Subcritical Conditions Research Articles

Comparative molecular dynamics study of liquid hydrogen annular jets in subcritical and supercritical environments

In this paper, the injection characteristics of a liquid hydrogen propellant jet under subcritical and supercritical operating conditions are comparatively studied at the molecular scale using molecular dynamics methods in the context of spacecraft propulsion systems. To control the temperature and pressure within the environment and nozzle respectively, two sub-computational domains are set up for separate modeling. The sub-computational domain merging process eliminates the broken molecular bonds by using the computational domain reduction in the x-direction. The jet velocity, density and other physical property distributions at different temperatures and pressures were comparatively studied. It is found that the rate of change of the radial density of the jet under supercritical conditions is small. Based on this, the surface area and volume of the liquid phase region of the jet were extracted and calculated. Compared to result of subcritical case, the volume of the liquid hydrogen jet in the supercritical environment is reduced by 1.9%, while the surface area is increase by 6.7%. To explain the variation in the surface area due to molecular diffusion, the molecular stress results revealed that the stress in the hydrogen jet in supercritical environment is only 88.9% of that in subcritical environment.

Numerical study on the effect of tangential intake design and inflow discharge on vertical dropshaft assessment using pressure and velocity distributions

Tangential intake structures with vertical dropshafts are commonly employed in stormwater and urban drainage systems. Vortex and plunging flows are the two typical flow patterns observed in dropshafts, with the former being the desired due to its favourable drainage performance and enhanced safety. The transition between these flow patterns is primarily influenced by the structural design and rate of flow discharge. To investigate the interception performance and the transition criteria between vortex and plunging flows, a series of numerical simulations were conducted. The effects of the approach channel slope, contraction ratio, and flow discharge were analyzed by comparing the evolution of the water surface, pressure and velocity distributions, and swirling distance across various scenarios. With a higher flow rate of 20 L/s, obvious water congestion was observed in dropshafts with contraction ratios below 0.4 and horizontal approach channels. However, the inflow channel was not congested in cases with sloping approach channels, indicating higher drainage efficiency of intake structures with slopping channels. Two hydraulic factors, namely, the minimum to maximum wall pressure ratios (Pmin/Pmax) and the minimum Vr/Vz ratios (Vr/Vz)min, were employed to differentiate between vortex flow and nonrotational flow. These two factors consistently led to the same conclusions under both subcritical and supercritical inflow conditions, indicating their validity and their potential application in the design and optimization of intake structures. The criteria of Pmin/Pmax > 0.2 and (Vr/Vz)min > 0.2 can be used as criteria for the formation of stable vortex flow.

Extraction of valuable metals from waste lithium iron phosphate batteries by subcritical water leaching method with synergistic waste polyvinyl chloride

In this paper, the effect of leaching valuable metals from waste lithium iron phosphate (LiFePO4) using waste polyvinyl chloride (PVC) as a source of hydrochloric acid under subcritical water treatment conditions was investigated. The effects of various parameters including temperature, reaction time, PVC/LiFePO4 mass ratio, and solid-to-liquid ratio on metal leaching were investigated. The results showed that the leaching of metals and dechlorination of PVC could be achieved simultaneously under subcritical hydrothermal conditions. The solid residues were analyzed using XRD, XPS and SEM-EDS elemental mapping to identify the compounds in the leached solids. ICP-OES analysis showed that optimal reaction conditions, including a temperature of 275 °C, a reaction time of 120 min, a PVC/ LiFePO4 mass ratio of 3:2, and a solid/liquid ratio of 1:10 (g/mL), could effectively leach 99.10 % of Li and 83.12 % Fe. Finally, the Li in the leachate was recovered in the form of Li3PO4 by precipitation. These findings suggest that subcritical water synergistic waste PVC technology is a promising and eco-friendly method to recover Li and Fe from waste lithium iron phosphate.

