As one of the key components of aircraft engines, turbine impellers experience extremely complex thermal loads under high-speed flight. Specifically at high-speed and high-altitude flight, turbine impellers must withstand the superposition of high-temperature gas impact and centrifugal force. Their temperature field distribution, thermal stress, and thermal deformation features have a direct relation to engine performance and structural safety of the impeller.
Introduction
New aircraft powerplants, especially turbofan engines with high performance, typically operate under extremely harsh conditions. As a central part with high-temperature and high-pressure gas and energy conversion, turbine impeller working environment temperature is typically above 1,400°C, and the rotational speed is 10,000 to 20,000 rpm. High-speed rotation and high thermal loads cause turbine impellers to face extreme thermal stress and thermal deformation. Besides being generated as a result of gas effect, thermal loads also closely correlate with centrifugal force, frictional action, and radiative heat transfer. Therefore, thermal load simulation and analysis of turbine impellers are of very important reference for improving the safety, reliability, and life of turbine impeller design.
Sources and Distribution Characteristics of Thermal Loads on Turbine Impellers
Turbine impellers are constantly under the coupling effect of the complicated thermal field and strong aerodynamic force in the high-temperature working state of aviation engines and gas turbines. The thermal loads not only come from numerous sources but also have extremely unbalanced distribution characteristics. The existence of these thermal loads will have a direct impact on the thermal stress distribution, material thermal fatigue life, and structural stability of the impeller. Therefore, careful scrutiny of their sources and modes of delivery is a valuable foundation on which to maximize cooling design, optimize material performance, and extend service life.
Main Sources of Thermal Loads
(1) Dominant Role of High-Temperature Gas Impact and Convective Heat Transfer
Highest thermal load on turbine impellers is provided by the high-speed and high-temperature gas emitted by the engine combustion chamber. When the gas passes through the first-stage high-pressure turbine, its thermal and kinetic energy are imparted directly onto the surface of the impeller by convective heat transfer, especially building continuous thermal (shock) on the windward side of the blade. Temperature rise on the blade surface is collectively determined by gas temperature, mass flow rate, heat transfer coefficient, and contact time, with the temperature difference some hundreds of degrees Celsius, and thus the major cause of heat responsible for non-uniform thermal expansion and thermal stress concentration.
(2) Temperature Diffusion Caused by Axial Heat Conduction
Not only are blades surface-heated but also transfer heat from the root to the disk through thermal conduction. During repeated heating, heat progressively diffuses axially to the disk mounting region. Under the condition of long-running high-power engines especially, this thermal conduction diffusion effect will cause the temperature at the hub region to progressively increase with an axial gradient field. This thermal buildup not only increases the disk’s thermal stress but also affects the material’s structural stability.
(3) High-Temperature Radiative Heat Transfer Strengthens Local Thermal Accumulation
In addition to convective heat transfer, the process of radiative heat transfer should not be neglected under high-temperature situations. Vast infrared radiative energy caused by combustion gases and the combustion chamber wall will irradiate the blades. Especially in the middle and tip areas of the windward side of the blade, due to the extensive surface area exposed to radiation and high radiative heat transfer coefficient, local thermal focusing is bound to occur, resulting in thermal spots and concentration of stress.
(4) Frictional Heat and Eddy Current Boundary Layer Effects
During the high-speed rotation, the frictional relative motion between the disk edge and blade trailing edge will generate heat to some extent. Special in the rotor system’s micro-clearance, non-uniform friction fit is likely to form local hot spots. The fluid shear layer and wake eddy zone near the disk can also cause the local thermal accumulation due to fluid viscous dissipation, which will aggravate the complex thermal condition.
Distribution Characteristics of Thermal Loads
(1) Significant Axial Temperature Gradient, with Tip Temperature Higher Than Root
Since the incoming air initially hits the tip area, and the tip is far from the disk cooling passage, its heat dissipating ability is comparatively weak, and thus a distinct temperature rising tendency from root to tip. This temperature gradient distribution is not conducive to the structure of the blade. Especially when the thermal expansion direction is the same as the centrifugal rotation, it would cause structure deformation, vibration unbalance, or even fatigue crack propagation.
(2) Asymmetry Between Hot and Cold Surfaces Leads to Imbalance in Thermal Stress Distribution
Under the asymmetric heat transfer conditions, the windward side of the blade is long-term exposed to the high-temperature gas impact, whose temperature is significantly higher than that of the leeward surface, thus inducing a transverse thermal stress gradient. The asymmetry of stress will form tensile-compressive cyclic fatigue stress at the blade root, cooling hole outlet, or air film covering layer edge and become a source of high-cycle fatigue failure.
