Gas Flow Behavior Research Articles

Hydrodynamic behaviour of micro/nanoscale Poiseuille flow under thermal creep condition

Current work investigates the effect of thermal creep on the behavior of rarefied gas flow through micro/nanochannels using the direct simulation Monte Carlo method. Thermal creep effects are studied on velocity profiles, streamwise velocity and pressure, and thermal mass flow rate. The strength of thermal creep is examined at different Knudsen number, channel pressure ratio, and bulk temperature. The thermal mass flow rate variation is investigated over a wide range of flow rarefaction from the slip to free molecular regime.

Numerical simulation of gas–solid flow with two fluid model in a spouted-fluid bed

The flow characteristics in a spouted-fluid bed differ from those in spouted or fluidized beds because of the injection of the spouting gas and the introduction of a fluidizing gas. The flow behavior of gas–solid phases was predicted using the Eulerian–Eulerian two-fluid model (TFM) approach with kinetic theory for granular flow to obtain the flow patterns in spouted-fluid beds. The gas flux and gas incident angle have a significant influence on the porosity and particle concentration in gas–solid spouted-fluid beds. The fluidizing gas flux affects the flow behavior of particles in the fountain. In the spouted-fluid bed, the solids volume fraction is low in the spout and high in the annulus. However, the solids volume fraction is reduced near the wall.

A numerical study of performance for tight gas and shale gas reservoir systems

Various analytical, semi-analytical, and empirical models have been proposed to characterize rate and pressure behavior as a function of time in tight gas and shale gas systems featuring horizontal wells with multiple hydraulic fractures. Despite a few analytical models, as well as a small number of published numerical studies, there is currently little consensus regarding the large-scale flow behavior over time in such systems, particularly regarding the dominant flow regimes and whether or not reservoir properties or volumes can be estimated from well performance data. We constructed a fit-for-purpose numerical simulator which accounts for a variety of production features pertinent to these systems—specifically ultra-tight matrix permeability, hydraulically fractured horizontal wells with induced fractures of various configurations, multiple porosity and permeability fields, and desorption. These features cover the production mechanisms which are currently believed to be most relevant in tight gas and shale gas systems. We employ the numerical simulator to examine various tight gas and shale gas systems and to identify and illustrate the various flow regimes which progressively occur over time. We perform this study at fine grid discretization on the order of 1 mm near fractures to accurately capture flow effects at all time periods. We visualize the flow regimes using specialized plots of rate and pressure functions, as well as maps of pressure and sorption distributions. We use pressure maps to visualize the various flow regimes and their transitions in tight gas systems. In a typical tight gas system, we illustrate the initial linear flow into the hydraulic fractures (i.e., formation linear flow), transitioning to compound formation linear flow, and eventually transforming into elliptical flow. We explore variations of possible shale gas system models. Based on diffusive flow (with and without desorption), we show that due to the extremely low permeability of shale matrix (a few nanodarcies), the flow behavior is dominated by the extent of and configuration of the fractures. This work expands our understanding of flow behavior in tight gas and shale gas systems, where such an understanding may ultimately be used to estimate reservoir properties and reserves in these types of reservoirs.

The emissions of gases from abandoned mines: role of atmospheric pressure changes and air temperature on the surface

Intrinsic parameters influence abandoned mine gas emissions. Independent of the presence or not of noxious gas in mine voids, meteorological factors influenced directly the gas outflow or inflow. Measurement stations were installed in the exits located in abandoned mines of the iron Lorraine basin, the Lorraine and the north east France coal basins to enhance the gas flow behaviour. Closure methods of the mines and exits from the abandoned mines are quite different. For the first, shafts are associated with adits. Some of them are closed with bars to avoid people entrance into abandoned mine workings, but gas flow is enabled. For the second, there are essentially shafts which are secured. Some of them are equipped with a vent to enable outflow of the mine gas in specific conditions. Mine gas parameters were measured. Besides, external parameters like temperature and atmospheric pressure were monitored. Gas flow can be influenced mainly by the temperature difference between external atmosphere and mine workings (in iron mines, which can be considered as an open system) while atmospheric pressure influences gas flow in the coal mines, which can be considered as closed systems. Modelling has confirmed the differential pressure value measured that exceeds friction losses.

