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### Research Papers

J. Turbomach. 2016;138(11):111001-111001-9. doi:10.1115/1.4033259.

The present activity was carried out in the framework of the Clean Sky European Research Project ITURB (optimal high-lift turbine blade aeromechanical design), aimed at designing and validating a turbine blade for a geared open-rotor engine. A cold-flow, large-scale, low-speed (LS) rig was built in order to investigate and validate new design criteria, providing reliable and detailed results while containing costs. This paper presents the design of an LS stage and describes a general procedure that allows to scale three-dimensional (3D) blades for LS testing. The design of the stator row was aimed at matching the test-rig inlet conditions and at providing the proper inlet flow field to the blade row. The rotor row was redesigned in order to match the performance of the high-speed (HS) configuration, compensating for both the compressibility effects and different turbine flow paths. The proposed scaling procedure is based on the matching of the 3D blade loading distribution between the real engine environment and the LS facility one, which leads to a comparable behavior of the boundary layer and hence to comparable profile losses. To this end, the datum blade is parameterized, and a neural-network-based methodology is exploited to guide an optimization process based on 3D Reynolds-averaged Navier–Stokes (RANS) computations. The LS stage performance was investigated over a range of Reynolds numbers characteristic of modern low-pressure turbines (LPTs) by using a multi-equation, transition-sensitive, turbulence model. Some comparisons with experimental data available within the project finally proved the effectiveness of the proposed scaling procedure.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111002-111002-12. doi:10.1115/1.4033260.

Large-amplitude deep surge instabilities are studied in a turbocharger compression system with a one-dimensional (1D) engine simulation code. The system consists of an upstream compressor duct open to ambient, a centrifugal compressor, a downstream compressor duct, a large plenum, and a throttle valve exhausting to ambient. As the compressor mass flow rate is reduced below the peak pressure ratio for a given speed, mild surge oscillations occur at the Helmholtz resonance of the system, and a further reduction in flow rate results in deep surge considerably below the Helmholtz resonance. At the boundary with mild surge, the deep surge cycles exhibit, for the particular system considered, a long cycle period containing four distinct flow phases, including quiet (stable), instability growth (mild surge), blowdown (reversal), and recovery. Further reductions in flow rate decrease the deep surge cycle period, eliminate the quiet flow phase, and shorten the duration of the instability growth phase. Simulated oscillations of nondimensional flow rate, pressure, and speed parameters show good agreement with the experimental results available in literature, in terms of deep surge cycle flow phases along with the amplitude and frequency of the resulting fluctuations. The predictions illustrate that the quiet and instability growth phases, exhibited by this compression system, disappear as the plenum volume is substantially reduced.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111003-111003-11. doi:10.1115/1.4033262.

Centrifugal blowers are widely used for gas compression in a variety of industrial fields; however, a wider operating range is required in these machines. Investigations on the generation mechanism of unsteady flow (i.e., surge) are very important to improve the operating range. The purpose of this study is to clarify the generation mechanism of pressure fluctuations in a multistage centrifugal blower equipped with inlet guide vanes (IGVs) upstream during the first stage under the IGVs partially open condition. These pressure fluctuations occur at flowrates when the slope of the total system head curve is steeply negative. According to our previous study on the detailed unsteady pressure measurements, this pressure oscillation is supposed to be the mild surge caused by the positive slope of the head curves at the second to the last stages. The slope of the total system head curve was kept negative due to the steeply negative slope of the head curve during the first stage. Thus, the whole compression system seemed to be stable. To confirm the validity of this hypothesis, system dynamic simulations based on Greitzer's lumped-parameter model were conducted using newly measured static pressure-rise characteristic curves of each stage in a four-stage centrifugal blower. In these simulations, the pressure-rise characteristic curves of the first stage and the second to last stages were modeled as two different actuator disks, and the stabilization/destabilization effects of each stage on the system dynamic characteristics were separately taken into account under the IGVs partially open condition. The system dynamic simulation reproduced the mild surge behavior of the system under the IGVs partially open condition when the slope of the total system head curve was still kept steeply negative. The calculated amplitude and frequency of the pressure fluctuations caused by the mild surge showed satisfactory agreement with the measured ones. However, the inception flowrate of the system instability in the simulation was approximately 7% smaller than that in the measurement. From these results, we confirmed that the pressure fluctuation occurred under the IGVs partially open condition was caused by the mild surge due to the positive slope of the pressure-rise characteristic during the second to last stage. In addition, we found that this mild surge was caused by the stall of the vaned diffusers during the second to last stage.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111004-111004-12. doi:10.1115/1.4033162.

