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### TECHNICAL PAPERS: Fuels & Combustion Technology

J. Eng. Gas Turbines Power. 2005;128(2):241-246. doi:10.1115/1.2056536.

Two-dimensional images of OH fluorescence, polycyclic aromatic hydrocarbons (PAHs) fluorescence, and laser-induced incandescence (LII) from soot were measured in a sooting diffusion flame. To obtain an accurate OH fluorescence image, two images were taken with the laser wavelength tuned to (“on”) and away from (“off”) the OH absorption line. An accurate OH fluorescence image was obtained by subtracting the off-resonance image from the on-resonance image. For the PAH fluorescence and LII measurements, temporally resolved measurements were used to obtain the individual images; the LII image was obtained by detecting the LII signal after the PAH fluorescence radiation had stopped and the PAH fluorescence image was obtained by subtracting the LII image from the simultaneous image of PAH fluorescence and LII. Based on the obtained images, the relative location of OH, PAH, and soot in the flame was discussed in detail. To investigate the PAH size distribution in a sooting flame using LIF, an estimation strategy for PAH size is proposed. Emission spectra were measured at several heights in the flame using a spectrograph. Since the emission wavelength of PAH fluorescence shifts toward longer wavelengths with increasing PAH size, the main PAH components in the emission spectra could be estimated. The results suggest that PAH grows and the type of PAH changes as the soot inception region was approached. Near the soot inception region, we estimated that the PAHs, which have over 16 carbon atoms, mainly constituted the emission spectrum.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

J. Eng. Gas Turbines Power. 2004;128(2):247-254. doi:10.1115/1.2061267.

The technological objective of this work is the development of a lean-premixed burner for natural gas. Sub-ppm $NOx$ emissions can be accomplished by shifting the lean blowout limit (LBO) to slightly lower adiabatic flame temperatures than the LBO of current standard burners. This can be achieved with a novel burner concept utilizing spatially periodic recirculation of combustion products: Hot combustion products are admixed to the injected premixed fresh mixture with a mass flow rate of comparable magnitude, in order to achieve self-ignition. The subsequent combustion of the diluted mixture again delivers products. A fraction of these combustion products is then admixed to the next stream of fresh mixture. This process pattern is to be continued in a cyclically closed topology, in order to achieve stable combustion of, for example, natural gas in a temperature regime of very low $NOx$ production. The principal ignition behavior and $NOx$ production characteristics of one sequence of the periodic process was modeled by an idealized adiabatic system with instantaneous admixture of partially or completely burnt combustion products to one stream of fresh reactants. With the CHEMKIN-II package, a reactor network consisting of one perfectly stirred reactor (PSR, providing ignition in the first place) and two plug flow reactors (PFR) has been used. The effect of varying burnout and the influence of the fraction of admixed flue gas has been evaluated. The simulations have been conducted with the reaction mechanism of Miller and Bowman and the GRI-Mech 3.0 mechanism. The results show that the high radical content of partially combusted products leads to a massive decrease of the time required for the formation of the radical pool. As a consequence, self-ignition times of 1 ms are achieved even at adiabatic flame temperatures of 1600 K and less, if the flue gas content is about 50–60% of the reacting flow after mixing is complete. Interestingly, the effect of radicals on ignition is strong, outweighs the temperature deficiency and thus allows stable operation at very low $NOx$ emissions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(2):255-263. doi:10.1115/1.2131887.

This study presents a novel multiobjective genetic-algorithm approach to produce a new reduced chemical kinetic reaction mechanism to simulate aviation fuel combustion under various operating conditions. The mechanism is used to predict the flame structure of an aviation fuel/$O2∕N2$ flame in both spatially homogeneous and one-dimensional premixed combustion. Complex hydrocarbon fuels, such as aviation fuel, involve large numbers of reaction steps with many species. As all the reaction rate data are not well known, there is a high degree of uncertainty in the results obtained using these large detailed reaction mechanisms. In this study a genetic algorithm approach is employed for determining new reaction rate parameters for a reduced reaction mechanism for the combustion of aviation fuel-air mixtures. The genetic algorithm employed incorporates both perfectly stirred reactor and laminar premixed flame data in the inversion process, thus producing an efficient reaction mechanism. This study provides an optimized reduced aviation fuel-air reaction scheme whose performance in predicting experimental major species profiles and ignition delay times is not only an improvement on the starting reduced mechanism but also on the full mechanism.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(2):264-270. doi:10.1115/1.2132379.

