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

J. Eng. Gas Turbines Power. 2008;130(6):061501-061501-5. doi:10.1115/1.2939004.

An experimental investigation was conducted to study the effects of increased ambient pressure (up to $6.89MPa$) and increased nozzle pressure drop (up to $2.8MPa$) on the cone angles for sprays produced by pressure-swirl atomizers having varying amounts of initial swirl. This study extends the classical results of DeCorso and Kemeny, (1957, “Effect of Ambient and Fuel Pressure on Nozzle Spray Angle  ,” ASME Transactions, 79(3), pp. 607–615). Shadow photography was used to measure cone angles at $x∕D0=10$, 20, 40, and 60. Our lower pressure results for atomizer swirl numbers of 0.50 and 0.25 are consistent with those of DeCorso and Kemeny, who observed a decrease in cone angle with an increase in nozzle pressure drop, $ΔP$, and ambient density, $ρair$, until a minimum cone angle was reached when $ΔPρair1.6∼100MPa(kg∕m3)1.6$ (equivalent to $200psi(lbm∕ft3)1.6$). Results for atomizers having higher initial swirl do not match the DeCorso and Kemeny results as well, suggesting that their correlation be used with caution. Another key finding is that an increase in $ΔPρair1.6$ to a value of $600MPa(kg∕m3)1.6$ leads to continued decrease in cone angle, but that a subsequent increase to $2000MPa(kg∕m3)1.6$ has little effect on cone angle. Finally, there was little effect of nozzle pressure drop on cone angle, in contrast to findings of previous workers. These effects are hypothesized to be due to gas entrainment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):061502-061502-8. doi:10.1115/1.2943180.

Laminar flame speeds and strain sensitivities of mixtures of $H2$ and air or air highly diluted with $N2$ ($O2:N2$ 1:9) have been measured for a range of equivalence ratios at high preheat conditions $(∼700K)$ using a nozzle generated, 1D, laminar, wall stagnation flame. The measurements are compared with numerical predictions based on three detailed kinetic models (GRIMECH 3.0 , a $H2∕CO$ mechanism from Davis (2004, “An Optimized Kinetic Model of H2∕CO Combustion  ,” Proc. Combust. Inst., 30, pp. 1283–1292) and a $H2$ mechanism from Li (2004, “An Updated Comprehensive Kinetic Model of Hydrogen Combustion  ,” Int. J. Chem. Kinet., 36, pp. 566–575)). Sensitivity of the measurements to uncertainties in boundary conditions, e.g., wall temperature and nozzle velocity profile (plug or potential), is investigated through detailed numerical simulations and shown to be small. The flame speeds and strain sensitivities predicted by the models for preheated reactants are in reasonable agreement with the measurements for mixtures of $H2$ and standard air at very lean conditions. For $H2$ and $N2$ diluted air, however, all three mechanisms significantly overpredict the measurements, and the overprediction increases for leaner mixtures. In contrast, the models underpredict flame speeds for room temperature mixtures of $H2$ with both standard and $N2$ diluted air, based on comparisons with measurements in literature. Thus, we find that the temperature dependence of the hydrogen flame speed as predicted by all the models is greater than the actual temperature dependence (for both standard and diluted air). Finally, the models are found to underpredict the measured strain sensitivity of the flame speed for $H2$ burning in $N2$ diluted air, especially away from stoichiometric conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Ceramics

J. Eng. Gas Turbines Power. 2008;130(6):061301-061301-8. doi:10.1115/1.2940988.

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as $500 μm$ of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2008;130(6):062101-062101-5. doi:10.1115/1.2943158.

High-temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of life assessment methods of high-temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated thermomechanical-fatigue (TMF) loading during start, steady state, stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between $450°C$ and $870°C$ on a Ni-base directional solidified superalloy. Strain ratio $ϕ$ was defined as shear strain range, $Δγ$, to normal strain range, $Δε$, and $ϕ$ varied from 0 to infinity. The “Blade wave form,” which simulated temperature and strain condition of the blade surface, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under the biaxial TMF loading showed strain ratio dependency giving shorter life with increasing $ϕ$. Considering biaxial stress effect on the failure life, an equivalent shear strain range was derived based on the $Γ$-plane theory, and the biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade wave form. It was concluded that creep damage gradually accumulated during cycles resulting in reduction in the TMF life. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, the failure lives were predicted within a factor of 1.5 on the observed life. It was found that the fatigue life of CoCrAlY coated material reduced $1∕2$ to $1∕3$ from that of the substrate. From observation of the longitudinal section of the coated specimens, many cracks started from the coating surface and penetrated into the substrate. It was concluded that the CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2008;130(6):062501-062501-8. doi:10.1115/1.2940354.

