Tests using various fuels with no load

The proposed MGT was tested without loading. Various fuels with LHV with various mixture ratios of methane (CH4) to carbon dioxide (CO2) were used.

The concentrations of CH4 in the fuel were in the range of 50–90%, and those of CO2 increased from 10% to 50%. The temperatures of the main components of the MGT were measured at a specified fuel and fuel flow rate against cylinder gauge pressure. Thermal efficiency was calculated accordingly. Finally, the performances of the MGT under the various test conditions were compared.

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5.1.1.1 Pressures and volume flow rates of fuel in the MGT

Fuel pressure was the pressure exerted by the cylinder (bottle); this pressure was adjusted using a control valve. Fuel pressure was determined by fuel volume flow rate. The volume flow rate of each fuel was measured as a function of pressure, and calculated from mass flow rate. (CH4 density is 0.7168 g/L at 0°C, 1 atm) The volume flow rate of each fuel was roughly linearly proportional to fuel pressure. Figure 5.1 plot rotational speeds as functions of volume flow rates for the various fuels with LHV.

In the test using fuel comprised of 90% CH4 and 10% CO2, the rotational rate approached 85,000 RPM as fuel pressure approached a maximum of 12bar.

Under the same pressure, a rate of only 47,500 rpm was reached with 60% CH4

and 40% CO2. The condition of 50% methane could not generate power when loading was applied. More specifically, the power of MGT at 40,000 RPM while applying 50% methane fuel was not able to drive the generator. The author present the results of 50% methane here were tending to offer more information for future research interest. Clearly, the combustible fuel concentration in the LHV fuels influenced the performance of MGT. According to Fig. 5.1, as expected, the fuel with the higher heating value performs better because it can supply more energy to MGT. Also, a higher pressure must be applied to the lower heating-value fuel to approach idleness at 45,000 RPM.

Since MGT performance was proportional to the CH4 concentration in fuel and volume flow rate, increasing volume flow rate increased MGT performance.

Furthermore, using a fuel with a LHV in the MGT originally designed for oil fuel may result in choking of both the turbine wheel and fuel supply pipe. This choking can occur in the nozzle throat or in the annulus at the turbine outlet. As rotational speed increases to >85,000 rpm, choking may markedly limit the

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rotational rate of the turbine wheel because the rotational speed increases as flow rate increases; however, the maximum fuel pressure reached at 85,000 rpm was 12 bar. Therefore, turbine choking did not occur during tests in this study.

Compared to the turbine choking, the choking at fuel supply pipe needs more attention. The original pipe which only have 0.5mm diameter was designed to deliver 0.05L liquid fuel per minute at 80,000 rpm. However, the experiment results showed that the volume of the fuel was 58L and the pressure was 12 bar when the maximum power was reached. Thus, in order to avoid choking condition, the fuel supply system should be modified to deliver more volume of fuel so that the pressure will be reduced and the security will be enhanced.

In this investigation, the equivalent ratio was given by O

The air mass flow rate of 321.12 g/min in the primary zone was determined from the compressor map at 45,000 RPM; the fuel mass flow rate was 26.87 g/min. The Air/Fuel ratio (A/F) was 16.7 less than the equivalent ratio of 18.1;

however, at 80,000 RPM, the A/F was 22.5, which was greater than the equivalent ratio of 18.1. Therefore, the A/F increased as rotational speed increased from 45,000 to 80,000 RPM.

Since the volume flow rate was proportional to cylinder pressure, the volume flow rate of fuel was given by the following linear regression equation (CH4 density is 0.7168 g/L at 0°C, 1 atm):

Pout

m& =5.54+4.2 (5.2)

The volume flow rates of the fuels with a LHV were then replaced by those of CH4. Figure 5.2 presents the corresponding results. At a particular compressor

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rotational speed, the volume flow rates of CH4 obtained for different fuels with LHV were almost equal, indicating that combustion was stable in the proposed MGT, even when different fuels were used.

