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### Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2017;139(8):081501-081501-12. doi:10.1115/1.4035734.

This paper reports the particle image velocimetry (PIV) measurement of swirling flow in a confined rectangular-shaped model combustion chamber. Water is used as the working fluid, and the average profiles of axial, radial, and magnitudes of velocity are given. Flow behavior is investigated and a rebound angle term is defined to investigate the direct effects of the noncircular chamber shape. Flow behavior near the walls is discussed in detail, as are other important swirling flow features such as the appearance of corner and central toroidal recirculation zones. Additionally, experimental data are compared with simulation results. Analyses were performed via commercial software STAR-CCM+ version 9.0. The large eddy simulation (LES) dynamic Smagorinsky subgrid scale, realizable k–ε model, and k–ω shear-stress transport (SST) detached eddy version were used as simulation tools. Three different test filters of 1.0, 2.2, and 3.0 were applied to the LES to identify improvements in accuracy. The overall best turbulence model is compared to the experimental result and reliability of such model is evaluated. The ability of such model was profound within the upstream and to some extent unreliable in downstream.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):081502-081502-14. doi:10.1115/1.4035820.

This paper presents an efficient approach to diesel engine combustion simulation that integrates detailed chemical kinetics into a quasidimensional fuel spray model. The model combines a discrete spray parcel concept to calculate fuel-air mixing with a detailed primary reference fuel chemical kinetic mechanism to determine species concentrations and heat release in time. Comparison of predicted pressure, heat release, and emissions with data from diesel engine experiments reported in the literature shows good agreement overall, and suggests that spray combustion processes can be predictively modeled without calibration of empirical burn rate constants at a significantly lower computational cost than standard multidimensional (CFD) tools.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):081503-081503-11. doi:10.1115/1.4035816.

Currently, the aviation sector is seeking for alternatives to kerosene from crude oil, as part of the efforts combating climate change by reducing greenhouse gas (GHG) emissions, in particular carbon dioxide (CO2), and ensuring security of supply at affordable prices. Several synthetic jet fuels have been developed including sustainable biokerosene, a low-carbon fuel. Over the last years, the technical feasibility as well as the compatibility of alternative jet fuels with today's planes has been proven However, when burning a jet fuel, the exhaust gases are a mixture of many species, going beyond CO2 and water (H2O) emissions, with nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC) including aromatic species and further precursors of particles and soot among them. These emissions have an impact on the local air quality as well as on the climate (particles, soot, contrails). Therefore, a detailed knowledge and understanding of the emission patterns when burning synthetic aviation fuels are inevitable. In the present paper, these issues are addressed by studying numerically the combustion of four synthetic jet fuels (Fischer–Tropsch fuels). For reference, two types of crude-oil-based kerosene (Jet A-1 and Jet A) are considered, too. Plug flow calculations were performed by using a detailed chemical-kinetic model validated previously. The composition of the multicomponent jet fuels was imaged by using the surrogate approach. Calculations were done for relevant temperatures, pressures, residence times, and fuel equivalence ratios φ. Results are discussed for NOx, CO as well as for benzene and acetylene as major soot precursors. According to the predictions, the NOx and CO emissions are within about ±10% for all fuels considered, within the parameter range studied: T = 1800 K, T = 2200 K; 0.25 ≤ φ ≤ 1.8; p = 40 bar; t = 3 ms. The aromatics free GtL (gas to liquid) fuel displayed higher NOx values compared to Jet A-1/A. In addition, synthetic fuels show slightly lower (better) CO emission data than Jet A-1/A. The antagonist role of CO and NOx is apparent. Major differences were predicted for benzene emissions, depending strongly on the aromatics content in the specific fuel, with lower levels predicted for the synthetic aviation fuels. Acetylene levels show a similar, but less pronounced, effect.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;139(8):081701-081701-10. doi:10.1115/1.4035738.

