Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2011;134(3):031201-031201-9. doi:10.1115/1.4003963.

The isentropic flow equations relating the thermodynamic pressures, temperatures, and densities to their stagnation properties are solved in terms of the area expansion and specific heat ratios. These fundamental thermofluid relations are inverted asymptotically and presented to arbitrary order. Both subsonic and supersonic branches of the possible solutions are systematically identified and exacted. Furthermore, for each branch of solutions, two types of recursive approximations are provided: a property-specific formulation and a more general, universal representation that encompasses all three properties under consideration. In the case of the subsonic branch, the asymptotic series expansion is shown to be recoverable from Bürmann’s theorem of classical analysis. Bosley’s technique is then applied to verify the theoretical truncation order in each approximation. The final expressions enable us to estimate the pressure, temperature, and density for arbitrary area expansion and specific heat ratios with no intermediate Mach number calculation or iteration. The analytical framework is described in sufficient detail to facilitate its portability to other nonlinear and highly transcendental relations where closed-form solutions may be desirable.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2011;134(3):031501-031501-8. doi:10.1115/1.4004237.

In recent years, lean-premixed (LP) combustors have been widely studied due to their potential to reduce NOx emissions in comparison to diffusion type combustors. However, the fact that the fuels and oxidizers are mixed upstream of the combustion zone makes LP type of combustors a candidate for upstream flame propagation (i.e., flashback) in the premixer that is typically not designed to sustain high temperatures. Moreover, there has been a recent demand for fuel-flexible gas turbines that can operate on hydrogen-enriched fuels like Syngas. Combustors originally designed for slower kinetics fuels like natural gas can potentially encounter flashback if operated with faster burning fuels like those containing hydrogen as a constituent. There exists a clear need in fuel-flexible lean-premixed combustors to control flashback that will not only prevent costly component damage but will also enhance the operability margin of engines. A successful attempt has been made to control flashback in an atmospheric LP combustor, burning natural gas-air mixtures, via the application of dielectric barrier discharge (DBD). A low-power DBD actuator was designed, fabricated and integrated into a premixer made out of quartz. The actuator was tuned to produce a low magnitude ionic wind with an intention to modify the velocity profile in the premixer. Flashback conditions were created by decreasing the air flow rate while keeping the fuel flow rate constant. Within this experimental setup, flashback happened in the core flow along the axis of the cylindrical premixer. Results show that the utilization of the DBD delays the occurrence of flashback to higher equivalence ratios. Improvements as high as about 5% of the flashback limit have been obtained without compromising the blowout limit. It is anticipated that this novel application of DBD will lead to future demonstrations of the concept under realistic gas turbine operating conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(3):031502-031502-7. doi:10.1115/1.4004402.

Bifurcation analysis is performed on experimental data obtained from a simple setup comprising ducted laminar premixed conical flames in order to investigate the features of nonlinear thermoacoustic oscillations. It is observed that as the bifurcation parameter is varied, the system undergoes a series of bifurcations leading to characteristically different nonlinear oscillations. Through the application of nonlinear time series analysis to pressure and flame (CH* chemiluminescence) intensity time traces, these oscillations are characterized as periodic, aperiodic, or chaotic oscillations, and subsequently the nature of the obtained bifurcations is explained based on dynamical systems theory. Nonlinear interaction between the flames and the acoustic modes of the duct is clearly reflected in the high speed flame images acquired simultaneously with pressure time series.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(3):031503-031503-9. doi:10.1115/1.4004262.

In support of the development of CFD for aeroengine combustion, quantitative measurements of spray properties and temperature were made. A generic swirling air blast injector was designed and built to produce well defined inlet conditions and for ease of numerical description for the CFD development. The measurements were performed in an optically accessible single sector combustor at pressures of 4 and 10 bar and preheat temperatures of 550 and 650 K, respectively. Jet A-1 was used as fuel. The burner air to fuel ratio was 20 and the pressure loss was set to 3%. Sauter mean diameter profiles and liquid mass flux distributions were generated from the phase Doppler anemometry measurements of the evaporating spray drop sizes and velocities. With planar measurements of Mie scattering and kerosene-LIF, the distribution of kerosene (liquid and vapor phase) was imaged. Temperatures were measured with OH-LIF. The burner was designed with a straight outlet to exhibit lifted flames. Hence initial distributions of size, velocity and density of the spray were measured before it entered the flame. Almost complete prevaporization was seen at least for the four bar flame. Compared with atmospheric investigations, the smaller diameters of the droplets and the small streamline curvature of the configuration led to a more uniform behavior of the spray.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(3):031504-031504-12. doi:10.1115/1.4004212.

