3.1. Visualization of Flow in the Curved Impinging Jet Field
When considering practical use of a combustor, flow near the ignition plugs in the ignition process is crucial, and there is a need to ascertain the flow field inside the burner with this combustor too. However, it is difficult to determine this through experiments using actual equipment, and thus collision with the combustor wall of fuel sprayed from the fuel injection nozzle was visualized using a container simulating the fuel injection part of the actual equipment.
Figure 5 shows a shadowgraph photo capturing the combustion field in which fuel sprayed from the fuel injection nozzle collides with the curved surface. This figure shows the state at (a) t = 3 ms and (b) t = 10 ms after the start of injection. In the results in
Figure 5a, it is evident that the spray flare angle and fuel–air mixture near the curved surface are adequate.
Figure 5b, where time has elapsed, shows how the fuel–air mixture has spread throughout the entire inside of the container.
Figure 6 shows the results when the condition of spray from the fuel injection nozzle was directly photographed, and high-speed measurement was carried out using PIV (Particle Image Velocimetry). Judging from these results, the fuel spray develops, after fuel injection, while involving the surrounding air, and after collision with the wall, the fuel–air mixture curls up. At t = 5.0 ms in particular, it is evident that the fuel–air mixture that has curled up along the wall is entrained again by the spray.
3.2. Ignition Characteristics of the Combustor
Ignition of the liquid fuel spray must be reliably realized by providing combustor function. In addition, that ignition happens due to a complex interplay of various factors such as state of the fuel spray, state of the surrounding air, and ignition conditions. If this ignition process is imperfect, the unit will not only fail to fulfill its function as a combustor but there will also be a risk of discharge of toxic unburned gas, and of fire or explosion. Thus, to improve these problems, it will be crucial to quantitatively evaluate ignitability due to fuel spray, and to understand the relationships between ignitability and various parameters which affect it.
With this combustor, the fuel nozzle is mounted in the radial direction of the combustor, and two glow plugs that act as the ignition source are mounted at positions separated by 10 mm on the left and right from the center part in the length direction of the combustor so there is intersection with the fuel spray. In this experiment, we examined the effect on ignition time when we varied the number of glow plugs or their position relative to the center.
Figure 7 shows the relationship between glow plug position and time to ignition. The effect of the number of glow plugs is also indicated as a parameter. Additionally, the horizontal axis indicates the glow plug position, and the L = 20 mm position is the center position of the fuel nozzle. As shown in the figure, at the glow plug position L = 20 mm, time to ignition is less than approximately 1 s, but when the glow plug position is moved away from the center part, it is evident that it is hard for the plugs to come into contact with the mixed gas of fuel spray and air, and the time to ignition increases. Additionally, in terms of differences due to the number of glow plugs, two glow plugs have a greater effect on time to ignition than one in the range L = 15–20 mm, but it is evident that, when L < 10 mm, the effect due to the number of glow plugs is smaller. This result shows that the time until ignition is shortened when the glow plug position is near the fuel nozzle center position, but considering practical equipment design, if the glow plug position is near the fuel nozzle center position, a flame is formed in this region, and the glow plug is present within the flame, so the plug is always overheated, and as a result there is a possibility that plug burning deterioration will be hastened; therefore, we want to determine the optimal glow plug position based on the results of these experiments.
3.3. Flame Stability of the Combustor
Figure 8 shows a flame morphology map for the combustor. The horizontal axis indicates the fuel flow rate (Q
fuel) and the vertical axis indicates the swirl airflow rate (Q
swirl). The other numerical values given in the figure indicate the equivalent ratio (φ) of the corresponding position. Here, the amount of air used to determine the equivalent ratio defined in this study was the sum of compressed air and swirling airflow. The blue solid line in the figure plots the values where the equivalent ratio decreases, immediately before the flame blows out, and the red solid line plots the values where the equivalent ratio increases, immediately before the flame is quenched in the fuel rich state. Therefore, the region bounded by the blue solid line and red solid line becomes the combustion region. In this combustion region, a flame with luminous flames that emits soot is formed in the red solid line region where the equivalent ratio is high, and, on the other hand, in the blue solid line region where the equivalent ratio is smaller than one, a flame with blue flames is formed.
