Energy Minimization Method Research Articles

Wetting and Capillarity Effects on Bubble Formation From Orifice Plates Submerged in Pools of Water

Abstract An experimental study of bubble growth from submerged orifice plates in pools of water is carried out to scale and correlate the effects of surface wettability and orifice diameter D0 on ebullience. Measurements of bubble growth on surfaces with nine different contact angles (38 deg ≤ θ ≤ 128 deg) and varying air flow rates (1–300 ml/min) were made using high speed videography and image processing. In the static or constant-volume regime, below a critical contact angle θc, the bubble base remains attached to the orifice, and the equivalent departure diameter Db is independent of θ. On the other hand, above the critical contact angle, the bubble base spreads on the surface resulting in larger Db. For θ > θc, Db is strongly dependent on θ and increases with it. Using minimum energy method, it is shown that the wettability effects can be scaled and correlated by a modified capillary length, defined as a function of the Laplace length and contact angle. The proposed correlation provides predictions of Db that agree with experimental data of this study as well as those available in the literature to within ±15%. Moreover, for a hydrophobic surface when D0 > twice the modified capillary length, the bubble grows inside the orifice; for a hydrophilic surface, this scales with twice the capillary length, and effect of θ is not seen.

The application of thermal-calculation methods in the design and syngas prediction of entrained-flow coal gasifiers

A novel thermal-calculation method for membrane wall entrained-flow coal gasifier based on the equilibrium model was proposed in this study, and the method takes into account the heat transfer of the gasifier. The equilibrium model was established by the Gibbs free energy minimization method to avoid solving the intermediate chemical reaction process. The concepts of effective radiation heating area of furnace and flame center modification factor were introduced into the thermal-calculation method. This study not only analyzes the influence of oxygen-to-coal and water-to-coal ratios under normal operating conditions, but also investigates the impact of structural characteristics of a membrane wall entrained-flow gasifier on gasification characteristics. The model was effectively used to predict the syngas composition, the syngas temperature at the outlet of the chamber, and the energy distribution of the gasifier. The syngas composition and heat loss of gasifier obtained by thermal-calculation method are in good agreement with the experimental data of industrial-scale plants. The results show that the oxygen-to-coal ratio significantly impacts the syngas composition and heat loss than the water-to-coal ratio. Increase in the effective radiation heating area of furnace results in the decrease in the syngas temperature at the outlet of chamber, thus reducing the heat loss of flue gas and improving the heat absorption of the membrane wall tube. However, simultaneously, it can also reduce the CO concentration in syngas. Reduction in the flame center modification factor is helpful to increase the CO concentration; however, it also causes colossal heat loss of flue gas.

The influence of carbon nanotube coated-carbon fibers on thermal residual stresses of Multi-Scale hybrid composites: Analytical approach

The effect of carbon nanotube (CNT) coated-carbon fibers on thermal residual stresses of multi-scale hybrid composites is assessed employing analytical approach. The model comprises carbon fiber, coating region and surrounding matrix, in which the coating region around core fiber encompasses CNTs and matrix. Considering three configurations of grown CNTs on the fiber surface including axially, radially and randomly oriented, the mechanical properties of various coating regions are acquired employing the Eshelby–Mori–Tanaka method in conjunction with an equivalent continuum approach. Utilizing the total energy minimization method, the closed-form solution of the thermal residual stresses of hybrid composite is obtained. The results disclose a noteworthy influence of CNT–coating on the reduction of interfacial stresses which precludes debonding at interface and attenuates the effect of thermal expansions mismatch between the carbon fiber and matrix. Furthermore, the results demonstrate that unlike radially oriented CNTs, the existence of axially and randomly oriented CNTs at the coating region has a remarkable diminishing effect on residual interfacial stresses. It is also shown that increasing the coating thickness leads to reduction of maximum interfacial stresses even at a constant CNT volume fraction. A close agreement exists between predicted outcomes by the proposed analytical approach and published data in the literature.

