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### Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2015;137(12):121201-121201-10. doi:10.1115/1.4030634.

A computationally efficient and cost effective simulation framework has been implemented to perform design space exploration and multi-objective optimization for a conceptual regenerative rotorcraft powerplant configuration at mission level. The proposed framework is developed by coupling a comprehensive rotorcraft mission analysis code with a design space exploration and optimization package. The overall approach is deployed to design and optimize the powerplant of a reference twin-engine light rotorcraft, modeled after the Bo105 helicopter, manufactured by Airbus Helicopters. Initially, a sensitivity analysis of the regenerative engine is carried out to quantify the relationship between the engine thermodynamic cycle design parameters, engine weight, and overall mission fuel economy. Second, through the execution of a multi-objective optimization strategy, a Pareto front surface is constructed, quantifying the optimum trade-off between the fuel economy offered by a regenerative engine and its associated weight penalty. The optimum sets of cycle design parameters obtained from the structured Pareto front suggest that the employed heat effectiveness is the key design parameter affecting the engine weight and fuel efficiency. Furthermore, through quantification of the benefits suggested by the acquired Pareto front, it is shown that the fuel economy offered by the simple cycle rotorcraft engine can be substantially improved with the implementation of regeneration technology, without degrading the payload-range capability and airworthiness (one-engine-inoperative) of the rotorcraft.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2015;137(12):121401-121401-13. doi:10.1115/1.4030499.

The focus of this paper is on the part load performance of a small scale (100 kWe) combined heat and power (CHP) plant fired by natural gas (NG) and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative microgas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with NG takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for preheating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant. Different biomass/NG energy input ratios are also modeled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate and (ii) higher primary energy savings (PESs) and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD), and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average, and mild climate conditions.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2015;137(12):121501-121501-7. doi:10.1115/1.4030521.

Reduction of engine-out NOx emissions to ultra-low levels is facilitated by enabling low temperature combustion (LTC) strategies. However, there is a significant energy penalty in terms of combustion efficiency as evidenced by the high levels of hydrocarbon (HC), carbon monoxide (CO), and hydrogen emissions. In this work, the net fuel energy lost as a result of incomplete combustion in two different LTC regimes is studied—partially premixed compression ignition (PPCI) using in-cylinder injection of diesel fuel and reactivity controlled compression ignition (RCCI) of port injected gasoline and direct injected diesel. A detailed analysis of the incomplete combustion products was conducted. Test results indicated that carbon monoxide (CO), hydrogen, and light hydrocarbon (HC) made up for most of the combustion in-efficiency in the PPCI mode, while heavier HC and aromatics were significantly higher in the RCCI mode.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121502-121502-8. doi:10.1115/1.4030517.

This investigation deals with the EF7 (TC) engine, a dual fuel engine equipped with a turbocharger system, consequently with a high probability of knock inception. In this study, an operating cycle of the engine was simulated using KIVA-3V code. Some modifications were carried out on the KIVA method of calculating pressure in the intake port in order to simulate turbocharger pressure correctly. Auto-ignition and knock were then simulated using the auto-ignition integral model. The modified code and the simulation were verified using three different methods; in-cylinder average pressure, gas temperature of the exhaust port, and auto-ignition timing. The simulation results using the auto-ignition integral model, as compared with the experimental data, proved to be reasonably accurate. Following this validation, the effect of the knock phenomenon on the engine heat transfer through the walls was investigated. The simulations showed that the rate of heat transfer through the walls under knocking conditions is about 2.2 times higher than that under normal conditions. However, it was also shown that the total heat transfer increases about 15%.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121503-121503-9. doi:10.1115/1.4030519.

The detection of fouling in exhaust gas recirculation (EGR) coolers of diesel engines should be fast and accurate. This would facilitate deciding an effective strategy to combat fouling and to prolong the lifetime of EGR coolers. In the present study, the propensity of soot deposition in a rectangular EGR cooler is modeled using Kalman filters. Noises, coherent feature of many deposition processes which can be resulted from measurement sensors such as thermocouples or incidental deposit flake-off, are also considered in the model. The Kalman filter minimizes the estimation error covariance by considering the measurement and process noise covariance matrices while it can simultaneously handle the noisy data. The results are characterized with measurement process noise covariance. The relation between these two defines the smoothness and shape of the estimated trend of fouling resistance. Comparisons of the experimental data and the resultant model confirmed the usefulness of the applied method for various operating conditions of an EGR cooler prone to particulate deposition of soot particles. The paper proceeds with the impact of such models in monitoring fouling and taking an appropriate mitigation approach in diesel engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121504-121504-10. doi:10.1115/1.4030520.

