CBM exploration: Permeability of coal owing to cleat and connected fracture

Coalbed methane (CBM) resources cannot be efficiently explored and exploited without a robust understanding of the permeability of fracture-size heterogeneities in coal. In this study, two sister coal samples were imparted with pre-developed cleat and connected fractures, and the permeability of the coal samples was measured under different conditions of controlled confining and gas pressures. Furthermore, the implications of the results for CBM exploration and exploitation were discussed. The permeability of coal with cleat development ranged from 0.001–0.01 mD, indicating ultra-low permeability coal. The gas migration in this coal changed from a linear flow to a non-linear flow, with the increase in gas pressure (>1 MPa). Thus, the permeability of the coal initially increased and then decreased. However, the Klinkenberg effect does not exist in this ultralow-permeability coal. For the coal sample with connected fracture, permeability ranged from 0.1–10 mD, which is larger by hundred orders of magnitude than that of the sample with cleat. For this coal, with a decrease in gas pressure (<1 MPa), the Klinkenberg effect significantly increased the permeability of the coal. With an increase in the applied confining pressure, both the Klinkenberg coefficient and permeability of the coal presented a decreasing trend. It is suggested that field fracture investigation is a prerequisite and indispensable step for successful CBM production. The coal beds that cleat network is well conductive to the connected fracture can be an improved target area for CBM production. During CBM production, a variety of flow regimes are available owing to the decrease in CBM reservoir pressure. In particular, under the low CBM reservoir pressure and low in situ geo-stress conditions, the gas migration in the CBM reservoir with connected facture development exhibits remarkable free-molecular flow. Thus, the reservoir permeability and predicted CBM production will be enhanced.

High-throughput super-resolution single particle trajectory analysis reconstructs organelle dynamics and membrane re-organization

Super-resolution imaging can generate thousands of single-particle trajectories. These data can potentially reconstruct subcellular organization and dynamics, as well as measure disease-linked changes. However, computational methods that can derive quantitative information from such massive datasets are currently lacking. Here we present data analysis and algorithms that are broadly applicable to reveal local binding and trafficking interactions and organization of dynamic sub-cellular sites. We applied this analysis to the endoplasmic reticulum and neuronal membrane. The method is based on spatio-temporal time window segmentation that explores data at multiple levels and detects the architecture and boundaries of high density regions in areas that are hundreds of nanometers. By statistical analysis of a large number of datapoints, the present method allows measurements of nano-region stability. By connecting highly dense regions, we reconstructed the network topology of the ER, as well as molecular flow redistribution, and the local space explored by trajectories. Segmenting trajectories at appropriate scales extracts confined trajectories, allowing quantification of dynamic interactions between lysosomes and the ER. A final step of the method reveals the motion of trajectories relative to the ensemble, allowing reconstruction of dynamics in normal ER and the atlastin-null mutant. Our approach allows users to track previously inaccessible large scale dynamics at high resolution from massive datasets. The algorithm is available as an ImageJ plugin that can be applied by users to large datasets of overlapping trajectories.

Kinetic comparative study on aerodynamic characteristics of hypersonic reentry vehicle from near-continuous flow to free molecular flow

A scaled model of the X38-like configuration was simulated under hypersonic conditions for the direct simulation Monte Carlo method and the unified gas kinetic scheme. The inflow conditions considered several flow regimes, from the near-continuum through the slip-transitional to the free molecular regime. Flow fields and surface properties were compared in detail between these two methods. Not only the density and temperature contours distribution but also the surface pressure, heat flux, friction distribution, both kinetic methods give fairly consistent results. Aerodynamics of the model were also achieved and compared. The results provided by both methods agreed with each other very well. The effects of the Knudsen number and angle of attack were assessed. It is meaningful to carry out comparative studies and accelerate both methods to further progress.

Gas Damping in Capacitive MEMS Transducers in the Free Molecular Flow Regime

We present a novel analysis of gas damping in capacitive MEMS transducers that is based on a simple analytical model, assisted by Monte-Carlo simulations performed in Molflow+ to obtain an estimate for the geometry dependent gas diffusion time. This combination provides results with minimal computational expense and through freely available software, as well as insight into how the gas damping depends on the transducer geometry in the molecular flow regime. The results can be used to predict damping for arbitrary gas mixtures. The analysis was verified by experimental results for both air and helium atmospheres and matches these data to within 15% over a wide range of pressures.