Technical benefits of the subcritical inlet condition for high-speed CO2 centrifugal compressor in the advanced power-generation cycle

In the CO2 power-generation cycles, the compressor inlet condition has significant impacts on the performances of turbomachinery as well as the economies of cycle systems. In this article, based on numerical simulation and system parametric analysis, we compared the subcritical and supercritical inlet conditions of the CO2 compressor, which is from a prototype in a MWt simple recuperated cycle (SRC). As the result, the advantages and particularities of the subcritical inlet condition for the CO2 compressor are discussed and confirmed from the aero/thermo-dynamics and thermal/technical economies. In the study, we found the efficiency and pressure ratios of the CO2 compressor were significantly improved as the inlet pressure reduced below the critical point. And the operation of transcritical compression would not be obviously impacted by phase changes at the far-blockage conditions. In the system parametric analysis, we found the thermal and technical economies of the SRC system obtained significant rises at the subcritical compressor inlet condition. The increase of system efficiency reached 2–3%, while the financial cost reduced by 7.7%. The structural and mechanical safety of key components was also improved. Thereby, the present study provides different views to the benefits of subcritical compressor inlet conditions in CO2 cycles.

Hole-driven dynamics of a three-dimensional gravitational liquid curtain

It is known that the disintegration of vertical liquid curtains (sheets) is affected crucially by the amplification of free edge holes forming inside the curtain. This paper aims to investigate the influence of the hole expansion dynamics, driven by the so-called rim retraction, on the breakup of a liquid curtain, in both supercritical (Weber number $We > 1$ ) and subcritical ( $We < 1$ ) conditions. The analysis is based on three-dimensional direct numerical simulations. For a selected supercritical configuration, the steady flow topology is first analysed. The investigation reveals the classic triangular shape regime of the steady curtain, due to the surface-tension-induced borders retraction towards its centre plane. The unsteady dynamics is then investigated as the curtain response to a hole perturbation introduced artificially in the steady flow configuration. The hole evolution determines a rim retraction phenomenon inside the curtain, which is influenced by both capillary and gravity forces. In supercritical conditions, the hole does not influence the curtain flow dynamics in the long-time limit. By reducing the Weber number slightly under the critical threshold ( $We=1$ ), the initial amplification rate of the hole area increases, due to the stronger retraction effect of surface tension acting on the hole rims. The free hole expansion in fully subcritical conditions ( $We < 1$ ) is investigated finally by simulating an edge-free curtain flow. As $We$ decreases progressively, the hole expands while it is convected downstream by gravity acceleration. In the range $0.4< We<1$ , the subcritical curtain returns to the intact unperturbed configuration after the hole expulsion at the downstream outflow. For $We<0.4$ , the surface tension force becomes strong enough to reverse the gravitational motion of the hole top point, which retracts upstream towards the sheet inlet section while expanding along the lateral directions. This last phenomenon causes finally the breakup of the curtain, which results in a columnar regime strictly resembling similar experimental findings of the literature.

Assessment of Aerodynamic Plates Subjected to Von Kármán Vortex Street for Enhancing the Wind Energy Generation in Blade-Less Devices

This study explores the feasibility of using an oscillating plate downstream of a cylindrical body to produce mechanical energy from a Von Kármán vortex street under sub-critical flow conditions (Re = 72,500). The study aims to quantify the impact of the plate length, its separation from the cylinder, and a machine damping factor on the power coefficient and the blade’s displacement to identify the optimal configuration. This preliminary assessment assumes that the plate oscillation is small enough to avoid changes in the vortex dynamics. This assumption allows the construction of a surrogate model using Computational Fluid Dynamics (CFD) to evaluate the effect of plate length and separation from the cylinder on the fluctuating lift forces over the plate. Later, the surrogate model, combined with varying machine damping factors, facilitates the description of the device’s dynamics through the numerical integration of an angular momentum equation. The results showed that a plate with a length of 0.52D, a separation of 5.548D from the cylinder, and a damping factor of 0.013 achieved a power coefficient of 0.147 and a perpendicular displacement of 0.226D. These results demonstrate a substantial improvement in the performance of blade-less generators.

