Chapter 1 Introduction
1.3 Scopes of the present study
This dissertation investigated the effects of using fuels with LHV on the performance of an annular MGT experimentally and numerically. The MGT adopted in this study was modified from a MW54 MK III, which is shown in Fig.
1.3(a). The original liquid fuel supply piping of this turbine engine was
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bifurcated into both the lubrication system for bearing and the liquid fuel system for combustion chamber. Figure 1.3(b) shows the feature of the engine that the bifurcation point is outside the engine case. This feature was the reason why we chose this engine, because such position of bifurcation point made the modification of fuel system easier.
This dissertation consisted of three parts. The first part of this study was experimental works that evaluated the combustion efficiency of an annular MGT while applying the LHV fuel. In the beginning of the experiment, the Jet A1 oil-fuel used gas turbine engine, WREN MW54, was modified to construct our gas-fuel used MGT system. LHV fuel used in this experiment was CH4 mixed with CO2. The CO2 was mixed in the LHV gas because in reality the bio-gas usually contains some impurity gases, due to the limitation of present biochemical technique that can’t obtain very pure methane gas from the purified marsh gas. The experiment results were used to observe and calculate the efficiency of the MGT under the different combinations of parameters, including the mixing ratio of the CO2, rotation speed of compressor wheel, pressure of fuel, and mass flow rate of the fuel. It is well known that the impurities of gas will dilute the concentration of pure methane. This dissertation tries to find the optimal operating condition to our proposed MGT to reach maximum combustion efficiency. Additionally, a remote control panel based on LabVIEW was established to construct a user-friendly monitor interface.
Since the detailed flow field and temperature distribution inside the combustion chamber are hard to measure through experiment while the MGT is running. In the second part, the corresponding simulations, aided by the commercial code CFD-ACE+, were carried out to investigate the cooling effect in a perforated combustion chamber and combustion behavior in an annular
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MGT when using LHV gas. The main purposes are to confirm that there were no hot spots occurred in the liners and the exhaust temperatures of combustor were lower than 800°C when MGT is operated under different conditions. The author found that there has been scarce published research on using the LHV fuel in an annular MGT numerically, except Yang et al. [36] had completed a numerical simulation process and model for a combustion chamber of the MGT with CFD-ACE+. Numerically, this dissertation first lightly modified the pre-existing numerical model for a much exact geometry to fit the existing facilities. A reliable steady state simulation result could help researchers to avoid inappropriate chamber design, which could cause hot spots on the chamber liner or could make the temperature of exhaust gas exceed the maximum allowable temperature that the turbine wheels can tolerate.
In the third part, the system identification of MGT was completed for future studies. The model identification process might be needed before the controller design research in the near future. The model identification process is prerequisite for controller design research in the near future. The measured data helped us identify the parameters of dynamic model in numerical simulation. This dissertation presents a novel distributed power supply system that can utilize renewable biogas. The completed micro biogas power supply system is small, low cost, easy to maintain and suited to household use. Finally, possible extensions of this study were suggested in the Future works section.
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Chapter 2
Experimental investigation
2.1 Briefing of the proposed micro gas turbine system
The MGT shown in Fig. 2.1 is basically composed of compressor (Radius:
27mm) [Fig. 2.1 (a)]; combustion chamber (Length: 52.4mm, Radius: 38mm) [Fig. 2.1 (b)]; and turbine wheel (Radius: 25mm) [Fig. 2.1 (c)]. The compressor compresses the incoming air into high pressure. The combustion chamber burns the fuel as well as produces high-pressure and high-velocity production gases.
The turbine is energized by the high-pressure and high-velocity gas flowing from the combustion chamber. The most common shaft design in a MGT is the single shaft design that a radial centrifugal compressor and turbine are attached to a shaft which was also adopted in this study. Generally, the micro turbines demonstrate the following characteristics:
■ Single stage radial compressor and turbine
■ Low pressure ratio which is typically from 2 to 4
■ High rotor speed which is often up to hundred thousands RPM
■ Minimal demand for cooling systems of rotor and turbine vanes
■ Heat recovering of exhaust gas for air pre-heating
■ Low cost of production
The MGT engines are optimized to generate large quantities of hot gas and the gas would be directed to pass through a second stationary guide vane and 2nd turbine, which is mounted on a separate shaft. The 2nd turbine only drives the load and it is considerably larger in diameter than the 1st stage. The 2nd turbine
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turns slower and, with large blades, can absorb a large amount of energy from the gas passing through it. This enables high torque to be produced at an RPM which is relatively easy to provide further reduction to suit the application. This configuration of two shafts allows considerable mismatch to be tolerated between power turbine and load. The heart of the power unit is a single stage gas turbine using centrifugal compressor and axial turbine. Power is drawn from the exhaust by a second turbine which supplies the drive to the propeller via the gearbox.
