Numerical Simulation results and discussions

The corresponding simulated mass flow rate of fuel could be divided into four mass-fraction conditions, that is, the highest one was 90 percent of methane and the lowest was 60 percent. All the rotation speed conditions were simulated according to the experimental works. Since a series of parametric studies was carried out, a reference case of steady state of methane concentration, 60%, at idle speed of MGT (45000 RPM) was chosen to serve as a detail illustration case [Fig 5.7 (a-d)]. Table 5.1 presented some important data of the simulation results.

Figures 5.8 to 5.11 demonstrated the simulation results of 45000 RPM condition while applying different concentration of CH4.

Figures 5.7(a) and 5.8 show simulation results of temperature distribution on the symmetric plane (0 degree cross-section) in the annular MGT combustor with different concentration of CH4, which demonstrated that with an appropriate value of air-fuel ratio, the temperature inside annular combustion chamber core reached 2106.7K (Table 5.2). Thus, combustion existed inside the combustion chamber. From the schematic view of MGT system, it can be realized that the fuel pipe in the combustion chamber is close to the liner wall.   

Thus, the gas fuel ought to burn right after it ejects from the fuel pipe and the

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temperature near the front wall of the combustor is expected to be higher. The simulation results shown in Figs. 5.7(a) indicated that the high-temperature zone did concentrate at the front of the combustion chamber. The combustor chamber could be divided into three zones: primary, intermediate, and dilution zones.

They were marked according to the position of liner holes shown in Fig. 5.7(a).

The other phenomenon shown in Fig. 5.7(a) was that the cool air flows via dilution holes on liners were fully functional. The jet flow, which discharged from dilution holes, cooled down the exhaust gas temperature effectively. The average exhaust temperature of combustor outlet was below the endurance temperature 800°C (1073K). In Fig. 5.8, the high temperature zone was moved toward the outlet of combustor as the methane concentration decreased. It was because that the fuel flow velocity was increased as the methane concentration was lowered that pushes the flame more downward. Furthermore, the more scattering distributed high temperature areas were induced by the lowered methane concentration. Also, the combustor outlet temperature (T3) was increased due to the downstream-shifted high-temperature zone with the decrease of methane concentration.

The simulation results shown in Figs. 5.7(b) and 5.9 demonstrated the distribution of CH4 mass fractions in primary zone. It was realized that the inverse fuel injection system could deliver the fuel properly to ensure that the combustion reaction occurred at the front of the combustion chamber. The fuel mixture of CH4 and CO2 was injected into a mixing tube, where it mixed with the concurrent air flow from the back of combustor. Then, the premixed fuel/air was exhausted from the mixing tube and immediately met with the air supplied from the hole of liner above to enter the primary zone. In the meantime, air was also supplied via the hole on the bottom surface of liner. Since such hole in the

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primary zone was not located on the symmetric plane, it couldn’t be observed in these figures. Because the air and fuel were mixed and burn in the primary zone, the mass fraction of CH4 decreased gradually from fuel tube exit to primary zone.

Eventually, the mass fraction of CH4 was near zero in the intermediate zone due to the strong dilution from air. In Figs 5.9, the CH4 mass fraction in primary zone was increased with CH4 concentration raised. Moreover, larger CH4 mass fraction produces much more severe reaction at interface zero methane mass fraction and mass diffusion zone of methane concentration gradient.

When the function of intermediate holes was discussed, Figs. 5.7 (c) and 5.10 showed the distribution of O2 mass fraction with different concentration of CH4 and indicated that large number of compressed air was induced to the combustion chamber via the intermediate holes. Under this condition, combustion would be done more completely and less dissociated product would occur. There were fourteen air inlets in the computational domain, which can be referred in Fig. 3.4 (a). Only four of them were located on the symmetric plane.

In Fig. 5.7 (c), each zone has one air inlet, and the last one is behind the fuel tube. At these air discharge holes, their oxygen mass fractions were 0.232 because they come from fresh air. The intense combustion is occurred in primary zone, the oxygen mass fraction is almost zero here. It increases gradually toward the intermediate zone and finally becomes 0.232 in the dilution zone since about 70% of total supply air entering the combustor flows into the two zones via the holes on the liner to dilute and cool the combustion products.

