Constant Volume Conditions Research Articles

A flamelet model for premixed combustion under variable pressure conditions

Studying premixed combustion under variable pressure conditions is important for modeling devices such as internal combustion engines. Various models have been developed for isobaric premixed combustion, but few models exist for variable pressure conditions. In this study, a new model is proposed for studying combustion at these conditions. The model is based on the flamelet progress variable approach that has been used extensively to study constant pressure premixed combustion. The quantities required by the combustion model are tabulated only at a reference pressure. At other pressure values, they are obtained using a quadratic logarithmic expansion in pressure. Expensive tabulation due to an additional pressure dimension is avoided, and only the coefficients for the expansion need to be tabulated. Additionally, under constant volume conditions, the burnt gas temperature may become larger than the unstretched premixed post-flame temperature due to compression of the gases. This effect is modeled by considering the mixture enthalpy. A polynomial expansion method is developed to obtain the temperature from the mixture enthalpy accurately at minimal computational cost. The model is validated against direct numerical simulation data with multi-step finite rate chemistry under isochoric conditions. Different turbulence intensities and length scales are studied to better assess the model performance. The model is shown to capture the effects on temperature due to the variable pressure.

A strain space soil model with evolving stiffness anisotropy

A complete formulation of the BRICK soil model in general strain space is presented herein for the first time. Like all elasto-plastic constitutive models, BRICK exhibits some anisotropic behaviour, owing to the development of plastic strains once its yield surfaces are engaged. However, an abundance of laboratory and field evidence demonstrates that stiffness anisotropy is also significant within the elastic domain. Because of the inseparable nature of strength and stiffness in BRICK, the simple use of an anisotropic elastic stiffness matrix would result in an unrealistically high degree of strength anisotropy. Therefore, in addition to the established BRICK formulation, this paper also presents a novel framework to introduce stiffness anisotropy by transforming the coordinate system in which the model is based. The transformed coordinate system evolves to enable a constant-volume condition during shearing at critical state, reflecting the reorganisation of the soil fabric. The superior performance of the enhanced BRICK model over the classic model is demonstrated by a variety of conventional and non-conventional laboratory tests on London Clay.

Characterization of low- and high-temperature oxidation processes under non-premixed diesel-engine-like conditions

Low- and high-temperature oxidation processes, including thermal auto-ignition under diesel-engine-like conditions (non-premixed mixtures), have been investigated. A special combustion chamber, characterized by constant volume and adiabatic conditions, has been used as an engine simulator. The investigated processes are very complex in nature, and depend significantly on the temperature and pressure. There are five characteristic regions of the process characterized by different delay times, reaction rates and number of recognizable oxidation reactions: region 1 corresponds to processes occurring at low initial pressures over a wide range of initial temperatures; region 2 corresponds to low initial temperatures over a wide range of initial pressures; region 3 corresponds to middle pressures and higher temperatures; region 4 corresponds to middle temperatures and higher pressures; and region 5 corresponds to high initial pressures and high temperatures. by analogy to a negative temperature coefficient (as discussed in the literature), a positive pressure coefficient has been introduced here. This indicates that in the selected range of pressures, the delay time of low-temperature oxidation processes is the shortest, and the rate of these reactions is the highest. Further increases in the pressure behind the positive pressure coefficient range increase the delay time and decrease the reaction rate. The positive pressure coefficient has been observed at lower temperatures (mostly corresponding to cool-flame reactions and transitions to blue flames). Generally, the ignition delay time reduces with increasing chamber temperature and pressure.

On the influence of inter-particle friction and dilatancy in granular materials: a numerical analysis

Mechanical behavior of granular soils is a classic research realm but still yet not completely understood as it can be influenced by a large number of factors, including confining pressure, soil density, loading conditions, and anisotropy of soil etc. Traditionally granular materials are macroscopically regarded as continua and their particulate and discrete nature has not been thoroughly considered although many researches indicate the macro mechanical behavior closely depends on the micro-scale characteristics of particles. This paper presents a DEM (discrete element method)-based micromechanical investigation of inter-particle friction effects on the behavior of granular materials. In this study, biaxial DEM simulations are carried out under both ‘drained’ and ‘undrained’ (constant volume) conditions. The numerical experiments employ samples having similar initial isotropic fabric and density, and the same confining pressure, but with different inter-particle friction coefficient. Test results show that the inter-particle friction has a substantial effect on the stress-strain curve, peak strength and dilatancy characteristics of the granular assembly. Clearly, it is noted that apart from the inter-particle friction, the shear resistance is also contributed to the dilation and the particle packing and arrangements. The corresponding microstructure evolutions and variations in contact properties in the particulate level are also elaborated, to interpret the origin of the different macro-scale response due to variations in the inter-particle friction.

