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The proficiency of hybrid nanoparticles in augmenting the heat transfer has fascinated many researchers to further analysing the working fluid. The present paper is focused on the MHD hybrid nanofluid flow with heat transfer on a moving plate with Joule heating. The combination of metal (Cu) and metal oxide (Al2O3) nanoparticles with water (H2O) as the base fluid is used for the analysis. Similarity transformation reduces the complexity of the PDEs into a system of ODEs, which is then solved numerically using the function bvp4c from MATLAB for different values of the governing parameters. Two solutions are obtained when the plate is moved oppositely from the free stream flow. Analysis of flow stability unveils the first solution as the real physical solution, which is realizable in practice. From physical perspective, the real solution must be available for all cases of λ which affirms the finding from stability analysis. An upsurge of suction’s strength and magnetic parameter enhances the heat transfer operation and extends the critical value λ c . Meanwhile, there is no change on the critical value when the Eckert number is added. This study is important in determining the thermal behavior of Cu-Al2O3/H2O when the physical parameters like magnetic field and Joule heating are embedded. The results are new and original with many practical applications in the modern industry.

A newly five-component equimolar high-entropy ceramic – (La0.2Eu0.2Gd0.2Y0.2Yb0.2)2Zr2O7 with a mixture phase of pyrochlore and fluorite structure was successfully synthesized. On this basis, a series of systematically designed and synthesized high or medium-entropy ceramics, accompanied by a large number of previous reports on multi-component rare earth zirconates, were summarized to jointly investigate the single-phase forming ability of high-entropy rare earth zirconates ceramics (HE-RE2Zr2O7 for short) in terms of entropy value and cationic radius difference ( δ ). Subsequently, the forming process and mechanism of dual-phase solid solution were analyzed. The conclusions indicated that the single-phase forming ability was more determined by the cationic radius difference rather than the entropy value，and the critical value of δ distinguishing single-phase between dual-phase zone was around 5.2%. In addition, the formation of dual-phase solid solution was essentially driven by the size disorder resulting from rattler effect of cations with significant difference in radius. This work is the first report on the single-phase forming ability of HE-RE2Zr2O7 and would pave new ways for predicting the formation of high-entropy phases.

In this study, the evaporation characteristics and drying patterns of various sessile ferrofluid droplets on certain substrate under horizontal magnetic fields of controlled intensities are reported. The effects of droplet concentration and magnetic field intensity on the duration of each evaporation stage and drying patterns of droplets have been systematically investigated. It turned out that a plateau appears at the initial stage of evaporation in the absence of magnetic field and it was found that the plateau value is positively correlated with the concentration of ferrofluid droplets. Under the external magnetic field, the evaporation time of droplets decreases, the stage of contact line retreat extends, the stage of late pinning mode shortens, and the deposition area of ferrofluid droplet decreases compared to that of without magnetics field. The deposition area increases gradually and becomes more uniform with the increase of magnetic field. The decrease of friction force which is due to the decrease of the number of nanoparticles at the contact line under external magnetic field is the main reason for the observed phenomena. We found that the coffee ring and the uniform deposition inside the droplet will be destroyed when the magnetic field intensity is higher than a critical value. Our work has a significant reference value for the evaporation of sessile magnetic fluid droplets under the applied magnetic field, especially when the drying pattern needs to be precisely controlled, such as in spray or biomedicine.

Structural damage from sample preparation processes such as cutting and polishing may change the pore structure of rocks. However, changes in pore structure caused by this structural damage from crushing and its effect on marine continental transitional shale have not been well documented. The changes of microscopic pore structure in marine continental transitional shale during the sample preparation have important research value for subsequent exploration and development of shale gas. In this study, the pore structures of transitional shale samples from the Shanxi-Taiyuan Formation of the Southern North China Basin under different degrees of damage were analyzed through low-temperature N2 adsorption experiments, combined with X-ray diffraction, total organic carbon, vitrinite reflectance analysis, and scanning electron microscopy. The results showed that (1) With increasing structural damage, the specific surface area (SSA) changed within relatively tight bounds, while the pore volume (PV) varied significantly, and the growth rate (maximum) exhibited a certain critical value with the crushing mesh number increasing from 20 to 200. (2) The ratio of SSA to PV can be used as a potential proxy for evaluating the influence of changes in the pore structure. (3) Correlation analysis revealed that the microscopic pore structure of marine continental transitional shale from the Shanxi-Taiyuan Formations is mainly controlled by organic matter and clay minerals. Clay minerals play a leading role in the development of microscopic pores and changes in pore structure.

In this paper, we present a stochastic hepatitis B virus (HBV) infection model and the dynamic behaviors of the model are investigated. When the fraction of vertical transmission μ ω ν C is not considered to be new infections, the existence and ergodicity of the stationary distribution of the model are obtained by constructing a suitable Lyapunov function, which determines a critical value ρ 0 s corresponding to the basic reproduction number of ODE system. This implies the persistence of the diseases when ρ 0 s > 1 . Meanwhile, the sufficient conditions for the extinction of the diseases are derived when ρ 0 T 0 . What is more, we give the specific expression of the probability density function of the stochastic model around the unique endemic quasi-equilibrium by solving the Fokker–Planck equation. Finally, the numerical simulations are illustrated to verify the theoretical results and match the HBV epidemic data in China.

Electrocoalescence is widely used to separate water from crude oil in petroleum industry. However, the electric field often forces water droplets distributed in chain shape, thus improving the danger of short circuit by bridging electrodes. To understand the mechanism of water chain phenomenon, an in-situ observation experiment were conducted to investigate the effect of voltage, frequency, water content and surfactant concentration. The results showed that the water chain length and growth rate of droplet diameter varied synchronously with the voltage and frequency. Additionally, the water chain length and growth rate of droplet diameter increased firstly and then decreased with the initial water content. The initial droplet diameter decreased with the surfactant concentration, which inhibit formation of water chains and coalescence. As the surfactant concentration reached a critical value, the initial droplet diameter remained constant, while the inhabitation of micelle for the coalescence strengthened with the surfactant concentration. These conclusions are helpful to understand the mechanism of formation of water chains.