Electrowetting-based control of wetting transition of a nanodroplet on pillar-arrayed surfaces

Two extreme wetting states, a highly non-wetting Cassie state and a wetting Wenzel state, can coexist or even mutually convert on patterned surfaces. Such a conversion process may be spontaneous or induced by external stimuli. This work studies the wetting transition of a nanodroplet on pillar-arrayed surfaces induced by an external electric field via an energy-minimization method in conjunction with molecular dynamics (MD) simulations. The simulation results reveal that driven by the electric field, the initial Cassie state could go through a partial wetting state, and eventually converts to the Wenzel state. The free-energy landscape reveals that there are multiple local energy minima, corresponding to multiple metastable states. For the metastable Cassie state, the wetting transition is irreversible, i.e., the droplet would remain in the Wenzel state when the electric field is removed. Conversely, the spontaneous dewetting transition from the Wenzel to the Cassie state can occur, if only the Cassie droplet is in a global energy minimum configuration. Thus, the stable Cassie wetting configuration is essential for triggering the spontaneous dewetting transition.

Energy minimization versus criteria-based methods in discrete cohesive fracture simulations

Energy minimization versus criteria-based methods in discrete cohesive fracture simulations

Prediction of the equilibrium compositions in the combustion products of a domestic boiler

In this study, two prediction models were developed to determine equilibrium compositions in the combustion products of a domestic boiler. These models are based on the Gibbs energy minimization and equilibrium constant methods. According to the domestic boiler standards (EN 15502–1 and EN 437), gaseous fuels (G20, G25, and NG) were selected to predict gaseous emissions. A novel investigation was realized for a domestic boiler's lean premixed burner in compliance with relevant standards. In the range of 0.6–0.9 equivalence ratio, the equilibrium compositions of the species were obtained.The proposed models were compared with CHEMKIN commercial software to verify the prediction of the results. Adiabatic flame temperatures and combustion products (NO, CO, N2, O2, CO2, and H2O) were estimated using prediction models. The estimated values showed good agreement with the results obtained from CHEMKIN software. Under efficient boiler operating conditions (ϕ=0.65, 0.7, and 0.75), minor deviations were observed comparing with the CHEMKIN results. For gaseous fuels, CO and NO emissions reach peak values near ϕ=0.9 and ϕ=0.85, respectively. Without using complex chemical kinetics mechanisms, models provide reliable estimates. The proposed models can be parametrically applied not only for boilers but also for other applications in combustion processes.

Combined autothermal and sorption-enhanced reforming of olive mill wastewater for the production of hydrogen: Thermally neutral conditions analysis

A thermodynamic study and an energy analysis are performed on the autothermal reforming (ATR) and sorption-enhanced autothermal reforming (SE-ATR) of olive mill wastewater (OMW) to produce green hydrogen. For comparative purposes, the traditional reforming (TR) and sorption-enhanced reforming (SER) are also assessed. The thermodynamic equilibrium compositions are calculated by using the Gibbs free energy minimization method. The effect of temperature, pressure and steam-to-carbon molar ratio (S/C) is assessed for the different reactor configurations. The energy analysis is done by determining the level of oxygen needed to operate under thermally neutral conditions.The results show a reduced hydrogen yield for ATR and SE-ATR compared to their non-autothermal counterparts, but a decreased energy requirement per mole of hydrogen produced. For the ATR, the thermally neutral operation requires a high amount of oxygen, which reduces the hydrogen yield. However, for SE-ATR, the exothermal sorption reaction provides nearly enough energy for the process to be thermally neutral by itself.

