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J. Turbomach. 2015;137(12):121001-121001-9. doi:10.1115/1.4031447.

An evaluation of the effect of freestream turbulence intensity on the rate of deposit accumulation for nozzle guide vanes (NGVs) was performed using the turbine reacting flow rig (TuRFR) accelerated deposition facility. The TuRFR allowed flows up to 1350 K at inlet Mach numbers of 0.1 to be seeded with coal fly ash particulate in order to rapidly evaluate deposit formation on CFM56 NGVs. Hot film and particle image velocimetry (PIV) measurements were taken to assess the freestream turbulence with and without the presence of a grid upstream of the NGVs. It was determined that baseline turbulence levels were approximately half that of the flow exiting typical gas turbine combustors and were reduced by approximately 30% with the grid installed. Deposition tests indicated that the rate of deposition increases as the freestream turbulence is increased, and that this increase depends upon the particle size distribution. For ash with a mass median diameter of 4.63 μm, the increase in capture efficiency was approximately a factor of 1.77, while for ash with a larger median diameter of 6.48 μm, the capture efficiency increased by a factor of 1.84. The increase in capture efficiency is due to the increased diffusion of particles to the vane surface via turbulent diffusion. Based on these results, smaller particles appear to be less susceptible to this mechanism of particle delivery. Overall, the experiments indicate that the reduction of turbulence intensity upstream of NGVs may lead to reduced deposit accumulation, and consequently, increased service life. A computational fluid dynamics (CFD) analysis was performed at turbulence levels equivalent to the experiments to assess the ability of built-in particle tracking models to capture the physics of turbulent diffusion. Impact efficiencies were shown to increase from 21% to 73% as the freestream turbulence was increased from 5.8% to 8.4%. An analysis incorporating the mass of the particles into the impact efficiency resulted in an increase of the mass-based impact efficiency from 17% to 27% with increasing turbulence. Relating these impact efficiencies directly to capture efficiencies, the predicted increase in capture efficiency with higher turbulence is less than that observed in the experiments. In addition, the variation in the impact efficiencies between the two ash sizes was smaller than the capture efficiency difference from experiments. This indicates that the particle tracking models are not capturing all of the relevant physics associated with turbulent diffusion of airborne particles.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121002-121002-11. doi:10.1115/1.4031341.

Detailed mapping of the sound field produced by a modern turbofan engine, with its multitude of overlapping noise sources, often requires a large number of microphones to properly resolve the directivity patterns of the constituent tonal and broadband components. This is especially true at high frequencies where the acoustic wavelength is short, or when shielding, scattering, and reflection of the sound field may be present due to installation effects. This paper presents a novel method for measuring the harmonic and broadband content of complex noncompact noise sources using continuously moving (referred to here as continuous-scan (CS)) microphones in conjunction with a state-of-the-art phase-referencing technique. Because the microphones are moving through the sound field produced by the noise sources, they effectively provide infinite spatial resolution of the sound directivity over the scan path. In this method, harmonic (i.e., shaft-coherent) content at the integer multiples of the instantaneous shaft rotational frequency is first extracted from the time signal using a tachometer signal and the Vold-Kalman (VK) filter. The residual broadband signal is then filtered in the time domain in fractional octave bands. The broadband spectra of the signals from the moving microphones are then computed at arbitrary positions along their scan paths using weighted averages (based on Chebyshev polynomial zero-crossings) and the assumption of a complex envelope that varies slowly over a spatial scale whose lower bound is set by the acoustic wavenumber. A benefit of this method is that the decomposition of the total measured sound field into a stochastic superposition of components preserves a meaningful phase definition for each “partial field” associated with a given shaft order (SO). This preservation of phase data enables the forward or backward projection of each of these partial fields using acoustical holography (AH). The benefits of the CS method are demonstrated using acoustic data acquired for a 22-in. scale-model fan stage run at the NASA Glenn Research Center's 9-foot by 15-foot wind tunnel. Two key outcomes of the work include (1) significant improvement in the spatial resolution of the measured sound field and (2) reduction in the overall data acquisition time. Additionally, the methods described here lead to new opportunities for noise source diagnostics and visualization.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121003-121003-10. doi:10.1115/1.4031355.

Conduction in thin disks can be modeled using the fin equation, and there are analytical solutions of this equation for a circular disk with a constant heat-transfer coefficient. However, convection (particularly free convection) in rotating-disk systems is a conjugate problem: the heat transfer in the fluid and the solid are coupled, and the relative effects of conduction and convection are related to the Biot number,  Bi, which in turn is related to the Nusselt number. In principle, if the radial distribution of the disk temperature is known then Bi  can be determined numerically. But the determination of heat flux from temperature measurements is an example of an inverse problem where small uncertainties in the temperatures can create large uncertainties in the computed heat flux. In this paper, Bayesian statistics are applied to the inverse solution of the circular fin equation to produce reliable estimates of Bi for rotating disks, and numerical experiments using simulated noisy temperature measurements are used to demonstrate the effectiveness of the Bayesian method. Using published experimental temperature measurements, the method is also applied to the conjugate problem of buoyancy-induced flow in the cavity between corotating compressor disks.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121004-121004-12. doi:10.1115/1.4031464.