Airfoil bearings offer many advantages over oil-lubricated bearings, but they have reliability issues during start∕stops (wear) and limited heat dissipation capability. To address these issues, a hybrid airfoil bearing (HAFB) combining hydrodynamic airfoil bearing with hydrostatic lift was introduced previously by one of the authors of this paper. Their studies show that HAFB has superior performance compared to its hydrodynamic counterpart in load capacity and cooling performance. In this article, the bearing stiffness and damping coefficients of HAFB are calculated using a linear perturbation method developed for HAFB. Simulations showed that feed parameter and supply pressure affect the dynamic characteristics of HAFB. With an increase in either the supply pressure or the feed parameter, the rotor centers itself and hence one sees a decrease in direct stiffness. Simulations showed that the cross-coupled stiffness could be reduced by increasing either the supply pressure or the feed parameter. Direct damping showed increasing trend with the supply pressure and the feed parameter. Frequency-domain analysis of the bearing coefficients was also performed. The direct damping showed marginal changes with supply pressure but showed rapid increase with increasing excitation frequencies. The damping converged to null values for all the pressures for supersynchronous excitations. The loss in damping with high stiffness values for high frequency excitation is a typical hardening effect of gas bearings. In almost all the cases, there are rapid decreases in cross-coupled stiffness and damping and the values show converging trends in supersynchronous regime.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062502-062502-12. doi:10.1115/1.2943149.

This paper presents a novel method of multiscale association for analyzing a turbogenerator accident having strange behaviors and serious consequence. Wave index (WI) and credibility of sensor fault are proposed based on multiscale analysis of the recorded data, and then the associational degree of WI is used to detect sensor fault. In addition, mechanism models are built to verify that detection. Furthermore, maximum likelihood method and neural network are applied to estimate the confidence interval of the fault sensor and the true signal. The estimation has been used to clearly explain the cause of this accident.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062503-062503-10. doi:10.1115/1.2795785.

Within the European research project (Advanced Transmission and Oil System Concepts), a systematic study of the separation efficiency of a typical aeroengine air∕oil separator design was conducted. The main objectives were to obtain a basic understanding of the main separation mechanisms and to identify the relevant parameters affecting the separation efficiency. The results of the study contribute to an optimized separator technology. Nonintrusive optical measurement techniques like laser diffraction and multiple wavelength extinction were applied to analyze the separation efficiency and identify potential optimization parameters. Oil mist with defined oil droplet size distribution was supplied to the breather. By simultaneously measuring particle size and oil concentration upstream and downstream of the breather, the separation mechanism was analyzed and the separation efficiency was assessed. In addition, the pressure drop across the separator was measured. The pressure drop is an important design feature and has to be minimized for proper sealing of the engine bearing chambers. The experimental programe covered a variation of air flow, oil flow, shaft speed, and droplet size. The main emphasis of the investigations was on the separation of small droplets with a diameter of up to $10μm$. The following trends on separation efficiency of small droplets were observed: The separation efficiency increases with increasing rotational speed, with increasing particle size, and with decreasing air flow rate. In parallel, the pressure drop across the breather increases with increasing speed and increasing air flow.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062504-062504-15. doi:10.1115/1.2943152.

