| 1. |
|
|
| 2. |
- Ackeskog, Hans, et al.
(författare)
-
An investigation of fluidized-bed scaling: heat transfer measurements in a pressurized fluidized-bed combustor and a cold model bed
- 1993
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 48:8, s. 1459-1473
-
Tidskriftsartikel (refereegranskat)abstract
- A method is proposed for predicting heat transfer coefficients in hot fluidized-bed combustors by translating results measured in a scaled-down, cold model bed. Provided the beds are scaled to hydrodynamic similarity, local heat transfer coefficients measured in the cold model bed can be translated into local hot-bed convective coefficients with the aid of existing correlations for the gas and particle convective components. To obtain the total hot-bed-to-surface coefficients, a radiative component is then added. The chief advantages of the proposed method are that existing convective heat transfer correlations can be applied locally in a bed, and that no a priori knowledge of the voidage close to the transfer surface is required. In a previous paper by Almstedt and Zakkay, measurements of the bubble activity in a pressurized fluidized-bed burning coal and in a scaled-down pressurized model bed operating at room temperature showed that a good hydrodynamic similarity can be obtained between the beds. The heat transfer translation method proposed here has been validated by comparing heat transfer coefficients measured in the same two beds, operating under scaled conditions. Average heat transfer coefficients for four different horizontal tube bundles, as well as local coefficients measured with probes in four different positions were compared. The results indicate good agreement between the hot-bed results measured and the results translated from the model bed measurements employing the proposed method. Furthermore, the present paper presents heat transfer measurements from the cold model bed for three different bed materials, at pressures of 0.1, 0.24 and 0.5 MPa and fluidization velocities ranging from 0.15 to 1.3 m/s. The results are in good accordance with existing theory, but indicate that the gas convective component (as well as the particle convective component) is significantly dependent on the fluidization velocity.
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Enwald, Hans, et al.
(författare)
-
Eulerian two-phase flow theory applied to fluidization
- 1996
-
Ingår i: International Journal of Multiphase Flow. - 0301-9322. ; 22:Suppl. 1, s. 21-66
-
Tidskriftsartikel (refereegranskat)abstract
- A general classification of two-phase flows and a number of possible ways to formulate two-fluid models are discussed. The two-fluid model is adopted, and a general procedure to develop such a model is presented. The local instantaneous equations of mass and momentum are derived together with the corresponding jump conditions. Volume, time and ensemble averaging procedures are discussed, and averaged equations and jump conditions are derived using a general averaging operator. A Reynolds decomposition and weighting procedure is applied to obtain the final equations. The equations necessary to close the system, so-called closure laws, are discussed. The mechanisms contributing to the viscosity of both phases and mixture viscosity models are presented. The particle pressure is discussed, and some simple models based on the modulus of elasticity concept are given. The interfacial momentum transfer term is discussed in detail, and a study of common models of the drag function is presented. A discussion of turbulence models for the gas and particulate phases is included. A summary and critical assessment of published work on simulations of hydrodynamics in bubbling and circulating fluidized beds are also presented.
|
|
| 8. |
- Enwald, Hans, et al.
(författare)
-
Fluid dynamics of a pressurized fluidized bed: comparison between numerical solutions from two-fluid models and experimental results
- 1999
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 54:3, s. 329-342
-
Tidskriftsartikel (refereegranskat)abstract
- A validation of four different two-fluid model closures was carried out to investigate the effect of gas-phase turbulence, drift velocity and three dimensionality on the fluid dynamics of a bubbling fluidized bed. At atmospheric conditions, it is verified that gas-phase turbulence has a negligible effect for the bed material and operating conditions used in the investigation, whereas the validation shows some evidence that the gas-phase turbulence has a significant contribution for higher pressures. The drift velocity shows no noticeable effect on the results at any pressure. A comparison between two- and three-dimensional calculations at atmospheric pressure shows that the three-dimensional effects appear to be considerable.
|
|
| 9. |
- Enwald, H., et al.
