Steady-state Gas Research Articles

Fault characterization and heat-transfer modeling to the Northwest of Nevado del Ruiz Volcano

The Nevado del Ruiz Volcano is an area of great interest for future geothermal exploitation in Colombia facing exploration challenges as hydrothermal fluids originate deep into the fractured basement. Fieldwork conducted on the Northwest of this volcano confirmed the existence of fault zones and enabled collection of thirty rock samples from outcrops. Permeability of these samples was then measured in the laboratory with a steady-state gas permeameter, taking into account the Klinkenberg correction. The Porchet method allowed to obtain an estimation of in situ hydraulic conductivity in fault zones directly in the field. Thermal conductivity and heat capacity of the rock units were estimated from previous studies. 2D heat transfer and groundwater flow in porous fractured medium were simulated in the study area, using the free modeling package OpenGeoSys with data collected during fieldwork.Three modeling scenarios were considered, investigating the influence of fault dip in groundwater flow and heat transfer processes. It was possible to reproduce the temperature measured at the Hotel Termales hot spring, where the Samaná Sur fault appears to behave as a pathway for geothermal fluids. It was additionally found that fault dip has an influence on the simulated temperatures. When the dip is in favor of the fluid flow (<90° facing fluid flow), the simulated surface temperature increases; on the other hand, if the dip of fault is opposite to the fluid flow (>90° facing fluid flow), the simulated surface temperature decreases.

Measurement of methane emission into environment during natural gas purging process

The main purpose of this study is to develop accurate equation for predicting methane emission into the environment during natural gas (NG) purging process. The process is carried out regularly in NG pressure reducing stations. For this purpose, a numerical investigation has been carried out to simulate NG exit flow from a purging valve during opening time. The simulation has been carried out using Ansys-Fluent code. To make the solution and results more similar to actual scenario, the valve is continuously opened in a transient turbulent flow. Initial condition, is assumed steady flow in the pipeline. Three-dimensional modeling is used to simulate the valve and connected pipe, and all of the effective parameters including, inlet pressure, pipeline diameter, valve diameter and purging process time (including the time which valve needs to get completely opened and also purging time) are investigated. For simplicity and also as the main component of NG is methane, methane is considered as working fluid (a real compressible gas). The numerical results show that discharging gas velocity is reached to a supersonic velocity at outlet section of valve. As the highest expected exit velocity is sonic velocity, the supersonic velocity is a surprised result. Looking at the streamlines show that this is due to a convergent-divergent nozzle occurrence (due to re-circulation zone near junction) in discharging pipe. Also results show that discharged mass flow rate has liner relation with pipeline pressure, second degree relation with valve diameter and has fourth-degree relation with valve to pipeline diameter ratio. To make the results more applicable for NG industry, two correlations have been developed for calculating the amount of released gas in steady state and unsteady state condition. Unsteady state correlation is valid for valve opening time and steady state correlation could be used while the valve is completely opened.

Decentralized Optimization of Multi-Area Electricity-Natural Gas Flows Based on Cone Reformulation

A large-scale integrated energy system can represent several subsystems representing areas that are tied by electricity and natural gas networks. Accordingly, we propose a decentralized optimal energy flow (DOEF) calculation as compared with a centralized solution method. The proposed approach demonstrates the merits of the decentralized operation and control of a multi-area integrated electricity-natural gas system (IEGS), in terms of large-scale modeling requirements, faster computations, and data management for local sensitivity analyses. Using the proposed decentralized structure, the communication burden is relatively light as individual area operators in a multi-area IEGS will make optimal dispatch decisions independently and the corresponding information is shared with adjacent subsystems. The reformulation of the second-order cone (SOC) is proposed using advanced sequential cone programming (SCP) to handle the nonlinear steady-state natural gas flow, which provides a feasible solution with a high degree of computational efficiency. Furthermore, an iterative alternating direction method of multipliers (I-ADMM) is adopted to manage the nonconvexity of integer variables, which guarantees a satisfactory convergence performance. Case studies on three multi-area IEGS validate the effectiveness of the proposed model in a multi-area IEGS.

