محاسبه دمای پره های توربین و برآورد عمر - تجزیه و تحلیل حساسیت

The overall operating cost of the modern gas turbines is greatly influenced by the durability of hot section components operating at high temperatures. In turbine operating conditions, some defects may occur which can decrease hot section life. In the present paper, methods used for calculating blade temperature and life are demonstrated and validated. Using these methods, a set of sensitivity analyses on the parameters affecting temperature and life of a high pressure, high temperature turbine first stage blade is carried out. Investigated uncertainties are: (1) blade coating thickness, (2) coolant inlet pressure and temperature (as a result of secondary air system), and (3) gas turbine load variation. Results show that increasing thermal barrier coating thickness by 3 times, leads to rise in the blade life by 9 times. In addition, considering inlet cooling temperature and pressure, deviation in temperature has greater effect on blade life. One of the interesting points that can be realized from the results is that 300 hours operation at 70% load can be equal to one hour operation at base load.

Increasing turbine inlet temperature is a means of improving efficiency, but this temperature exceeds allowable temperature of metal parts. In addition, the gas turbine hot parts operate in a harmful condition of centrifugal and gas pressure forces and thermal cycling. Subsequently, most of the life problems are encountered in this area. Blade metal temperature distribution and temperature gradients are the most important parameters determining blade life. Therefore, accurately predicting blade heat transfer parameters is essential for precisely predicting blade life. As mentioned above, one of the most important loads for calculating blade life is temperature distribution. In cooled turbines, in order to calculate blade temperature precisely, internal coolant, external hot gas, and metal conduction should be simulated simultaneously by conjugate heat transfer (CHT) method. There have been increased research efforts in applying the CHT methodology to simulate gas turbine blade heat transfer. Some of them are on modeling C3X and MarkII vanes in a single solver [1], [2], [3], [4], [5], [6] and [7]. Although three-dimensional (3-D) modeling of vanes and blades with complex cooling passages is time-consuming, there are some studies [8], [9], [10] and [11] which used 3-D solver and CHT method to calculate the temperature distribution of vanes and blades with more complex internal cooling passages. In addition, there are some studies [12], [13], [14], [15] and [16] in which blade simulated by conjugate (or coupled) heat transfer method using one-dimensional (1-D) simulation for internal cooling passages. Short calculation time is the most important reason that in these works, 1-D solver is used for simulation of internal cooling passages. Dewey and Hulshof [12] carried out aero-thermal analysis for combustion turbine F-Class life prediction. In order to get both temperatures and stresses right, they used combination of through-flow (BLADE-CT) and computational fluid dynamics (CFD) (FLOTRAN) to analyze the external gas flow, the Cooling Passage Flow (CPF) program to perform the cooling flow analysis and ANSYS program to analyze the heat conduction to calculate distribution of temperatures and stress. Zecchi et al. [13] presented a simulation tool to analyze cooling system of gas turbine. This tool couples energy, momentum and mass flow conservation equations together with experimental correlations for heat transfer and pressure losses. They validated this tool with experimental data using conjugate heat transfer methodology. In addition, they carried out sensitivity analysis to boundary conditions variation in order to show how uncertainty on data can affect metal temperature distribution. Takahashi et al. [14] performed a 3-D steady-state numerical analysis of thermal conjugation for inside and outside fields of the blade, which consists of convection heat transfer around the blade, thermal conduction in the blade material combined with a one-dimensional thermo-flow calculation for internal blade cooling rib-roughened passages. The 1-D calculation utilized correlations of friction and heat transfer in the rib-roughened cooling passages derived from large-eddy simulation in ribbed rectangular channels. In this study, effects of inlet temperature profiles, mass flow rate, and temperature of internal cooling air on the blade local temperature are also presented. Coutandin et al. [15] used iterative process involving external fluid dynamic simulations (CFD), internal flow network code and finite element conductive model (FEM) to design an advanced double wall cooling system and validated their results with experimental data. Amaral et al. [16] applied conjugate heat transfer method using 1-D aero-thermal model based on friction and heat transfer correlations for lifetime prediction of a high-pressure turbine blade operating at a very high inlet temperature. Their CHT method is validated on two test cases: a gas turbine rotor blade without cooling and one with five cooling channels evenly distributed along the camber line. The abovementioned studies investigated the calculation procedure of blade temperature distribution. Temperature distribution is one of the various loads that affect blade life. For calculating blade life, in addition to temperature, some other conditions and parameters should also be considered. Pressure distribution, rotating velocity and support conditions are some other factors that determine blade life. Furthermore, in order to calculate blade life, failure mechanisms should be