In this paper we first establish a new critical point theorem which may produce a relationship between the sign-changing critical point, the Morse index, the upper bound of the critical value, the value of the type inf k ≤ dim ⁡ Y ∞ Y ⊂ E , ⁡ sup Y ⁡ G and the number of nodal domains. An unified approach is introduced for these composite properties. As one application of the abstract theorem, we study the existence of infinitely many sign-changing bound states for the following nonlinear stationary Schrodinger equation: − Δ u + a ( x ) u = | u | p − 2 u , u ∈ H 1 ( R N ) , where N ≥ 2 , p > 2 and p 2 N / ( N − 2 ) : = 2 ⁎ when N > 2 ; the potential a ( x ) ∈ C ( R N , R ) is a nonnegative function verifying suitable decay assumptions, but being not symmetric or periodic. We obtain the information on the precise estimates of the energies, the Morse indices and the number of nodal domains for these sign-changing bound states. We believe that the abstract theorem in this article will have more applications.

Flash event caused by a DC electric field/current was applied to the crack healing in 8 mol % Y2O3 stabilized cubic ZrO2 polycrystals (8Y-CSZ). The flash event, which occurred by applying the DC power higher than a critical value of 100 mW/mm3, successfully healed the microcrack within several minutes without any healing agents at a furnace temperature of 800 °C. As compared to the healing treatment under static annealing, the healing phenomena were accelerated about 2 times under the flash treatment even at the same temperatures, suggesting that the enhanced healing phenomena cannot be explained only by the temperature effect. Since the rate of grain growth was accelerated under the flash treatment, the flash healing would be accelerated through the current-enhanced diffusional processes. This study shows for the first time that the flash event has a potential to apply to the crack healing process in the ceramic materials and composites.

In this study, a new method of microwave-induced discharge over biomass-derived char (i.e., bio-char) spherical carriers was proposed. Bio-char spherical carriers were obtained through rational forming. The discharge intensity and temperature increase under discharge were measured to elaborate the quantitative relationship between microwave discharge and the temperature increase. The effect of bio-char properties on microwave-induced discharge was investigated. The results revealed that discharge achieved the highest electric–thermal conversion efficiency of 61.04%, and the heat generation ratio through the discharge in the initial 10 min was up to 69.08%. The graphitisation degree and transmission depth of microwaves can affect discharge intensity. A critical light intensity inducing hot spot effect exists under different working conditions, and exceeding the critical value led to the hot spot effect and a sudden increase in bed temperature, with a corresponding range of 2.48–5.76 V. The maximum temperature under microwave discharge reached 981 °C, with a heating rate of 376.4 °C/min. The method is potential in the field of value-added utilization of bio-char and microwave-promoted chemical processes.

Oil recovery is closely related to the sweep and displacement efficiencies in oil reservoirs. Therefore, several measures have been devised to enhance oil recovery based on these two factors. In heterogeneous heavy oil reservoirs, expansion of the swept volume of medium- and low-permeability layers is a suitable approach for enhancing oil recovery. However, the oil displacement efficiency is also an important factor. With regard to enhancing oil recovery, the selection of displacement agents will depend on whether the sweep efficiency or displacement efficiency is to be adjusted and on how these factors can be controlled to maximise the increase in oil recovery. In this study, the pixel method is used to distinguish between the sweep efficiency and displacement efficiency, and the joint effect of these factors is analysed. A heterogeneous microscopic model is used to conduct oil displacement experiments using polymer solutions with different viscosities and surfactant–polymer systems with similar viscosities and different interfacial tensions. The results indicate that under heterogeneous conditions, increasing the viscosity of the polymer solution leads to a slight expansion in the swept volume and an increase in the oil displacement efficiency in the low-permeability area; however, the overall sweep efficiency remains almost unchanged. There is a critical point for sweep efficiency: under the experimental conditions, when the oil viscosity is 305 mPa s, the critical viscosity of the polymer solution is 125 mPa s, and when the oil viscosity is 1050 mPa s, the critical value is 205 mPa s. When the viscosity of polymer solution reaches the critical value, for the surfactant–polymer system, a reduction in the oil–water interfacial tension has the same effect as a substantial increase in the polymer solution viscosity; moreover, the joint effect of the sweep efficiency and displacement efficiency can be enhanced. Therefore, for heterogeneous heavy oil reservoirs, the focus should be on improving oil displacement efficiency under certain sweep conditions rather than on expanding the swept volume. The findings of this study can help for better understanding of the selection of displacements agent in heterogeneous heavy oil reservoirs for enhanced oil recovery.

The mechanical properties of complex concentrated alloys (CCAs) depend on their formed phases and corresponding microstructures. The data-driven prediction of the phase formation and associated mechanical properties is essential to discovering novel CCAs. The present work collects 557 samples of various chemical compositions, comprising 61 amorphous, 167 single-phase crystalline, and 329 multi-phases crystalline CCAs. Three classification models are developed with high accuracies to category and understand the formed phases of CCAs. Also, two regression models are constructed to predict the hardness and ultimate tensile strength of CCAs, and the correlation coefficient of the random forest regression model is greater than 0.9 for both of two targeted properties. Furthermore, the Shapley additive explanation (SHAP) values are calculated, and accordingly four most important features are identified. A significant finding in the SHAP values is that there exists a critical value in each of the top four features, which provides an easy and fast assessment in the design of improved mechanical properties of CCAs. The present work demonstrates the great potential of machine learning in the design of advanced CCAs.

In this work, we describe a simulation framework for fluid movement in a corrugated sawtooth channel whose walls are undergoing periodic repeated oscillations. The sawtooth geometry of the channel walls creates a fluid ratchet by generating an anisotropy in the fluid impedance. The simulations are developed using an immersed boundary method, and we present numerical results for both Newtonian and non-Newtonian fluids. These results are in agreement with physical studies of ratchets in the literature and with general flow behaviors expected for non-Newtonian fluids. In particular, we find enhanced mean flow rates for non-Newtonian fluids up to a critical value of the Weissenberg number. Existence of such a critical value has been shown for non-Newtonian flows in other environments, but has not been explored computationally for fluid ratchets. We also provide results which highlight the difference in movement of ratcheted non-Newtonian versus Newtonian fluids.

