Hydrodynamics Of Gas Research Articles

Hydrodynamics of a Granular Gas in a Heterogeneous Environment

We analyze the transport properties of a low density ensemble of identical macroscopic particles immersed in an active fluid. The particles are modeled as inelastic hard spheres (granular gas). The non-homogeneous active fluid is modeled by means of a non-uniform stochastic thermostat. The theoretical results are validated with a numerical solution of the corresponding the kinetic equation (direct simulation Monte Carlo method). We show a steady flow in the system that is accurately described by Navier-Stokes (NS) hydrodynamics, even for high inelasticity. Surprisingly, we find that the deviations from NS hydrodynamics for this flow are stronger as the inelasticity decreases. The active fluid action is modeled here with a non-uniform fluctuating volume force. This is a relevant result given that hydrodynamics of particles in complex environments, such as biological crowded environments, is still a question under intense debate.

Models for the dynamics of dust-like matter in the self-gravity field: The method of hydrodynamic substitutions

Models for the dynamics of a dust-like medium in the self-gravity field are investigated. Solutions of the corresponding problems are constructed by the method of hydrodynamic substitutions generalizing the Cole–Hopf substitutions. The method is extended to multidimensional ideal and viscous fluid flows with cylindrical and spherical symmetries for which exact solutions are constructed. Solutions for the dynamics of self-gravitating dust with arbitrary initial distributions of both fluid density and velocity are constructed using special coordinate transformations. In particular, the problem of cosmological expansion is considered in terms of Newton’s gravity theory. Models of a one-dimensional viscous dust fluid flow and some problems of gas hydrodynamics are considered. Examples of exact solutions and their brief analysis are provided.

Effect of particle stress tensor in simulations of dense gas–particle flows in fluidized beds

A two-fluid model based on the kinetic theory of granular flow for the rapid-flow regime and the Coulomb friction law for the quasi-static regime is applied to predict the hydrodynamics of dense gas–particle flow in a three-dimensional fluidized bed. Two different models for the particle stress tensor that use different constitutive equations in the elastic-inertial regime are examined to assess their ability to predict bed dynamics. To understand how particle stress models affect structural features of the flow, a quantitative analysis is performed on some important aspects of the mechanics of bubbling beds that have received relatively little attention in the literature. Accordingly, different flow regimes are identified in the context of fluidized beds through the dimensionless inertial number, and the main characteristics of each regime are discussed. In addition, how the particle stress tensor manifests itself in the bubble characteristics, natural frequency of the bed, and particle Reynolds stress are investigated, all of which help to better understand the complex dynamics of the fluidized bed. The numerical results are validated against published experimental data and demonstrate the significant role of the stress tensor in the elastic-inertial regime.

Hydrodynamics of gas–liquid flow in micropacked beds: Pressure drop, liquid holdup, and two‐phase model

Hydrodynamics of gas–liquid two‐phase flow in micropacked beds are studied with a new experimental setup. The pressure drop, residence time distribution, and liquid holdup are measured with gas and liquid flow rates varying from 4 to 14 sccm and 0.1 to 1 mL/min, respectively. Key parameters are identified to control the experimentally observed hydrodynamics, including transient start‐up procedure, gas and liquid superficial velocities, particle and packed bed diameters, and physical properties of the liquids. Contrary to conventional large packed beds, our results demonstrate that in these microsystems, capillary forces have a large effect on pressure drop and liquid holdup, while gravity can be neglected. A mathematical model describes the hydrodynamics in the micropacked beds by considering the contribution of capillary forces, and its predictions are in good agreement with experimental data. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4694–4704, 2017

CFD simulation of bubbling fluidized bed: Effects of bed column geometry on hydrodynamics of gas–solid mixing

AbstractIn this study, multiphase computational fluid dynamics (CFD) is employed to simulate and study the hydrodynamics behavior of gas‐solid mixing in a new bed column design of bubbling fluidized bed called swirl bed. The new fluidized bed is a swirl tube reactor design based on swirl tube technology. The results are compared with the conventional straight tube rector which is the scaled down version of the existing pilot plant gasifier. It is found that the usage of the swirl tube reactor design results in better lateral solid mixing inside the bed. Furthermore an increase in residence time of the particles is also observed due to the swirl motion of the particles.

