0

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

J. Eng. Gas Turbines Power. 2010;132(7):071501-071501-9. doi:10.1115/1.4000119.

The development of a dynamic thickened flame (TF) turbulence-chemistry interaction model is presented based on a novel approach to determine the subfilter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount, which thickens the flame to a scale that can be resolved on the large eddy simulation (LES) grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial computational fluid dynamics code FLUENT . Validation in the approach is conducted by comparing model results to experimental data collected in a laboratory-scale burner. The burner is based on an enclosed scaled-down version of the low swirl injector developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied, as well as the cold-flow characteristics of the combustor. Planar laser induced fluorescence of the hydroxyl molecule was collected for the combusting condition, as well as the velocity field data using particle image velocimetry. Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):071502-071502-7. doi:10.1115/1.4000120.

An experimental investigation of the flow field of a 12 burner annular combustor and a single burner combustor with the same burner was performed. It has revealed the aerodynamic effect, which causes the discrepancies in the flame transfer function behavior measured at the same operating conditions in the single and the annular combustion chambers. The results have shown significant differences in the flow field. In particular, it is seen that for the investigated system in the annular combustor a free swirling jet flow forms, while in the single burner configuration, a swirling wall jet flow regime exists. In this paper, we discuss the physical mechanism and show how to generalize an earlier finding, which identified a critical confinement value for a given swirler. We propose a new correlation for coswirling burners, which explains the changes found for the investigated system. It compares also well with the experimental data from other burner geometries. The correlation should allow to design single burner tests as to match the annular combustor flow regime.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):071503-071503-8. doi:10.1115/1.4000141.

An unconfined strongly swirled flow is investigated for different Reynolds numbers using particle image velocimetry (PIV) and large eddy simulation (LES) with a thickened-flame (TF) model. Both reacting and nonreacting flow results are presented. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the experimental data. Also the predicted root mean square (rms) fluctuations exhibit a double-peak profile with one peak in the burnt and the other in the unburnt region. The measured and predicted heat release distributions are in qualitative agreement with each other and exhibit the highest values along the inner edge of the shear layer. The precessing vortex core (PVC) is clearly observed in both the nonreacting and reacting cases. However, it appears more axially elongated for the reacting cases and the oscillations in the PVC are damped with reactions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):071504-071504-9. doi:10.1115/1.4000268.

Lean-direct-injection (LDI) combustion is being considered at the National Energy Technology Laboratory as a means to attain low $NOx$ emissions in a high-hydrogen gas turbine combustor. Integrated gasification combined cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent $CO2$ separation. The IGCC’s air separation unit produces a volume of $N2$ roughly equivalent to the volume of $H2$ in the gasifier product stream, which can be used to help reduce peak flame temperatures and $NOx$ in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and it has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on $NOx$ emissions. From a peak flame temperature perspective, greater $NOx$ reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower $NOx$ production potential for fuel-side dilution due to its smaller flame size, whereas air dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal $NOx$ generation. For more complex staged-air LDI combustor designs, the dilution of the primary combustion air at fuel-rich conditions can result in the full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for $NOx$ reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with the dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and $NOx$ emissions. Strategies for accurate computational modeling of LDI combustors’ stability characteristics are also discussed.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2010;132(7):072301-072301-11. doi:10.1115/1.4000263.

