EPFL-GTT: Group of Thermal Turbomachinery

The research scope of GTT includes efforts to increase reliability, efficiency and power output of gas turbines and aero engines. The most important areas of research are the following: fluid-structure interaction at forced response and flutter for turbine and compressor blades, cooling techniques for gas turbine blades, flow control using plasma.

Numerical and experimental investigation of the shock-vane interaction in a transonic compressor

Stefan Kristukat

A numerical and experimental investigation of the shock-IGV (inlet guide vane) interaction in a transonic compressor is conducted at the Laboratoire de Thermique Appliquée et de Turbomachines (LTT) at the Ecole Polytechnique Fédérale de Lausanne (EPFL). The objective of the presented research work is to determine the impact of the upstream propagating shock pattern of an axial transonic compressor rotor on an aerodynamically loaded IGV and the influence of this interacting process on the compressor flow field for different operating points and aerodynamic setups. For the approach to the subject, a literature review shed light on the different research efforts over the past years. A large numerical study is conducted including the work with two different flow solvers. A numerical 3D transient analysis (commercial code) was conducted in order to provide information about the influence of the axial spacing on the overall mass flow through the stage. A numerical Q-3D transient analysis (industry owned code) on one streamline at a constant rotor inlet Mach number is performed in order to compare the shock-IGV interaction for different vane types similar to what is known in literature. A semi-analytical model was developed in order to evaluate in a fast approach the impact of a rotor bow shock on an upstream situated vane. The results of the numerical investigation can be summarized as following: The overall results of this investigation show that the upstream vane of a transonic compressor rotor transforms the rotor forcing function into an excitation with vane shape depending orientation and force. The time averaged flow field of the IGV does not show a measurable change of the exit flow angle due to the shock-IGV interaction for the studied operating points and axial distances. The 3D transient calculations do not show any influence of the examined axial spacing between vane and blade on the mass flow rate through the stage. The results of the Q3D analysis are compared with the semi-analytic approach and the measurements, both showing with some restrictions good accordance to the model. The results of the numerical investigation give an accurate prediction of the transient aerodynamic load of the upstream vane. An analytic tool was developed which uses three input parameters, the rotor relative inlet Mach number, the stagger angle of the rotor and the axial spacing between vane and rotor in order to provide an estimation of the amplitude of the pressure variation on the upstream situated vane. An extensive modification of the existing test facility of the LTT precedes the experimental study. The main steps of the approach are: A subsonic axial compressor test facility was modified to allow test runs at transonic rotor inlet conditions. A perfect leak tightness of the closed circuit was accomplished to enable its operation in heavy gas. Heavy gases like refrigerants have been used in many studies for compressor testing. While respecting certain restrictions during the use of heavy gas, a reduction of operation and manufacturing costs of the facility is beneficial. A high molecular weight of the gas reduces the sonic speed to the half of that of air thus reducing the dimension and the rotational speed of the rotor significantly and consequently reduces the demand in the material quality of the blading. A 1.5 stage transonic compressor had been designed and manufactured for the heavy gas (R134a) in a first step. Preliminary measurements showed that the influence of the downstream stator makes it difficult to separate the influence of shock-IGV interaction on the compressor flow field from loss generation mechanisms downstream of the rotor. Thus, the stator was removed and measurements were conducted in a one stage transonic compressor. Modifications on the construction of the testing setup are done in a way that the distance between IGV and rotor can be varied by means of spacers. Thus the flow can be determined for up to four different axial distances at varying rotational speed for different operating points. For transonic operation the time-dependent load of the IGV (inlet guide vane) due to the forcing function of the rotor bow shock was recorded with a high speed data acquisition system. The results of the experimental investigation can be summarized as following: No influence of the axial spacing on the efficiency and the mass flow was observed in the investigation. The operation characteristics remained equal between the minimum and the maximum vane-blade spacing. The measured global values of the stage are close to the values the design was made for, although the overall efficiency was generally lower than predicted. The experimental results of the pressure variation on the upstream vane agree well with the predicted values. The study in general showed a good matching between the numerical and the experimental investigation taking into account a rotor blade shape which is not optimized to the extend. The shock-vane interaction is responsible for an excitation of the entire vane blade. With decreasing distance between the rotor leading edge and vane trailing edge the excitation increases. The strength and orientation of the reflected shock/pressure wave part depends upon the vane shape. A more sophisticated design of the rotor blade shape should be in the focus of further investigations using this test facility at transonic rotor inlet conditions. Within this investigation a tool was developed in order to predict the load variation on the vane driven by the transient rotor forcing function. The tool maps the fact that the pressure on the vane surface oriented against rotational direction of the rotor exceeds the pressure of the passage shock due to the amplification effect of the reflection of the shock on a surface. Its possible application not only for compressors but also for turbine related topics should be an objective for similar investigations.

EPFL2008

Turbulent scale resolving modelling of rotating stall in low-pressure steam turbines operated under low volume flow conditions

Ivan William McBean, Benjamin Megerle, Peter Ott

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Unsteady computational fluid dynamics (CFD) has been applied to simulate the rotating stall phenomenon in two model turbines. It is shown that the investigated flow field presents a challenge to conventional Reynolds-averaged Navier–Stokes equations simulations. The modelling has been enhanced by applying scale-resolving turbulence modelling, which can simulate large-scale turbulent fluctuations. With this type of simulation a qualitative and quantitative agreement between CFD and measurement for the unsteady and time averaged flow field has been achieved. The results of the numerical investigation allow for a detailed insight into the dynamic flow field and reveal information on the nature of the excitation mechanism. It is concluded that the CFD approach developed can be used to assess LSMB blade designs prior to model turbine tests to check whether they are subjected to vibration under LVF

2015

Numerical and experimental investigation of the shock-vane interaction in a transonic compressor

Stefan Kristukat

EPFL2008