Endwall and leading-edge film cooling of turbine blades in a hydrogen-fueled rotating detonation combustor-turbine coupled system

This study utilizes three-dimensional numerical simulations to demonstrate that combining circular endwall holes with vertical-inclined leading-edge film cooling effectively reduces turbine blade temperatures and enhances flow stability within a hydrogen-fueled rotating detonation combustor-turbine system by leveraging the upstream detonation wave to facilitate downstream jet diffusion.

The Gist

Imagine you are trying to build a super-fast, super-efficient car engine. But instead of burning fuel smoothly like a normal car, this engine uses tiny, continuous explosions that spin around in a circle to generate power. This is called a Rotating Detonation Combustor (RDC). It's like a ring of fire that never stops spinning, pushing the engine forward with incredible force.

The problem? These spinning explosions are incredibly hot—hot enough to melt metal in a split second. If you attach a standard turbine (the part that turns the engine's power into motion) right behind this ring of fire, the heat will destroy it instantly.

This paper is about designing a super-cooling suit for the turbine blades so they can survive this fiery environment. Here is how the researchers solved the puzzle, explained simply:

1. The Challenge: The "Firehose" of Heat

Think of the turbine blades as delicate leaves in a hurricane. But instead of wind, they are hit by a "firehose" of superheated gas and shockwaves (pressure waves) from the spinning explosions.

• The Heat: The gas is over 3,000°C (hotter than lava).
• The Shock: It's not just hot; it's being hit by invisible, hammer-like shockwaves that rattle the blades.

2. The Solution: The "Cooling Suit"

To save the blades, the team designed a system that sprays a thin layer of cool air over the blades, like a mist on a hot day. They tested two main parts of this suit:

Part A: The "Base" (Endwall Cooling)

The blades are attached to the floor and ceiling of the engine (the endwalls). These areas get the hottest because the heat gets trapped there.

• The Test: They compared two ways to spray the cooling air:
  1. Slot Holes: Long, narrow slits (like a window screen).
  2. Circular Holes: Small, round dots (like a showerhead).
• The Winner: The Circular Holes.
  • Why? Imagine trying to water a garden. A showerhead (circular holes) covers the ground evenly and uses less water than a long, leaky hose (slots). The researchers found that circular holes used 19% less cooling air but did the exact same job of keeping the metal cool. This saves fuel and makes the engine more efficient.

Part B: The "Helmet" (Leading-Edge Cooling)

The front tip of the blade (the leading edge) takes the first hit of the explosion. It needs special protection.

• The Test: They compared two angles for the cooling jets:
  1. Vertical: Shooting straight out, like a fountain.
  2. Vertical-Inclined: Shooting out at a slight angle, like a skier leaning into a turn.
• The Winner: The Vertical-Inclined scheme.
  • The Analogy: Imagine trying to keep a blanket on a windy day. If you hold it straight up (vertical), the wind blows it right off your shoulders. But if you lean it slightly forward (inclined), the wind actually pushes the blanket tighter against your body.
  • In the engine, the "wind" is the spinning explosion. The angled holes let the cool air "stick" to the blade better, forming a protective shield that doesn't get blown away by the shockwaves.

3. The "Surprise" Discovery

The researchers expected the spinning explosions to make cooling harder. They thought the chaos would blow the cool air away.

• The Twist: They found the opposite! The chaotic, pulsing nature of the explosion actually helped mix the cool air with the hot air better than a smooth, steady flow would. It's like how shaking a bottle of salad dressing mixes the oil and vinegar faster than just letting it sit. The explosion's "shake" helped the cooling suit work more effectively.

The Bottom Line

By combining the circular holes on the base and the angled holes on the front tip, the researchers created a "thermal armor" for the turbine blades.

• They dropped the temperature from a melting 3,000°C down to a survivable 2,000°C.
• They made the engine more stable and efficient.
• They proved that this new type of "explosion engine" can actually be built to last, opening the door for faster, more powerful aircraft and rockets in the future.

In short: They figured out how to keep a metal blade from melting while it's being pummeled by a ring of fire, using a clever mix of round holes and angled sprays.

Technical Summary

1. Problem Statement

The integration of Rotating Detonation Combustors (RDCs) with gas turbines offers significant advantages in thermodynamic efficiency and power density. However, the unique operating conditions of RDCs—characterized by continuous circumferential detonation waves, high-frequency shock wave impingement, and extreme thermal loads (peak heat fluxes exceeding 9–25 MW/m²)—pose severe challenges for downstream turbine components.

Specifically, turbine blades and endwalls are subjected to:

• Extreme Thermal Loads: Temperatures can exceed material melting points, leading to thermal fatigue.
• Complex Flow Dynamics: The interaction between oblique shock waves and turbine blades creates unsteady pressure gradients and localized high-temperature zones (e.g., horseshoe vortices).
• Cooling Inefficiency: Conventional film cooling strategies, often designed for steady-flow combustors, may fail under the pulsating, shock-dominated flow of an RDC.