A parametric study of a solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system design is conducted with the intention of determining the thermodynamically based design space constrained by modern material and operating limits. The analysis is performed using a thermodynamic model of a generalized SOFC-GT system where the sizing of all components, except the fuel cell, is allowed to vary. Effects of parameters such as pressure ratio, fuel utilization, oxygen utilization, and current density are examined. Operational limits are discussed in terms of maximum combustor exit temperature, maximum heat exchanger effectiveness, limiting current density, maximum hydrogen utilization, and fuel cell temperature rise. It was found that the maximum hydrogen utilization and combustor exit temperature were the most significant constraints on the system design space. The design space includes the use of cathode flow recycling and air preheating via a recuperator (heat exchanger). The effect on system efficiency of exhaust gas recirculation using an ejector versus using a blower is discussed, while both are compared with the base case of using a heat exchanger only. It was found that use of an ejector for exhaust gas recirculation caused the highest efficiency loss, and the base case was found to exhibit the highest overall system efficiency. The use of a cathode recycle blower allowed the largest downsizing of the heat exchanger, although avoiding cathode recycling altogether achieved the highest efficiency. Efficiencies in the range of 50–75% were found for variations in pressure ratio, fuel utilization, oxygen utilization, and current density. The best performing systems that fell within all design constraints were those that used a heat exchanger only to preheat air, moderate pressure ratios, low oxygen utilizations, and high fuel utilizations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072302-072302-9. doi:10.1115/1.4000300.

In most compressors the flow is adiabatic, but in low-speed turbochargers, the compression process has both heat transfer and work input. This paper examines different compressor efficiency definitions for such diabatic flows. Fundamental flaws in the use of the isentropic efficiency for this purpose are identified, whereas the polytropic efficiency can be used with or without heat transfer without ambiguities. The advantage of the polytropic approach for a practical application is demonstrated by analyzing the heat transfer in a turbocharger compressor. A simple model of the heat transfer allows a correction for this effect on the polytropic efficiency at low-speed to be derived. Compressor characteristics that have been corrected for this surprisingly large effect maintain a much higher efficiency down to low-speeds.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2010;132(7):072401-072401-10. doi:10.1115/1.4000128.

Three-dimensional numerical simulations of the effect of fouling on an axial compressor stage were carried out. As a case study, the NASA Stage 37 was considered for the numerical investigation, which was performed by means of a commercial computational fluid dynamic code. The numerical model was validated against the experimental data available from literature. Computed performance maps and main flow field features showed a good agreement with the experimental data. The model was considered representative of a realistic compressor stage. The model was then used to simulate the occurrence of fouling by imposing different combinations of added thickness and surface roughness levels. The effect of fouling on compressor performances was studied. Reductions in the flow coefficient and in the pressure coefficient were found to be of the same order of magnitude of the experimental results found in literature. The model developed seems to overcome some of the limitations of other models found in literature that tend to significantly underestimate the actual values of performance reduction. The numerical results were also used to analyze and debug the stage performance scaling procedure used in stage-stacking models in order to represent fouling in multistage compressors. The analysis highlighted that scaling can adequately represent the behavior of the fouled stage in the choked flow region, but it does not capture the reduction in the maximum of the pressure coefficient, which is instead revealed by the numerical simulations. Finally, blockage due to fouling was investigated both qualitatively and quantitatively.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072402-072402-10. doi:10.1115/1.4000299.

Mixed operation with both centrifugal and reciprocating compressors in a compression plant poses significant operational challenges as pressure pulsations and machine mismatches lead to centrifugal compressors’ instabilities or poor performance. Arrangements with reciprocating compressors placed in series with centrifugal compressors generally lead to higher suction/discharge pulsations on the centrifugal compressor than conventional parallel operation. This paper demonstrates that by properly analyzing and designing the interconnecting piping between the compressors, utilizing pulsation attenuation devices, and matching the compressors’ volumetric-flow rates, a satisfactory functional compression system design can be achieved for even the worst cases of mixed centrifugal and reciprocating compressor operation. However, even small analysis errors, design deviations, or machine mismatches result in a severely limited (or even inoperable) compression system. Also, pulsation attenuation often leads to a significant pressure loss in the interconnect piping system. Utilizing analysis tools in the design process that can accurately model the transient fluid dynamics of the piping system, the pulsation attenuation devices, and the compressor machine behaviors is critical to avoid potentially costly design mistakes and minimize pressured losses. This paper presents the methodology and examples of such an analysis using a 1D transient Navier–Stokes code for complex compression piping networks. The code development, application, and example results for a set of mixed operational cases are discussed. This code serves as a design tool to avoid critical piping layout and compressor matching mistakes early in the compressor station design process.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2010;132(7):072501-072501-8. doi:10.1115/1.4000092.

