PCA will be manning Booth #209 at the ASME Turbo Expp in San Antonio, Texas, June 3rd to 7th 2013. Chris Robinson, Mick Casey, Hamid Hazby and Ian Woods will be attending the conference and will be pleased to welcome visitors to the booth.
PCA staff are involved in a number of technical papers as lead and supporting authors:-
INVESTIGATION OF EFFICIENT CFD METHODS FOR THE PREDICTION OF BLADE DAMPING
R. Elder, I. Woods, S. Patil, W. Holmes, R. Steed, B. Hutchinson
ABSTRACT Accurate and efficient prediction of blade damping is one essential element in the engineering of durable and reliable compressors and turbines. Over the years, a variety of empirical and linearised methods have been developed and used, and have served well. Recently, the development of efficient unsteady CFD methods combined with an expansion in available and affordable computing power has enabled CFD analysis of blade damping. This paper looks at the prediction of aerodynamic blade damping using some recently developed CFD methods.
Unsteady CFD methods are used to predict the fluid flow in a transonic fan rotor, with tip Mach number of about 1.4. Deformation of the blade is determined from a mechanical pre-stressed modal analysis. In this investigation, blade motion for the first bending moments is prescribed in the CFD code, for a range of nodal diameters. After periodic unsteady solutions are obtained, damping coefficients are calculated based on the predicted blade forces and the specified blade motion.
Traditional unsteady CFD methods require the simulation of many blades in a given row, depending on the nodal diameter. For instance, for a nodal diameter of four, a wheel with 22 blades would require simulation of eleven blades. Computational methods have been developed which now enable simulation of only a few (1 or 2) blades per row yet yield the full sector solution, thus providing considerable savings in computing time and machine resources. The properties of the available methods vary, but one method, the Fourier Transformation method, has the property that it is frequency preserving, and hence suitable for the present task.
Fourier Transformation predictions, for a variety of nodal diameters, are compared with full sector predictions. Positive damping was predicted for this range of nodal diameters at design speed near peak efficiency operating condition indicating a stable system. The Fourier Transformation predictions for blade aerodynamic damping match very closely the reference full sector solutions. The Fourier transformation methods also provide solutions 3.5 times faster than average periodic reference cases.
A ZONAL CALCULATION METHOD FOR AXIAL GAS TURBINE DIFFUSERS
A.Hirschmann, M. Casey, M. Montgomery
The axial exhaust downstream of a gas turbine in a combined cycle plant includes an annular diffuser with struts and a closed hub carrying the turbine rotor bearing. Flow separation occurs at the blunt end of the hub and can also occur on the casing wall and on the struts. A zonal method for the computation of the flow in such diffusers is described. A throughflow code is used for the axisymmetric core flow, a lag-entrainment-integral-method for the blockage of the boundary layers and the wake of the hub. For cases with separated flow a semi-inverse procedure for the coupling is needed. Additional empirical information is required such as a term related to the closing of the hub separation, correlations for the skin friction and the form factor and an estimate of the losses based on a dissipation coefficient.
Experimental data from annular diffuser test cases from the literature and from a typical turbine diffuser are used to validate the method. The location of the separation in the diffuser is calculated correctly and the prediction of the pressure recovery is promising, suggesting that this is a useful tool for the preliminary design of highly loaded annular configurations.
A NEW PARTIAL ADMISSION METHOD FOR TURBOCHARGER TURBINE CONTROL AT OFF-DESIGN
Sebastian Challand, Mirko Ilievski, Michael Casey, Markus Schatz
A new method for turbocharger control in automotive applications is presented. It is called MEDUSA (Multiple Exhaust Duct with Source Adjustment, European patent application number: 21326 - EP ) and is a partial admission system consisting of several separate flow channels that connect the exhaust duct of the engine and individual nozzle segments of the turbine. By opening or closing the individual flow channels using external valves, the turbine flow can be adjusted, hence allowing the whole turbocharger to be controlled. Due to the use of external valves, the system is considerably more robust than other variable geometry systems based on variable inlet guide vanes and thus becomes suitable for application to spark-ignition motors at high temperature.
