Low-pressure Limit Research Articles

One-step construction of flexible conductive-piezoelectric nanoresistance network material for pressure sensing and positioning

Traditional pressure sensors with localization functions are often performed through arrays of numerous sensors and dot-matrix electrodes. However, their application is limited because of the restrictions of miniaturization and flexibility as well as expensive multi-step preparations. Here we present a simple one-step method to establish the conductive-piezoelectric nanoresistance networks by preparing electrospun polyaniline/polyacrylonitrile (PANI/PAN) nanofibers (NFs). The nanoresistance networks constituted of PANI nanoscale conductive paths and PAN piezoelectric elements can be used as a new substitute material for sensors arrays and dot-matrix electrodes of traditional sensors for pressure sensing and localization based on the Wheatstone bridge principle. The performance of conductive-piezoelectric PANI/PAN integrated electrospun nanofibrous membrane (IENFM) for force sensing and positioning is appropriately explored and assessed. The IENFM shows high linear sensitivity of 1.71 V N−1 and low detectable pressure limit (0.1 N). Moreover, the maximum output voltage can be traced back to the position where pressure is exerted on the PANI/PAN IENFM by detecting output voltages distribution. Thus, our work opens a new sight for the application of the integrated sensing material for pressure localization.

Challenges in Kinetic modeling of ammonia pyrolysis

Ammonia pyrolysis reactions have implications for its ignition and oxidation in engines and gas turbines. In the present work, the chemistry of ammonia pyrolysis is investigated by kinetic modeling and theory. Rate constants for key reactions are carefully evaluated based on available experimental and theoretical results. The high pressure limit k1,∞ for NH2 + H (+M) ⇄ NH3 (+M) (R1) is calculated to be essentially the collision frequency, indicating that dissociation of ammonia in combustion processes will be at or close to the low pressure limit even at engine and gas turbine conditions. The chemical kinetic model is validated against reported shock tube measurements of NH3, NH2, and NH in ammonia pyrolysis. Predictions are in good agreement with observations for dilute conditions (≤ 0.5% NH3), but the model appears to underpredict the NH3 consumption rate at longer times in less dilute mixtures. At short reaction times, thermal dissociation of NH3, together with the NH3 + H reaction, controls conversion. At longer times, secondary reactions involving NH2 and NH become important due to their impact on the radical pool. Predictions become sensitive to formation and consumption of diazene (tHNNH and cHNNH). Several of the key steps in the ammonia pyrolysis mechanism are radical-radical reactions that are difficult to measure accurately and challenging to calculate theoretically, and a more comprehensive experimental characterization is desirable to support further model development.

Process parameters design method of drainage gas recovery technology in gas-driven pump for coalbed methane production

The main types of non-rod pumps such as the electrical submarine, piston, and jet pumps often have high discharge capacities. In this paper, the process parameters design method of a new non-rod gas-driven pump is proposed for carrying coal power, low discharge capacity, and low lift height drainage gas recovery in coalbed methane production. The gas-driven pump is driven by high-pressure gas and is mainly composed of a liquid storage chamber, liquid inlet valve, and liquid outlet valve. The change in gas pressure simultaneously opens or closes the liquid inlet and outlet valves, leads the liquid into the pump, and then discharges it. When the pressure of driven gas is below the lower pressure limit, the liquid inlet valve opens to allow the liquid to flow into the pump. When the pressure of driven-gas is above the upper-pressure limit, the liquid outlet valve opens to allow the liquid to drain from the pump. Based on the working mechanism of the gas-driven pump, we established a calculation model for the lower and upper limits of the driven gas pressure where the liquid inlet and outlet valves open respectively. Furthermore, we determined a model to calculate the pressure changes in the high-pressure gas tank on the ground during the drainage process. The engineering application on the pilot test wells shows that the gas-driven pump production system works well in liquid production and is a novel type of drainage gas recovery in coalbed methane production. The new calculation model was established as the solution to this lift processing scheme.

Instability of the body-centered cubic lattice within the sticky hard sphere and Lennard-Jones model obtained from exact lattice summations.

A smooth path of rearrangement from the body-centered cubic (bcc) to the face-centered cubic (fcc) lattice is obtained by introducing a single parameter to lattice vectors of a cuboidal unit cell. As a result, we obtain analytical expressions in terms of lattice sums for the cohesive energy where the interaction is described by a Lennard-Jones (LJ) interaction potential or a sticky hard-sphere (SHS) model with a r^{-n} long-range attractive term. These lattice sums are evaluated to computer precision by expansions in terms of a fast converging Bessel function series. Applying the whole range of lattice parameters for the SHS and LJ potentials we prove that the bcc phase is unstable (or, at best, metastable) toward distortion into the fcc phase in the low temperature and pressure limit. Even if more accurate potentials are used, such as the extended LJ potential for argon or chromium, the bcc phase remains unstable. This strongly indicates that the appearance of a low temperature bcc phase for several elements in the periodic table is due to higher than two-body forces in atomic interactions.