A Novel Model for Oil–Water Stratified Flow in Horizontal Wells With a Curved Interface Based on Dynamic Contact Angle

Abstract During the process of oil production and transportation, oil–water two-phase flow is a common occurrence. Well completion optimization and production design are greatly affected by the prediction accuracy of two-phase flow characteristics. In this paper, a novel model is proposed to predict the influence of interface shape on stratified flow. Dynamic contact angle theory and minimum energy method are introduced to solve the momentum equations with a curved interface and dispersed phase holdup in the lower water layer or the upper oil layer, respectively. When the Eotvos number is lower than ten, the interface shape changes from a flat surface to a curved surface, and the flow area of the upper water layer and the lower oil layer will increase and decrease, respectively. Results show that the dynamic contact angle and pressure gradient are greatly affected by oil superficial velocity, oil viscosity, and pipe diameter. By comparing the prediction with available experiment results, the validity of the model is evaluated. Results show that the novel model has an overall good prediction performance for the pressure gradient, with an average percentage error of 13.12%. While the average percentage error of Liu's model and two-fluid model are 22.89% and 34.98%, respectively. The novel model is a unified model that could be used to solve the problem with a curved/flat interface. It will also promote the oil well production design and horizontal well completion optimization.

Performance investigation of a non-combustion heat carrier biomass gasifier for various reforming methods of pyrolysis products

ABSTRACT Conventional biomass gasifiers such as fixed bed gasifiers, fluidized bed gasifiers, and entrained flow gasifiers have been widely studied in the literature covering biomass gasification systems. However, there are few published studies on non-combustion heat carrier biomass gasifier technology in the literature. Therefore, further studies are needed to reveal potential advantages, disadvantages, and possible improvement opportunities for this gasifier type. In this work, we investigate the effects of various operating parameters: reformation temperature, steam-to-biomass ratio, and steam-to-oxygen ratio on important performance indicators of non-combustion biomass gasifiers. To estimate the pyrolysis products, we use stoichiometric methods based on published data in the literature. The reforming of pyrolysis products is estimated using the Gibbs free energy minimization method using Aspen Plus. In addition, calculator blocks, sensitivity blocks, and design specifications are used to obtain the results. The results reveal that the desired hydrogen to carbon monoxide ratio can be obtained for steam reforming and autothermal reforming of pyrolysis products while a low hydrogen-to-carbon ratio (less than 1.5) is obtained for partial oxidation of pyrolysis products. However, partial oxidation can be beneficial in increasing the heating value of the syn-gas. The results presented in this study will be useful in improving the current configuration of non-combustion heat carrier biomass gasifiers. In future studies, the effects of gasifying agents on the tar content of the syn-gas, impurities in the syn-gas, start-up time of the gasifier, and size of the gasifier should be investigated.

Simulation and Optimization Study of an Ultra-Low-Field Bell-Shaped Head MRI Electromagnet

Ultra-low magnetic field systems have attracted increasing attention in recent years because they lack cryocoolers and exhibit lightweight, low cost, and mobilization convenience. In contrast to permanent magnet systems, electromagnetic systems can easily vary background magnetic fields, thus offering opportunities on multiple scales of time and distance to characterize the molecular dynamics and transport properties of complex liquids in bulk or embedded in confined environments. An unconventional ultra-low field bell-shaped head MRI magnet with a dedicated compact structure is needed for applications in clinics, intensive care units, and mobile vans and evaluating stroke in the hyperacute and acute stages. In this work, we use the discrete minimum energy (DME) method to design the ultra-low field bell-shaped head MRI electromagnet. This method involves obtaining a DME current map, initial coil extraction, and coil adjustment. The DME map is used to determine the initial current distribution of the MRI electromagnet over a predefined domain subject to constraints. Then, initial coil extraction aims to extract the initial regularized rectangle coil layout from the DME map. Considering that the extraction process decreases magnetic field homogeneity in the region of interest (ROI), coil adjustment is then implemented. This step adjusts the position and volume of the initial coil layout to meet the homogeneity requirement. A bell-shaped electromagnet ( $$< 20$$ mT, 2 MA/m $$^2$$ ) with a homogeneity of 7 ppm over 260 mm DSV is designed in this work.

Adhesion Process of Biomimetic Myelin Membranes Triggered by Myelin Basic Protein.