Experimental and numerical study on effects of deflectors on flow field distribution and desulfurization efficiency in spray towers

The desulfurization efficiency of a spray tower that uses Wet Flue Gas Desulfurization (WFGD) technology can be improved by adding deflectors. This paper experimentally and numerically investigates the performance of a Deflector Spray Tower (DST) and then compares the results with an Open Spray Tower (OST) and Tray Spray Tower (TST). An experimental facility (i.e., 1:15 scale pilot plant) is established based on the WFGD system of an actual 600MW coal-fired power plant. The experiment simulates gas and liquid hydrodynamics inside the spray tower. The continuous phase is described in an Eulerian framework and liquid droplet motion is computed by the Lagrange approach. Sulfur dioxide absorption is modeled on the two-film theory, appropriate empirical correlations and semi-empirical correlations. The effect of the tray on the flow field is considered under porous media conditions. A comparison of experimental and numerical results validates the numerical simulations, showing that the Euler-Lagrange approach accurately predicts the flue flow field and liquid droplet distribution. Here, results show that deflectors positively affect flow field structure in the spray tower. The deflectors rearrange the single-phase flow field of the gas and partially solve the bias flow due to the single inlet. Therefore, peak velocity in the tower decreases such that the pressure drop for DST is 14.1% lower than the pressure drop for OST. In addition, compared with TST, DST is superior in tower resistance, with a decrease of approximately 39.7%. The SO2 desulfurization absorption model is verified in an actual 660MW power plant. Here, desulfurization efficiency is 89.6% for OST, 92.5% for TST and 96.7% for DST at the design parameter.

Assessment of gas permeability coefficient of porous materials

Abstract The results of experimental research upon the assessment of gas permeability of porous materials with respect to the gas flow. The conducted research applied to natural materials with an anisotropic gap-porous structure and - for comparative purposes - to model materials such as pumice and polyamide agglomerates. The research was conducted with the use of a special test stand that enables measuring the gas permeability with respect to three flow orientations compared with symmetric cubic-shaped samples. The research results show an explicit impact of the flow direction on the permeability of biochar, which results from their anisotropic internal structures. The permeability coefficient of such materials was determined and an experimental evaluation of the value of this coefficient was conducted with respect to the gas stream and the total pressure drop across the porous deposit. The process of gas permeability was considered in the category of hydrodynamics of gas flow through porous deposits. It is important to broaden the knowledge of gas hydrodynamics assessment in porous media so far unrecognised for the development of a new generation of clean energy sources, especially in the context of biogas or raw gas production.

Assessment of porous material anisotropy and its effect on gas permeability

The results of experimental research upon the assessment of porous material anisotropy and its effect on gas permeability of porous materials with respect to the gas flow. The conducted research applied to natural materials with an anisotropic gap-porous structure and - for comparative purposes - to model materials such as coke, pumice and polyamide agglomerates. The research was conducted with the use of a special test stand that enables measuring the gas permeability with respect to three flow orientations compared with symmetric cubic-shaped samples. The research results show an explicit impact of the flow direction on the permeability of materials porous, which results from their anisotropic internal structures. The anisotropy coefficient and permeability effective coefficient of such materials was determined and an experimental evaluation of the value of this coefficient was conducted with respect to the gas stream and the total pressure drop across the porous deposit. The process of gas permeability was considered in the category of hydrodynamics of gas flow through porous deposits. It is important to broaden the knowledge of gas hydrodynamics assessment in porous media so far unrecognised for the development of a new generation of clean energy sources, especially in the context of biogas or raw gas production.

Experimental investigation on gas-solid hydrodynamics of coarse particles in a two-dimensional spouted bed

The gas-solid hydrodynamics of coarse particles in a two-dimensional spouted bed (2DSB) are measured using 6mm glass beads to obtain quantitative experimental results. A one-dimensional mathematical model based on gas-solid mass and momentum balance is used to characterize the spout-annulus interaction and to further interpret the experimental results. The superficial velocity and static bed height are in the range of 2.58 to 4.39m/s and 8.0 to 13.3cm, respectively. Pressure fluctuation periodicity of the bed pressure drop is characterized by power spectral density (PSD) analysis. The whole-field particle velocity profiles in 2DSB are obtained by high speed CCD camera and PIV technique. The results show that particle motion in the spout in the present 2DSB experiments is dominated by the balance of drag force, solid stress and gravitation. The existence of obvious solid stress from particle impaction and friction leads to energy dissipation, which limits the increase of vertical particle velocity along the spout axis (vyc) in the spout and results in a flat stage of vyc with slight fluctuation near the bottom of the fountain. The solid stress also contributes to the nearly flat peak of lateral profile of downward particle velocity in the annulus. These detailed results are helpful for understanding the gas hydrodynamics and particle motion behavior of coarse particles in 2DSB, and for verification of the extended CFD-DEM coupling method for particle size approaching fluid cell size.