Subcritical Water Hydrolysis of Cotton Fibers to Nanocellulose for Producing Poly(vinyl alcohol)/Cellulose Biocomposite

AbstractCellulose nanofibers (CNs) are isolated from cotton wool using subcritical water in a semiflow hydrothermal reactor to achieve green hydrolysis. The yield and crystallinity index (CI) of nanocellulose is optimized via varying temperatures and hydrolysis time while maintaining the pressure at 10 MPa. Under the subcritical condition, water dissociates into H3O+ and OH−, allowing protonation of the β‐glycosidic bond to hydrolyze cellulose strands into short chain fibers. Hydrolysis rates enhance with increasing temperatures, further disintegrating cellulose into smaller nanofibrils. At 180 °C, amorphous cellulose dissolution occurs, increasing the crystallinity without disintegrating the crystalline fibrous network. The concentration of H3O+ increases at 230 °C, further hydrolyzes cotton wool into uniform nanofibril cellulose with 14.3 ± 1.2 nm diameter. The tensile strength and elongation at break enhance using cellulose nanofibers obtained at 220 and 230 °C due to the small nanocellulose sizes that allow efficient dispersion in the poly(vinyl alcohol) (PVA) matrix.

Phytochemical studies of white mulberry fruits (Morus alba L.).

Medicinal preparations made from plant materials have been widely used for many years due to their high pharmacological efficacy and safety of use. Therefore, a study of white mulberry fruits (Morus alba L.) for the content of substances is very important for the pharmaceutical industry such as flavonoids, alkaloids, polysaccharides, minerals, vitamins, and amino acids. White mulberry has a wide distribution area around the world, including in Kazakhstan, especially in the southern regions of the country (Almaty, Zhambyl, and Turkestan regions). The composition of the fruits of this plant is significantly influenced by the area where the trees grow, and therefore, the establishment of a specific composition of biologically active substances is very important. In the course of this study, such methods as gas chromatography were used-mass spectrometry of an extract obtained using carbon dioxide under subcritical conditions, atomic absorption, gravimetric, and spectrophotometric methods. As a result, for the first time in Kazakhstan, the composition of white mulberry fruits (Morus alba L.), namely, biologically active substances, has been identified, such as alkaloids, flavonoids, vitamins, macro- and microelements, and amino acids and fatty acids; in addition, the percentage composition of the above compounds has been determined. The results of the study show a comparative analysis of the component composition of white mulberry fruits (Morus alba L.) in various areas of tree growth, including outside of Kazakhstan. The obtained data testify to the great possibilities of using this raw material in medicine, pharmacology, and the food industry.

Degradation and Extraction of Organochlorine Pollutants from Environmental Solids under Subcritical Water Conditions.

A subcritical water degradation and extraction method was developed to remediate environmental soils contaminated by highly recalcitrant organochlorine pollutants. Hydrogen peroxide was used to effectively decompose organochlorine pollutants under subcritical water conditions. As a method optimization study, the static wet oxidation of chlorophenols was first performed in subcritical water with and without added hydrogen peroxide. Complete oxidation was achieved using an added oxidant, and thus, the oxidation and extraction of chlorophenols from a sand matrix was then attempted. Complete oxidation and extraction with added oxidant were achieved within 30 min at 100 °C. We then investigated the subcritical water degradation and extraction of dieldrin, mirex, and p,p'-DDD. These organochlorine pesticides were not as easily oxidized as the chlorophenols, and the benefit of adding hydrogen peroxide was only clearly observed at 200 °C. Approximately a 20% increase in degradation was noted for each pesticide and insecticide at this temperature. Unfortunately, this difference was not observed with an increase in temperature to 250 °C, except in some cases, where the amount of degradation byproducts was reduced. Dieldrin and p,p'-DDD were essentially destroyed at 250 °C, while all the pesticides and the insecticides were completely removed from the sand at this temperature. The proposed method was then used to remediate a soil sample highly contaminated with DDT. The soil was obtained from the grounds of an old DDT mixing facility in Virginia and has been aging for several decades. Not only was 100% removal of DDT from this soil achieved using the proposed method at 250 °C, but also, the extracted DDT was completely destroyed during the process. The proposed remediation method, therefore, demonstrates a high potential as an efficient and environmentally sound technique for the detoxification of soils.