The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. The exhaust-gas heat-recovery systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. The inverted Brayton cycle (IBC) considered as a potential exhaust-gas heat-recovery system is a little-studied approach, especially when applied to small automotive power-plants. Hence, a model of the inverted Brayton cycle using finite-time thermodynamics (FTT) is presented to study heat recovery applied to a highly downsizing automotive internal combustion engine. IBC system consists of a turbine, a heat exchanger (HE), and compressors in sequence. The use of IBC turbine is to fully expand the exhaust gas available from the upper cycle. The remaining heat in the exhaust after expansion is rejected by the downstream heat exchanger. Then, the cooled exhaust gases are compressed back up to the ambient pressure by one or more compressors. In this paper, the exhaust conditions available from the engine test bench data were introduced as the inlet conditions of the IBC thermodynamic model to quantify the power recovered by IBC, thereby revealing the benefits of IBC to this particular engine. It should be noted that the test bench data of the baseline engine were collected by the worldwide harmonized light vehicles test procedures (WLTP). WLTP define a global harmonized standard for determining the levels of pollutants and CO2 emissions, fuel consumption. The IBC thermodynamic model was simulated with the following variables: IBC inlet pressure, turbine pressure ratio, heat exchanger effectiveness, turbomachinery efficiencies, and the IBC compression stage. The aim of this paper is to analysis the performance of IBC system when it is applied to a light-duty automotive engine operating in a real-world driving cycle.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2017;139(8):081901-081901-6. doi:10.1115/1.4035903.

Continuously increasing hot gas temperatures in heavy duty gas turbines lead to increased thermal loadings of the hot gas path materials. Thermal barrier coatings (TBCs) are used to reduce the superalloys temperature and cooling air needs. Until now 6–8 wt. % yttria stabilized zirconia (YSZ) is the first choice material for such coatings, but it is slowly reaching its maximum temperature capability due to the phase transformation at high temperature and sintering. New thermal barrier coating material with increased temperature capability enables the next generation of gas turbine with >60% combined cycle efficiency. Such material solutions have been developed through a multistage selection process. In a first step, critical material performance requirements for thermal barrier coating performance have been defined based on the understanding of standard TBC degradation mechanisms. Based on these requirements, more than 30 materials were a preselected and evaluated as potential coating materials. After carefully reviewing their properties both from literature data and laboratory test results on raw materials, five materials were selected for coating manufacturing and laboratory testing. Based on the coating manufacturing trials and laboratory test results, two materials have been selected for engine testing, in a first step in GT26 Birr Test Power Plant and afterward in customer engines. For such tests, the original coating thickness has been increased such to achieve coating surface temperature ∼100 K higher than with a standard thermal barrier coating. Both coatings performed as predicted in both GT26 Birr Test Power Plant and customer engines.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;139(8):082501-082501-10. doi:10.1115/1.4035735.

The mechanical performances of turbocharger rotor bearings system are strongly coupled with the thermal effects of lubrication. This paper built an integrated three-dimensional thermohydrodynamic model for the rotor and semifloating ring bearings. The thermal viscosity and non-Newtonian effects of lubricant oil are involved. Three experimental cases with different oil supply temperatures and pressures are conducted to validate the numerical results. The prediction coincides well with the measured results. Subsynchronous responses jumping between the conical and cylindrical mode shapes happens. The thermal results show that the heat conduction and expansion of the solid parts can affect the temperature fields and clearances of the oil films. Furthermore, for the bearings with axial grooves, the underdeveloped thermal boundary layers exist in the inner film at high rotational speed. The complexity and heterogeneity of the oil film temperature and viscosity reveal the essentiality and significance of the three-dimensional thermohydrodynamic analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082502-082502-13. doi:10.1115/1.4035893.

This article presents the application of statistical methods to guide the rotordynamic design of a turbogenerator shaft-line. One of the basic requirements is all shaft components must survive the event of a short circuit at the terminals of the generator. This is typically assessed via a transient response simulation of the complete machine train (including generator's electrical model) to check the calculated response torque against the allowable value. With an increasing demand of a shorter design cycle and competition in performance, cost, footprint, and safety, the probabilistic approach is starting to play an important role in the power train design process. The main challenge arises with the size of the design space and complexity of its mapping onto multiple objective functions and criteria which are defined for different machines. In this paper, the authors give an example demonstrating the use of statistical methods to explore (design of experiment (DoE)) and understand (surface response methods) the design space of the combined cycle power train with respect to the typically most severe constraint (fault torque torsional response), which leads to a quicker definition of a turbogenerator's arrangement. Further statistical analyses are carried out to understand the robustness of the chosen design against future modifications as well as parameters' uncertainties.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(8):082601-082601-14. doi:10.1115/1.4035808.