Objectives In 2018, our clinical laboratory was alerted to back-to-back plasma sodium critical value callback failures on the same patient, occurring on different shifts and involving different technologists. Therefore, we set forth to investigate the root cause for the critical value callback failures. Design and methods We conducted a thorough investigation focused on the processes associated with critical value identification and notification for plasma sodium measurement performed on the Siemens Vista. Results Our investigation uncovered a flaw in the Siemens CentraLink middleware software. A default dark blue bar in the top row of the results review display was determined to obscure the red color which highlights critical values for lab staff identification. Sodium was disproportionately impacted by this flaw, as it is commonly ordered as part of metabolic panels, and is listed first among the panel analytes in the top row of the CentraLink display. Retrospective data review comparing critical callback failure rates for sodium to potassium and hemoglobin confirmed that sodium had significantly higher critical callback failure rates than these other analytes. After alerting the product manufacturer, Siemens programmed the CentraLink display so that the top row was blank and devoid of patient results, so that the blue color in the top row would no longer obscure the red visual cue of a patient’s critical result. Sodium critical value callback failures were reduced to 0% after this middleware display correction. Conclusions Middleware design flaws can have unexpected consequences on clinical laboratory operations. We encourage clinical laboratories to closely examine user interfaces utilized by laboratory staff, and be wary of potential impacts that the display format may have on results reporting.

Three-dimensional (3D) direct numerical simulations (DNS) of the viscous incompressible fluid flow through a helical pipe with circular cross section were performed. The flow is governed by three parameters: the Dean number (or the Reynolds number), curvature, and torsion. First, we obtained steady solutions by steady 3D calculations, where dual solutions were found, one was uniform in the pipe axial direction and the other varied very slowly, if torsion exceeded a critical value. Then, the instability of the steady solutions obtained was studied by unsteady 3D calculations. We obtained critical Reynolds numbers of steady to unsteady transition by observing the behaviors of the unsteady solutions. The present results of the critical Reynolds number nearly agreed with those by the 2D linear stability analysis (Yamamoto et al. [9] ) except for the lowest critical Reynolds number region, where the present study gave the critical Reynolds number much less than that obtained by the 2D linear stability analysis. We found the vortical structures in the form of a standing wave slightly above the marginal instability state, which is a trigger of explosive 3D instability.

In this paper, we consider a reaction-diffusion equation with continuous delay and spatial variable coefficients which models the evolution of a single species. We establish a sharp threshold dynamic result: there exists a critical value λ ( a ) such that if λ ( a ) 0 the positive steady state solution of the equation is globally attractive, while if λ ( a ) ≥ 0 the trivial steady state is globally attractive. To this end, we analyze the ω-limit set of the equation and prove that it is a singleton. Moreover, we apply our method to obtain global attractivity of the positive steady state of a spatially nonlocal diffusive logistic model.

Because of coal seam excavation and periodic disturbance of back-and-forth coal cutting, the surrounding rock is subjected to tiered cyclic unloading and loading. Based on this, a seepage experiment under constant axial stress and tiered cyclic unloading-loading confining pressure was carried out, and two new damage variables were proposed. These results showed that, the accumulative plastic deformations and corresponding permeability increased step-wise with unloading levels, permeability decreased with cycle number in the same unloading level when the unloading level was small, it was the opposite when the unloading level was large. The Kaiser effect and the Felicity effect actually coexisted during the experiment. As the unloading level increased, the relationship between dissipated energy and cycle number converted from logarithm to linear relationship. Two damage variables showed that the critical value of the damage variables for coal failure Dc≈0.9.

The strip formation process on a glass surface in 3D printing of viscous droplets is presented with the relevant strip characteristics modelled. PEDOT:PSS polymer inks at different viscosities are used. It is found that unlike the symmetric circulation flow developed by a single droplet deposition, strip-printing by multiple droplets is achieved by the coalescence of droplets spreading unequally in different directions. Whereas the liquid viscous cohesion retards the flow in the direction aligning with the centre line of the coalesced droplets, the side spreading driven by liquid surface tension fills up the space on the target surface along the bridge between two coalesced droplets. When the deposition distance or space of two consecutive droplets is set at greater than a critical value, this flow inequality results in strips of scallop shape. With a deposition distance below the critical value, the strips formed tend to have parallel edges and the strip width increases with a decrease in the deposition distance. However, if the distance is set below a saturate level at which the liquid surface tension cannot withstand the deposited liquid volume, bulges are detrimentally formed randomly along the printed strips. Because of the viscous retardation to the spreading of drop liquid, the strips printed using a higher viscosity ink is narrower than that of a lower viscosity. Based on the strip formation process, the relevant characteristics of the printed strips are mathematically modelled using dimensional analysis. The developed models are found in good agreement with the experimental data and can be used to guide the selection of the operating parameters for strips with desired characteristics.

The impact of gyrotactic microorganisms in the mixed convection stagnation point flow of Cu-Al2O3/water hybrid nanofluid towards an immovable plate is numerically examined. The boundary layer equation coupled with the energy, nanoparticle volume fraction and conservation equations for microorganisms are simplified into a set of ordinary differential equations by adopting similarity transformation. The validation results are in a good agreement with previous studies approximately by 0% difference relative error. The results imply that the dual solutions only exist in the opposing flow up to a critical value/turning point. Beyond this turning point, the solutions are unavailable which reflects the separation of flow. The hybrid nanofluid is essential in delaying the separation process where the results show that λ c , h n f = − 3.5804 > λ c , C u = − 3.3903 > λ c , A l = − 3.2016 . Moreover, the advantage of this hybrid Cu-Al2O3/water nanofluid is in the augmentation of heat and mass transfer rates including the density of microorganisms which is higher than the Cu- and Al2O3-water nanofluids.

The crack growth behaviour of silicon cell during entire solar photovoltaic module manufacturing process is numerically studied in this work using finite element analysis. In this investigation, the inherently present micro-cracks in the silicon cells are introduced systematically in the finite element model by considering their influencing parameters such as location, length and origination. The cracks in the silicon cell near the solder region are found to be more critical ones through mode-I stress intensity factor. The critical micro-cracks initiate the fracture even during the PV module manufacturing process, when the stress intensity factor reaches the critical value called the fracture toughness of the silicon cell. To substantiate the findings from this work, the residual stress evolution without cracks in the silicon cell is also numerically simulated and correlated with the experimental observations reported. Finally, the extended finite element method is employed further to confirm the fracture phenomena in the silicon cell for critical lengths and locations.