“Rumble” is a self-excited combustion instability, usually occurring at the start-up of aero-engines with fuel-spray atomizers at sub-idle and idle conditions, and exhibiting low limit frequencies in the range of $50Hzto150Hz$. Entropy waves at the (nearly) choked combustor outlet are supposed to be the key feedback mechanism for the observed self-excited pressure oscillations. The experimental study presented here aims at clarifying the role of entropy waves in the occurrence of rumble. A generic air-blast atomizer with a design prone to self-excitation has been incorporated into a thermoacoustic combustor test rig with variable outlet conditions. The thermoacoustic response of the flame was characterized by recording the $OH*$ chemiluminescence, the dynamic pressures, the dynamic temperatures, and by applying PIV. The measurements have shown the occurrence of periodic hot spots traveling with the mean flow with considerable dispersion. Measurements have been conducted with an open-ended resonance tube in order to eliminate the impact of entropy waves on the mechanism of self-excitation. The oscillation obtained, comparable in amplitude and frequency, proved that self-excitation primarily depends on convective time delays of the droplets in the primary zone and thus on the atomization characteristics of the nozzle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):271-280. doi:10.1115/1.2056535.

The purpose of this paper is to aid systems analysts in the design, modeling, and assessment of advanced, gasification-based power generation systems featuring air separation units (ASUs) integrated with gas turbines adapted for syngas fuel. First, the fundamental issues associated with operating a gas turbine on syngas will be reviewed, along with the motivations for extracting air from the turbine-compressor and/or injecting nitrogen into the turbine expander. Configurations for nitrogen-only and air-nitrogen ASU integration will be described, including the benefits and drawbacks of each. Cryogenic ASU technology will be summarized for both low-pressure and elevated-pressure applications and key design and integration issues will be identified and discussed. Finally, membrane-based ASU technology will be described and contrasted with cryogenic technology in regard to system design and integration.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Controls, Diagnostics & Instrumentation

J. Eng. Gas Turbines Power. 2004;128(2):281-287. doi:10.1115/1.2056507.

A diagnostic method consisting of a combination of Kalman filters and Bayesian Belief Network (BBN) is presented. A soft-constrained Kalman filter uses a priori information derived by a BBN at each time step, to derive estimations of the unknown health parameters. The resulting algorithm has improved identification capability in comparison to the stand-alone Kalman filter. The paper focuses on a way of combining the information produced by the BBN with the Kalman filter. An extensive set of fault cases is used to test the method on a typical civil turbofan layout. The effectiveness of the method is thus demonstrated, and its advantages over individual constituent methods are presented.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2004;128(2):288-293. doi:10.1115/1.2056529.

The main goal of this work is the transient analysis of hybrid systems based on solid oxide fuel cells (SOFC). The work is divided into three parts: in the first, the fuel cell transient models are presented and discussed, whereas in the subsequent parts of the paper the anodic recirculation system (Part B: Ferrari, M.L., Traverso, A., Massardo, A.F., 2004, ASME Paper No. 2004-GT-53716) and the entire hybrid transient performance (Part C: Magistri, L., Ferrari, M.L., Traverso, A., Costamagna, P., Massardo, A.F., 2004, ASME Paper No. 2004-GT-53845) are investigated. In this paper the transient behavior of a solid oxide fuel cell is analyzed through the use of two different approaches: macroscopic and detailed SOFC models. Both models are presented in this paper, and their simulation results are compared to each other and to available experimental data. As a first step the transient response of the fuel cell was studied using a very detailed model in order to completely describe this phenomenon and to highlight the critical aspects. Subsequently, some modifications were made to this model to create an apt simulation tool (macroscopic fuel cell model) for the whole plant analysis. The reliability of this model was verified by comparing several transient responses to the results obtained with the detailed model. In the subsequent papers (Parts B and C), the integration of the macroscopic fuel cell model into the whole plant model will be described and the transient study of the hybrid plant will be presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2006;128(2):294-301. doi:10.1115/1.1852565.