The demand for increased thrust, higher engine efficiency, and reduced fuel consumption has increased the turbine inlet temperature and pressure in modern gas turbine engines. The outcome of these higher temperatures and pressures is the potential for unconsumed radical species to enter the turbine. Because modern cooling schemes for turbine blades involve injecting cool, oxygen-rich air adjacent to the surface, the potential for reaction with radicals in the mainstream flow, and augmented heat transfer to the blade arises. This result is contrary to the purpose of film cooling. In this environment, there is a competing desire to consume any free radicals prior to the flow entering the rotor stage while still maintaining surface temperatures below the metal melting temperature. This study evaluated various configurations of multiple cylindrical rows of cooling holes in terms of both heat release and effective downstream cooling. Results were evaluated based on net heat flux reduction (NHFR) and a new wall absorption (WA) parameter which combined the additional heat available from these secondary reactions with the length of the resulting flame to determine which schemes protected the wall more efficiently. Two particular schemes showed promise. The two row upstream configuration reduced the overall augmentation of heat by creating a short, concentrated reaction area. Conversely, the roll forward configuration minimized the local heat flux enhancement by spreading the reaction area over the surface being cooled.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121505-121505-8. doi:10.1115/1.4030516.

With the development at infrared guidance weapon, the survival of the ship, especially in high risk areas, is facing serious challenges. In order to improve its survival ability, infrared suppression system emerges. Marine gas turbine exhaust ejector system is its core component, which is responsible for reducing or even eliminating the infrared radiation signal of marine gas turbine exhaust system. Based on collecting data on many sorts of ejectors, we sort out literature related to gas turbine exhaust ejector. From the view of ejector structure, the paper briefly describes the development of gas turbine exhaust ejector used on ships in domestic and foreign. Put forward two major structural innovations: the structure of nozzle changes from circular to rectangular and diffuser adopts multilevel structure. A new type of marine gas turbine exhaust ejector was designed. Ejector model is simplified. Use numerical simulation method to predict the single-stage ejector and multistage ejectors. Further structural optimization plan and design can be made based on this essay.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121506-121506-8. doi:10.1115/1.4030793.

Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulations (LES) of a Siemens scaled combustor are compared against comprehensive experimental data. The steady RANS simulation modeled one quarter of the geometry with 8 M polyhedral cells using the shear stress transport (SST) k-ω model. Unsteady LES were performed on the quarter geometry (90 deg, 8 M cells) as well as the full geometry (360 deg, 32 M cells) using the wall-adapting local eddy-viscosity (WALE) subgrid model and dynamic evaluation of model coefficients. Aside from the turbulence model, all other models are identical for the RANS and LES. Combustion was modeled with the flamelet generated manifold (FGM) model, which represents the thermochemistry by mixture fraction and reaction progress. RANS simulations are performed using Zimont and Peters turbulent flame-speed (TFS) expressions with default model constants, as well as the kinetic rate from the FGM. The flame-speed stalls near the wall with the TFS models, predicting a flame brush that extends to the combustor outlet, which is inconsistent with measurements. The FGM kinetic source model shows improved flame position predictions. The LES predictions of mean and rms axial velocity, mixture fraction, and temperature do not show improvement over the RANS. All three simulations overpredict the turbulent mixing in the inner recirculation zone, causing flatter profiles than measurements. This overmixing is exacerbated in the 90 deg case. The experiments show evidence of heat loss, and the adiabatic simulations presented here might be improved by including wall heat-loss and radiation effects.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121507-121507-7. doi:10.1115/1.4030839.