The differential bearing between the low-pressure turbine (LPT) and high-pressure turbine (HPT) shafts is one of the most vulnerable parts in a turbomachinery engine. Unfortunately, it is also one of the most difficult parts to monitor for damage existence signatures, because the signal-to-noise ratio at the normal sensor locations is extremely low. In addition, the speed variations in both the LPT and HPT can further deteriorate the damage signature extracted by conventional analysis methods. In this paper, we developed a “synthesized synchronous sampling” technique to enhance the detection of differential bearing damage signature. Combining this technique together with the conventional acceleration enveloping technique, we are able to detect differential bearing damage at a much earlier stage, thus providing early warnings of the machinery health conditions.

Topics: Bearings , Vibration
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072502-072502-9. doi:10.1115/1.4000130.

Fretting fatigue is a random process that continues to be a major source of damage associated with the failure of aircraft gas turbine engine components. Fretting fatigue is dominated by the fatigue crack growth phase and is strongly dependent on the magnitude of the stress values in the contact region. These stress values often have the most influence on small cracks where traditional long-crack fracture mechanics may not apply. A number of random variables can be used to model the uncertainty associated with the fatigue crack growth process. However, these variables can often be reduced to a few primary random variables related to the size and location of the initial crack, variability associated with applied stress and crack growth life models, and uncertainty in the quality and frequency of nondeterministic inspections. In this paper, an approach is presented for estimating the risk reduction associated with the nondestructive inspection of aircraft engine components subjected to fretting fatigue. Contact stress values in the blade attachment region are estimated using a fine mesh finite element model coupled with a singular integral equation solver and combined with bulk stress values to obtain the total stress gradient at the edge of contact. This stress gradient is applied to the crack growth life prediction of a mode I fretting fatigue crack. A probabilistic model of the fretting process is formulated and calibrated using failure data from an existing engine fleet. The resulting calibrated model is used to quantify the influence of inspection on the probability of fracture of an actual military engine disk under real life loading conditions. The results can be applied to quantitative risk predictions of gas turbine engine components subjected to fretting fatigue.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2010;132(7):072801-072801-10. doi:10.1115/1.4000262.

Experimental measurements have been taken on a production four-cylinder, multipoint (fuel) injection spark-ignition engine, $1.2 dm3$ displacement with a four-valve per cylinder aluminum head, and a 60 kW at 5500 rpm rated power. The aim of the investigation was to understand the behavior of the cooling system of a small automotive engine, which was operated for a prolonged period at high speed under full or part load, then brought to idle for a short period and finally shut down. In this study, the effects of different loads, idle operation time, and lengths of the engine-radiator piping were analyzed. In particular, experimental tests were carried out with the engine running at 4000 rpm under different brake mean effective pressure values in the range 496 to 1133 kPa. In all experimental tests the engine was brought to idle in 5 s, and measurements were repeated for different values of the idle operation time ranging from 1 s to 80 s. Test data of coolant conditions and metal temperature at 26 points of the engine head and liner were recorded. The cooling circuit was instrumented with transparent tubes at the radiator inlet and photographs of the vapor phase moving to the radiator were taken during experimental tests. The volume of leaked coolant as a function of time was also measured. Additional tests were carried out to evaluate the effects of different lengths of the engine-radiator piping on the after-boiling phenomenon. Finally, in order to make the results applicable also to nonautomotive engines, measurements were repeated without the standard cabin heater and the associated piping. The investigation results show that as the engine is shut down and coolant flow stops, the head metal may be hot enough to vaporize a fraction of the coolant contained in the cylinder head passages, causing the pressure within the cooling circuit to rise above the threshold value of the radiator cap pressure valve and, consequently, an important quantity of the coolant to be expelled.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072802-072802-7. doi:10.1115/1.4000267.

