Zixiang Sun

Efficient Finite Element Analysis/Computational Fluid Dynamics Thermal Coupling for Engineering Applications

An efficient finite element analysis/computational fluid dynamics (FEA/CFD) thermal coupling technique has been developed and demonstrated. The thermal coupling is achieved by an iterative procedure between FEA and CFD calculations. Communication between FEA and CFD calculations ensures continuity of temperature and heat flux. In the procedure, the FEA simulation is treated as unsteady for a given transient cycle. To speed up the thermal coupling, steady CFD calculations are employed, considering that fluid flow time scales are much shorter than those for the solid heat conduction and therefore the influence of unsteadiness in fluid regions is negligible. To facilitate the thermal coupling, the procedure is designed to allow a set of CFD models to be defined at key time points/intervals in the transient cycle and to be invoked during the coupling process at specified time points. To further enhance computational efficiency, a “frozen flow” or “energy equation only” coupling option was also developed, where only the energy equation is solved, while the flow is frozen in CFD simulation during the thermal coupling process for specified time intervals. This option has proven very useful in practice, as the flow is found to be unaffected by the thermal boundary conditions over certain time intervals. The FEA solver employed is an in-house code, and the coupling has been implemented for two different CFD solvers: a commercial code and an in-house code. Test cases include an industrial low pressure (LP) turbine and a high pressure (HP) compressor, with CFD modeling of the LP turbine disk cavity and the HP compressor drive cone cavity flows, respectively. Good agreement of wall temperatures with the industrial rig test data was observed. It is shown that the coupled solutions can be obtained in sufficiently short turn-around times (typically within a week) for use in design.

Efficient FEA/CFD Thermal Coupling for Engineering Applications

An efficient finite element analysis/computational fluid dynamics (FEA/CFD) thermal coupling technique has been developed and demonstrated. The thermal coupling is achieved by an iterative procedure between FEA and CFD calculations. Communication between FEA and CFD calculations ensures continuity of temperature and heat flux. In the procedure, the FEA simulation is treated as unsteady for a given transient cycle. To speed up the thermal coupling, steady CFD calculations are employed, considering that fluid flow time-scales are much shorter than those for the solid heat conduction and therefore the influence of unsteadiness in fluid regions is negligible. To facilitate the thermal coupling, the procedure is designed to allow a set of CFD models to be defined at key time points/intervals in the transient cycle and to be invoked during the coupling process at specified time points. To further enhance computational efficiency, a “frozen flow” or “energy equation only” coupling option was also developed, where only the energy equation is solved while the flow is frozen in CFD simulation during the thermal coupling process for specified time intervals. This option has proven very useful in practice, as the flow is found to be unaffected by the thermal boundary conditions over certain time intervals. The FEA solver employed is an in-house code, and the coupling has been implemented for two different CFD solvers; a commercial code and an in-house code. Test cases include an industrial low pressure (LP) turbine and a high pressure (HP) compressor, with CFD modeling of the LP turbine disc cavity and the HP compressor drive cone cavity flows, respectively. Good agreement of wall temperatures with the industrial rig test data was observed. It is shown that the coupled solutions can be obtained in sufficiently short turn-around times (typically within a week) for use in design.Copyright

Numerical Simulation of Natural Convection in Stationary and Rotating Cavities

In compressor inter-disc cavities with a central axial throughflow it is known that the flow and heat transfer is strongly affected by buoyancy in the centrifugal force field. As a step towards developing CFD methods for such flows, buoyancy-driven flows under gravity in a closed cube and under centrifugal force in a sealed rotating annulus have been studied. Numerical simulations are compared with the experimental results of Kirkpatrick and Bohn (1986) and Bohn et al (1993). Two different CFD codes have been used and are shown to agree for the stationary cube problem. Unsteady simulations for the stationary cube show good agreement with measurements of heat transfer, temperature fluctuations, and velocity fluctuations for Rayleigh numbers up to 2 × 1010 . Similar simulations for the rotating annulus also show good agreement with measured heat transfer rates. The CFD results confirm Bohn et al’s results, showing reduced heat transfer and a different Rayleigh number dependency compared to gravity-driven flow. Large scale flow structures are found to occur, at all Rayleigh numbers considered.Copyright