A Comprehensive Review of Organic Rankine Cycles

It has been demonstrated that energy systems driven by conventional energy sources like fossil fuels are one of the main causes of climate change. Organic Rankine cycles can help to reduce that impact, as they can be operated by using the industrial waste heat of renewable energies. The present study presents a comprehensive bibliographic review of organic Rankine cycles. The study not only actualizes previous reviews that mainly focused on basic cycles operating on subcritical or supercritical conditions, but also includes the analysis of novel cycles such as two-stage and hybrid cycles and the used fluids. Recuperative and regenerative cycles are more efficient than reheated and basic single-stage cycles. The use of two-stage cycles makes it possible to achieve higher thermal efficiencies and net power outputs of up to 20% and 44%, respectively, compared with those obtained with single-stage cycles. Theoretical studies show that hybrid systems, including Brayton and organic Rankine cycles, are the most efficient; however, they require very high temperatures to operate. Most organic Rankine cycle plants produce net power outputs from 1 kW up to several tens of kW, mainly using microturbines and plate heat exchangers.

Band-gap alteration of Zn2SnO4 nanostructures for optical and photo-luminescent applications

Present manuscript reports the hydrothermal synthesis of pristine (Zn2SnO4) and copper doped Zn2SnO4 spinals under subcritical conditions. XRD pattern identifies the cubic phase of pristine and its doped nanostructures. Tauc plot and density functional theory (DFT) studies claim the lowering of band gap values up to 2.5 eV upon the insertion of transition metal copper (Cu) into the host lattice. Blue shift occurs due to Moss−Burstein effect upon excess concentration of Copper is realized. Both pristine and Cu doped Zn2SnO4 present light green and dark blue colour upon the irradiation of different UV (252 nm and 365 nm) lamps, respectively. Besides this, Zn2SnO4 shows effectiveness in hosting Cu ions, thus could act as a prospective green/blue emitter.

Catalytic Hydrothermal Deoxygenation of Stearic Acid with Ru/C: Effects of Alcohol- and Carboxylic Acid-Based Hydrogen Donors.

Catalytic hydrothermal processing is a promising technology for the production of biofuels used in transportation to alleviate the energy crisis. An important challenge for these processes is the need for an external supply of hydrogen gas to accelerate the deoxygenation of fatty acids or lipids. It follows that in situ-produced hydrogen can improve process economics. This study reports on the use of various alcohol and carboxylic acid amendments as sources for in situ hydrogen production to accelerate Ru/C-catalyzed hydrothermal deoxygenation of stearic acid. Addition of these amendments significantly increases yields of liquid hydrocarbon products, including the major product heptadecane, from stearic acid conversion at subcritical conditions (330 °C, 14-16 MPa during the reaction). This research provided guidance for simplifying the catalytic hydrothermal process of biofuel production, making the production of the desired biofuel in one pot possible without the need for an external H2 supply.

Recovery of rare earth elements from phosphogypsum using subcritical water extraction

Phosphogypsum (PG) is generated in massive amount in the production of fertilizer. It has been indicated that PG is a promising secondary source for rare earth elements (REEs). This study examined the leaching of REEs, including yttrium (Y), lanthanum (La), cerium (Ce) and neodymium (Nd) from PG utilizing subcritical water extraction (SWE). Hydrochloric acid (HCl) was selected as the leaching reagent, and the leaching efficiency increased with increasing HCl concentration and decreasing solid to liquid ratio (S/L). Complete leaching of REEs (100.36% of Y, 97.50% of La, 100.06% of Ce and 96.58% of Nd) was achieved under experimental conditions of 0.1 mol/L HCl, 10 g/L, 100 °C, initial pressure of 10 kg/cm2, and 5 min. The total energy consumption in SWE was 4.2 × 106 kJ/kg. Leaching efficiency of Y, La, and Nd slightly increased as temperature increased to 125 °C, and decreased when further increased to 150 °C. The behavior was due to solubility of anhydrite as affected by temperature and pressure under subcritical conditions, and the enhanced mass transfer. From comparisons, SWE could be an efficient and green technology for the recovery of REEs from PG.