MERGER HYDRODYNAMICS OF THE LUMINOUS CLUSTER RX J1347.5–1145

ABSTRACT We present an analysis of the complex gas hydrodynamics in the X-ray-luminous galaxy cluster RX J1347.5–1145 caught in the act of merging with a subcluster to its southeast using a combined 186 ks Chandra exposure, 2.5 times greater than previous analyses. The primary cluster hosts a sloshing cold front spiral traced by four surface brightness edges west, southeast, east, and northeast from the primary central dominant galaxy, suggesting that the merger is in the plane of the sky. We measure temperature and density ratios across these edges, confirming that they are sloshing cold fronts. We observe the eastern edge of the subcluster infall shock, confirming that the observed subcluster is traveling from the southwest to the northeast in a clockwise orbit. We measure a shock density contrast of and infer a Mach number of 1.25 ± 0.08 and a shock velocity of km s−1. Temperature and entropy maps show cool, low-entropy gas trailing the subcluster in a southwestern tail, consistent with core shredding. Simulations suggest that a perturber in the plane of the sky on a clockwise orbit would produce a sloshing spiral winding counterclockwise, opposite to that observed. The most compelling solution to this discrepancy is that the observed southeastern subcluster is on its first passage, shock-heating gas during its clockwise infall, while the main cluster’s clockwise cold front spiral formed from earlier encounters with a second perturber orbiting counterclockwise.

Investigation of gas–solids flow in a circulating fluidized bed using 3D electrical capacitance tomography

The hydrodynamics of gas–solids flow in the bottom of a circulating fluidized bed (CFB) are complicated. Three-dimensional (3D) electrical capacitance tomography (ECT) has been used to investigate the hydrodynamics in risers of different shapes. Four different ECT sensors with 12 electrodes each are designed according to the dimension of risers, including two circular ECT sensors, a square ECT sensor and a rectangular ECT sensor. The electrodes are evenly arranged in three planes to obtain capacitance in different heights and to reconstruct the 3D images by linear back projection (LBP) algorithm. Experiments were carried out on the four risers using sands as the solids material. The capacitance and differential pressure are measured under the gas superficial velocity from 0.6 m s−1 to 3.0 m s−1 with a step of 0.2 m s−1. The flow regime is investigated according to the solids concentration and differential pressure. The dynamic property of bubbling flows is analyzed theoretically and the performance of the 3D ECT sensors is evaluated. The experimental results show that 3D ECT can be used in the CFB with different risers to predict the hydrodynamics of gas–solids bubbling flows.

On the stability of Taylor bubbles inside a confined highly porous medium

This work focuses on the local hydrodynamics of a multiphase gas–liquid flow forced into an innovative medium of high porosity (96%): an open cell solid foam. The gas (nitrogen) and liquid (ethanol) phases are injected at constant flow-rates in a millichannel to form a well-controlled Taylor flow which enters the porous medium. Based on a fluorescence technique, the apparent liquid holdup in the porous medium is quantified, and its evolution in time and along the porous medium extracted from spatiotemporal diagrams. The analysis of the main frequency, when varying the gas–liquid flow-rate ratio, leads to the identification of two hydrodynamic regimes. A model based on a scaling analysis is proposed to quantify the dimensionless numbers describing the transition between both regimes. It points out that the bubble length fixed by the Taylor flow is the control parameter. The model prediction of the critical bubble length at which the transition occurs is in good agreement with the experimental observations.

Characterization of gas–liquid–solid fluidized beds by S statistics

The hydrodynamics of a gas–liquid–solid fluidized bed was investigated by applying the S statistics method to pressure fluctuations measured under various operating conditions in a laboratory-scale bed. S statistics tests reveal the existence of three transition velocities, especially at low gas velocities. Four distinct fluidization regimes, namely, the compacted bed, agitated bed and coalesced and discrete bubble regimes were detected. A comparison of reconstructed attractors of pressure fluctuations measured at different axial positions along the riser and with various solid loadings showed significant differences in the signals compared before fluidization, especially at minimum liquid agitation velocity. Close to the minimum liquid fluidization velocity and high liquid velocities, the variation in particle size has an insignificant effect on the bed hydrodynamics. Therefore, S statistics is a reliable method to demarcate different fluidization regimes and to characterize the influence of various operating conditions on the hydrodynamics of gas–liquid–solid fluidized beds. The method is applicable in large-scale industrial installations to detect dynamic changes within a bed, such as regime transitions or agglomeration.