A new energy-based life prediction framework for calculation of axial and bending fatigue results at various stress ratios has been developed. The purpose of the life prediction framework is to assess the behavior of materials used in gas turbine engines, such as Titanium 6Al-4V (Ti 6Al-4V) and Aluminum 6061-T6 (Al 6061-T6). The work conducted to develop this energy-based framework consists of the following entities: (1) a new life prediction criterion for axial and bending fatigue at various stress ratios for Al 6061-T6, (2) the use of the previously developed improved uniaxial energy-based method to acquire fatigue life prior to endurance limit region (Scott-Emuakpor, 2007, “Development of an Improved High Cycle Fatigue Criterion  ,” ASME J. Eng. Gas Turbines Power, 129, pp. 162–169), (3) and the incorporation of a probabilistic energy-based fatigue life calculation scheme to the general uniaxial life criterion (the first entity of the framework), which is capable of constructing prediction intervals based on a specified percent confidence level. The precision of this work was verified by comparison between theoretical approximations and experimental results from recently acquired Al 606-T6 and Ti 6Al-4V data. The comparison shows very good agreement, thus validating the capability of the framework to produce accurate uniaxial fatigue life predictions for commonly used gas turbine engine materials.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062505-062505-10. doi:10.1115/1.2966391.

Microturbomachinery implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and wear quickly during transient rub events. Flexure pivot tilting pad bearings offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability; and when modified for hydrostatic pressurization, demonstrate superior rotordynamic performance over other bearing types. External pressurization stiffens gas bearings thus increasing system critical speeds, albeit reducing system damping. Most importantly, measurements demonstrate that external pressurization is not needed for rotor supercritical speed operation. In practice, the supply pressure could be shut off at high rotor speeds with substantial gains in efficiency. This paper introduces a simple strategy, employing an inexpensive air pressure regulator to control the supply pressure into the hybrid bearings, to reduce or even eliminate high amplitudes of rotor motion while crossing the system critical speeds. Rotor speed coast-down tests with the pressure controller demonstrate the effectiveness of the proposed approach. A simple on-off supply pressure control, i.e., a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed. A rotordynamic analysis integrating predicted bearing force coefficients forwards critical speeds in agreement with the test results. Predicted rotor responses for the controlled supply conditions show an excellent correlation with measured data. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of flexure pivot hybrid gas bearings.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2008;130(6):061601-061601-9. doi:10.1115/1.2940989.

This paper presents an example of the use of fuzzy logic combined with influence coefficients applied to engine test-cell data to diagnose gas-path related performance faults. The approach utilizes influence coefficients, which describe the changes in measurable parameters due to changes in component condition such as compressor efficiency. Such approaches usually have the disadvantages of attributing measurement noise or sensor errors to changes in engine condition and do not have the ability to diagnose more faults than the number of measurement parameters that exist. These disadvantages usually make such methods impractical for anything but simulated data without measurement noise or errors. However, in this example, the influence coefficients are used in an iterative approach, in combination with fuzzy logic, to overcome these obstacles. The method is demonstrated using eight examples from real-world test-cell data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):061602-061602-8. doi:10.1115/1.2966390.

The measurement of pressure within both stationary and rotating frames of reference is a fundamental requirement when studying the flow field through turbomachinery blading. Measurement of pressure within the rotating frame presents a particular challenge, as centrifugal acceleration of the sensor can have a significant impact on sensor calibration, and therefore accuracy of the resulting measurements. In this paper the telemetric calibration of pressure sensors at up to 12,000 g is described, and the impact on calibration of membrane size, sensor body shape, and sensor mounting direction is discussed. The program of work reported in this paper focuses on experimental issues associated with rotating pressure measurement. The combined effect of centrifugal load and pressure on integrally temperature compensated silicon pressure sensors is presented. Experimental results are given that provide insight into the influence of acceleration on pressure readings. Implementation of acceleration into sensor calibration is presented. Supplementary finite element calculations enable impact of sensor body shape to be taken into account during the evaluation of sensor acceleration-to-pressure sensitivity ratio. Different sensors with varied membrane sizes and acceleration force directions are examined and compared.

