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

J. Eng. Gas Turbines Power. 2014;136(10):101501-101501-11. doi:10.1115/1.4027357.

The most uncertain and challenging part in the design of a gas turbine has long been the combustion chamber. There has been a large number of experimentations in industry and universities alike to better understand the dynamic and complex processes that occur inside a combustion chamber. This study concentrates on gas turbine combustors, as a whole, and formulates a theoretical design procedure for staged combustors, in particular. Not much of the literature currently available in the public domain provides intensive study on designing staged combustors. The work covers an extensive study of the design methods applied in conventional combustor designs, which includes the reverse flow combustor and the axial flow annular combustors. The knowledge acquired from this study is then applied to develop a theoretical design methodology for double staged (radial and axial) low emission annular combustors. Additionally, a model combustor is designed for each type, radial and axial, of staging using the developed methodology. A prediction of the performance of the model combustors is executed. The main conclusion is that the dimensions of the model combustors obtained from the developed design methodology are within the feasibility limits. The comparison between the radially staged and the axially staged combustor has yielded the predicted results such as a lower NOx prediction for the latter and a shorter combustor length for the former. The NOx emission results of the new combustor models are found to be in the range of 50–60 ppm. However, the predicted NOx results are only very crude and need further detailed study.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101502-101502-8. doi:10.1115/1.4027128.

Previous research has been conducted showing significant benefits on combustion efficiency and stability by creating high gravity-loaded combustion environments. Ultracompact combustor systems decrease the size and weight of the overall engine by integrating the compressor, combustor, and turbine stages. In this system, the core flow is split and a portion is routed into a circumferential direction to be burned at a high equivalence ratio. Fuel and air are brought into the cavity and combusted in a high g-loaded environment driven by air injection. Computational research showed that the hole diameter of the air injection jets are directly related to g-loading within the cavity. An experimental rig was built where the air injection rings could be changed to contain one of three different jet hole diameters to verify this result. The smallest air injection diameter achieved the highest g-loading in the cavity, which is consistent with the computational fluid dynamics (CFD) results. However, the flame stability within the cavity was affected by the air injection jet becoming too large or too small for a particular equivalence ratio. Video taken at 8000 Hz was used to capture the flame structure, revealing that the flame was not stable even before lean blow out conditions were achieved. Additionally, the direction that the air jets swirled in the cavity was found to have an impact on the combustion dynamics. When flow swirled counterclockwise and impacted the suction side of the turbine vane, the cavity had a more uniform fully developed flow field, as opposed to the pressure side impact. Finally, liquid fuel testing was done to test the atomization and mixing of JP-8 in a g-loaded environment. The results showed that increasing the cavity g-load increased the residence time the fuel stayed in the cavity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101503-101503-8. doi:10.1115/1.4027256.

Experiments under two intake air swirl levels (swirl ratios of 0.55 and 5.68) were conducted in order to investigate the early flame development of combustion in a single-cylinder spark-ignition direct-injection engine. The engine was equipped with a quartz insert in the piston, which provided an optical access to its cylinder through the piston. The crank angle resolved combustion images through the piston window and in-cylinder pressure measurements of 250 cycles were simultaneously recorded for both swirl levels at a specified engine speed and low load condition. The early development, size, and spatial characteristics extracted from the flame images were analyzed as a function of crank angle degrees after the ignition. The experimental results revealed that the early flame development was strongly influenced by the highly directed swirl motion of intake-air into the combustion cylinder. The location of the start of the flame kernel relative to the spark plug position also changed intermittently at different swirl levels. While the structure of the early flame was found to be similar for both swirl levels, the starting location of the flame showed a vast difference in how the flame progressed. In general, the flame kernel was formed two crank-angle degrees after spark timing for the high swirl level, which was four crank-angle degrees earlier than that of the low swirl case. For the low swirl flow, the early combustion showed more cycle-to-cycle variation in terms of both the flame size and centroid location. It was quantitatively shown that increasing the swirl ratio from 0.55 to 5.68 could reduce the cycle-to-cycle variation of the early flame structure, resulting in about three to four crank-angle degrees advance of the peak pressure location and a 1% improvement for the coefficient of variation (COV) of the indicated mean effective pressure (IMEP).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101504-101504-10. doi:10.1115/1.4027273.