The myelin sheath—a multi-double-bilayer membrane wrapped around axons—is an essential part of the nervous system which enables rapid signal conduction. Damage of this complex membrane system results in demyelinating diseases such as multiple sclerosis (MS). The process in which myelin is generated in vivo is called myelination. In our study, we investigated the adhesion process of large unilamellar vesicles with a supported membrane bilayer that was coated with myelin basic protein (MBP) using time-resolved neutron reflectometry. Our aim was to mimic and to study the myelination process of membrane systems having either a lipid-composition resembling that of native myelin or that of the standard animal model for experimental autoimmune encephalomyelitis (EAE) which represents MS-like conditions. We were able to measure the kinetics of the partial formation of a double bilayer in those systems and to characterize the scattering length density profiles of the initial and final states of the membrane. The kinetics could be modeled using a random sequential adsorption simulation. By using a free energy minimization method, we were able to calculate the shape of the adhered vesicles and to determine the adhesion energy per MBP. For the native membrane the resulting adhesion energy per MBP is larger than that of the EAE modified membrane type. Our observations might help in understanding myelination and especially remyelination—a process in which damaged myelin is repaired—which is a promising candidate for treatment of the still mostly incurable demyelinating diseases such as MS.

Thermodynamic Analysis of Dry Reforming of Methane for Valorization of Landfill Gas and Natural Gas

Dry reforming of methane (DRM) is a promising technology to convert carbon dioxide (CO2) and methane (CH4), two major greenhouse gases into syngas (a mixture of carbon monoxide (CO) and hydrogen (H2)). A thermodynamic equilibrium analysis for DRM with a focus on carbon formation is carried out in Aspen Plus using the Gibbs free energy minimization method. The effects of feed CO2/CH4 ratio (0.5–3), reaction temperature (773–1373 K), and system pressure (0.1–10 atm) on the equilibrium conversion, product distribution, and solid carbon formation are investigated. From the analysis, it was found that the optimal operating conditions of 1 atm, 1123 K, and a feed ratio (CO2:CH4) of 1:1, minimized carbon formation, produced syngas at a H2/CO ratio of 1 (sufficient for downstream Fischer–Tropsch synthesis), while minimizing energy requirements. It is found that adding small amounts of oxygen or water significantly reduced carbon formation, minimized loss in syngas production, and reduced energy requirements. Three application scenarios were simulated to reflect the valorization of vented and flared natural gas and landfill gas (LFG). It was found that using captured CO2 with natural gas and LFG produced favorable results and therefore may be an opportunity for commercial DRM.

Heat-assisted incremental bending of metal plates with unsymmetrical curvatures

Small-batched metal plates with free-form surfaces are widely used in different manufacturing industries; however, it is unrealistic to produce such metal plates based on traditional die stamping and rolling processes. Over the last few decades, numerous efforts have been adopted to produce the metal plates with free-form surfaces with acceptable quality, efficiency, and cost through flexible forming processes. In the present paper, a heat-assisted incremental bending process was proposed to manufacture metal plates with large unsymmetrical curvatures. The test metal plates were supported by some hydraulic cylinders, and the punch moved up and down to press the metal plates according to the loading path determined by the minimum energy method. In addition, an induction heating system was applied to heat the metal plates at the punching position. One objective metal plate with large unsymmetrical curvature is deformed based on the heat-assisted incremental bending experiments. The obtained results indicate that the proposed thermo-mechanical flexible forming process could produce complex metal plates with good accuracy and high efficiency.