Commentary by Dr. Valentin Fuster

Research Papers: Power Engineering

J. Eng. Gas Turbines Power. 2008;130(6):063001-063001-7. doi:10.1115/1.2720544.

One of the most significant contributors to the overall uncertainty of a performance test of a combined cycle steam turbine is the uncertainty of the primary flow measurement. ASME performance test codes provide many alternative methods for determining flow. In two actual combined cycle tests performed in 2005, the following three alternate methods were used to determine the high-pressure (HP) steam flow into the combined cycle steam turbines: (1) Derivation from measured HP feedwater flow using calibrated PTC 6 throat tap nozzles, (2) derivation from low-pressure (LP) condensate using calibrated PTC 6 throat tap nozzles, and (3) derivation from LP condensate using calibrated orifice metering sections. This paper describes the design, calibration, and installation of each flow meter involved, the methods used to calculate the HP steam flow, the estimated uncertainty of the HP steam flow derived using each method, and the actual test results using each method. A comparison of the methods showed that there are distinct advantages with one of the methods and that very low uncertainties in HP steam flow can be achieved if sufficient attention is applied to the design, calibration, and installation of all flow meters involved. Note that the information in this paper was originally published in ASME Paper PWR2006-88074 and presented at the 2006 ASME Power Conference in Atlanta, GA. For detailed diagrams, figures, and tabulations of data and analysis, please refer to the published proceedings from that conference.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2008;130(6):062801-062801-7. doi:10.1115/1.2938271.

Recently, an in-cylinder injection method has been considered for the improvement of thermal efficiency in natural-gas and hydrogen spark-ignition (SI) engines. However, the SI and combustion processes of gaseous jets are not well understood. The present study aims to provide fundamental data for the development of direct-injection SI gas engines. The ignition, combustion, and flame behavior of high-pressure and intermittent hydrogen and natural-gas jets in a constant volume combustion chamber were investigated. The effects of injection pressure, nozzle size, ambient pressure, and spark location were also investigated for various spark timings and equivalence ratios.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062802-062802-11. doi:10.1115/1.2938274.

A general form of the stress-strain constitutive relation was introduced for the application of two nonlinear $k-ε$ turbulence models, namely, the algebraic Reynolds stress model of Gatski and Speziale (1993, “On Explicit Algebraic Stress Models for Complex Turbulent Flows  ,” J. Fluid Mech., 254, pp. 59–78) and the cubic model of Lien (1996, “Low Reynolds Number Eddy-Viscosity Modeling Based on Non-Linear Stress-Strain/Vorticity Relations  ,” Proceedings of Third Symposium on Engineering Turbulence Modeling and Measurements, Crete, Greece), to the numerical analysis of flow fields in a test engine with flat-piston and bowl-in-piston arrangements, under swirling and no-swirling flow motored conditions. The model capabilities in capturing turbulent flow features were compared to those of the upgraded linear RNG $k-ε$ model, which was previously indicated as a good compromise between accuracy and computational cost. Evaluations were made on the basis of the predicted flow evolution throughout the whole engine cycle, as well as of the comparison between the numerical and experimental results. Furthermore, the effect of the stress-strain relationship on the predicted averaged turbulence quantities and anisotropy-invariant values were examined, in addition to the sensitivity of the nonlinear models to the mesh quality. Finally, prospects concerning possible improvements of turbulence eddy-viscosity models were presented. The predictions were made by a newly developed CFD code embedding various accuracy-order finite-volume discretization schemes. Modified wall boundary conditions with respect to the conventional logarithmic-function approach were used, so as to make the local equilibrium hypothesis virtually ineffective.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062803-062803-9. doi:10.1115/1.2939010.

Torque-based engine control systems usually employ a produced torque estimation feedback in order to verify that the strategy target torque has been met. Torque estimation can be performed using static maps describing the engine behavior or using models describing the existing relationships between signals measured on the engine and the indicated torque produced. Signals containing information on the combustion development, suitable for this purpose, are, among others, the ion-current signal, the vibration signals obtained from accelerometers mounted on the engine block, or the instantaneous engine speed fluctuations. This paper presents the development and the identification process of an engine-driveline torsional behavior model that enables indicated torque estimation from instantaneous engine speed measurement. Particular attention has been devoted to the interactions between indicated and reciprocating torques, and their effects over instantaneous engine speed fluctuations. Indicated and reciprocating torques produce, in fact, opposite excitations on the driveline that show opposite effects on the engine speed wave form: For low engine speed, usually indicated torque prevails, while the opposite applies for higher engine speed. In order to correctly estimate indicated torque from engine speed measurement, it is therefore necessary to correctly evaluate the reciprocating torque contribution. Reciprocating torque is usually described using a wave form as a function of crank angle, while its amplitude depends on the value of the reciprocating masses. As mentioned before, knowledge of the reciprocating masses is fundamental in order to obtain correct estimation of the indicated torque. The identification process that has been set up for the engine-driveline torsional model enables to evaluate the relationship between torques applied to the engine and the corresponding engine speed wave form even without knowing the value of the reciprocating masses. In addition, once this model has been set up, it is possible to estimate with high precision the value of the reciprocating masses. Particular attention has also been devoted to the feasibility of the application of the identified model onboard for torque estimation; for this reason, the model has been developed in a very simple form. The approach proved to be effective both on gasoline and diesel engines, both for engine mounted on a test cell and onboard, with different engine configurations. Examples of application are given for some of the configurations investigated.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062804-062804-8. doi:10.1115/1.2940353.