2.1.1 Transmission System
The MGT thermal system is very similar to heavy duty turbines. However, due to the lower inertia of the compressor-turbine-generator shaft, it easily gets a high rotation speeds and may reach 100,000 rpm. Consequently, the MGT usually uses gear speed reducers to diminish the rotation speed to match the AC power grid frequency of the power generator. The gearbox adopted in this study consists of a large power turbine which drives the exhaust gas from the gas generator engine and reduction gear [Fig. 2.2(a)]. The power turbine is mounted on a shaft running in high temperature bearings. Lubrication and cooling for these bearings comes from a fuel takeoff system supplied via a tube mounted above the engine, feeding via the rear of the gearbox. The input shaft drives a gearbox comprising a two stage reduction using helical gears, fully hardened for long life. The gear box lubrication system is “wet sump” that holds the oil within the gearbox and the oil circulates by the action of the gears and requires no external oil tank or services. The gearbox is charged with 20ml of automobile transmission oil. In normal use the oil needs replacement every 20 hrs continuous running or 120 times start-up.
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2.1.2 Fuel supply
A 20 Kg steel cylinder which had mounted a valve to control the value of the pressure gauge was adopted in this study [Fig 2.2(b)]. A 1/8 inch Teflon pipe was used to connect the cylinder to the MGT because the pipe is heat-resistant.
The advantage of using the cylinder is that the cylinder is easy to replace while the gas is run out and easy to change with different fuels for the experiment.
2.1.3 Start system
A blower was adopted to replace the start motor. The functions of the blower were pushing the compressor of MGT rotate and supplying enough air to mix with gaseous fuel in the MGT [Fig 2.2(c)]. Additionally, the spark plug was modified to ignite the fuel more easily. The modified spark plug could be activated with a 1.5V~3V battery or a voltage regulator. If the voltage is too high, the spark plug would fail.
2.1.4 Lubrication
The MGT works at a very high rotational speed, so the lubrication system is very important. In the original system the lubricating oil is pre-mixed with the oil fuel. When the fuel emits to combustion chamber, the lubricating oil and gas would splash to the bearings. Fig 2.2(d) shows the lubrication oil in the study.
In this study, when the MGT was modified to use gaseous fuel, the lubricating oil could not be pre-mixed with the gaseous fuel. Thus, the lubricating oil pipe should be an independent component. The lubricating oil would be emitted to the bearings directly by a pump that was controlled by a digital program. The program was scheduled to pump the lubricating oil every
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few seconds in order to protect the bearings.
2.2 Experiment layout
The experiment layout is shown in Fig. 2.3(a). Figure 2.3(b) shows the transmission gear box and generator, which are placed on a test frame made of 6061 aluminum alloy. Each part is fixed onto the frame. The generator is a commercial product, which was taken from a motorcycle [37]. The generated current was rectified to direct one for commercial use and the load was a set of high-power headlight, which is about 200W. The blower used to drive the MGT was connected to the MGT air inlet via a rubber tube. When the MGT reaches stable RPM, the rubber tube was removed and the turbine wheel kept rotating by absorbing power from the air combustor. The generator was connected to the MGT via a transmission gearbox, which transforms the power;
generated by the MGT; into axial work. This axial work drove the generator to produce electrical power. Fuel pressure and flow rate at inlet nozzles of the combustion chamber can be adjusted using a pressure valve and flow meter. The rotational speed of the MGT was measured using an induction tachometer installed close to the compression fans. The temperatures at the compressor inlet and outlet, turbine inlet and outlet and exhaust were measured using thermocouples installed on the MGT. All measurement data were stored on disk via a laptop-controlled data-acquisition system.