Diesel-ignited gasoline dual fuel combustion experiments were performed in a single-cylinder research engine (SCRE), outfitted with a common-rail diesel injection system and a stand-alone engine controller. Gasoline was injected in the intake port using a port-fuel injector. The engine was operated at a constant speed of 1500 rev/min, a constant load of 5.2 bar indicated mean effective pressure (IMEP), and a constant gasoline energy substitution of 80%. Parameters such as diesel injection timing (SOI), diesel injection pressure, and boost pressure were varied to quantify their impact on engine performance and engine-out indicated specific nitrogen oxide emissions (ISNOx), indicated specific hydrocarbon emissions (ISHC), indicated specific carbon monoxide emissions (ISCO), and smoke emissions. Advancing SOI from 30 degrees before top dead center (DBTDC) to 60 DBTDC reduced ISNOx from 14 g/kW h to less than 0.1 g/kW h; further advancement of SOI did not yield significant ISNOx reduction. A fundamental change was observed from heterogeneous combustion at 30 DBTDC to “premixed enough” combustion at 50–80 DBTDC and finally to well-mixed diesel-assisted gasoline homogeneous charge compression ignition (HCCI)-like combustion at 170 DBTDC. Smoke emissions were less than 0.1 filter smoke number (FSN) at all SOIs, while ISHC and ISCO were in the range of 8–20 g/kW h, with the earliest SOIs yielding very high values. Indicated fuel conversion efficiencies were ∼ 40–42.5%. An injection pressure sweep from 200 to 1300 bar at 50 DBTDC SOI and 1.5 bar intake boost showed that very low injection pressures lead to more heterogeneous combustion and higher ISNOx and ISCO emissions, while smoke and ISHC emissions remained unaffected. A boost pressure sweep from 1.1 to 1.8 bar at 50 DBTDC SOI and 500 bar rail pressure showed very rapid combustion for the lowest boost conditions, leading to high pressure rise rates, higher ISNOx emissions, and lower ISCO emissions, while smoke and ISHC emissions remained unaffected by boost pressure variations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101505-101505-7. doi:10.1115/1.4027358.

In this paper, the effects of the start of injection (SOI) timing and exhaust gas recirculation (EGR) rate on the nitrogen oxides (NOx) emissions of a biodiesel-powered diesel engine are studied with computational fluid dynamics (CFD) coupling with a chemical kinetics model. The KIVA code coupling with a CHEMKIN-II chemistry solver is applied to the simulation of the in-cylinder combustion process. A surrogate biodiesel mechanism consisting of two fuel components is employed as the combustion model of soybean biodiesel. The in-cylinder combustion processes of the cases with four injection timings and three EGR rates are simulated. The simulation results show that the calculated NOx emissions of the cases with default EGR rate are reduced by 20.3% and 32.9% when the injection timings are delayed by 2- and 4-deg crank angle, respectively. The calculated NOx emissions of the cases with 24.0% and 28.0% EGR are reduced by 38.4% and 62.8%, respectively, compared to that of the case with default SOI and 19.2% EGR. But higher EGR rate deteriorates the soot emission. When EGR rate is 28.0% and SOI is advanced by 2 deg, the NOx emission is reduced by 55.1% and soot emission is controlled as that of the case with 24% EGR and default SOI. The NOx emissions of biodiesel combustion can be effectively improved by SOI retardation or increasing EGR rate. Under the studied engine operating conditions, introducing more 4.8% EGR into the intake air with unchanged SOI is more effective for NOx emission controlling than that of 4-deg SOI retardation with default EGR rate.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101506-101506-7. doi:10.1115/1.4027275.

While steel pistons have been in use for a long time in commercial vehicle diesel engines, the first series production applications for passenger car diesel engines are currently imminent. The main reason for the use of steel pistons in high speed diesel engines is not, as maybe initially hypothesized, the increasing requirements on the component strength due to increasing mechanical loads, but rather challenges based on the actual CO2-legislation. The increasing requirements to reduce the fuel consumption necessitate new innovative technologies. The imminent penalties for exceeding the prescribed CO2 emissions seem to make the steel piston a viable alternative today, despite its higher manufacturing costs. So far, the CO2-benefits using steel pistons were mainly ascribed to the reduced friction between piston and cylinder liner due to no thermal interference.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101507-101507-10. doi:10.1115/1.4027359.

