A combustor design applied to the micro turbine Chuan-Sheng Chen 1, Tzu-Erh Chen 1*, Hong-Chia Hong 1 1 Chung-Shan Institute of Science and Technology, Aeronautical Systems Research Division, Taichung, Taiwan; * Corresponding Author: tipo22038259@hotmal.com The objective of this paper is to introduce a gas fuel combustor design process and methods for micro turbine generators, including design principles, procedures, and the selection of combustor and fuel nozzle types, and significant design parameters. The CFD method was used in this research to analyze the physical model and verify the combustor design. The achievements of this study show that the combustion efficiency of the design is more than 99 %, the pressure loss is about 3.4 %, the pattern factor is 0.17, and the emission of NOx is 27 ppm, all which satisfy the goals of the original design requirements. Key Words: Gas turbine combustor; micro turbine generator; combustor design; gas fuel; low NOx emission. 1. INTRODUCTION With the modern industrial development, the demand of energy is increasing day by day. Electricity is one of the most commonly used energy, so how to supply the electricity uninterruptedly becomes an urgent issue worldwide. Because the long distance electricity transportation requires not only considerable energy consumption, but also a great amount of maintenance expenses, it is more suitable to build the individual local electricity generation system for remote areas instead of large-scale power plants. A micro gas turbine generator, which is to be used to meet the demand of power requirements for remote areas, has to generate electricity with high efficiency and low pollutant emissions. As known today, Capstone US and Swedish Turbec have already developed such kind of micro gas turbine generators [1]. Capstone predicts that the small-scale generator has a market of over 14,700 million dollars, among which micro gas turbine generators occupy 1,500 million dollars [2]. There exists villages in Taiwan located in remote mountain areas where electric fence wire is difficult to reach. To provide uninterrupted electricity for these areas, micro gas turbine generators can be good choices. Additionally, the livestock industry in Taiwan produces a large amount of biogas every year, which can be used as a fuel for micro gas turbines. Therefore, it is high time to develop micro gas turbine generators. Combustor is one of main components of gas turbine engines [3]. The design of a combustor for micro gas turbines is discussed in this research, which includes the design procedure, analysis method, and preliminary design achievements [4]. The micro gas turbine breathes and compresses air into a high pressure state through the compressor, and the compressed high pressure air will be mixed with fuels, marsh gas and natural gas, etc., and burned in the combustor, the burned gas with high temperature and pressure will flow to the turbine to produce work to generate power out of generators and drive the compressor in turn. Furthermore, heat exchanger is incorporated into micro gas turbine systems so that the compressed air, before entering the combustor, will be preheated by the high-temperature burned gas discharging from the turbine [5][6]. Accordingly, by preheating the
compressed air, the thermal efficiency of the micro gas turbine engine raises. By taking advantages of the high temperature of burned gas discharging from micro gas turbine engine, which can then be ducted into another heat exchanger and further recycling the waste heat, the benefits of usable energy is increased substantially[7]. The structure of micro gas turbine engine system is shown in Fig. 1. Fig. 1 The structure of micro gas turbine engine system. The combustor plays an important role in micro gas turbine engine system to engine performance and reliability. The design of combustor must meet the requirements of high combustion efficiency, low pressure loss, good flame stability, high reliability, low pollution, and uniform temperature distribution at the outlet surface. [8]. The combustor design parameters, including combustor type, fuel nozzle, combustor detail geometry, cooling slot, and swirler sizing, will affect the performance of combustor, among which the design and analysis of a high efficiency, low pollution, gas phase fuel used combustors are focused in this research. 2. DESIGN METHOD 2.1. Combustor Design Principles The typical geometry of gas turbine engine is disposed as Fig. 2 shown. The basic design requirements of gas turbine engine combustor are listed below: (1) simplified geometry, light weight, (2) high combustion efficiency, (3) low pressure loss, (4) uniform temperature distribution at outlet, (5) no hot spot at combustor wall, (6) low exhaust emissions, and (7) easy to ignite and stable combustion. 2.2. Combustor Design Process Fig. 2 The typical geometry of a gas turbine engine combustor [9]. The first step of combustor design is to select the type of combustor and fuel nozzle. With the
benefit of smaller volume and less cooling air required, an annular combustor is selected in this research. The fuel nozzle can be classified into liquid spray nozzles and gas spray nozzles according to the fuel state. The main task for fuel nozzle is to lead the fuel into the combustor. Without atomizing or vaporizing, the main task for the gas spray nozzle is to mix the fuel with air properly, and to ensure to burn rapidly in the combustor. The second step is to determine the performance parameters of the combustor according to the task demand, including flow-rate of air, the fuel type and flow-rate, pressure and temperature at both inlet and outlet of combustor, combustion efficiency, pressure loss, uniformity of temperature distribution at outlet, and NOx emission etc. The definition of some critical parameters mentioned above is described as follows: Flow-rate of air: The flow-rate of the combustor intake when gas turbine engine is operated at the design point. The fuel type and fuel flow-rate: Natural gas is used as fuels during design process. The terminal goal is using marsh gas as fuels for the micro gas turbine generator. The flow-rate of fuel can be converted from the heating value of fuel and the output work that the micro gas turbine provides. Combustion efficiency: The combustion efficiency is defined as the ratio of the enthalpy increases of combustor through oxidation of fuel/air mixture and the theoretical heat release of fuels. The combustor efficiency of modern turbine engine is above 99% or more, the theoretical value is calculated as follows: ( ) the and represent the flow-rate of air and fuel that enter the combustor