In this study, a combustion facility was constructed that includes a flexible fuel supply system to produce synthesis gas using a maximum of three components. The rig with lean premixed burner is able to operate at up to five bars. The length of the inlet plenum and the outlet boundary conditions of the combustion chamber are adjustable. Experiments were carried out under a broad range of conditions, with variations in fuel components including hydrogen, methane and carbon monoxide, equivalence ratios, thermal power and boundary conditions. The dynamic processes of self-excited combustion instabilities with variable fuel components were measured. The mechanisms of coupling between the system acoustic waves and unsteady heat release were investigated. The results show that instability modes and flame characteristics were significantly different with variations in fuel components. In addition, the results are expected to provide useful information for the design and operation of stable syngas combustion systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):031505-031505-12. doi:10.1115/1.4004254.

A new modeling formulation for turbulent chemistry interactions in large-eddy simulation (LES) is presented that is based on a unique application of the linear-eddy model (LEM) that includes large scale strain effects. This novel application of the LEM may be used to predict turbulent flame extinction limits due to both small and large scale strain effects. Statistics from this modeling formulation may be used to generate an inexpensive run-time model for LES predictions. This paper presents the LEM modeling formulation and demonstrates the capabilities of the approach for augmenter conditions. A methodology is also presented for formulating an LES-linear-eddy model (LES-LEM) subgrid model based on the simulation data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):031506-031506-11. doi:10.1115/1.4004255.

Providing better fuel flexibility for future gas turbine generations is a challenge as the fuel range is expected to become significantly wider (natural gas, syngas, etc.). The technical problem is to reach a wide operational window, regarding both operational safety and low emissions. In a previous paper, an approach to meet these requirements has already been presented. However, in this previous study it was difficult to exactly quantify the improvement in operational safety due to the fact that the flashback phenomena observed were not fully understood. The present continuative paper is focused on a thorough investigation of operational safety also involving the influence of pressure on flashback and the emissions of the proposed burner concept. To gain better insight into the character of the propagation and to visualize the path of the flame during its upstream motion, tests were done on an atmospheric combustion test rig providing almost complete optical access to the mixing section as well as the flame tube. OH* chemiluminescence, HS-Mie scattering and ionization detectors were applied and undiluted H2 was used as fuel for the detailed analysis. To elaborate on the influence of pressure on the stability behavior, additional tests were conducted on a pressurized test rig using a downscaled burner. OH* chemiluminescence, flashback and lean blow out measurements were conducted in this campaign, using CH4 , CH4 /H2 mixtures and pure H2 . The conducted experiments delivered the assets and drawbacks of the fuel injection strategy, where high axial fuel momentum was used to tune the flow field to achieve better flashback resistance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):031507-031507-8. doi:10.1115/1.4004720.

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bars. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30 deg. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2 , CO, CO2 , and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX ) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bars and 5 mm at 6.5 and 8 bars. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model. This model was also able to predict some key features of the flow such as the change of penetration depth with the pressure.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2011;134(3):031701-031701-10. doi:10.1115/1.4004395.

At off-design conditions, engine performance model prediction accuracy depends largely on its component characteristic maps. With the absence of actual characteristic maps, performance adaptation needs to be done for good imitations of actual engine performance. A nonlinear multiple point genetic algorithm based performance adaptation developed earlier by the authors using a set of nonlinear scaling factor functions has been proven capable of making accurate performance predictions over a wide range of operating conditions. However, the success depends on searching the right range of scaling factor coefficients heuristically, in order to obtain the optimum scaling factor functions. Such search ranges may be difficult to obtain and in many off-design adaption cases, it may be very time consuming due to the nature of the trial and error process. In this paper, an improvement on the present adaptation method is presented using a least square method where the search range can be selected deterministically. In the new method, off-design adaptation is applied to individual off-design point first to obtain individual off-design point scaling factors. Then plots of the scaling factors against the off-design conditions are generated. Using the least square method, the relationship between each scaling factor and the off-design operating condition is generated. The regression coefficients are then used to determine the search range of the scaling factor coefficients before multiple off-design points performance adaptation is finally applied. The developed adaptation approach has been applied to a model single-spool turboshaft engine and demonstrated a simpler and faster way of obtaining the optimal scaling factor coefficients compared with the original off-design adaptation method.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2011;134(3):031801-031801-7. doi:10.1115/1.4005115.