As future downsized boosted engines may employ multiple combustion modes, the goal of the current work is the definition of valving strategies appropriate for moderate to high load spark ignition (SI) combustion and at low to moderate loads for spark assisted compression ignition (SACI) combustion for an engine with variable valve timing capability and fixed camshaft profiles. The dilution and unburned gas temperature requirements for SACI combustion can be markedly different from those of SI; therefore it is important to ensure that a given valving strategy is appropriate for operation within both regimes. This paper compares one-dimensional (1D) thermodynamic simulations of rated engine operation with positive valve overlap (PVO) and a baseline negative valve overlap (NVO) camshaft design in a boosted automotive engine with variable valve timing capability. Several peak lifts and valve open durations are investigated to guide the down-selection of camshaft profiles for further evaluation under SACI conditions in a companion paper. While the results of this study are engine specific, rated performance predictions show that the duration of both the intake and exhaust camshafts significantly impacts the ability to achieve high load operation. While it was noted that the flow through the exhaust valves chokes for the majority of the exhaust stroke for peak exhaust lifts less than 8 mm, the aggressive engine rating of 194 kW at 5250 rpm could be achieved with peak intake lifts as low as 4 mm and baseline duration. Therefore, camshafts with peak lifts of 8/4 mm exhaust/intake were down-selected to facilitate multimode combustion operation with high levels of PVO. Analysis of high load operation with the down-selected camshafts indicates that peak unburned gas temperatures remain low enough to mitigate end-gas knock, while other variables such as peak cylinder pressure, turbine inlet temperature, and turbocharger speed are all predicted to be within acceptable limits.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101508-101508-11. doi:10.1115/1.4027360.

Spark assisted compression ignition (SACI) is a combustion mode that may offer significant efficiency improvements compared to conventional spark-ignited combustion systems. Unfortunately, SACI is constrained to a relatively narrow range of dilution levels and top dead center temperatures. Both positive valve overlap (PVO) and negative valve overlap (NVO) strategies may be utilized to attain these conditions at low and intermediate engine loads. The current work compares 1D thermodynamic simulations of PVO valving strategies and a baseline NVO strategy in a downsized boosted automotive engine with variable valve timing capability. As future downsized boosted engines may employ multiple combustion modes, the goal of this work is the definition of valving strategies appropriate for SACI combustion at low to moderate loads and spark ignition (SI) combustion at moderate to high loads for an engine with fixed camshaft profiles. PVO durations, valve opening timings, and peak lifts are investigated at low to moderate loads and are compared to a baseline NVO configuration in order to assess valving strategies appropriate for multimode combustion operation. A valvetrain kinematic model is used to translate the desired valve lift profiles into camshaft profiles while a kinematic analysis is used to calculate piston to valve clearances and to define the practical limits of the PVO strategies. The NVO and PVO strategies are also compared to throttled SI operation at part load to assess the overall efficiency benefit of operating under the thermodynamic conditions of the SACI combustion regime. While the results of this study are engine specific, there are several camshaft profiles that are appropriate for the use of PVO rebreathing type valve events. For the range of PVO valve events examined and taking into consideration piston to valve interference, the use of high exhaust and low intake lifts with early exhaust valve opening timing and long PVO durations enables high levels of internal exhaust gas recirculation (EGR) with relatively low pumping losses.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101509-101509-16. doi:10.1115/1.4027276.

The use of close-coupled post injections is an in-cylinder soot-reduction technique that has much promise for high efficiency heavy-duty diesel engines. Close-coupled post injections, short injections of fuel that occur soon after the end of the main fuel injection, have been known to reduce engine-out soot at a wide range of engine operating conditions, including variations in injection timing, exhaust gas recirculation (EGR) level, load, boost, and speed. While many studies have investigated the performance of post injections, the details of the mechanism by which soot is reduced remains unclear. In this study, we have measured the efficacy of post injections over a range of load conditions, at constant speed, boost, and rail pressure, in a heavy-duty optically-accessible research diesel engine. Here, the base load is varied by changing the main-injection duration. Measurements of engine-out soot indicate that not only does the efficacy of a post injection decrease at higher engine loads, but that the range of post-injection durations over which soot reduction is achievable is limited at higher loads. Optical measurements, including the natural luminescence of soot and planar laser-induced incandescence of soot, provide information about the spatiotemporal development of in-cylinder soot through the cycle in cases with and without post-injections. The optical results indicate that the post injection behaves similarly at different loads, but that its relative efficacy decreases due to the increase in soot resulting from longer main-injection durations.

