| 1. |
- Golriz, Mohammad R., et al.
(författare)
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Influence of fluidization velocity on bed defluidization in fluidized bed combustors
- 2005
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Ingår i: Proceedings of the 18th International Conference on Fluidized Bed Combustion. - New York : American Society of Mechanical Engineers. ; , s. 551-556
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Konferensbidrag (refereegranskat)abstract
- Effects of superficial gas velocity and bed particle size on bed defluidization during biomass combustion were investigated. Sampled bed particles from four different large-scale circulating- and bubbling fluidized bed combustors, using biomass as fuel, were collected and analyzed. The bed particles from each fluidized bed unit were divided into small and large particle size fractions. The results indicate no significant difference in elemental compositions between small and large coated bed particles but the ratio of coating thickness to the mean particle diameter was higher for the small particles compared to the large ones. Controlled fluidized bed agglomeration tests revealed strong influence from fluidization velocity on initial defluidization temperatures at lower velocities, but little effect at higher velocities. Influence of bed particle size on initial defluidization temperature varied depending on operating conditions. Finally, a model based on viscous flow sintering is proposed for the relation between agglomeration temperature and superficial gas velocity. The model predictions are in good agreement with experimental data.
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| 2. |
- Golriz, Mohammad R., et al.
(författare)
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Mass transport inside a flowing electrolyte direct methanol fuel cell
- 2004
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Ingår i: ASME 2004 2nd International Conference on Fuel Cell Science, Engineering and Technology, FUELCELL 2004. - : American Society of Mechanical Engineers (ASME). - 0791841650 - 9780791841655 ; , s. 107-112
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Konferensbidrag (refereegranskat)abstract
- In this work, analytical mass transfer models are developed for two different types of Direct Methanol Fuel Cells (DMFCs). One type is a conventional assembly with a proton exchange membrane (PEM) and the other utilizes a flowing electrolyte in addition to a PEM to reduce methanol crossover. These models are used to predict methanol crossover behaviour that is a major issue affecting the efficiencies of PEM-DMFCs. It is shown that using flowing electrolyte DMFCs can lead to a significant decrease in methanol crossover with a corresponding increase in electrical efficiency of the cell. Combined with the experiments carried out in a previous work, the simulation showed significant efficiency improvements when using a flowing electrolyte DMFC compared to a traditional PEM assembly.
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| 3. |
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| 4. |
- Kjeang, Erik, et al.
(författare)
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A Parametric Study of Methanol Crossover in a Flowing Electrolyte Direct Methanol Fuel Cell
- 2006
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Ingår i: Journal of Power Sources. - 0378-7753 .- 1873-2755. ; 153, s. 89-99
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Tidskriftsartikel (refereegranskat)abstract
- Direct methanol fuel cells (DMFCs) have significant potential to become a leading technology for energy conversion in a variety of applications. However, problems, such as methanol crossover reduce the efficiency and open circuit voltage of the cells. The novel design of flowing electrolyte-direct methanol fuel cells (FE-DMFCs) addresses this issue. Methanol molecules are effectively removed from the membrane electrode assembly (MEA) by the flowing electrolyte, and the unused fuel can be utilized externally. In this paper, a general 3D numerical computational fluid dynamics (CFD) model is established to simulate methanol crossover by convection–diffusion in the FE-DMFC. Illustrations of methanol concentration distribution and methanol molar flux densities are presented, and the performance is compared to conventional DMFCs. The results indicate that methanol crossover can be reduced significantly. A parameter study is performed where the influences of anode fuel feed concentration, electrolyte channel thickness and electrolyte volumetric flow rate on methanol crossover are evaluated. In addition, effects of various electrolyte channel orientations are determined. According to the simulations, counter flow is the superior choice of channel orientations to minimize crossover.
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| 5. |
- Kjeang, Erik, et al.
(författare)
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Modeling methanol crossover by diffusion and electro-osmosis in a flowing electrolyte direct methanol fuel cell
- 2005
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Ingår i: Fuel Cells. - Weinheim : Wiley-VCH Verlagsgesellschaft. - 1615-6846 .- 1615-6854. ; 5:4, s. 486-498
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Tidskriftsartikel (refereegranskat)abstract
- A CFD model is created to analyze methanol transport in a flowing electrolyte direct methanol fuel cell (FE-DMFC) by solving the 3D advection-diffusion equation, with consideration of electro-osmosis. The average methanol flux at the anode and cathode surfaces is simulated and compared to equivalent direct methanol fuel cells. Methanol crossover is defined as methanol flux at the cathode surface, and the results reveal that methanol crossover can be drastically reduced by the flowing electrolyte. The performance of the FE-DMFC at peak power current density is evaluated, and diffusion is shown to be the dominant contribution, although electro-osmosis increases with current density. The power consumption of the electrolyte pump is shown to be negligible compared to the cell power output. This indicates that thin electrolyte channels with high flow rates could further improve the efficiency
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| 6. |
- Koksal, M., et al.