(författare)
-
Simulation of the fluid dynamics of a bubbling fluidized bed. Experimental validation of the two-fluid model and evaluation of a parallel multiblock solver
- 1999
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 54:3, s. 311-328
-
Tidskriftsartikel (refereegranskat)abstract
- A mesh refinement study and validation of two-fluid model closures were carried out for a bubbling fluidized bed application. The mesh refinement study indicates that a higher degree of mesh refinement is required for atmospheric than for pressurized fluidization. Statistical bubble parameters (bubble frequency, mean bubble rise velocity, mean pierced bubble length and mean bubble volume fraction) were evaluated. The simulated statistical bubble quantities are computed from voidage signals derived from the transient multidimensional solution of two-fluid models. The algorithm for computing these quantities is taken directly from the evaluation program treating the measurement signals. To remedy the long simulation times required to obtain acceptable statistical values, a parallel version of the two-fluid model solver was developed, based on a domain decomposition method for distributed memory computers. A number of problems related to the parallelization are investigated. These are optimal treatment of velocity components on multi-block boundaries, frequency of data exchange at multi-block boundaries, local errors at multi-block boundaries and simulation time requirements.
|
|
| 10. |
|
|
| 11. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
A Novel Multigrid Approach for Lagrangian Modeling of Fuel Mixing in Fluidized Beds
- 2010
-
Ingår i: Proceedings of the International Conference on Multiphase Flow, Tampa, 2010.
-
Konferensbidrag (refereegranskat)abstract
- This paper presents a novel Larangian approach to model fuel mixing in gas-solid fluidized beds. In the mixing process, fuel particles are considerably larger than the inert bed material and therefore, the commonly used Largangian particle algorithms are not able to simulate the phenomenon properly. In the proposed model two grids are used for simulations and the information between the two grids is exchanged using an algorithm presented in the paper. In addition, a statistical procedure is developed to analyze the results obtained from the simulations. The effects of initial distribution of bed material, inlet gas velocity and amount of the bed material on the fuel mixing are investigated. It is concluded that initial location of fuel particles affects their preferential positions. Also, increasing the fluidization velocity and the amount of the bed material influences the flow structure and configuration of the fuel particles.
|
|
| 12. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
A novel multigrid technique for lagrangian modeling of fuel mixing in fluidized beds
- 2011
-
Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 66:22, s. 5628-5637
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a novel Lagrangian approach to model fuel mixing in gas-solid fluidized beds. In the mixing process, fuel particles are considerably larger than the inert bed material and therefore, the present work proposes three grids to account for the difference in size between the fuel particles and inert solids. The information between the grids is exchanged using an algorithm presented in the paper. A statistical method has been developed to analyse the distribution of the fuel particles in the bed. The results for the preferential positions, velocity vectors and horizontal dispersion coefficients are compared with experimental data in a bed applying simplified scaling relationships for different operating conditions. The effects of initial bed height and inlet gas velocity on the fuel mixing are investigated.It is found that the proposed Lagrangian modeling can capture the complex pattern of the movement of the fuel particles, in spite of the large difference in diameter between inert and fuel particles.
|
|
| 13. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
A Study of Fuel Particle Movement in Fluidized Beds
- 2013
-
Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 52:16, s. 5791-5805
-
Tidskriftsartikel (refereegranskat)abstract
- Lagrangian simulations are performed to investigate the process of fuel mixing in fluidized-bed energy converters. The computations are carried out for a narrow (0.4 m) and a wide (1.2 m) bed. Movement of a limited number of large and light particles in a bulk of heavy and small particles is studied using a multigrid technique proposed by Farzaneh et al. Preferential positions and the dispersion coefficient of the fuel particles are obtained under different operating conditions. In addition, detailed information on the motion of the fuel particles in the form of upward and downward velocity is obtained. Furthermore, in an attempt to investigate the effect of the inlet boundary conditions on the process of fuel mixing, two boundary conditions are employed: a uniform velocity profile at the air distributor and a non-uniform velocity profile obtained by including the air supply system in the computational domain.It is observed that the numerical simulations which include the air supply system in the computational domain, improve the prediction of the hydrodynamic behavior of the bed. However, regarding the averaged movement pattern of the fuel particles, the effect of the boundary condition employed is not significant in the 0.4 m bed. As for the wide 1.2 m bed, the simulation results differ substantially from the experiments when the uniform velocity profile is employed as inlet boundary condition. Including the plenum in the simulations considerably improves the results, but they are still not in a perfect agreement with the experiments.
|
|
| 14. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
Eulerian-Eulerian-Lagrangian Simulation of Fuel Mixing in Fluidized beds
- 2013
-
Ingår i: 8th International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013.