Multistructural Variational Reaction Kinetics of the Simplest Unsaturated Methyl Ester: H-Abstraction from Methyl Acrylate by H, OH, CH3, and HO2 Radicals.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH3/HO2 radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol-1. Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO2, we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

Deployment and Exploration of a Gas Storage Well Pattern Based on the Threshold Radius

AbstractTo tackle the problem that wells that are deployed in a specific pattern based on the requirements of gas reservoir development are not suitable for gas storage, we have conducted concentrically circular injection and production simulation experiments for gas storage, discovered the existence of a threshold radius, denoted by Rt, and derived the expression for Rt. Based on the analysis and discussion results, we propose a strategy for deploying gas storage wells in specific patterns. The expression for Rt shows that it is affected by factors such as the gas storage gas production/injection time, the upper pressure limit, the lower pressure limit, the bottomhole flow pressure at the ends of injection and production, the and permeability. The analysis and discussion results show that the Rt of a gas storage facility is much smaller than the Rt for gas reservoir development. In the gas storage facilities in China, the Rt for gas production is less than the Rt for the gas injection, and Rt increases with the difference in the operating pressure and with permeability K. Based on the characteristics of Rt, we propose three suggestions for gas storage well pattern deployment: (1) calculate Rt according to the designed functions of the gas storage facility and deploy the well pattern according to Rt; (2) deploy sparser, large‐wellbore patterns in high‐permeability areas and denser, small‐wellbore patterns in high‐permeability areas; and (3) achieve the gas injection well pattern by new drilling, and the gas production well pattern through a combination of the gas injection well pattern and old wells. By assessing a gas storage facility with a perfect well pattern after a number of adjustments, we found that the Rt of the 12 wells calculated in this paper is basically close to the corresponding actual radius, which validates our method. The results of this study provide a methodological basis for well pattern deployment in new gas storage construction.

Operation of Large RF Driven Negative Ion Sources for Fusion at Pressures below 0.3 Pa

The large (size: 1 m × 2 m) radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in deuterium. The plasma will be generated by inductively coupling an RF power of up to 800 kW into the source volume. Under consideration for future neutral beam heating systems, like the one for the demonstration reactor DEMO, is an even lower filling pressure of 0.2 Pa. Together with the effect of neutral gas depletion, such low operational pressures can result in a neutral gas density below the limit required for sustaining the plasma. Systematic investigations on the low-pressure operational limit of the half-ITER-size negative ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility were performed, demonstrating that operation is possible below 0.2 Pa. A strong correlation of the lower pressure limit on the magnetic filter field topology is found. Depending on the field topology, operation close to the low-pressure limit is accompanied by strong plasma oscillations in the kHz range.

Distribution of Electrons and Ions Near an Absorbing Spherical Body in a Nonequilibrium Plasma

The applicability of the collision kinetic model of point sinks, viz., the linearized theory of screening of the electric field produced by a charged dust particle, based on the Vlasov equations for electrons and ions in a nonequilibrium plasma, which are supplemented with collision terms in the Bhatnagar–Gross–Crook form and effective point sinks to dust particle, is analyzed. The criterion for the applicability of the collision kinetic model of point sinks is the smallness of the deviation of the electron and ion number densities near an absorbing spherical body from unperturbed values. The distributions of electron and ion number densities obtained using the collision kinetic model of point sinks and the orbit motion limited approach are compared. It is shown that the latter approach is applicable only in the low-pressure limit, and upon an increase in pressure, the Coulomb asymptotics of the potential, which is proportional to the frequency electron and ion collisions with neutral atoms (molecules), renders the orbit motion limited approach inapplicable for calculating the ion distribution. It is established that the domain of applicability of the collision kinetic model of point sinks is close to the applicability domain of the Debye–Huckel theory.