Topics: Engines , Stress , Soot , Cylinders
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101510-101510-7. doi:10.1115/1.4027278.

High fidelity models that balance accuracy and computation load are essential for real-time model-based control of homogeneous charge compression ignition (HCCI) engines. Gray-box modeling offers an effective technique to obtain desirable HCCI control models. In this paper, a physical HCCI engine model is combined with two feed-forward artificial neural network models to form a serial architecture gray-box model. The resulting model can predict three major HCCI engine control outputs, including combustion phasing, indicated mean effective pressure (IMEP), and exhaust gas temperature (Texh). The gray-box model is trained and validated with the steady-state and transient experimental data for a large range of HCCI operating conditions. The results indicate that the gray-box model significantly improves the predictions from the physical model. For 234 HCCI conditions tested, the gray-box model predicts combustion phasing, IMEP, and Texh with an average error of less than 1 crank angle degree, 0.2 bar, and 6 °C, respectively. The gray-box model is computationally efficient and it can be used for real-time control application of HCCI engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101511-101511-10. doi:10.1115/1.4027290.

Turbochargers are a key technology to deliver fuel consumption reductions on future internal combustion engines. However, the current industry standard modeling approaches assume the turbine and compressor operate under adiabatic conditions. Although some state of the art modeling approaches have been presented for simulating the thermal behavior, these have focused on thermally stable conditions. In this work, an instrumented turbocharger was operated on a 2.2 liter diesel engine and in parallel a one-dimensional lumped capacity thermal model was developed. For the first time this paper presents analysis of experimental and modeling results under dynamic engine operating conditions. Engine speed and load conditions were varied to induce thermal transients with turbine inlet temperatures ranging from 200 to 800 °C; warm-up behavior from 25 °C ambient was also studied. Following a model tuning process based on steady operating conditions, the model was used to predict turbine and compressor gas outlet temperatures, doing so with an RMSE of 8.4 and 7.1 °C, respectively. On the turbine side, peak heat losses from the exhaust gases were observed to be up to double those observed under thermally stable conditions due to the heat accumulation in the structure. During warm-up, the model simplifications did not allow for accurate modeling of the compressor, however on the turbine side gas temperature prediction errors were reduced from 150 to around 40 °C. The main benefits from the present modeling approach appear to be in turbine outlet temperature prediction, however modeling improvements are identified for future work.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101512-101512-6. doi:10.1115/1.4027295.

Thermodynamics is the key discipline for determining and quantifying the elements of advanced engine designs, which lead to high efficiency. In spite of its importance, thermodynamics is often not given full consideration in understanding engine operation for high efficiency. By fully utilizing the first and second laws of thermodynamics, detailed understanding of the engine features that provide for high efficiency may be determined. Of all the possible features that contribute to high efficiency, the results of this study show that highly diluted engines with high compression ratios provide the greatest impact for high efficiencies. Other important improvements, which increase the efficiency include reduced heat losses, optimal combustion phasing, reduced friction, and reduced combustion duration. Thermodynamic quantification of these concepts is provided. For one comparison, the brake thermal efficiency increased from about 34% for the conventional engine to about 48% for the engine with one set of the above features. One aspect that contributes to these improvements is the importance of the increase of the ratio of specific heats. In addition, these design features often result in low emissions due to the low combustion temperatures.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101513-101513-10. doi:10.1115/1.4027296.

