Case of pure methane as the fuel

At the beginning, the case to be discussed is that the composition of fuel used is pure methane, or α =100% in Fig. 4.1. The variations of flame configuration as a function of incoming flow velocity are shown in Fig. 4.2. It can be seen that, the envelope flames, transition region, and wake flames appear in order when inflow velocity increases. These flames are discussed in details as follows.

4.2.1 Envelope Diffusion Flame

The envelope flames surrounding the porous cylinder and spreading

to downstream in the low-speed flow regime, from 0.41 to 1.24 m/sec, are shown in Figs. 4.2(a) and 4.2(b).

The definitions of stand-off distance and flame thickness in the forward stagnation region for envelope flame are graphically illustrated in Fig. 2.18. Tables 4.2 and 4.3 list the values of flame thickness and stand-off distance. It can be seen that both of them are decreased as incoming airflow velocity increases. An increase of incoming flow velocity represents an increase in flame stretch so that the flame thickness becomes thinner. In the meantime, the convection of air flow becomes stronger to push flame front toward the burner surface. For this case, the maximum flame thickness and stand-off distance are 2.4 mm and 1.7 mm, whereas the minimum ones are 1.3 mm and 1 mm, respectively, occurred in the neighborhood of transition velocity (Uin = 1.24 m/s).

Both minimum values can be regarded as the critical ones, behind which no flame front can be sustained ahead of the cylindrical burner.

However, from these tables, the changes of the flame thickness and stand-off distance with the incoming velocity are step-wise rather than continuous. It is attributed to the insufficiency in pixel and resolution of experimental photographs. Remind that one pixel width is 0.4 mm approximately as mentioned in Chapter 2.4.3. Therefore, the errors for these two values are expected relatively large.

Figure 4.3 shows the two measured temperatures ahead of the burner as a function of incoming velocity. Their positions are 2.5 mm and 5.0 mm in front of the burner forward surface as shown in Fig. 2.9. From this figure, it can show that the flame is moving toward the burner surface as the incoming flow velocity increases. Also, in the flow velocity

range, 0.41~1.00 m/s, the thermocouple at 2.5mm location is inside the flame zone according to the data of flame thickness and stand-off distance in Table 4.2 and 4.3. The one of Uin =0.51 m/s is almost at the middle of the flame zone, therefore, it can catches the maximum flame temperature, 1098K. As Uin greater than 1.00 m/s, the thermocouple at 2.5mm location is out of the flame zone, so its temperature shows a drop.

For an example, it is 657 K as Uin = 1.09 m/s comparing to 882K as Uin = 1.00 m/s. When it approaches the transitional velocity (Uin= 1.24 m/s), the temperature is lowered to 487 K. As for the thermocouple at the 5 mm ahead of the cylinder surface, it is always outside the flame zone from the values given by Table 4.2 and 4.3. Therefore, it shows an asymptotic decreasing trend as the flame is shifting toward the burner surface due to the increase of incoming flow velocity.

Figure 4.4 shows the seven temperature measurements downstream of the burner as a function incoming flow velocity. The measuring positions are 2.5, 5, 7.5, 15, 30, 45 and 60 mm behind the burner rear surface, respectively; see Fig. 2.10. It can be seen that in this range the combustion plumes, where the active reactions are still maintained, on the two sides of cylinder are shifted inward toward the center line due to their interactions with the outer convective flow. Since the centerline theoretically is a symmetrical line, it behaves like an adiabatic boundary.

Therefore, the temperatures from x= 2.5 to 60 mm along the center line are increased as long as it is an envelop flame. Also, the temperature at the same position increases as the flow velocity increases. It is because that the larger velocity provides a greater shear force at the interface between the flame and air flow that pushes the flame inward more.

4.2.2 Transition Flame

When the inflow velocity increases up to 1.26 m/sec, the flame front is broken suddenly and is retreated to far downstream of the rear surface of the cylindrical burner. It is defined as flame transition region (see Fig.

4.1), whose flame configurations are shown in Figs. 4.2(c) and 4.2(d). In this region, the lift-off flame front in the range of Uin between 1.24 to 1.53 m/ sec oscillates back-and-forth without a specified frequency because the balance position changes all the time and it can only survive for 5 to 8 seconds. These behaviors are shown in Fig. 4.5, which is made up 9 pictures at different time under the fixed incoming airflow velocity 1.26 m/s. It is found that the flame behind the cylinder lacks a recirculating flow to stabilize it that might be the reason why the flame is unable to stay behind the cylinder. So, we call the flames in this range are in a transition process.

4.2.3 Wake Flame

When the inflow velocity exceeds 1.56 m/sec, the flame front retreats along the cylinder surface, then the flame front can stabilize on the rear part of the cylinder as shown in Figs. 4.2(e) and 4.2(f). Its existing range is from Uin = 1.56 to 2.63 m/s. The ejecting fuel from the forward burner surface is mixed with the incoming oxidizer to make up a flammable mixture, which is carried to downstream and subsequently ignited by the recirculated hot gas behind the cylinder to initiate the reaction to form the wake flame.

The definition of attached angle for wake flame is shown in Fig. 2.18.

At Uin= 1.56 m/s, the wake flame transition limit velocity, its attached

angle is 154^°. As incoming airflow velocity increases, the flame attached angle is decreased as shown in Table 4.4. An increase in incoming airflow velocity intensifies the stretch and convection that let the flame front shift inward and downstream. However, more fuels are carried downstream and the mixing with air is better by the faster incoming flow that makes the wake flame front become stronger. Apparently, the latter effect outweighs the former one, so the flame front moves forward along the burner surface to cause a smaller attached angle.

Figure 4.6 shows the flame temperature along the vertical centerline behind the burner. It can be seen that in the range of Uin = 1.56 to 2.06 m/s, two combustion plumes following their respective flame fronts exist on the two sides of rearward cylinder (Figs. 4.2(e) and 4.2(f)). The combustion plumes are shifted inward due to the shear by the outer air flow. Eventually, two plumes are merged on the centerline. Therefore, the temperatures from x= 2.5 to 45 mm along the center line initially are increased. Up to 60mm, it is found that its temperature is lower than that at 45 mm. Apparently, the combustion plumes end between 45 and 60mm, and then they become thermal plume. For example, T = 1240K at 60mm of measuring position comparing to T=1317K at 45mm when Uin = 1.77 m/s. The temperature increases as the inflow velocity increases at the same position, the reason is similar to that occurred in envelop flame, mentioned above.

Comparing to temperature variation in the range of U =1.56 to 2.06, the temperatures at 2.5, 5, 7.5, 15, 30 and 45mm increase very slightly as inflow velocity increases between U = 2.06 to 2.63 m/s. However, the ones at 60mm increase sharply in that range. Especially at U = 2.63

in

in

in

m/s, the maximum inflow velocity can be reached at the present experiment, the temperature at 60mm of measuring position is higher that measured 45mm, indicating that the combustion flume is stretched to become longer in that inflow range.