The mitigation of blast shock with water has broad application prospects. Understanding the mitigation effects on the reflected overpressure of the explosion shock with water surrounding an explosive in a confined space is of great significance for military explosives safety applications. To estimate the effects of the parameters on the reflected overpressure of blasted shock wave, a series of experiments were carried out in confined containers with spherical explosives immersed in a certain thickness of water, and numerical simulations were conducted to explore the corresponding mechanisms. The results reveal that the reflected overpressure is abnormally aggravated at a small scaled distance. This aggravation is due to the high impulse of the bulk accelerated water shell converted from the explosion. With increasing scaled distance, the energy will be gradually dissipated. The mitigation effects will appear with the dispersed water phase front impacting at a larger scaled distance, except in the case of a dense water phase state. A critical scaled distance range of 0.7–0.8 m/kg1/3 for effective mitigation was found. It is suggested that the scaled distance of space walls should be larger than the critical value for a certain water-to-explosive weight ratio range (5–20).

Fluid dynamics of a self-propelled biomimetic underwater vehicle (BUV) with pectoral fins is investigated by an immersed boundary (IB) method. Typically, the BUV with a pair of pectoral fins starts from rest and attains a constant mean velocity as the mean longitudinal force is zero. The capability and accuracy of the IB method to deal with the interaction between the fluid and complex moving body are firstly validated. Then we carry out a parametric study to understand the effect of key governing parameters on the dynamic response of the BUV. It is found that with the increase of motion frequency or rolling amplitude, the pectoral fin propulsors can induce larger forward velocity so that the BUV takes less time to attain its stable periodic swimming state. Although the pectoral fin is a very complicated lifting surface, a linear relationship between forward Reynolds number (final swimming velocity is used as velocity scale) and frequency Reynolds number (product of motion frequency and fin chord length is used as velocity scale) can be established when the frequency Reynolds number is above a critical value. A linear relationship between forward Reynolds number and rolling amplitude is also found within the studied range of rolling amplitude. Furthermore, a small-density-ratio BUV is sensitive to the surrounding flow with more rapid evolution process of self-propulsion. Whereas, BUV with a large density ratio is more stable. The implications of the hydrodynamic analysis on the bio-inspired engineering design of BUV with pectoral fins are also discussed.

One of the mechanisms of destruction of a solid is the formation and growth of “micro” and “macro” cracks. This task is especially acute for space objects. A microcrack in the body of the spacecraft practically does not affect its mechanical strength, but it can lead to depressurization of the body, which can lead to the loss of life support for the crew. Cracks can have different causes of their occurrence, but the mechanism of their growth in modern fracture mechanics is associated with the chosen criterion. For example, in the force criterion, crack growth begins when the stress intensity factors also reach a certain threshold critical value characteristic of a given material. This paper presents the results of calculating stress intensity factors for spatial branching cracks. The calculations were carried out on the basis of the original author's program, which makes it possible to determine the stress and displacement fields in a three-dimensional elastic body weakened by a system of arbitrarily oriented cracks. The program is based on the method of discontinuous displacements, implemented for three-dimensional problems of fracture mechanics in an elastic medium, implemented in C ++ codes. The main purpose of the work is to demonstrate the possibility of determining the stress intensity factors for a spatial branching crack and their dependence on the geometric parameters of the problem.

The study presents the vortex shedding and force coefficients for a system of two revolving cylinders, diameter (D) at Reynolds number (Re) equal to 100. The outer cylinder orbits the inner cylinder while the inner cylinder rotates on its own axis. The effect of varying Keulegan-Carpenter number ( K = 2 π ∙ U ∞ ω ∙ D ) is studied for a fixed center-to-center distance of the cylinders (L) = 4D as illustrated in Fig. 1 . The numerical study has been carried out using ANSYS Fluent. For higher values of K , the inertial force of the free stream is the dominating force on the cylinders. For lower K values, the drag force on the outer cylinder is dominating the flow properties. For intermediate values of K , the vortex of the outer cylinder folds in the region between the cylinders. There is a critical value K = Kc which tips the dominating force from inertial force to drag force of the revolving cylinder.

Experiments have found substantial morphological differences between buoyancy-driven flames developing on the upper and lower surfaces of inclined burning plates. These differences cannot be explained on the basis of existing analytical solutions of steady semi-infinite flames, which provide identical descriptions for the top and bottom configurations. To investigate the potential role of flame instabilities in the experimentally observed flow differences, a temporal linear stability analysis is performed here. The problem is formulated in the limit of infinitely fast reaction, taking into account the non-unity Lewis number of the fuel vapor. The stability analysis incorporates non-parallel effects of the base flow and considers separately spanwise traveling waves and Gortler-like streamwise vortices. The solution to the stability eigenvalue problem determines the downstream location at which the flow becomes unstable, characterized by a critical value of the relevant Grashof number, whose value varies with the plate inclination angle. The results for the flame formed on the underside of the fuel surface indicate that instabilities emerge farther downstream than they do for a flame developing over the top of the fuel surface, in agreement with experimental observations. Increased buoyancy-induced vorticity production is reasoned to be responsible for the augmented instability tendency of topside flames.

Ambient condition, especially the wind condition, is an important factor to determine the behavior of hydrogen diffusion during hydrogen release. However, only few studies aim at the quantitative study of the hydrogen diffusion in a wind-exist condition. And very little researches aiming at the variable wind condition have been done. In this paper, the hydrogen diffusion in different wind condition which including the constant wind velocity and the variable wind velocity is investigated numerically. When considering the variable wind velocity, the UDF (user defined function) is compiled. Characteristics of the FGC (flammable gas cloud) and the HMF (hydrogen mass fraction) are analyzed in different wind condition and comparisons are made with the no-wind condition. Results indicate that the constant wind velocity and the variable wind velocity have totally different effect for the determination of hydrogen diffusion. Comparisons between the constant wind velocity and the variable wind velocity indicate that the variable wind velocity may cause a more dangerous situation since there has a larger FGC volume. More importantly, the wind condition has a non-negligible effect when considering the HMF along the radial direction. As the wind velocity increases, the distribution of the HMF along the radial direction is not Gaussian anymore when the distance between the release hole and the observation line exceeds to a critical value. This work can be a supplement of the research on the hydrogen release and diffusion and a valuable reference for the researchers.