-
Konferensbidrag (refereegranskat)abstract
- In this paper, we combine Eulerian-Lagrangian and Eulerian-Eulerian frameworks to track a limited number of fuel particles in a bulk of inert particles in a gas-solid fluidized bed. The gas and the inert phase are treated as interpenetrating continua and resolved within the Eulerian-Eulerian framework, whereas the fuel particles are regarded as a discrete phase. In the method, the forces acting on a fuel particle are calculated by using the velocity and pressure fields of the mixture of the inert solid particles and gas.. To validate the numerical method, the results are compared with experimental data in the form of preferential positions, velocity vectors and dispersion coefficient of the fuel particles. The effects of a number of operating parameters (e.g. the fluidization velocity and the amount of bed material) on the mentioned properties of the fuel particles are studied. In addition, to accurately formulate the inlet boundary condition the air supply system is included in the computational domain. It is observed that the proposed numerical technique is able to correctly capture the behaviour of fuel particles in fluidized beds.
|
|
| 15. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
Simulation of Fuel Mixing in Fluidized Beds using a Combined Tracking Technique
- 2013
-
Ingår i: Fluidization XIV, 2013, Noordwijkerhout, The Netherlands.
-
Konferensbidrag (refereegranskat)abstract
- This paper presents an Eulerian-Eulerian-Lagrangian (E-E-L) numerical method to track a limited number of fuel particles in a bulk of inert particles in a gas-solid fluidized bed. The gas and the inert phases are treated as the interpenetrating continua and resolved within the Eulerian-Eulerian framework, whereas the fuel particles are regarded as a discrete phase. To validate the numerical method, the results are compared with experimental data in the form of preferential positions, velocity vectors and the dispersion coefficient of the fuel particles. It is observed that the proposed numerical technique is able to capture the behavior of fuel particles in fluidized beds.
|
|
| 16. |
- Farzaneh, Meisam, 1982, et al.
(författare)
-
The crucial role of frictional stress models for simulation of bubbling fluidized beds
- 2015
-
Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 270:Part A, s. 68-82
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we combine Eulerian-Lagrangian and Eulerian-Eulerian frameworks to simulate the behavior of a limited number of fuel particles in a bulk of inert particles in a bubbling gas-solid fluidized bed. The gas and the inert phase are treated as interpenetrating continua and resolved within the Eulerian-Eulerian framework, whereas the fuel particles are regarded as a discrete phase. The forces acting on a fuel particle are calculated by using the velocity and pressure fields of the inert solid and gas phases. We assume that the hydrodynamics of the bed are predominantly governed by the motion of the inert solid and gas phases. Therefore, emphasis in this work is on a correct description of the stress tensor of the inert particulate phase and, in particular, on the modeling of frictional stresses, which is of primary importance for continuum simulations of bubbling fluidized beds. Performance of two of the traditionally used frictional stress theories (Schaeffer [12] and Srivastava and Sundaresan [13]) and of the one more recently proposed (Jop et al. [18]) is investigated and the corresponding results are compared with experimental findings in the form of position and velocity of the fuel particles. In addition, preferential positions, the dispersion coefficient, and the average cycle time of the fuel particles motion are obtained by the simulations and compared with experiments. It is observed that the results of the visco-plastic model proposed by Jop et al. [18] are in good agreement with the experiments for prediction of the bed hydrodynamics and the movement of the fuel particles. The other two models underestimate the frictional stresses in the inert solid phase, leading to erroneous predictions of the stress tensor of the inert particulate phase and thus of the entire system.
|
|
| 17. |
- Gustavsson, Mattias, et al.
(författare)
-
Numerical simulation of fluid dynamics in fluidized beds with horizontal heat exchanger tubes
- 2000
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 55:4, s. 857-866
-
Tidskriftsartikel (refereegranskat)abstract
- A numerical code, Gemini, based on the implicit multifield method (IMF) of Harlow and Amsden for Eulerian two-fluid modelling, is used to simulate the fluid dynamics of bubbling fluidized beds, assuming no turbulence in the gas or solid phase. The paper gives a formulation of the equations of motion and empirical closure laws in general curvilinear coordinates for calculation of the fluid dynamics in beds with complex internal geometries. A special discretization method for general curvilinear structured grids with multiblock connectivity is implemented, and two-dimensional non-stationary calculations are performed for a bed with a cross-sectional width of 0.3 m, containing two horizontal heat exchanger tubes. The local visible bubble flow and the gas and particle motion around the tubes are briefly discussed and compared with experimental fluid dynamic results at different pressure levels.
|
|
| 18. |
- Gustavsson, Mattias, et al.