A continuous multicomponent fuel flame propagation and chemical kinetics model has been developed. In the multicomponent fuel model, the theory of continuous thermodynamics was used to model the properties and composition of fuels such as gasoline. The difference between the current continuous multicomponent fuel model and previous similar models in the literature is that the source terms contributed by chemistry in the mean and the second moment transport equations have been considered. This new model was validated using results from a discrete multicomponent fuel model. In the flame propagation and chemical kinetics model, five improved combustion submodels were also integrated with the new continuous multicomponent fuel model. To consider the change in local fuel vapor mixture composition, a “primary reference fuel (PRF) adaptive” method is proposed that formulates a relationship between the fuel vapor mixture PRF number (or research octane number) and the fuel vapor mixture composition based on the mean molecular weight and/or variance of the fuel vapor mixture composition in each cell. Simulations of single droplet vaporization with a single-component fuel (iso-octane) were compared with multicomponent fuel cases.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072803-072803-6. doi:10.1115/1.4000290.

Experimental and numerical studies were performed to investigate the simultaneous reduction in $NOx$ and CO for stoichiometric diesel combustion with a three-way catalyst. A single-cylinder engine was used for the experiments and KIVA simulations were used in order to characterize the combustion efficiency and emissions of throttled stoichiometric diesel combustion at 0.7 bar boost pressure and 90 MPa injection pressure. In addition, the efficiency of emission conversion with three-way catalysts in stoichiometric diesel combustion was investigated experimentally. The results showed CO and $NOx$ emissions can be controlled with the three-way catalyst in spite of the fact that CO increases more at high equivalence ratios compared with conventional diesel combustion (i.e., lean combustion). At a stoichiometric operation, the three-way catalyst reduced CO and $NOx$ emissions by up to 95%, which achieves lower emissions compared with conventional diesel combustion or low temperature diesel combustion, while keeping better fuel consumption than a comparable gasoline engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072804-072804-6. doi:10.1115/1.4000292.

An end pumped passively $Q$-switched laser igniter was developed to meet the ignition system needs of large bore lean burn stationary natural gas engines. The laser spark plug used an optical fiber coupled diode pump source to axially pump a passively $Q$-switched Nd:YAG laser and transmit the laser pulse through a custom designed lens. The optical fiber coupled pump source permits the excitation energy to be transmitted to the spark plug at relatively low optical power, less than 250 W. The $Q$-switched laser then generates as much as 8 mJ of light in 2.5 ns, which is focused through an asymmetric biconvex lens to create a laser spark from a focused intensity of approximately $225 GW/cm2$. A single cylinder engine fueled with either natural gas only or hydrogen augmented natural gas was operated with the laser spark plug for approximately 10 h in tests spanning 4 days. The tests were conducted with fixed engine speed, fixed boost pressure, no exhaust gas recirculation, and laser spark timing advance set at maximum brake torque timing. Engine operational and emissions data were collected and analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072805-072805-9. doi:10.1115/1.4000293.

A two-pronged experimental and computational study was conducted to explore the formation, transport, and vaporization of a wall film located at the piston surface within a four-valve, pent-roof, direct-injection spark-ignition engine, with the fuel injector located between the two intake valves. Negative temperature swings were observed at three piston locations during early injection, thus confirming the ability of fast-response thermocouples to capture the effects of impingement and heat loss associated with fuel film evaporation. Computational fluid dynamics (CFD) simulation results indicated that the fuel film evaporation process is extremely fast under conditions present during intake. Hence, the heat loss measured on the surface can be directly tied to the heating of the fuel film and its complete evaporation, with the wetted area estimated based on CFD predictions. This finding is critical for estimating the local fuel film thickness from measured heat loss. The simulated fuel film thickness and transport corroborated well temporally and spatially with measurements at thermocouple locations directly in the path of the spray, thus validating the spray and impingement models. Under the strategies tested, up to 23% of fuel injected impinges upon the piston and creates a fuel film with thickness of up to $1.2 μm$. In summary, the study demonstrates the usefulness of heat flux measurements to quantitatively characterize the fuel film on the piston top and allows for validation of the CFD code.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072806-072806-7. doi:10.1115/1.4000298.

