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In order to obtain the influencing rule of secondary combustion on emissions and combustion oscillation characteristics of gas turbine axial staged combustor in non-premixed combustion mode and explore a load increasing mode with stable low emission, an axial staged combustor for F-class gas turbines is selected for experimental study. The results show that CO consumption is restrained and CO emission increases sharply when secondary fuel is added at a lower combustor outlet temperature. The addition of secondary fuel and the increase of secondary equivalence ratio lead to the reduction of NOx emission, but the increase of load can weaken the ability of secondary fuel to reduce NOx emission. The addition of secondary fuel and the increase of secondary equivalence ratio restrain the combustion oscillation in the low frequency band (75—90 Hz). When the secondary equivalence ratio is higher than a certain threshold (0.19), the addition of secondary fuel can restrain higher frequency(175—210 Hz) combustion oscillation. In addition, by comprehensively considering the influence of secondary combustion on emissions and combustion oscillation, the operating range and load increasing mode of low emissions and stable combustion of axial staged combustor in the higher load range (20%—50% load) are obtained, which provides a reference for stable low emission operation of the unit during load increasing.

Fatigue crack is one of the major faults of the gas turbine blades. Previous modeling studies on fatigue crack growth of blades mainly focus on a single numerical or analytical approach considering different sizes of cracks under various fatigue loadings. However, the actual crack propagation process is time-varying. The iteration and update of the model are necessary to evaluate the fatigue life of blades precisely. This paper proposes a hybrid modeling method to study blade crack growth under combined high and low cycle fatigue (CCF) loadings. The method emphasizes the combination and interaction between the finite element (FE) numerical simulation and the analytical calculation based on the fracture mechanics model. The current crack propagation length of the blade FE model is calculated from the crack growth rate obtained by the stress intensity factor (SIF) range according to Paris law. Then, a new SIF range is resolved from the FE model with the updated crack length. The proposed method is verified by the first-stage compressor blade from an in-service gas turbine. Results show that the crack growth is faster using the proposed hybrid modeling method than the traditional method. The update of the SIF range under CCF loadings cannot be ignored when predicting the fatigue life of the blade. Also, a sensitivity analysis is carried out. Suggestions are given on how to set the crack extension length at each stage during modeling, especially when the blade approaches failure. [ABSTRACT FROM AUTHOR]

Detection of combustion instability is crucial for the safety and reliability of gas turbines. In this paper, the recurrence quantification analysis (RQA) and multi-fractal analysis (MFA) methods are applied to investigate the transition process from combustion noise to combustion instability in an industrial-scale combustor. Based on the dynamic pressure (DP) obtained from high pressure and high temperature tests, a novel method is proposed to construct early detection indicators (EDI) of combustion instability. The method is mainly based on the three-dimensional map of the recurrence rate, Hurst exponent, and root mean square ratio. A regression method and SVM are applied to define the classification boundary. For three test cases, the results showed that the proposed EDI can effectively detect the onset of combustion instability. Compared to the conventional method based on the root mean square levels of dynamic pressure, the EDI has capability to forecast the onset of combustion instability approximately a few hundred milliseconds in advance. [ABSTRACT FROM AUTHOR]

The effects of heat treatment (H1 and H2) on the creep behavior and microstructures of Mar-M247 at 871 °C/250 MPa are studied. The results show that the as-cast microstructure is composed of eutectics, γ phase, γ' phase, MC and M23C6 carbides, while new M6C appears in heat-treated microstructures, indicating the transformation of carbides after heat treatments. The Mar-M247 is excellent, with over 1500 h of creep life, and H1 is 48% higher than H2. The addition of post-brazing and diffusion heat treatment in H2 is detrimental to creep resistance; the two steps promoted the transformation of MC into M23C6 in advance. The increase and coarsening of M23C6 would consume more γ-phase-forming elements, weakening the solution strengthening at grain boundaries. As a result, the resistance of the grain boundary and γ/γ' interface to dislocation motion is significantly reduced, leading to the cracks' initiation and propagation along the grain boundaries. [ABSTRACT FROM AUTHOR]