Turbocharging technique, together with engine downsizing, will play a fundamental role in the near future as a way to reach the required maximum performance while reducing engine displacement and, consequently, CO2 emissions. However, performing an optimal control of the turbocharging system is very difficult, especially for small engines fitted with a low number of cylinders. This is mainly due to the high turbocharger operating range and to the fact that the flow through compressor and turbine is highly unsteady, while only steady-flow maps are usually provided by the manufacturer. In addition, in passenger cars applications, it is usually difficult to optimize turbocharger operating conditions because of the lack of information about pressure/temperature in turbine upstream/downstream circuits and turbocharger rotational speed. This work presents a methodology suitable for instantaneous turbocharger rotational speed determination through a proper processing of the signal coming from an accelerometer mounted on the compressor diffuser or a microphone faced to the compressor. The presented approach can be used to evaluate turbocharger speed mean value and turbocharger speed fluctuation (due to unsteady flow in turbine upstream and downstream circuits), which can be correlated to the power delivered by the turbine. The whole estimation algorithm has been developed and validated for a light-duty turbocharged common-rail diesel engine mounted in a test cell. Nevertheless, the developed methodology is general and can be applied to different turbochargers, both for spark ignited and diesel applications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):121508-121508-7. doi:10.1115/1.4030969.

Multimode combustion (MMC) concepts using homogeneous charge compression ignition (HCCI) gasoline combustion at low loads and spark assisted compression ignition (SACI) gasoline combustion at medium loads have the potential for improved fuel efficiency relative to spark ignition (SI) gasoline combustion. Two MMC concepts are compared in this paper with respect to fuel efficiency and tailpipe NOx emissions. The first concept uses stoichiometric HCCI and SACI to allow standard three-way catalyst (TWC) operation. The second concept also uses HCCI and SACI, but cycles between lean and rich combustion and uses a TWC with increased oxygen storage capacity (OSC) for potentially even greater fuel efficiency improvement. This paper performs a preliminary comparison of the two MMC concepts by analyzing two scenarios: (1) cycling between stoichiometric HCCI at 2 bar BMEP (brake mean effective pressure) and stoichiometric SACI at 3 bar BMEP, and (2) cycling between lean HCCI at 2 bar BMEP and rich SACI at 3 bar BMEP. The effects of excess oxygen ratio during HCCI operation and the frequency of oxygen depletion events on TWC performance and fuel efficiency are investigated. Results show that MMC lean/rich cycling can achieve better fuel efficiency than stoichiometric HCCI/SACI cycling. NOx emissions are moderately higher, but may still be low enough to meet current and future emission regulations.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2015;137(12):121701-121701-13. doi:10.1115/1.4031113.

This paper demonstrates the application of an integrated rotorcraft multidisciplinary design and optimization framework, deployed for the purpose of preliminary design and assessment of optimum regenerative powerplant configurations for rotorcraft. The proposed approach comprises a wide range of individual modeling theories applicable to rotorcraft flight dynamics, gas turbine engine performance, and weight estimation as well as a novel physics-based stirred reactor model, for the rapid estimation of various gas turbine gaseous emissions. A single-objective particle swarm optimizer (sPSO) is coupled with the aforementioned rotorcraft multidisciplinary design framework. The overall methodology is deployed for the design space exploration and optimization of a reference multipurpose twin engine light (TEL) civil rotorcraft, modeled after the Bo105 helicopter, employing two Rolls Royce Allison 250-C20B turboshaft engines. Through the implementation of single-objective optimization, notionally based optimum regenerative engine design configurations are acquired in terms of engine weight, mission fuel burn, and mission gaseous emissions inventory, at constant technology level. The acquired optimum engine configurations are subsequently deployed for the design of conceptual regenerative rotorcraft configurations, targeting improved mission fuel economy, enhanced payload-range capability as well as improvements in the rotorcraft overall environmental footprint, while maintaining the required airworthiness requirements. The proposed approach essentially constitutes an enabler in terms of focusing the multidisciplinary design of conceptual rotorcraft powerplants to realistic, three-dimensional operations and toward the realization of their associated engine design trade-offs at mission level.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2015;137(12):122501-122501-9. doi:10.1115/1.4030524.