Study of Gas/Liquid Behavior Within an Aeroengine Bearing Chamber

Aeroengine bearing chambers typically contain bearings, seals, shafts and static parts. Oil is introduced for lubrication and cooling and this creates a two phase flow environment that may contain droplets, mist, film, ligaments, froth or foam and liquid pools. Some regions of the chamber contain a highly rotating air flow such that there are zones where the flow is gravity dominated and zones where it is rotation dominated. The University of Nottingham Technology Centre in Gas Turbine Transmission Systems, is conducting an ongoing experimental program investigating liquid and gas flow behavior in a relevant highly rotating environment. Previously reported work by the UTC has investigated film thickness and residence volume within a simplified chamber consisting of outer cylindrical chamber, inner rotating shaft and cuboid off-take geometry (termed the generic deep sump). Recently, a more aeroengine relevant bearing chamber offtake geometry has been studied. This geometry is similar to one investigated at Purdue University and consists of a “sub-sump” region approached by curved surfaces linked to the bearing chamber. The test chamber consists of an outer, stationary cylinder with an inner rotating shaft. The rig runs at ambient pressure and the working fluid (water) is introduced either via a film generator on the chamber wall or through holes in the shaft. In addition to visual data (high speed and normal video), liquid residence volume within the chamber and film thickness were the two numerical comparators chosen. Data was obtained for a number of liquid supply rates, scavenge ratios and shaft rotation speeds. The data from the current model is compared to that from the earlier studies. The data shows that in contrast to the previously reported generic deep sump study, the residence volume of the curved wall deep sump (CWDS) designs is far less sensitive to shaft speed, liquid supply rate and scavenge ratio. The method of liquid supply only makes a significant difference at the lowest scavenge ratios. Residence volume data for the Nottingham CWDS is comparable, when appropriately scaled, to that for the Purdue design. The film thickness data shows that at the lower shaft speeds investigated the flow is gravity dominated whereas at higher shaft speeds shear dominates.

CFD simulation of gas–solid flow with a cluster structure-dependent drag coefficient model in circulating fluidized beds

To model the effect of clusters on hydrodynamics of gas and particles phases in risers, the interfacial drag coefficient is taken into account in computational fluid dynamic simulations by means of a two-fluid model. The momentum and energy balances that characterize the clusters in the dense phase and dispersed particles in the dilute phase are described by the multi-scale resolution approach. The model of cluster structure-dependent (CSD) drag coefficient is proposed on the basis of the minimization of energy dissipation by heterogeneous drag (MEDHD) in the full range of Reynolds number. The model of CSD drag coefficient is then incorporated into the two-fluid model to simulate flow behavior of gas and particles in a riser. The distributions of volume fraction and velocity of particles are predicted. Simulated results are in agreement with experimental data published in the literature.

Assessment of gas flow characteristics and reaction kinetics of chemical vapour deposition reactors for the production of nano‐hybrid fibres

ABSTRACTChemical vapour deposition (CVD) had been used to produce carbon nanotube (CNT) coated carbon fibre, referred to in this work as nano‐hybrid fibres. In order to understand the gas flow behaviour inside the CVD reactor, computational fluid dynamics (CFD) has been utilised to simulate the process in a horizontal CVD reactor together with a vertical CVD reactor for comparison. For simplicity purposes, the CNT synthesis was represented by ferrocene decomposition into iron, which acts as the catalyst for CNT formation. The gas flow behaviour inside the horizontal reactor was asymmetrical in nature and seen to be dominated by recirculation effects compared with the vertical reactor, which was symmetrical and generally lacking in circulation. The impact of the gas flow characteristics on the reaction rates of the ferrocene decomposition and mass fraction of chemical species was also investigated. The kinetic rate of reaction in the horizontal reactor was found to be 2–3 orders of magnitude larger in the horizontal reactor compared with the vertical reactor. The results provide valuable insight into the best approach for producing nano‐hybrid fibres in a one‐step continuous process. © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.