Zero-dimensional robust model of an SOFC with internal reforming for hybrid energy cycles

We present a mathematical model of a tubular solid oxide fuel cell (SOFC) and its implementation in an in-house zero-dimensional code named COM-GAS. The proposed zero-dimensional robust model of the SOFC with internal reforming for hybrid energy cycles allows the prediction of basic SOFC parameters such as current, voltage, voltage losses, combustion composition, power output, efficiency etc. The voltage generated by the SOFC was determined based on the extended Nernst equation. Exhaust gas composition was estimated based on equilibrium in the steady-state water gas shift reaction. Numerical simulations of both power output and voltage were compared with available experimental data, and differences did not exceed 5% in most cases. High efficiency, low emission, and fuel flexibility allows SOFCs to be coupled with gas turbines (GTs), representing a remarkable solution for hybrid cycles. In particular, when using the coupled SOFC/GT, the integrated hybrid cycle efficiency can be significantly increased (to 55%–58%) depending on its type. The newly-proposed double-pressurised SOFC/GT, not previously studied, can be used for applications requiring a low power range and low emissions.

Evaluation of a graded exercise test to determine peak fat oxidation in individuals with low cardiorespiratory fitness.

The maximal capacity to utilise fat (peak fat oxidation, PFO) may have implications for health and ultra-endurance performance and is commonly determined by incremental exercise tests employing 3-min stages. However, 3-min stages may be insufficient to attain steady-state gas kinetics, compromising test validity. We assessed whether 4-min stages produce steady-state gas exchange and reliable PFO estimates in adults with peak oxygen consumption < 40 mL·kg-1·min-1. Fifteen participants (9 females) completed a graded test to determine PFO and the intensity at which this occurred (FATMAX). Three short continuous exercise sessions (SCE) were then completed in a randomised order, involving completion of the graded test to the stage (i) preceding, (ii) equal to (SCEequal), or (iii) after the stage at which PFO was previously attained, whereupon participants then continued to cycle for 10 min at that respective intensity. Expired gases were sampled at minutes 3-4, 5-6, 7-8, and 9-10. Individual data showed steady-state gas exchange was achieved within 4 min during SCEequal. Mean fat oxidation rates were not different across time within SCEequal nor compared with the graded test at FATMAX (both p > 0.05). However, the graded test displayed poor surrogate validity (SCEequal, minutes 3-4 vs. 5-6, 7-8, and 9-10) and day-to-day reliability (minutes 3-4, SCEequal vs. graded test) to determine PFO, as evident by correlations (range: 0.47-0.83) and typical errors and 95% limits of agreement (ranges: 0.03-0.05 and ±0.09-0.15 g·min-1, respectively). In conclusion, intraindividual variation in PFO is substantial despite 4-min stages establishing steady-state gas exchange in individuals with low fitness. Individual assessment of PFO may require multiple assessments.

Solving Highly Detailed Gas Transport MINLPs: Block Separability and Penalty Alternating Direction Methods

Detailed modeling of gas transport problems leads to nonlinear and nonconvex mixed-integer optimization or feasibility models (MINLPs) because both the incorporation of discrete controls of the network and accurate physical and technical modeling are required to achieve practical solutions. Hence, ignoring certain parts of the physics model is not valid for practice. In the present contribution we extend an approach based on linear relaxations of the underlying nonlinearities by tailored model reformulation techniques yielding block-separable MINLPs. This combination of techniques allows us to apply a penalty alternating direction method and thus to solve highly detailed MINLPs for large-scale, real-world instances. The practical strength of the proposed method is demonstrated by a computational study in which we apply the method to instances from steady-state gas transport including both pooling effects with respect to the mixing of gases of different composition and a highly detailed compressor station ...

Steady state and transient simulation for electricity‐gas integrated energy systems by using convex optimisation

The increasing interdependence between electric power and natural-gas systems requires integrated simulation tools applicable to the coupled system. Therefore, this study proposes steady-state and transient simulation tools for electricity-gas integrated energy systems (IESs) by using convex optimisation. Steady-state IES simulation is designed for long-term planning studies while transient IES simulation, which includes line-pack storage, is designed for real-time operational research. The proposed convex steady-state gas flow model introduces binary variables to represent gas flow directions, and employs the second-order cone (SOC) to relax non-convex pipeline gas flow equations. As a result, steady-state gas flow analysis is formulated as a mixed integer SOC programming problem. The proposed transient gas flow model employs the implicit finite difference method to transform partial differential equations into algebraic difference equations. Moreover, the short-term gas flow directions are specified and SOC relaxations are used. Simulation results on an integrated IEEE 24-node system and Belgian 20-node gas system verify the effectiveness of the proposed SOC models. The impacts of wind power forecast errors on multi-period gas network operations are also investigated.