The present paper investigates numerically the hydrodynamic instabilities occurring in a cylindrical container filled with a conducting viscous fluid and submitted to an axial magnetic field. The axisymmetric swirling flow produced by rotation of the bottom disk, in which a vortex breakdown bubble occurred on the axis of symmetry. This flow structure represents one of the most important instabilities. The governing Navier–Stokes and potential equations are solved by using the finite-volume method. For both steady-state and oscillatory regimes, various combinations of the top, bottom, and side walls conductivity are considered. The effects of the magnetic field and wall electrical conductivities on the position of vortex breakdown and his disappearance is developed. The results obtained showed that the vortex breakdown is suppressed beyond the magnitude of the magnetic field exceeds a critical value. The stability diagram (Ha c r –Re) corresponding to the vortex breakdown disappearance for electrically conducting and insulating walls is obtained.

This paper presents an experimental and theoretical study on limit conditions for smoldering combustion of biomass fuel. A fuel rod was burned in an oxidizer flow in a Tsuji-Yamaoka type, counterflow configuration, and the spread rate and temperature distribution were measured. The spread rate and maximum temperature increased with an increase in oxygen mass fraction in the oxidizer flow. Smoldering spread was not possible when the oxygen mass fraction was below a lower critical value, whereas a smoldering-to-flaming transition (SFT) occurred when the mass fraction was above an upper critical value. A one-dimensional model was then developed based on experimental observations to describe steady smoldering spread. The limit conditions were predicted by assessing the existence of solution. It was found that extinction occurs due to finiteness of char-oxidation rate, while the SFT limit can be predicted by considering gas-phase combustion of pyrolysis products.

The present work concern with the comprehensive study of heat transfer from a horizontal heated cylinder rotating in water coefficient. Above this critical value, the heat transfer coefficient is just a function of Earlier work done by Researcher has shown the importance of the critical Reynolds number in determining the heat transfer Reynolds and Prandtl number while Grashof number has negligible effect in the heat transfer coefficient. The work has shown the variation of Nusselt number with different angular position on the cylinder circumference with peak value of local Nusselt number at the bottom and lowest at the top for a stationary cylinder, and the variation tend to shift on the graph as Reynold number is increased with lowest Nu number going more down and highest Nu number going up. This variation tends to flatten out with increase in Reynold number.

In order to examine extinction of carbon particle in the course of combustion, an attempt has been made to obtain its critical condition. Main concern has been put on the particle extinction in the lower limit that can occur at the end of the particle combustion although not only the lower limit but also the upper limit of the critical conditions has been obtained. By conducting asymptotics, with focusing on the temporal variation of the particle temperature, an analytical expression has been obtained for the limit of the particle diameter, as functions of the pressure ratio, oxygen mass-fraction, ambient temperature, and/or radiative heat flux. An approximate expression is also obtained from the analytical solution. It is found that the approximate expression can fairly represent the limits. In addition, use has been made of the Arrhenius plot of a comprehensive parameter, consisting of the particle diameter and pressure ratio, in order to identify regions for the particle combustion sustained, which are next to those for the particle extinction. Comparisons have also been conducted by use of experimental data in the literature, with presenting a fair degree of agreement, as far as the trend and approximate magnitude are concerned. It has been confirmed that the formulation has captured the essential feature, that the reduction in particle size does not necessarily favor the particle combustion, and that the particle extinction can occur when the particle diameter is reduced to the critical value.

An extensive investigation is performed to reveal the flow and heat transfer characteristics of a thermomicropolar hybrid nanofluid over a shrinking surface. For a wide range of physical parameters, the dual solutions of the shear stress, gradient of microrotation and heat transfer are presented. When the dual solutions fail to occur beyond a critical value of the shrinking parameter, the boundary layer separation takes place. However, this causes mechanical damages to a system. The critical values of the shrinking parameter, shear stress, gradient of microrotation and heat transfer are expressed in explicit functions of the physical parameters. Results show that these functions can predict the solutions with a good accuracy. Since these expressions can easily be used in order to identify the drawbacks of a system with a wide range of flow parameters, it might play an important role for the development of technologies.

To prevent the formation of biofilm or to remove already formed biofilm on the transparent walls of the closed cultivation systems, it is important to ensure that there are sufficient wall shear stresses to disrupt the stability of the biofilm. A pilot-scale flat-panel photobioreactor and laboratory tubular system were used to determine the critical value of the wall shear stress. The formation of biofilm on transparent walls of the cultivation systems reduces light penetration into the cultivation medium, which can subsequently reduce the production of microalgae, since light radiation a key parameter influencing the growth of microalgae. To study the influence of hydrodynamic conditions on the prevention of biofilm formation, a numerical model of a flat-panel photobioreactor was validated based on the experimental data. Using the validated model, a critical value of wall shear stress that above which avoids the formation of biofilm was defined to be 0.2 Pa. To refine the values from the numerical model, and to investigate the influence of hydrodynamic conditions on the process of disruption of the formed and stabilized biofilm, a simple experimental tubular system with the one-dimensional flow was developed. For one-dimensional flow, it is easier to describe the hydrodynamic behaviour and more precisely define the parameters influencing biofilm formation. The experiments in this simplified tubular system were focused on removing already formed biofilm. The formed biofilm on the transparent wall was completely removed from the surface when the wall shear stress reached a value of 53 Pa. However, the stability of the biofilm was already disturbed at a value of 6 Pa. The resulting values of critical wall shear stress could be used to select the operating conditions of the cultivation systems or to adjust their geometry affecting the hydrodynamic conditions. According to the velocity distribution based on the measured values, the thickness of the biofilm was less than the thickness of the viscous sublayer. Thus, the prevention of formation, disruption, or even removal of biofilm can be ensured by shear forces in the viscous sublayer.

