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### Research Papers

J. Turbomach. 2016;138(10):101001-101001-11. doi:10.1115/1.4033016.

Effusion cooling is one of the most effective and widespread techniques to prevent combustor liner from being damaged. However, most recent developments in combustion techniques, resulting from increasingly stricter air pollution regulations, have highlighted the necessity of reducing the amount of air available for effusion cooling while keeping an adequate level of protection. Adoption of compound angles in effusion cooling is increasingly recognized by jet engine manufacturers as a powerful solution to meet new combustor requirements. Therefore, understanding the flow behavior and developing methods able to provide accurate estimates of wall temperatures is of a major importance. This study assesses the capability of a high-level Reynolds-averaged Navier–Stokes (RANS) method, differential Reynolds stress model (DRSM), in conjunction with a generalized gradient diffusion hypothesis (GGDH), and of a hybrid RANS–large eddy simulations (LES) method, zonal detached eddy simulation (ZDES), to predict overall film effectiveness. Both approaches are compared with the experimental data from Zhang et al. (2009, “Cooling Effectiveness of Effusion Walls With Deflection Hole Angles Measured by Infrared Imaging,” Appl. Therm. Eng., 29(5), pp. 966–972) and with a classical well-known RANS model (k–ω shear-stress transport (SST) model). Despite the fact that some discrepancies are found, both approaches have proved suitable and reliable for predicting wall temperatures (relative errors of about 5%). Moreover, a new method to deal with ZDES length scales for unstructured grids is proposed. ZDES applicability and its general advantages and drawbacks are also discussed. Finally, an in-depth analysis of the film structure is carried out on the basis of the ZDES simulations. The principal structures are identified (an asymmetric main vortex (AMV) and a counter rotating vortex pair, CRVP), and the film formation mechanisms are presented. Significant spanwise-homogeneous distributions of surface overall film cooling effectiveness are observed.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101002-101002-9. doi:10.1115/1.4033163.

For an unpowered turbofan in flight, the airflow through the engine causes the fan to freewheel. This paper considers the flow field through a fan operating in this mode, with emphasis on the effects of blade row losses and deviation. A control volume analysis is used to show that windmilling fans operate at a fixed flow coefficient which depends on the blade metal and deviation angles, while the blade row losses are shown to determine the fan mass flow rate. Experimental and numerical results are used to understand how the loss and deviation differ from the design condition due to the flow physics encountered at windmill. Results are presented from an experimental study of a windmilling low-speed rig fan, including detailed area traverses downstream of the rotor and stator. Three-dimensional computational fluid dynamics (CFD) calculations of the fan rig and a representative transonic fan windmilling at a cruise flight condition have also been completed. The rig test results confirm that in the windmilling condition, the flow through the fan stator separates from the pressure surface over most of the span. This generates high loss, and the resulting blockage changes the rotor work profile leading to modified rotational speed. In the engine fan rotor, a vortex forms at the pressure surface near the tip and further loss results from a hub separation caused by blockage from the downstream core and splitter.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101003-101003-8. doi:10.1115/1.4032280.

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centerline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely, the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ. To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra1) at which the drag rose above the smooth blade case was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested. The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101004-101004-11. doi:10.1115/1.4033186.

The operation during compressor surge of a medium speed marine diesel engine was examined on a test bed. The compressor of the engine's turbocharger was forced to operate beyond the surge line, by injecting compressed air at the engine intake manifold, downstream of the compressor during steady-state engine operation. While the compressor was surging, detailed measurements of turbocharger and engine performance parameters were conducted. The measurements included the use of constant temperature anemometry for the accurate measurement of air velocity fluctuations at the compressor inlet during the surge cycles. Measurements also covered engine performance parameters such as in-cylinder pressure and the impact of compressor surge on the composition of the exhaust gas emitted from the engine. The measurements describe in detail the response of a marine diesel engine to variations caused by compressor surge. The results show that both turbocharger and engine performance are affected by compressor surge and fast Fourier transform (FFT) analysis proved that they oscillate at the same main frequency. Also, prolonged steady-state operation of the engine with this form of compressor surge led to a non-negligible increase of NOx emissions.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101005-101005-11. doi:10.1115/1.4032924.