A shock-tube study of the N2O + M ⇄ N2 + O + M (M = Ar) rate constant using N2O laser absorption near 4.6 µm

The low-pressure limit rate constant k1,0 of the reaction N2O + M ⇄ N2 + O + M was measured in shock-heated mixtures of 0.2% N2O/Ar using 4.56-µm laser absorption of N2O in the temperature range 1546–2476 K near 1.3 atm. Modeling the N2O profiles with a detailed kinetic analysis, which considered non-ideal pressure variations, provided k1,0 values that were best fit by the expression k1,0=1.01×1015exp(−30,050/T), with k1,0 in cm3mol–1s–1 and T in K. Estimated k1,0 uncertainties at 1546, 1821, and 2230 K were respectively 13.0%, 8.9%, and 9.0%. By combining the results of the present study with previous low-temperature data measured in flow/static reactors, the best fit over the temperature range 850–2500 K was determined to be (k1,0 in cm3mol–1s–1, T in K)(R1)k1,0=(1.04±0.04×1015)exp[(−30,098±90)/T]. This is the first study of k1,0 using N2O infrared laser absorption. Through the high signal-to-noise ratios of the experimental N2O profiles and careful consideration of dP/dt effects, this k1,0 determination is in excellent agreement with the large body of historical k1,0 data but demonstrates significantly less scatter than all previous measurements.

Hydrogen oxidation near the second explosion limit in a flow reactor

This work investigates the oxidation of hydrogen near its second explosion limit in a turbulent flow reactor at pressures of 1 to 8 bar, temperatures of 950 K and an equivalence ratio of 0.035. The concentrations of H2, O2 and H2O are measured along the reactor and simulated using several kinetic models from the literature. These experiments demonstrate evident negative pressure dependence from roughly 1 to 4 bar, with further increases in pressure resuming its positive impact on reaction rates. The simulated and measured species concentrations along the reactor generally agree within a factor of 2.Further investigation is then conducted to measure the rate coefficient of reaction H + O2 (+ M) = HO2 (+M) (R2), which is one of the most sensitive reactions in hydrogen's oxidation chemistry at these conditions. This investigation is conducted by using nitric oxide (NO) as a dopant and measuring the resulting, quasi-steady-state concentrations of NO2. The rate coefficients are obtained at 950 – 1010 K. Combined with literature results, an Arrhenius expression is proposed, k2,0N2 = 4.50 × 1020 (T/K)−1.73 [cm6 mole−2 s−1], for the reaction rate at the low-pressure limit over 500 K – 2000 K with N2 as the bath gas. Simulations using the models from the literature with the proposed Arrhenius expression for this reaction then demonstrate improved agreement with the experiments.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

AbstractDecades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle.Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations.Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

A Shock-Tube Study of the Rate Constant of PH3 + M ⇄ PH2 + H + M (M = Ar) Using PH3 Laser Absorption.

Phosphine (PH3) is a highly reactive and toxic gas. Prior experimental investigations of PH3 pyrolysis reactions have included only low-temperature measurements. This study reports the first shock-tube measurements of PH3 pyrolysis using a new PH3 laser absorption technique near 4.56 μm. Experiments were conducted in mixtures of 0.5% PH3/Ar behind reflected shock waves at temperatures of 1460-2013 K and pressures of ∼1.3 and ∼0.5 atm. The PH3 time histories displayed two-stage behavior similar to that previously observed for NH3 decomposition, suggesting by analogy that the rate constant for PH3 + M ⇄ PH2 + H + M (R1) could be determined. A simple three-step mechanism was assembled for data analysis. In a detailed kinetic analysis of the first-stage PH3 decomposition, values of k1,0 were obtained and best described by (in cm3·mol-1·s-1) k1,0 = 7.78 × 1017 exp(-80,400/RT), with units of cal, mol, K, s, and cm3. Agreement between the 1.3 and 0.5 atm data confirmed that the measured k1,0 was in the low-pressure limit. Agreement of the experimental k1,0 with ab initio estimates resolved the question of the main pathway of PH3 decomposition: it proceeds as PH3 ⇄ PH2 + H instead of PH3 ⇄ PH + H2.

Low-Pressure Limit of Competitive Unimolecular Reactions.

Barker and Ortiz found unusual falloff effects in the flux coefficients of the competitive unimolecular reactions of 2-methylhexyl radicals, and they concluded that this might have important effects on the rate constants of reactions with higher thresholds. To study this effect, we carried out master equation calculations of the same reaction system to learn whether this effect shows up in measurable rate constants, and the answer is yes. We also studied specially designed mechanisms to reveal that the various reactive pathways connecting the reagents can have a large effect on the rate constants, causing them to be quite different than if the reactions proceeded independently, and that reactions with significantly higher barriers may nevertheless have larger rate constants. This provides a new perspective for interpreting and predicting the kinetics of competitive unimolecular reactions.