Nonlinear evolution equations, correct up to fourth order in wave steepness, are derived for a pair of obliquely interacting wave packets in the presence of a thin pycnocline. These evolution equations are then employed to perform stability analysis of a pair of obliquely interacting uniform wave trains. Figures plotted here reveal that the growth rate of instability decreases with the increase in pycnocline depth and also with the increase in density difference across the pycnocline. When the angle of interaction between the two wave packets is less than certain critical value the growth rate of instability decreases with the increase in angle and beyond this critical value the result is reversed.

The post-buckling behavior of a compressed, simply supported, shearable elastic rod on a nonlinear elastic foundation is studied. For the critical values of auxiliary parameter the eigenvalues are double so that the Liapunov-Schmidt method leads to two bifurcation equations. These equations describe the post-buckling behavior for the values of auxiliary parameter near the critical ones. The occurrence of secondary bifurcations, depending on the nonlinearity of foundation and shear rigidity, is investigated. It is shown that these effects have a significant influence on the post-buckling behavior of the rod. The type of bifurcations, stability, mode interactions and asymptotic expansions of primary and secondary branches are determined. It is shown that the secondary branches can be stable for the nonlinear hardening elastic foundation. The complex bifurcation analysis reveals that for every critical value of the auxiliary parameter, there are eleven different post-buckling behaviors, depending on the nonlinearity of foundation and shear rigidity.

We study the structure of stationary solutions of a micro-electro mechanical system with fringing field. It is known that there is a positive critical value such that no stationary solutions exist for the applied voltage beyond this critical value, at least two stationary solutions exist for the applied voltage below this critical value and there is a unique stationary solution at this critical value. In this paper, we verify that there are exactly two solutions below this critical value analytically. Moreover, the stability of the smaller stationary solutions is derived.

Low temperature performance is key for catalysts used in selective catalytic reduction of NOx with ammonia (NH3-SCR). The best performing NH3-SCR catalysts are based on small pore copper loaded zeolites. A key question remains which structural and compositional parameters make a copper zeolite catalytically active in this application. In this work a collection of 83 different zeolite samples having 25 different frameworks and compositional variations was evaluated to uncover the critical structural and compositional parameters governing catalytic activity at 150 °C. The Si/Al ratio, the content of double six ring building units, the framework density and pore dimensionality were identified as having a strong impact on the catalytic activity of zeolites. A critical value for each of these parameters was determined. This knowledge was used to predict MOZ, SZR and GME as active NH3-SCR catalysts. They were synthesized respecting the compositional prerequisites and their catalytic evaluation confirmed the predicted high catalytic activity. ispartof: Journal Of Catalysis vol:390 pages:224-236 status: published

Two-dimensional uniform vortices that rotate without deforming in an isochoric, inviscid fluid are investigated. They have polygonal symmetry and are usually considered of the same kind of the famous Kirchhoff elliptical one. By following this idea, their shapes and rotation speeds are calculated through a new integral approach. It is heuristically proved, by examining the co-rotating streamlines, that the similitude with the elliptical vortex is not complete, due to the appearance of external singularities in the Schwarz functions of the polygonal boundaries. Once the effects of these singularities have been accounted for, exactly steady vortices are obtained. In this way, for any number of “sides”, a continuous family of rotating, steady polygonal vortices is obtained, parametrized in the ratio between minimum and maximum radial coordinates of the boundary. Solutions exist above a critical value of this ratio, at which the boundary ceases to be differentiable.

A steady flow and heat transfer of a hybrid nanofluid past a permeable moving surface is investigated. In this study, 0.1 solid volume fraction of alumina (Al2O3) is fixed, then consequently, various solid volume fractions of copper (Cu) are added into the mixture with water as the base fluid to form Cu-Al2O3/water hybrid nanofluid. The similarity equations are obtained by converting the governing equations of the hybrid nanofluid using the technique of similarity transformation. The bvp4c function available in Matlab software is used to solve the similarity equations numerically. The numerical results are obtained for selected parameters and discussed in detail. It is found that hybrid nanofluid enhances the heat transfer rate compared to the regular nanofluid. The results show that two solutions exist up to a certain value of the moving parameter and suction strengths. The critical value in which the solution is in existence decreases as nanoparticle volume fractions increase. The temporal stability analysis is conducted in determining the stability of the dual solutions, and it is revealed that only one of them is stable and physically reliable.

To judge the coal and gas outburst (hereinafter referred to as ‘outburst’) risk of working faces in coal roadways according to the index of drilling cuttings weight (DCW), the influence factors and critical value of DCW were investigated. The study was conducted by deducing the theoretical calculation formula for DCW and using a set of self-designed, self-machined measuring apparatus for DCW which can be used to simulate the working face in a real coal roadway. The theoretical calculation formula for DCW indicates that the DCW increases with the increase of in situ stress and gas pressure while it also increases with decreasing coal strength. Among the three factors, with the change of gas pressure, the change in DCW is the smallest. On this basis, so that the critical value of DCW for judging the outburst risk of coal masses can better indicate the outburst risk at a working face, the tests for measuring DCWs under different outburst risk levels were carried out by using a set of apparatus for outburst risk which can be used to simulate the working face in a coal roadway by taking gas pressure as a controllable factor. The test results show that, with increasing outburst risk in a simulated coal seam, the DCW linearly increases at first and then suddenly rises. It also implies that the gas pressure exerts significant influences on DCW only when the outburst risk of a coal mass increases to a certain degree. There is a linear relationship between DCW and initial expansion energy of released gas (IEERG). On this basis, according to the critical values of IEERG indicating weak outburst and strong outburst risks, the critical values of DCW for outburst risk are judged. By using discriminant analysis, it can be obtained that the critical value of DCW when a weak outburst occurred is 4.88 kg/m. The critical value can be used to distinguish data samples with measured DCWs having weak outburst risk, whose accuracy reaches 89.47%. The critical value of DCW when a strong outburst occurred is 7.55 kg/m, which can accurately (100%) distinguish between data samples from weak and strong outbursts based on the measured DCWs. The final judgement accuracy of outburst risk is 93.75%.