A large-eddy-simulation-based numerical investigation of a turbulent gaseous jet in crossflow (JICF) is presented. The present work focuses on cases with a steady crossflow and two different jet-to-crossflow velocity ratios, 2 and 4, at the same jet centerline velocity of 160 m/s. Emphasis is placed on the detailed flow evolution and scalar mixing in a compressible, turbulent environment. Various flow characteristics, including jet trajectories, jet-center streamlines, vortical structures, and intrinsic instabilities, as well as their relationships with the mixing process, are examined. Mixing efficiency is quantified by the decay rate of scalar concentration, the probability density function (PDF), and the spatial and temporal mixing deficiencies. Depending on the jet-to-crossflow velocity ratios, the wake vortices downstream of the injector orifice can either separate from or connect to the main jet plume, and this has a strong impact on mixing efficiency and vortex system development. Statistical analysis is applied to explore the underlying physics, with special attention at the jet-center and transverse planes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082602-082602-13. doi:10.1115/1.4035809.

The present work extends Part I of our study to investigate the flow dynamics and scalar mixing of a turbulent gaseous jet in an oscillating crossflow. Attention is first given to intrinsic flow instabilities under a steady condition. Both power spectral density and proper orthogonal decomposition analyses are applied. For the case with a jet-to-crossflow velocity ratio of 4, the two most dynamic modes, corresponding to jet Strouhal numbers of around 0.1 and 0.7, are identified as being closely linked to the shear-layer vortices near the injector orifice and the vertical movement in the jet wake region, respectively. The effect of oscillation imposed externally in the upstream region of the crossflow is also examined systemically at a jet-to-crossflow velocity ratio of 4. A broad range of forcing frequencies and amplitudes are considered. Results reveal that the dominant structures observed in the case with a steady crossflow are suppressed by the harmonic excitations. Flapping–detaching motions, bearing the forcing frequencies and their subharmonics, become dominant as the forcing amplitude increases. The ensuing flow motions lead to the formation of a long, narrow jet plume and a relatively low mixing zone, which substantially alters the mixing efficiencies as compared to the case with a steady crossflow.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082603-082603-8. doi:10.1115/1.4035821.

This paper focuses on the off-design performance of a turbofan engine with an interstage turbine burner (ITB). The ITB is an additional combustion chamber located between the high-pressure turbine (HPT) and the low-pressure turbine (LPT). The incorporation of ITB in an engine can provide several advantages, especially due to the reduction in the HPT inlet temperature and the associated NOx emission reduction. The objective is to evaluate the effects of the ITB on the off-design performance of a turbofan engine. The baseline engine is a contemporary classical turbofan. The effects of the ITB are evaluated on two aspects: first, the influences of an ITB on the engine cycle performance; second, the influences of an ITB on the component characteristics. The dual combustors of an ITB engine provide an extra degree-of-freedom for the engine operation. The analysis shows that a conventional engine has to be oversized to satisfy off-design performance requirement, like the flat rating temperature. However, the application of an ITB eases the restrictions imposed by the off-design performance requirements on the engine design, implying that the off-design performance of an ITB engine can be satisfied without sacrificing the fuel efficiency. Eventually, the performance of the ITB engine exhibits superior characteristics over the baseline engine at the studied operating points over a flight mission.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082604-082604-15. doi:10.1115/1.4035822.