(författare)
-
Two-fluid modelling of cooling-tube erosion in a fluidized bed
- 2000
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 55:4, s. 867-879
-
Tidskriftsartikel (refereegranskat)abstract
- The fluid dynamics of a bubbling fluidized bed is simulated in general curvilinear co-ordinates using an Eulerian two-fluid model. Computations are performed at different pressure levels for a bed with a cross-sectional width of 0.3 m in which two heat exchanger tubes were inserted. A monolayer kinetic energy dissipation model is implemented to simulate the erosion of cooling tube surfaces. The local visible bubble flow and particle motion close to the tubes are correlated with the modelled instantaneous erosion rate. Experiments were conducted in order to validate the erosion simulations, and fair qualitative agreement between experimental and numerical results was obtained.
|
|
| 19. |
|
|
| 20. |
|
|
| 21. |
- Johansson, Bert, et al.
(författare)
-
Application of wedge-shaped hot-film probes in a gas-particle flow
- 2003
-
Ingår i: Experimental Thermal and Fluid Science. - 0894-1777. ; 27:2, s. 187-191
-
Tidskriftsartikel (refereegranskat)abstract
- A wedge-shaped hot-film probe is used to perform measurements of turbulence intensity and an estimate of the turbulent kinetic energy as well as the Eulerian spectra, macro- and micro-scales of the gas phase in a horizontal pipe flow carrying pulp fibres or spherical particles. In addition, the paper gives a summary of a method for handling the poor frequency response of the probe and discusses how to sort out "bad samples", occurring when the probe is hit by particles.
|
|
| 22. |
|
|
| 23. |
- Johansson, Klas, et al.
(författare)
-
Experimental validation of CFD models for fluidized beds: Influence of particle stress models, gas phase compressibility and air inflow models
- 2006
-
Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 61:5, s. 1705-1717
-
Tidskriftsartikel (refereegranskat)abstract
- This work compares numerical simulations of fluid dynamics in fluidized beds using different closure models and air feed system models. The numerical results are compared to experiments by means of power spectral density distributions of fluctuating pressure signals and bubble statistics obtained from capacitance probe measurements. Two different particle rheology models are tested in combination with two different values of the maximum particle volume fraction. The first particle model predicts the particle pressure by an exponential power law and assumes a constant particle viscosity (CPV), and the second model predicts the stresses using the kinetic theory of granular flow (KTGF). Furthermore, two model approaches for the air inflow are evaluated. The first inflow model includes the coupling between the air-feed system and the fluidized bed in the simulation, and the second model assumes a constant mass flow of gas into the fluidized bed. Finally, the influence of the compressibility of the gas phase on the numerical predictions is investigated. The numerical simulations are made using the CFX-4.4 commercial flow solver.The simulations show that the KTGF model gives a more evenly distributed bubble flow profile over the bed cross-section, while the CPV model gives a more parabolic bubble flow profile, with a higher bubble flow in the central part of the bed. This work shows that the KTGF model results are in significantly better agreement with the experiments. It is furthermore shown that the modelling of the air-feed system is crucial to for predicting the overall bed dynamic behaviour.
|
|
| 24. |
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
|
|
| 28. |
- Ljus, Camilla, et al.
(författare)
-
Turbulence modification by particles in a horizontal pipe flow
- 2002
-
Ingår i: International Journal of Multiphase Flow. - 0301-9322. ; 28:7, s. 1075-1090
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements were made of turbulence intensities and turbulent energy spectra in a fully developed, turbulent air-particle pipe flow. The influence of the particles on the turbulence was studied. Measurements were made with spherical particles and particles with a large aspect ratio (pulp fibres). There is a significant change in turbulence intensity at higher particle concentrations with loading ratios of m = 0.1 and 0.03. The measurements show that the turbulence intensity increases close to the centre of the pipe while the turbulence intensity decreases close to the pipe wall for the spherical particles. These results are in agreement with earlier measurements found in the literature. For the fibres, the turbulence intensity decreases over the whole pipe cross-section. Fibre flocs, however, give variations in the mean velocity that result in the production of turbulence in the lower part of the channel.
|
|
| 29. |
- Löfstrand, Hans, et al.