Horizontal spacing of horizontal extraction gas wells can be designed to achieve a 90% pumping rate of the total generated landfill gas (LFG) from given waste properties (viz: gas permeability, landfill gas generation and non-homogeneity with depth), cover characteristics and vacuum pressure. However, cover characteristics and vacuum pressure are also important design parameters and different combinations of them result in different distributions of gas pressure in the waste, some of which would induce problematic air intrusion while others might pose threat to cover stability. This paper uses the maximum gas pressure directly below cover to distinguish these combinations, and provides the first study of the effects of the above parameters on potential outcomes. The ability of the overlying cover to resist LFG emission from the landfilled waste is suggested not to exceed a critical value, otherwise the maximum gas pressure below it would become at least 1 kPa larger than atmospheric pressure. A design formula for this critical value is proposed with respect to typical values of waste properties, vacuum pressure and the buried depth of horizontal wells in wide ranges. Together with consideration of recovery efficiency, the proposed method can be used to design a horizontal extraction gas collection system and a cover system in better working condition, and to evaluate the maximum gas pressure below cover. These applications are illustrated by a worked example.

A solar cavity receiver filled with a porous layer now is popularly found in solar energy industry. As its thermal performance will influence the efficiency of solar energy utilization systems considerably, deep understanding on it is urgent for the development of solar energy industry. In the present work, the influences of thickness, effective thermal conductivity and permeability of a porous layer on thermal performance of a porous solar cavity receiver are investigated comprehensively for the first time. The analysis is carried out not only from the viewpoint of the first law of thermodynamics but also from the second law of thermodynamics. Three new findings are obtained: (1) a critical value of thickness of the porous layer, (2) a critical value of effective thermal conductivity of the porous layer and (3) a critical value of permeability of the porous layer, across which the thermal performance of the solar receiver will alter significantly. In addition, some comparison is conducted between the observations in this research and those in previous publications to check the effects of the configuration and flow driven mechanism of a porous solar cavity receiver on its thermal performance for the first time.

To reveal the fuel ignition delay differences among diluent gases, non-reactive gas Ar, He, N2 and reactive gas CO2 four gases were selected to compare the ethylene ignition delay difference on a shock tube rig. The tested condition temperature and pressure range is 1074–1832 K and 2 atm. Comparison results show that the ethylene ignition delay discrepancy was not pronounced among Ar, He and N2 these non-reactive dilution gases. In terms of CO2 dilution, a critical CO2 concentration was observed. When the CO2 concentration was lower than this critical value, no distinguished ignition delay difference was found. Conversely, the ignition delay difference was pronounced once the CO2 concentration was above this critical value. Particularly, the difference was specially apparent in low temperature conditions. Reaction dynamic analysis shows that the heat effect is predominant in low temperature while the chemical effect is predominant in high temperature in the reaction of CO2 participation. Additionally, the CH* and OH* radical emission signal intensity turns smaller and profile curve turns irregular as the CO2 ratio increases. The CO2 shows a strong inhibition effect which is closely related with H radical consumption reaction CO2+H CO + OH.

Zirconia ceramics were prepared by oscillatory pressure sintering (OPS) and hot pressing (HP). The result revealed that OPS could enhance densification compared to HP when sintering temperature was higher than a critical value. The onset temperature for rapid grain growth was found to be same for both techniques. However, rate of grain growth in OPS was lower than that in HP. Furthermore, the result also showed that samples prepared by OPS exhibited higher hardness than those prepared by HP when sintering temperature was higher than the critical value. The improved hardness was solely due to the higher density of the samples prepared by OPS.

This contribution experimentally investigates the heat transfer and the consequent auto-ignition of thermally thick PMMA (polymethyl methacrylate) exposed to linearly declining heat flux (HF) coupling with a theoretically derived model. Thermal decomposition in solid was ignored and critical temperature was utilized. Both scenarios neglecting and considering surface heat loss were studied in the proposed model. An asymptotic solution, extending the derivation method from constant to time-declining heat flux, was obtained to explicitly estimate the surface temperature and ignition time. The credibility of the established model was validated through comparison with experimental data and a previously developed numerical model. The results show that in both considered scenarios ignition is confined in a region where the decreasing rate of heat flux is lower than a critical value. No ignition occurs if the decreasing rate increases beyond this region. The newly developed model is compatible with the correlations of constant heat flux, and it provides identical accuracy in predicting ignition time when compared with other existing models in the literature. A constant surface heat loss coefficient was used in the model, and it was found little error is introduced by invoking this approximation. Also, a parametric study was implemented, indicating that the critical temperature affects the ignition time prediction significantly. Meanwhile, the effects of surface heat loss on surface temperature and ignition time were quantitatively evaluated.

Vegetation occurring along river margins deeply alters the hydrodynamic flow structure, inducing a characteristic shear layer profile and triggering the onset of large scale vortices, which, in turn, govern the lateral exchanges across the vegetated interface. In such systems, vegetative drag plays a key role, regulating the velocity difference between the vegetated area and the adjacent main channel. In this study, the link between vegetation density, differential velocity ratio, and presence of large scale coherent structures is experimentally explored. Tests were performed considering different vegetation density and focusing on the lateral distributions of velocity statistics and presence of coherent turbulent structures. Results showed that, with decreasing vegetation density, the favorable conditions for the onset of large scale structures can disappear with direct effects on velocity distributions and efficiency of lateral momentum transport. Based on the test results, a critical value of differential velocity ratio is suggested.

The formation of controllable size and dripping frequency in electrohydrodynamic (EHD) atomization with electric periodic dripping regime are of much interest and importance because of significant and wide applications, such as micro-encapsulation and ink-printing. In the present study, the experimental and theoretical works were carried out to explore droplet formation in periodic dripping regime in presence of an electric field. The dimensionless electric charge carried by each droplet produced is smaller than the 50 % of critical value of the Rayleigh limit, where charge-to-mass ratio of droplets was obtained through the deflection distance in the presence of an electric field. The droplet in electric periodic dripping regime usually undergoes oscillating deformation, and finally forms a spherical droplet below the tip no more than ten times out diameter of tube. The droplet size tens of microns to one hundreds of microns decreases with an increase in applied potential. In the electric dripping mode, droplets size is independent of flow rate and affected by flow rate due to adsorption of surface active species in micro-dripping. The simplified model to predict droplets size was derived from the balance of electric, surface tension and gravity forces. The droplets size calculated in good agreement with the experiments. Meanwhile, the dripping frequency of droplets with rang of a few to several hundred hertz obtained from time-resolved images is highly dependent of liquid flow rate and electric potential. The largest dripping frequency was predicted and in reasonable agreement with the experimental results. In electric periodic dripping regime drop-on-demand droplets in size and dripping frequency further our understanding on the formation of identical droplets and are beneficial to many practical applications.