The oil film thickness on the bearing chamber wall directly affects the wall heat transfer efficiency, so a fundamental study on the motion of oil film on the rotating cylinder has been conducted to this end. On the one hand, the rotating cylinder test rig was designed, and an ultrasonic measurement system was established to measure the dynamic oil film thickness. On the other hand, the unsteady oil film heat and mass transfer movement model was also established, and the numerical simulation to solve oil film motion by using computational fluid dynamic (CFD) commercial software was carried out. Meanwhile, on the basis of study on the oil film formation process and film thickness verification, the oil film distributions on the chamber wall with rotation speed and oil flow rate were analyzed and studied. Results show that the oil film on the rotating chamber wall experiences a development process from the oil film formation to basic stability, about 1.0 s in this paper. And comparison between the numerical and experimental data shows that the maximum error between experimental data and numerical simulation is 7.76%. Moreover, for the oil film distributions in the stable state, oil film thickness shows a trend of decreasing with the increasing of rotation speed, but increasing with the increasing of oil flow rate. The research here will provide the basis for subsequent study of the interaction between oil film motion and the wall heat transfer.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122502-122502-10. doi:10.1115/1.4030633.

Computational fluid dynamics (CFD) analysis, which solves the full three-dimensional (3D) Navier–Stokes equations, has been recognized as having promise in providing a more detailed and accurate analysis for oil-film journal bearings than the traditional Reynolds analysis, although there are still challenging issues requiring further investigation, such as the modeling of cavitation and the modeling of conjugate heat transfer effects in the CFD analysis of bearings. In this paper, a 3D CFD method for the analysis of journal bearings considering the above two effects has been developed; it employs three different cavitation models, including the Half-Sommerfeld model, a vaporous cavitation model, and a gaseous cavitation model. The method has been used to analyze a two-groove journal bearing and the results are validated with experimental measurements and the traditional Reynolds solutions. It is found that the CFD method which considers the conjugate heat transfer and employs the gaseous cavitation model gives better predictions of both bearing load and temperature than either the traditional Reynolds solution or CFD with other cavitation models. The CFD results also show strong recirculation of the fresh oil in the grooves, which has been neglected in the traditional Reynolds solution. The above results show conclusively that the present 3D CFD method considering the conjugate heat transfer and employing the gaseous cavitation model provides an efficient tool for more detailed and accurate analysis for bearing performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122503-122503-15. doi:10.1115/1.4031344.

Tilting pad journal bearings (TPJBs) supporting high-performance turbomachinery rotors have undergone steady design improvements to satisfy ever stringent operating conditions that include large specific loads, due to smaller footprints, and high surface speeds that promote flow turbulence and hence larger drag power losses. Simultaneously, predictive models continuously evolve to include minute details on bearing geometry, pads and pivots' configurations, oil delivery systems, etc. In general, predicted TPJB rotordynamic force coefficients correlate well with experimental data for operation with small to moderately large unit loads (1.7 MPa). Experiments also demonstrate bearing dynamic stiffnesses are frequency dependent, best fitted with a stiffness-mass like model whereas damping coefficients are adequately represented as of viscous type. However, for operation with large specific loads (>1.7 MPa), poor correlation of predictions to measured force coefficients is common. Recently, an experimental effort (Gaines, J., 2014, “Examining the Impact of Pad Flexibility on the Rotordynamic Coefficients of Rocker-Pivot-Pad Tiling-Pad Journal Bearings,” M.S. thesis, Mechanical Engineering, Texas A&M University, College Station, TX) produced test data for three TPJB sets, each having three pads of unequal thickness, to quantify the effect of pad flexibility on the bearings' force coefficients, in particular damping, over a range of load and rotational speed conditions. This paper introduces a fluid film flow model accounting for both pivot and pad flexibility to predict the bearing journal eccentricity, drag power loss, lubricant temperature rise, and force coefficients of typical TPJBs. A finite element (FE) pad structural model including the Babbitt layer is coupled to the thin film flow model to determine the mechanical deformation of the pad surface. Predictions correlate favorably with test data, also demonstrating that pad flexibility produces a reduction of up to 34% in damping for the bearing with the thinnest pads relative to that with the thickest pads. A parametric study follows to quantify the influence of pad thickness on the rotordynamic force coefficients of a sample TPJB with three pads of increasing preload, $r¯p$  = 0, 0.25 (baseline) and 0.5. The bearing pads are either rigid or flexible by varying their thickness. For design considerations, dimensionless static and dynamic characteristics of the bearings are presented versus the Sommerfeld number (S). Pad flexibility shows a more pronounced effect on the journal eccentricity and the force coefficients of a TPJB with null pad preload than for the bearings with larger pad preloads (0.25 and 0.5), in particular for operation with a small load or at a high surface speed (S > 0.8).