A Practical Guide to Fluorescence Temporal and Spatial Correlation Spectroscopy

ABSTRACT The aim of this article is to introduce the basic principles behind the widely used microscopy tool: fluorescence fluctuation correlation spectroscopy (FFCS). We present the fundamentals behind single spot acquisition (FCS) and its extension to spatiotemporal sampling, which is implemented through image correlation spectroscopy (ICS). The article is an educational guide that introduces theoretic concepts of FCS and some of the ICS techniques, followed by interactive exercises in MATLAB. There, the learner can simulate data time series and the application of various FFCS techniques, as well as learn how to measure diffusion coefficients, molecular flow, and concentration of particles. Additionally, each section is followed by a short exercise to reinforce learning concepts by simulating different scenarios, seek verification of outcomes, and make comparisons. Furthermore, we invite the learner throughout the article to consult the literature for different extensions of FFCS techniques that allow measurements of different physicochemical properties of materials. Upon completion of the modules, we anticipate the learner will gain a good understanding in the field of FFCS that will encourage further exploration and adoption of the FFCS tools in future research and educational practices.

Kinetic comparative study on Aerodynamic Characteristics of Hypersonic Reentry Vehicle from Near-continuous Flow to Free Molecular Flow

A scaled model of the X38-like configuration was simulated under hypersonic conditions by the direct simulation Monte Carlo method and the unified gas kinetic scheme. The inflow conditions considered several different flow regimes, from the near-continuum though the slip-transitional to the free molecule regime. Flow fields and surface properties were compared in details between two methods. Not only the density and temperature contours distribution but also the surface pressure, heat flux, friction distribution, both kinetic methods give fairly consistent results. Aerodynamics of the model under hypersonic rarefied conditions were also obtained and compared. The results given by both methods agreed with each other very well. The effects of the Knudsen number and angle of attack were assessed. At the absence of experimental results, it is meaningful to carry out comparative studies and accelerate both methods to further progress.

MEMS Membranes with Nanoscale Holes for Analytical Applications

Micro-electro-mechanical membranes having nanoscale holes were developed, to be used as a nanofluidic sample inlet in novel analytical applications. Nanoscopic holes can be used as sampling points to enable a molecular flow regime, enhancing the performance and simplifying the layout of mass spectrometers and other analytical systems. To do this, the holes must be placed on membranes capable of consistently withstanding a pressure gradient of 1 bar. To achieve this goal, a membrane-in-membrane structure was adopted, where a larger and thicker membrane is microfabricated, and smaller sub-membranes are then realized in it. The nanoscopic holes are opened in the sub-membranes. Prototype devices were fabricated, having hole diameters from 300 to 600 nm, a membrane side of 80 μm, and a simulated maximum displacement of less than 150 nm under a 1 bar pressure gradient. The obtained prototypes were tested in a dedicated vacuum system, and a method to calculate the effective orifice diameter using gas flow measurements at different pressure gradients was implemented. The calculated diameters were in good agreement with the target diameter sizes. Micro-electro-mechanical technology was successfully used to develop a novel micromembrane with nanoscopic holes, and the fabricated prototypes were successfully used as a gas inlet in a vacuum system for mass spectrometry and other analytical systems.

Low-Speed DSMC Simulations of Hotwire Anemometers at High-Altitude Conditions

Numerical simulations of hotwire anemometers in low-speed, high-altitude conditions have been carried out using the direct simulation Monte Carlo (DSMC) method. Hotwire instruments are commonly used for in-situ turbulence measurements because of their ability to obtain high spatial and temporal resolution data. Fast time responses are achieved by the wires having small diameters (1–5 μm). Hotwire instruments are currently being used to make in-situ measurements of high-altitude turbulence (20–40 km). At these altitudes, hotwires experience Knudsen number values that lie in the transition-regime between slip-flow and free-molecular flow. This article expands the current knowledge of hotwire anemometers by investigating their behavior in the transition-regime. Challenges involved with simulating hotwires at high Knudsen number and low Reynolds number conditions are discussed. The ability of the DSMC method to simulate hotwires from the free-molecular to slip-flow regimes is demonstrated. Dependence of heat transfer on surface accommodation coefficient is explored and discussed. Simulation results of Nusselt number dependence on Reynolds number show good agreement with experimental data. Magnitude discrepancies are attributed to differences between simulation and experimental conditions, while discrepancies in trend are attributed to finite simulation domain size.