Investigating the effect of the production method on the ceramic properties of gas turbine combustion chamber chambers V94.2
abstract
V94.2 gas turbines with rated power of 159MW and efficiency of 34.9% are considered as one of the heavy class gas turbines, which are very popular among power plant users in thermal power plants. These turbines, which fit in the form of single-axis turbines, have two combustion chambersIt is silhouette-shaped with 8 burners on each. The temperature of the inlet fluid is 1060 degrees Celsius and the inner cover of the combustion chamber is made of ceramic, which can be easily replaced. In this research, mullite is mixed with tubular alumina with a certain ratio and granularity along with nano silica solution and necessary additives, and after shaping, it is transferred to the freezing tunnel at a temperature of minus 75 degrees Celsius and finally for 10 hours at a temperature of 1550°C goes through the sintering stage. The physical, mechanical and thermomechanical properties of the samples such as density, percentage of open porosity, flexural strength at room temperature and high temperatures, as well as the coefficient of thermal expansion of the samples produced by the freeze casting method were measured simultaneously with a brick sample produced by the traditional method. The results showed that the production method has an important effect on the properties of bricks. The bending strength of the samples produced by the freecasting method at the temperature of 1200 and 1400 degrees Celsius was 10.9 and 6.8 MPa, respectively, and for the bricks produced by the traditional method, it was 8.7 and 5.6 MPa. The resistance to thermal shock of two samples after 30 cycles from 1020 Cº to cold water showed that the average crack length in samples produced by freeze casting method is 15 mm, while it reaches 50 mm in the traditional method, which is the reason The joints are mullite between the particles that make up the brick. All the tests were done at Wuhan University, China, which is a member of ILAC and approved by Tavanir Company.
Keywords – alumina, V94.2 gas turbines, ceramic chamber, mullite.
Introduction
Today, sol-gel technology is used to make parts, coatings (films), fibers, particles or composite materials with precise shapes and high surface quality. However, a major disadvantage of this method is the very high shrinkage of the gel formed during the drying process. Using the freeze casting method is a way to overcome this defect of the sol-gel method and to make parts without
crack or shrinkage close to zero [1]. Surveys show that currently some reputable companies producing complex engineering parts such as Siemens and General Motors use this method to produce their products [2, 3, 4]. Production in the traditional way is that the raw materials are mixed with granulation and certain proportions after weighing with glue, and after shaping by a press, they are transferred to the dryer and finally baked to achieve the required strength. be made
Each turbine has two combustion chambers (flame tube) whose inner diameter is 2.3 meters (Figures 1 and 2). These containers have two types of versions 3 and 5. As seen in Figure 2, version 3 has two rows of arched ceramics and 9 rows of grooved ceramics that are held by their own holders. In version 5, three types of ceramics are used.
Figure 1: Schematic of V94.2 gas turbine with combustion chamber
In section 1, a row of large-sized arc- shaped ceramics is installed, and in section 3, a row of small-sized grooved ceramics is installed. But in part 2, versions 3 and 5, which include the majority of consumable ceramics, are used with large grooves, which are completely the same in both versions.
Figure 2: Gas turbine combustion chamber V94.2
Figure 3 shows the schematic of the bricks of both versions. In order for air to enter the combustion chamber, there is about 4 mm of empty space between the ceramics. Also, in the upper part of the combustion chamber, there are 8 burners that create hot gas
Figure 3: Schematic of combustion chamber ceramics of V94.2 gas turbines (1) two upper rows of version 3 – (2) common between versions 3 and 5 – (3) middle row of version 5 – (4) upper row of version 5
Raw materials and testing method,
mullite with different granulations from Treibacher company and tubular alumina from Alteo company were used as the source of AL203. Table 1 shows the chemical analysis of raw materials used. Tubular alumina with different grain size from less than 45 microns to 2.5 mm and mullite with grain size of 0.5-1.7 mm are mixed together with nano silica solution and various additives for shaping
Table 1: Chemical composition of raw materials used
In the freeze casting method, raw materials with different ratios and granulations are mixed with silica gel and various additives in a mixer and poured into the mold after weighing. After the forming operation, it goes to the freezing tunnel at -75 °C by the vibrating table and the press, and during the 2-hour stop, the mold is disassembled. The part removed from the dried mold goes through the sintering process at a temperature of about 1550 ºC for 10 hours in a shuttle furnace to achieve the necessary strength. Various properties such as BD, MOR, HMOR, Young’s modulus, thermal expansion, thermal conductivity, XRF+XRD (simultaneously) and thermal shock resistance were measured at Wuhan University, China, from samples produced by freeze casting and traditional methods.
Results and discussion
The X-ray diffraction results of the samples after baking at 1550 ºC showed that stoichiometric mullite with a ratio of 3:2 and corundum phase were present in both samples. The amount of amorphous or glassy phase in samples A and B was 3.93% and 4.97%, respectively. The physical and mechanical properties of the samples are given in Table 2
The bulk density of the samples depends on parameters such as the type and amount of phases and the porosity of the samples. The bulk density of samples A and B are 2.92 and 2.90 grams per cubic centimeter, respectively, and there is no significant difference. The bending strength of two samples at room temperature and high temperature (1200 and 1400 degrees Celsius) shows that both samples have good resistance to applied forces. However, it can be seen that the strength of samples A at temperatures of 1200 Cº and 1400 Cº is 10.9 and 6.8 MPa, respectively, and for sample B it is 8.7 and 5.6 MPa. This means that sample A shows more resistance against applied stresses at high temperatures
The thermal shock resistance of the samples was measured from Cº 1020 into running cold water. The results showed that after 30 cycles sample A did not suffer any failure and the average crack length was less than 15 mm, while sample B had an average crack length of less than 50 mm. The reason for this is the difference in the production method. In the freeze casting method, nano-silica particles are used and these particles create a thin film around the particles that make up the piece, and after heating it turns into the mullite phase [5, 6]. The resulting mullite phase, as a connecting phase between particles, increases the thermal shock resistance of the part due to its properties such as high ductility, good creep resistance, and high thermal shock resistance [7, 8, 9].
Table 2: Measurement results of physical properties produced by freeze casting method ( A ) and production by traditional method ( B )
Conclusion:
The sample produced by freeze casting method has good physical properties such as density, percentage of open porosity, bending strength at room temperature and high temperatures, coefficient of thermal expansion and coefficient of thermal conductivity, and it is similar to samples by traditional method, but it has higher resistance to thermal shock than It is a traditional method. Considering the longevity of these ceramic chambers in the combustion chamber of gas turbines, it is considered a very effective parameter
Reference
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