Modeling the hydrodynamics of cocurrent gas–solid downers according to energy-minimization multi-scale theory

Cocurrent gas–solid downer reactors have many applications in industry because they possess the technological advantages of a lower pressure drop, shorter residence time, and less solid backmixing when compared with traditional circulating fluidized bed risers. By introducing the concept of particle clusters explicitly, a one-dimensional model with consideration of the interphase interactions between the fluid and particles at both microscale and mesoscale is formulated for concurrent downward gas–solid flow according to energy-minimization multi-scale (EMMS) theory. A unified stability condition is proposed for the differently developed sections of gas–solid flow according to the principle of the compromise in competition between dominant mechanisms. By optimizing the number density of particle clusters with respect to the stability condition, the formulated model can be numerically solved without introducing cluster-specific empirical correlations. The EMMS-based model predicts well the axial hydrodynamics of cocurrent gas–solid downers and is expected to have a wider range of applications than the existing cluster-based models.

Electro-Hydrodynamics and Kinetic Modeling of Dry and Humid Air Flows Activated by Corona Discharges

The present work is devoted to the 2D simulation of a point-to-plane Atmospheric Corona Discharge Reactor (ACDR) powered by a DC high voltage supply. The corona reactor is periodically crossed by thin mono filamentary streamers with a natural repetition frequency of some tens of kHz. The study compares the results obtained in dry air and in air mixed with a small amount of water vapour (humid air). The simulation involves the electro-dynamics, chemical kinetics and neutral gas hydrodynamics phenomena that influence the kinetics of the chemical species transformation. Each discharge lasts about one hundred of a nanosecond while the post-discharge occurring between two successive discharges lasts one hundred of a microsecond. The ACDR is crossed by a lateral dry or humid air flow initially polluted with 400 ppm of NO. After 5 ms, the time corresponding to the occurrence of 50 successive discharge/post-discharge phases, a higher NO removal rate and a lower ozone production rate are found in humid air. This change is due to the presence of the HO2 species formed from the H primary radical in the discharge zone.

Ozonation of hospital raw wastewaters for cytostatic compounds removal. Kinetic modelling and economic assessment of the process

The kinetics of the ozone consumption for the pretreatment of hospital wastewater has been analysed in order to determine the reaction rate coefficients between the ozone and the readily oxidisabled organic matter and cytostatic compounds. The wastewater from a medium size hospital was treated with ozone and peroxone methodologies, varying the ozone concentration, the reaction time and the hydrogen peroxide doses. The analysis shows that there are four cytostatic compounds, i.e. irinotecan, ifosfamide, cyclophosphamide and capecitabine, detected in the wastewaters and they are completely removed with reasonably short times after the ozone treatment. Considering the reactor geometry, the gas hydrodynamics, the mass transfer of ozone from gas to liquid and the reaction of all oxidisable compounds of the wastewater it is possible to determine the chemical ozone demand, COzD, of the sample as 256mgO3L−1 and the kinetic rate coefficient with the dissolved organic matter as 8.4M−1s−1. The kinetic rate coefficient between the ozone and the cyclophosphamide is in the order of 34.7M−1s−1 and higher for the other cytostatics. The direct economic cost of the treatment was evaluated considering this reaction kinetics and it is below 0.3€/m3 under given circumstances.

Effect of bubble–particle interaction models on flow predictions in three-phase bubble columns

The hydrodynamics of gas–liquid–solid flows are investigated by applying event- and time-driven three-dimensional Euler–Lagrange–Lagrange simulations. Thereby, the focus is put on the modeling of the interaction between ascending gas bubbles and solid particles, which are suspended in a liquid. Specifically, the following approaches are used: (i) coupling via a simple drag force modification (Mitra-Majumdar et al., 1997), (ii) solely elastic collisions, (iii) a multistage collision model (Ralston et al., 1999), (iv) coupling via advanced drag models, (v) bubble–particle cohesion models, and (vi) no interactions between the phases. The setup of the simulations is based on the experimental measurements of Gan (2013), who used a laboratory-scale cylindrical bubble column, neutrally buoyant acrylic beads of 3mm in diameter, and a solids hold-up of 1.6vol%.The results indicate a vertical transport of solid particles from the lowest column quarter towards the upper column region. Further, the effect of the particles on the suspension׳s viscosity has a significant impact on the local gas hold-up distribution. While the drag force modification model of Mitra-Majumdar et al. (1997) leads to uniform solid hold-ups and lower bubble velocities, the elastic collision model and the multistage collision model showed similar flow predictions. Compared to the experimental measurements of Gan, the velocities at the column׳s center are underpredicted for the solid phase and overpredicted for the gas phase. The inclusion of the advanced drag model of Holloway et al. (2010), a bubble–particle cohesion force model, as well as a fine computational grid significantly improves the predictions.

