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- Bahador, Mehdi, et al.
(författare)
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A conjugate heat transfer model for heat load prediction in combustion devices
- 2006
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Ingår i: Collection of Technical Papers - 9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference Proceedings. ; 3, s. 1813-1819
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Konferensbidrag (refereegranskat)abstract
- Different phenomena such as complex flow field and heat release by combustion are involved in the heat transfer process in combustion chambers. This paper concerns prediction of heat load and wail temperature in a gas turbine combustor by taking different phenomena into account. Two dimensional axi-symmetric models were used to model the flow field and combustion in a premised combustor with two different cooling schemes. The k-ε turbulence model and Eddy Dissipation Concept (EDC) were used for modeling turbulent flow and combustion, respectively. In the modeling of heat transfer through the walls, a conjugate heat transfer formulation was applied. The temperatures calculated by the models were compared with experimental data. The results showed that although worse agreement was found in some parts, however generally the trends of the temperature variations predicted very well. In addition, radiative heat transfer has been included in the study. The results showed that radiative heat transfer in simple and ribbed duct cooling schemes can increase the average inner wall temperature by to 33 and 40 K, respectively.
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| 3. |
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| 4. |
- Bahador, Mehdi, et al.
(författare)
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Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers
- 2008
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 51:9-10, s. 2411-2417
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Tidskriftsartikel (refereegranskat)abstract
- Ash is produced in combustion of biomass. Sonic part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also call affect the thermal radiation. The effects of fly ash particles oil the thermal radiation are considered ill this investigation. By analyzing sampled data in all electrostatic filter, a realistic particle size distribution is found. Although the optical data oil biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fc coal fly ash can be applied for file biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles oil thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential oil the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important oil the radiative heat Source and radiative heat flux to the wall whereas their absorption effect is important and call increase the radiative heat Source and wall heat fluxes.
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| 7. |
- Bahador, Mehdi, et al.
(författare)
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On heat load calculations in gas turbine combustors
- 2004
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Ingår i: Computational Studies. - 1462-6063. ; 5, s. 345-357
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Konferensbidrag (refereegranskat)abstract
- Combined convective and radiative heat transfer governs the heat load on the combustor wall. This paper reviews engineering simplified models and more advanced ones. The latter ones are based on CFD approaches and a variety of methods to solve the radiative transfer equation. The principles of heat transfer in the combustor including the cooling aspects are briefly described. Details of the advanced modelling procedures are outlined and some results are discussed.
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| 8. |
- Bahador, Mehdi
(författare)
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On Radiative Heat Transfer Modeling with Relevance for Combustor and Biomass Furnaces
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermal radiation sometimes is the dominating heat transfer mode in combustion chambers and furnaces and therefore in the design of many relevant industrial facilities prediction of it is necessary. During previous years many research efforts in the radiative heat transfer field have originated and many methods and models from simple to complex have been developed. However, the field is still open for investigation. The objectives of this thesis are to study the radiative heat transfer in combustion chambers and furnaces by both empirical and numerical methods and it has been attempted to cover some important topics in the thermal radiation field. The thesis focuses on empirical methods for heat load prediction in combustion chambers. Such a method was used to predict effects of combustor modifications in a micro gas turbine. In another study, the heat load in a gas turbine combustor was predicted by numerical methods. Both convective and radiative heat transfer were modeled. The radiative heat transfer was modeled by the discrete ordinates method and the spectral line weighted sum of grey gases model. The predicted results showed good agreement with experimental data. There are some information available on prediction of thermal radiation in coal fired boilers and the interaction of thermal radiation and particles. In this thesis, part of the study is focused on the thermal radiation in biomass boilers. Using experimental data, particle size distributions for fly ash (which is the most important particle in biomass systems) and char were extracted. The data were used to predict the scattering and absorption coefficients and phase function (using Mie scattering theory) of particles. The properties were used in some test case studies and also in a complete modeling for these types of boilers (flow field, combustion and radiation). In modeling of thermal radiation in gaseous systems, studies were carried out by the exponential wide band model and the Beer Lambert's relation for both the spectral and band analyses. In addition, the different high temperature spectroscopic databases were used to calculate the total emissivities of H2O and CO2 and they were compared with Hottel's emissivity charts. Besides, computer codes for solution of the RTE using the discrete ordinates method and the finite volume method in axisymmetric and three dimensional geometries were developed.
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| 9. |
- Bahador, Mehdi, et al.
(författare)
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Thermal analysis of a heat recovery system for externally fired micro gas turbines
- 2007
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Ingår i: Proceedings of the ASME Turbo Expo. ; 3, s. 1023-1030
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Konferensbidrag (refereegranskat)abstract
- Several serious problems such as material durability and fouling in the High Temperature Heat Exchanger (HTEH) for Externally Fired Micro Gas Turbines (EFMGT) cause the low thermal efficiency. In this study for increasing the thermal efficiency, a duct around a cylindrical fixed bed combustor which burns wood pellets is proposed and two different designs, empty and porous material filled, are investigated. A heat transfer model, based on coupling between radiative and convective modes at the combustor and duct sides is developed to evaluate the important geometrical parameters in the different designs. The predicted results for the empty duct show that although an increase of the combustion length increases the temperature of air at the duct outlet, an increase of the combustor diameter is more effective. In addition, an increase of the duct cross section is the most effective way and according to the predictions, the pressure drop in this case is still acceptable. The porous duct design shows a significant increase in the air temperature at the duct outlet. However, the pressure drop is high. The investigation shows the possibility of reduction of the pressure drop with the same amount of heat transfer by selecting suitable particle size and porosity. Copyright © 2007 by ASME.
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| 10. |
- Klason, Torbern, et al.
(författare)
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Investigation of radiative heat transfer in fixed bed biomass furnaces
- 2008
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Ingår i: Fuel. - : Elsevier BV. - 1873-7153 .- 0016-2361. ; 87:10-11, s. 2141-2153
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an investigation of the radiative heat transfer process in two fixed bed furnaces firing biomass fuels and the performance of several widely used models for calculation of radiative heat transfer in the free-room of fixed bed furnaces. The simple optically thin (OT) model, the spherical harmonic P-1-approximation model, the grey gas model based on finite volume discretization (FGG), and the more accurate but time consuming spectral line weighted-sum-of-grey-gases (SLW) model are investigated. The effective mean grey gas absorption coefficients are calculated using an optimised version of the exponential wide band model (EWBM) based on an optical mean beam length. Fly-ash and char particles are taken into account using Mie scattering. In the investigated updraft small-scale fixed bed furnace radiative transfer carries heat from the bed to the free-room, whereas in the cross-current bed large-scale industry furnace, radiative transfer brings heat from the hot zones in the free-room to the drying zone of the bed. Not all the investigated models can predict these heat transfer trends, and the sensitivity of results to model parameters is fairly different in the two furnaces. In the small-scale furnace, the gas absorption coefficient predicted by using different optical lengths has great impact on the predicted temperature field. In the large-scale furnaces, the predicted temperature field is less sensitive to the optical length. In both furnaces, with the same radiative properties, the low-computational-cost P-1 model predicts a temperature field in the free-room similar to that by the more time consuming SLW model. In general, the radiative heat transfer rates to the fuel bed are not very sensitive to the radiative properties, but they are sensitive to the different radiative heat transfer models. For a realistic prediction of the radiative heat transfer rate to the fuel bed or to the walls, more computationally demanding models such as the FGG or SLW models should be used.
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