(författare)
-
Dimensionless expansion model for bubbling fluidized beds with and without internal heat exchanger tubes
- 1995
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 50, s. 245-253
-
Tidskriftsartikel (refereegranskat)abstract
- A bed expansion model is presented for bubbling fluidized beds without tubes and for beds with staggered horizontal heat exchanger tubes. There are indications that the model also works for inline tube configurations. The bed expansion ratio is modelled as a function of a dimensionless drag force and, in the case of tubes present in the bed, a dimensionless horizontal and vertical tube pitch function. The data used to derive the model cover a wide range of operating conditions, with varying fluidization velocities, pressures, particle materials (Geldart groups B and D), bed geometries and tube configurations. The dimensionless drag force also takes temperature effects into account, and the model shows good agreement with data from a freely bubbling large-scale atmospheric fluidized bed boiler operating at 850 degrees C.
|
|
| 30. |
- Magnusson, Anna, 1974, et al.
(författare)
-
Dual fibre optical probe measurements of solids volume fraction in a circulating fluidized bed
- 2005
-
Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 151:1-3, s. 19-26
-
Tidskriftsartikel (refereegranskat)abstract
- The calibration of a dual fibre optical probe was experimentally investigated in a circulating fluidized bed. The particle volume fraction registered by the optical probe was compared to pressure drop measurements for a range of operating conditions. A theoretical calibration theory was applied to the probe signal to obtain the local and instantaneous particle volume fraction. It was found that glare points on the particles and the beam length from the probe to the particle determines the curvature of the calibration function to a large extent.
|
|
| 31. |
|
|
| 32. |
- Olowson, Per, et al.
(författare)
-
Hydrodynamics of a bubbling fluidized bed: influence of pressure and fluidization velocity in terms of drag force
- 1992
-
Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 47:2, s. 357-366
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles have been carried out in a pressurized fluidized bed. Based on the results, it is demonstrated that the representation of in-bed hydrodynamics in terms of particle drag force facilitates a comparison between the influence of pressure and fluidization velocity. By calculating a "potentially available drag force" corresponding to the different operating conditions, it is shown that most of the in-bed parameters, such as bed expansion, bubble volume fraction, bubble rise velocity and local visible bubble flow rate, fall on single curves when plotted versus this force. Deviations occur due to bubble instability, which is largely a pressure-dependent effect. Some mechanisms that together govern bubble instability and splitting are established: (a) The through-flow velocity of gas through the bubbles decreases considerably as the pressure increases. (b) The fluctuations of the through-flow velocity of gas through the bubbles are of the same order of magnitude at all the operating conditions investigated. (c) A given fluctuation in gas velocity has a higher relative influence on the fluctuation in particle drag force at high pressures than at low pressures. In addition, the overall bed behaviour becomes less stable at high Reynolds numbers, i.e. at high pressures and fluidization velocities. The bubble size at a given location in the bed is determined by a complex balance between bubble splitting and coalescence. Both splitting and coalescence are governed by fluctuations in the particle drag force caused by fluctuations in gas velocity. These velocity fluctuations are largely caused by the gas short-cutting between adjacent bubbles. Due to increased coalescence, the bubble flow is redistributed towards the centre of the bed cross-section with both increasing pressure and fluidization velocity.
|
|
| 33. |
|
|
| 34. |
- Olowson, Per, et al.
(författare)
-
Influence of pressure on the minimum fluidization velocity
- 1991
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 46:2, s. 637-640
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of pressure on minimum fluidization velocity has been measured in a pressurized fluidized bed. Three different bed materials, corresponding to Geldart's group B or group D particles, were investigated at pressures between 0.1 and 1.6 MPa. The onset of fluidization was determined both visually and by pressure drop measurements. The results show a clear decrease in the minimum fluidization velocity with increasing pressure. The experimental results are in good accord with results obtained using the Ergun equation. A number of simplified correlations derived from the Ergun equation were also investigated. While many of these give a good description of the pressure effect, their accuracy varies significantly for different bed materials.
|
|
| 35. |
- Olsson, Elisabet, et al.