Scour around the single emergent cylinder due to subcritical and supercritical approach flow conditions

ABSTRACT Flume experiments with a uniform cohesionless sand with d 50 = 0.9 m m were utilized in the current study to examine the local scour process around a single emergent cylinder under various geometrical circumstances and with a various flow characteristic that simulates an inundating tsunami flow. This single emergent cylinder represents a piloti-type column located over a movable bed material because piloti-type columns are used to elevate the building structures from the ground level. In the experiments, it was noted that the maximum scour depth probably occurs at the cylinder’s upstream face, resembling local scour around a scaled-down bridge pier in an open channel. Additionally, the scour hole around the cylinder is shallower the smaller they are, as are the horseshoe vortices that surround them. Furthermore, the effects of the surface bow-wave, hydraulic jump detachment and the wall-jet over the front face of the emerging cylinder were used to identify the relationship concerning the scouring pattern and selected flow characteristics over a piloti-type emergent column under tsunami flow. Empirical formulae have been finally developed to determine the maximum scour depth in front of the cylinder, which is beneficial to characterize the stability of the piloti-type columns, considering the specified Froude number conditions.

Component composition and antimicrobial activity of subcritical CO2 extract of Ferula asafoetida L., growing in the territory of Kazakhstan

Ferula asafoetida L. has been studied for centuries by many scientists as food products and in traditional medicine, which are relevant for Kazakhstan as well. Recent studies have shown that Ferula asafoetida L. has neuroprotective, memory-enhancing, digestive enzymatic, antioxidant, antispasmodic, hypotensive, hepatoprotective, antimicrobial, antitumor, cytotoxic, and anthelmintic effects. The aim of this study is to determine the component composition by GC-MS method and to study the antimicrobial properties of Ferula asafoetida L. extract obtained by CO2 extraction, which grows in Kazakhstan. Materials and methods. To determine the possibility of using Ferula asafoetida L., we carried out the component composition of the extract obtained by CO2 extraction in subcritical conditions of the underground part of Ferula asafoetida L. by a certain GC-MS method and also investigated the antimicrobial effect of this extract. Results. The plant raw materials were collected in accordance with GACP requirements. Conducted subcritical CO2 extraction of plant raw materials showed a 2.5% extraction yield. The study of the component composition by GC-MS revealed 46.3% of sulfate compounds. The determination of antimicrobial activities showed high efficacy against gram-positive (Staphylococcus aureus subsp. Aureus, Bacillus subtilis subsp. Spizizenii), gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae subsp. pneumoniae, Salmonella enterica subsp. enterica) and fungi (Candida albicans, Aspergillus niger). Conclusions. The possibility of using the obtained CO2 extract of Ferula asafoetida L. in the field of pharmaceutical products as a substance and a drug that has a huge antimicrobial effect

Determination of Chemical Composition and Antimicrobial Activity of the CO2 Extract of Eryngium planum L.

The article presents parameters for obtaining a carbon dioxide extract from the subterranean part of Eryngium planum that contains a valuable set of organic substances and has a certain antimicrobial effect. Methods. Raw materials were collected in the Almaty region (Republic of Kazakhstan). The CO2 extract of Eryngium planum herbs was obtained under subcritical conditions. A gas chromatograph with a mass spectrometric detector was used to determine the compositional breakdown of the extract. Antimicrobial activity was determined by two methods: the micromethod of serial dilutions and the disk-diffusion method. Three microbial test strains were used: Staphylococcus aureus ATCC 6538-P, Escherichia coli ATCC 8739, and Candida albicans ATCC 10231. Results. To extract biologically active substances from the subterranean part of Eryngium planum L., we have chosen carbon dioxide extraction technology, a technology for processing carbon dioxide (CO2) raw materials, which allows us to extract various substances in high concentrations. Carbon dioxide extraction technology is an effective and environmentally safe way to isolate various biologically active substances contained in medicinal plant raw materials. In the composition of the CO2 extract of Eryngium planum L. 43 components were identified, the main of which are α-linolenic acid, 8.30%; myristic acid, 6.40%; caryophyllene, 6.92%; spatulous, 6.62%; and other main identified compounds and their percentage. Conclusions. The study showed that the CO2 extract of Eryngium planum L. contains biologically active compounds that have a pronounced antimicrobial effect against clinically significant microorganisms, such as Escherichia coli, Staphylococcus aureus, and Candida albicans.