External deposition on a slot film cooled nozzle guide vane, subjected to nonuniform inlet temperatures, was investigated experimentally and computationally. Experiments were conducted using a four-vane cascade, operating at temperatures up to 1353 K and inlet Mach number of approximately 0.1. Surveys of temperature at the inlet and exit planes were acquired to characterize the form and migration of the hot streak. Film cooling was achieved on one of the vanes using a single spanwise slot. Deposition was produced by injecting sub-bituminous ash particles with a median diameter of 6.48 μm upstream of the vane passage. Several deposition tests were conducted, including a baseline case, a hot streak-only case, and a hot streak and film cooled case. Results indicate that capture efficiency is strongly related to both the inlet temperature profiles and film cooling. Deposit distribution patterns are also affected by changes in vane surface temperatures. A computational model was developed to simulate the external and internal flow, conjugate heat transfer, and deposition. Temperature profiles measured experimentally at the inlet were applied as thermal boundary conditions to the simulation. For deposition modeling, an Eulerian–Lagrangian particle tracking model was utilized to track the ash particles through the flow. An experimentally tuned version of the critical viscosity sticking model was implemented, with predicted deposition rates matching experimental results well. Comparing overall deposition rates to results from previous studies indicates that the combined effect of nonuniform inlet temperatures and film cooling cannot be accurately simulated by simple superposition of the two independent effects; thus, inclusion of both conditions in experiments is necessary for realistic simulation of external deposition.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101006-101006-10. doi:10.1115/1.4032905.

This paper presents the first detailed experimental performance data for a new centrifugal process compressor test rig. Additional numerical simulations supported by extensive pressure measurements at various positions allow an analysis of the operational and loss behavior of the entire stage and its components. The stage investigated is a high flow rate stage of a single-shaft, multistage compressor for industrial applications and consists of a shrouded impeller, a vaneless diffuser, a U-bend, and an adjoining vaned return channel. Large channel heights due to high flow rates induce the formation of highly three-dimensional flow phenomena and thus enlarge the losses due to secondary flows. An accurate prediction of this loss behavior by means of numerical investigations is challenging. The published experimental data offer the opportunity to validate the used numerical methods at discrete measurement planes, which strengthens confidence in the numerical predictions. CFD simulations of the stage are initially validated with global performance data and extensive static pressure measurements in the vaneless diffuser. The comparison of the pressure rise and an estimation of the loss behavior inside the vaneless diffuser provide the basis for a numerical investigation of the flow phenomena in the U-bend and the vaned return channel. The flow acceleration in the U-bend is further assessed via the measured two-dimensional pressure field on the hub wall. The upstream potential field of the return channel vanes allows an evaluation of the resulting flow angle. Measurements within the return channel provide information about the deceleration and turning of the flow. In combination with the numerical simulations, loss mechanisms can be identified and are presented in detail in this paper.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101007-101007-11. doi:10.1115/1.4032925.

Detailed heat transfer coefficient (HTC) and film cooling effectiveness (Eta) distribution on a squealer-tipped first stage rotor blade were measured using an infrared technique. The blade tip design, obtained from the Solar Turbines, Inc., gas turbine, consists of double purge hole exits and four ribs within the squealer cavity, with a bleeder exit port on the pressure side close to the trailing edge. The tests were carried out in a transient linear transonic wind tunnel facility under land-based engine representative Mach/Reynolds number. Measurements were taken at an inlet turbulent intensity of Tu = 12%, with exit Mach numbers of 0.85 (Reexit = 9.75 × 105) and 1.0 (Reexit = 1.15 × 106) with the Reynolds number based on the blade axial chord and the cascade exit velocity. The tip clearance was fixed at 1% (based on engine blade span) with a purge flow blowing ratio, BR = 1.0. At each test condition, an accompanying numerical study was performed using Reynolds-averaged Navier–Stokes (RANS) equations solver ansys fluent to further understand the tip flow characteristics. The results showed that the tip purge flow has a blocking effect on the leakage flow path. Furthermore, the ribs significantly altered the flow (and consequently heat transfer) characteristics within the squealer-tip cavity resulting in a significant reduction in film cooling effectiveness. This was attributed to increased coolant–leakage flow mixing due to increased recirculation within the squealer cavity. Overall, the peak HTC on the cavity floor increased with exit Mach/Reynolds number.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101008-101008-9. doi:10.1115/1.4032926.

