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Combustion of fossil fuels has been continuously generating over 70% of the total energy for our human society, concurrently emitting vastly greenhouse gases and ecologically harmful pollutants. To...

This study investigates the influence of burner configuration on nonpremixed flameless combustion (FLC) of propane in a cylindrical furnace. Two nozzle patterns are adopted, i.e., a central fuel/ox...

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.

We studied the boundary layer effects on flame temperature measurements using line-of-sight (LOS) absorption spectroscopy. The primary objective is to quantify the measurement uncertainty of the LOS-determined temperature, which is influenced by the thermal and species boundary layers existing in standard laminar premixed flames. The boundary layer thickness (δ), central flame temperature (Tc) and species concentration (Xc) are the major factors investigated in this work. Typical absorption lines of H2O in the wavelength range of 1.4–2.9 μm were examined under different boundary layer conditions (δ/L = 0–50%, L is the optical path length above the flame; Tc = 1400–2200 K; and Xc = 14–20%). As a result, the thermal boundary layer is observed to contribute mainly to the temperature deviation of the LOS measurement, which increases with δ and Tc. In comparison, the variation of Xc between 14% and 20% has a negligible effect on the temperature measurement in the presence of both thermal and species boundary layers. A systematical investigation of all the selected line pairs reveals that a properly selected line pair reduces the temperature deviation by a maximum of 16.8% under typical laminar flame conditions. The particular line pair centered at 4029.52 cm−1 and 4030.73 cm−1, which could be detected by a single tunable semiconductor laser, is recommended for the LOS temperature measurement over a temperature range of 1400–2200 K. Finally, we performed a case study of five representative temperature measurements in laminar flames and successfully corrected the LOS-determined temperature by taking into account the boundary layer effects.

We performed a comprehensive laser absorption/extinction study of temperature and soot volume fraction (SVF) in C2H4/air premixed sooting flames. Laser-absorption two-line thermometry at 2.5 μm provided a temperature uncertainty of 50 K compared with that of 90 K using conventional thermocouples. Laser extinction of soot at 633 nm was first validated against the previous measurements using laser-induced incandescence. All of the measurements were conducted at four representative C2H4 flame conditions (equivalence ratio Φ = 1.78, 1.95, 2.14, and 2.38). In addition, a CFD (computational fluid dynamics) framework coupling a skeletal mechanism (56 species and 428 reactions) with the Moss–Brookes model was developed for interpreting the experimental data. The current CFD simulations well predicted the temperature and SVF distribution along the centerline of flame. It is of interest to observe that the SVF depends on the Reynolds number of reactants by investigating the SVFs at different heights above the burne...

This paper reports the combustion and emission characteristics of the premixed MILD combustion of propane established by a single jet burner in a laboratory-scale cylindrical furnace. Measurements ...

This paper reports a comprehensive investigation into the occurrence and identification of nonpremixed MILD combustion of C3H8/air in a cylindrical furnace. The combustion mode is altered by varying burner arrangement, air-fuel nozzle separation, equivalence ratio and thermal input. Based on the experimental observations, flow analysis and chemical calculations, a new criterion is proposed to identify the occurrence of nonpremixed MILD combustion using local heat release rate (HRR) particularly for CFD simulations. Importantly, unlike the case of CH4, the nonpremixed MILD combustion of C3H8 exhibits a negative HRR (HRR−) region that departs distantly from the positive HRR (HRR+) region, due to complex pyrolysis. This is distinct from the conventional flame where the HRR− region is adjecent to the HRR+ one. The new criterion notably improves the mode identification of nonpremixed combustion operating in various furnaces at different conditions. Moreover, the current study highlights the importance of local HRR in linking the experimental observation, CFD simulation and chemical calculation of MILD combustion.