(författare)
-
Hydrodynamics of a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry
- 1995
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 50:4, s. 581-592
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of the bubble hydrodynamics were carried out in a cold pressurized bed with horizontal tubes. The mean bubble rise velocity, the bubble frequency, the mean pierced length, the bubble volume fraction, and the visible bubble flow rate were measured using capacitance probes. The absolute gas velocity through the bubbles was measured using Pitot-static pressure probes. The bed expansion ratio was determined by measuring the pressure difference between the freeboard and the bed at different heights and extrapolating the pressure difference down to zero. The fluctuations in the pressure drop over the entire bed height were also measured, and the power spectral density distribution of these fluctuations was calculated. The influence of pressure, fluidization velocity, and tube-bank geometry on the bubble behaviour and gas-flow distribution were studied. The bed has a cross-section of 0.2 m x 0.3 m. It was operated at pressures between 0.1 and 1.6 MPa, at excess gas velocities of 0.2 and 0.6 m/s. Three different tube-bank geometries were used, one with a fairly dense pitch and two with more sparse configurations, and comparisons are also made with previous results obtained without tubes in the bed. The bed material was silica sand with a mean particle diameter of 0.7 mm. For the three tube banks investigated here, all the measured parameters except the mean pierced length consistently increased with increasing excess gas velocity. The mean pierced length increased with increasing excess gas velocity at low pressures, while the velocity effect at high pressures was less obvious. When the pressure was increased, the mean pierced length first increased to a maximum value, at p = 0.4 MPa for the low excess gas velocity and at p = 0.2 MPa for the high excess gas velocity, then decreased again as the pressure was increased further. The bubble frequency and the bed expansion increased as the pressure was increased. The absolute gas velocity through the bubbles as well as the gas velocity relative to the bubbles decrease as the pressure was increased. For the sparse tube banks at the lower excess gas velocity, the mean bubble rise velocity, the bubble volume fraction and the visible bubble flow rate increased as the pressure was increased. For the dense tube bank, however, these parameters showed a maximum at p = 0.4 MPa. At the higher excess gas velocity, these parameters showed a maximum at about p = 0.2-0.4 MPa for all three tube banks. It appears that, at this velocity, the presence of tubes prevents a further increase in these parameters. This behaviour differs significantly from the behaviour without tubes. From the power spectral density distributions of the pressure fluctuations over the entire bed height and from visual observation, it appears that the bed is slugging, or close to slugging, at atmospheric pressure for all the tube configurations. When the pressure is increased, the power spectral density distribution becomes wider as the large bubbles/slugs break down and the bed moves toward a more dispersed bubbling behaviour.
|
|
| 36. |
- Olsson, Elisabet, et al.
(författare)
-
Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry
- 1995
-
Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 50:20, s. 3231-3245
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of local instantaneous bed-to-tube heat transfer were carried out in a cold pressurized bed with horizontal tubes. The influence of fluidization velocity and pressure was studied at different circumferential positions around a horizontal tube within a tube bank for three different tube-bank geometries. The signal From a heat transfer sensor was compared with capacitance probe signals sampled simultaneously in an adjacent position. The capacitance probe registers the alternating passage of bubbles and dense phase at the probe, and the respective contributions to the local instantaneous heat transfer from the gas and particle convection were thus identified by comparison with the heat transfer signal. The local time-averaged heat transfer coefficients at the different circumferential positions were determined from the instantaneous heat transfer signals, for the different operating conditions, as were the spatial-averaged heat transfer coefficient for the tube. The time-averaged heat transfer results were correlated with the hydrodynamic properties of the bed obtained in a previous investigation under the same operating conditions, and a strong coupling between the local mean bubble frequency and the local time-averaged heat transfer coefficient was obtained. The heat transfer results show a significant increase of the bed-to-tube heat transfer coefficient with increasing pressure, while results from a previous erosion study in the same bed show that, at high pressures, the tube erosion decreases with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels. The heat flux was measured using flush-mounted sensors on a heated copper cylinder replacing one of the tubes in the horizontal tube banks. The bed has a cross-section of 0.2 m x 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. Of the three tube-bank geometries used, one had a fairly dense pitch and two had more sparse configurations. The bed material was silica sand with a mean particle diameter of 0.7 mm.
|
|
| 37. |
|
|
| 38. |
- Sasic, Srdjan, 1968, et al.
(författare)
-
Dynamics of fibres in a turbulent flow field - a novel particle-level simulation technique
- 2009
-
Ingår i: 14th International Conference on Modelling Fluid Flow, CMFF 09 Budapest.
