The paper describes the development and validation of an efficient and cost effective method for the prediction of the NOx emissions of turbulent gas turbine burners in the early burner design phases, which are usually focused on the optimization of the swirler aerodynamics and the fuel-air mixing. Since the method solely relies on nonreacting tests of burner models in the water channel, it can be applied before any test equipment for combustion experiments exists. In order to achieve optimum similarity of fuel-air mixing in the water channel tests with engine operation the model is operated at the engine momentum ratio. During the laser induced fluorescence (LIF) measurements the water flow representing the fuel is doped with fluorescent dye, a plane perpendicular to the length axis near the burner exit plane is illuminated with a 5W Ar-ion laser, and the fluorescence is recorded with a video camera from downstream. From the video sequence,s the local probability density functions (PDF) of the dye concentration fluctuations are calculated from the data. Furthermore, the time mean velocity fields are measured with particle image velocimetry (PIV). The PDFs of the local equivalence ratio are derived from the LIF data. Assuming flamelets, the NOx generation in the entire equivalence ratio range observed in the water channel tests is computed using the unstrained freely propagating one-dimensional flame model in Cantera and the GRI3.0 reaction scheme. Although neither flame stretch nor post flame NOx generation were considered, the computed NOx values were in excellent agreement with the experimental data from perfectly premixed combustion experiments. The local time averaged NOx mole fraction is obtained by integrating the flamelet NOx over the mixture PDF. Finally the global NOx emission of the burner at the considered operating point is obtained by spatial integration, considering the measured velocity field. The method was validated using a conical swirl burner with two fuel injection stages, allowing the degree of premixedness to be adjusted over a wide range, depending on the specific fuel injection scenario. For the case with fuel injection along the air inlet slots NOx values slightly above the minimum NOx limit for perfectly premixed combustion were computed. This is consistent with the emission measurements and indicates the finite mixing quality of this injection method. In the partially premixed regime the configurations with potential for low NOx emissions were reliably identified with the LIF and PIV based water channel method. The method also shows the steep increase of the NOx emissions with the decreasing degree of premixing observed in the experiments, however, quantitative predictions would have required a postprocessing of the data from the LIF mixing study with a higher spatial resolution than available.

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

This paper describes the results of an experimental study to understand the influence of inlet flow disturbances on the dynamics of combustion process in bluff body stabilized diffusion flames of liquid petroleum gas and air. The results show the influence of weak disturbances created by the change in incoming pipe length on the amplitude of pressure oscillations and the phase angle between pressure and heat release. It is seen that the phase delay increases as the entry length increases. The rms value of pressure, however, generally falls with the increase in length. The phase angle is seen to be in the second quadrant, showing that the heat release oscillations damp the pressure oscillations. Therefore, the decrease in the phase angle results in the reduction in damping and hence an increase in pressure fluctuations. The dominant frequencies of combustion oscillations are found to be the low frequency oscillations, and the frequency of oscillations increases with a decrease in the inlet pipe length and an increase in the flow Reynolds number. It is suggested that such low frequency oscillations are driven by vortex shedding at the wake of the bluff body, which energizes the diffusion and mixing process.

To explore the mechanism of differential pressure fluctuation inducing cross flow between subchannels in the tight-lattice rod bundle, an evaluation method is presented, which permits the prediction in detail of the unsteady differential pressure fluctuation behavior between subchannels. The instantaneous fluctuation of differential pressure between two subchannels in gas-liquid slug flow regime is deemed as a result of the intermittent nature of slug flow in each subchannel. The method is based on the detailed numerical simulation result of two-phase flow that pressure drop occurs mainly in the liquid slug region and it is, however, negligibly small in the bubble region. The instantaneous fluctuation of differential pressure between two subchannels is associated with pressure gradient in the liquid slug for each channel. In addition to a hydrostatic gradient, acceleration and frictional gradients are taken into account to predict pressure gradient in the liquid slug. This method used in conjunction with the numerical simulation code works satisfactorily to reproduce numerical simulation results for instantaneous fluctuation of differential pressure between two modeled subchannels. It is shown that the static head, acceleration, and frictional pressure drops in the liquid slug are main contributions to the fluctuation of differential pressure between subchannels.