The development of a hydrogen-fueled engine using external mixture injection (e.g., using port or manifold fuel injection) with high efficiency and high power is dependent on the control of backfire. This work has developed a method to control backfire by reducing the valve overlap period while maintaining or improving engine performance. For this goal, a single-cylinder hydrogen-fueled research engine with a mechanical continuous variable valve timing system was developed. This facility provides a wide range of valve overlap periods that can be continuously and independently varied during firing operation. By using this research engine, the behavior of backfire occurrence and engine performance are determined as functions of the valve overlap period for fuel-air equivalence ratios between 0.3 and 1.2. The results showed that the developed hydrogen-fueled research engine with the mechanical continuous variable valve timing system has similar performance to a conventional engine with fixed valve timings, and is especially effective in controlling the valve overlap period. Backfire occurrence is reduced with a decrease in the valve overlap period, and is also significantly decreased even under operating conditions with the same volumetric efficiency. These results demonstrate that decreasing the valve overlap period may be one of the methods for controlling backfire in a hydrogen-fueled engine while maintaining or improving performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062805-062805-11. doi:10.1115/1.2943193.

The simulation of heat release, flame propagation speeds, and pollutant formation was carried out in both a turbocharged compressed natural gas (CNG) engine and a multivalve naturally aspirated bifuel engine running on either CNG or gasoline. The predictive tool used for investigation is based on an enhanced fractal geometry concept of the flame front, which is able to capture the modulation of turbulent to laminar burning speed ratio throughout the overall combustion phase without introducing flame kernel growth or burnout submodels. The prediction model was applied to a wide range of engine speeds, loads, relative air-fuel ratios, and spark advances, and the obtained results were compared to experimental data. These latter were extracted from measured in-cylinder pressure by an advanced diagnostics technique that was previously developed by the authors. The results confirmed a quite accurate prediction of burning speed even without any kind of tuning, with respect to different currently available fractal as well as nonfractal approaches for the simulation of flame-turbulence interaction. Furthermore, the computational code proved to be capable of capturing the effects of fuel composition, different combustion-chamber concepts, and operating conditions on engine performance and emissions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062806-062806-9. doi:10.1115/1.2943194.

Expanding the range of HCCI operation will be critical for maximizing the fuel economy benefits in future vehicle applications. The mixture stratification, both thermal and compositional, can have very tangible impact on HCCI combustion, and gaining a deeper insight into these effects is critical for expanding the HCCI range of operation. This paper presents results of the comprehensive experimental investigation of the mixture preparation effects on a single-cylinder gasoline HCCI engine with exhaust reinduction. The effects include the type of mixture preparation (external mixing versus direct injection), charge motion, and injection timing. A combination of pressure-based combustion diagnostics, emission analysis, and heat flux measurements on the combustion chamber wall quantifies the effects on combustion and provides insight into reasons for observed engine behavior. As an example, the instantaneous temperature and heat flux measurements show the fuel impingement locations and allow assessing the fuel film dynamics and their effect on mixture stratification. The effects of direct injection and partial closing of the swirl control valve are relatively small compared with extending the injection timing late into the intake process or completely closing the swirl control valve and allowing charge storage in the intake port.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062807-062807-18. doi:10.1115/1.2939002.

