Date of Award
Master of Science (MS)
In order to accurately estimate radiative heat transfer in turbulent combustion systems, one needs to take into account the non-linear interactions between the temporally fluctuating species and temperature variables and the radiation field. Simply employing time-averaged values of these variables in the radiation calculations to estimate absorption and emission (as is commonly done in the modeling community) can result in gross errors in the estimation of the radiative fluxes. Therefore, models that account for these Turbulence-Radiation Interactions (TRI) have been proposed in the literature to improve the fidelity of the radiative transfer calculations. TRI models accomplish this by computing appropriate time-averaged representations of the absorption and emission terms by taking into consideration the interactions and relationships between these terms and the fluctuating species and temperature fields. However, knowledge of the specie and temperature fluctuation statistics is key to developing these TRI relationships.
In this thesis, statistical analysis of high-fidelity experimental measurements in five oxy-fuel flames with fuel jet Reynolds numbers ranging from 12,000 to 18,000 and fuel compositions in the range (50% H2-50% CH4 to 40% H2-60%CH4) were first carried out to formulate TRI models of absorption and emission. Statistical analysis of the measurements showed that in spite of the high concentrations of the radiatively participating gases in these flames, the temporal variations in the absorption field were determined to be insignificant. However, strong fluctuations in the emission field were observed and was found to correlate well with the root-mean-square of temperature.
Next, a TRI model for emission based on this experimentally observed correlation was implemented as a User-Defined Function (UDF) in the computation fluid dynamic code ANSYS FLUENT. Time-averaged simulations of the five flames were then carried out to examine the impact of the new TRI model on the radiation field. Turbulence was modeled employing the realizable k-epsilon model and non-adiabatic mixture fraction relationships were employed to represent the chemistry. The radiative properties of the mixture were determined employing a weighted-sum-of-gray gases model developed at the University of North Dakota. The predicted temperature and CO2 mole fractions agreed well with the experimental measurements suggesting the adequacy of our modeling procedure. The radiant fraction in these flames without accounting for the effects of TRI was 8%. However, including the TRI model predicted a radiant fraction of 16% as a result of significant enhancement in the emission term. Therefore, numerical simulations that do not adequately account for the TRI effects in these flames can significantly under-estimate the resulting wall radiative fluxes. Further, despite the absence of fluctuations in the absorption term, the magnitude of the absorption term was nearly equal to that of the emission term across all flames. This also indicates that the “optically thin” radiation approximation (which neglects absorption) that has traditionally been employed to simulate similar laboratory flames can again result in a significant over-estimation of the radiative fluxes. Finally, our preliminary calculations indicate that despite the importance of TRI models for wall radiative flux estimations, the impact of their inclusion on the flame temperature and specie field predictions was negligible.
Rahman, Md Ashiqur, "Modeling The Interactions Between Turbulence And Radiation In Oxy-Combustion Flames" (2017). Theses and Dissertations. 2323.