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2015;137(12):122601-122601-10. doi:10.1115/1.4030383.

In this article, we describe the use of proper orthogonal decomposition (POD) to investigate how the dominant wake structures of a bluff-body-stabilized turbulent premixed flame are affected by the heat released by the flame itself. The investigation uses a validated large eddy simulation (LES) to simulate the dynamics of the bluff-body's wake (Blanchard et al., 2014, “Simulating Bluff-Body Flameholders: On the Use of Proper Orthogonal Decomposition for Wake Dynamics Validation,” ASME J. Eng. Gas Turbines Power, 136(12), p. 122603; Blanchard et al., 2014, “Simulating Bluff-Body Flameholders: On the Use of Proper Orthogonal Decomposition for Combustion Dynamics Validation,” ASME J. Eng. Gas Turbines Power, 136(12), p. 121504). The numerical simulations allow the effect of heat release, shown as the ratio of the burned to unburned temperatures, to be varied independently from the Damköhler number. Five simulations are reported with varying fractions of the heat release ranging from 0% to 100% of the value of the baseline experiment. The results indicate similar trends reported qualitatively by others, but by using POD to isolate the dominant heat release modes of each simulation, the decomposed data can clearly show how the previously reported flow structures transition from asymmetric shedding in the case of zero heat-release to a much weaker, but fully symmetric shedding mode in the case of full heat release with a much more elongated and stable wake.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122602-122602-6. doi:10.1115/1.4030515.

This paper is aimed to show the effects of partial stall on the fracture of the first stage rotating blades of the gas turbine compressors of an onshore gas refinery. The first part of the paper deals with the results of finite element modeling (FEM) of stress distribution and stress concentration areas on the blades under its first to third natural frequencies. Comparison of the stress concentration areas with the fractured blades shows that the blades have been fractured due to resonance under the first and second natural frequencies. The second part of the paper deals with the computational fluid dynamics (CFD) simulation of air flowing through the blades to determine the most probable sources of vibrational loads as the aerodynamic forces. Results of CFD simulations show that the operation of the gas turbines under 40–50% of their nominal output power—which has been very regular in the history of operation of the turbines—increases the possibility of stall at the tip side of the first stages rotating blades. The vortices shedding due to downwash flow at the tip side of the blades causes flow instability and increases the aerodynamic vibrational forces on the blades, which finally makes them to experience a kind of high cycle fatigue (HCF).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122603-122603-9. doi:10.1115/1.4030518.

Careful experimental measurements can capture small changes in compressor total pressure ratio (TPR), which arise with subtle changes in an experiment's configuration. Research facilities that use unconditioned atmospheric air must account for changes in ambient compressor inlet conditions to establish repeatable performance maps. A unique dataset from a three-stage axial compressor has been acquired over the duration of 12 months in the Midwest U.S., where ambient conditions change significantly. The trends show a difference in compressor TPR measured on a cold day versus a warm day despite correcting inlet conditions to sea level standard day. To reconcile these differences, this paper explores correcting the compressor exit thermodynamic state, Reynolds number effects, and variations in rotor tip clearance (TC) as a result of differences in thermal growth.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122604-122604-11. doi:10.1115/1.4030815.

Performance deterioration in gas turbine engines (GTEs) depends on various factors in the ambient and the operating conditions. For example, humidity condensation at the inlet duct of a GTE creates water mist, which affects the fouling phenomena in the compressor and varies the performance. In this paper, the effective factors on the short-term performance deterioration of a GTE are identified and studied. GTE performance level is quantified with two physics-based performance indicators, calculated from the recorded operating data from the control system of a GTE over a full time between overhaul (TBO) period. A regularized particle filtering (RPF) framework is developed for filtering the indicator signals, and an adaptive neuro-fuzzy inference system (ANFIS) is then trained with the filtered signals and the effective ambient and the operating conditions, i.e., the power, the air mass flow, and the humidity condensation rate. The trained ANFIS model is then run to simulate the GTE performance deterioration in different conditions for system identification. The extracted behavior of the system clearly shows the dependency of the trend of performance deterioration on the operating conditions, especially the humidity condensation rate. The developed technique and the results can be utilized for GTE performance prediction, as well as for suggesting the optimum humidity supply at the GTE intake to control the performance deterioration rate.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122605-122605-10. doi:10.1115/1.4030836.