Explaining the Lack of Mesh Convergence of Inviscid Adjoint Solutions near Solid Walls for Subcritical Flows

Numerical solutions to the adjoint Euler equations have been found to diverge with mesh refinement near walls for a variety of flow conditions and geometry configurations. The issue is reviewed, and an explanation is provided by comparing a numerical incompressible adjoint solution with an analytic adjoint solution, showing that the anomaly observed in numerical computations is caused by a divergence of the analytic solution at the wall. The singularity causing this divergence is of the same type as the well-known singularity along the incoming stagnation streamline, and both originate at the adjoint singularity at the trailing edge. The argument is extended to cover the fully compressible case, in subcritical flow conditions, by presenting an analytic solution that follows the same structure as the incompressible one.

Reactive transport modelling under supercritical conditions

The utilisation of supercritical geothermal fluids depends on the assessment of the long-term effects of fluid-rock interaction on flow and transport behaviour. Reactive transport simulations are used for these assessments quantitatively based on high-quality thermodynamic and chemical data, understanding of the thermochemical constraints, and capability for accurate, stable, and rapid simulations. This work explores the precipitation/dissolution behaviour of basalt in water under supercritical conditions (400 °C and 502 bar) based on a comprehensive laboratory study conducted by Passarella (2021).The mineral reactions in the experiment were found to be largely kinetically controlled based on the model results. The final model dissolution rate constants were generally within an order of magnitude of the values calculated using the Arrhenius Law at 400 °C, based on published k25 and Ea values, paired with the reactive surface area estimates following the work of White and Peterson (1990). This indicates that the reaction rates at the experimental supercritical conditions are consistent with those observed at subcritical conditions for each mineral. It is theorised that this is because the properties of supercritical water at high pressures are comparable to subcritical water, specifically in terms of viscosity and the dielectric constant. A similar review of the final model precipitation rate constants showed better agreement with neutral dissolution rates extended to precipitation than the estimates following Horiuti (1957) for the experimental conditions.Mineral alterations during the experiment depended primarily on glass dissolution based on the model. Fluid saturation relative to the other minerals increased from the release of glass dissolution products, affecting their dissolution rates. Glass dissolution was also the primary source of components for secondary mineral formation, including celadonite, which also depended on olivine dissolution to provide magnesium. Upon glass depletion, the dissolution rates of the remaining primary minerals increased, and celadonite dissolved. Chlorite was deposited later in the modelled experiment than the other secondary minerals.

Isomerization and epimerization of glucose and galactose in arginine solution and phosphate buffer under subcritical fluid conditions.

Reaction of glucose or galactose was performed in arginine solution or phosphate buffer (pH 7.0) using a batch reactor at 110°C. The yields of products, pH, and absorbances at 280 and 420nm were measured during the reaction. Fructose, mannose, and allulose were formed from glucose; tagatose, talose, and sorbose were done from galactose. The reaction proceeded more rapidly in arginine solution than in phosphate buffer. In arginine solution, yields of fructose and tagatose were 20% and 16%, respectively, after 30-min reaction; in phosphate buffer, they were 14% and 10%, respectively. However, in both reaction media, the pH drop and increase in absorbances continued even after the yield became almost constant. The absorbance increased particularly in the latter half of the reaction due to formation of browning products. Therefore, to avoid browning, the reaction should be stopped as soon as possible after the yield approaches its maximum value.

Biodiesel production from crude palm oil under subcritical methanol conditions: Experimental investigation and kinetic model

Subcritical methanol approach is an environmentally benign method to produce biodiesel owing to no wastewater and soap formation during the process. However, biodiesel production from low-cost feedstock, i.e., crude palm oil (CPO) under subcritical methanol conditions, has not been evaluated well. Hence, this study aims to investigate a subcritical methanol technique for biodiesel production from CPO. Transesterification was performed using a batch reaction vessel under varied temperatures (160–200 °C), catalyst loadings (0.25–0.75 wt%), methanol-to-CPO molar ratios (5:1–30:1), and reaction times (10–60 min). The result showed that a biodiesel yield of 91.41 % was obtained at 180 °C within 50 min. The reaction kinetics of the transesterification process under subcritical methanol conditions was also deduced, and the reaction was observed to obey Arrhenius's behavior. The developed kinetic model was observed to fit the experimental data well.