2.3 Measurement instrumentation
2.3.1 Data Acquisition
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Data acquisition is the process of gathering or generating information in an automated fashion from analog and digital measurement sources such as sensors and devices under test. Data acquisition uses a combination of PC-based measurement hardware and software to provide a flexible and user-defined measurement system. Oftentimes, the researcher must calibrate sensors and signals before a data acquisition device acquires them. National Instruments [38], a leader in PC-based data acquisition, offers a complete family of proven data acquisition hardware devices, and powerful and easy-to-use software that extends to many languages and operating systems. NI CompactDAQ delivers fast and accurate measurements in a small, simple, and affordable system. A CompactDAQ Chassis shown in Figure 2.4(a), which is a product of NI company was adopted because of the following advantages: Plug-and-play installation and configuration, AC power supply and USB cable connect, mounting kits available for panel, enclosure, DIN-rail and desktop development, A380 metal construction, more than 5 MS/s streaming analog input per chassis, and Hi-speed USB-compliant connectivity to PC. Different types of signal process modules were chosen to complete the data acquisition system, including NI 9203 Analog Input Module, NI 9211 Thermocouple Differential Analog Input Module, NI 9263 Analog Output Module, and NI 9215 Simultaneous Analog Input. All of the modules are shown in Fig 2.4(b-e) and the specifications of these modules are shown in Table 2.1.
Table 2.1 Specifications of the data acquisition modules Model Signal Type Channels
Max Sampling
Rate
Resolution Signal Input Ranges
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NI 9203 Current 8 500 k 16 bits ±20 mA NI 9211 Thermocouple 4 15 24 bits ±80 mA NI 9263 Voltage 4 100 k 16 bits ±10 V NI 9215 Voltage 4 100 k 16 bits ±10 V
2.3.2 Temperature measurement
The thermocouple probes were set up at the compressor outlet, combustion chamber, turbine wheel outlet, and nozzle outlet in order to observe the combustion situation. The exhaust temperature of the MGT engine was approximately 600-800℃ which could be obtained from the numerical results.
Therefore, a K-typed Chromel (chromium-nickel alloy) alumel (aluminium- nickel alloy) thermocouple shown in Fig 2.5(a) was used to measure temperature in the range of -200℃ to 1370℃, with an accuracy of +/-2.2℃ or 0.75% of the measurement.
2.3.3 Turbine RPM measurement
An electromagnetic induction kind of tachometer shown in Fig. 2.5(b) was used to measure the rpm of the compressor. When the generator turbine blades passed the sensor, the sensor would produce a pulse; then the pulse signal would be delivered to the data acquisition system. Using a conversion program, the pulse signal can be transformed to represent the rpm of the compressor.
2.3.4 Fuel pressure measurement
A ball flow meter was installed on the fuel line before the injection in the combustor chamber [Fig. 2.5(c)]. The flow meter was calibrated with an
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accuracy of 1% of the full scales.
2.3.5 Fuel flow rate measurement
The fuel supply system is the main component to start the MGT, and the flow rate of the fuel not only needs to be sensed but also to be controlled. A mass flow controller (MFC) is a closed-loop device that sets, measures, and controls the flow of a particular gas or liquid. In this study, a TC-1350 MFC produced by Tokyo Keiso Company was adopted [Fig. 2.5(d)]. The TC-1350 MFC was calibrated to measure and control the methane fuel.
2.4 Test stand operating procedures
Safety Notes
(1) During operation and for a time afterwards parts of the engine are hot enough to cause serious bums.
(2) Always have a fire extinguisher on hand when running; CO2 or BCF are ideal – dry powder, foam or water are not recommended.
(3) This engine must not be used near flammable gases, liquid or materials.
(4) Keep all spectators away from the side and rear of the engine to a distance of at least 10M radius, as shown. If operating from a pit area, take special care as safety distance is often difficult to maintain.
(5) Maximum exhaust temperature is 800°C and should not be altered.
Turning on the MGT:
(1) Connect the blower to the MGT air inlet using the rubber tube and connect the fuel pipe.
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(2) Connect the lubricating oil pipe. The oil pump is used to fill the pipe with lubricating oil, ensuring that no air enters the pipe as air would plug the pipe and stop the pump. Then, connect the spark plug to the power supply and ensure that the spark plug ignites.
(3) Set up all measure sensors for RPM, temperature, pressure, flow rate and power output, and connect these sensors to the data Acquisition device.
(4) Use propane to ignite the MGT, and then change the liquid fuel to a fuel with a low-heating value. Turn on the valves of both of the bottles, and apply the appropriate pressure for each fuel.
(5) When MGT is ignited, quickly switch the three-way valve to let in the low-heating value fuel; simultaneously increase the fuel pressure and the blower flow rate slowly. If MGT stalls, then turn off the flow valve, apply a higher pressure to the low-heating value fuel and redo step 4.