For small-scale organic Rankine cycles (ORCs) to be a competitive technology, it is reasonable to assume that the same turbine design will be implemented into a range of different applications. It is therefore critical to be able to predict turbine off-design performance over a range of different operating conditions while utilizing different working fluids. Similitude theory can be used for this purpose, and it has been well validated for ideal gases. However, the same cannot be said for its applications to the organic fluids found within ORCs. This paper considers a candidate subsonic turbine design operating with R245fa and the corresponding turbine performance map. Similitude theory is used to predict the performance of the same turbine operating at different inlet conditions using R245fa, R123, and R1234yf. The similitude predictions are compared to computational fluid dynamics (CFD) results obtained using ansys CFX. The original similitude theory using turbine total inlet conditions was found to only apply within a small range of operating conditions, so a modified similitude theory has been suggested that uses the choked flow conditions instead. This modified similitude theory agrees with the CFD predictions to within 2%, right up until the choked mass flow rate. Further studies considering supersonic turbines are required to establish the applicability of similitude for applications beyond the choked pressure ratio.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122606-122606-11. doi:10.1115/1.4030838.

This paper develops and validates a power flow behavioral model of a gas turbine engine (GTE) composed of a gas generator and free power turbine. The behavioral model is suitable for supervisory level (optimal) controller development of the engine itself or of electrical power systems containing gas-turbine-generator pairs as might be found in a naval ship or terrestrial electric utility plant. First principles engine models do not lend themselves to the supervisory level control development because of their high granularity. For the behavioral model, “simple” mathematical expressions that describe the engine's internal power flows are derived from an understanding of the engine's internal thermodynamic and mechanical interactions. These simple mathematical expressions arise from the balance of energy flow across engine components, power flow being the time derivative of energy flow. The parameter fit of the model to a specific engine such as the GE LM2500 detailed in this work utilizes constants and empirical fits of power conversion efficiencies obtained using data collected from a high-fidelity engine simulator such as the Gas Turbine Simulation Program (GSP). Transient response tests show that the two-norm normalized error between the detailed simulator model and behavioral model outputs to be 2.7% or less for a GE LM2500.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122607-122607-7. doi:10.1115/1.4030907.

There have been numerous studies reporting film effectiveness for film rows in isolation, which have led to correlations that are used for preliminary design. Many applications require multiple film cooling rows. Although there is some published data which deal with the combined effect of multiple rows, in most design situations the additive effect is computed using correlations for single rows. The most widely used method is the Sellers superposition method. In many applications, the method gives accurate results. Although the method is to some extent physically based, energy is not conserved within the model, and in certain situations this limitation can be shown to lead to an underprediction of the film effectiveness. In this paper, a new energy-based method for predicting the additive effect of multiple film cooling rows is outlined. The physical basis and limitations of the model are discussed. Predictions conducted using the new method are compared with computational fluid dynamics (CFD) data and contrasted with the Sellers method. In situations where energy conservation is required to avoid underprediction of effectiveness the method is shown to be advantageous.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(12):122608-122608-7. doi:10.1115/1.4030837.

Effusion cooling represents one of the most innovative techniques to limit and control the metal temperature of combustors liner, and recently, attention has been paid by the scientific community on the characterization and the definition of design practices of such devices. Most of these studies were focused on the heat transfer on the hot side of effusion cooling plates, while just few contributions deal with the effusion plates cold side convective cooling. This paper reports a numerical survey aimed at the characterization of the convective cooling at the effusion plates cold side. Several effusion holes spacing is accounted for in conjunction with representative operating conditions. The study led to the development of an empirical correlation for the prediction of the cold side heat transfer coefficient enhancement factor, EF: it expresses the EF related to each extraction hole as a function of the pressure ratio β and the effusion plate porosity factor σ.

Commentary by Dr. Valentin Fuster