For efficient and accurate unsteady flow analysis of blade row interactions, a space–time gradient (STG) method has been proposed. The development is aimed at maintaining as many modeling fidelities (the interface treatment in particular) of a direct unsteady time-domain method as possible while still having a significant speed-up. The basic modeling considerations, main method ingredients and some preliminary verification have been presented in Part I of the paper. Here in Part II, further case studies are presented to examine the capability and applicability of the method. Having tested a turbine stage in Part I, here we first consider the applicability and robustness of the method for a three-dimensional (3D) transonic compressor stage under a highly loaded condition with separating boundary layers. The results of the STG solution compare well with the direct unsteady solution while showing a speed up of 25 times. The method is also used to analyze rotor–rotor/stator–stator interferences in a two-stage turbine configuration. Remarkably, for stator–stator and rotor–rotor clocking analyses, the STG method demonstrates a significant further speed-up. Also interestingly, the two-stage case studies suggest clearly measurable clocking dependence of blade surface time-mean temperatures for both stator–stator clocking and rotor–rotor clocking, though only small efficiency variations are shown. Also validated and illustrated is the capacity of the STG method to efficiently evaluate unsteady blade forcing due to the rotor–rotor clocking. Considerable efforts are directed to extending the method to more complex situations with multiple disturbances. Several techniques are adopted to decouple the disturbances in the temporal terms. The developed capabilities have been examined for turbine stage configurations with inlet temperature distortions (hot streaks), and for three blade-row turbine configurations with nonequal blade counts. The results compare well with the corresponding direct unsteady solutions.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121005-121005-9. doi:10.1115/1.4031356.

The effects of blade deformation under running conditions on the performance of a highly loaded transonic mixed flow impeller were investigated. Two impellers were manufactured, one using the “running” blade profiles as designed and one using the converted “unrunning” or “cold” geometry. Both impellers were tested experimentally and investigated numerically. The test data taken with smooth casing showed that at maximum speed, the isentropic efficiency and pressure ratio of the running geometry was higher than the unrunning geometry by about 0.4% and 1.4%, respectively. However, the difference in performance diminished in the presence of recirculating casing treatment. Numerical calculations suggested that the differences at high speeds were mainly due to the variation in the impeller tip clearance. The calculations using deformed blade profiles under centrifugal load only, predicted performance differences which were about twice as high as the measured values. However, closer predictions were obtained when the effects of pressure loads on blade deformation were included using closely coupled fluid-structural analyses.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121006-121006-9. doi:10.1115/1.4031267.

This study investigates on heat transfer enhancement in pin fin cooling channels. Experiments are conducted in a staggered pin fin array consisting of 15 rows. Heat transfer measurements are conducted in the pin fin cooling channel using the transient liquid crystal technique. The reference temperature is approximated by the fluid bulk temperature, acquired by thermocouples at specific positions. Thermal inertia of the used thermocouples is considered. One other problem that occurs while using relatively long thermocouples in short aspect ratio ducts is the heat conduction along the wires, the so-called stem effect. This can lead to erroneous temperature measurements. The impact of the thermocouple immersion length on the temperature measurement is investigated. A detailed assessment of the space and timewise varying temperature distribution is conducted for the appropriate reference temperature. This paper gives an overview about the experimental setup and the used transient measurement technique. Results are represented in terms of temperature distribution, heat transfer distribution, and averaged Nusselt number at the endwall.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(12):121007-121007-10. doi:10.1115/1.4031424.

Multiple high-fidelity time-accurate computational fluid dynamics simulations were performed to investigate the effects of upstream stator loading and rotor shock strength on vortex shedding characteristics in a single-stage transonic compressor. Three loadings on the upstream stator row of decreased, nominal, and increased loading in conjunction with three axial spacings of close, mid, and far were studied for this analysis. The time-accurate urans code turbo was used to generate periodic, quarter annulus simulations of the blade row interaction (BRI) compressor rig. It was observed that vortex shedding was synchronized to the passing of a rotor bow shock. Results show that vortex strength increases linearly with stator loading and rotor bow shock strength. “Normal” and “large” shock-induced vortices formed on the stator trailing edge (TE) immediately after the shock passing, but the large vortices were strengthened at the TE due to a low-velocity region on the suction surface. This low-velocity region was generated upstream on the suction surface from a shock-induced thickening of the boundary layer or separation bubble. The circulation of the large vortices was greater than the normal vortices by a factor of 1.7, 1.83, and 2.04 for decreased, nominal, and increased deswirler loadings. At decreased loading, only 24% of the measured vortices were considered large, while at nominal loading 58% were large. A model was developed to predict shock-induced vortex circulation from a known rotor bow shock strength, stator diffusion factor, and near-wake parameters. The model predicts the average vortex circulation very well, with 5% difference between predicted and measured values. An understanding of the unsteady interactions associated with blade loading and rotor shock strength in transonic stages will help compressor designers account for unsteady flow physics at design and off-design operating conditions.

Commentary by Dr. Valentin Fuster

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