Thermal/Turbulence Time Scale Ratios in Low Exothermic Reacting and Non-reacting Flows

Abstract

The aim of this work is to contribute to the study of chemistry effects on the turbulent scalar field that is, here, the temperature, through a better knowledge of the ratio between the scalar time scale and the mechanical time scale (R_t) in the distributed reaction regime characterized by a Damköhler number lower than unity. The numerical studies are performed by calculating the order of magnitude of R_t in both low exothermic reacting and non-reacting case. Propane has been chosen as a typical hydrocarbon for this study. The molar fractions are computed by the IEM (Interaction Exchange with the Mean) model in which the probability density function is calculated from its transport equation. The models and the numerical simulations are used to describe a jet stirred reactor with subsonic jet injection. The predictions are validated by comparison with experimental data for temperature and concentrations fields. It is pointed that in the non-reacting case, R_t parameter is around unity; the temporal scales dynamics and scalar are coupled. In this case, the transport of the passive scalar is ensured by the dynamic field. In the reactive case, however, this coupling is not assured any more. Indeed, the chemical reaction, characterized by a non-linear source term, affects the scalar field by decreasing its characteristic scales, which results in R_t lower than unity. The CFD-PDF predictions were within engineering accuracy of experimental data. It can be summarized that the results of exercise are satisfactory, and the CPU-time and RAM memory savings encouraging.

Keywords:

• CFD-probability density function
• Combustion
• Jet-stirred reactor
• Low Damköhler number
• Reaction mechanism
• Temperature field
• Turbulence modeling