-
Konferensbidrag (refereegranskat)abstract
- A particle-level simulation technique has been developed for modelling the flow of fibres in turbulent flow. A single fibre is conceived here as a chain of segments, thus enabling the model fibre to have all the degrees of freedom (translation, rotation, bending and twisting) needed to realistically reproduce the dynamics of real fibres. Equations of motion are solved for each segment, accounting for the forces that describe interaction with the fluid, the forces that describe contact with other fibres and those that maintain integrity of the fibre. The motion of the fluid is resolved as a combination of 3D mean flow velocities obtained from a CFD code and fluctuating turbulent velocities derived from the Langevin equation. A case of homogeneous turbulence is treated in this paper. In addition, the paper discusses issues related to numerical stability of the proposed model.The results obtained here show that fibre flocs can be created also in the absence of attractive forces. Inter-fibre contacts and the individual fibre features (such as flexibility and equilibrium shapes) are shown to govern the physics behind formation and breaking up of fibre flocs. Highly irregular fibre shapes and stiff fibres lead to strong flocculation. The modelling framework applied in this work aims at making possible a numerical model applicable for solving fibre flow problems at industrial scale.
|
|
| 39. |
- Sasic, Srdjan, 1968, et al.
(författare)
-
Dynamics of fibres in a turbulent flow field – A particle-level simulation technique
- 2010
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X. ; 31:6, s. 1058-1064
-
Tidskriftsartikel (refereegranskat)abstract
- A particle-level simulation technique has been developed for modelling the flow of fibres in a turbulent flow field. A single fibre is conceived here as a chain of segments, thus enabling the model fibre to have all the degrees of freedom (translation, rotation, bending and twisting) needed to realistically reproduce the dynamics of real fibres. Equations of motion are solved for each segment, accounting for the interaction forces with the fluid, the contact forces with other fibres and the forces that maintain integrity of the fibre. The motion of the fluid is resolved as a combination of 3D mean flow velocities obtained from a CFD code and fluctuating turbulent velocities derived from the Langevin equation. A case of homogeneous turbulence is treated in this paper.The results obtained show that fibre flocs in air-fibre flows can be created even when attractive forces are not present. In such a case, contacts between fibres, properties of an individual fibre (such as flexibility and equilibrium shapes) and properties of the flow of the carrying fluid are shown to govern the physics behind formation and breaking up of fibre flocs. Highly irregular fibre shapes and stiff fibres lead to strong flocculation. The modelling framework applied in this work aims at making possible a numerical model applicable for designing processes involving transport of fibres by air at industrial scale.
|
|
| 40. |
- Sasic, Srdjan, 1968, et al.
(författare)
-
Individual fibre in an arbitrary flow field - true direct numerical simulation (DNS) using an implicit immersed boundary method
- 2008
-
Ingår i: 11th International Conference on Multiphase Flow in Industrial Plants.
-
Konferensbidrag (refereegranskat)abstract
- A novel immersed boundary method for three-dimensional, time-dependent flows is presented in this work and applied to simulating the behaviour of an individual fibre in various flow regimes. The fibre is placed in a periodic box and has either a fixed position or is allowed to move freely (including translation and rotation) through the domain. The immersed boundary method mirrors the velocity field along the normal of the local triangulated immersed boundary segment to guarantee that the fluid takes into account the immersed body accurately. As a result of the procedure, there is a fictitious velocity field inside the immersed boundary, mirroring the boundary layer. Care is taken to solve the velocity field in such a way that the mass is conserved in the cells containing the immersed boundary. The method applied is second-order accurate and is intended to be used for fully resolving the flow field around arbitrary moving bodies immersed in a fluid. The immersed boundary method is employed on a selection of different fibre shapes, aiming at predicting the behaviour of real fibres in realistic flow situations. The force exerted by the fluid is directly calculated by integrating the pressure and viscous forces over the objects immersed. The resulting coarse-grained drag and lift force functions can be employed in larger scale calculations of fluid-fibre flows.
|
|
| 41. |
- van Wachem, Berend, 1972, et al.
(författare)
-
Methods for multiphase computational fluid dynamics
- 2003
-
Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 96:1-3, s. 81-98
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents an overview of the physical models for computational fluid dynamic (CFD) predictions of multiphase flows. The governing equations and closure models are derived and presented for fluid-solid flows and fluid-fluid flows, both in an Eulerian and a Lagrangian framework. Some results obtained with these equations are presented. Finally, the capabilities and limitations of multiphase CFD are discussed.
|
|
| 42. |
- Wiman, Jan, et al.
