Numerische Simulation und messtechnische Evaluierung der Schadstoffemissionen aus Stückholzfeuerungen

Dissertation von David Schütz
Universität Stuttgart, 2012

This thesis deals with CFD-simulation (Computational Fluid Dynamics) of wood combustion in conventional wood log stoves for domestic use (< 25 kW). In order to evaluate the model, extensive measurements within the combustion chamber were carried out. Furthermore, the emission performance of the wood log stove was measured in detail. The evaluated model contributes important data to a fast and economical development of low-emission wood-log-fired stoves.
The typical characteristic of the combustion process in a wood log stove is the mixing of the devolatilization products of wood and air. This intermixture is mainly influenced by the turbulent fluid flow within the stove and takes places at relatively low combustion temperatures. In the CFD-model, the RNG k-eps; turbulence-model was used in order to describe the turbulent fluid flow within the combustion chamber. In addition the Eddy Dissipation Concept and a global reaction-mechanism were used to describe the combustion process in the gas phase. The radiative heat transfer was modeled by the Discrete-Ordinates Method and soot was taken into account with the Tesner and Magnussen model. By taking material properties of the components used for the combustion chamber into account it was possible to calculate the heat-losses occurring all over the surface of the stove.
Inside the flame, laser absorption spectrometry was used to detect the concentration of CO and CH4 locally, which are the two most important species of the combustion process. By using these results the combustion-gas-release-model could be validated, which describes the composition of the gas-mixture escaping the wood under combustion-conditions. Furthermore, the modeled CO burnout and the temperature distribution inside the combustion chamber could be verified by using the results of the measurements.
By modifying the air ratio, the inflow conditions for secondary air and the ratio of primary air to secondary air, several combustion-stages could be realized inside the used test-stand wood log stove. The emission performance of the stove was qualitatively compared with the calculations of the CFD-model. It turns out that the simulated emissions of CO2, O2 and CO match well with the measurement data. The modeling of the TVOC-emissions requires more detailed reaction mechanisms. There is no problem with adapting the soot-model of Tesner and Magnussen in a way that it delivers correct results for stationary operating conditions. However, in order to come up with a reliable prediction of the soot-emissions for a spectrum of different operating conditions, the development of more precise soot-models, which are especially designed for wood-combustion, will be required.

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