Pressure pulsations into a centrifugal compressor can move its operating point into surge. This is concerning in pipeline stations where centrifugal compressors operate in series/parallel with reciprocating compressors. Sparks (1983, “On the Transient Interaction of Centrifugal Compressors and Their Piping Systems,” ASME Paper No. 83-GT-236); Kurz et al. (2006, “Pulsations in Centrifugal Compressor Installations,” ASME Paper No. GT2006-90700); and Brun et al. (2014, “Impact of the Piping Impedance and Acoustic Characteristics on Centrifugal Compressor Surge and Operating Range,” ASME J. Eng. Turbines Power, 137(3), p. 032603) provided predictions on the impact of periodic pressure pulsation on the behavior of a centrifugal compressor. This interaction is known as the “compressor dynamic response” (CDR) theory. Although the CDR describes the impact of the nearby piping system on the compressor surge and pulsation amplification, it has limited usefulness as a quantitative analysis tool, due to the lack of prediction tools and test data for comparison. Testing of compressor mixed operation was performed in an air loop to quantify the impact of periodic pressure pulsation from a reciprocating compressor on the surge margin (SM) of a centrifugal compressor. This data was utilized to validate predictions from Sparks’ CDR theory and Brun’s numerical approach. A 50 hp single-stage, double-acting reciprocating compressor provided inlet pulsations into a two-stage 700 hp centrifugal compressor. Tests were performed over a range of pulsation excitation amplitudes, frequencies, and pipe geometry variations to determine the impact of piping impedance and resonance responses. Results provided clear evidence that pulsations can reduce the surge margin of centrifugal compressors and that geometry of the piping system immediately upstream and downstream of a centrifugal compressor will have an impact on the surge margin reduction. Surge margin reductions of over 30% were observed for high centrifugal compressor inlet suction pulsation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082605-082605-11. doi:10.1115/1.4035848.

The thrust vectoring performance of a novel nozzle mechanism was numerically investigated. The nozzle was designed for supersonic, air-breathing engines using published engine data, isentropic relationships, and piecewise quartic splines. The mechanism utilizes two staggered, adjustable ramps. A baseline inviscid numerical simulation without ramps verified the nozzle design by comparing the results to the analytical data. Nine ramp configurations were analyzed under steady-state turbulent viscous conditions, using two sets of inlet parameters corresponding to inlet conditions with and without an afterburner (AB). The realizable $k$$ε$ model was used to model the turbulence field. Area-weighted integrals of the exit flow showed superior flow deflection with the nonafterburning inlet flow parameters. Calculations of the mean flow deflection angles showed that the flow can be deflected as much as 30 deg in a given direction with the largest ramp length and angle values. The smallest ramp length (less than 5% of the nozzle length) demonstrated as much as 21 deg in flow deflection.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082606-082606-14. doi:10.1115/1.4035841.

Widespread adoption of renewable energy technologies will arguably benefit from the availability of economically viable distributed thermal power conversion systems. For this reason, considerable efforts have been dedicated in recent years to R&D over mini-organic Rankine cycle (ORC) power plants, thus with a power capacity approximately in the 3–50 kW range. The application of these systems for waste heat recovery from diesel engines of long-haul trucks stands out because of the possibility of achieving economy of production. Many technical challenges need to be solved, as the system must be sufficiently efficient, light, and compact. The design paradigm is therefore completely different from that of conventional stationary ORC power plants of much larger capacity. A high speed turbine is arguably the expander of choice, if high conversion efficiency is targeted, thus high maximum cycle temperature. Given the lack of knowledge on the design of these turbines, which depends on a large number of constraints, a novel optimal design method integrating the preliminary design of the thermodynamic cycle and that of the turbine has been developed. The method is applicable to radial inflow, axial and radial outflow turbines, and to superheated and supercritical cycle configurations. After a limited number of working fluids are selected, the feasible design space is explored by means of thermodynamic cycle design calculations integrated with a simplified turbine design procedure, whereby the isentropic expansion efficiency is prescribed. Starting from the resulting design space, optimal preliminary designs are obtained by combining cycle calculations with a 1D mean-line code, subject to constraints. The application of the procedure is illustrated for a test case: the design of turbines to be tested in a new experimental setup named organic rankine cycle hybrid integrated device (ORCHID) which is being constructed at the Delft University of Technology, Delft, The Netherlands. The first turbine selected for further design and construction employs siloxane MM (hexamethyldisiloxane, C6H18OSi2), supercritical cycle, and the radial inflow configuration. The main preliminary design specifications are power output equal to 11.6 kW, turbine inlet temperature equal to 300 °C, maximum cycle pressure equal to 19.9 bar, expansion ratio equal to 72, rotational speed equal to 90 krpm, inlet diameter equal to 75 mm, minimum blade height equal to 2 mm, degree of reaction equal to 0.44, and estimated total-to-static efficiency equal to 77.3%. Results of the design calculations are affected by considerable uncertainty related to the loss correlations employed for the preliminary turbine design, as they have not been validated yet for this highly unconventional supersonic and transonic mini turbine. Future work will be dedicated to the extension of the method to encompass the preliminary design of heat exchangers and the off-design operation of the system.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082607-082607-8. doi:10.1115/1.4035842.

