Numerical Predictions of Mild and Conventional Combustions in an Mcf with the Fuel-Jet Reynolds Number Modified Edc Constants

Abstract

This study contributes to the ongoing endeavors aimed at refining the EDC model for accurate prediction of turbulent flames. A comprehensive discussion of the fine-structures model and the mean reaction rate is provided within the context of the Eddy Dissipation Concept (EDC). The study investigates various timescale ratios as functions of turbulence Reynolds number and finds that assuming a unity reacting fraction for fine-structures used in, such as Ansys Fluent, leads to an overestimation of the mean reaction rate, even when utilizing the standard EDC model. Therefore, modifications to the EDC constants are necessary to bring accurate and reliable model predictions. Complying with the EDC theory and keeping CD1=0.1296 unchanged in this study, a modification method based on the fuel jet Reynolds number is employed to globally adjust the EDC constants and predict a Moderate or Intense Low-oxygen Dilution (MILD) combustion furnace (MCF) operating with non-preheated fuel and air. Compared to the standard EDC model with a limiting turbulence Reynolds number of 21.7, the limiting turbulence Reynolds number is 0.45 for the MCF with the fuel jet Reynolds number modified EDC model indicating the validity region of the EDC model is broadened. Compared with the predictions by both the standard and the suggested EDC constants from literature, the EDC constants modified with the fuel jet Reynolds number yield combustion predictions that accurately reproduce the experimental results from literature. However, it should be noted that some discrepancies may arise due to the globally modified EDC constants, the underestimation of turbulence and the some regions with low turbulence Reynolds number. All predicted chemical related quantities, especially temperature, OH mole fractions, Damköhler numbers, heat of reactions, and net reaction rates, exhibit increasing trends when the MCF transitioning from MILD to conventional furnace conditions.

Keywords: MILD combustion, Eddy Dissipation Concept (EDC), Mean reaction rate, Jet Re-based EDC modification, Limiting turbulence Reynolds number, Damköhler number