(6) Turn on the lubrication pump. Carefully measure temperature, and maintain temperature < 500°C.
(7) When the MGT reaches 40000RPM, the blower and rubber tube can be removed.
(8) Adjust the rotation rate to 40000-80000RPM, and measure power outputs (voltage and current), temperature, pressures and flow rate.
Shutdown the MGT:
(1) Slowly turn off the flow rate control valve and lubrication pump.
(2) Connect the blower to the MGT air inlet via the rubber tube, and use the air sucked by the blower to cool the MGT to 50°C.
(3) Turn off valves on fuel supply the bottles.
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2.5 Parameters of tests
The parameters of the tests included fuel concentration (the mixing ratio of CH4 and CO2) and rotation speed of the turbine. The fuel mixtures contained 90% CH4 with 10% CO2, 80% CH4 with 20% CO2, 70% CH4 with 30% CO2, 60% CH4 with 40% CO2 and 50% CH4 with 50% CO2. In the experiments, stable RPM, 45,000 RPM, was reached. At each assigned time step, RPM was gently increased by a value of 5000 RPM. After the MGT was stabilized, then held the specific RPM and the engine would be maintained in this stable condition for 10 seconds to measure the data. The experiments were tested with six different fuels, and then each fuel was tested to confirm its maximum rotation speed. Each experimental condition was held at least twice for data consistency.
2.6 Uncertainty Analysis
The accuracy of the experimental data should be confirmed before the analyses of experimental results are carried out because the exactitude of the data may not be very good. Uncertainty analysis (or error analysis) is a procedure used to quantify data validity and accuracy [39]. Experimental measuring results are always in errors. Experimental errors can be classified into fixed (systematic) error and random (non-repeatability) error respectively [39].
Fixed error is the same for each reading and can be removed by proper calibration and correction. Random error is different for every reading and hence cannot be removed. The objective of uncertainty analysis is to estimate the probable random error in experimental results.
Reliable estimation can be primarily categorized into single-sample and
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multi-sample experiments. If experiments can be repeated enough times by enough observers and diverse instruments, the reliability of the results can be assured by the use of statistics [40]. Such repetitive experiments are called multi-sample experiments. Relatively, when uncertainties are not found by repetition because of time and costs, this would be called single-sample experiments.
2.6.1 The Asymmetric Uncertainties of Thermocouple
Room temperatures are measured by a 1mm diameter K-typed thermocouple, whose signals are sent to a PC-record. The accuracy of the thermocouple itself without coating is ±0.2%. Due to the effects of conduction, convection, and radiation, it is worthwhile to check the correctness of gas temperature measured by such K-typed thermocouple. Via an application of energy balance, i.e.,
Energy in = Energy out, or
Convection to the junction of thermocouple = Radiation from the junction of thermocouple + Conduction loss from the probe
Because of the fine thermocouple (1mm), the conduction term can be neglected. Then, the steady-state energy equation can be rewritten as follows.
0
In practice, the flame temperature is much higher than the wall temperature of thermocouple, so the absorption term,αTw4, from the relatively low wall temperature of thermocouple can be removed from Eq. (2.1). According to Eq.
(2.2), the expression of correlation is given as:
h
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where Tg = the true gas temperature
Tt = the temperature measured by thermocouple probe ε = emissivity of the thermocouple
σ = Stefan Boltzmann constant
=
h convection heat transfer coefficient at thermocouple wire surface
Now, the analysis method of uncertainty can be utilized to obtain the uncertainty in the flame temperature from the correlation associated with h, Tt, and ε. The relationship between temperature and error is shown in Fig. 2.6.
2.6.2 Uncertainty Analysis of mass flow controller
The apparatuses must be corrected by other standard instruments to make sure that they can normally operate and let the inaccuracy of the experimental results reduce to minimum. In this study, the major sensor in the experiment was the mass flow controller (MFC). The measurement range of the TC-1350 MFC adopted in this study was 0.6-100L/min±0.2%. The author also used different type of MFC, series TC-3100, which had wider measurement range as the standard correction apparatus to correct the TC-1350 MFC. All the uncertainties in different flow rate were between -0.05% and 0.12%.
2.6.3 The Experimental Repeatability
To verify experimental accuracy, perform one test using the specified mixed
To verify experimental accuracy, perform one test using the specified mixed