Researchers have been using one-dimensional based models of diesel particulate filters (DPFs) for over two decades with good success in comparison to measured experimental data. Recent efforts in literature have expanded the classical model to account for the effects of varying soot layer thickness on the flow area of the gases. However, some discrepancies exist with respect to this formulation and the physical phenomena modeled in the channel equations. In addition, there is still some discussion regarding the calculation of the gas temperature within the soot and wall layers. As a result, this paper presents a model to discuss these different phenomena to remove or validate previous assumptions. In specific, formulation of the flow equations in area-conserved format (or quasi-one-dimensional) allows the model to account for the changes in the gaseous area as a function of soot loading. In addition, imposing thermodynamic equilibrium at the interface of the channels and wall layers allows the model to capture the thermal entrance lengths. These tasks were undertaken to illustrate whether or not the results justify the effort is worthwhile and this additional complexity needs to be incorporated within the model. By utilizing linear density interpolation in the wall to increase the computational efficiency of the code, it was determined that the classical model assumptions of neglecting soot thickness and gas temperature in the wall are valid within the range of typical DPF applications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062808-062808-6. doi:10.1115/1.2939003.

An extension to a phenomenological submodel for soot formation to include soot agglomeration effects is developed. The improved submodel was incorporated into a commercial computational fluid dynamics code and was used to investigate soot formation in a heavy-duty diesel engine. The results of the numerical simulation show that the soot oxidation process is reduced close to the combustion chamber walls, due to heat loss, such that larger soot particles and clusters are predicted in an annular volume at the end of the combustion cycle. These results are consistent with available in-cylinder experimental data and suggest that the cylinder of a diesel engine must be split into several volumes, each of them with a different role regarding soot formation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(6):062809-062809-13. doi:10.1115/1.2925679.

A rather complete mathematical model for a common-rail injection-system dynamics numerical simulation was developed to support experimentation, layout, and control design, as well as performance optimization. The thermofluid dynamics of the hydraulic-system components, including rail, connecting pipes, and injectors was modeled in conjunction with the solenoid-circuit electromagnetics and the mechanics of mobile elements. One-dimensional flow equations in conservation form were used to simulate wave propagation phenomena throughout the high-pressure connecting pipes, including the feeding pipe of the injector nozzle. In order to simulate the temperature variations due to the fuel compressibility, the energy equation was used in addition to mass conservation and momentum balance equations. Besides, the possible cavitation phenomenon effects on the mass flow rate through the injector bleed orifice and the nozzle holes were taken into account. A simple model of the electromagnetic driving circuit was used to predict the temporal distribution of the force acting on the pilot-valve anchor. It was based on the experimental time histories of the current through the solenoid and of the associated voltage that is provided by the electronic control unit to the solenoid. The numerical code was validated through the comparison of the prediction results with experimental data, that is, pressure, injected flow rate, and needle lift time histories, taken on a high performance test bench Moehwald-Bosch MEP2000-CA4000. The novel injection-system mathematical model was applied to the analysis of transient flows through the hydraulic circuit of a commercial multijet second-generation common-rail system, paying specific attention to the wave propagation phenomena, to their dependence on solenoid energizing time and rail pressure, as well as to their effects on system performance. In particular, an insight was also given into the model capability of accurately predicting the wave dynamics effects on the rate and mass of fuel injected when the dwell time between two consecutive injections is varied.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2008;130(6):061201-061201-9. doi:10.1115/1.2943196.

Ejectors are commonly employed in gas turbine exhaust systems for reasons such as space ventilation and IR suppression. Ejectors may incorporate bends in the geometry for various reasons. Studies have shown that the bend has a deteriorating effect on the performance of an ejector. This work was aimed to investigate the effect of exhaust gas swirl on improving the performance of a bent ejector. Four short oblong ejectors with different degrees of bend in the mixing tube and four swirl conditions were tested in this study. The primary nozzle, in all cases, was composed of a circular to oblong transition. Testing was performed at ambient and hot primary flow with $0deg$, $10deg$, $20deg$, and $30deg$ swirl angles. It was observed that the swirl had a strong affect on the performance of a bent ejector. Improvement of up to 55%, 96%, and 180% was obtained in the pumping ratio, pressure rise, and total efficiency, respectively, with a $20deg$ swirl in the exhaust gas.

Commentary by Dr. Valentin Fuster