Evolution of pd patterns in polyethylene insulation cavities under AC voltage

In the framework of developing defect-based life models, in which breakdown is explicitly associated with partial discharge (PD)-induced damage growth from a defect, ageing tests and PD measurements were carried out in the laboratory on polyethylene (PE) layered specimens containing artificial cavities. PD activity was monitored continuously during aging. A quasi-deterministic series of stages can be observed in the behavior of the main PD parameters (i.e. discharge repetition rate and amplitude). The evolution of the parameters reflects the physicalchemical changes taking place at the dielectric/cavity interface during the aging process. PD activity shows similar time behavior under constant cavity gas volume and constant cavity gas pressure conditions, suggesting that the variation of PD parameters may not be attributed to the variation of the gas pressure. It is speculated that the change of PD activity is related to the composition of the cavity gas, as well as to the properties of dielectric/cavity interface.

Thickness distribution and design of a multi-stage process for sheet metal incremental forming

Sheet incremental forming (ISF) is a promising technology. It is inexpensive and does not require particular dies. Sheet thinning, however, has always been one of the forming defects which impede the process’s wide application. Although a multi-stage forming process is supposed to be effective to deal with this problem, it is still uncertain how the process can reduce thickness thinning and there is no applicable rule to determine the favorable number of forming stages. In this work, based on a truncated cone, a finite element method (FEM) model for a double-pass forming was established first. Unlike simplifications in previous studies, with the process of the three-dimensional coordinates in numerical controlled (NC) machining code, the tool trajectory in this simulation model is the same as that in real work. With this approach, it was expected to gain reliability of the simulation result, and then this simulation result and a single-pass forming result were analyzed. The results of the analysis indicate that more uniform thickness distribution in the double-forming process largely benefits from the increase of the total plastic deformation zone. Finally, under the condition of constant volume in deformation, an equation was proposed to work out the right number of necessary forming stages and the rule of this equation was verified with a relatively complex product.

DEM Simulations of Undrained Triaxial Behavior of Granular Material

AbstractThe paper presents results of three-dimensional discrete element method (DEM) simulations of axisymmetric undrained tests on loose assemblies of polydisperse spheres using a periodic cell. In the work reported, undrained tests were modeled by deforming the samples under constant volume conditions. The undrained (effective) stress paths are shown to be qualitatively similar to published physical experimental results. The onset of liquefaction (or temporary liquefaction) is identified by a redundancy index equal to unity, which defines the transition from solidlike to liquidlike behavior. This corresponds to a critical mechanical coordination number slightly in excess of 4. The results of the simulations also suggest that a reversal in the direction of the undrained stress path does not necessarily indicate temporary liquefaction. The undrained behavior obtained by the DEM simulations is found to be dependent on strain rate and the so-called temporary liquefaction phenomenon is only observed if the ...

Experimental Research on Hydrogen Storage Characteristics of TBAB Hydrates

The hydrogen storage characteristics of Tetra Butyl Ammonium Bromide (abbr. TBAB) hydrates were studied in a high pressure gas hydrate experimental apparatus. The effects of temperature and pressure on hydrogen storage characteristics under the condition of constant volume were discussed. The results showed that lower reaction temperature or higher reaction pressure can cause to a rapid formation of hydrogen hydrate and a larger hydrogen storage density, Comparing with tetrahydrofuran (abbr. THF) hydrates under same temperature and pressure, TBAB hydrates have faster hydrogen storage rate and larger hydrogen storage density.

Effective Stress Change and Post-Earthquake Settlement Properties of Granular Materials Subjected to Multi-Directional Cyclic Simple Shear

In order to investigate the effect of cyclic shear direction on the properties of saturated granular materials, such as effective vertical stress reduction and post-earthquake settlement, several series of multi-directional cyclic simple shear tests under constant volume conditions are performed on Toyoura sand and granulated blast furnace slag (GBFS), as an alternative material. The GBFS has particular properties such as light weight, high shear strength and high permeability, and it is considered to be one of the most promising materials in geotechnical engineering. From the test results, it is clarified that the shear strain amplitude has a significant effect on the changes in the effective stress of granular materials. However, at higher levels of shear strain amplitude, the cyclic shear direction has little influence on the effective stress reduction. It is found that the vertical strain, after the cyclic shearing of the GBFS samples, was lower than that of the Toyoura sand under the same test conditions. Finally, to evaluate the changes in effective stress under uni-directional and multi-directional cyclic simple shear conditions, an estimation method is represented by a function of cumulative shear strain G* and resultant shear strain Γ. The validity of this proposed model is confirmed by comparing the experimental and the calculated data obtained under multi-directional cyclic simple shear conditions.