In many industrial areas, downsizing the pumping system is a decisive aim of the designers. The reasons could be multiple means; in a single-stage pump, increasing the power density of the pump means actually reducing the production costs. The main goal of this study was the comparison in terms of power density of a conventionally designed single-stage pump with a novel design concept based on the counter-rotating (CR) principle. In order to simplify the experimental investigations for the present study, the volute geometry was fixed instead of reducing the pump outflow diameters for a fixed design point. The energy concentration was then increased by raising the developed hydraulic power within the same envelope. The design of the impellers was carried out with an in-house design tool, based on inverse design method. Numerical results highlight the advantageousness of the new layout, in terms of power concentration, compared to the conventional impeller. Numerical predictions are also in significant agreement with the experimental investigation results, obtained in a specifically developed CR motors test rig. The experimental optimization of the rotational speed ratio of the CR impellers has shown the possibility to further increase the head in off-design condition and thereby the pump power density.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111005-111005-9. doi:10.1115/1.4033264.

This study deals with the studies of the effect of double flow-control devices (DFCDs) on turbine vane film cooling. Aiming for improving film effectiveness, two semispheroid DFCDs per pitch were attached to the vane surface upstream of the cooling hole. Although the DFCDs were successfully applied to the flat-plate film cooling in the previous study, the applicability to the turbine vane was to be investigated. In order to observe the flow field in detail, Reynolds-averaged Navier–Stokes (RANS) simulation was conducted first. The DFCDs were installed upstream of each cooling hole of the pressure and suction sides of the vane to investigate the effect of the device position. In this paper, the effects of blowing ratio and cooling hole pitch were also investigated. The results obtained by CFD showed that the vortex generated from DFCD suppressed lift-off of the secondary air. As a result, the film effectiveness became significantly higher than that without DFCD condition. Moreover, the improvement in the film effectiveness by DFCD was observed by both of the pressure and suction sides of the turbine vane. Based on the findings through RANS simulation, adiabatic effectiveness and total pressure loss coefficient measurement were performed in a linear cascade test facility. The experiment confirmed that the film effectiveness was improved when DFCDs existed.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111006-111006-12. doi:10.1115/1.4033266.

Large eddy simulations validated with the aid of direct numerical simulation (DNS) are used to study the concerted action of reduced frequency and flow coefficient on the performance of the T106A low-pressure turbine profile. The simulations are carried out by using a discretization in space and time that allows minimizing the accuracy loss with respect to DNS. The reference Reynolds number is 100,000, while reduced frequency and flow coefficient cover a range wide enough to provide valid qualitative information to designers. The various configurations reveal differences in the loss generation mechanism that blends steady and unsteady boundary layer losses with unsteady wake ingestion losses. Large values of the flow coefficient can alter the pressure side unsteadiness and the consequent loss generation. Low values of the flow coefficient are associated with wake fogging and reduced unsteadiness around the blade. The reduced frequency further modulates these effects. The simulations also reveal a clear trend of losses with the wake path, discussed by conducting a loss-breakdown analysis that distinguishes boundary layer from wake distortion losses.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111007-111007-10. doi:10.1115/1.4033267.