Large Eddy Simulation of Gas - Second Order Moment of Particle Approach for Riser

Flow behavior of gas and particles is performed by means of gas–solid two-fluid model with the large eddy simulation for gas and the second order moment for particles in the riser. This study shows that the computed solids volume fractions of two cases are compared with the experimental data using a two-dimensional model. The gas and solid velocity is computed.

CO2 Injectivity in a Multi-lateral Horizontal Well in a Low Permeability Coal Seam: Results from a Field Trial

Enhanced coalbed methane (ECBM) production by CO2 injection offers the potential of increasing recovery of the gas in place over primary recovery methods and at the same time storing CO2. A field ECBM trial via CO2 storage has been carried out in the east margin of the Ordos Basin, Shanxi province, China. A unique aspect of the trial was using a multi-lateral horizontal well, which was clearly beneficial for production or injection for low permeability formations. This paper described a reservoir model constructed to simulate the CO2 injection behaviour using the coal seam reservoir simulator SIMED II. The simulation work included history matching the bottomhole pressure while using injection rate as control. A complexity in the modelling was representation of the multi-lateral horizontal well as its branches were not aligned with x or y coordinates. Thus approximation was taken to represent the well in the model. The simulation result showed good match for some data but could not match well the whole range of data, suggesting possible well opening or closing to flow during injection and shut-in periods. It has been suggested that the permeability decrease due to gas adsorption induced coal swelling may play an important role in the gas flow behaviour, especially near the wellbore where the amount of CO2 adsorbed was highest. However, due to the coarse grid size applied in this finite difference model and the difficulty to refine them near the well branches while maintaining overall size of the model, the permeability loss due to adsorption induced coal swelling was diluted by the relatively large grid size. Thus this may not accurately reflect the permeability change and the quality of the history matching results are affected. A suggestion is to use refined grid blocks near wellbore to examine the impact of swelling on the overall CO2 flow behaviour for this project.

Simulation of Gas-Solid Two-Phase Flow Using LES-SOM Model

Flow behavior of gas and particles is predicted by the large eddy simulation of gas-second order moment of solid model (LES-SOM model) in the simulation of flow behavior in CFB. This study shows that the simulated solid volume fractions and velocity along height using a two-dimensional model are in agreement with experiments. The second-order moments of particles are computed.

Limiting self-similar motion of a solid particle in a free-molecular gas flow escaping from an orifice

A problem of motion of a single spherical solid particle in the far wake of a free-molecular gas flow escaping from an orifice is considered. It is shown that the spatial distributions of macroscopic parameters of the gas are completely determined by functions of one variable: coordinate along an arbitrary straight line normal to the axis of symmetry. Based on this property, a dimensionless equation of particle motion is derived, which has self-similar solutions: trajectories of motion and particle coordinates (traces) on the target for different initial conditions. Conditions of determining the gas flow behavior on the basis of particle traces on the target are considered.

Issues in the integration of active control turbochargers with internal combustion engines

The application of active control means to regulate the flow of exhaust gas in a turbocharger turbine is a concept developed by the Turbomachinery Group at Imperial College, London. It is a new concept the first results of which were made public in 2004. This paper presents the theoretical grounding behind the idea, its development and the elements required for a successful implementation of active control for a turbocharger turbine and the integration of such a turbocharger system within an internal combustion engine. This paper is intended to fill a gap in the theoretical understanding of the issues pertaining to the concept of Active Control for Turbocharger Turbines. The discussion is led towards a theory summarising the flow physics and their effect on the behaviour of the exhaust gas flow occurring during turbocharger turbine inlet geometrical changes and the implications from the periodic nature of these geometric changes in particular with respect to cycle performance results both for the turbocharger and for the engine. This paper is written with the purpose of presenting a realistic context of ACT operation by identifying and considering those parameters relevant to the operation and successful application of ACT to an internal combustion engine. In addition, the requirements for a dedicated ACT control strategy which can be effective in the ACT-engine integrated environment are, also, highlighted.