The paper presents a theoretical assessment of this innovative control system, based on one dimensional considerations and CFD simulations. The CFD-calculations of the MEDUSA-system are compared to those of a turbocharger turbine controlled with a variable inlet nozzle. The results indicate that the performance and operating range of the new system is comparable, or even better, than the currently used variable nozzle systems, especially at low load conditions. This indicates that further experimental work is justified as it could become considerably more effective than the typical waste gate systems used in spark ignition engines and provides a new solution for the turbocharger control in these applications. So far, only radial turbines have been considered for application of this method but it could also be used for mixed-flow or axial turbines.
LOW-FLOW-COEFFICIENT CENTRIFUGAL COMPRESSOR DESIGN FOR SUPERCRITICAL CO2
C. Lettieri, N. Baltadjiev, M. Casey, Z. Spakovszky
This paper presents a design strategy for very low flow coefficient multi-stage compressors operating with supercritical CO2 for Carbon Capture and Sequestration (CCS) and Enhanced Oil Recovery (EOR). At flow coefficients less than 0.01 the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance.
The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale re-injection compressor currently in service. Three-dimensional, steady, supercritical CO2 CFD simulations of the full stage with leakage flows are carried out with the NIST real gas model. The design study suggests that a non-dimensional impeller exit width parameter b2*=(b2/R)ϕ of 6 yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the non-dimensional impeller exit width parameter b2* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO2, the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.
GT2013 – 95179
ON KINEMATIC RELAXATION AND DEPOSITION OF WATER DROPLETS IN THE LAST STAGES OF LOW PRESSURE STEAM TURBINES
J. Starzmann, P. Kaluza and M. V. Casey, Frank Sieverding
In the first part of the paper steady two-phase flow predictions have been performed for the last stage of a model steam turbine to examine the influence of drag between condensed fog droplets and the continuous vapour phase. In general, droplets due to homogeneous condensation are small and thus kinematic relaxation provides only a minor contribution to the wetness losses. Different droplet size distributions have been investigated to estimate at which size inter-phase friction becomes more important.
The second part of the paper deals with the deposition of fog droplets on stator blades. Results from several references are repeated to introduce the two main deposition mechanisms which are inertia and turbulent diffusion. Extensive post-processing routines have been programmed to calculate droplet deposition due to these effects for a last stage stator blade in three-dimensions. In principle the method to determine droplet deposition by turbulent diffusion equates to that of Yau and Young  and the advantages and disadvantages of this relatively simple method are discussed.
The investigation includes the influence of different droplet sizes on droplet deposition rates and shows that for small fog droplets turbulent diffusion is the main deposition mechanism. If the droplets size is increased inertial effects become more and more important and for droplets around 1 µm inertial deposition dominates. Assuming realistic droplet sizes the overall deposition equates to about 1% to 3% of the incoming wetness for the investigated guide vane at normal operating conditions.
PREDICTION OF FLOW IN AN EXHAUST GAS TURBINE DIFFUSER WITH A SCALE-ADAPTIVE-SIMULATION MODEL
Silke Volkmer, Markus Schatz, Michael Casey, Matthew Montgomery
The prediction of the flow in a gas turbine exhaust diffuser of a combined cycle power plant is particularly difficult as maximum performance is obtained with highly loaded diffusers, which operate close to boundary layer separation. CFD (computational fluid dynamics) simulations then need to cope with complex phenomena such as smooth wall separation, recirculation, reattachment, blockage and free shear layer mixing. Recent studies based on the RANS (Reynolds-Averaged-Navier-Stokes) approach demonstrate the challenge for two-equation turbulence models to predict separation and mixing of the flow correctly in such highly loaded diffusers and identify that more accurate methods are needed. Hence, the application of a hybrid Scale-Adaptive Simulation (SAS) is investigated and the CFD results are compared with experimental results from an in-house test rig.
In the present study the flow in a model exhaust diffuser (for heavy-duty gas turbine diffuser applications typical Reynolds number 1.5x106 and inlet Mach number 0.6) is examined with unsteady RANS (URANS) simulations with the SST (Shear Stress Transport) model as well as a hybrid Scale Adaptive Simulation (SAS) model. The SAS model switches from URANS to a mode similar to a Large Eddy Simulation (LES) in unsteady flow regions to resolve various scales of detached eddies. The current study shows that with the SST model similar results are obtained with RANS and URANS simulations, whereas the more complex SAS model leads to a much better resolution of the unsteady fluctuations. However, the time-averaged results of the SAS calculations overpredict the blockage of the separation and hub wake. This results in an underprediction of the pressure recovery and the mixing of the flow compared to the simpler two-equation models and also compared to experimental results.