J. Turbomach. 2019;141(2):020201-020201-1. doi:10.1115/1.4042566.

This special issue of the Journal of Turbomachinery contains a collection of 11 papers originally presented at the 17th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery (ISROMAC 17) which was held on the island of Maui in December 2017. ISROMAC has been held biennially in Hawaii since 1985, and the conference has a long history of scientific exchange in all areas related to rotating machinery. The 16th and 17th Symposia focused on cross-disciplinary work across all scientific areas covered by the 30 forums of the conference.

Commentary by Dr. Valentin Fuster

Research Papers

J. Turbomach. 2019;141(2):021001-021001-8. doi:10.1115/1.4041036.

The ingestion and deposition of solid particulates within gas turbine engines has become a very significant concern for both designers and operators in recent times. Frequently aircraft are operated in environments where sand, ash, dust, and salt are present, which can drive damage mechanisms from long term component degradation to in-flight flame-out. Experiments are presented to assess deposition characteristics of sodium chloride (NaCl) at gas turbine secondary air system temperature conditions in horizontal pipe flow. Monodisperse NaCl particles were generated in the size range 2.0–6.5 µm, with gas temperatures 390–480 °C, and metal temperatures 355–730 °C. Two engine-representative surface roughnesses were assessed. An experimental technique for the measurement of deposited NaCl based on solution conductivity was developed and validated. Experiments were carried out under isothermal and nonisothermal/thermophoretic conditions. An initial experimental campaign was conducted under ambient and isothermal conditions; high temperature isothermal results showed good similarity. Under thermophoretic conditions, deposition rates varied by up to several orders of magnitude compared to isothermal rates.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021002-021002-7. doi:10.1115/1.4041672.

Objective of this paper is to analyze the consequences of borescope blending repairs on the aeroelastic behavior of a modern high pressure compressor (HPC) blisk. To investigate the blending consequences in terms of aerodynamic damping and forcing changes, a generic blending of a rotor blade is modeled. Steady-state flow parameters like total pressure ratio, polytropic efficiency, and the loss coefficient are compared. Furthermore, aerodynamic damping is computed utilizing the aerodynamic influence coefficient (AIC) approach for both geometries. Results are confirmed by single passage flutter (SPF) simulations for specific interblade phase angles (IBPA) of interest. Finally, a unidirectional forced response analysis for the nominal and the blended rotor is conducted to determine the aerodynamic force exciting the blade motion. The frequency content as well as the forcing amplitudes is obtained from Fourier transformation of the forcing signal. As a result of the present analysis, the change of the blade vibration amplitude is computed.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021003-021003-9. doi:10.1115/1.4041081.

This article presents a brief review of the experimental and theoretical state of the art regarding the leakage flow prediction of brush seals. The authors model a computational fluid dynamics (CFD)-based approach for the leakage flow of brush seals. The brush seal is treated by modeling its real geometrical structure, namely numerous bristles in an array in transverse flow. The fluid domain is segregated into discrete volumes surrounding each bristle. Two different discretization schemes are chosen to study their influence on the leakage behavior. Furthermore, for each scheme multiple inter-bristle distances, pressure ratios and turbulence models are evaluated. In addition, the influence of irregular arrangement configurations, which forms a quasi-chaotic inner structure, is studied. The results gained are compared to other authors' experimental and numerical data.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021004-021004-7. doi:10.1115/1.4041132.

For heat transfer measurements on the center blade of a linear cascade, the infrared measurement technique was set up. As a highly challenging condition, the angular dependency of the infrared signal was identified. Beside a shallow angle of view, limited by geometric conditions, the curved blade surface necessitated the consideration of this dependency. Therefore, a powerful in-situ calibration method was set up, which accounts for the angular dependency implicitly. In contrast to usual procedures, the correlation of the measured infrared intensity and the temperature was calibrated by a separate calibration function for each position on the blade. In all, three different calibration approaches were proceeded and assessed. Initial measurements in low-speed test conditions delivered physically more reasonable results, using a local calibration compared to a usual global calibration. By means of these data, an evaluation of the aerodynamic characteristic of the cascade was enabled. With few modifications, the procedure is capable to deliver high-precision heat transfer measurements in the high-speed cascade wind-tunnel at the Institute of Jet Propulsion.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021005-021005-11. doi:10.1115/1.4041380.

The unsteady blade row interaction (UBRI) is inherent and usually has a large effect on performance in multistage axial compressors. The effect could be considered by using the average-passage equation system (APES) in steady-state environment by introducing the deterministic correlations (DC). How to model the DC is the key in APES method. The primary purpose of this study is to develop a DC model for compressor routine design. The APES technique is investigated by using a 3D viscous unsteady and time-averaging Computational fluid dynamics (CFD) flow solver developed in our previous studies. Based on DC characteristics and its effects on time-averaged flow, an exponential decay DC model is proposed and implemented into the developed time-averaging solver. Steady, unsteady, and time-averaging simulations are conducted on the investigation of the UBRI and the DC model in the first transonic stage of NASA 67 and the first two stages of a multistage compressor. The DC distributions and mean flow fields from the DC model are compared with the unsteady simulations. The comparison indicates that the proposed model can take into account the major part of UBRI and provide significant improvements for predicting compressor characteristics and spanwise distributions of flow properties in axial compressors, compared with the steady mixing plane method.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021006-021006-9. doi:10.1115/1.4041082.