(författare)
-
Erosion of horizontal tubes in a pressurized fluidized bed - Influence of pressure, fluidization velocity and tube-bank geometry
- 1995
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 50:21, s. 3345-3356
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of local tube erosion were carried out in a cold pressurized bed with horizontal tubes. The influence of fluidization velocity, pressure and circumferential position was studied at different locations within tube banks for three different tube-bank geometries. The erosion results were correlated with the hydrodynamic properties of the bed obtained in a previous investigation under the same operating conditions. At high pressures, the erosion decreases with increasing pressure. Preliminary results from heat transfer measurements in the bed show a significant increase of the bed-to-tube heat transfer coefficient with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels. The erosion tests were carried out using target tubes coated with a thin layer of stearin. This coating wears rapidly and, thus, an exposure time of 1 h per operating condition was sufficient to obtain an accurately measurable erosion. This exposure time compares favourably with those reported in most other investigations using more erosion-resistant tube materials. The bed has a cross-section of 0.2 m x 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. Three different tube-bank geometries were used, one with a fairly dense pitch and two with more sparse configurations. The bed material was silica sand with a mean particle diameter of 0.7 mm and a shape factor of approximately 0.8. The erosion results presented here are generally in good agreement with results reported for real tubes under hot conditions, both from atmospheric and pressurized fluidized bed combustors.
|
|
| 43. |
- Wiman, Jan, et al.
(författare)
-
Hydrodynamics, erosion and heat transfer in a pressurized fluidized bed: influence of pressure, fluidization velocity, particle size and tube bank geometry
- 1997
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 52:16, s. 2677-2695
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of hydrodynamics, local tube erosion and local instantaneous bed-to-tube heat transfer were carried out in a cold pressurized fluidized bed, with two horizontal tube banks having different tube packings. The influence of pressure, fluidization velocity, particle size and tube bank geometry was studied. Two size distributions of silica sand were used, one with a mean particle diameter of d(p) = 0.7 mm and one with d(p) = 0.45 mm. The bed has a cross-section of 0.2 m x 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. The results show that, if plotted vs the excess gas velocity, the hydrodynamic behaviour is similar for the two different particle sizes. However, the smaller particles generally give rise to less erosion than the larger particles, as an effect of their momentum being lower at a given particle velocity. The small particles also give a higher heat transfer than the large particles, as a result of a higher particle convection. The hydrodynamic behaviour, erosion levels and local heat transfer differ significantly between the two tube banks. The denser tube bank causes an earlier transition to a turbulent bed behaviour with increasing pressure or fluidization velocity. The dense tube bank gives rise to considerably less erosion but also gives a somewhat lower heat transfer than the more sparse tube bank, at corresponding operating conditions. The tube erosion is strongly related to the bubble rise velocity. The heat transfer coefficient is generally coupled to the bubble frequency, except for the high excess gas velocity with the dense tube bank where, at high pressures, the bed assumes a strongly turbulent behaviour and no distinct bubble pattern exists. The results indicate that the most severe erosion will occur in sparsely packed parts of a tube bank. For the sparse tube bank investigated, at high pressures, the erosion decreases with increasing pressure. The bed-to-tube heat transfer coefficient generally increases with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels.
|
|
| 44. |
- Wiman, Jan, et al.
(författare)
-
Influence of pressure, fluidization velocity and particle size on the hydrodynamics of a freely bubbling fluidized bed
- 1998
-
Ingår i: Chemical Engineering Science. - 0009-2509. ; 53:12, s. 2167-2176
-
Tidskriftsartikel (refereegranskat)abstract
- The hydrodynamics have been studied in a cold, freely bubbling, pressurized fluidized bed. The bed has a cross-section of 0.2 m x 0.3 m and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. The bed material was silica sand with a mean particle diameter of d(p) = 0.45 mm. Comparisons were made with previous results obtained with particles of d(p) = 0.7 mm. The hydrodynamic results are similar for the two different particle sizes when plotted vs the excess gas velocity. The results also show that the bed expansion, bubble rise velocity, bubble volume fraction and visible bubble flow rate fall on single curves if plotted vs a dimensionless potentially available drag force, while the bubble frequency, the mean pierced length and the through-flow velocity of gas through the bubble do not. The dimensionless drag force is a suitable scaling parameter as long as the particles do not respond to the gas-phase velocity fluctuations and as long as the dense phase does not expand. At high pressures, an increased gas-particle interaction, in combination with turbulent fluctuations in the gas phase, can be used to explain the increased bubble instability, with a corresponding increased bubble splitting and dense phase expansion. The gas-particle interaction also increases with decreasing particle size, which may help explain the maximum stable bubble size for group A particles observed by many workers.
|
|
