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Reacting Flow Lab

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The research work performed in this group, led by Prof. Zhuyin Ren is aimed at developing models and computational methodologies for the calculation of turbulent and reactive flows, especially turbulent combustion. For non-reactive turbulent flows, CFD plays an important role in the design of engineering equipment such as aircraft wings and gas-turbine compressors. While CFD is also used for turbulent combustion, the models currently in use in industry fall far short of the required accuracy and level of description. Our work is aimed at developing advanced models that provide a detailed description of the combustion chemistry and of the turbulence-chemistry interactions. This work focuses on PDF methods applied to turbulent flames; the combination of PDF methods with LESdimension reduction and storage/retrieval methodologies for combustion chemistry; stochastic modeling of acceleration ; and turbulent mixing models in turbulent flows.

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Now in its eighth year, the 2011 Combustion Art Competition Awards held at a recent meeting of the Combustion Institute brought together a bunch of pyrotechnic scientists eager to show off their red-hot creations. New Scientisttakes a look at the winners and runners-up.

 

Transported PDF method

A variety of modelling methods have been proposed to predict the complex, highly non-linear, interaction between turbulence and chemical reaction in both premixed and non-premixed flames. Among these, PDF methods have the advantages of representing convection and reaction exactly without modelling assumptions, and they have been demonstrated to be an effective approach for calculating turbulent reactive flows.

The PDF calculations performed on TNF workshop target flames (simple jet flames, piloted jet flames, bluff body flames) clearly demonstrate the ability of PDF method to account accurately for strong turbulent combustion interactions such as local extinction and re-ignition. In recent PDF calculations on turbulent lifted flames in a vitiated coflow, the numerical model is found to capture the most interesting feature of this particular lifted flame: the strong sensitivity of liftoff height to coflow temperature. More work is performed on piloted jet flames and bluff body flames to investigate the effect of detailed chemical mechanisms and mixing models.

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Recently, the Lagrangian investigations of local extinction, re-ignition and auto-ignition in piloted jet flame and lifted jet flames are performed. The processes of local extinction, re-ignition and auto-ignition are illustrated by the particle trajectories in composition space.

Chemistry accelaration

The goal of this research is the development of Lagrangian stochastic models of acceleration in homogeneous turbulence (with and without mean deformations) that agree with the available data from DNS and experiments. We have identified a class of second-order Markovian stochastic models (conditional on a log-normal representation for the Lagrangian dissipation) that are exactly consistent with Gaussian (one-point, one-time) velocity statistics and conditionally Gaussian acceleration statistics (the Reynolds (2003) model belongs to this class). Preliminary calculations of two-time cross-correlations (below) using the Reynolds (2003) model and a cubic acceleration model (an easily identifiable nonlinear model in the class) show that the former has an unphysical cusp at the origin (as opposed to smooth behavior generated by the latter). DNS data are going to be used as a discriminator among different stochastic models in the class.

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