5.1.1.2 Temperature of unloaded MGT

Temperatures were measured at the compressor inlet (T1), compressor outlet (T2), turbine inlet (T3) and turbine outlet (T4). Figures 5.3 (a-d) indicate that the histories of all temperatures and the corresponding rotational speeds for different LHV fuel. The inlet and outlet temperatures (T3 and T4) of the turbine declined as rotational speed increased because as compressor RPM increased, an increased amount of air enters the combustion chamber, and the turbine was cooled by this additional air. Liou and Leong [52], who experimented with a particular MGT using oil fuel, observed the same phenomenon at 40,000–120,000 RPM. However, when this MGT operated at >120,000 RPM, temperature increased as rotational speed increased.

Turbine inlet (T3) and turbine outlet (T4) temperatures varied slightly as the concentration of CH4 in fuels varied (Figs. 5.3(a) and (b)). This trend could influence thermal efficiency, which was discussed in Section 5.1.4. Additionally, the difference between T3 and T4 varied minimally with the fuel with a LHV because, at a given compressor rotational speed, the amounts of CH4 burned were the same for all fuels (Fig. 5.2). T3 and T4 were markedly perturbed at the start of operation because the blower and fuel were manually controlled before the MGT reached stability. Thus, a red dash line was added in Fig. 5.3 to separate an unstable duration of the MGT from the stable one. Excessively increasing the air flow rate markedly reduced T3 and T4. However, an excessive fuel flow rate rapidly increased T3 and T4.

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Figure 5.4(a) presents the surface (case) conditions of a combustion chamber using oil fuel; Fig. 5.4(b) presents those of a combustion chamber using gaseous fuel. In Fig. 5.4 (b), The surface condition (color changed) on both the primary and intermediate zones of combustion chamber have been affected by high temperature while applying the LHV fuel, whereas such phenomenon was not appeared when using liquid fuel. Clearly, combustion chamber temperature with methane exceeds that with liquid fuel because the combustion chamber was designed to burn liquid fuel. When gaseous fuel was used, the cooling system was inadequate. Moreover, this experimental conclusion can also be deduced that it is easier to burn methane than oil fuel.

Based on the schematic view of the MGT system (Fig. 3.2), the fuel pipe in the combustion chamber is close to the liner wall. Thus, the gas fuel should burn immediately after it is ejected from the fuel pipe and the temperature near the liner wall should be higher than using oil fuel. This phenomenon has been predicted from the numerical results of temperature distribution in the combustion chamber; discussed in section 5.2.

5.1.1.3 Thermal efficiency

The thermal efficiency of the proposed MGT system combined with the adiabatic efficiencies of the compressor and turbine were estimated thermodynamically by assuming a typical and simple Brayton cycle. The reason for selecting this simple Brayton cycle is that this equation is suitable for a single-shaft and low compression-ratio gas turbine that is suggested by Sonntag et al. [53]. The efficiency of the air-standard Brayton cycle is

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According to Sonntag [53], because the working fluid does not complete a thermodynamic cycle in the MGT, the internal combustion engine operates on a so-called open cycle. However, when analyzing internal-combustion engines, devising closed cycles that closely approximate open cycle is advantageous.

The thermal efficiency of the proposed MGT was derived from the efficiency of the air-standard Brayton cycle in Eq. (5.3). Figure 5.5 shows thermal efficiency versus compressor rotational speed. For the fuels with different concentration of CH4, efficiency decreased as rotational speed increased. Based on Eq. (5.3), the thermal efficiency was affected by T4, T3, and T2 (compressor outlet temperature); that is, thermal efficiency decreased because the decrease in T2 was larger than the decrease in T3 and T4 at high RPMs. Fuels with lower concentration of methane had more thermal efficiency (Figs. 5.5). Theoretically, thermal efficiency should not increase with the decrease of methane concentration in fuel. The T3 in the experiment condition of using 60% methane was larger than using 90% methane. This phenomenon could be observed in Figs. 5.3. The inferences was that the pressures and flow rates of fuels with lower concentration of methane were larger than those of fuels with higher concentration, and high inject pressure of fuel could cause major variation to temperature distribution in combustion chamber because the fuel pipe exit was very close to the front wall of chamber.

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