A novel metal-based thermal barrier coating was tested in a spark-ignition engine. The coating was applied to the surface of aluminum plugs and exposed to in-cylinder conditions through ports in the cylinder wall. Temperatures were measured directly behind the coating and within the plug 3 and 11 mm from the surface. In-cylinder pressures were measured and analyzed to identify and quantify knock. Test results suggest the coating does not significantly reduce overall heat transfer, but it does reduce the magnitude of temperature fluctuations at the substrate surface. It was found that heat transfer can be reduced by reducing the surface roughness of the coating. The presence of the coating did not promote knock.

Commentary by Dr. Valentin Fuster

### Research Papers: Nuclear Power

J. Eng. Gas Turbines Power. 2010;132(7):072901-072901-7. doi:10.1115/1.4000339.

The melt droplets’ crust formation modeling, which is used in current fuel coolant interaction (FCI) codes, is rather basic. In the paper the development of the melt droplet heat transfer model, which enables the treatment of the material properties’ influence on the steam explosion, is presented. The model is complex enough to adequately predict the crust development during the melt droplets’ cooling in the premixing phase. At the same time the model is simple enough that it can be practically implemented into FCI codes and is thus being an optimal model for FCI applications. Fragmentation criteria are derived in order to take into account the influence of the formed crust on the steam explosion process. The derived criteria are based on experimental results and the thin plate approximation. To enable the use of the model and the fragmentation criteria in FCI codes with Eulerian formulation, adequate transport equations for model parameters are given.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):072902-072902-7. doi:10.1115/1.4000343.

The estimation of piping failure frequency is an important task to support the probabilistic risk analysis and risk-informed in-service inspection of nuclear power plant systems. This paper describes a hierarchical or two-stage Poisson-gamma Bayesian procedure and applies this to estimate the failure frequency using the Organization for Economic Co-operation and Development/Nuclear Energy Agency pipe leakage data for the United States nuclear plants. In the first stage, a generic distribution of failure rate is developed based on the failure observations from a group of similar plants. This distribution represents the interplant (plant-to-plant) variability arising from differences in construction, operation, and maintenance conditions. In the second stage, the generic prior obtained from the first stage is updated by using the data specific to a particular plant, and thus a posterior distribution of plan specific failure rate is derived. The two-stage Bayesian procedure is able to incorporate different levels of variability in a more consistent manner.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Gas Turbines Power. 2010;132(7):074501-074501-7. doi:10.1115/1.4000287.

The methods employed to perform rotordynamics calculations of industrial machines are rather standard and usually allow forecasting the dynamic behavior of the considered machines. Anyhow, in some cases, in order to obtain high level of accuracy, the model has to be updated to fit experimental results, and standard modeling methods have to be improved. In this paper, the updating of the torsional model of a steam turbogenerator is presented. In order to fit the eigenfrequencies calculated using the standard model and the natural frequencies measured on-field, a modeling improvement is proposed, considering partially the dynamics of the components usually modeled as rigid disks. The proposed method has also the aim to preserve the physical meaning of the model. Finally, the new model is updated, and a very good fitting is obtained between eigenfrequencies and experimental natural frequencies.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):074502-074502-4. doi:10.1115/1.4000288.