Exergy-based analyses are important tools for studying and evaluating energy conversion systems. Conventional exergy-based analyses provide us with important information on the design and operation of a system. However, further insight into the improvement potential of plant components and the overall plant, as well as into component interactions, is important when optimal operation is required. This necessity led to the development of advanced exergy-based analyses, in which the exergy destruction as well as the associated costs and environmental impacts are split into avoidable/unavoidable and endogenous/exogenous parts. Based on the avoidable exergy destruction, costs and environmental impacts potential and strategies for improvement are revealed. The objective of this paper is to demonstrate the application, the advantages, and the information obtained from an advanced exergoeconomic analysis by applying it to a complex plant, i.e., to a combined cycle power plant. The largest parts of the unavoidable cost rates are calculated for the components constituting the gas turbine system and the low-pressure steam turbine. The combustion chamber has the second highest avoidable investment cost and the highest avoidable cost of exergy destruction. In general, the investment cost of most of the components is unavoidable, with the exception of some heat exchangers. Similarly, most of the cost of exergy destruction is unavoidable, with the exception of the expander of the gas turbine system and the high-pressure and intermediate-pressure steam turbines. The advanced exergoeconomic analysis reveals high endogenous values, which suggest that improvement of the total plant can be achieved by improving the design of individual components, and lower exogenous values, which means that component interactions are in general of lower significance for this plant.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2012;134(3):032401-032401-9. doi:10.1115/1.4004403.

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Fouling is caused by the adherence of particles to airfoils and annulus surfaces. Particles that cause fouling are typically smaller than 2 to 10 microns. Smoke, oil mists, carbon, and sea salts are common examples. Fouling can be controlled by appropriate air filtration systems, and can often be reversed to some degree by detergent washing of components. The adherence of particles is impacted by oil or water mists. The result is a build up of material that causes increased surface roughness and to some degree changes the shape of the airfoil (if the material build up forms thicker layers of deposits), with subsequent deterioration in performance. Fouling mechanisms are evaluated based on observed data, and a discussion on fouling susceptibility is provided. A particular emphasis will be on the capabilities of modern air filtration systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032402-032402-8. doi:10.1115/1.4004722.

A wide-ranging analysis was performed by GE Oil & Gas and the University of Florence to investigate the effects on the estimation of centrifugal compressor performance induced by a different choice of the total temperature measurement section. With this goal in mind, the study focused on the analysis of a commonly found discrepancy between the measurements at the impeller outlet section and at the stage exit section. Based on the experimental data collected on a centrifugal impeller, three main physical phenomena were analyzed and discussed in further detail. First, the effect of the heat exchange was examined, and its influence on the total temperature variation throughout the machine was extrapolated. Next, the influence of the heat-exchange phenomena affecting the temperature sensors was evaluated by means of numerical models and physical assumptions. Finally, the effects on the temperature measurement of the flow structure at the impeller outlet were investigated. In particular, a corrective model to account for the thermal inertia of the thermocouples normally applied in this section was applied to the experimental data. The corrected temperatures at the investigated measurement sections were then compared, and their influence on the correct stage performance estimation is discussed in this study.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2011;134(3):032501-032501-8. doi:10.1115/1.4004261.

Plasticity effects and crack-closure modeling of small fatigue cracks were used on a Ti-6Al-4V alloy to calculate fatigue lives under various constant-amplitude loading conditions (negative to positive stress ratios, R) on notched and un-notched specimens. Fatigue test data came from a high-cycle-fatigue study by the U.S. Air Force and a metallic materials properties handbook. A crack-closure model with a cyclic-plastic-zone-corrected effective stress-intensity factor range and equivalent-initial-flaw-sizes (EIFS) were used to calculate fatigue lives using only crack-growth-rate data. For un-notched specimens, EIFS values were 25-μm; while for notched specimens, the EIFS values ranged from 6 to 12 μm for positive stress ratios and 25-μm for R = −1 loading. Calculated fatigue lives under a wide-range of constant-amplitude loading conditions agreed fairly well with the test data from low- to high-cycle fatigue conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(3):032502-032502-9. doi:10.1115/1.4004487.