respectively,,, and are the constant pressure heat capacity of mixture at inlet, outlet, and of unburned mixture,,, and are the temperature at inlet, outlet, and of unburned mixture, LHV is the low-order heating value of the selected fuel. Pollutant limitation: NOx is the main pollutant for micro gas turbine. NOx is mainly the combination of NO and NO 2. Many methods have been applied to the combustor and fuel nozzle design which can reduce the percentage of NOx in the burned gas, for example, by lowering the temperature of reaction zone, burning the mixture with fuel lean condition, and reducing the residence time of fuel/air mixture, etc. [10]. After confirming the type and performance parameters of the combustor, the third step is to determine the geometry of combustor by the combustion intensity (Qvc) of combustor. Qvc is a parameter that measures the volume of combustor required, and defined as the heat release per hour of the unburned gas as shown below: is the volume of combustor. Qvc for aircraft gas turbine engine combustor commonly ranges from 0.7-2. For industrial gas turbine engine combustors, Qvc needs to be lowered to 0.07-0.2 after (1) (2)
being operated for a long time on the ground. The critical geometry of combustor needed to be decided are listed below. Interval of the fuel nozzle: The distribution of temperature, which is affected by the form of flame (united or not), at the outlet of combustor, is significant for gas turbine engine. Shorter the distance between nozzles, more homogeneous the outlet temperature, which cost more and weigh more as well. Usually, the interval of fuel nozzle depends on the head height of combustor, the ratio of them ranges from 0.7-1.5, and the smaller the engine, the larger the ratio. Length of combustor: The combustion chamber needs to be long enough for reactant to react completely to reach high combustion efficiency. Generally, the length to height ratio of the combustor is among 2-6. 3. ANALYSIS METHOD 3.1. Model Description In order to look insight the design and understand the temperature, pressure, flow distribution and performance of the combustor, the 3D numerical simulation model of combustor is constructed. According to the performance and geometric design parameters, the preliminary physical model of combustor and fuel nozzle can be produced as shown in Fig 3. The geometric parameters of combustor are listed as table 1. (a) (b) Fig. 3(a) 3-D model of combustor and; (b) fuel nozzle. Table 1 The parameters of combustor geometry. Geometry Parameter Inner diameter, R i (m) Outer diameter, R O (m) Done head width, Hd (m) Length, Lc (m) Gaps of entrance (m) Interval of the fuel nozzle (m) Qvc (MJ/m 3 /Pa/h) Natural gas was used as the fuel in this research. The fuel was injected directly into the combustor through 2 pilot fuel nozzles. After the flame is ignited in the combustor, the first stage or
second stage main burners will turn on in accordance with the engine operating mode. The sketch of combustor zone definition and dilution hole position are shown as Fig 4. 3.2. Numerical Method Fig. 4 The sketch of combustor zone definition and dilution hole position. A commercial CFD software (CFX, ANSYS group, America) [15] was used to simulate fuel injection and mixture formation in the engine. 3-D viscous incompressible flow is considered in this research. Navier-Stokes equation prefaces as a governing equation for mass, momentum, and energy. The SIMPLE (Semi Implicit Method for Pressure Linked Equations) algorithm is used as the coupling between pressure and speed. The1st order upwind scheme is used for saving the iteration period. The RNG derived k-ε model was applied to modeling the small scale of turbulent motions. Multiple step reaction mechanism EDM (eddy dissipation model) was used as the combustion model with P1 radiation model. Maximum total number of cells and the minimum size of cells were 1340,000. 4. RESULT AND DISCUSSION Fig. 5(a) is the temperature distribution of the combustor. Except some local high temperature spots can be observed at the outlet of fuel nozzle, the temperature distribution is uniform at the radius direction. Fig. 5(b) is the outlet temperature distribution map of the combustor. The highest temperature, about 1325 K, is located close to the hub of turbine. The temperature decreased with increasing radius, and at central portion of the turbine, temperature distribution is quite even. The average temperature at the outlet of combustor is 1227 K, and the pattern factor is 0.17. (a) (b)
Fig 5(a) The temperature distribution and (b) outlet temperature distribution map of combustor. Fig. 6 shows the species distribution at the axial direction. The upstream end of the combustor, where the fuel is injected, produced a lot of carbon monoxide (Fig. 6(a)) due to insufficient oxygen and fuel rich combustion. At the secondary zone, compressed fresh air is participating to the combustor through dilution holes 1 to 4, and keeps the flame sustained at secondary and dilution zones. Additionally, because of the fuel lean combustion at primary zone, the temperature of flame maintains at a relatively low level to prevent the formation of NOx so that only a small amount of nitrous oxides is formed. After the fresh air is supplied at secondary zone, the reaction becomes severe and the temperature of flame rises, which causes the nitrous oxides formed rapidly at this section. Luckily, the excess air with relatively low temperature has injected into dilution zone, the overall amount of NOx formation has been reduced, and a RQL (rich-quench-lean) NOx emission reduction environment is formed at this section. 5. CONCLUSION Fig. 6 The distribution of (a)co and (b) NO of combustor at the axial direction. A high combustion efficiency combustor (>99%) of a micro gas turbine generator is designed, and the emission of NOx is 27 ppm, which meet the design point setup at the beginning and achieve the goal of low emissions. With the building of data bases from the rig test, the numerical models could be modified by comparing with the results from CFD to more accurately simulate the realistic situations for the future. As a result, we ve eventually built the guideline for the combustor and nozzle design. Acknowledgments The authors are grateful to the Bureau of Energy Ministry of Economic Affairs for supporting this research under grant number 105-E0207. References [1] Guillermo, P., James, B. D., Edelman, E., Mark, G. G., and Jeffrey, W. W. (1999). U.S. Patent No. 6,405,522 B1. Washington, DC: U.S. Patent and Trademark Office. [2] Capstone, Capstone 2013 annual report, 2013
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