On Radiative Heat Transfer Modeling with Relevance for Combustor and Biomass Furnaces

University dissertation from Lund University (Media-Tryck)

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.