Prediction of flow behavior of particles in a tapered bubbling fluidized bed using a second-order moment-frictional stresses model

Flow behavior of gas and particles is simulated by means of the second-order moments method (SOM) in a tapered bubbling fluidized bed. The frictional stress model proposed by Johnson et al. (1990) is used to predicate the frictional stresses of particles. A drag coefficient model for gas–monosized particles flow is proposed for predicting the interaction between gas and solid phase in the tapered bubbling fluidized beds. Distributions of concentration, velocity and moments of particles are predicted in the tapered bubbling fluidized bed. The ratio of the vertical normal second-order moment to the lateral normal second-order moment increases with the increase of tapering angle. The averaged ratios are larger than unity in the inner section (IS region) in the cylinder of the bed. It is, however, reverse in the outside of the cylinder (OS region). The second-order moments are direction-dependence, and the anisotropy of the normal components of the second-order moments is obvious in tapered bubbling fluidized beds. A comparison has been made between the simulations using proposed model and the experimental data. Simulated concentration of particles, bed pressure drop and bed expansion agreed reasonably with experimental measurements.

Role of rough surface topography on gas slip flow in microchannels

We conduct a lattice Boltzmann simulation of gas slip flow in microchannels incorporating rough surface effects as characterized by fractal geometry with a focus on gas-solid interaction. The gas slip flow in rough microchannels, which is characterized by Poiseuille number and mass flow rate, is evaluated and compared with smooth microchannels. The effects of roughness height, surface fractal dimension, and Knudsen number on slip behavior of gas flow in microchannels are all investigated and discussed. The results indicate that the presence of surface roughness reduces boundary slip for gas flow in microchannels with respect to a smooth surface. The gas flows at the valleys of rough walls are no-slip while velocity slips are observed over the top of rough walls. We find that the gas flow behavior in rough microchannels is insensitive to the surface topography irregularity (unlike the liquid flow in rough microchannels) but is influenced by the statistical height of rough surface and rarefaction effects. In particular, decrease in roughness height or increase in Knudsen number can lead to large wall slip for gas flow in microchannels.

Implications of the figure of merit in environmental SEM

A proper understanding of the behavior of gas flow in an environmental scanning electron microscope together with its interactions with the electron beam is important for work in this field. Further discussion on a figure of merit introduced recently is reported together with a critical review of some prior literature, with particular attention to issues found in error. A concise attempt is made to consolidate the relevant terminology in order to avoid further misinterpretations in this field.

Simulation of flow behavior of particles by cluster structure-dependent drag coefficient model for chemical looping combustion process: Air reactor modeling

A computational fluid dynamic (CFD) model for the air reactor of chemical looping combustion (CLC) technology has been developed, with special focus on flow of clusters. To model effects of clusters on flow behavior of gas and particles in a riser of air reactor, the model of cluster structure-dependent (CSD) drag coefficient is proposed on the basis of the minimization of energy dissipation by heterogeneous drag (MEDHD). The momentum balances that characterize the clusters in the dense phase and dispersed particles in the dilute phase are proposed. The CSD drag coefficient model is incorporated into the two-fluid model. The distributions of concentration and velocity of particles are predicted in a riser of air reactor. The Ergun/Wen and Yu drag model underestimates the distribution of concentration of particles in the riser. Dynamic evolution of clusters is reproduced and the profile of concentration and solid fluxes is analyzed. Predictions using the CSD drag coefficient model are in agreement with experimental data published in the literature. The distributions of O2 concentration and temperatures of gas phase and particles are predicted with CuO as oxygen carrier in the AR.