Use of CFD for Thermal Coupling in Aeroengine Internal Air Systems Applications

With the rapid progress of computational fluid dynamics (CFD) and computer technology, CFD has been increasingly used for aero-engine component temperature predictions. This paper presents a review of the latest progress in this aspect with emphasis on internal air system applications. The thermal coupling methods discussed include the traditional finite element analysis (FEA), the conjugate heat transfer, FEA/CFD coupling procedure and other thermal coupling techniques. Special attention is made to identify the merits and disadvantages between the various methodologies. Discussion is further extended on the steady and transient thermal coupling applications.

Coupled Aerothermal Modeling of a Rotating Cavity With Radial Inflow

Flow and heat transfer in an aero-engine compressor disc cavity with radial inflow has been studied using computational fluid dynamics (CFD), large eddy simulation (LES) and coupled fluid/solid modelling. Standalone CFD investigations were conducted using a set of popular turbulence models along with 0.2° axisymmetric and a 22.5° discrete sector CFD models. The overall agreement between the CFD predictions is good, and solutions are comparable to an established integral method solution in the major part of the cavity. The LES simulation demonstrates that flow unsteadiness in the cavity due to the unstable thermal stratification is largely suppressed by the radial inflow. Steady flow CFD modelling using the axisymmetric sector model and the Spalart-Allmaras turbulence model was coupled with a finite element (FE) thermal model of the rotating cavity. Good agreement was obtained between the coupled solution and rig test data in terms of metal temperature. Analysis confirms that use of a small radial bleed flow in compressor cavities is effective in reducing thermal response times for the compressor discs and that this could be applied in management of compressor blade clearance.Copyright

3D coupled fluid-solid thermal simulation of a turbine disc through a transient cycle

Thermal analysis of a turbine disc through a transient test cycle is demonstrated using 3D computational fluid dynamics (CFD) modeling for the cooling flow and 3D finite element analysis (FEA) for the disc. The test case is a 3D angular sector of the high pressure (HP) turbine assembly of a civil jet engine and includes details of the coolant flow around the blade roots. Proprietary FEA and CFD solvers are used to simulate the metal and fluid domains, respectively. Coupling is achieved through an iterative loop with smooth exchange of information between the FEA and CFD simulations at each time step, ensuring consistency of temperature and heat flux on the coupled interfaces between the metal and fluid domains. The coupled simulation can be completed within a few weeks using a PC cluster with multiple parallel CFD executions. The FEA/CFD coupled result agrees well with corresponding rig test data and the baseline 3D and 2D FEA solutions, which have been calibrated using test data. Provision of upstream boundary conditions and modeling of rapid transients are identified as areas of uncertainty. Averaging of CFD solutions and relaxation is used to overcome difficulties caused by CFD oscillations associated with flow unsteadiness. The present work supports the continued use and development of the FEA/CFD coupling method for industrial applications.Copyright

Coupled Aero-Thermal Modeling of a Rotating Cavity with Radial Inflow

Flow and heat transfer in an aero-engine compressor disc cavity with radial inflow has been studied using computational fluid dynamics (CFD), large eddy simulation (LES) and coupled fluid/solid modelling. Standalone CFD investigations were conducted using a set of popular turbulence models along with 0.2° axisymmetric and a 22.5° discrete sector CFD models. The overall agreement between the CFD predictions is good, and solutions are comparable to an established integral method solution in the major part of the cavity. The LES simulation demonstrates that flow unsteadiness in the cavity due to the unstable thermal stratification is largely suppressed by the radial inflow. Steady flow CFD modelling using the axisymmetric sector model and the Spalart-Allmaras turbulence model was coupled with a finite element (FE) thermal model of the rotating cavity. Good agreement was obtained between the coupled solution and rig test data in terms of metal temperature. Analysis confirms that use of a small radial bleed flow in compressor cavities is effective in reducing thermal response times for the compressor discs and that this could be applied in management of compressor blade clearance.Copyright

Velocity pick-up and discharge coefficient for round orifices with cross flow at inlet:

This study investigates the flow through round orifices with cross flow at the inlet. Emphasis is placed on the change in tangential velocity component for orifices with low L/D. A definition of velocity pick-up is developed based on the orifice exit to cross-flow tangential velocity ratio. Steady, incompressible, and 3D computational fluid dynamics models with the SST k - ω turbulent model are employed to calculate orifice flows with different geometrical and flow conditions. Stationary orifices and axial orifices in a rotating disk are considered. Computational fluid dynamics solutions are compared with experimental results and published correlations for discharge coefficients, and good agreement is generally demonstrated. It is found that the non-dimensional velocity pick-up depends strongly on the ratio of characteristic times for flow to travel across the orifice in the tangential direction and that for the flow to pass through the orifice. A correlation of velocity pick-up as a function of this ratio is given.

Coupled Aero-Thermo-Mechanical Simulation for a Turbine Disc Through a Full Transient Cycle

Use of computational fluid dynamics (CFD) to model the complex, 3D disk cavity flow and heat transfer in conjunction with an industrial finite element analysis (FEA) of turbine disk thermomechanical response during a full transient cycle is demonstrated. The FEA and CFD solutions were coupled using a previously proposed efficient coupling procedure. This iterates between FEA and CFD calculations at each time step of the transient solution to ensure consistency of temperature and heat flux on the appropriate component surfaces. The FEA model is a 2D representation of high pressure and intermediate pressure (IP) turbine disks with surrounding structures. The front IP disk cavity flow is calculated using 45 deg sector CFD models with up to 2.8 million mesh cells. Three CFD models were initially defined for idle, maximum take-off, and cruise conditions, and these are updated by the automatic coupling procedure through the 13,000 s full transient cycle from stand-still to idle, maximum take-off, and cruise conditions. The obtained disk temperatures and displacements are compared with an earlier standalone FEA model that used established methods for convective heat transfer modeling. It was demonstrated that the coupling could be completed using a computer cluster with 60 cores within about 2 weeks. This turn around time is considered fast enough to meet design phase requirements, and in validation, it also compares favorably to that required to hand-match a FEA model to engine test data, which is typically several months. [DOI: 10.1115/1.4003242]

Numerical Simulation of Complex Air Flow in an Aeroengine Gear Box

The internal gear box (IGB) of an aeroengine represents a severe challenge in computational fluid dynamics (CFD). In the present study, an axisymmetric CFD model was assessed to investigate the complex internal air flow in an aeroengine IGB. All the non-axisymmetric components and geometry features inside the gear box, such as bearings, gears, bolts and slots, as well as the radial drive system and vent pipes, were simulated using porous media models. Their flow resistance was estimated either by empirical correlations or by preparatory CFD studies and comparison with measurements. To evaluate the CFD technique adopted in the present investigation, a separate bolt windage study was conducted using a similar axisymmtric CFD model with the porous media approach. Good agreement of the bolt windage with other workers’ rig test data was observed. The present application of the porous media approach into a complex gear box flow represents a first attempt to use state of art CFD to assist an industrial design. Both maximum take-off (MTO) and ground idle (GI) running conditions were investigated. The complex flow patterns in the gear box were obtained. The results show a similar dimensionless performance of intermediate pressure (IP) and high pressure (HP) gears between the two operating conditions. For the present gear box arrangement under investigation, the CFD results suggest that the airflows induced by the HP gear and HP bearing are higher than their IP counterparts. A comparison with power absorption rig test data for the similar HP crownwheel in isolation shows that an assumption of pressure loss coefficient of 10 for the porous media of bevel gears may be appropriate, as the HP gear torque coefficient obtained in the CFD prediction is equal to 0.05, very close to its expected value. In addition, the effects of an assumed stationary IP gear and a large seal clearance on the HP gear performance were also investigated. The numerical results show that their impacts are insignificant, probably due to the strong pumping effects of the HP gear. Further discussion on the possible influence of the airflow on the oil motion within the gearbox and assistance to improve the traditional internal airflow models used for bearing chamber sealing analysis was also made. Three dimensional geometry modeling and inclusion of the oil phase are considered feasible. Such further investigations would aid the understanding of the interaction between the induced airflow due to the rotating components and oil motion, and their impact on oil scavenging behaviour and ‘windage’ contribution to heat oil.Copyright