We perform a molecular dynamics simulation to investigate the effect of grain size on the material properties of nanocrystalline graphene, which inherently behaves like brittle material. From a series of simulations, our results reveal a transition from an inverse pseudo Hall-Petch to a pseudo Hall-Petch behavior in nanocrystalline graphene at a critical grain size. We show that since the grain boundary junctions serve as the stress concentration sites where cracks preferentially nucleate, the strength decreases with an increasing density of grain boundary junctions. This inverse pseudo Hall-Petch behavior is observed by decreasing the grain size in nanocrystalline graphene, where the grain size is larger than the critical value. In contrast, when the grain size is reduced below a critical value, the nanocrystalline graphene sustains tensile loads, even after cracks initiate due to a gradual evolution of the cracks. This indicates that the stress concentration of grain boundary junctions decreases with a decreasing grain size by stress cancellation between 5 and 7 defects. We believe that our findings provide a comprehensive and in-depth understanding of the mechanical properties and deformation process of nanocrystalline graphene.

Non-premixed laminar or turbulent flames can undergo extinction when scalar dissipation rates exceed a critical value. When local extinction occurs, negative edge flames that act to expand the region of extinction are initially present. Steady negative edge flames have been previously studied in a combustor designed to support the negative propagation velocity of the flame edge, but unsteady negative edge flames have not received significant attention. The current study uses numerical simulations in a simple flowfield to produce unsteady negative edge flames which oscillate in response to imposed strain rate fluctuations in order to understand the dynamic response of extinction edge propagation. An energy balance through the edge shows that the unsteady behavior can be well described by a simple extension of the energy balance from steady flames. Furthermore, the edge velocity was calculated, and a physical interpretation of the extinction velocity was derived using the energy budget. The time scale of the imposed strain rate fluctuations was varied to show a limited response time for the extinction edge flame dynamics.

The interaction of turbulence–combustion inside the flame field is studied in a Launder-Sharma Low Reynolds Number (LS-LRN) k−e/EDM framework, while suitable coefficients have been utilized in the code. A finite volume method (FVM) with staggered grids was applied to discrete set of governing equations. SIMPLE algorithm is applied with a fine grid resolution. The convective terms are discretized using Power Law Scheme (PLS). The system of governing equations is solved simultaneously using numerical or TDMA finite difference methods (Tri-Diagonal Matrix Algorithm). By implementation of the Zeldovich and Westbrook-Dryer mechanisms, NOx and CO concentrations were obtained, respectively. It is illustrated that the implemented LS-LRN-k−e/EDM method with the new coefficients by shorter runtimes has very good agreement with previously published experimental measurements. Increasing the H2O diluent at the inlet leads to an increase in the temperature, which increases the NOx and CO near the entrance, but gradually towards the outlet of the combustion chamber. The energy absorbed by H2O leads to a severe decrease in temperature and subsequent reduction in the amount of NOx and CO emissions. With increasing H2O diluent, changes in temperature are not very significant, while changes in pollutants CO and especially NOx, are remarkable. With the increase of H2O diluent, the maximum amount of CO emission displaces towards the inlet of the combustion chamber. However, it should be noted that, at a specific value of H2O diluent, the length of the combustion chamber should not be less than critical value, causing the exhaust of the pollutant with large volume to the environment. After the critical point, the increase in the length of the chamber has little effect on reduction of the pollutant exhaust. However, by increasing the H2O diluent, enclosures with smaller length can be utilized.

We report atomic-scale observations on the interaction of {101ˉ2} deformation twins with 14H long-period stacking ordered (LPSO) phase in a magnesium alloy. It was found that the interaction strongly depends on the thicknesses of LPSO plates as well as the thickness ratios between LPSO plates and twins. We observed three size-dependent structure responses of LPSO to incoming twins: (1) pure shearing of thin LPSO plates in line with the incursive {101ˉ2} twins; (2) twin-to-dislocation ‘switch’ from the alpha-Mg matrix to LPSO when LPSO/twin thickness ratios are below a critical value of 0.17 ± 0.01; and (3) elastic deformation of LPSO to accommodate the propagation of incoming twin when LPSO/twin thickness ratios are larger than 0.17 ± 0.01. Moreover, the inter-plate spacing of LPSO also influences the propagation modes of twins by controlling nucleation sites of new twins. These size-dependent interactions are accomplished by local structural transition of face-centered cubic units of LPSO during thin plate shearing and formation of gliding dislocations during LPSO deformation and twin blocked. The atomic-scale observations provide fundamental insights into these interaction modes and, hence, the precipitation strengthening mechanisms in Mg alloys under both quasi-static and dynamic loadings.

An analytical procedure predicting a debond (interface crack) onset and growth in an adhesive joint between two beams or plates is developed and applied to a specific configuration often used in reinforcement tests in civil engineering. The procedure is based on Timoshenko beam theory and Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with the Coupled Criterion of the Finite Fracture Mechanics (CC-FFM) for mixed-mode fracture. First, a sixth order differential equation in the shear stresses along the adhesive layer is deduced and solved, leading to closed form expressions for both shear and normal stresses in the adhesive. Then, the critical value of the applied load necessary to produce debonding is predicted by coupling a stress and an energy condition based on: (i) the stress distribution produced in the interface before the debond onset and (ii) the energy released during the debonding process along the interface. Although the developed procedure can be applied to several types of joints with different geometries, materials and loads (e.g., double lap joint tests including adherents made of steel or composites), herein it is applied to the double pull-push shear test where the debond onset and growth between a Carbon Fibre Reinforced Polymer (CFRP) laminate and a concrete block occurs. For such a case, the debond is produced under predominant fracture mode II; nevertheless, it is shown that relevant normal (peeling) stresses associated to mode I may appear as well. A comparison of the present solution with a previous one by the shear-lag model is provided as well.