Modelling and Experimental Investigations on Thermal Radiation in Combustion Environments

Abstract: Thermal radiation is an important physical phenomenon in combustion environments. For the understanding of existing- and the design of new combustion environments computational modelling is a useful tool as it can describe the different transport phenomena. This thesis has focused on studying thermal radiative property models of the participating media, gases and particles. Two specific combustion environments have also been studied, from a thermal radiation perspective. The focus is on radiative property models that are useful specifically for engineering purposes. These property models are often developed from more advanced spectral models by means of correlative or other methods to simplify the treatment of the radiative transfer. One such model is the non-correlated statistical narrow band (SNB) model, which drops spectral relation between intensity and transmissivity, which is used in the correlated SNB model. By doing so a significant decrease in computational demand can be achieved. The accuracy of this model has been questioned in the literature although it has been suggested as an appropriate option for sooty environments. The assessment of the model revealed that it only gives good predictions in large geometries and highly sooty environments. Because of the model’s limited applicability and a still rather high computational demand it is not recommended for use even in sooty environments. A simpler model, and computationally much faster, called the weighted sum of gray gas (WSGG) model accompanied the assessment of the non-correlated SNB model. The WSGG model was shown to be a better choice than the non-correlated SNB model in sooty environments. A gray gas property model was compared with a WSGG model in combustion environments resembling the ones found in grate fired furnaces. In these environments, which also contains particles, the gray model was shown to give predictions of wall heat flux in close agreement with the WSGG model. This shows how the simplest model, which the gray model is, can sometimes be a suitable choice, especially when particles are present. Particle property models were evaluated in various combustion environments. A common approach is to use Planck mean coefficients to represent the particle properties. The use of Planck mean coefficients for fly-ash particles, common in furnaces combusting solid fuels, is shown to give large errors in prediction of both radiative heat flux and the source term in particular. The two investigated combustion environments are those found in grate fired furnaces and environments with high CO mole fraction. The grate fired furnace was studied in both modelling- and measurement work. Specifically the importance of particle radiation was investigated in the grate fired combustion environment. A preliminary study of the grate fired furnace was a parametric study. The parameters investigated were different particles originating from combustion of biomass and municipal solid waste, different furnace size, and boundary emissivities. The most significant effect on the overall radiative heat transfer is that of particles from municipal solid waste; moderate effects are seen when particles come from biomass. Increased furnace size most affected the heat flux to the hot bed and source term compared with a case without any particles. The choice of emissivity can be as important as considering particles or not. The measurement and modelling work were carried out on a 400 kW grate fired furnace combusting biomass. The boundary temperatures, flue gas temperature, gas mole fraction, particle mass-size distribution, and wall irradiation were measured. A so-called indirect wall irradiation was retrieved when the furnace was modelled with the implemented measured parameters. This indirect wall irradiation was compared with the directly measured wall irradiation. The study revealed that particle radiation is very important in the evaluated furnace, it doubles the wall irradiation in the hot flame zone compared with irradiation only from gases. The CO contribution to the total directional radiation was studied in environments with high mole fractions of CO, often found in gasifiers. CO is normally disregarded in environments where the fuel is fully oxidised, as it is a weak radiating species and small mole fractions of CO exist in these types of environments. The evaluation reveal that disregarding CO is still a good approach even in gasifier environments. These environments still contain small volume fractions of CO2 and H2O, which compared with CO are much stronger radiators. The rotational-vibrational bands of these two species overlaps the important fundamental band of CO reducing the importance of CO to the total directional radiative heat flux.