A promising candidate for CO2 neutral power production is semiclosed oxyfuel combustion combined cycles (SCOC-CC). Two alternative SCOC-CCs have been investigated both with recirculation of the working fluid (WF) (CO2 and H2O) but with different H2O content due to different conditions for condensation of water from the working fluid. The alternative with low moisture content in the recirculated working fluid has shown the highest thermodynamic potential and has been selected for further study. The necessity to use recirculated exhaust gas as the working fluid will make the design of the gas turbine quite different from a conventional gas turbine. For a combined cycle using a steam Rankine cycle as a bottoming cycle, it is vital that the temperature of the exhaust gas from the Brayton cycle is well-suited for steam generation that fits steam turbine live steam conditions. For oxyfuel gas turbines with a combustor outlet temperature of the same magnitude as conventional gas turbines, a much higher pressure ratio is required (close to twice the ratio as for a conventional gas turbine) in order to achieve a turbine outlet temperature suitable for combined cycle. Based on input from the optimized cycle calculations, a conceptual combustion system has been developed, where three different combustor feed streams can be controlled independently: the natural gas fuel, the oxidizer consisting mainly of oxygen plus some impurities, and the recirculated working fluid. This gives more flexibility compared to air-based gas turbines, but also introduces some design challenges. A key issue is how to maintain high combustion efficiency over the entire load range using as little oxidizer as possible and with emissions (NOx, CO, unburnt hydrocarbons (UHC)) within given constraints. Other important challenges are related to combustion stability, heat transfer and cooling, and material integrity, all of which are much affected when going from air-based to oxygen-based gas turbine combustion. Matching with existing air-based burner and combustor designs has been done in order to use as much as possible of what is proven technology today. The selected stabilization concept, heat transfer evaluation, burner, and combustion chamber layout will be described. As a next step, the pilot burner will be tested both at atmospheric and high pressure conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101514-101514-10. doi:10.1115/1.4027361.

Dual-fuel reactivity controlled compression ignition (RCCI) combustion has shown high thermal efficiency and superior controllability with low NOx and soot emissions. However, as in other low temperature combustion (LTC) strategies, the combustion control using low exhaust gas recirculation (EGR) or a high compression ratio at high load conditions has been a challenge. The objective of this work was to examine the efficacy of using dual direct injectors for combustion phasing control of high load RCCI combustion. The present computational work demonstrates that 21 bar gross indicated mean effective pressure (IMEP) RCCI is achievable using dual direct injection. The simulations were done using the KIVA3V-Release 2 code with a discrete multicomponent fuel evaporation model, coupled with sparse analytical Jacobian solver for describing the chemistry of the two fuels (iso-octane and n-heptane). In order to identify an optimum injection strategy a nondominated sorting genetic algorithm II (NSGA II), which is a multiobjective genetic algorithm, was used. The goal of the optimization was to find injection timings and mass splits among the multiple injections that simultaneously minimize the six objectives: soot, nitrogen oxide (NOx), carbon monoxide (CO), unburned hydrocarbon (UHC), indicated specific fuel consumption (ISFC), and ringing intensity. The simulations were performed for a 2.44 liter, heavy-duty engine with a 15:1 compression ratio. The speed was 1800 rev/min and the intake valve closure (IVC) conditions were maintained at 3.42 bar, 90 °C, and 46% EGR. The resulting optimum condition has 12.6 bar/deg peak pressure rise rate, 158 bar maximum pressure, and 48.7% gross indicated thermal efficiency. The NOx, CO, and soot emissions are very low.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101515-101515-9. doi:10.1115/1.4026801.

