REPRESENTATION OF DIFFERENTIAL MOLECULAR DIFFUSION BY USING LAMINAR FLAMELET AND MODELING OF POOL FIRE BY USING TRANSPORTED PDF METHOD

A  combustion simulation involves various physiochemical processes, such as molecular and turbulent diffusion, smoke and soot formation, thermal radiation, chemical reaction mechanisms, and kinetics. In the last decade, computational fluid dynamics (CFD) has been increasingly used in combustion modeling. It is critically important to improve and enhance the predictive capabilities of combustion models. This work presents an analysis of two types of diffusion flames: the momentum-dominant jet flames and buoyancy-controlled pool fires. The gap between the existing knowledge of differential molecular diffusion in turbulent high momentum jet flow and the practical applications has been reduced. The importance of mixing modeling in pool fire simulations has been revealed, and enhancement for predicting fire extinction limits has been proposed.

Modeling differential molecular diffusion in turbulent non-premixed combustion remains a great challenge for flamelet models. The laminar flamelet is a key component of a flamelet model for turbulent combustion. One significant challenge that has not been well addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion problems. Laminar flamelet is generated typically based on two conceptual burner configurations, the opposed jet burner, and the Tsuji burner. They are commonly considered equivalent when dealing with the description of laminar flamelet structures. A difference between them is revealed in this work for the first time when they are used to represent differential molecular diffusion. The traditionally opposed jet burner yields an almost fixed equal diffusion location in the mixture fraction space for the transport of different elements. The Tsuji burner can produce a continuous variation of the equal diffusion location in the mixture fraction space with a slight extension. This variation of the equal diffusion location is shown to be an essential characteristic of turbulent non-premixed combustion, as demonstrated in a laminar jet mixing layer problem, a turbulent jet mixing layer problem, and a turbulent jet non-premixed flame. The Tsuji burner is thus potentially a more suitable choice than the opposed jet burner for laminar flamelet generation that can be consequently used in flamelet modeling of differential molecular diffusion for turbulent non-premixed combustion.

Capturing fire extinction limits in simulations is essential for developing predictive capabilities for fire. In this work, the combined large-eddy simulation (LES) and transported probability density function (PDF) methods are assessed for the predictions of fire extinction. The University of Maryland line burner is adopted as a validation test case. The NIST Fire Dynamics Simulator (FDS) code for LES is combined with an in-house PDF code called HPDF for the fire simulations. The simulation results were verified by using the available experimental data. The combustion efficiency under the different oxygen depletion levels in the oxidizer is analyzed. Fire extinction occurs when the oxygen depletion level reduces to a certain level. The model’s capability to capture this extinction limit is assessed by using the experimental data. Different mixing models and model parameters are examined. It is found that the fire extinction limit is very sensitive to the different mixing models and mixing parameters. The level of sensitivity is higher than in momentum-driven turbulent flames, which suggests the importance of mixing modeling in fire simulations. The existing mixing models need further enhancement for predicting fire extinction.