The present work is part of a detailed aerothermal investigation in a model of a rotating internal cooling channel performed in a novel facility setup which allows test conditions at high rotation numbers ($Ro$). The test section is mounted on a rotating frame with all the required instrumentation, resulting in a high spatial resolution and accuracy. The channel has a cross section with an aspect ratio of 0.9 and a ribbed wall with eight ribs perpendicular to the main flow direction. The blockage of the ribs is 10% of the channel cross section, whereas the rib pitch-to-height ratio is 10. In this first part of the paper, the flow over the wall region between the sixth and seventh ribs in the symmetry plane is investigated by means of two-dimensional particle image velocimetry (PIV). Tests were carried out at a Reynolds number ($Re$) of 15,000 in static and rotating conditions, with a maximum $Ro$ of 0.77. Results are in good agreement with the data present in literature at the same Reynolds number and with rotation numbers of 0 (static conditions) and 0.38 in a channel with the same geometry as in the present work. When $Ro$ is increased from 0.38 to 0.77, the main velocity and turbulence fields show important changes. At a rotation number of 0.77, although the extension of the recirculation bubble after the sixth rib on the trailing side does not vary significantly, it covers the full inter-rib area on the leading side in the streamwise direction. The turbulence intensity on the leading side shows a low value with respect to the static case but roughly at the same level as in the lower $Ro$ case. On the trailing side, the maximum value of the turbulence intensity slightly decreases from $Ro$  = 0.38 to $Ro$  = 0.77, the wall shear layer is restabilized along the second half of the pitch due to the high rotation, and the secondary flows are redistributed causing spanwise vortex compression. The observed result is the rapid decay of turbulent fluctuations in the second half of the inter-rib area.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2016;138(10):101009-101009-10. doi:10.1115/1.4032927.

The present two-part work deals with a detailed characterization of the flow field and heat transfer distribution in a model of a rotating ribbed channel performed in a novel setup which allows test conditions at high rotation numbers (Ro). The tested model is mounted on a rotating frame with all the required instrumentation, resulting in a high spatial resolution and accuracy. The channel has a cross section with an aspect ratio of 0.9 and a ribbed wall with eight ribs perpendicular to the main flow direction. The blockage of the ribs is 10% of the channel cross section, whereas the rib pitch-to-height ratio is 10. In this second part of the study, the heat transfer distribution over the wall region between the sixth and seventh ribs is obtained by means of liquid crystal thermography (LCT). Tests were first carried out at a Reynolds number of 15,000 and a maximum Ro of 1.00 to evaluate the evolution of the heat transfer with increasing rotation. On the trailing side (TS), the overall Nusselt number increases with rotation until a limit value of 50% higher than in the static case, which is achieved after a value of the rotation number of about 0.3. On the leading side (LS), the overall Nusselt number decreases with increasing rotation speed to reach a minimum which is approximately 50% of the one found in static conditions. The velocity measurements at Re= 15,000 and Ro= 0.77 provided in Part I of this paper are finally merged to provide a consistent explanation of the heat transfer distribution in this model. Moreover, heat transfer measurements were performed at Reynolds numbers of 30,000 and 55,000, showing approximately the same trend.

Commentary by Dr. Valentin Fuster