Hybrid fuel cell/gas turbine systems provide an efficient means of producing electricity from fossil fuels with ultra low emissions. However, there are many significant challenges involved in integrating the fuel cell with the gas turbine and other components of this type of system. The fuel cell and the gas turbine must maintain efficient operation and electricity production while protecting equipment during perturbations that may occur when the system is connected to the utility grid or in stand-alone mode. This paper presents recent dynamic simulation results from two laboratories focused on developing tools to aid in the design and dynamic analyses of hybrid fuel cell systems. The simulation results present the response of a carbonate fuel cell/gas turbine, or molten carbonate fuel cell/gas turbine, (MCFC/GT) hybrid system to a load demand perturbation. Initial results suggest that creative control strategies will be needed to ensure a flexible system with wide turndown and robust dynamic operation.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2005;128(2):302-311. doi:10.1115/1.2061287.

The gas turbine engine is a complex assembly of a variety of components that are designed on the basis of aerothermodynamic laws. The design and operation theories of these individual components are complicated. The complexity of aerothermodynamic analysis makes it impossible to mathematically solve the optimization equations involved in various gas turbine cycles. When gas turbine engines were designed during the last century, the need to evaluate the engines performance at both design point and off design conditions became apparent. Manufacturers and designers of gas turbine engines became aware that some tools were needed to predict the performance of gas turbine engines especially at off design conditions where its performance was significantly affected by the load and the operating conditions. Also it was expected that these tools would help in predicting the performance of individual components, such as compressors, turbines, combustion chambers, etc. At the early stage of gas turbine developments, experimental tests of prototypes of either the whole engine or its main components were the only method available to determine the performance of either the engine or of the components. However, this procedure was not only costly, but also time consuming. Therefore, mathematical modelling using computational techniques were considered to be the most economical solution. The first part of this paper presents a discussion about the gas turbine modeling approach. The second part includes the gas turbine component matching between the compressor and the turbine which can be met by superimposing the turbine performance characteristics on the compressor performance characteristics with suitable transformation of the coordinates. The last part includes the gas turbine computer simulation program and its philosophy. The computer program presented in the current work basically satisfies the matching conditions analytically between the various gas turbine components to produce the equilibrium running line. The computer program used to determine the following: the operating range (envelope) and running line of the matched components, the proximity of the operating points to the compressor surge line, and the proximity of the operating points at the allowable maximum turbine inlet temperature. Most importantly, it can be concluded from the output whether the gas turbine engine is operating in a region of adequate compressor and turbine efficiency. Matching technique proposed in the current work used to develop a computer simulation program, which can be served as a valuable tool for investigating the performance of the gas turbine at off-design conditions. Also, this investigation can help in designing an efficient control system for the gas turbine engine of a particular application including being a part of power generation plant.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):312-317. doi:10.1115/1.2131888.

Background: Power generation from gas turbines is penalized by a substantial power output loss with increased ambient temperature. By cooling down the gas turbine intake air, the power output penalty can be mitigated. Method of Approach: The purpose of this paper is to review the state of the art in applications for reducing the gas turbine intake air temperature and examine the merits from integration of the different air-cooling methods in gas-turbine-based power plants. Three different intake air-cooling, methods (evaporative cooling, refrigeration cooling, and evaporative cooling of precompressed air) have been applied in two combined cycle power plants and two gas turbine plants. The calculations were performed on a yearly basis of operation, taking into account the time-varying climatic conditions. The economics from integration of the different cooling systems were calculated and compared. Results: The results have demonstrated that the highest incremental electricity generation is realized by absorption intake air-cooling. In terms of the economic performance of the investment, the evaporative cooler has the lowest total cost of incremental electricity generation and the lowest payback period (PB). Concerning the cooling method of pre-compressed air, the results show a significant gain in capacity, but the total cost of incremental electricity generation in this case is the highest. Conclusions: Because of the much higher capacity gain by an absorption chiller system, the evaporative cooler and the absorption chiller system may both be selected for boosting the performance of gas-turbine-based power plants, depending on the prevailing requirements of the plant operator.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2004;128(2):318-325. doi:10.1115/1.2056532.

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions to occur downstream of the combustor is increased. This may take place when partially reacted species leave the combustor and encounter film-cooled surfaces. One common feature on turbine endwalls is a step between various engine components and seals. Such step features produce recirculating flows which when in the vicinity of film-cooled surfaces may lead to particularly severe reaction zones due to long fluid residence times. The objective of this paper is to study and quantify the surface heat transfer implications of such reacting regions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm) flows over a film-cooled backward-facing step. The test article contained two sets of 35 deg film cooling holes located downstream of a step. The film-cooling holes could be supplied with different gases, one side using air and the other nitrogen allowing for simultaneous testing of reacting and inert cooling gases. A mixture of ethylene and argon provided a fuel-rich free stream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damköhler numbers (ratio of characteristic flow time to chemical time) from near zero to 30. The experimental results show that for conditions relevant for future engine technology, adiabatic flame temperatures can be approached along the wall downstream of the step leading to potentially significant increases in surface heat flux. A computational study was also performed to investigate the effects of cooling-jet blowing ratio on chemical reactions behind the film-cooled step. The blowing ratio was found to be an important parameter governing the flow structure behind the backward-facing step, and controlling the characteristics of chemical-reactions by altering the local equivalence ratio.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Industrial and Cogeneration