Kinetics of the OH + NO2 reaction: effect of water vapour and new parameterization for global modelling

Abstract. The effect of water vapour on the rate coefficient for the atmospherically important, termolecular reaction between OH and NO2 was determined in He–H2O (277, 291, and 332 K) and N2–H2O bath gases (292 K). Combining pulsed-laser photolytic generation of OH and its detection by laser-induced fluorescence (PLP-LIF) with in situ, optical measurement of both NO2 and H2O, we were able to show that (in contrast to previous investigations) the presence of H2O increases the rate coefficient significantly. We derive a rate coefficient for H2O bath gas at the low-pressure limit (k0H2O) of 15.9×10-30 cm6 molecule−2 s−1. This indicates that H2O is a more efficient collisional quencher (by a factor of ≈6) of the initially formed HO–NO2 association complex than N2, and it is a factor of ≈8 more efficient than O2. Ignoring the effect of water vapour will lead to an underestimation of the rate coefficient by up to 15 %, e.g. in the tropical boundary layer. Combining the new experimental results from this study with those from our previous paper in which we report rate coefficients obtained in N2 and O2 bath gases (Amedro et al., 2019), we derive a new parameterization for atmospheric modelling of the OH + NO2 reaction and use this in a chemical transport model (EMAC) to examine the impact of the new data on the global distribution of NO2, HNO3, and OH. Use of the new parameters (rather than those given in the IUPAC and NASA evaluations) results in significant changes in the HNO3∕NO2 ratio and NOx concentrations (the sign of which depends on which evaluation is used as reference). The model predicts the presence of HOONO (formed along with HNO3 in the title reaction) in concentrations similar to those of HO2NO2 at the tropical tropopause.

Quantum chemical and kinetic study of the CCl2 + HCl → CHCl3 insertion reaction

The CCl2 + HCl → CHCl3 (1) insertion reaction has been investigated by ab initio molecular orbital and kinetic calculations. The geometries of reactants, products and transition state were optimized with a large number of density functional theory (DFT) formulations combined with the 6-311++G(3df,3pd) basis set. The reaction barriers calculated with G4(MP2)//DFT and G4//DFT theories of 1.16 ± 0.27 and 0.61 ± 0.26 kcal mol−1 are consistent with the barrier height of ΔE0# = 1.54 ± 0.30 kcal mol−1 estimated from the room temperature experimental rate constant. Calculated enthalpies of reaction of −53.97 ± 0.09 (G4(MP2)//DFT), −55.27 ± 0.09 (G4//DFT) and −56.52 ± 3.90 kcal mol−1 (DFT) agree well with experimental values. The obtained rate constants over the 300–2000 K temperature range at the high and low pressure limit can be expressed as k1,∞(G4(MP2)//DFT) = (4.8 ± 1.8) × 10−14 (T/300 K)2.12±0.19 and k1,∞(G4//DFT) = (9.8 ± 3.4) × 10−14 (T/300 K)1.77±0.16 in cm3 molecule−1 s−1, and k1,0 = [HCl] 3.40 × 10−27 (T/300 K)−6.57 exp(−2218 K/T) cm3 molecule−1 s−1. Falloff curves for the intermediate pressure range obtained for the reactions (1, −1) with these rate constants and a derived central broadening factor of Fcent = 0.16 + 0.84 exp(−T/331 K) + exp(−7860 K/T) were compared with reported experimental rate data.

Microcanonical Rate Constants for Unimolecular Reactions in the Low-Pressure Limit.

Low-pressure-limit microcanonical rate constants, κ0(E,J), describe the rate of activating bath gas collisions in a unimolecular reaction and are calculated here using classical trajectories and quantized thresholds for reaction. The resulting semiclassical rate constants are two-dimensional (in total energy E and total angular momentum J) and are intermediate in complexity between the four-dimensional state-to-state collisional energy and angular momentum transfer rate constant, R(E',J';E,J), and the highly averaged thermal rate constant, k0. Results are presented for CH4 (+M), C2Hx (+M), x = 3-6, and H2O (+M), where κ0(E,J) is shown generally to be a sensitive function of the bath gas, temperature, and initial state of the unimolecular reactant. Strong variations in κ0 with respect to E and J lead to complex trends in relative microcanonical bath gas efficiencies. This underlying complexity may complicate the search for simple explanations for observed trends in relative thermal bath gas efficiencies. A different measure of the microcanonical collision efficiency that describes the energy range of activating collisions is introduced that supports the empirical decomposition of collisional activation into separable translational-to-vibrational and rotational-to-vibrational activation mechanisms. The two mechanisms depend differently on mass, temperature, and the J-dependence of the threshold energy for reaction, with rotational-to-vibrational activation favored for heavier baths and for reactions with rigid transition states. Finally, κ0 is used to test the accuracy of several two-dimensional models for R that were proposed for use in master equation studies.