Figure 9 shows a direct photograph (combustion chamber side surface portion and combustion chamber outlet portion) of each equivalent ratio of flames at the positions (
A to
H) shown in the flame morphology map of
Figure 8. In φ = 0.75 (
A) in
Figure 9a, the luminous flame disappears in the entire flame due to the increase in the swirling air flow rate with respect to the fuel flow rate and, conversely, the blue flame is predominantly formed to the vicinity of the upstream part of the combustor. The flame recedes and the flame length becomes shorter. When φ = 1.33 (
B) in
Figure 9b, the flame transitions from a blue flame to a luminous flame and approaches the combustion limit (lower part), so the flame becomes unstable and begins to vibrate. When φ = 2.20 (
C) in
Figure 9c, the flame is a luminous flame, so the flame becomes a spiral state due to the influence of the swirling air flow and reaches the combustion limit (lower part), so it is in a very unstable state. become. In φ = 1.05 (
D) in
Figure 9d, the luminous flame is mixed in the blue flame, but it is in a very stable state. In φ = 1.20 (
E) in
Figure 9e and φ = 1.45 (
F) in
Figure 9f, the luminous flame gradually becomes dominant, and the length of the flame also extends downstream of the combustor. Furthermore, at φ = 0.96 (
G) in
Figure 9g and φ = 1.02 in
Figure 9h, the flame appears in the blue flame behind the luminescent flame, but it is approaching the combustion limit (upper part). The flame oscillates in the length direction. For the above reasons, it was confirmed that the swirling air flow is an important parameter for stabilizing the flame.
3.4. Transition Flame and Temperature Fluctuations in the Combustion Chamber
From the combustion state with a stable blue flame where no soot is produced, the swirl airflow rate begins to change due to external factors, and it is advantageous from a practical standpoint to understand the process whereby combustion becomes unstable, i.e., the combustion behavior of the transition flame. Thus, as shown in the flame map indicated in
Figure 10, temperature changes and flame morphology inside the combustor were observed for regions A, B, and C when the fuel flow rate Q
fuel was fixed at 4, 5, or 6 mL/min, and the swirl airflow rate Q
swirl was varied in the range 15–60 L/min.
First, focus on region A in which Q
swirl is changed by fixing it at Q
fuel = 4 mL/min.
Figure 11 shows a direct photograph of the flame and temperature fluctuations at each point in region A.
Figure 11a shows the result of Q
swirl = 15 L/min (φ = 1.70). This point is near the limit of the red solid line region where the equivalent ratio is high, as shown in
Figure 10. This flame periodically oscillates and burns in the axial direction. When the flame stays in the upstream part of the combustor, it is a blue flame, but when the flame extends to the downstream part, it changes to a flame in which a luminous flame is mixed. The temperature in the combustion chamber in this flame state is stagnant at around 600 °C.
Figure 11b shows the result of Q
swirl = 35 L/min (φ = 1.08). In the flame state, the vibration combustion shown above is cured, and the flame is stagnant in the state of blue flame in the upstream part of the combustor and is burning stably. It can be seen that the temperature in the combustion chamber at this time is gradually rising because the flame is stagnant.
Figure 11c shows the result of Q
swirl = 55 L/min (φ = 0.79). The flame morphology shows a blue flame near the upstream part of the combustor, but the flame gradually extends to the downstream side, and the color of the flame becomes lighter. The temperature in the combustion chamber in such a flame state tends to gradually decrease.
Next, focus on region B where Q
swirl is changed by fixing Q
fuel = 5 mL/min.
Figure 12 shows a direct photograph of the flame and temperature fluctuations at each point in region B.
Figure 12a shows the result of Q
swirl = 15 L/min (φ = 1.86). Since this point is near the limit of the red solid line region as in
Figure 11a, the flame is very unstable. In this region, blue flames and luminous flames extending to the downstream part of the combustor repeatedly appear while vibrating, and then a small explosion occurs near the upstream part. At this time, the temperature in the combustion chamber rises when the flame is present but, once the flame disappears, the temperature decreases.
Figure 12b shows the result of Q
swirl = 40 L/min (φ = 1.24). The flame morphology at this time is similar to that in
Figure 11b, and it can be seen that the flame stagnates in the state of blue flame in the upstream part of the combustor and burns stably. Therefore, the temperature in the combustion chamber is gradually rising.
Figure 12c shows the result of Q
swirl = 60 L/min (φ = 0.93). The flame morphology shows the same flame behavior as in
Figure 11c. Therefore, the temperature in the combustion chamber at this time also tends to gradually decrease.
Furthermore, the region C where Q
fuel = 6 mL/min is fixed and Q
swirl is changed is examined.
Figure 13 shows a direct photograph of the flame and temperature fluctuations at each point in region C.
Figure 13a shows the result of Q
swirl = 20 L/min (φ = 2.24). Since this point has the same flame behavior as described above in
Figure 11a and
Figure 12a, the temperature fluctuation inside the combustor is also similar.
Figure 13b shows the result of Q
swirl = 45 L/min (φ = 1.38). As for the flame morphology in this state, as shown in
Figure 11b and
Figure 12b, the flame is stable in the state of blue flame in the upstream part of the combustor, so the temperature in the combustion chamber also rises gradually.
Figure 13c shows the result of Q
swirl = 60 L/min (φ = 1.12). Since this point has not yet reached the blue solid line region, as shown in
Figure 10, the flame fluctuates slightly in the axial direction of the combustor but remains relatively stable. Therefore, the temperature in the combustion chamber fluctuates up and down as the flame repeatedly moves in the axial direction of the combustor.