J. Eng. Gas Turbines Power. 2006;128(2):326-335. doi:10.1115/1.1765122.

In recent years, deregulation in the power generation market worldwide combined with significant variation in fuel prices and a need for flexibility in terms of power augmentation specially during periods of high electricity demand (summer months or noon to 6:00 p.m.) has forced electric utilities, cogenerators and independent power producers to explore new power generation enhancement technologies. In the last five to ten years, inlet fogging approach has shown more promising results to recover lost power output due to increased ambient temperature compared to the other available power enhancement techniques. This paper presents the first systematic study on the effects of both inlet evaporative and overspray fogging on a wide range of combined cycle power plants utilizing gas turbines available from the major gas turbine manufacturers worldwide. A brief discussion on the thermodynamic considerations of inlet and overspray fogging including the effect of droplet dimension is also presented. Based on the analyzed systems, the results show that high pressure inlet fogging influences performance of a combined cycle power plant using an aero-derivative gas turbine differently than with an advanced technology or a traditional gas turbine. Possible reasons for the observed differences are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(2):336-343. doi:10.1115/1.2131889.

To attain the highest economic and energy-saving characteristics of gas turbine cogeneration plants, it is necessary to rationally determine capacities and numbers of gas turbines and auxiliary equipment in consideration of their operational strategies corresponding to seasonal and hourly variations in energy demands. Some optimization approaches based on the mixed-integer linear programing have been proposed to this design problem. However, equipment capacities have been treated as continuous variables, and correspondingly, performance characteristics and capital costs of equipment have been assumed to be continuous functions with respect to their capacities. This is because if equipment capacities are treated discretely, the number of integer variables increases drastically and the problem becomes too difficult to solve. As a result, the treatment of equipment capacities as continuous variables causes discrepancies between existing and optimized values of capacities and expresses the dependence of performance characteristics and capital costs on capacities with worse approximations. In this paper, an optimal design method is proposed in consideration of discreteness of equipment capacities. A formulation for keeping the number of integer variables as small as possible is presented to solve the optimal design problem easily. This method is applied to the design of a gas turbine cogeneration plant, and its validity and effectiveness are clarified.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):344-353. doi:10.1115/1.2132378.

By being exposed to atmospheric conditions gas turbines are inevitably subjected to sources of fouling. The resulting degradation can be partially recovered by cleaning the compressor. Based on open literature and patents, the different approaches leading to the most advanced method of compressor online washing have been compiled. The origins of online washing and the development trends over the decades are outlined, and the current systems are categorized. The introduction of system categories has been justified by a field survey. Additionally, the main design parameters of online washing systems are summarized.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Manufacturing, Materials & Metallurgy

J. Eng. Gas Turbines Power. 2004;128(2):354-361. doi:10.1115/1.2056527.

Alloy selection and alloy design both require consideration of an array of material attributes, including in-service properties, weldability, and fabricability. Critical properties of modern heat-resistant alloys for gas turbine applications include high-temperature strength, thermal stability, oxidation resistance, and fatigue resistance. In this paper, the properties of 12 solid-solution-strengthened and six age-hardenable heat-resistant alloys are compared. Weldability is an important attribute and can be a major limiting factor in the use of certain alloys. Weldability test methods are discussed, and the resistance of alloys to solidification cracking and strain-age cracking is compared. The use of weldability testing in the development of modern heat-resistant alloys is discussed with several examples cited. Finally, alloy selection for gas turbine components is outlined, taking into account both alloy properties and fabricability.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2004;128(2):362-369. doi:10.1115/1.2056530.