The high pressure (HP) rotor tip and over-tip casing are often life-limiting features in the turbine stages of current gas turbine engines. This is due to the high thermal load and high temperature cycling at both low and high frequencies. In the last few years, there have been numerous studies of turbine tip heat transfer. Comparatively fewer studies have considered the over-tip casing heat transfer. This is in part, no doubt, due to the more onerous test facility requirements to validate computational simulations. Because the casing potential field is dominated by the passing rotor, to perform representative over-tip measurements a rotating experiment is an essential requirement. This paper details the measurements taken on the Oxford turbine research facility (OTRF), an engine-scale rotating turbine facility which replicates engine-representative conditions of Mach number, Reynolds number, and gas-to-wall temperature ratio. High density arrays of miniature thin-film heat-flux gauges were used with a spatial resolution of 0.8 mm and temporal resolution of ∼120 kHz. The small size of the gauges, the high frequency response, and the improved processing methods allowed very detailed measurements of the heat transfer in this region. Time-resolved measurements of $TAW$ and $Nu$ are presented for the casing region (−30% to +125% $CAX$) and compared to other results in the literature. The results provide an almost unique data set for calibrating computational fluid dynamics (CFD) tools for heat transfer prediction in this highly unsteady environment dominated by the rotor over-tip flow.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111008-111008-10. doi:10.1115/1.4033292.

This paper presents an experimental investigation on the performances of a new film cooling structure design, in which a ramp is placed upstream of a cylindrical film hole and a cylindrical cavity with two diagonal impingement holes is set at the inlet of the film hole to generate a swirling coolant flow entering the film hole. The experiments are carried out by two undisturbed measurement techniques, planar laser induced fluorescence (PLIF) and time-resolved particle image velocimetry (TR-PIV) in a water tunnel. The effects of the upstream ramp angle, blowing ratio (BR), and coolant impingement angle on the film cooling performances of a flat plate are studied at three ramp angles (0 deg, 15 deg, and 25 deg), two coolant swirling directions (clockwise and counterclockwise), two impingement angles (15 deg and 30 deg), and three BRs (0.6, 1.0, and 1.4). The experimental results show that at high BRs, the combination structures of the upstream ramp with the swirling coolant flow generated by the impingement angles can significantly improve film cooling performances; the best combination is at a 30 deg impingement angle and a 25 deg ramp angle. This can be explained by the fact that the swirling flow is significantly pressed on to the wall by means of the upstream ramp. Using the analogous analysis of heat and mass transfer, the adiabatic film effectiveness averaged over a cross section is obtained; the analysis indicates that at high BRs, the combined effect of a ramp with a large angle of 25 deg with 30 deg impingement angle can increase the film effectiveness up to 30% when compared to the test case without a ramp at the exit of the film hole. The images captured by PLIF exhibit an interesting phenomenon, i.e., the swirling of the coolant in different directions can influence the counter vortex pair (CVP) in rotating layers, and the coolant swirling in a clockwise direction enhances the right mixing of the CVP with coolant ejection, whereas the coolant swirling in a counterclockwise direction enhances the left-mixing of the CVP with coolant ejection.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(11):111009-111009-15. doi:10.1115/1.4033463.

A direct numerical simulation (DNS) of spanwise-rotating turbulent channel flow was conducted for four rotation numbers: $Rob=0$, 0.2, 0.5, and 0.9 at a Reynolds number of 8000 based on laminar centerline mean velocity and Prandtl number 0.71. The results obtained from these DNS simulations were utilized to evaluate several turbulence closure models for momentum and heat transfer transport in rotating turbulent channel flow. Four nonlinear eddy viscosity turbulence models were tested and among these, explicit algebraic Reynolds stress models (EARSM) obtained the Reynolds stress distributions in best agreement with DNS data for rotational flows. The modeled pressure–strain functions of EARSM were shown to have strong influence on the Reynolds stress distributions near the wall. Turbulent heat flux distributions obtained from two explicit algebraic heat flux models (EAHFM) consistently displayed increasing disagreement with DNS data with increasing rotation rate.

Commentary by Dr. Valentin Fuster