A compare review about equilibrium conditions of semi-clathrate hydrate: experimental measurements visions and thermodynamic modeling aspects

The formation of gaseous (gas) hydrates for storage, separation and transportation application is essential. In this regard, a comprehensive study of this case is essential. Semi-clathrate hydrates have higher temperature stability and are formed in a stable range. The purpose of this study is review the experimental and modeling of the semi-clathrate hydrates, to investigate the equilibrium conditions for the formation/dissociation of them based on the type of thermodynamic promoters like TBAB, TBAC, TBAF, TBANO3 and TBP groups. This review is consist of 4 overall section, at first an introduction to semi-clathrate hydrates has defined. After that, the experimental research has discussed through different gas systems such as CO2, CH4, N2, H2 etc. Also, the target of each study, like the CO2 capture, separation of CH4, formation/dissociation equilibrium conditions, are expressed. Then, in the modeling section, the different types of thermodynamic modeling like, equaling fugacity, intelligence computing, Gibbs free energy minimization and Chen-Guo method are collected. At final section, a comparison between types of promoter showed that the addition of TBAF to aqueous solution has the best promotion effect on the CO2 clathrate hydrate formation. Also, the results of comparing the concentration of promoters have shown that up to a certain amount of TBAB, the salt's role as a promoter and by addition concentration of promoter, has an inhibition effect. Also, besides the results of the comparison different promoters on equilibrium conditions of different gaseous hydrates, have indicated that, TBAB has the most significant impact on carbon dioxide hydrate.

Phase content prediction in polymer‐derived ceramics with metal additives

AbstractIn this study, silicon oxycarbide (SiOC) is selected as the base polymer to derive a SiOC ceramic (PDC) matrix, and four transition metals M (M = Ni, Mo, Co, and Zr) are individually introduced into the SiOC base to form various SiOC/M systems. SiOC‐Ni, SiOC‐MoCx, and SiOC‐CoSix are obtained by pyrolysis at 1100°C, whereas SiOC‐ZrOx forms upon pyrolysis at 1400°C. The selected SiOC/M systems encompass four different types of phase separation pathways—pure metal, metal carbide, metal silicide, and metal oxide (SiC‐SiO2‐C‐Ni, SiC‐SiO2‐C‐MoCx, SiC‐SiO2‐C‐CoSix, and SiC‐SiO2‐C‐ZrOx). The driving force for crystallization has been analyzed using a Gibbs free energy minimization method and phase fractions of these different PDC systems are calculated based on the lever rule. This work also reveals the energetics related to the quaternary systems and provides guidance to synthesizing metal‐containing PDCs with desired phase contents. In addition, we have examined the broad applicability of the phase content prediction method for a variety of other SiOC/M systems.

Modeling and simulation of pine sawdust gasification considering gas mixture reflux

In this study, a comprehensive model was developed by Aspen Plus software to simulate the pine sawdust gasification process on the basis of the Gibbs free energy minimization method. The accuracy of this model was validated by experimental data and another model. In addition, the computational experiments were carried out. The influence of temperature, equivalence ratio (ER), steam to biomass ratio (S/B) and the reflux ratio of gas mixture on the gas composition, heating value and the carbon conversion efficiency was investigated. Results showed that the predicted values were in good agreement with experimental data. The root means square error (RMSE) of each gas composition except methane ranged from 3.72 % to 8.30 % when varying gasification temperature and also ranged from 2.75 % to 9.11 % when varying steam to biomass ratio, and values of methane were both below 20 %. As for the influence of parameters, temperature had a significant effect on biomass gasification. The increase of this parameter not only can enhance the carbon conversion efficiency and the production of H2, but inhibit the generation of CO and CH4. In addition, equivalence ratio was directly proportional to the carbon conversion efficiency and the molar fraction of CO and CO2. The effect of steam to biomass ratio was similar with that of temperature. And the reflux operation of gas mixture could promote the gaseous products’ quality by promoting the generation of CO and reducing the molar fraction of CO2.