Development of an endoscopic-laser PIV/DIA technique for high-temperature gas–solid fluidized beds

To enable the investigation of the hydrodynamics of gas–solid fluidized bed reactors at high-temperatures and reactive conditions, a new non-invasive experimental technique has been developed, based on the extension of Particle Image Velocimetry (PIV) coupled with Digital Image Analysis (DIA) using dedicated high-temperature endoscopes for image recording and laser illumination. The new endoscopic-laser PIV/DIA technique (ePIV/DIA) allows to determine simultaneously information from the emulsion and bubble phase with high spatial and temporal resolution in a pseudo-2D columns.The ePIV/DIA technique with single-source laser illumination has first been thoroughly validated at cold-flow conditions by comparison with a standard PIV/DIA technique with LED-illumination, showing very good agreement of the results in terms of particle velocity and solids mass flux profiles. The technique has subsequently been applied to fluidized beds in the bubbling fluidization regime at elevated temperatures to demonstrate its unique possibilities to measure detailed bed hydrodynamics at elevated temperatures.

Instantaneous heat transfer rate around consecutive Taylor bubbles

While many studies examine the hydrodynamics of two-phase gas–liquid slug flow, the details of the heat transfer mechanism of this flow regime remain largely unknown. Analyzing the detailed hydrodynamics around a single Taylor bubble provides a basis for understanding the heat transfer mechanism. Propagation of a single Taylor bubble is approximately steady; however, in undeveloped slug flow, acceleration and coalescence of consecutive bubbles is unsteady and highly complicated. The present study is aimed to investigate the effects of developing slug flow, particularly in the region where consecutive bubbles coalesce, on the instantaneous heat transfer for different flow regimes and heating locations. To enable controlled flow conditions, experiments were carried out for two consecutive Taylor bubbles rising in a vertical pipe. Measurements of the instantaneous heat transfer coefficient as a function of the leading bubble location were carried out for different separation distances between the two consecutive bubbles. These measurements were performed for various liquid flow rates, corresponding to laminar, transitional and turbulent background flow regimes. The experiments were conducted at several locations corresponding to different heating lengths. Furthermore, a comparison between the heat transfer coefficients of a bubble that has undergone coalescence with those corresponding to a single Taylor bubble with a similar length was performed. For all conditions, it was found that the heat transfer rate is augmented in the presence of an accelerated trailing bubble. The variation of the heat transfer rates is discussed with relation to the local flow hydrodynamics.

Hydrodynamics and pressure loss of concurrent gas–liquid downward flow through sieve plate packing

Gas–liquid concurrent downward flow through a new structured packing was investigated by experiment systematically. The experimental packing consisted of 21 sieve plates. Each plate was 190mm×190mm in side length and 1mm in thickness. Three sets of packing with different sizes of sieve holes were tested. During experiment, four flow states have been observed, i.e., the trickling flow at low gas and liquid flux, the continuous flow at low gas but high liquid flux, the semi-dispersed flow at high gas flux and the completely dispersed flow as gas flux increases further. Another important phenomenon observed is the occurrence of pulsing flow, i.e., in some range of gas and liquid flow fluxes, both two phases will no longer flow smoothly through the sieve plates but flow downward in pulse regularly. Through extensive experiment, the rough boundaries for flow regime transition were obtained. Then, the pressure losses of gas flow and gas–liquid two phase flow in non-pulsing flow regime were systematically measured. The analysis for pressure differences measured at different locations show that for a given packing, the pressure loss through each plate is nearly the same. Its magnitude depends on the gas and liquid flow rates. The comparisons between pressure losses of different packings indicate that the pressure loss is associated with the hole diameter, hole pitch, free area ratio of sieve plate and the mounting distance between two adjacent plates. Finally, considering these factors, the correlations for pressure drops were developed, which approximates the experimental values with an average deviation less than 10%.