= 0.714 and 0.833), but also at stoichiometric and fuel-rich equivalence ratios. In these cases it is accompanied by significant amounts of NO2.
A common feature observed from both fuels at all equivalence ratios was the rapid formation of NO at distances shorter than 100 mm from the atomiser nozzle. This is observed especially under fuel-lean conditions ( = 0.714 and 0.833), but also at stoichiometric and fuel-rich equivalence ratios. In these cases it is accompanied by significant amounts of NO2.
In the first 150 mm of the furnace, the flame temperature profile exhibits a similar pattern at all equivalence ratios at a given furnace wall temperature, as it is largely determined by radiation from the furnace walls. The fuel is injected in a cone of (fuel-atomising air) mixture, surrounded by the secondary air stream. In this cone the conditions are more fuel-rich than in the overall combustion system ( = 2.03 in the spray cone alone). Injection and the onset of ignition create a region of high turbulence, where mixing of the atomisation and secondary air streams is accomplished. At the end of this zone the oxygen supplied in both air streams has not been consumed totally, although its concentration decreases from 20.9 % to approximately 7.0 %; at all sequivalence ratios (see Figures 70 and 71). This implies the consumption of larger amounts of oxygen at the lower (fuel-lean) equivalence ratios. Significant amounts of CO are also present (see Figures 82 and 83).
Nitric oxide found in this stage is formed by the fuel-NO mechanism. The extent of the fuel-N conversion by this mechanism is dependent on various factors, such as flame temperature, equivalence ratio or residence time. The influence of the equivalence ratio will be examined in paragraphs 5.1.3. to 5.1.5.
It was found that at 900 °C furnace wall temperature no large differences exist in the conversion of fuel-N to NOX before 200 - 250 mm at all equivalence ratios from both fuels studied (between 30 and 40 % for fuel M1 and slightly higher for fuel G1, as shown in Figures 53 and 54). Increasing the furnace wall temperature (thus, the flame temperature) alters the fuel-N to NO conversion at short distances. At 1,100 °C furnace wall temperature the conversion is higher at fuel-lean equivalence ratios than in stoichiometric or fuel-rich mixtures at a given distance from the atomiser nozzle. Larger differences were encountered at 1,200 °C furnace wall temperature, as the rates of reaction are increased by higher flame temperatures and higher oxygen availability under fuel-lean conditions.
Although the participation of the prompt-NO mechanism cannot be totally dismissed, especially since amounts of N2O are formed (Figures 90 and 92) the fuel-NO mechanism is deemed to be predominant at this stage (see section "4.1.1.b. Reaction of N2 with hydrocarbon fractions: Prompt-NO" in chapter I for the role of N2O in the prompt-NO mechanism). If the prompt-NO mechanism prevailed, similar concentrations of NO would be encountered from both fuels as this process depends on the reaction of hydrocarbon radicals with atmospheric nitrogen. In addition, experimental work performed with a low-nitrogen diesel fuel, reported in chapter IV, showed a similarly low amount of NO formed, thus ruling out a large contribution of the prompt-NO mechanism.
A possible participation of the thermal-NO mechanism is discussed in paragraph 5.1.6.
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