Case of methane/nitrogen fuel mixture

After complete the above experimental study for pure methane case, the following parametric studies are based on the variation of methane-to-nitrogen mass ratio in the fuel mixture, which α varies from 90% to 40%. For each α, the incoming airflow velocities changes from 0.41 to 2.63 m/s as well. It is intended to find the relations between the mass fraction of methane in the fuel mixture (α) and inflow velocity (Uin) for flame transition behaviors. The complete flame configuration map iss shown in Fig. 4.1, which can be divided into two regimes. In the lower methane mass fraction regime, (α≤60%), the envelop flame directly transforms into wake flame, whereas in the higher methane mass fraction regime, (α >60%), there exists a transition zone between envelop and wake flames. Two cases, α = 60% and α = 80%, are selected to demonstrate their features.

4.3.1 α = 80%

4.3.1.1 Envelope Diffusion Flame

The envelop flame characteristics is similar to α = 100%. In this case, the maximum flame thickness and stand-off distance are 2 mm and 1.7 mm occurred at 0.41 m/s of incoming airflow velocity, whereas the minimum ones are 1.3 mm and 1 mm, respectively at the neighborhood of

transition velocity (Uin = 1.14 m/s), as shown in Table 4.2 and 4.3. The corresponding envelope flame photographs for α = 80% are shown in Fig 4.7(a) and 4.7(b). Comparison with α=100%, the difference in maximum flame thickness is 0.4mm at the same incoming airflow velocity 0.41 m/s.

It is because the flame becomes weaker when the methane mass ratio is lowered so that the envelop flame front becomes thinner. However, the flame thicknesses at the respective transition velocities, Uin= 1.24m/s and 1.14 m/s, for both α=100% and α=80% are the same; 1.3mm. The possible reason is that the flame stretch effect approaches the limit at the transition velocity.

Comparing the stand-off distance with α=100%, it finds that the stand-off distance strongly depends on the incoming airflow velocity, but not on the methane to nitrogen mass ratio. It is because that the stand-off distance, or quench distance, is decided by the velocity gradient at the wall and the wall temperature.

Figure 4.8 shows the two measured temperatures ahead of the burner as a function of incoming velocity. Comparing α=100% (Fig.4.3) with the present case, the temperatures are 1,098 K and 916 K, respectively, at 2.5mm location under Uin =0.51 m/s, because the thermocouple is at the outside of flame zone for α=80% according to Table 4.2 and 4.3. For other incoming airflow velocities, the trend is the same. So does the thermocouple at 5mm upstream of the burner.

Figure 4.9 shows the seven temperature measurements downstream of the burner as a function incoming flow velocity. It shows that the variation trend for α=80% is the same as that in α=100%, only with the lower corresponding temperatures. These temperature measurements

indicate that the flame become weaker as methane to nitrogen mass ratio decreases.

4.3.1.2 Flame Transition

At α=80%, the transition velocity that the envelope flame is transformed into a transition flame, is 1.16 m/s, which is lower than 1.26 m/s of α=100%. Of course, it also indicates that the flame at α=80% is weaker. Therefore, the envelope flame is more easily to blown off comparing to that of α=100%.

As methane to nitrogen mass ratio is below 60%, the envelope flame will transform into wake one directly without any appearance of transition flame when incoming velocity gradually increases to a limiting value, therefore the limit of flame transition behaviors exists between α=60% and α=70%.

Figure 4.10 shows the transition flame configuration at Uin = 1.24m/s.

The behaviors are similar to those described in Fig. 4.5.

4.3.1.3 Wake Flame

When the inflow velocity exceeds 1.46 m/sec, the flame front can stabilize on the rear surface of the cylinder as shown in Figs. 4.7(e) and 4.7(f). At Uin= 1.46 m/s, the wake flame transition limit velocity, its attached angle is 149^° (see Table 4.4). Comparing the variation of attached angle as a function of incoming flow velocity with α=100%, the trend is similar. However, the angle may not always be larger for the case with higher α under the same Uin. For example, the attach angle is 143^° for α=70%, whereas it is 141^° for α=90% and 80% under Uin = 1.58 m/s.