The Effects of Sample Position and Gas Flow Pattern on the Sintering of a 7xxx Aluminum Alloy

The effects of sample position and gas flow pattern on the sintering of a 7xxx aluminum alloy Al-7Zn-2.5Mg-1Cu in flowing nitrogen have been investigated both experimentally and numerically. The near-surface pore distribution and sintered density of the samples show a strong dependency on the sample separation distance over the range from 2 mm to 40 mm. The open porosity in each sample increases with increasing separation distance while the closed porosity remains essentially unchanged. A two-dimensional computational fluid dynamics (CFD) model has been developed to analyze the gas flow behavior near the sample surfaces during isothermal sintering. The streamlines, velocity profile, and volume flow rate in the cavity between each two samples are presented as a function of the sample separation distance at a fixed nitrogen flow rate of 6 L/min. The CFD modeling results provide essential details for understanding the near-surface pore distribution and density of the sintered samples. It is proposed that the different gas flow patterns near the sample surfaces result in variations of the oxygen content from the incoming nitrogen flow in the local sintering atmosphere, which affects the self-gettering process of the aluminum compacts during sintering. This leads to the development of different near-surface pore distributions and sintered densities.

A cluster structure-dependent drag coefficient model applied to risers

Cluster structures affect macroscopic hydrodynamic behavior in gas–solid risers. The moment and energy balances for the dense phase and dilute phase are presented by the multi-scale resolution approach to investigate the dependence of drag coefficient on structure parameters. The modified model of cluster structure-dependent (CSD) drag coefficient is proposed on the basis of the minimization of energy dissipation by heterogeneous drag (MEDHD). Unlike previous works on CSD drag coefficient model, the modified CSD drag model takes wall friction into account. The closure for the drag coefficient depends not only on flow behavior of gas and particles but also on the wall friction. The structure-dependent drag coefficients calculated from the approach of the minimization of energy dissipation by drag force are then incorporated into the two-fluid model to simulate the behavior of gas–solid flow in a riser. The distributions of concentration and velocity of particles are predicted. Simulated results are in agreement with experimental data published in the literature. The effect of the wall friction on flow behavior of particles is analyzed.

The application of active control for turbocharger turbines

In this paper, the motivation behind the development of an active control turbocharger is presented, along with the initial thinking that led to the basic concept of applying active flow control at the inlet to a turbocharger turbine. In addition, the concept of active control for turbochargers is analysed in depth with the purpose of presenting a theoretical basis for any subsequent application of this type of control of exhaust gas flow into a turbocharger turbine by providing the fundamental thermo-fluids background. Secondly, the aim was not only to merely present a theory summarising the behaviour of the exhaust gas flow occurring during turbocharger turbine inlet geometrical changes, but to also present the implications from the periodic nature of these geometric changes, in particular with respect to cycle performance results both for the turbocharger and for the engine. The effects of the application of active control turbochargers were demonstrated through testing of the first prototype active control turbocharger built and tested at the aerodynamic test facility at Imperial College and through the experimental data collected. In this first attempt, at the most favourable amplitudes tested, the power recovered reached a maximum value of 7.5% increase over the equivalent variable geometry turbocharger performance, although this depended on the phasing of the turbine inlet area variation in relation to the energy content variation of the incoming exhaust pulse.

Dynamic Simulation of Flow Behavior in a New Spray Granulation Tower

Aiming to understand the asphalt granulation and separation processes in a new technique of solvent deasphalting, this work performs a detailed numerical study of the gas flow behaviors in a spray granulation tower with array nozzles. The influence of nozzle distances and jet angles on the fluid dynamics of the tower, especially on the double swirling flow, is investigated. The simulation results reveal many interesting phenomena, which include a flow consisting of two distinct regimes, a multistage flow development of jet array and a strong dependence of the velocity distribution on the nozzle arrangement.