Integrated electricity-heat-gas modelling and assessment, with applications to the Great Britain system. Part II: Transmission network analysis and low carbon technology and resilience case studies

With the increasing interactions between the heat, electricity and gas sectors due to the introduction of low carbon heating technologies, there is a need for models to assess the inter-sector interactions at a network level. This paper presents a novel integrated electricity-heat-gas transmission network model that considers electrical and gas network flows coupled with the fuel requirements for the heating sector. The latter is modelled at a nodal network level and on a half-hourly basis, building upon the high-resolution temporal and spatial heat demand model presented in Part I. In particular, here the modelling is developed further to include an integrated heat-electricity-gas optimization to assess the operation of different heating technologies, and particularly hybrid ones. More specifically, DC power flow modelling is coupled with steady-state gas network energy and transportation cost optimization to assess gas-electricity price interactions, and with transient gas flow modelling for gas network operational studies. Numerical case studies are performed on the GB energy system, considering the evolution towards a low carbon future, the operation of hybrid dual-fuel electric heat pump/gas boiler technologies, and the ability to alleviate gas network constraints. Resilience case studies considering nodal gas price implications under gas network supply shocks are also considered. The results show how pathways to electrify heating and decarbonise the power sector can lead to a 75% reduction in carbon emissions of the heat and electricity sectors and that using hybrid heating technologies can reduce conventional generation peaks by 24%. Additionally, it is shown that exploiting gas demand response can provide an additional resilience option to the gas network.

A parametric study of dispersed laminar gas-particle flows through vertical and horizontal channels

Particle diameter, particle phase material density and inlet particle volume fraction are three important parameters governing the flow physics of dispersed gas-particle flows. In this work, an inhouse numerical solver is developed to investigate the effects of particle diameter (Stokes number), particle phase material density, inlet particle volume fraction and inlet phase velocities in the flow characteristics of gas-particle flows through vertical and horizontal channels and also in open domains. It is found that, for a constant inlet particle volume fraction, lower diameter particles attain a higher steady state velocity at any section inside the channel than the higher diameter particles; while the corresponding steady state gas velocity at any section increases with increase in particle diameter. On the other hand, for a constant particle diameter, the steady state gas phase velocity at any section decreases with increase in inlet particle volume fraction. Significant changes in both gas and particle velocity and volume fraction profiles have also been observed with inlet slip, i.e., when the velocities of both the phases at inlet are distinct as opposed to being equal, keeping all other flow and physical parameters invariant.

Characterization of an innovative device controlling gaseous exchange in packages for food products

The work describes both the characterization carried out and the experimental results from an innovative device allowing the gas exchange across a sealed package. The device is especially suitable for fresh fruit and vegetable preservation during cold storage, as it allows for the management of the headspace atmosphere inside sealed packages containing food products presenting metabolic activity.Characterization results confirm the device's suitability for managing the bidirectional gas flow. For each tested device, the hydro-dynamical parameter and the diffusion parameter have been measured to define its steady-state behaviour. The steady-state gas concentrations reached in the package show the aptness of the devices for use with specific products; these equilibrium gas concentrations depend on the device type and on the mass of product contained in the package.The results of simulated storage demonstrate that some products (pomegranate arils and table grapes) require preconditioning of the package free volume to reach the optimal gas concentrations in a short time. Other products (black truffle) with very high metabolic activity could reach the optimal condition without package preconditioning depending on the free volume.

Determination of Gas Pressure Distribution in a Pipeline Network using the Broyden Method

A potential problem in natural gas pipeline networks is bottlenecks occurring in the flow system due to unexpected high pressure at the pipeline network junctions resulting in inaccurate quantity and quality (pressure) at the end user outlets. The gas operator should be able to measure the pressure distribution in its network so the consumers can expect adequate gas quality and quantity obtained at their outlets. In this paper, a new approach to determine the gas pressure distribution in a pipeline network is proposed. A practical and user-friendly software application was developed. The network was modeled as a collection of node pressures and edge flows. The steady state gas flow equations Panhandle A, Panhandle B and Weymouth to represent flow in pipes of different sizes and a valve and regulator equation were considered. The obtained system consists of a set of nonlinear equations of node pressures and edge flowrates. Application in a network in the field involving a large number of outlets will result in a large system of nonlinear equations to be solved. In this study, the Broyden method was used for solving the system of equations. It showed satisfactory performance when implemented with field data.

