Stability and emission characteristics of nonpremixed MILD combustion from a parallel-jet burner in a cylindrical furnace

Abstract

This experimental study investigates the stability and emission characteristics of the nonpremixed MILD combustion from a parallel jet burner firing propane (C3H8) in a cylindrical furnace without preheating and dilution. Forty cases are examined by varying the fuel-air nozzle separation S (= 8 mm–105 mm), global equivalence ratio Φ (= 0.6–1.0) and thermal input Pin (= 10 and 15 kW) to highlight the impact of S. The investigation is carried out based on the measurements of in-furnace temperature and exhaust species concentrations, as well as the visual inspection into the furnace. It is demonstrated that the NO emission (ENO) decreases generally with increasing either S (at S > 8 mm) or Φ (at least for Φ ≤ 1.0). Interestingly, the combustion for S = 8 mm behaves like a premixed counterpart and so is distinct from the other cases: e.g., this combustion is fully flameless or stable in the MILD regime over a wider range of Φ and its ENO has a weaker dependence on the furnace temperature. By contrast, for S = 45 mm, the flameless mode takes place only at Φ = 1.0 due to the small mixing zone under this air-fuel arrangement, which is revealed by aerodynamic analysis. Hence, the corresponding experimental ENO-S relationship appears to have a turning point around S = 45 mm. For the present study, all the measured NO emissions for S = 8–105 mm fall as the overall furnace temperature rises, which seemingly differs from the existing knowledge. To explain this and other results, the numerical analysis from computational fluid dynamics (CFD) is performed. It is found that the variations of NO emissions depends on the maximum temperature within the furnace rather than the average furnace temperature. Finally, the present study approves that the fuel-air jet separation should be taken as a critical operational parameter when designing the parallel multiple jet burner for nonpremixed MILD combustion.