Direct injection of gaseous fuel has emerged to be a high potential strategy to tackle both environmental and fuel economy requirements. However, since the electronic gaseous injection technology is rather new for automotive applications, limited experience exists on the optimum configuration of the injection system and the combustion chamber. To facilitate the development of these applications computer models are being developed to simulate gaseous injection, air entrainment, and the ensuing combustion. This paper introduces a new method for modeling the injection process of gaseous fuels in multidimensional simulations. The proposed model allows holding down grid requirements, thus, making it compatible with the three-dimensional simulation of an internal combustion engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):074503-074503-3. doi:10.1115/1.4000338.

In this research paper, a safety analysis has been carried out for the conceptual design of a compact sized pressurized water reactor (PWR) core that utilizes a tristructural-isotropic (TRISO) fuel particle with an inventive composition. The use of TRISO fuel in PWR technology improves integrity of the design due to its fission fragments retention ability, as this fuel provides first retention barrier within the fuel itself against the release fission fragments. Hence, addition of one more reliable barrier in well established PWR technology makes this design concept safer and environment friendly. A small amount of Pu-240 has been added in the fuel for excess reactivity control. This addition of Pu-240 in TRISO fuel reduces the number of burnable poison and control rods required for reactivity control, and completely eliminates the requirement of soluble boron system. The suggested design operates at much lower temperature and pressure than a standard PWR power reactor, and the presence of TRISO fuel ensures the retention of fission fragments at elevated temperatures. All reactivity coefficients were found negative for the designed core, and the shutdown margin has also been increased with the suggested TRISO fuel composition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):074504-074504-5. doi:10.1115/1.4000150.

In order to meet stringent emission standards, it is essential to have a precise control of air-fuel ratio (AFR) under cold start and warm-up conditions. This requires an understanding of the fuel transport dynamics in the intake system during these conditions. This study centers on estimating the parameters of a fuel transport dynamics model during engine operation at different thermal conditions ranging from cold start to fully warmed-up conditions. A method of system identification based on perturbing fuel injection rate is used to find fuel dynamics parameters in a port fuel injected (PFI) spark ignition engine. Since there was no cold chamber available to prepare cold start conditions, a new method was utilized to simulate cold start conditions. The new method can be applied on PFI engines, which use closed valve injection timing. A four-cylinder PFI engine is tested for different thermal conditions from $−15°C$ to $82°C$ at a range of engine speeds and intake manifold pressures. A good agreement is observed between simulated and experimental AFR for 52 different transient operating conditions presented in this study. Results indicate that both fuel film deposit factor $(X)$ and fuel film evaporation time constant $(τf)$ decrease with increasing coolant temperature or engine speed. In addition, an increase in the intake manifold pressure results in an increase in $X$ while causes a decrease in $τf$.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;132(7):074505-074505-6. doi:10.1115/1.4000368.

An integrated head assembly (IHA) is equipped with the missile shield to absorb the missile energy from postulated control element drive mechanism (CEDM) missile during the dynamic event of accidental conditions. Once a CEDM nozzle breaks, reactor coolant jet discharges from the broken nozzle, then it impinges at the bottom of the CEDM, and gives a thrust force to the CEDM missile until it impacts on the missile shield. After the missile impacting on missile shield, it is necessary to evaluate the structural responses on the local area of the missile shield, as well as behaviors of overall IHA structure. The jet has been previously assumed to be a single-phase flow. However, in order to reduce excessive conservatism for the jet characteristic, the jet is assumed to be a two-phase critical flow, and accordingly Fauske slip equilibrium model is applied to estimate the jet velocity. In this paper, jet impingement models are proposed to estimate the missile velocity depending on jet expansions and size of objects. With the calculated missile velocities using the jet impingement models, the nonlinear CEDM missile impact analysis is performed to investigate structural responses of the missile shield of advanced power reactor 1400. Finally, the results show that the structural integrity of the missile shield and the IHA can be maintained due to CEDM missile impact.

Topics: Missiles
Commentary by Dr. Valentin Fuster