4.3. Flame time (t_f)

Groups 1 and 2 show an increase of the Flame Time with the furnace temperature, their values rising to become nearly coincidental (Figure 28), although the increase is greater for Group 1 (fuels M2 and M3) than for Group 2 (fuels G4 and G5).

Figure 28: Calculated Flame Time for a standard 1 mm initial diameter droplet

However, the trend shown by Group 3 is that of a net increase in the values of t_f, although at a lower rate than other groups. Finally the duration of its Flame Time decreases between 750 and 800 °C furnace temperature.

In general, the Flame Time increases with the furnace temperature. This can be explained in terms of the evaporation of fuel that takes place in order to form an ignitable mixture during the Pre-ignition Delay. At low furnace temperatures much fuel evaporates and diffuses into the surrounding air before an ignitable mixture is accomplished. Little mass of fuel is left in the droplet when ignition starts, and thus vaporisation and combustion of this remaining fuel take a shorter time to complete. As the furnace temperature rises the ignitable mixture is attained more rapidly, and the mass of fuel left in the droplet after ignition onset is large. Longer periods of time are required to vaporise and burn this fuel.

4.4. Total Combustion Time (t)

Figure 29: Calculated Total Combustion Time for a standard 1 mm initial diameter droplet

Group 3 (fuel G3) exhibits a distinct pattern of steady decrease of t values, thus becoming the sample with the shortest value of the Total Combustion Time at 800 °C.

4.5. Ignition Temperature (T_i)

Figure 30: Graph showing the Ignition Temperature from experiments with fuel M3 at 800 °C vs d_o

The values of T_i are correlated to the inverse of the initial droplet diameter (Taylor and Burgess (1988)) according to:

Calculation of the Ignition Temperature for a standard 1 mm diameter droplet at 800 °C provided the values listed in Table 5:

4.6. Critical Diameter for Ignition ()

Although no clear trend of behaviour could be ascertained from the results obtained, at 800 °C the fuels of each given group show similar values of .

4.7. Peak temperature (T_c)

In general, the tendency is that of increasing Peak Temperature for larger initial diameter and higher furnace temperature, although such pattern was not thoroughly obeyed in all experiments. Larger amounts of fuel give rise to higher heat release which is added to that stemming from the hot surrounding atmosphere.

A relationship between the Peak Temperature (T_c) and the initial droplet diameter can be ascertained from the experiments with fuel M2 at 700 °C. The relationship takes a quadratic form at 750 °C (T_c = 794 + 266 , r² = 0.9249). It can be seen in Figure 31.

Figure 31: Relationship between the Peak Temperature and the square of the initial droplet diameter from experiments with fuel M2 at 750 °C

Other samples did not cause an increase of the Peak Temperature. In addition, visual observations did not reveal the appearance of solid residues except for fuel M2.

However, the combustion of all samples investigated is accompanied by the ejection of considerable amounts of soot. Soot emerges from the internal side of the flame, which agrees with observations previously made by other researchers (Law (1989)) . Its formation occurs as convective streams are established in the droplet centre at the onset of ignition, forming a fuel-rich zone that promotes soot formation. This effect is enhanced by the aromatic compounds contained in the fuels, which average 23.4 % for heavy vacuum gas oils (fuels G2, G3 and G4) and 32.9 % in heavy coker gas oils (fuels M2 and M3). The layer of soot particles is eventually expelled in the outward flow of the combustion gases. It settles on the thermocouple wires and burns out.

4.8. Conclusions

Three groups of fuels with distinct combustion behaviours have been distinguished among the samples studied:

1. Group 1: Includes fuels M2 and M3
2. Group 2: Comprises fuels G4 and G5
3. Group 3: Includes fuel G3.

The following conclusions can be drawn:

1. As the furnace temperature rises up to 800 °C Group 3 (fuel G3) becomes the most easily ignitable sample, followed by Group 1 (M2 and M3) and Group 2 (fuels G4 and G5), the latter displaying the longest t_i among the results obtained. The generic boiling ranges of the two groups of fuels suggest opposite behaviour, since heavy vacuum gas oils contain larger amounts of volatile matter which should assist rapid ignition.

2. The Flame Time of Group 3 is also the shortest, as it shows a net decrease at 800 °C. However, Groups 1 and 2 experience an increase of t_f up to similar values in experiments at 800 °C.

3. As a result, Group 3 yields the shortest Total Combustion Time (t) at 800 °C, whereas both values of Group 2 are coincidental and intermediate between those of Group 1.

4. The Ignition Temperature of Groups 2 and 3 (ie, all heavy vacuum gas oils) measured at 800 °C lie within the same range, approximately 363 ± 4 °C, which is higher than that of Group 1 (heavy coker gas oils) whose T_i lie at around 318 ± 4 °C.

5. As expected the Critical Diameter for Ignition decreased as the furnace temperature was increased for all samples. However, this variable could not be correlated mathematically to any other parameter, possibly due to the inherent uncertainty of its measurement. At 800 °C furnace temperature the Critical Diameter for Ignition of all fuels converge to a narrow range of values at about 0.49 mm.

6. Coke burn-out was present in experiments with fuel M2 only.

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