Computational fluid dynamics simulation of surge in a three stage axial compressor

A three dimensional unsteady Reynolds-Averaged Navier-Stokes solver was used to perform multistage unsteady simulations of a three and half stage compressor. Previously published research presented the simulation of the same compressor with mal-scheduling of the variable stator vanes (VSV) and predicted a rotating stall pattern in all stages. The nominal VSV schedule compressor was simulated to provide a reference case for comparison purposes. The compressor’s behaviour in the nominal case seemed to behave against conventional wisdom with mass flow and pressure fluctuations representing compressor surge. However further analysis showed that inherent design feature in the compressor which had a highly loaded third stator was the primary cause of surge initiation, a situation which was eased with mal-scheduling by unloading that stator. Full analysis of the simulation results of the nominal case and discussion are presented in this paper

Improved modelling capabilities of the airflow within turbine case cooling systems using smart porous media

Impingement cooling is commonly employed in gas turbines to control the turbine tip clearance. During the design phase, Computational Fluid Dynamics is an effective way of evaluating such systems but for most Turbine Case Cooling (TCC) systems resolving the small scale and large number of cooling holes is impractical at the preliminary design phase. This paper presents an alternative approach for predicting aerodynamic performance of TCC systems using a “smart” porous media to replace regions of cooling holes. Numerically (CFD) defined correlations have been developed, which account for geometry and local flow field, to define the porous media loss coefficient. These are coded as a user defined function allowing the loss to vary, within the calculation, as a function of the predicted flow and hence produce a spatial variation of mass flow matching that of the cooling holes. The methodology has been tested on various geometrical configurations representative of current TCC systems and compared to full cooling hole models. The method was shown to achieve good overall agreement whilst significantly reducing both the mesh count and the computational time to a practical level

Effects of blade damage on the performance of a transonic axial compressor rotor

Gas turbine axial compressor blades may encounter damage during service for various reasons. Debris from casing or foreign objects may impact blades causing damage near the rotor’s tip. This may result in deterioration of performance and reduction in the surge margin. Ability to assess the effect of damaged blades on the compressor performance and stability is important at both the design stage and in service. The damage to compressor blades breaks the cyclic symmetry of the compressor assembly. Thus computations have to be performed using the whole annulus. Moreover, if rotating stall or surge occurs, the downstream boundary conditions are not known and simulations become difficult. This paper presents an unsteady CFD analysis of compressor performance with tip curl damage. Tip curl damage typically occurs when rotor blades hit a loose casing liner. The computations were performed up to the stall boundary, predicting rotating stall patterns. The aim is to assess the effect of blade damage on stall margin and provide better understanding of the flow behaviour during rotating stall. Computations for the undamaged rotor are also performed for comparison. A transonic axial compressor rotor is used for the time-accurate numerical unsteady flow simulations, with a variable choked nozzle downstream simulating an experimental throttle. One damaged blade was introduced in the rotor assembly and computations were performed at 60% of the design rotational speed. It was found that there is no significant effect on the compressor stall margin due to one damaged blade despite the differences in rotating stall patterns between the undamaged and damaged assemblies

Numerical investigation of VSVs mal-schedule effects in a three-stage axial compressor

Variable Stator Vanes (VSVs) are commonly used in multi-stage axial compressors for stage matching at part load operations and during start up. Improper VSVs settings or malfunction of the controlling actuator system can lead to compressor instabilities including rotating stall and surge. It is important to be able to predict the aerodynamic behaviour of compressors in such events to either produce tolerant designs or incorporate diagnosis and recovery systems. This paper presents a numerical study of a compressor operating near the stall boundary for a mal-scheduled VSVs case. A high-speed three-stage axial compressor with Inlet Guide Vanes (IGV) is used in the investigation because of its relative simplicity and availability of geometry and aerodynamic data. A 3D RANS viscous unsteady time-accurate flow solver was used to perform the full annulus simulation with a downstream variable nozzle to control outflow boundary conditions. The unstructured mesh contained about 25 million grid points and the simulation was performed on a high performance computing cluster for many engine rotations. Rotating stall with one single cell covering several passages in all three rotors was predicted which propagated at approximately half of the shaft speed. Full analysis of the flow features is presented in the paper