Air/gas foil bearings (AFB) have shown a promise in high-speed micro to mid-sized turbomachinery. Compared to rolling element bearings, AFBs do not require oil lubrication circuits and seals, allowing the system to be less complicated and more environment-friendly. Due to the smaller number of parts required to support the rotor and no lubrication/seal system, AFBs provide compact solution to oil-free turbomachinery development.While foil bearing technology is mature in small industrial machines and power generation turbines, its application to aero-propulsion systems has been prohibited due to the reliability issues relevant to unique aero-propulsion environments such as severe rubbing due to the very slow acceleration of typically heavy rotors. This paper presents a hybrid air foil bearing (a combination of hydrostatic and hydrodynamic) with 102 mm in diameter designed for aero-propulsion applications, and preliminary test results on start-stop friction characteristics and thermal behavior at low speeds below 10,000 rpm are presented. The bearing could withstand 1000 start/stop cycles with 6 rev/s2 acceleration under a static load of 356 N (43.4 kPa).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(3):032503-032503-7. doi:10.1115/1.4004236.

Contact interfaces with dry friction are frequently used in turbomachinery. Dry friction damping produced by the sliding surfaces of these interfaces reduces the amplitude of bladed-disk vibration. The relative displacements at these interfaces lead to fretting-wear which reduces the average life expectancy of the structure. Frequency response functions are calculated numerically by using the multi-harmonic balance method (mHBM). The dynamic Lagrangian frequency-time method is used to calculate contact forces in the frequency domain. A new strategy for solving nonlinear systems based on dual time stepping is applied. This method is faster than using Newton solvers. It was used successfully for solving Nonlinear CFD equations in the frequency domain. This new approach allows identifying the steady state of worn systems by integrating wear rate equations a on dual time scale. The dual time equations are integrated by an implicit scheme. Of the different orders tested, the first order scheme provided the best results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032504-032504-8. doi:10.1115/1.4004404.

This paper introduces an approach for considering manufacturing variability leading to a nonaxisymmetric blading in the computational fluid dynamics simulation of a high-pressure compressor stage. A set of 150 rotor blades from a high-pressure compressor stage was 3D scanned in order to obtain the manufacturing variability. The obtained point clouds were parameterized using a parametric blade model, which uses typical profile parameters to translate the geometric variability into a numerical model. Probabilistic simulation methods allow for the generation of a sampled set of blades that statistically corresponds to the measured one. This technique was applied to generate 4000 sampled blades in order to investigate the influence of a nonaxisymmetric blading. It was found that the aerodynamic performance is considerably influenced by a variation of the passage cross section. Nevertheless, this influence decreases with an increasing number of independently sampled blades and, thus, independently shaped passage cross sections. Due to its more accurate consideration of the geometric variability, the presented methodology allows for a more realistic performance analysis of a high-pressure compressor stage.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032505-032505-7. doi:10.1115/1.4004439.

Since heavier gases exert larger effects on rotordynamic stability, stability evaluation is important in developing or designing high-pressure compressors. To evaluate the rotor stability during operation, an excitation test using a magnetic bearing is the most practical method. In stability analysis, labyrinth seals can produce significant cross coupling forces, which particularly reduce the damping ratio of the first forward mode. Therefore, forward modes should be distinguished from backward modes in the excitation test. One method that excites only the forward modes, not the backward modes (and vice versa), is the use of a rotating excitation. In this method, the force is simultaneously applied to two axes to excite the rotor in circular orbits. Two trigonometric functions, i.e., cosine and sine functions, are used to generate this rotation force. Another method is the use of a unidirectional excitation and a mathematical operation to distinguish the forward whirl from the backward whirl. In this method, a directional frequency response function that separates the two modes in the frequency domain is obtained from four frequency response functions by using a complex number expression for the rotor motion. In this study, the latter method was employed to evaluate the rotor stability of a high-pressure compressor. To obtain the frequencies and damping ratios of the eigenvalues, the curve fitting based on system identification methods, such as the prediction error method, was introduced for the derived frequency response functions. Firstly, these methods were applied to a base evaluation under a low-pressure gas operation, in which the stability mainly depends on the bearing property. Using the obtained results, the bearing coefficients were estimated. Next, the same methods were applied to stability evaluations under high-pressure gas operations. The destabilizing forces were also estimated from the test results and compared with the calculation results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032506-032506-11. doi:10.1115/1.4004719.