Mixture rules and falloff are now major uncertainties in experimentally derived rate parameters for H + O2 (+M) ↔ HO2 (+M)

Rate constants for the title reaction for different bath gas species, M, are essential to accurate combustion predictions. Most experimental studies of the title reaction assume the reaction is in the low-pressure limit. Furthermore, experimental studies for M = H2O usually rely on measurements in H2O/A mixtures, separate measurements in a reference bath gas M = A, and an assumed mixture “rule,” which estimates rate constants in the mixture from those of pure bath gases. We present results from master equation calculations to quantify the uncertainties due to these assumptions in experimental interpretations. Our calculations indicate potential errors due to falloff and mixture rule assumptions that would be imperceptible experimentally over typical variations of pressure and composition yet introduce substantial uncertainties (reaching ∼ 50%) in reported rate constants, which often involve extrapolation to zero pressure and unity mole fraction. Going forward, we recommend that experimentally determined pseudo-second-order rate constants for H + O2 ↔ HO2 for the experimental pressure and mixture composition (which are independent of these assumptions) be reported, that experiments used to derive rate constants in the low-pressure limit or for M = H2O be conducted at lower pressures and higher H2O fractions (where these assumptions are more accurate), and that uncertainty analysis consider uncertainties due to falloff and mixture rule assumptions.

Compressibility in a Variable Generalised Chaplygin Gas

AbstractConsidering the Panigrahi and Chatterjee model (2017) for variable generalised Chaplygin gas, in this paper we found for this kind of exotic matter an analytic expression for the adiabatic compressibility βs. It was analyzed the behaviour of the adiabatic compressibility in the limit of high and low pressure. The derived equation for βs was used to deduce the value of the heat capacity at constant pressure Cp for variable generalised Chaplygin gas.

Recognitions on the flow mechanism of shale during the plug pulse decay measurement

When using the plug pulse decay method to measure shale permeability, a comprehensive analysis of various factors influencing the measurement and analysis precision is conducive to improving the accuracy of the test result. A great number of nano-scale pores are developed in shale, so the lower the test pressure is, the more easily the non-Darcy flow tends to emerge. In order to ensure the gas is in the form of Darcy flow in the process of measurement, we put forward the lower pressure limit of Darcy flow in pores according to the definition of Knudsen number. Then, the gas desorption-flow coupling model for the gas flow process during the plug pulse decay measurement was established by considering the effects of gas on the adsorption in organic pores. After the partial differential equation set was solved and derived, the corresponding permeability calculation method was proposed. Finally, after the plug pulse decay measurement, the supporting nitrogen isothermal adsorption test and methane isothermal adsorption test were conducted on shale samples. And the following research results were obtained. First, during the plug pulse decay measurement, the flow of gas in rock samples is one-dimensional linear, so when establishing the flow equation, the gas pseudopressure can be replaced with the gas pressure to simplify the calculation. Second, the adsorption of nitrogen in shale is much less than that of methane in shale, so the influence of nitrogen desorption on the flow is negligible when the pressure difference (i.e. <5%) of upstream and downstream changes within 5% of the initial pore pressure. In conclusion, the available industrial standards don't take into consideration the influence of gas adsorption, but the shale permeability measurement can still satisfy the accuracy requirement.

Drop expansion driven by bubbling on microscale patterned substrates under low air pressure

• The two kinds of drop behavior under low air pressure were investigated. • The critical air pressure for drop expansion was analyzed and experimentally verified. • The bubbling mechanism on microscale patterned substrates was illustrated. • Two stages of drop expansion driven by bubbling was demonstrated. The growth of bubbles has been extensively studied for decades. However, the bubbling inside a single drop on complex surfaces and its subsequent effects on drop are not well understood yet. A systematical investigating of bubbling mechanism in drops on customized microscale pore-patterned surfaces is presented, and the results demonstrate that drop behaviors can be controlled under low air pressure. As the pressure decreases, the drop behaves via two approaches, depending on the competition between the critical chamber pressure and the lower limit of pressure. When the former one is higher, drop prefers to expand, otherwise, evaporation occurs. Further studies suggest that the drop expansion consists of two stages: pinning mode and pinning to depinning transition. This work provides new insights in understanding of bubbling physics on complex patterned substrates, opens a general route for the controlling of drop behaviors under low air pressure, and can also find potential applications in optimization design for self-cleaning patterned surface.