(författare)
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HEAT TRANSFER IN CIRCULATING FLUIDIZED BEDS WITH SECONDARY AIR INJECTION
- 2006
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Ingår i: International Heat Transfer Conference, IHTC-13. - 1567002250
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Konferensbidrag (refereegranskat)abstract
- In this study, we present an empirical model for bed-to-surface heat transfer in Circulating Fluidized Beds (CFB) with secondary air (SA) injection. The model is based on a correlation that relates the heat transfer coefficient to cross-sectionally averaged solids holdup. The average solids holdup was correlated with the operating and design parameters of CFBs with SA injection using three dimensionless parameters: Archimedes number, Gs/rp(U0-USA) and (1-HSA/Hr). The correlations were obtained using least square analysis for a wide range of data available in the literature as well as the data obtained in this study for two types of SA injectors: radial and tangential. The data used in developing the correlations cover a range of 2.0 to 8.6 m/s for superficial gas velocity, 5 to 100 kg/m2s for solids circulation rate, 0-0.6 for secondary to primary air ratio, and 60 to 300 mm for particle diameter (Groups A and B). The predictions obtained with the correlations show good agreement with the experimental data. Furthermore, an experimental correlation, developed in our previous work for prediction of wall-to-bed heat transfer coefficient in CFB risers operated with SA was used in a parametric study to investigate the effects of design and operating parameters on heat transfer. The results of the parametric study indicate that the heat transfer coefficient increases with increasing secondary air ratio, solids circulation rate, and height of secondary air injection port. The tangential SA injector was found to result in a significant increase in heat transfer coefficient compared with radial injector and non-SA operation due to increase in average solids holdup. This is consistent with the available data in the literature.
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| 7. |
- Koksal, M., et al.
(författare)
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Thermal Aspects a Circulating Fluidized Bed with Air Staging
- 2005
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Ingår i: International Journal of Energy Research. - : John Wiley & Sons, Ltd.. - 0363-907X .- 1099-114X. ; 29:10, s. 923-935
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Tidskriftsartikel (refereegranskat)abstract
- In this study, effects of air staging on wall-to-bed heat transfer were investigated in a laboratory scale circulating fluidized bed (0.23m ID, 7.6m high). The bed was operated under ambient conditions with silica sand particles (dp=89 mm, rp=2650 kgm_3). Two different designs of secondary air (SA) injectors were used for air staging: radial and tangential. Bed-to-wall heat transfer measurements were carried out at three elevations above the SA injection port. The results indicate that similar to non-SA operation, the heat transfer with air staging depends strongly on the cross-sectional average suspension density. Tangential secondary injection was found to increase the bed-to-wall heat transfer above the SA injection port significantly due to increased suspension density compared with non-SA operation.
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| 8. |
- Maria, Boman, et al.
(författare)
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The Solar Contribution to the Energy Use for Heating of Residential Buildings
- 2004
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Ingår i: Proceedings of the CIB World Building Congress.
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Konferensbidrag (refereegranskat)abstract
- The energy use of residential buildings in cold and mild climates with the influence of the solar gaincan be simulated with the support of TEKLA 2.0. The model is adjusted to single-zone buildings thathave a uniform internal temperature. The calculations are based on the standard EN 832, whichincorporates dated or undated references and provisions from other publications. The predicted resultscorrespond well with the measured data from a monitored occupied residential reference building InUmeå, Sweden.A parameter study was made with TEKLA 2.0 on simulated data of the reference building for climateconditions in Umeå, Kiruna and Lund, Sweden. It indicates that the parameters that have the mostinfluence on the thermal performance of the building according to the components are the U-value ofthe wall, the thickness of roof insulation and the U-value of windows. The parameters that have themost influence on the solar gain are the daily average global solar radiation and the directions of thewindows provided.
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| 9. |
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| 10. |
- Mwaba, MG, et al.
(författare)
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A semi-empirical correlation for crystallization fouling on heat exchange surfaces
- 2006
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Ingår i: Applied Thermal Engineering. - Oxford : Pergamon Press. - 1359-4311 .- 1873-5606. ; 26:4, s. 440-447
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Tidskriftsartikel (refereegranskat)abstract
- Crystallization fouling experiments are reported in this paper. In general the results show four distinct time regions: a region where the fouling thermal resistance is zero, a region where the fouling thermal resistance is negative, a region where the fouling resistance increases steadily and a region where the fouling thermal resistance remains constant. Each of these regions can be matched with different development phases in the crystallization fouling process: nucleation phase (regions 1 and 2), growth phase and asymptotic phase. Overall the fouling curves exhibit an S-curve, characteristic of the logistic curve. A new correlation based on the solution of the logistic equation is formulated. The correlation parameters are evaluated and related to the crystallization parameters. The results of this work can find application in predicting the evolution of the fouling scale layer in heat exchangers that are in operation. The predictions can assist heat exchanger operators to plan cleaning schedules.
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