The steady laminar flamelet model (SLFM) (Peters, 1984, “Laminar Diffusion Flamelet Models in Non-Premixed Turbulent Combustion,” Prog. Energy Combust. Sci., 10(3), pp. 319–339; Peters, 1986, “Laminar Flamelet Concepts in Turbulent Combustion,” Symp. (Int.) Combust., 21(1), pp. 1231–1250) has been shown to be reasonably good for the predictions of mean temperature and the major species in turbulent flames (Borghi, 1988, “Turbulent Combustion Modeling,” Prog. Energy Combust. Sci., 14(4), pp. 245–292; Veynante and Vervisch, 2002, “Turbulent Combustion Modeling,” Prog. Energy Combust. Sci., 28(3), pp. 193–266). However, the SLFM approach has limitations in the prediction of slow chemistry phenomena like NO formation (Benim and Syed, 1998, “Laminar Flamelet Modeling of Turbulent Premixed Combustion,” Appl. Math. Model., 22(1–2), pp. 113–136; Heyl and Bockhorn, 2001, “Flamelet Modeling of NO Formation in Laminar and Turbulent Diffusion Flames,” Chemosphere, 42(5–7), pp. 449–462). In the case of SLFM, the turbulence and chemistry are coupled through a single variable called scalar dissipation, which is representative of the strain inside the flow. The SLFM is not able to respond to the steep changes in the scalar dissipation values and generally tends to approach to the equilibrium solution as the strain relaxes (Haworth et al., 1989, “The Importance of Time-Dependent Flame Structures in Stretched Laminar Flamelet Models for Turbulent Jet Diffusion Flames,” Symp. (Int.) Combust., 22(1), pp. 589–597). A pollutant like NO is formed in the post flame zones and with a high residence time, where the scalar dissipation diminishes and hence the NO is overpredicted using the SLFM approach. In order to improve the prediction of slow forming species, a transient history of the scalar dissipation evolution is required. In this work, a multiple unsteady laminar flamelet approach is implemented and used to model the NO formation in two turbulent diffusion flames using detailed chemistry. In this approach, multiple unsteady flamelet equations are solved, where each flamelet is associated with its own scalar dissipation history. The time averaged mean variables are calculated from weighted average contributions from different flamelets. The unsteady laminar flamelet solution starts with a converged solution obtained from the steady laminar flamelet modeling approach. The unsteady flamelet equations are, therefore, solved as a post processing step with the frozen flow field. The domain averaged scalar dissipation for a flamelet at each time step is obtained by solving a scalar transport equation, which represents the probability of occurrence of the considered flamelet. The present work involves the study of the effect of the number of flamelets and also the different methods of probability initialization on the accuracy of NO prediction. The current model predictions are compared with the experimental data. It is seen that the NO predictions improves significantly even with a single unsteady flamelet and further improves marginally with an increase in number of unsteady flamelets.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2014;136(10):101601-101601-9. doi:10.1115/1.4027279.

The integrated control of a homogenous charge compression ignition (HCCI) combustion phasing, load, and exhaust aftertreatment system is essential for realizing high-efficient HCCI engines, while maintaining low hydrocarbon (HC) and carbon monoxide (CO) emissions. This paper introduces a new approach for integrated HCCI engine control by defining a novel performance index to characterize different HCCI operating regions. The experimental data from a single cylinder engine at 214 operating conditions is used to determine the performance index for a blended fuel HCCI engine. The new performance index is then used to design an optimum reference trajectory for a multi-input multi-output HCCI controller. The optimum trajectory is designed for control of the combustion phasing and indicated mean effective pressure (IMEP), while meeting catalyst light-off requirements for the exhaust aftertreatment system. The designed controller is tested on a previously validated physical HCCI engine model. The simulation results illustrate the successful application of the new approach for controller design of HCCI engines.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2014;136(10):102601-102601-9. doi:10.1115/1.4027204.

A unique methodology and test rig was designed to evaluate the degradation of damaged nozzle guide vanes (NGVs) in a transonic annular cascade in the short duration facility at the Royal Military College. A custom test section was designed which featured a novel rotating instrumentation suite. This permitted 360 deg multispan traverse measurements downstream from unmodified turbine NGV rings from a Rolls-Royce/Allison A-250 turbo-shaft engine. The downstream total pressure was measured at four spanwise locations on both an undamaged reference and a damaged test article. Three performance metrics were developed in an effort to determine characteristic signatures for common operational damage such as trailing edge bends or cracked trailing edges. The highest average losses were observed in the root area, while the lowest occurred closer to the NGV tips. The results from this study indicated that multiple spanwise traverses were required to detect localized trailing edge damage. Recommendations are made for future testing and to further develop performance metrics.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):102602-102602-10. doi:10.1115/1.4027371.

