MILD combustion — Moderate or Intense Low-oxygen Dilution — is a regime where fuel burns in air so diluted with hot combustion products that temperatures stay low and uniform. No visible flame, low NOx, high efficiency. Industrial furnaces use it. But how it ignites has been unclear: does the fuel find the air, or does the fuel find the hot products?
The authors (arXiv:2603.22927) run direct numerical simulations of a three-stream mixing layer: fuel, air, and hot products. Four cases with varying dilution levels separate the MILD and non-MILD regimes.
The answer depends on the regime. In MILD conditions, ignition is driven by mixing with hot products — the thermal energy from the recirculated combustion gases triggers autoignition. The flame index and chemical explosive mode analysis show a premixed-autoignition mode dominates. In non-MILD conditions, ignition involves substantial premixed-deflagrative heat release from fuel-air mixing instead.
The through-claim: MILD combustion reverses the normal ignition hierarchy. Conventional combustion ignites because fuel meets oxygen. MILD combustion ignites because fuel meets heat. The hot products aren't waste — they're the ignition source. Dilution doesn't suppress combustion; it changes which mixing partner matters.