This paper present the development of an oil-free turbocharger (TC) supported on gas foil bearings (GFBs) and its performance evaluation in a test rig driven by a diesel vehicle engine (EG). The rotor-bearing system was designed via a rotordynamic analysis with dynamic force coefficients derived from the analysis of the GFBs. The developed oil-free TC was designed using a hollow rotor with a radial turbine at one end and a compressor wheel at the other end, a center housing with journal and thrust GFBs, and turbine and compressor casings. Preliminary tests driven by pressurized shop air at room temperature demonstrated relatively stable operation up to a TC speed of 90,000 rpm, accompanied by a dominant synchronous motion of ∼20 μm and small subsynchronous motions of less than 2 μm at the higher end of the speed range. Under realistic operating conditions with a diesel vehicle engine at a maximum TC speed of 136,000 rpm and a maximum EG speed of 3140 rpm, EG and TC speeds and gas flow properties were measured. The measured time responses of the TC speed and the turbine inlet pressure demonstrated time delays of ∼3.9 and ∼1.3 s from that of the EG speed during consecutive stepwise EG speed changes, implying the GFB friction and rotor inertia led to time delays of ∼2.6 s. The measured pressures and temperatures showed trends following second-order polynomials against EG speed. Regarding TC efficiency, 4.3 kW of mechanical power was supplied by the turbine and 3.3 kW was consumed by the compressor at the top speed of 136,000 rpm, and the power loss reached 22% of the turbine power. Furthermore, the estimated GFB power losses from the GFB analysis were approximately 25% of the total power loss at higher speeds, indicating the remainder of the power loss resulted from heat transfer from the exhaust gas to the surrounding solid structures. Incidentally, as the TC speed was increased from 45,000 to 136,000 rpm, the estimated turbine inlet power increased from 19 to 79 kW, the compressor exit power increased from 7 to 26 kW, and the TC output mass flow rate from the compressor increased from 21 to 74 g/s. The average TC compressor exit power was estimated at ∼34% of the turbine inlet power over this range.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032507-032507-8. doi:10.1115/1.4004721.

The design of high cycle fatigue resistant bladed disks requires the ability to predict the expected damping of the structure in order to evaluate the dynamic behavior and ensure structural integrity. Highly sophisticated software codes are available today for this nonlinear analysis, but their correct use requires a good understanding of the correct model generation and the input parameters involved to ensure a reliable prediction of the blade behavior. The aim of the work described in this paper is to determine the suitability of current modeling approaches and to enhance the quality of the nonlinear modeling of turbine blades with underplatform dampers. This includes an investigation of a choice of the required input parameters, an evaluation of their best use in nonlinear friction analysis, and an assessment of the sensitivity of the response to variations in these parameters. Part of the problem is that the input parameters come with varying degrees of uncertainty because some are experimentally determined, others are derived from analysis, and a final set are often based on estimates from previous experience. In this investigation the model of a commercial turbine bladed disk with an underplatform damper is studied, and its first flap, first torsion, and first edgewise modes are considered for 6 EO and 36 EO excitation. The influence of different contact interface meshes on the results is investigated, together with several distributions of the static normal contact loads, to enhance the model setup and, hence, increase accuracy in the response predictions of the blade with an underplatform damper. A parametric analysis is carried out on the friction contact parameters and the correct setup of the nonlinear contact model to determine their influence on the dynamic response and to define the required accuracy of the input parameters.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2011;134(3):032801-032801-9. doi:10.1115/1.4005114.

The benefits of oxygen enhancement in conjunction with EGR on emissions were investigated in a single-cylinder direct injection diesel engine. Cylinder pressure, NOx , and particulate were measured for EGR sweeps with and without oxygen enhancement. In all cases, the total flow of oxygen to the cylinder was maintained constant. This was achieved by increasing cylinder pressure for typical EGR (N-EGR) and by adding oxygen to the intake stream for oxygen-enhanced EGR (O-EGR). The results show that O-EGR produced a substantially better combination of NOx and particulate than N-EGR. In the N-EGR cases, the EGR dilutes the oxidizer causing lower NOx and higher particulate. In O-EGR, flame temperature reduction leading to lower NOx is achieved by a combination of higher molar specific heats of CO2 and H2 O and dilution. Particulate emissions decreased or remain constant with increasing O-EGR. In addition to the obvious challenge of providing a source of oxygen to an engine, two operational challenges were encountered. First, as O-EGR was increased, the ratio of specific heats (Cp /Cv ) of the cylinder intake charge decreased and decreased the compression temperature, causing significant changes in ignition delay. These changes were compensated for in the experiments by increasing intake temperature but would be challenging to manage in transient engine operation. Second, the increased water concentration in the exhaust created difficulties in the exhaust system and was suspected to have produced a water emulsion in the oil.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032802-032802-8. doi:10.1115/1.4004717.