Experimental study of syngas combustion at engine-like conditions in a rapid compression machine

As the world energy demand and environmental concern continue to grow, syngas is expected to play an important role in future energy production. It represents a viable energy source, particularly for stationary power generation, since it allows for a wide flexibility in fossil fuel sources, and since most of the harmful contaminants and pollutants can be removed in the post-gasification process prior to combustion. In this work, two typical mixtures of H 2, CO, CH 4, CO 2 and N 2 have been considered as representative of the producer gas coming from wood gasification, and its turbulent combustion at engine-like conditions is made in a rapid compression machine in order to improve current knowledge and provide reference data for modeling and simulation of internal combustion engines. Methane as main constituent of the natural gas is used as reference fuel for comparison reasons. Single compression and compression- expansion events were performed as well direct light visualizations from chemiluminescence emission. There is an opposite behavior of the in-cylinder pressure between single compression and compression–expansion strokes. For single compression, peak pressure decreases as the ignition delay increases. In opposite, for compression–expansion the peak pressure increases as the ignition delay increases. This opposite behavior has to do with the combustion duration under constant volume conditions. Conclusion can be drawn that higher pressures are obtained with methane–air mixture in comparison to both typical syngas compositions. These results could be endorsed to the heat of reaction of the fuels, air to fuel ratio and burning velocity. Another major finding is that syngas typical compositions are characterized by high ignition timings due to its low burning velocities. This could compromise the use of typical syngas compositions on high rotation speed engines.

Replica Temperatures for Uniform Exchange and Efficient Roundtrip Times in Explicit Solvent Parallel Tempering Simulations.

The efficiency of parallel tempering simulations is greatly influenced by the distribution of replica temperatures. In explicit solvent biomolecular simulations, where the total energy is dominated by the solvent, specific heat is usually assumed to be constant. From this, it follows that a geometric distribution of temperatures is optimal. We observe that for commonly used water models (TIP3P, SPC/E) under constant volume conditions and in the range of temperatures normally used, the specific heat is not a constant, consistent with experimental observations. Using this fact, we derive an improved temperature distribution which substantially reduces the round-trip times, especially when working with a small number of replicas.

Interactions of multiple processes during CBM extraction: A critical review

Abstract Coal permeability models are required to define the transient characteristics of permeability evolution in fractured coals during CBM recovery. A broad variety of models have evolved to represent the effects of sorption, swelling and effective stresses on the dynamic evolution of permeability. In this review, we classify the major models into two groups: permeability models under conditions of uniaxial strain and permeability models under conditions of variable stress. The performance of these models is evaluated against analytical solutions for the two extreme cases of either free shrinking/swelling or constant volume. For the case of free shrinking/swelling none of the swelling/shrinking strain contributes to the change in coal permeability because effective stresses do not change. Conversely, for the case of constant volume the full swelling/shrinking strain contributes to the change in coal permeability because the coal is completely constrained from all directions. Therefore, these two solutions represent the lower bound and the upper bound behaviors of permeability evolution, respectively. Review of laboratory observations concludes that although experiments are conducted under conditions of free shrinking/swelling the observed response is closest to that for constant volume condition. Similarly, review of in-situ observations concludes that coal gas reservoirs behave close to the constant volume condition although these observations are made under undefined in-situ stress and constraint conditions anticipated to be intermediate between free swelling and constant volume (i.e. for uniaxial strain). Thus comparison of these laboratory and field observations against the spectrum of models indicates that current models have so far failed to explain the results from stress-controlled shrinking/swelling laboratory tests and have only achieved some limited success in explaining and matching in situ data. Permeability models under uniaxial strain are more appropriate for the overall behavior of coal gas reservoirs under typical in situ conditions while models representing variable stress conditions are more appropriate for behavior examined under typical laboratory conditions. Unlike permeability models under the uniaxial strain condition, models under the constant volume condition are effective-stress based and can be used to recover the important non-linear responses due to the effective stress effects when mechanical influences are rigorously coupled with the gas transport system. Almost all the permeability models are derived for the coal as a porous medium, but used to explain the compound behaviors of coal matrix and fracture. We suggest that the impact of coal matrix-fracture compartment interactions has not yet been understood well and further improvements are necessary.