Thermal stability, fire behavior, and fumes emission of polyethylene nanocomposites with halogen‐free fire retardants

AbstractThe synergism between nanoparticles and different kinds of halogen‐free fire retardants leading to reduction in flammability and smoke generation of polyethylene was investigated. The composites were evaluated by thermogravimetric analysis, oxygen index test, cone calorimeter measurements, and a single‐chamber test. Moreover, the semivolatile and volatile compounds evolved in the thermal degradation processes of polyethylene were determined using a steady state tube furnace and gas chromatograph with mass spectrometer. Morphological and structural characteristics as well as thermomechanical properties of the composites were characterized using various techniques including scanning electron microscopy (SEM), X‐ray diffraction (XRD), and dynamic mechanical thermal analysis (DMTA). The obtained data were compared to the results received for polyethylene and polymer with bromine‐containing fire retardants. The incorporation of nanofillers and halogen‐free fire retardants caused a reduction in combustibility as a consequence of the formation of char. Furthermore, the presence of nanoparticles has a beneficial effect on the inhibition of dripping. The polyethylene with carbon nanotubes, zinc borate, and aluminum hydroxide exhibited better combined properties of fire behavior and production of nontoxic smoke compared to the remaining composition and reference materials.

X3 expansion tube driver gas spectroscopy and temperature measurements

The University of Queensland’s X3 facility is a large, free-piston driven expansion tube used for super-orbital and high Mach number scramjet aerothermodynamic studies. During recent development of new scramjet test flow conditions, experimentally measured shock speeds were found to be significantly lower than that predicted by initial driver performance calculations. These calculations were based on ideal, isentropic compression of the driver gas and indicated that loss mechanisms, not accounted for in the preliminary analysis, were significant. The critical determinant of shock speed is peak driver gas sound speed, which for a given gas composition depends on the peak driver gas temperature. This temperature may be inaccurately estimated if an incorrect fill temperature is assumed, or if heat losses during driver gas compression are significant but not accounted for. For this study, the ideal predicted peak temperature was 3750 K, without accounting for losses. However, a much lower driver temperature of 2400 K is suggested based on measured experimental shock speeds. This study aimed to measure initial and peak driver gas temperatures for a representative X3 operating condition. Examination of the transient temperatures of the driver gas and compression tube steel wall during the initial fill process showed that once the filling process was complete, the steady-state driver gas temperature closely matched the tube wall temperature. Therefore, while assuming the gas is initially at the ambient laboratory temperature is not a significant source of error, it can be entirely mitigated by simply monitoring tube wall temperature. Optical emission spectroscopy was used to determine the driver gas spectra after diaphragm rupture; the driver gas emission spectrum exhibited a significant continuum radiation component, with prominent spectral lines attributed to contamination of the gas. A graybody approximation of the continuum suggested a peak driver gas temperature of 3200 K; uncertainty associated with the blackbody curve fit is ±100 K. However, work is required to quantify additional sources of uncertainty due to the graybody assumption and the presence of contaminant particles in the driver gas; these are potentially significant. The estimate of the driver gas temperature suggests that driver heat losses are not the dominant contributor to the lower-than-expected shock speeds for X3. Since both the driver temperature and pressure have been measured, investigation of total pressure losses during driver gas expansion across the diaphragm and driver-to-driven tube area change (currently not accounted for) is recommended for future studies as the likely mechanism for the observed performance gap.

Assessment of a novel numerical model for combustion and in-flight heating of particles in an industrial furnace

The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2 MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178 K higher compared to a particle, which trajectory led through zones with lower gas temperatures.