A fully established elastohydrodynamic lubricating (EHL) film between the piston and the liner surfaces during normal engine operation minimizes piston slap and prevents adhesive wear. Wear cannot be prevented in the initial engine start up due to the absence of EHL film. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start-up cycles, shear heating affects the lubricant viscosity and other characteristics considerably. This study models 2D piston skirts EHL by incorporating shear heating effects due to lubricant flow between the skirts and liner surfaces. The hydrodynamic and EHL film profiles are predicted by solving the 2D Reynolds equation and using the inverse solution technique, respectively. The temperature distribution within the oil film is given by using the 2D transient thermal energy equation with heat generated by viscous heating. The numerical analysis is based on an energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to low and high viscosity grade engine oils to investigate the adverse effects of the rising temperatures on the load carrying capacity of such lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study optimizes the viscosity-grade of an engine lubricant to minimize the adhesive wear of the piston skirts and cylinder liner at the time of initial engine start up.

Commentary by Dr. Valentin Fuster

Research Papers: Nuclear Power

J. Eng. Gas Turbines Power. 2011;134(3):032901-032901-6. doi:10.1115/1.4004596.

In this paper, kernel principal component analysis (KPCA) is studied for fault detection and identification of the instruments in nuclear power plants. A KPCA model for fault isolation and identification is proposed by using the average sensor reconstruction errors. Based on this model, faults in multiple sensors can be isolated and identified simultaneously. Performance of the KPCA-based method is demonstrated with real NPP measurements.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2012;134(3):032902-032902-7. doi:10.1115/1.4004598.

An analytic investigation of the thermal exchanges in channels is conducted with the prospect of building a simple method to determine the Nusselt number in steady, laminar or turbulent and monodimensional flow through rectangular and annular spaces with any ratio of constant and uniform heat rate. The study of the laminar case leads to explicit laws for the Nusselt number, while the turbulent case is solved using a Reichardt turbulent viscosity model resulting in an easy to solve one-dimensional ordinary differential equation system. This differential equation system is solved using a matlab based boundary value problems solver (bvp4c). A wide range of Reynolds, Prandtl, and radius ratios is explored with the prospect of building correlation laws allowing the computing of Nusselt numbers for any radius ratio. Those correlations are in good agreement with the literature. The correlations are also compared with a CFD analysis made on a case extracted from the Réacteur Jules Horowitz.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2012;134(3):034501-034501-5. doi:10.1115/1.4004437.

The multihole tube is an important component used for lean premixed prevaporized low-emission combustion in micro gas turbines, as it plays a key role in establishing uniform fuel-air mixture before flowing into the combustor. Recognizing that poor fuel-air mixing leads to increased emissions, it is therefore imperative to characterize the extent of fuel-air mixing at the exit of the multihole tube. In the present investigation, mixing characterization experiments were conducted by mapping the distribution of fuel-air equivalence ratios at the tube exit with gas analysis technique. Two different multihole tube configurations were tested and compared using aviation kerosene. Experiments were performed under atmospheric pressure, with an inlet air temperature of 480 K and an overall fuel-air equivalence ratio of 0.6. While the baseline configuration yielded the maximum magnitude of equivalence ratio deviation close to 35% at the tube exit, the modified configuration demonstrated much improved mixing uniformity with the maximum extent of equivalence ratio deviation being reduced to ∼10%. A three-dimensional computational fluid dynamics simulation was also carried out to illustrate the resulting flow field associated with the baseline configuration and suggest the needed configuration modifications for performance improvement. Experimental and computational results indicate that the matching of fuel atomization and flow field is the primary factor affecting fuel-air mixture uniformity. By optimizing the flow rate ratio of the axial jet air in the nozzle section to the swirling jet air in the tube section as well as the axial jet momentum, enhanced fuel-air mixture uniformity can be achieved.

Commentary by Dr. Valentin Fuster

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