Is the quasi-steady state a real behaviour? A micromechanical perspective

Whether the so-called quasi-steady state is a real material response is a fundamental yet controversial question in the study of undrained shear behaviour of sand. An attempt is made here to clarify the question from a micromechanical viewpoint by means of a grain-scale modelling technique combined with statistical analyses. The study shows that the quasi-steady state is a real behaviour rather than a test-induced phenomenon; it is a transition state, and can be regarded as the result of spatial rearrangement of discrete particles sheared under the constant-volume condition. The quasi-steady state has distinct features that make it different from the steady state at both the macro scale and micro scale. During the loading process, the average number of contacts per particle decreases with strain until the quasi-steady state emerges, and after that it increases gradually to an approximately constant value at large deformations associated with the steady state. This result suggests that the loss of contacts is most pronounced at the quasi-steady state. The study also shows that the contact normal forces and particle rotations play a major role in the deformation process, whereas the contributions of contact tangential forces and particle sliding appear to be minor.

A study of the hydro-mechanical behaviour of compacted crushed argillite

The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, as a possible sealing and backfill material in the French geological high-level radioactive waste disposal. In this study, the effects of the grain size distribution and the microstructure on the hydro-mechanical behaviour of the compacted crushed argillite have been investigated. The volume change properties were investigated by running one-dimensional compression tests under constant water content (2.4–2.8%) with loading–unloading cycles. Under various vertical stresses, water flooding tests were carried out under constant–volume condition. Depending on the vertical stress level, either swelling or collapse behaviour was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identified: finer samples exhibit stiffer compression behaviour and higher swelling potential. To provide a microstructure insight into the macroscopic behaviour feature observed, both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) observations were performed, evidencing that: (i) at the same dry density, the size of inter-aggregate pores is larger for the coarser crushed material; (ii) mechanical compression only reduces the inter-aggregate porosity in the stress range considered; (iii) the micro-mechanisms governing the flooding under constant–volume condition include the swelling of the clay particles, the increase of the intra-aggregate pores and the collapse of the inter-aggregate pores. The results show a strong effect of the grain size distribution on the hydro-mechanical behaviour and thus the close link between the microstructure and the hydro-mechanical behaviour.

Sampling the structure of calcium carbonate nanoparticles with metadynamics

Metadynamics is employed to sample the configurations available to calcium carbonate nanoparticles in water, and to map an approximate free energy as a function of crystalline order. These data are used to investigate the validity of bulk and ideal surface energies in predicting structure at the nanoscale. Results indicate that such predictions can determine the structure and morphology of particles as small as 3-4 nm in diameter. Comparisons are made to earlier results on 2 nm particles under constant volume conditions which support nanoconfinement as a mechanism for enhancing the stability of amorphous calcium carbonate. Our results indicate that crystalline calcitelike structure is thermodynamically preferred for nanoparticles as small as 2 nm in the absence of nanoconfinement.

Hydro-mechanical constitutive model for unsaturated compacted bentonite–sand mixture (BSM): Laboratory tests, parameter calibrations, modifications, and applications

A bentonite–sand mixture (BSM) is one of the clay-based sealing components proposed for use in a Canadian deep geological repository (DGR) for used nuclear fuel. Numerical modelling to assess the overall design of the proposed DGR requires characterisation of the hydraulic–mechanical (H–M) of each of the components of the sealing system, including the BSM. The BSM currently under consideration is a 50/50 mixture (by dry mass) of bentonite and well-graded silica sand, compacted to a dry density of at least 1.67 Mg/m 3. This paper presents the H–M constitutive model parameters, calibrated for BSM specimens under saturated and unsaturated conditions, based on various laboratory tests. A set of parameters for an elastoplastic model for unsaturated soil, Basic Barcelona Model (BBM), have been determined to simulate the mechanical behaviour of the BSM specimen. A set of parameters for van Genuchten’s Soil–Water Characteristic Curve (SWCC) and Kozeny’s hydraulic permeability model have been determined to simulate the hydraulic behaviour of the BSM specimen. Using a finite element computer code, CODE_BRIGHT, these sets of parameters have been used to simulate H–M processes in BSM specimens during water infiltration under constant volume (CV) and constant mean stress (CMS) boundary conditions. The key features of the selected constitutive models that are different from the laboratory tests of the BSM specimen have been summarised. The functions to improve the capability of the selected constitutive models to match the laboratory test results of the BSM specimen have been proposed.

