Abstract
MILD combustion is gaining interest in recent times because it is attractive for “green” combustion technology. However, its fundamental aspects are not well understood. Recent progresses made on this topic using direct numerical simulation data are presented and discussed in a broader perspective. It is shown that a revised theory involving at least two chemical timescales is required to describe the inception of this combustion not only showing both autoignition and flame characteristics but also a strong interaction between these two phenomena. The reaction zones have complex morphological and topological features and the most probable shape is pancake-like structure implying micro-volume combustion under MILD conditions unlike the sheet-combustion in conventional cases. Relevance of the MILD (micro-volume) combustion to supersonic combustion is explored theoretically and qualitative support is shown and discussed using experimental and numerical Schlieren images.
Highlights
The world total primary energy supply (TPES) has increased from 6.2Btoe (Billion ton of oil equivalent) in 1973 to about 13.8Btoe in 2016 (International Energy Agency, 2019
This behavior is contrary to what is observed for the inception of MILD combustion in the direct numerical simulation (DNS) results shown in Figure 5B for the three non-premixed cases
Since the interest is in the inception stage of MILD combustion, the variation of YOH with corresponding θ is shown in Figure 6 for the samples collected from the first 5% of the computational domain in regions with large heat release rate, which are marked using NZ = NZ/ max(NZ) > 0.2
Summary
The world total primary energy supply (TPES) has increased from 6.2Btoe (Billion ton of oil equivalent) in 1973 to about 13.8Btoe in 2016 (International Energy Agency, 2019). The efficiency gain comes from the energy recovered by recirculating hot gases and the emission reduction is because of reduced oxygen level in and temperature rise across combustion zones under MILD conditions This mode of combustion is said to occur when the fuel-air mixture temperature, Tr, is higher than the reference auto-ignition temperature, Tign, for a given mixture and the temperature rise, T = (Tb − Tr), is smaller than Tign (Cavaliere and de Joannon, 2004), where Tb is the burnt gas temperature. One expects the autoignition process to be dominant under this condition but direct numerical simulation (DNS) studies showed the presence of autoignition fronts with premixed and non-premixed flames and their interactions (Minamoto, 2013; Doan, 2018) This challenges the use of conventional flame theories and models for MILD combustion.
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