Reduced gas accretion on super-Earths and ice giants

A large fraction of giant planets have gaseous envelopes that are limited to about 10 % of their total mass budget. Such planets are present in the Solar System (Uranus, Neptune) and are frequently observed in short periods around other stars (the so-called Super-Earths). In contrast to these observations, theoretical calculations based on the evolution of hydrostatic envelopes argue that such low mass envelopes cannot be maintained around cores exceeding five Earth masses. Instead, under nominal disc conditions, these planets would acquire massive envelopes through runaway gas accretion within the lifetime of the protoplanetary disc. In this work, we show that planetary envelopes are not in hydrostatic balance, which slows down envelope growth. A series of 3-dimensional, global, radiative hydrodynamical simulations reveal a steady state gas flow, which enters through the poles and exits in the disc midplane. Gas is pushed through the outer envelope in about 10 orbital timescales. In regions of the disc that are not significantly dust-depleted, envelope accretion onto cores of about five Earth masses can get stalled as the gas flow enters the deep interior. Accreted solids sublimate deep in the convective interior, but small opacity-providing grains are trapped in the flow and do not settle, which further prevents rapid envelope accretion. The transition to runaway gas accretion can however be reached when cores grow larger than typical Super-Earths, beyond 15 Earth masses, and preferably when disc opacities are below kappa=1 cm^2/g. These findings offer an explanation for the typical low-mass envelopes around the cores of Super-Earths.

In Silico Analysis of the Regulation of the Photosynthetic Electron Transport Chain in C3 Plants.

We present a new simulation model of the reactions in the photosynthetic electron transport chain of C3 species. We show that including recent insights about the regulation of the thylakoid proton motive force, ATP/NADPH balancing mechanisms (cyclic and noncyclic alternative electron transport), and regulation of Rubisco activity leads to emergent behaviors that may affect the operation and regulation of photosynthesis under different dynamic environmental conditions. The model was parameterized with experimental results in the literature, with a focus on Arabidopsis (Arabidopsis thaliana). A dataset was constructed from multiple sources, including measurements of steady-state and dynamic gas exchange, chlorophyll fluorescence, and absorbance spectroscopy under different light intensities and CO2, to test predictions of the model under different experimental conditions. Simulations suggested that there are strong interactions between cyclic and noncyclic alternative electron transport and that an excess capacity for alternative electron transport is required to ensure adequate redox state and lumen pH. Furthermore, the model predicted that, under specific conditions, reduction of ferredoxin by plastoquinol is possible after a rapid increase in light intensity. Further analysis also revealed that the relationship between ATP synthesis and proton motive force was highly regulated by the concentrations of ATP, ADP, and inorganic phosphate, and this facilitated an increase in nonphotochemical quenching and proton motive force under conditions where metabolism was limiting, such as low CO2, high light intensity, or combined high CO2 and high light intensity. The model may be used as an in silico platform for future research on the regulation of photosynthetic electron transport.

Assessing relative contributions of transport mechanisms and real gas properties to gas flow in nanoscale organic pores in shales by pore network modelling

It is well-known that the movement of gas in organic nanoscale pores of typical shales must be modelled by capturing real gas flow behaviours in the full range of flow regimes, gas ad-/de-sorption and its effect on the flow, and surface diffusion while properly accounting for real gas PVT and viscosity changes as affected by the confined pore space. So far, no comprehensive model has been developed to enable the evaluation of the relative contributions of each of these physical aspects in a realistic organic pore space. In this work, a steady-state pore-network gas flow model that accounts for all of the listed aspects is developed to allow an assessment of their flow contributions in organic pores. The gas flow model is applied to three pore/throat network models, which are constructed from the same realistic pore network but with different average pore radii at 15.6, 3.2 and 1.56 nanometres, respectively, to calculate apparent gas permeability for each model at gas pressures ranging from 5 to 70MPa. Analytical solution is applied to calculate the apparent gas permeability at the same gas pressures for three cylindrical pores with pore radii equal to the average pore radii of respective pore networks. For both the single pores and the pore networks, results show that when the average or single pore radius is larger than 10nm, there is little influence on apparent gas permeability no matter what gas property, either real or ideal gas, is considered, nor is the surface diffusion, in the full pressure range. However, when the pore radius is smaller than 5nm, the apparent gas permeability is notably influenced by the gas property and the surface diffusion. Furthermore when the pore radius is less than 2nm, the gas permeability will be significantly underestimated if the surface diffusion is neglected. It is found that the influence of both critical temperature and pressure in the confined pore space, which deviate from the expected values in wide space, is insignificant and negligible on shale gas permeability. The relative contributions of the gas property and the surface diffusion, respectively, are shown to follow different trends for the single pores and the pore networks within the range of the pressures. An analysis shows that the differences can be attributed to the mixture of large and small pores and throats in a pore network that effectively suppresses the stronger effects of the gas property and the surface diffusion in the smaller pores and throats. This indicates the importance to consider spatial pore size distribution and pore connectivity when seeking to estimate effective properties.

Numerical investigation of heat and mass transfer flow under the influence of silicon carbide by means of plasma-enhanced chemical vapor deposition vertical reactor

The effect of characteristics flow (contour of velocity), mass transfer (Sherwood number) and heat transfer (Nu number) on the growth rate of silicon carbide by means of plasma-enhanced chemical vapor deposition vertical reactor is investigated. The species transport and thermal fluid transport with chemical reaction are taken into account. The steady-state laminar fluid flow and gas flow having ideal behavior are considered. A mixture of silane and propane (2% molar) as main reactant gases and hydrogen (96% molar) as propellant gas are injected into the reactor. Four different diameters of shower head, three different substrate rotation speeds and five different temperatures of the substrate are used. The finite volume method is employed to solve the problem. The governing equations are solved by upwind differencing scheme. The assumption of speed–pressure coupling leads to use of semi-implicit method for pressure-linked equations to solve the governing equation. It is found that the deposition rate reduces with the shower head diameter and value of substrate temperature and enhances with rotational speed of the substrate. Furthermore, the best shower head diameter to achieve maximum rate of deposition is 1 mm. At the end, a comparison as a limiting case of the considered problem with the existing studies is made. Comparing the results of this experiment with prior studies has shown acceptable consistency.

The Perihelion Emission of Comet C/2010 L5 (WISE)

Abstract The only Halley-type comet discovered by the Wide-Field Infrared Survey Explorer (WISE), C/2010 L5 (WISE), was imaged three times by WISE, and it showed a significant dust tail during the second and third visits (2010 June and July, respectively). We present here an analysis of the data collected by WISE, putting estimates on the comet’s size, dust production rate, gas production (CO+CO2) rate, and active fraction. We also present a detailed description of a novel tail-fitting technique that allows the commonly used syndyne–synchrone models to be used analytically, thereby giving more robust results. We find that C/2010 L5's dust tail was likely formed by strong emission, likely in the form of an outburst, occurring when the comet was within a few days of perihelion. Analyses of the June and July data independently agree on this result. The two separate epochs of dust tail analysis independently suggest a strong emission event close to perihelion. The average size of the dust particles in the dust tail increased between the epochs, suggesting that the dust was primarily released in a short period of time, and the smaller dust particles were quickly swept away by solar radiation pressure, leaving the larger particles behind. The difference in CO2 and dust production rates measured in 2010 June and July is not consistent with “normal” steady-state gas production from a comet at these heliocentric distances, suggesting that much of the detected CO2 and dust was produced in an episodic event. Together, these conclusions suggest that C/2010 L5 experienced a significant outburst event when the comet was close to perihelion.

Rate dependency of permeability in tight rocks

Matrix permeability of tight rocks is a challenging parameter to measure. The gas permeability measurements of low-permeability rocks are sensitive to pressure, effective stress, temperature, and testing technique. However, the influence of gas flow rate on permeability measurements of tight rocks is unknown.In this work, steady-state gas permeability experiments are conducted on four shale and siltstone samples using methane and nitrogen as flowing fluid. The gas permeability is measured at several flow rates, while mean pressure and mean effective stress are held constant. The measurements are repeated at several mean pressure values. The tests are designed to study the influence of mean pressure as well as flow rate on gas permeability. After the gas permeability measurements, the samples are saturated with water to measure water permeability. Subsequently, the samples are sheared in a triaxial cell and similar gas and water permeability tests are repeated to compare the permeability behavior before and after failure. The results indicate strong rate sensitivity in permeability of tight rocks. Measured permeability is observed to rise as flow rate increases and reaches a constant value at higher rates. Permeability of the failed samples shows a similar behavior, although with higher permeability values. The rate dependency of gas permeability seems to follow a trend, similar to what is reported in the literature as the transition between pre-laminar and laminar flow regimes. Based on the experimental results, a discussion is provided on the rate at which the transition between pre-laminar and laminar flow occurs. Finally, by combining the theories on rate dependent flow regimes and pressure dependent (Knudsen number) flow regimes, a more complete picture of gas permeability in porous media is proposed.