identified. During gas turbine operation, each component has its own failure modes. For instance, vanes failure modes are thermal fatigue, low cycle fatigue and corrosion. In the case of blades, failure mechanisms are low cycle fatigue, high cycle fatigue, thermal fatigue, environmental attack and creep [17] and [18]. Consequently, life estimation of gas turbine hot section blades consists of two main parts; creep and fatigue calculation and environmental attack consideration. In most life estimation investigations, creep and fatigue lives are major parts of procedure and other failure mechanisms like corrosion are in second order of importance [19] and [20]. The critical part of a gas turbine that determines the hot section life is the turbine 1st stage blade [17], [18], [19], [20] and [21]. Severe states of stress and temperature and corrosive condition in gas turbine 1st stage are the reasons for this claim. There are many studies, which considered the aforementioned failure mechanisms for predicting blade life. Hashemi and Carlton [22] predicted the blade life of a steam turbine by estimating creep and fatigue life as main failure mechanisms. Their life prediction system used a linear damage accumulation method in order to calculate the total life of blade. Greitzer [23] utilized a turbine blade life and durability approach based on variability in design and operating factors. The lifetime was modeled considering thermo-mechanical low cycle fatigue and creep. The study showed that deviation in cooling air temperature was the most important factor determining blade life. Hou et al. [24] used a non-linear FE method to determine the steady state and transient stresses in a blade, and in this manner, determine the cause of failure in fir-tree region. This approach utilized cyclic symmetry model, centrifugal forces due to rotation velocity, and a 3-D temperature distribution. The common cause of failure was shown to be a combination of low cycle fatigue (LCF) and high cycle fatigue (HCF). Castillo et al. [25] also performed a similar blade failure investigation for another gas turbine blade. Their analysis results show that the major part of damages in the hot section blade is due to creep failure. There are many uncertainties in life simulation of turbine blades. In gas turbine operation, deviation of some parameters from set points can affect hot section life. For example, a blockage or leakage in coolant passage would affect coolant mass flow rate. This can greatly influence hot section temperature distribution and resultant life. Furthermore, some deviations in manufacturing process can affect life. In addition, there are usually some uncertainties in calculations of cycle, turbomachinery and heat transfer parameters, which are crucial boundary conditions for life estimation. Unfortunately, there are a few studies on the effect of aforementioned factors on blade life. Roos [26] conducted a set of sensitivity analysis of the trailing edge ejection slot width on the cooling effectiveness in a cast nozzle guide vane. For internal coolant passage, he utilized a pipe network-based approach. His investigation showed that reducing the slot size causes a corresponding decrease in coolant mass flow rate and consequently an increase in blade temperature. Espinosa et al. [27] evaluated the effect of reducing the cooling airflow rate on the temperature distribution on the blade's surface. The results show a clear dependence of temperature distribution, related to the cooling effectiveness, on the coolant flow rate in the cooling channels. Haubert et al. [28] evaluated effects of design parameters on the predicted blade life. They concluded that the heat transfer parameters are the most critical variables affecting blade life and the least critical parameter was blade geometry. The main concern of this study is to investigate the effects of some important uncertainties or deviations on blade temperature and life. Cooling inlet boundary conditions that are obtained from secondary air system (SAS) analysis, and material specifications of blade coating, which are specified during the manufacturing process, are some of these deviations. In addition, since mechanical drive gas turbines operate in part load conditions, the effect of change in load on blade temperature and life is also investigated.

In this study, numerical methodologies for conjugate heat transfer and life estimation are developed and validated against experimental data. The results of both heat transfer and life show good agreement with experimental data. The methods are developed to improve the fidelity of durability analyses for internally cooled airfoils. Heat transfer and life through typical turbine blade are predicted and the computational results are fully analyzed. The heat transfer results show that maximum blade temperature at the reference case is 960 ˚C and because of inlet temperature radial pattern, occurs at 70% span of blade leading edge. In addition, the life estimation results demonstrate that the minimum life occurs at the same point as maximum temperature. This indicates that the most dominant factor for blade creep life is temperature. Furthermore, the weakest point for fatigue failure mechanism is the fir-tree region of the blade. Uncertainties of some parameters, which affect turbine blade temperature and life, are also investigated. Results show that adding 300 µm TBC coating on the blade leads to 9 times increase in life in comparison with the reference case (100 µm TBC). In addition, deviation in SAS temperature has profound effect on the blade life. Furthermore, change of loading level in mechanical drive application gas turbines can increase the reliability and availability.