There is a critical gap in understanding how to effectively protect turbine blades in a fully coupled 3D RDC-turbine system, particularly regarding the optimization of endwall and leading-edge cooling strategies under detonation flow conditions.

2. Methodology

The study employs three-dimensional, unsteady numerical simulations using ANSYS Fluent to analyze the coupled flow field of a hydrogen-air RDC and a downstream turbine stage.

• Computational Domain: A full-annular model consisting of an upstream RDC (40 mm length, 30–40 mm diameter) and a downstream turbine stator (12 blades, 10 mm height).
• Physics Models:
  • Combustion: Species transport model with a global one-step hydrogen-air reaction mechanism (validated against detailed chemistry and experimental thrust data).
  • Turbulence: Realizable $k-\epsilon$ turbulence model with non-equilibrium wall functions.
  • Solver: Density-based solver with second-order upwind spatial discretization and AUSM flux splitting.
• Cooling Configurations Tested:
  1. Endwall Cooling: Comparison between slot holes and circular holes positioned at the hub and tip to target horseshoe vortex regions.
  2. Leading-Edge Cooling: Comparison between vertical holes and vertical-inclined holes (combined scheme) on the blade leading edge.
• Boundary Conditions:
  • RDC inlet: Stoichiometric H₂/Air premixed gas (100 g/s, 300 K).
  • Turbine outlet: Pressure outlet (0.1 MPa).
  • Cooling inlets: Pressure or mass flow inlets with coolant temperatures of 300 K.
• Validation: The numerical setup was verified against experimental data for RDC thrust performance, detonation wave propagation speeds, and flat-plate film cooling efficiency benchmarks.

3. Key Contributions

This study makes several novel contributions to the field of RDC-turbine integration:

• First 3D Coupled Analysis: It provides a comprehensive 3D simulation of the transient interaction between rotating detonation waves and film-cooled turbine blades, moving beyond simplified 2D models.
• Optimization of Endwall Geometry: It systematically compares slot vs. circular holes for endwall cooling, identifying the optimal trade-off between cooling performance and air consumption.
• Leading-Edge Strategy under Pulsation: It evaluates vertical vs. vertical-inclined leading-edge cooling specifically under the unsteady, shock-driven environment of an RDC, revealing how detonation waves influence secondary flow attachment.
• Mechanism Insight: It elucidates how the upstream rotating detonation flow field actually facilitates the downstream diffusion of secondary film cooling jets, a counter-intuitive finding compared to steady-flow assumptions.

4. Key Results

A. Endwall Cooling Performance

• Circular vs. Slot Holes: While both configurations effectively reduce endwall temperatures, circular holes are superior. They consume 19.2% less cooling air than slot holes while maintaining comparable cooling efficiency (60.4% vs. 62.6%).
• Thermal Reduction: Endwall cooling successfully reduced the temperature of post-detonation products from >3000 K to <2000 K before they reached the turbine blades.
• Flow Stability: The cooling scheme mitigated intense pressure fluctuations and reduced shock wave formation between blades, enhancing overall flow stability.

B. Leading-Edge Cooling Performance

• Vertical vs. Vertical-Inclined: The vertical-inclined scheme demonstrated superior performance.
  • Steady Flow: It achieved higher average cooling efficiency (e.g., 82.24% vs. 80.86% at 3 g/s) and better secondary flow attachment (cold film height ~1.3d vs. 1.7d for vertical).
  • Unsteady Detonation Flow: Under the influence of rotating detonation waves, the vertical-inclined scheme showed greater stability. The vertical scheme suffered from "cold air accumulation" ahead of the shock wave, which was then swept away, causing efficiency drops in the wake regions. The inclined scheme maintained better adhesion, resulting in a higher average efficiency (50.32% vs. 45.66%).
• Optimal Mass Flow: A mass flow rate of 3 g/s was identified as optimal; increasing to 5 g/s caused flow detachment, reducing efficiency.

C. Combined Cooling Strategy

• The combination of circular endwall holes and vertical-inclined leading-edge holes provided comprehensive thermal protection.
• The detonation flow field was found to enhance the mixing and diffusion of secondary cooling jets into the mainstream, improving coverage in specific regions compared to steady-flow assumptions.

5. Significance

This research addresses a critical barrier to the practical engineering application of Rotating Detonation Engines. By demonstrating that specific film cooling geometries (circular endwall holes and inclined leading-edge holes) can effectively manage the extreme, unsteady thermal loads of an RDC-turbine system, the study:

1. Enables Durability: Provides a viable thermal protection strategy to prevent turbine blade failure due to thermal fatigue and melting.
2. Improves Efficiency: Reduces the parasitic loss of cooling air (by ~19% via circular holes), preserving overall engine cycle efficiency.
3. Guides Design: Offers specific design guidelines for the next generation of RDC-integrated gas turbines, moving beyond trial-and-error to physics-based optimization of cooling strategies in detonation environments.

In conclusion, the study validates that a carefully optimized film cooling system can successfully protect turbine components in the harsh environment of a hydrogen-fueled rotating detonation combustor, paving the way for more efficient and compact propulsion systems.