Vortex generators are known to be effective in augmenting the mixing of fuel jets with air. The configuration investigated in this study is a tubular air passage with fuel injection from one single orifice placed in the side wall. In the range of typical gas turbine combustor inlet temperatures, the performance vortex generator premixers (VGPs) have already been investigated for natural gas as well as for blends of natural gas and hydrogen. However, for highly reactive fuels, the application of VGPs in recuperated gas turbines is particularly challenging because the high combustor inlet temperature leads to potential risk with regard to premature self-ignition and flame flashback. As the current knowledge does not cover the temperature range far above the self-ignition temperature, an experimental investigation of the operational limits of VGPs is currently being conducted at the Thermodynamics Institute of the Technical University of Munich, Garching, Germany, which is particularly focused on reactive fuels and the thermodynamic conditions present in recuperated gas turbines with pressure ratios of 4–5. For the study presented in this paper, an atmospheric combustion VGP test rig has been designed, which facilitates investigations in a wide range of operating conditions in order to comply with the situation in recuperated microgas turbines (MGT), namely, global equivalence ratios between 0.2 and 0.7, air preheating temperatures between 288 K and 1100 K, and air bulk flow rates between 6 and 16 g/s. Both the entire mixing zone in the VGP and the primary combustion zone of the test rig are optically accessible. High-speed OH* chemiluminescence imaging is used for the detection of the flashback and blow-off limits of the investigated VGPs. Flashback and blow-off limits of hydrogen in a wide temperature range covering the autoignition regime are presented, addressing the influences of equivalence ratio, air preheating temperature, and momentum ratio between air and hydrogen on the operational limits in terms of bulk flow velocity. It is shown that flashback and blow-off limits are increasingly influenced by autoignition in the ultrahigh temperature regime.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(8):082801-082801-9. doi:10.1115/1.4035849.

Diluting spark-ignited (SI) stoichiometric combustion engines with excess residual gas improves thermal efficiency and allows the spark to be advanced toward maximum brake torque (MBT) timing. However, flame propagation rates decrease and misfires can occur at high exhaust gas recirculation (EGR) conditions and advanced spark, limiting the maximum level of charge dilution and its benefits. The misfire limits are often determined for a specific engine from extensive experiments covering a large range of speed, torque, and actuator settings. To extend the benefits of dilute combustion while at the misfire limit, it is essential to define a parameterizable, physics-based model capable of predicting the misfire limits, with cycle to cycle varied flame burning velocity as operating conditions change based on the driver demand. A cycle-averaged model is the first step in this process. The current work describes a model of cycle-averaged laminar flame burning velocity within the early flame development period of 0–3% mass fraction burned. A flame curvature correction method is used to account for both the effect of flame stretch and ignition characteristics, in a variable volume engine system. Comparison of the predicted and the measured flame velocity was performed using a spark plug with fiber optical access. The comparison at a small set of spark and EGR settings at fixed load and speed, shows an agreement within 30% of uncertainty, while 20% uncertainty equals ± one standard deviation over 2000 cycles.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(8):082802-082802-8. doi:10.1115/1.4035817.

The performance of a methane direct injection engine was investigated under various fuel injection timings and injection pressures. A single-cylinder optical engine was used to acquire in-cylinder pressure data and flame images. An outward-opening injector was installed at the center of the cylinder head. Experimental results showed that the combustion characteristics were strongly influenced by the end of injection (EOI) timing rather than the start of injection (SOI) timing. Late injection enhanced the combustion speed because the short duration between the end of injection and the spark-induced strong turbulence. The flame propagation speeds under various injection timings were directly compared using crank-angle-resolved sequential flame images. The injection pressure was not an important factor in the combustion; the three injection pressure cases of 0.5, 0.8, and 1.1 MPa yielded similar combustion trends. In the cases of late injection, the injection timings of which were near the intake valve closing (IVC) timing, the volumetric efficiency was higher (by 4%) than in the earlier injection cases. This result implies that the methane direct injection engine can achieve higher torque by means of the late injection strategy.

Commentary by Dr. Valentin Fuster