This paper investigates the vibration characteristics of rotor displacement signals in a magnetic bearing system under conditions when rotor contact with auxiliary bearings is possible. Since these signals may be used for feedback control, it is necessary to determine how they may affect the ability of the controller to regain rotor levitation. An experimental system is used to demonstrate the sensitivity of the rotor nonlinear dynamic behavior to unbalance, which is sufficient to cause contact during rotor run-up through rigid-body and flexural mode critical speeds. Complex rotor dynamics may involve contact with more than one auxiliary bearing or bush. Application of appropriate rotating forces to the rotor through a magnetic bearing is also shown to induce similar contact dynamics. Thus, an alternative procedure for assessing the nonlinear rotor dynamic behavior is established with the potential for identification of appropriate control forces. The contact dynamics are also considered in the presence of auxiliary bearing misalignment. Misalignment may arise through physical translation of a housing or through steady-state offset errors in sensor measurements. A misalignment of 50% of the nominal radial clearance is applied at an auxiliary bearing. Various contact modes are evident as the rotor is run up in speed. During rundown, different contact dynamics may be encountered and the level of such hysteresis is assessed.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2004;128(2):370-376. doi:10.1115/1.2056531.

New materials are being evaluated to replace type 347 stainless steel in microturbine recuperators operating at higher temperatures in order to increase the efficiency of the microturbine. Commercial alloys 120 and 625 are being tested along with potentially lower cost substitutes, such as Fe-20Cr-25Ni and Fe-20Cr-20Ni. Long-term testing of these materials at 650–700 °C shows excellent corrosion resistance to a simulated exhaust gas environment. Testing at 800 °C has been used to further differentiate the performance of the various materials. The depletion of Cr from foils of these materials is being used to evaluate the rate of attack. Although those alloys with the highest Ni and Cr contents have longer lives in this environment, lower alloyed steels may have sufficient protection at a lower cost.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2006;128(2):377-387. doi:10.1115/1.1805548.

In this research, combustion characteristics of gasoline compression ignition engines have been analyzed numerically and experimentally with the aim of expanding the high load operation limit. The mechanism limiting high load operation under homogeneous charge compression ignition (HCCI) combustion was clarified. It was confirmed that retarding the combustion timing from top dead center (TDC) is an effective way to prevent knocking. However, with retarded combustion, combustion timing is substantially influenced by cycle-to-cycle variation of in-cylinder conditions. Therefore, an ignition timing control method is required to achieve stable retarded combustion. Using numerical analysis, it was found that ignition timing control could be achieved by creating a fuel-rich zone at the center of the cylinder. The fuel-rich zone works as an ignition source to ignite the surrounding fuel-lean zone. In this way, combustion consists of two separate auto-ignitions and is thus called two-step combustion. In the simulation, the high load operation limit was expanded using two-step combustion. An engine system identical to a direct-injection gasoline (DIG) engine was then used to validate two-step combustion experimentally. An air-fuel distribution was created by splitting fuel injection into first and second injections. The spark plug was used to ignite the first combustion. This combustion process might better be called spark-ignited compression ignition combustion (SI-CI combustion). Using the spark plug, stable two-step combustion was achieved, thereby validating a means of expanding the operation limit of gasoline compression ignition engines toward a higher load range.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):388-396. doi:10.1115/1.2056027.

In this study we report the development and validation of phenomenological models for predicting direct injection (DI) diesel engine emissions, including nitric oxide (NO), soot, and unburned hydrocarbons (HC), using a full engine cycle simulation. The cycle simulation developed earlier by the authors (D. Jung and D. N. Assanis, 2001, SAE Transactions: Journal of Engines, 2001-01-1246) features a quasidimensional, multizone, spray combustion model to account for transient spray evolution, fuel–air mixing, ignition and combustion. The Zeldovich mechanism is used for predicting NO emissions. Soot formation and oxidation is calculated with a semiempirical, two-rate equation model. Unburned HC emissions models account for three major HC sources in DI diesel engines: (1) leaned-out fuel during the ignition delay, (2) fuel yielded by the sac volume and nozzle hole, and (3) overpenetrated fuel. The emissions models have been validated against experimental data obtained from representative heavy-duty DI diesel engines. It is shown that the models can predict the emissions with reasonable accuracy. Following validation, the usefulness of the cycle simulation as a practical design tool is demonstrated with a case study of the effect of the discharge coefficient of the injector nozzle on pollutant emissions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):397-402. doi:10.1115/1.2061307.

