO2 mixing, which due to SF as nucleation sites promoted the formation of CaCO3 and Mc during CO2 mixing.