Turbine Efficiency: How Slots and Gaps Impact Engine Performance
"Understanding the critical role of combustor-turbine slot and midpassage gaps can unlock gains in heat transfer and overall turbine effectiveness."
In the relentless pursuit of enhanced turbine engine efficiency, engineers are increasingly focused on managing high turbine inlet temperatures while minimizing the need for cooling. The parts that make up the hot gas path hardware downstream of the combustor endure searing combustion gas temperatures, surpassing the metal melting point. Cooling is essential to ensure the turbine can survive these conditions, yet extracting coolant—typically from the compressor—reduces overall efficiency, since this coolant performs no useful work in the turbine stage.
A critical aspect of turbine design involves incorporating high-pressure coolant to mitigate the effects of hot gas ingestion. This is achieved by allowing coolant to leak through intentional gaps between turbine components. The nozzle guide vane section, situated downstream of the combustor, usually consists of single or double-airfoil sections assembled into a ring. Gaps between each airfoil section (midpassage gaps) and the turbine vane ring (upstream slots) are unavoidable consequences of manufacturing and assembly. They also accommodate thermal expansion during varying operating conditions.
While these leakage paths are primarily designed to prevent hot gas ingestion, they also provide an opportunity to strategically cool turbine components. However, until recently, studies of endwall heat transfer and turbine aerodynamic loss largely overlooked these leakage paths. Emerging research now highlights the significant impact of slots and gaps on endwall film-cooling effectiveness and heat transfer. In this article, we’ll delve into a study that examines a realistic endwall slot and gap within a scaled-up nozzle guide vane cascade, focusing on how these features influence heat transfer.
How Do Combustor-Turbine Slots and Midpassage Gaps Affect Turbine Performance?
The study, conducted using a large-scale cascade, simulated a combustor-turbine interface slot and a midpassage gap to closely mimic real-world conditions. Heat transfer coefficients were measured along the endwall of a first-stage vane and within the midpassage gap to determine the effects of leakage flow. Key findings indicated that increased combustor-turbine leakage flows led to a slight increase in endwall surface heat transfer coefficients. However, the presence of the midpassage gap resulted in notably high heat transfer near the passage throat, where flow exits the gap.
- Upstream Slot: A two-dimensional slot modeled the interface between the combustor and turbine, positioned upstream of the vane.
- Midpassage Gap: Represented the gap between vane sections, running through the vane passage and dividing it into pressure-side and suction-side platforms.
- Realistic Conditions: The setup aimed to replicate the geometric and flow conditions found in actual turbine engines.
Balancing Cooling and Efficiency in Turbine Design
The ongoing advancements in turbine design require a balanced approach to cooling and efficiency. By understanding and carefully managing the effects of combustor-turbine slots and midpassage gaps, engineers can optimize cooling strategies, minimize losses, and improve the performance and durability of modern turbine engines. This detailed analysis is a step forward in enhancing the design and operational effectiveness of gas turbines, impacting both industrial and aerospace applications.