During port-fuel–injected (PFI) spark-ignition (SI) engine startup and warm-up fuel accounting continues to be a challenge. Excess fuel must be injected for a near stoichiometric combustion charge. The “extra” fuel that does not contribute to the combustion process may stay in the intake port or as liquid films on the combustion chamber walls. Some of this combustion chamber wall liquid fuel is transported to the engine’s oil sump and some of this liquid fuel escapes combustion and evolves during the expansion and exhaust strokes. Experiments were performed to investigate and quantify this emerging in-cylinder fuel vapor post-combustion cycle by cycle during engine startup. It is believed that this fuel vapor is evaporating from cylinder surfaces and emerging from cylinder crevices. A fast in-cylinder diagnostic, the fast flame ionization detector, was used to measure this behavior. Substantial post-combustion fuel vapor was measured during engine startup. The amount of post-combustion fuel vapor that develops relative to the in-cylinder precombustion fuel charge is on the order of one for cold starting (0 °C) and decreases to $∼13$ for hot starting engine cycles. Fuel accounting suggests that the intake port puddle forms quickly, over the first few engine cranking cycles. Analysis suggests that sufficient charge temperature and crevice oxygen exists to at least partially oxidize the majority of this post-combustion fuel vapor such that engine out hydrocarbons are not excessive.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):403-413. doi:10.1115/1.2130730.

A simple two-zone mass transfer model was used to predict the mean squish velocity history at the rim of a conventional bowl-in-piston combustion chamber. The chamber’s geometry produces gas flow that converges radially inwards (“squish”) as TDC (top dead center) is approached. The squish flow generates turbulence, which can be used to enhance the combustion rate. When compared with PIV (particle image velocimetry) measurements, the peak squish velocity at the bowl rim was 12% less than the value predicted by the simple mass transfer model. After a thorough examination, the assumption of uniform density in the simple model was strongly suspected to be the cause of this discrepancy. Improvements were made to the simple model to account for density variations that are caused by nonuniform heat transfer in the combustion chamber. The revised model yielded velocities that were in close agreement with PIV measurements.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):414-420. doi:10.1115/1.2130731.

The homogeneous charge compression ignition (HCCI) combustion fueled by dimethyl ether (DME) and compressed natural gas (CNG) was investigated. The experimental work was carried out on a single-cylinder diesel engine. The results show that adjusting the proportions of DME and CNG is an effective technique for controlling HCCI combustion and extending the HCCI operating range. The combustion process of HCCI with dual fuel is characterized by a distinctive two-stage heat release process. As CNG flow rate increases, the magnitude of peak cylinder pressure and the peak heat release rate in the second stage goes up. As DME flow rate increases, the peak cylinder pressure, heat release rate, and $NOx$ emissions increase while $THC$ and $CO$ emissions decrease.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):421-433. doi:10.1115/1.2130732.

Increasingly stringent diesel particulate emissions standards have reestablished international interest in diesel filters, whose first series application dates back to 1985. Modern diesel engine technology, with computerized engine management systems and advanced, common rail injection systems, needs to be fully exploited to support efficient and durable diesel filter systems with catalytic aids, as standard equipment in passenger cars. Efficient system and components’ optimization requires the use of mathematical models of diesel filter performance. The three-dimensional model for the regeneration of the diesel particulate filter presented in this paper has been developed as an engineering tool for the detailed design optimization of SiC diesel filters of modular structure. The 3-D modeling is achieved by interfacing an existing 1-D model to commercial finite element method software for the computation of the 3-D temperature field within the whole filter assembly, including the adhesive of the filter blocks, the insulation mat, and the metal canning. The 3-D model is applied to real-world component optimization studies of diesel filter systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):434-445. doi:10.1115/1.2056007.

A general conservative numerical model for the simulation of transmission-line unsteady fluid dynamics has been developed and applied to high-pressure injection systems. A comprehensive thermodynamic approach for modeling acoustic cavitation, i.e., cavitation induced by wave propagation, was proposed on the basis of a conservative homogeneous two-phase barotropic flow model of a pure liquid, its vapor, and a gas, both dissolved and undissolved. A physically consistent sound speed equation was set in a closed analytical form of wide application. For the pure-liquid flow simulation outside the cavitation regions, or in the absence of these, temperature variations due to compressibility effects were taken into account, for the first time in injection system simulation, through a thermodynamic relation derived from the energy equation. Nevertheless, in the cavitating regions, an isothermal flow was retained consistently with negligible macroscopic thermal effects due to vaporization or condensation, because of the tiny amounts of liquid involved. A novel implicit, conservative, one-step, symmetrical, and trapezoidal scheme of second-order accuracy was employed to solve the partial differential equations governing the pipe flow. It can also be enhanced at a high-resolution level. The numerical model was applied to wave propagation and cavitation simulation in a high-pressure injection system of the pump-line-nozzle type for light and medium duty vehicles. The system was relevant to model assessment because, at part loads, it presented cavitating flow conditions that can be considered as severe, at least for a diesel injection system. The predicted time histories of pressure at two pipe locations and of injector needle lift were compared to experimental results, substantiating the validity and robustness of the developed conservative model in simulating acoustic cavitation inception and desinence with great accuracy degree. Cavitation transients and the flow discontinuities induced by them were numerically predicted and analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):446-454. doi:10.1115/1.2032447.