However, the discrepancy is quite small that the measurement error may

take the responsibility. Finally, it is found that the variation of attached angle change is not so obvious at the present cylinder burner with 30mm- diameter under different methane to nitrogen ratios.

Figure 4.11 shows the temperature distributions at the seven measuring positions in the rear of the cylinder burner for wake flame.

Totally speaking, the temperatures for α=100% are slightly higher than those in the present case. It is because the flame is weaker for α=80%.

At Uin = 1.46 to 2.06 m/s, the variation trend of temperature generally is similar to that α=100% (Fig. 4.6), but with few exceptions. The temperatures at 45mm and 60mm of measuring positions are found lower than the one at 30 mm. The reason is that the combustion plume becomes is shortened as methane to nitrogen ratio decreases. As U is greater than 2.06 m/s, the temperature varying trend is still generally

in

similar to that of α=100%, except those at the 15mm, 35mm, 45mm and 60mm of the measuring position at Uin = 2.25m/s. It is found that the temperatures at these points are decreased rather than increased as inflow velocity exceeds 2.25m/s. It is also because the combustion plume becomes smaller comparing to that of α=100%.

4.3.2 α = 60%

4.3.2.1 Envelope Diffusion Flame

The corresponding photographs for α = 60% are shown in Fig 4.12(a) and 4.12(b). In this case, the maximum flame thickness and stand-off distance are 1.7 mm at Uin=0.41 m/s, whereas the both minimum ones are 1 mm at the neighborhood of transition velocity (Uin = 1.04 m/s), as shown in Table 4.2 and 4.3. Comparing with α=100%, the difference in maximum flame thickness is 0.7mm at the same incoming airflow

velocity 0.41 m/s, the reasons are the same as mentioned in Chapter 4.3.1.1. The differences in flame thickness between α=100% and α=60% are 0.3mm at both critical Uin = 1.24m/s and 1.04 m/s. This may indicate that the stretch effect is more apparent for the flame with a lower methane to nitrogen mass ratio. For the comparison of stand-off distance, the trend is similar to that α=80% with α=100%, referred in Chapter 4.3.1.1. When α is below 60%, the measurement of temperature by inserting the probe into the envelop flame front causes it blown off in the front of cylinder burner, indicating that the flame in this regime is quite weak and it cannot sustain with the quenching effect of the thermocouple junction. Therefore, in this experiment the envelope flame temperature distributions in the front of the cylinder burner are not measured at 60%, 50% and 40%.

Figure 4.13 shows the seven temperature measurements downstream of the burner as a function incoming flow velocity. The trend for the temperature variation at these measuring positions of α=60% is similar to that of α=100%, as expected, the temperatures for α=60% is lower.

4.3.2.2 Wake Flame

At α=60%, the flame front can stabilize on the rear surface of the cylinder as shown in Figs. 4.12(c) and 4.12(d) when the inflow velocity exceeds 1.06 m/sec. At Uin= 1.06 m/s, the wake flame transition limit velocity, its attached angle is 152^°(see Table 4.4). Comparing the variation of attached angle as a function of Uin with α=100%, the trend is similar. The attached angle of α=100% is slight higher than that of α=60% at the same velocity. For example, at Uin= 1.77 m/s, it is at 141^° for α=100%, and 135^° for α=60%.

The temperature distributions in the rear of the cylinder burner are shown in the Fig. 4.13. The temperatures at the present case are slightly lower than those at α=100%, the reasons are the same as mentioned in Chapter 4.3.1.3. As Uin gradually increase up to 1.87 m/s, the general trend of temperature variation is similar to that of α=100%, but the temperatures at 45mm and 60mm of measuring position are found lower than the one at 30 mm as Uin is below 1.48m/s. It is because that the combustion plume becomes smaller comparing to that of α=100% as methane to nitrogen ratio decreases. Furthermore, as U is greater than 1.87 m/s, the

in

temperature trend is decreasing due to the termination of combustion plume.