Hadron-quark mixed phase in the quark-meson coupling model

We explore the possibility of a structured hadron-quark mixed phase forming in the interior of neutron stars. The quark-meson coupling (QMC) model, which explicitly incorporates the internal quark structure of the nucleon, is employed to describe the hadronic phase, while the quark phase is described by the same bag model as the one used in the QMC framework, so as to keep consistency between the two coexisting phases. We analyze the effect of the appearance of hadron-quark pasta phases on the neutron-star properties. We also discuss the influence of nuclear symmetry energy and the bag constant $B$ in quark matter on the deconfinement phase transition. For the treatment of the hadron-quark mixed phase, we use the energy minimization method and compare it with the Gibbs construction. The finite-size effects like surface and Coulomb energies are taken into account in the energy minimization method; they play crucial roles in determining the pasta configuration during the hadron-quark phase transition. It is found that the finite-size effects can significantly reduce the region of the mixed phase relative to that of the Gibbs construction. Using a consistent value of $B$ in the QMC model and quark matter, we find that hadron-quark pasta phases are formed in the interior of massive stars, but no pure quark matter can exist.

Modelling of a soft multi-chambered climbing robot and experiments

Soft robots have wide potential applications prospects in unstructured environments owing to their being able to imitate forms of motion of creatures in climbing and creeping through small spaces. By utilising the high flexibility of soft materials, a pneumatic soft climbing robot was designed. At first, a model for soft climbing robots with a stiffness gradient was designed according to the drive mode of pneumatic networks in soft robots. Afterwards, the visco-mechanical properties of robots at the contact surface were analysed and also the deformation characteristics of cavities were discussed by using the method of minimum potential energy. Subsequently, through simulation and use of the finite element method, the optimal number of cavities in an actuator required by a climbing robot was calculated and also the climbing behaviours of the robot were analysed. Finally, by employing 3D printing and layer-by-layer casting, a prototype soft climbing robot was prepared to perform climbing experiments. The research is expected to provide a new method for monitoring complex unstructured environments.

Electronic, structural and optical properties of zincblend and wurtizite cadmium selenide (CdSe) using density functional theory and hubbard correction

Zinc blend (zb) and wurtizite (wz) structure of cadmium selenide (CdSe) is determined using density-functional theory within local density approximation (LDA), generalized gradient approximation (GGA), Hubbard-correction (GGA+U) and Hybrid functional approximation (PBE0 or HSE06). The first principle pseudopotential plane wave is used and the relaxed atomic position for the CdSe in zb and wz structure was obtained by using total energy and force minimization method following the Hellmann Feynman approach. The convergence test of total energy with respect to cutoff energy and k-point sampling is performed . The equilibrium lattice constant and unit cell volume of CdSe in both phases are calculated and the obtained value is compared` with experimental values. In addition the band gap of CdSe is analyzed using DFT within LDA, GGA, DFT+U and PBE0 to approximate the unknown exchange correlation functional. The band gap values obtained using LDA and GGA are severally under estimated due to their poor approximation of exchange-correlation potential. This problem was improved by using projector augmented-wave pseudopotential within Hubbard-correction (GGA+U) and the hybrid functional approximation. Optical properties: complex and real parts of dielectric function, energy loss spectrum and absorption coefficient of CdSe in both ZB and WZ phase were studied.

Decomposition Characteristics of SF6 under Arc Discharge and the Effects of Trace H2O, O2, and PTFE Vapour on Its By-Products

The research on decomposition characteristics of SF6 and its by-products have great significance to the operation, maintenance, condition assessment and fault diagnosis of power equipment. In this paper, the particle composition models of SF6, SF6/polytetrafluoroethylene (PTFE), SF6/PTFE/O2, SF6/PTFE/H2O, and SF6/PTFE/O2/H2O were established by using Gibbs free energy minimization method, and the effects of trace H2O and O2 impurities and PTFE vapour on SF6 by-products were studied by the models. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF6 by-products. It was found that when PTFE vapour is involved in the arc plasma, the main by-product after arc quenching is CF4, and the molar fractions of C2F6 and C3F8 are very low. When O2 is involved, the main by-products are SOF2, SO2 and SO2F2, and a small amount of CO and CO2 was also produced. When H2O is involved, the main by-products in simulation are SOF2, SO2 and HF, and a small amount of SO2, CO2, CO, SO2F2 and H2 was also produced. The experimental results are in good agreement with the above results.