G030061 単軸クリープ試験片へのスリット導入と変形一様性([G03006]材料力学部門一般セッション(6):高温・環境)

Specific numerical analyses have been required at designing modern high temperature machines and structures. Since analytical results usually depend on a constitutive equation of creep, it is necessary to derive a high-precision equation. Creep properties on deformation can be normally obtained using constant load creep machines, but constant stress creep tests are more suitable to acquire high-precision creep data. In order to keep the true stress constant, an applied load or the lever ratio should be changed during the test duration. Conventional uniaxial creep specimens have ridges attached on both sides of gauge length to measure deformation outside an electric furnace. For ductile materials, however, even if the constant stress creep testing machine are used, these ridges constrain area reduction around them, and unequal-uniform deformation breaks the constant volume condition in the gauge portion during deformation. Authors have proposed introducing slits into the ridge to release the circumferential constraint by the extensometer ridges. In this study, taking into account a realistic analytic condition, the authors carried out elastic creep Finite Element (FE) analyses to investigate an optimum depth of the slit. Subsequently, a series of constant stress creep tests using SUS304 steel are conducted to examine the effect of introducing slits on the performance of uniform deformation. The analytical and the experimental results show that the introduction of these slits into the ridges would improve uniform deformation effectively.

Solvation of hydrophobes in water and simple liquids

The solvation of nonpolar molecules in water and that in simple liquids are compared and contrasted. First, solvation thermodynamics is reviewed in a way that focuses on how the enthalpy and entropy of solvation depend on the choice of microscopic volume change v in the solvation process--including special choices v being zero (fixed-volume condition) and v being the partial molecular volume of a solute molecule (fixed-pressure condition)--and how the solvation quantities are related with temperature derivatives of the solvation free energy. Second, the solvation free energy and the solvation enthalpy of a Lennard-Jones (LJ) atom in model water are calculated in the parameter space representing the solute size and the strength of the solute-solvent interaction, and the results are compared with those for an LJ atom in the LJ solvent. The solvation diagrams showing domains of different types of solvation in the parameter space are obtained both for the constant-volume condition and for the constant-pressure condition. Similarities between water and the simple liquid are found when the constant-volume solvation is considered while a significant difference manifests itself in the fixed-pressure solvation. The domain of solvation of hydrophobic character in the parameter space is large in the constant-volume solvation both for water and for the simple liquid. When switched to the constant-pressure condition accompanying a microscopic volume change, the hydrophobic domain remains large in water but it becomes significantly small in the simple liquid. The contrasting results are due to the smallness of the thermal pressure coefficient of water at low temperatures.

Rheology of dense granular mixtures: Particle-size distributions, boundary conditions, and collisional time scales

We computationally investigate the dependence of the rheology of dense sheared granular mixtures on their particle size distribution. We consider the simplest case of a binary mixture of two different sized particles where the fraction of large particles is varied from one simulation to the next while the total solid mass is kept constant. We find that the variation of the rheology with the particle size distribution depends on the boundary conditions. For example, under constant pressure conditions the effective friction coefficient μ(∗) (the ratio between shear and pressure stresses at the boundary) increases mildly with the average particle size. On the other hand, under constant volume conditions, μ(∗) has a nonmonotonic dependence on the average particle size that is related to the proximity of the system solid fraction to the maximum packing fraction. Somewhat surprisingly, then, μ(∗) scales with a dimensionless shear rate (a generalized inertial number) in the same way for either boundary condition. We show that, for our system of relatively hard spheres, these relationships are governed largely by the ratio between average collision times and mean-free-path times, also independent of boundary conditions.

Image‐based calculation of perfusion and oxyhemoglobin saturation in skeletal muscle during submaximal isometric contractions

The relative oxygen saturation of hemoglobin and the rate of perfusion are important physiological quantities, particularly in organs such as skeletal muscle, in which oxygen delivery and use are tightly coupled. The purpose of this study was to demonstrate the image-based calculation of the relative oxygen saturation of hemoglobin and quantification of perfusion in skeletal muscle during isometric contractions. This was accomplished by establishing an empirical relationship between the rate of radiofrequency-reversible dephasing and near-infrared spectroscopy-observed oxyhemoglobin saturation (relative oxygen saturation of hemoglobin) under conditions of arterial occlusion and constant blood volume. A calibration curve was generated and used to calculate the relative oxygen saturation of hemoglobin from radiofrequency-reversible dephasing changes measured during contraction. Twelve young healthy subjects underwent 300 s of arterial occlusion and performed isometric contractions of the dorsiflexors at 30% of maximal contraction for 120 s. Muscle perfusion was quantified during contraction by arterial spin labeling and measures of muscle T(1). Comparisons between the relative oxygen saturation of hemoglobin values predicted from radiofrequency-reversible dephasing and that measured by near-infrared spectroscopy revealed no differences between methods (P = 0.760). Muscle perfusion reached a value of 34.7 mL 100 g(-1) min(-1) during contraction. These measurements hold future promise in measuring muscle oxygen consumption in healthy and diseased skeletal muscle.