The aerodynamic impact of installing a horizontal pylon in front of a contra-rotating open rotor engine, at take-off, was studied. The unsteady interactions of the pylon's wake and potential field with the rotor blades were predicted by full-annulus URANS CFD calculations at 0 deg and 12 deg angle of attack (AoA). Two pylon configurations were studied: one where the front rotor blades move down behind the pylon (DBP), and one where they move up behind the pylon (UBP). When operating at 12 deg AoA, the UBP orientation was shown to reduce the rear rotor tip vortex sizes and separated flow regions, whereas the front rotor wake and vortex sizes were increased. In contrast, the DBP orientation was found to reduce the incidence variations onto the front rotor, leading to smaller wakes and vortices. The engine flow was also time-averaged, and the variation in work done on average midspan streamlines was shown to depend strongly on variation in incidence, along with a smaller contribution related to change of radius.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021007-021007-8. doi:10.1115/1.4041748.

Recent studies have shown that in a prewarming, respectively, warm-keeping operation of a steam turbine, the blades and vanes transport most of the heat to the thick-walled casing and rotor. Thereby, a thermal bottle-neck arises at the connection between the blade root and the rotor. The thermal contact resistance (TCR) at these interfaces affects the temperature distribution and thus the thermal stresses in the rotor. The present paper introduces an experimental setup, which is designed to quantify the TCR at the blade-rotor-connection of a steam turbine. An uncertainty analysis is presented, which proves that the average measurement uncertainties are less than one percent. The experiments especially focus on the investigation of the contact pressure, which is a function of the rotational speed. Therefore, the results of several steady-state measurements under atmospheric and evacuated atmosphere using a high temperature-resistant chromium-molybdenum steel are presented. For the evaluation of the TCR, a numerical model of the specimen is developed in addition to a simplified 1D approach. The results show a significantly increasing TCR with decreasing contact pressure, respectively, rotational speed.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021008-021009-8. doi:10.1115/1.4042284.

Experimental studies have been conducted on a modified T106 low pressure turbine (LPT) profile in an annular 1.5 stage axial turbine rig at the Chair of Thermal Turbomachines and Aeroengines, Ruhr-Universität Bochum. The rig setup allows the highly resolved measurement of unsteady wake–stator flow interaction in both space and time. Incoming wakes are generated by a variable-speed driven rotor equipped with cylindrical bars. In the present paper, an experimental approach to the investigation of unsteady phenomena is proposed. Time-averaged and instantaneous measurement data from 2D flow field traverses at the stator exit are provided for the analysis of the periodically unsteady vortex formation, displacement, and suppression. Additional time-accurate blade pressure data are used to study the relationship between the flow structures downstream of the stator row and the immediate intermittent wake impact on the blades. Bar wake kinematics is also discussed in relation to the observations.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021009-021008-9. doi:10.1115/1.4042283.

In this work, we present the results of the numerical investigations of periodic wake–secondary flow interaction carried out on a low pressure turbine (LPT) equipped with modified T106-profile blades. The numerical predictions obtained by means of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations using a k-ω-model have been compared with measurements conducted in the same configuration and showed a good agreement. Based on the verified numerical data, the Q-criterion has been employed to characterize the secondary flow structures and accurately identify their origin. An analysis of the fundamental wake kinematics and the unsteady vortex migration revealed dominant interaction mechanisms such as the circumferential fluctuation of the pressure side horseshoe vortex (HSV) and its direct interaction with the passage vortex (PV) and the concentrated shed vortex (CSV). Finally, a correlation with the total pressure loss coefficient is provided and a link to the incoming wake structures is given.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021010-021010-11. doi:10.1115/1.4042163.

Ultra-high bypass ratio (UHBR) engines are designed as compact as possible and are characterized by a short asymmetric air inlet and heterogeneous outlet guide vanes (OGVs). The flow close to the fan is therefore circumferentially nonuniform (or distorted) and the resulting noise might be impacted. This is studied here at take-off conditions by means of a simulation of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations of a full-annulus fan stage. The model includes an asymmetric air inlet, a fan, heterogeneous OGVs, and homogeneous inlet guide vanes (IGVs). Direct acoustic predictions are given for both inlet and aft noises. A novel hydrodynamic/acoustic splitting method based on a modal decomposition is developed and is applied for the aft noise analysis. The noise mechanisms that are generally considered (i.e., interaction of fan-blade wakes with OGVs and fan self-noise) are shown to be impacted by the distortion. In addition, new sources caused by the interaction between the stationary distortion and the fan blades appear and contribute to the inlet noise.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2019;141(2):021011-021011-9. doi:10.1115/1.4041806.

This investigation represents work on a model to determine heat flows on turbochargers. Recently, a power-based method has been developed to compare adiabatic and hot gas tests from radial turbines and compressors. Moreover, this method has shown the ability to correct standard measurements in terms of heat flows. In this investigation, a wastegate turbocharger has been investigated from a small gasoline engine. For validation purposes of the isentropic efficiencies, a conjugate-heat-transfer (CHT) simulation has been carried out on the turbine. Results have shown that isentropic efficiencies fit well for values of turbine inlet temperatures of 600 °C between corrected data and the simulation. For other temperatures, the differences between the determined values and CHT are greater. The differences rise with higher temperatures generally. So, the objective of the investigation is to improve the existing method for determining turbocharger heat transfers. Hence, an additional dependency of turbine inlet temperatures has been implemented in the approach and tested for T3 = 400 °C, 600 °C, 800 °C, and 950 °C. The modification has shown better results and smaller differences to CHT simulation. Especially, at low speeds where the former approach has had big differences, the modification improves the distribution for the investigated turbine inlet temperatures.

Commentary by Dr. Valentin Fuster

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