J. Hartick

Interaction of turbulence and radiation in confined diffusion flames

A new approach for modeling turbulence-radiation interaction in confined diffusion flames is proposed. In addition to the balance equations of velocity and mixture fraction f, equations of mean, variance, and covariance of the heat-release rate g and mixture fraction are solved. Coupling with the chemistry model is achieved by means of a two-dimensional PDF of mixture fraction and heat-release rate. The proposed approach is open for improvement by more sophisticated submodels. Validity of this approach is illustrated by simulation of a 20-kW axisymmetric confined swirling combustion system. The results are compared to experimental data from LDV and Raman/Rayleigh measurements obtained from the same combustion system. Particularly, investigations of the influence of different coupling models on the local source terms of the radiation and of the nitrogen oxide production are discussed. The main results of this study are (1) the thin eddy assumption is valid for this system; (2) there are considerable fluctuations of the heat-release rate g with constant mixture fraction f at all locations because of the individual history of the burned gas eddies; (3) the coupling model has only negligible influence on the spatial temperature- velocity- and mixture-fraction fields and on the overall integrated radiation-transfer power; (4) the coupling model has strong influence on the local nitrogen oxide production and on the total nitrogen oxide emission. The present study demonstrates that for middle-sized enclosed diffusion flames, the complex phenomenon of turbulence-radiation interaction can be simulated efficiently without use of tedious stochastic approaches.

Comparison of UV Raman Scattering Measurements in a Turbulent Diffusion Flame with Reynolds-Stress Model Predictions

Simultaneous space- and time-resolved measurements of CO2, O2, N2, CH4 and H2O have beem made using spontaneous Raman scattering in a turbulent CH4/N2 diffusion flame. A narrowband XeCl-excimer-laser working at 308 nm and an intensified multichannel camera were used to give full information about all major species and spectral background. Knowledge of the background is very important for measurement accuracy. this paper presents new experimental results containing concentration, temperatures mixture-fractions f, f-plots and rms of fluctuations of f in comparison to numerical predicted results obtained with a Reynolds-stress flame model. The agreement is very good in the rich zones and poor in the flame zones. Strong fluorescence background from OH radicals was found in the high temperature regions of the flame resulting in smaller numbers of evaluatable spectra.

Turbulence-radiation interaction in confined combustion systems

A new approach for modeling turbulence-radiation interaction is proposed. The formulation is based upon equations for statistical moments. Additional to the balance equations for the velocity and mixture fraction, equations for the mean, variance, covariance of heat release rate, and mixture fraction is solved. The coupling with the chemistry model is formulated by means of a two dimensional pdf of mixture fraction and heat release rate. The proposed approach is open for improvement by more sophisticated submodels. A natural gas fired combustion chamber is designed and constructed, and the temperature field measured by CARS spectroscopy. The main features of the modified combustion system are discussed. The comparison of experimental temperatures with the numerical simulation of the combustion system shows the good quality of our approach. The modeling of the two dimensional pdf is found to be most suitable for the hot region near the burner, where most radiation effects take place.