Analysis of Local Turbulent Reaction Rates from CFD Predictions of a 2MWt Natural Gas-Fired Turbulent Diffusion Flame

Breithaupt, Peter P. The present study is concerned with mathematical modelling of natural gas flames. The attention is focused on CFD-predictions in the near burner zone of a 2MW, type-2 swirling natural gas fired turbulent diffusion flame. A combustion model for turbulent diffusion flames generally calculates local turbulent reaction rates, which can be used in computational-fluiddynamics (CFD) software packages as source terms for the species mass and the enthalpy transport equation. Unfortunately, turbulent reaction rates are difficult to measure directly, and profiles of time-averaged temperature, species concentration, velocity components and mean local velocity fluctuations are commonly used to validate the model by comparison of predicted results with experimental data: "The better the agreement, the better the combustion model used". On the other hand, measurements and/or predictions of stationary, time-mean flow and species properties do not always serve the combustion engineer's need to know "where something is happening". Therefore, the first objective of this paper is to show that turbulent reaction rates generated by means of CFD predictions may be used for analysis required for engineering purposes. The strength of the widely applied eddy-break-up (EBU) combustion model to predict swirl-stabilised natural gas flames is demonstrated. However, modern gas-fired combustion technologies incorporate less intense stabilisation which results in less distinct "main-flame-zones" of high heat release and high peak flame temperatures. Consequently, the second objective of this paper is to propose a "novel" combustion model. A statistical analysis of the predicted reaction rates yields a formulation for an improved Arrhenius law-type model, called "turbulent-kinetic" (TK) combustion model. The proposed model accounts not only for the strong temperature dependency of chemical reactions at lower flame temperatures but as well for small scale turbulent fluctuations within the chemical reaction zone. The "novel" model stays attractive for use in CFD applications where computational speed is a critical consideration. At present the proposed combustion model is validated against detailed inflame measurements from a variety of semi-industrial natural gas flames.