This paper investigates the application of fault diagnosis (FD) approach for improving performance of compressors within exact operating point determination. Detecting of sensor fault or failure status is more important in the compressor for safety-critical application. No work has previously been reported on the use of the FD system within a compressor surge-suppressing system. Therefore, the main contribution of this paper is presenting different and complementary techniques for surge-suppressing studies via sensor FD. By data acquisition from a nonlinear Moore–Greitzer model, a neural network (NN) and innovation complex decision logic provide residual generation and evaluation blocks in an analytical redundancy FD system, respectively. The proposed FD deals with the most-common sensor faults and failures in seven different scenarios according to their nature, such as bias, cutoff, loss of efficiency, and freeze.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):102603-102603-9. doi:10.1115/1.4027362.

Thermal barrier coatings (TBCs), attributed to their inherent brittleness, are vulnerable to damage by impacting foreign objects when kinetic energy of the objects surpasses certain limits. The damage is termed foreign object damage (FOD) and results in various issues to coatings as well as to substrates, from plastic impression to delamination to spallation to cracking, depending on the severity of impact. The FOD experiments were conducted utilizing a ballistic impact gun for vane airfoil components coated with 220 μm-thick, 7% yttria–stabilized zirconia (7YSZ) by electron beam physical vapor deposit (EB-PVD). The testing was performed with impact velocities ranging from 150 m/s to Mach 1 using 1.6-mm hardened chrome-steel ball projectiles. The resulting FOD was in the forms of impact impressions, cone cracking, and delamination of the coatings/substrates. Prediction of delamination crack size as a function of impact velocity was made based on an energy-balance approach through a quasistatic, first-order approximation. The prediction was in reasonable agreement with experimental data considering a presumable compaction of the TBCs upon impact.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):102604-102604-5. doi:10.1115/1.4027965.

In this paper, the entity model of a 1.5 MW offshore wind turbine blade was built by Pro/Engineer software. Fluid flow control equations described by arbitrary Lagrange–Euler (ALE) were established, and the theoretical model of geometrically nonlinear vibration characteristics under fluid–structure interaction (FSI) was given. The simulation of offshore turbulent wind speed was achieved by programming in Matlab. The brandish displacement, the Mises stress distribution and nonlinear dynamic response curves were obtained. Furthermore, the influence of turbulence and FSI on blade dynamic characteristics was studied. The results show that the response curves of maximum brandish displacement and maximum Mises stress present the attenuation trends. The region of the maximum displacement and maximum stress and their variations at different blade positions are revealed. It was shown that the contribution of turbulence effect (TE) on displacement and stress is smaller than that of the FSI effect, and its extent of contribution is related to the relative span length. In addition, it was concluded that the simulation considering bidirectional FSI (BFSI) can reflect the vibration characteristics of wind turbine blades more accurately.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2014;136(10):102501-102501-7. doi:10.1115/1.4027217.

A main goal of noncontacting mechanical seals is to provide minimal leakage during operation. This may be achieved by specifying a small clearance between the mating faces that is just enough to avoid rubbing contact while allowing some tolerable leakage. The amount of leakage flow is reduced through the acceleration and deceleration of the fluid through a tortuous path, so the sealing performance depends on the geometric characteristics of the leakage path. This study focuses on annular hole-pattern seals, which are noncontacting mechanical seals commonly used in high pressure compressors. A design of experiments (DOE) approach is used to investigate the effects of various geometric variables on the leakage rate of a hole-pattern seal during normal operating conditions. The design space, defined by the ranges of hole diameter, hole depth, axial space between holes and number of holes in circumferential direction, is discretized using the simple random sampling method. Then, steady-state computational fluid dynamics (CFD) simulations are performed at each design point to evaluate seal performance. To better understand the sensitivity of the hole-pattern seal leakage rate with respect to design variables selected, response surfaces are built through its values at design points using quadratic polynomial fitting. The results show that the optimal solution had a better leakage control ability over the base model design. It is believed that the results of this study will assist in improving the design of annular hole-pattern seals.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):102502-102502-8. doi:10.1115/1.4027928.