From the above results, it can be seen that the flame behavior formed in the combustor due to the change in the swirling air flow has a great influence on the temperature inside the combustor, and the approximate temperature change is determined by the form of the flame morphology.
3.5. Effects of the Equivalent Ratio Variation on Flame Length and Temperature in the Combustion Chamber
When a combustor is considered as a heat source for thermal spraying, the temperature produced at the heat source part is extremely important. This becomes a crucial factor for determining the melted and semi-melted state of the thermal spray material, and has a major impact on coating performance. On the other hand, the temperature in the combustion chamber has a major effect on flame morphology, as described above, and here we varied the equivalent ratio, and examined effects on flame length and temperature in the combustion chamber.
First, we examined the effect of changes in the equivalent ratio on flame length.
Figure 14 shows the relationship between the equivalent ratio and flame length when, as parameters, swirl airflow Q
swirl is fixed at 20, 30, 40, and 50 L/min, and the fuel flow rate Q
fuel is varied. It was found that, when the equivalent ratio is increased, the flame suddenly begins to extend at around φ = 1.16 with Q
swirl = 20 L/min, at φ = 1.5 with Q
swirl = 30 L/min, and around φ = 1.6 with Q
swirl = 40 L/min. Overall; when the flame is backed up in the upstream part (L < 100 mm), the swirl airflow is more dominant than the fuel flow rate, and thus the flame formed in the combustor is in the blue flame state but when the flame extends, luminous flames mix in with the blue flames and, as the equivalent ratio increases, so does the ratio of luminous flame. Due to the relationship between the equivalent ratio φ and the flame length shown in
Figure 14, the flame length changes sharply from a specific equivalent ratio φ in general; until the equivalent ratio is around one, the flame is a premixed flame. It forms a blue flame, which stagnates upstream of the combustor. After that, when the equivalent ratio is increased, the bright flame is mixed in the blue flame, and the flame becomes unstable by transitioning from the premixed flame to the diffuse flame. It is considered that the length of the flame extends to the downstream side at this time because the fuel that could not be burned in the upstream part of the combustor moves to the downstream side and combustion occurs there. Furthermore, if the swirling air flow, which is a variable, is increased, the flame is maintained in the upstream part of the combustor even if the equivalent ratio is high, so it is considered that the rapid change in the flame length is delayed.
Figure 15 shows the relationship between the equivalent ratio, and the combustion chamber internal temperature and flame length, when fixing at Q
fuel = 6 mL/min while varying Q
swirl. Representative photos of the flame morphology are also shown in the figure. The position for measurement of temperature in the combustion chamber is 30 mm downstream from the upstream part of the combustion chamber, and 5 mm from the combustion chamber wall in the radial direction; for the temperature measurement value, we time averaged data collected at 10 s intervals with a sampling cycle of 10 ms. Furthermore, for the measurement value of flame length, photos of flame behavior were taken at 1 s intervals with a high-speed camera, and flame length was determined from the captured image data and then averaged to obtain the final value. When the equivalent ratio φ is increased, the flame gradually extends to the combustor downstream side, and from around φ = 1.5 the flame suddenly extends to the downstream side and begins to vigorously pulsate. Regarding the temperature near the wall of the combustion chamber corresponding to this flame behavior, on the other hand, the flame is backed up in the combustor upstream part up to φ < 1.5 and, due to the effects of swirl airflow, the flame fluctuates near the wall of the combustion chamber, and thus the temperature exhibits a value of approximately 1000 °C. However, when φ > 1.5, it is evident that the effects of the swirl airflow diminish, and flames concentrate overall in the center direction, so there is a sudden decrease in temperature near the combustion chamber.
Next,
Figure 16 shows the relationship between the equivalent ratio, and the combustion chamber internal temperature and flame length, when fixing Q
swirl = 20 L/min and varying Q
fuel. Just as when Q
swirl was varied while Q
fuel was kept fixed, as shown in
Figure 15, there was a sudden increase starting near φ = 1.2, and a gentle increase thereafter. On the other hand, the temperature near the combustion chamber wall increases with in-creasing φ and peaks at φ = 1.16. After that, the temperature switches to a declining trend. These results are the same as those for flame length and variation in temperature near the combustion chamber wall shown in
Figure 14, but when flame behavior is observed for φ in the range 1.16 < φ < 1.5, the flame moves from the combustion chamber wall to the combustion chamber center due to the increase in Q
fuel; thus, it is thought that the temperature near the combustion chamber wall suddenly starts to decrease.
For the above reasons, the change in the equivalent ratio becomes a factor determining the dominance of fuel flow or swirl airflow in the narrow combustion chamber and, for this reason, flame behavior varies greatly; thus, it was found that the temperature in the combustion chamber is also affected.