No abstract

Moderate or intense low-oxygen dilution (MILD) combustion is a promising concept to reduce emissions and increase efficiency. It requires high levels of dilution and preheating of reactants, which is realized by mixing reactants with products. In order to extend the application areas of MILD combustion, adequate computational models must be developed. In this study, one of the candidate models, flamelet generated manifolds (FGM), has been assessed in terms of applicability to MILD combustion. Based on the results of this assessment, a novel multistage (MuSt) FGM method has been developed. The need for developing such a method mainly stems from the existence of different combustion stages in the MILD regime, which cannot be represented by a single progress variable. In the MuSt-FGM approach, each stage of combustion is modeled using a different progress variable, without increasing the dimension of the lookup table. The MuSt-FGM approach has been tested by conducting a priori study, and simulating both 1D laminar and 2D turbulent flames. Proving successful in all three tests, the MuSt-FGM method emerges as a promising tool for modeling not only MILD combustion, but also other systems where combustion is characterized by different stages.Keywords: Multistage flamelet generated manifolds (FGM), Moderate or intense low-oxygen dilution (MILD), Non-premixed combustion, Autoignition, Jet in hot coflow (JHC).2

The energy demand in the world is ever increasing, and for some applications combustion is still the only reliable source, and will remain as such in the foreseeable future. To be able to mitigate the environmental effects of combustion, we need to move to cleaner technologies. Moderate or intense low oxygen dilution (MILD) combustion is one of these technologies, which offer less harmful emissions, especially nitric oxide and nitrogen dioxide (NOx). It is achieved by the recirculation of the flue gases into the fresh reactants, reducing the oxygen content, and thereby causing the oxidation reactions to occur at a milder pace, as the acronym suggests. This results in a flameless combustion process and reduces the harmful emissions to negligible amounts. To assist in the design and development of combustors that work in the MILD regime, reliable and efficient models are required. In this study, modeling of the effects of temperature variation in the oxidizer of a MILD combustion case is tackled. The turbulent scales are fully resolved by performing direct numerical simulations (DNS), and chemistry is modeled using multistage flamelet generated manifolds (MuSt-FGM). In order to model the temperature variations, a passive scalar which is created by normalizing the initial temperature in the oxidizer is defined as a new control variable. During flamelet creation, it was observed that not all the compositions are autoigniting. Several approaches are proposed to solve this issue. The results from these cases are compared against the ones performed using detailed chemistry. With the best performing approach, the ignition delay is predicted fairly well, but the average heat release rate is over-predicted. Some possible causes of this mismatch are also given in the discussion.

DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: