Combustor Metal Fatigue Scenarios: A Comprehensive Guide

Combustor metal fatigue scenarios are a critical aspect of gas turbine engine design and operation, as they can lead to catastrophic failures if not properly managed. This comprehensive guide delves into the technical details and advanced analytical techniques required to understand and mitigate these scenarios.

Factors Affecting Fatigue Life in Gas Turbine Engines

The NASA report provides a detailed analysis of the factors that affect fatigue life in gas turbine engines, including:

1. Burner Outlet Temperatures (BOT): BOT is the most sensitive random variable affecting fatigue life, with a sensitivity analysis showing that concentrating engineering resources on reducing the mean value and/or reducing the scatter of BOT may be the most efficient way to increase fatigue life.
2. Burner Profile: The burner profile can significantly impact the temperature distribution and thermal gradients within the combustor, which in turn affect the fatigue life of the metal components.
3. Dwell Times: The duration of exposure to high temperatures and stresses can greatly influence the accumulation of fatigue damage over the engine’s operating cycles.
4. Seal Clearances: Variations in seal clearances can affect the cooling air flow and heat transfer within the engine, impacting the thermal and mechanical stresses experienced by the metal components.
5. Material Properties: The fatigue life of the metal components is highly dependent on the material properties, such as tensile strength, yield strength, and fatigue resistance.
6. Number of Missions: The number of missions or operating cycles at which the disk is retired can have a significant impact on the overall fatigue life.

Comprehensive Approach to Fatigue Life Prediction

The NASA report highlights the importance of determining all engine parameters that affect fatigue life and the need for detailed knowledge of each discipline that influences disk life, including:

1. Primary Hot Gas Flow: Understanding the primary hot gas flow path and its impact on the thermal and mechanical loading of the metal components.
2. Secondary Cooling Air Flow: Analyzing the secondary cooling air flow and its role in mitigating the thermal stresses experienced by the metal components.
3. Heat Transfer: Modeling the complex heat transfer processes within the engine to accurately predict the temperature distribution and thermal gradients.
4. Structural Mechanics: Applying advanced structural mechanics principles to analyze the mechanical stresses and deformations experienced by the metal components.
5. Life Prediction: Utilizing fatigue life prediction models, such as the Coffin-Manson relationship and the Basquin equation, to estimate the fatigue life of the metal components.
6. Materials: Characterizing the material properties, including tensile strength, yield strength, and fatigue resistance, to accurately predict the fatigue life.
7. Inspection: Developing reliable inspection techniques to detect and monitor the onset of fatigue damage in the metal components.
8. Failure Analysis: Conducting detailed failure analysis to understand the root causes of fatigue failures and inform future design and operational decisions.
9. Design: Incorporating the insights gained from the above disciplines into the design of the gas turbine engine to optimize the fatigue life of the metal components.

Advanced Analytical Techniques for Fatigue Life Prediction

The NASA report and the research gate paper present advanced analytical techniques for predicting fatigue life, including:

1. First-Order Reliability Methods (FORM): The NASA report used FORM to compute the probability of failure (Pf) and the sensitivity of the fatigue life to the engine parameters for the first-stage turbine disk rim.
2. Material Damping Parameter: The research gate paper presents a methodology for evaluating fatigue life of metals in both low and high cycles using the material damping parameter, which is a measure of the energy dissipated per cycle of deformation.

Managing Fatigue Risk in Process Safety

The AIChE process safety metrics guide provides a comprehensive framework for managing fatigue risk in process safety, including:

1. Fatigue Risk Metrics: Monitoring fatigue risk using metrics such as the percentage of employees working overtime, the number of extended shifts, and the percentage of employees reporting fatigue.
2. Fatigue Risk Education: Providing fatigue risk education to all affected employees to help them identify and reduce fatigue risk.

Conclusion

Combustor metal fatigue scenarios are a critical aspect of gas turbine engine design and operation, and proper management requires a comprehensive understanding of the factors that affect fatigue life, detailed knowledge of each discipline that influences disk life, and the use of advanced analytical techniques to predict fatigue life. The NASA report, research gate paper, and AIChE process safety metrics guide provide valuable insights and recommendations for managing combustor metal fatigue scenarios.

References:

1. Cruse, T.A., Mahadevan, S., Tryon, R.G., “Fatigue Reliability of Gas Turbine Engines, Part I – Method- ologies Developed”, Engineering Fracture Mechanics, under review.
2. Fatigue analysis of metals using damping parameter | Request PDF
3. Process Safety Metrics – AIChE
4. https://ntrs.nasa.gov/api/citations/19970041395/downloads/19970041395.pdf
5. https://www.researchgate.net/publication/303321397_Fatigue_analysis_of_metals_using_damping_parameter
6. https://www.aiche.org/sites/default/files/docs/pages/ccps_process_safety_metrics_-_april_2018.pdf