At present, directional solidification (DS) made blades are commonly used in high performance turbine for their better high temperature mechanical, especially in creep properties compared with the equiaxed grain (EG) blades made by conventional casting method. To predict DS blades' fatigue life accurately, one of the practical ways is to conduct tests on full-scale blades in a laboratory/bench environment. In this investigation, two types of full scale turbine blades, which are made from DZ22B by DS method and K403 by conventional casting method, respectively, were selected to conduct high temperature combined low and high cycle fatigue (CCF) tests on a special design test rig, to evaluate the increase of fatigue life benefitted from material change. Experimental results show that different from EG blades, DS blades' fracture section is not located on the position where the maximum stress point lies. By comparing fatigue test results of the two types of blade, it can be found that the fatigue properties among different regions of the DS blade are different, and its fatigue damage is not only related to the stress field, but also affected by different parts material's fatigue properties.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):102503-102503-7. doi:10.1115/1.4027929.

Part I of this investigation is mainly focused on fatigue tests of full scale turbine blades, based on the observation of the phenomena that some directional solidification (DS) blades do not fracture at their maximum stress region, and it has been revealed that there exists material's fatigue property variation among different regions of DS blades. For more in-depth and quantitative study on the fatigue property variation, Part II of this investigation designs and fabricates four types of DS bladelike specimens (including platform-, shroud-, body-, and rootlike specimens), which imitate the geometry, microstructure, and stress features of a full scale turbine blade on its four typical regions, to conduct the low cycle fatigue (LCF) tests. Test results show that the bodylike specimen has the best fatigue performance, and under the same stress state, the fatigue life of root-, shroud-, and platformlike specimens are 29.1%, 28.5%, and 13.7% of the bodylike specimen, respectively. The large material's fatigue property variation among different regions of DS blades should be considered in future blade life design.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2014;136(10):101701-101701-9. doi:10.1115/1.4027936.

Supercritical carbon dioxide (SCO2) Brayton cycles have the potential to offer improved thermal-to-electric conversion efficiency for utility scale electricity production. These cycles have generated considerable interest in recent years because of this potential and are being considered for a range of applications, including nuclear and concentrating solar power (CSP). Two promising SCO2 power cycle variations are the simple Brayton cycle with recuperation and the recompression cycle. The models described in this paper are appropriate for the analysis and optimization of both cycle configurations under a range of design conditions. The recuperators in the cycle are modeled assuming a constant heat exchanger conductance value, which allows for computationally efficient optimization of the cycle's design parameters while accounting for the rapidly varying fluid properties of carbon dioxide near its critical point. Representing the recuperators using conductance, rather than effectiveness, allows for a more appropriate comparison among design-point conditions because a larger conductance typically corresponds more directly to a physically larger and higher capital cost heat exchanger. The model is used to explore the relationship between recuperator size and heat rejection temperature of the cycle, specifically in regard to maximizing thermal efficiency. The results presented in this paper are normalized by net power output and may be applied to cycles of any size. Under the design conditions considered for this analysis, results indicate that increasing the design high-side (compressor outlet) pressure does not always correspond to higher cycle thermal efficiency. Rather, there is an optimal compressor outlet pressure that is dependent on the recuperator size and operating temperatures of the cycle and is typically in the range of 30–35 MPa. Model results also indicate that the efficiency degradation associated with warmer heat rejection temperatures (e.g., in dry-cooled applications) are reduced by increasing the compressor inlet pressure. Because the optimal design of a cycle depends upon a number of application-specific variables, the model presented in this paper is available online and is envisioned as a building block for more complex and specific simulations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(10):101702-101702-11. doi:10.1115/1.4028002.

The growth in the importance of interruptible sources of energy is increasing the concerns of many electricity market regulators with respect to the reliability and stability of electricity supply. Decisions such as that to increase the number of reserve markets, their reserve requirements, or the role of reserve prices in the final electricity price have meant that generation plants are currently often operating with strategies to obtain not only large energy market quotes but also reserve ones. In this paper, a mixed integer linear programming (MILP) model is proposed to obtain the energy and reserve dispatch of a real combined cycle plant (CCP) to optimize its use on a weekly or annual basis. The dispatch is optimal in the sense that it maximizes the joint energy and reserve profits, including an estimation of the energy and reserve prices. The detailed technical and economic characteristics of the plant have been considered, such as start-ups, shut-downs, minimum hours for steam generation, supplementary firing, or natural gas contracts. The cases studies validate the main features of the mathematical model and analyze the computational efficiency in a realistic simulation.

Commentary by Dr. Valentin Fuster

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