A compound diesel homogeneous charge compression ignition (HCCI) combustion system has been developed based on the combined combustion strategies of multiple injection strategy and a mixing enhanced combustion chamber design. In this work, a STAR-CD based, multidimensional modeling is conducted to understand and optimize the multiple injection processes. The parameters explored included injection timing, dwell time, and pulse width. Insight generated from this study provides guidelines on designing the multipulse injection rate pattern for optimization of fuel-air mixing. Various heat release modes created by different injection strategies are investigated by experimental comparison of combustion efficiency, heat loss, and thermal efficiency. It is demonstrated that the process of fuel evaporation and mixing are strongly influenced by pulse injection parameters. Through control of the parameters, the stratification and autoignition of the premixed mixture, and the heat release mode can be controlled. The dispersed mode of heat release created only by the compound diesel HCCI combustion is a flexible mode in combustion control. The thermal efficiency with this mode can reach approximately to as high as that of conventional diesel combustion, while the $NOx$ and smoke emissions can be reduced simultaneously and remarkably.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(2):455-462. doi:10.1115/1.2056537.

In spark ignition engines, initial flame kernel is wrinkled by a progressively increasing bandwidth of turbulence length scales until eventually the size of the flame kernel is sufficient for it to experience the entire turbulence spectrum. In the present study, an effective rms turbulence velocity as a function of time, estimated by integrating the nondimensional power spectrum density (psd) function for isotropic turbulence, is utilized to analyze the statistical distribution of flame front curvatures and turbulent burning velocities of flames propagating in methane-air premixtures. The distributions of flame front curvatures show these to become more dispersed as the effective turbulence velocity increases, and result in increased burning of premixtures. A decrease in the Markstein number also results in a further increase in curvature dispersion and enhanced burning, in line with the flame stability analysis.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J. Eng. Gas Turbines Power. 2004;128(2):463-467. doi:10.1115/1.2031267.

Saudi Aramco has focused more attention in recent years on fuel-gas conditioning for land-based industrial and aeroderivative combustion gas turbines (CGTs). Hydrocarbon dew points and entrained solids are two important fuel quality issues that frequently trouble CGT operators, partly because they cannot be guaranteed by the fuel suppliers and they are rarely monitored by the operators. This paper addresses these issues and offers some practical advice to optimize the design and operation of fuel gas delivery systems. Saudi Aramco has been testing an automated on-line dew point monitor capable of detecting both hydrocarbon and aqueous dew points in natural gas. Dew point monitoring, conducted at three locations on the fuel gas grid, revealed wide variations in the hydrocarbon and aqueous dew points due to a variety of conditions. Gas production and pipeline operations accounted for the most dramatic variations in dew points, but exposure of pipelines to the weather can also be important. Measurement of pipeline solids for the purpose of sizing and placement of particle filters have also been explored. Pipeline scraping, gas velocities, length of pipeline span, pipeline junctions, and control valves are all considerations for solid control strategies. The optimized design and operation of a CGT fuel system is highly dependent on dew point control and efficient removal of entrained pipeline solids. Practical experience in monitoring hydrocarbon and aqueous dew points, pipeline solids control, and optimizing fuel conditioning equipment are considered.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(2):468-471. doi:10.1115/1.2061327.

This paper describes a test program designed to assess the suitability of a TF40-based gas turbine generator set (genset) for use in the Advanced Hull Form Inshore Demonstrator (AHFID). An overall description of the test program and its objectives is given. Test results are presented. The relevance of the test to the greater program goal of demonstrating the viability of rim-driven propulsion technology is discussed. The genset demonstration test was successfully completed in 2003. The package performed as designed and met all test objectives.

Commentary by Dr. Valentin Fuster