(3) Cooling Structure Directly Affects the Uniformity of Thermal Load Distribution
Advanced turbine blades typically employ complex integrated cooling channel structures (e.g., serpentine channel, impingement chamber, rib cooling, etc.) to reduce thermal load peaks. Reasonable distribution density and diameter design of cooling holes can reduce the temperature of local overheating areas by 100～200℃ and considerably improve uniformity of the thermal field. Practices have shown that the regulation effect on the temperature difference field by internal cooling structures is far stronger than by external surface spray cooling, and the distribution laws are important references for subsequent thermal stress simulation.
Construction of Thermal Load Simulation Model
In an attempt to profoundly investigate the thermal load distribution law borne by turbine impellers in high-temperature environments and thermal stress evolution mechanism caused by them, this paper establishes a complete thermal-structural coupling simulation model using the finite element method. The model not only considers the multi-source thermal boundary effects under real service conditions but also introduces the nonlinear variation law of the material properties at high temperature to ensure the engineering applicability and physical reasonableness of the simulation results.
Simulation Object and Basic Assumptions
(1) Setting of Impeller Geometric Model
The current paper selects a specific type of single-stage high-pressure turbine impeller as the object of simulation. Its composition includes a disk, blade root, blades, and internal cooling channels, whose global form is that of an ordinary free-form surface structure. In the simulation, such minor geometric details as tip air film holes are not taken into consideration to increase the efficiency of the calculation but without losing the essential cooling channels to genuinely represent the inner heat transfer process.
(2) Material Parameters and Thermophysical Properties
Single-crystal Ni-based superalloy CMSX-4 is selected as the material model with creep strength at high temperature. Material parameters in simulation change dynamically with temperature and include especially:
- Thermal conductivity (λ): Decreases from 15 W/m·K to 9 W/m·K with increasing temperature;
- Thermal expansion coefficient (α): 15.1×10⁻⁶/K at 1000℃;
- Elastic modulus (E): Considering the softening effect caused by temperature, the E value gradually decreases from 180 GPa to 100 GPa;
- Creep behavior: A time-dependent material model based on the Norton-Bailey formula is employed to simulate plastic flow of the blades at constant temperature and elevated stress.
Boundary Conditions and Load Setting
(1) External Thermal Load
The gas flow acts on the blade surface as convective heat transfer, the heat transfer coefficient is 1200~1800 W/m²·K and the temperature is 1350°C. This range of values is referenced to the actual working conditions of high-pressure section in general aviation engine. The heat transfer boundary is mainly imposed on the blade windward surface, tip, trailing edge, and other areas.
(2) Internal Cooling Boundary
It directs cooling air into internal channels of the blade. The cooling air temperature is 650°C and the heat transfer coefficient is 900~1100 W/m²·K, covering the surface of the internal cooling cavity and cooling channel surface to form a two-way temperature difference drive of “hot outside and cold inside”.
(3) Rotation and Initial Conditions
The impeller rotational speed is held constant at 15,000 rpm, considering the influence of centrifugal force and thermal-mechanical coupling on rotation during simulation. The global model temperature is set to 1000°C as an initial temperature condition to simulate the thermal (shock) condition of sudden loads or cold start during conventional steady-state operation.
Numerical Methods and Simulation Process
(1) Modeling and Mesh Generation
The platform of ANSYS Workbench is employed for 3D modeling and analysis. Following geometric importation, a hybrid structured-unstructured mesh strategy is employed, and local encryption calculation is performed in local stress concentration regions like the blade tip, root, and cooling channel corners. The mesh quantity is regulated to around 1.5 million elements to achieve the balance between simulation accuracy and computational efficiency.
(2) Transient Thermal-Structural Coupling Simulation
The transient analysis module is started to model the thermal (shock) process within the time span of 0~30 seconds, and synchronous monitoring of the temperature field distribution and stress reaction of all key parts is carried out. Thermal field solution results are applied as the initial load of structural analysis to achieve full coupling solution of multi-physical fields. In the meantime, by using the automatic control of the time step and the convergence criterion of the nonlinear solver, stable analysis of high-temperature nonlinear behaviors (such as material softening and creep) at any time during the process of simulation is ensured.
(3) Model Verification and Parameter Sensitivity Analysis
To verify the reliability of the model, this paper compares certain simulation results with experimental thermal fatigue test data. They correlate well in temperature rise curve, thermal deformation trend, and stress peak position. Furthermore, through the capability of adjusting boundary parameters such as gas temperature and rotating speed, the sensitivity of blade temperature field and thermal stress field is investigated, providing a quantitative reference for subsequent structural optimization and cooling system adjustment.
Conclusion
The turbine impellers have complex thermal load problems at high-speed working conditions, and the development laws of their temperature field, thermal stress, and thermal deformation directly affect the safety of the engine and impeller life. This paper has studied thoroughly the thermal load behavior of turbine impellers in high-speed operation by numerical simulation methods and presents improvement schemes through optimal cooling design and material selection.