A conventional turbine engine typically had a swirling mechanism that increases recirculation of the flow. Such recirculation could promote fluid mixing and retain the combustion flame. Simulation results (Figs. 5.7(d) and 5.11) demonstrated that although the annular combustion chamber did not have

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a swirling generator, the design still could create an appropriate recirculation zone to confine the flame at the chamber center, and prevent the flame expanding to generate a hot spot. In Fig. 5.7(d), the first recirculation in primary zone was called central recirculation zone which was created due to the inlet air to the combustor had gained tangential component. The tangential component could develop a centrifugal force on flow particles to deflect rather than moving in the axial direction. Another recirculation occurs at the above the bottom liner wall between the two air inlets located at the primary and intermediate zones, respectively. It was resulted from two different air flow velocities via the inlets.

It also could be observed that the largest velocity occurred at the exit and the maximum and average values were 55.2 and 42.4 m/s, respectively (Table 5.1).

The air flow velocities between the liner walls and casting were relatively smaller. The acceleration as air passes the high temperature area of liner was still observed. Also, the flow velocities at the air inlet were quite great because the area change. Additionally, Fig. 5.11 shows the velocity flow field for each specified concentration of methane at 45000 rpm. From Table 5.1, the outlet temperature was increased with a decreased of methane concentration. However, the variation (1065K~980K) was insignificant, so did the local density.

Therefore, the variation of average outlet velocity was coherent with that of outlet mass flow rate. In other word, the outlet velocity was increased as the CH4

concentration decreased.

Figures 12 (a-d) are the curved charts with different methane concentration and different compressor speed. These figures show the influence of compressor speed are more distinct than the methane concentration does to the parameters of the maximum temperature in combustor, the maximum temperature on liner walls, the maximum temperature at combustor outlet, and the outlet maximum

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velocity.

Temperature control and limit is the most important issue while operate the MGT. Overheat condition could damage the MGT seriously. The combustor outlet, which was next to turbine, emitted high temperature gas. The critical endurance temperature of exhaust gas was set at 800℃ (1073K) [54]. Moreover, the turbine blade is made of Inconel whose allowance temperature is about 1336 (=1609K). Table 5.1 showed the simulation results of temperature at T3 and the maximum temperature on liner walls. When different methane concentration of simulated fuel was employed, all predictions did not exceed the maximum temperature that the turbine wheel can tolerate. Additionally, Table 5.1 also showed the temperature distribution of the combustor liner at maximum reachable speed condition while applying different concentration of CH4. A combustor wall made of Inconel must be capable of tolerating high temperatures.

The maximum endurance temperature is roughly 1600K; therefore, all the simulated conditions were not burned out. Since the distributions of CH4 mass fraction, O2 mass fraction and velocity in the combustion chamber were unavailable due to the deficiency of experiment facilities in this study, temperature data were the only information that could be made comparison between the numerical predictions and experiment measurements. Figures 5.13 (a-d) demonstrate the results of maximum temperature at turbine inlet (T3) by using the four-step simulation and the experimental data at different methane concentrations. From these figures, the predicted results of four-step mechanisms are more superior than the experimental measurements. In addition, the chemical mechanism used in this study was a reduction one (4-step) that the simplified chemical reactions were expected to be faster than the actual

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chemical reactions. Thus, the four-step reaction mechanism over predicted the maximum temperature in combustor outlet with the experimental data is expected. The results showed that, among these simulation conditions, the heat damages to turbine blade were less than those to liners. One of the reasons was that outlet was away from the solid zone of liners and then it suffered less over-heating problem as mentioned maximum previously. The other one was that the cool air flows via dilution holes on liners were fully functional. The jet flow discharged from dilution holes cool down the exhaust gas temperature effectively. When designing a combustion chamber, hot spots must be prevented as they would burn through the combustion chamber and cause the engine to fail.