With the need to reduce carbon emissions such as C O 2 , hydrogen is being examined as potential “clean” fuel for the future. One potential strategy is lean premixed combustion, where the fuel and air are allowed to mix upstream before entering the combustor, which has been proven to curb N O x formation in natural gas fired engines. However, premixing hydrogen and air may increase the risk of autoignition before the combustor, resulting in serious engine damage. A flow reactor was set up to test the ignition delay time of hydrogen and air at temperatures relevant to gas turbine engine operations to determine maximum possible mixing times. The results were then compared to past experimental work and current computer simulations. The current study observed that ignition is very sensitive to the initial conditions. The ignition delay times follow the same general trend as seen in previous flow reactor studies: ignition within hundreds of milliseconds and relatively low activation energy. An experimentally derived correlation by Peschke and Spadaccini (1985, “Determination of Autoignition and Flame Speed Characteristics of Coal Gases Having Medium Heating Values,” Research Project No. 2357-1, Report No. AP-4291) appears to best predict the observed ignition delay times. Homogenous gas phase kinetics simulations do not appear to describe ignition well in these intermediate temperatures. Therefore, at the moment, only the current empirical correlations should be used in predicting ignition delay at engine conditions for use in the design of gas turbine premixers. Additionally, fairly large safety factors should still be considered for any design to reduce any chance of autoignition within the premixer.

A rapid compression machine was used to study the soot formation process under diesel enginelike conditions. The apparatus creates accurately controlled conditions at the end of compression (uniform mixture, temperature, and well-defined mixture composition) and, by decoupling chemistry with mixing, provides an unambiguous data interpretation for kinetics study. The soot evolution was studied by the line-of-sight absorption method (at 632.8 nm ), which measured the soot volume concentration evolution in the initial stage of soot growth before the optical path became opaque. For a rich butane mixture at fuel equivalence ratio of 3, the ignition delay showed a negative temperature dependence at intermediate temperatures. The soot volume fraction showed an initial exponential growth, with a growth rate depending on the compressed charge fuel concentration. A substantial amount of soot was formed after the soot cloud became opaque. By weighing the total soot particles after the experiment, only ∼ 10 - 15 % of the soot mass was formed when the beam transmission was reduced to 5%. The final soot mass was ∼ 15 - 18 % of the total carbon mass for compressed charge density of 250 mol ∕ m 3 and temperature from 740 to 930 K .

Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NO x burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying high-speed video recordings, the laser Doppler anemometry and the laser induced fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechanism of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.

Fuel placement and air-fuel mixing in a generic aeroengine premix module employing plain jet liquid fuel injection into a counter-swirling double-annular crossflow were investigated at different values of air inlet pressure (6 bar, 700 K and 12 bar, 700 K) and liquid-to-air momentum flux ratio, both parameters being a function of engine power. Kerosene Jet A-1 was used as liquid fuel. Measurement techniques included LDA for investigation of the airflow and Mie-scattering laser light sheets and PDA for investigation of the two-phase flow. Measurements were taken at various axial distances from the fuel nozzle equivalent to mean residence times of up to 0.47 ms. It was found that the initial fuel placement reacts very sensitively to a variation of liquid-to-air momentum flux ratio. Susceptibility of the spray to dispersion due to centrifugal forces and to turbulent mixing is primarily a function of the fuel droplet diameters, which in turn depend on operating pressure. The data are interpreted by evaluation of the corresponding Stokes numbers.

Fuel-air mixing in a direct injection spark ignition (DISI) engine occurs in a highly unsteady, turbulent and three-dimensional flow. As a result, any cycle-to-cycle unsteady variation in the mixing process can directly impact the performance of the DISI engine. To study the unsteady process in these engines, we have developed and implemented a large-eddy simulation (LES) approach with an innovative subgrid scalar mixing model based on the linear-eddy mixing (LEM) model into a commercial IC engine code (KIVA-3V). Time-averaged results of the simulations using the new LES version (KIVALES) are compared to the steady-state predictions of the original KIVA-3V. Significantly different in-cylinder turbulent fuel-air mixing is predicted by these two methods. Analysis shows that KIVALES resolves spatial features larger than the grid and that the subgrid kinetic energy adjusts to the LES resolution. As a result, KIVALES captures a highly unsteady, anisotropic fuel-air mixing process whereas a more diffused mixed field is predicted by the original KIVA-3V. This ability of KIVALES is attributed to the subgrid closure which scales the subgrid dissipation with the local grid size and thus, decreases the overall dissipation in the flow.

The advantages of hydrogen fueled internal combustion engines are well known, certainly concerning the ultra-low noxious emissions (only NO x is to be considered). Disadvantages are the backfire phenomenon and the gaseous state of hydrogen at atmospheric conditions. A complete control of the mixture formation is necessary and therefore a test engine with sequential port injection was chosen. The tests are carried out on a single-cylinder CFR engine with the intention to use the results to optimize a 6 and 8-cylinder engine with multipoint injection. Different positions of the injector against the intake air duct are examined (represented as different junctions). A numerical simulation CFD code (FLUENT) is used under “stationary” conditions (continuous injection) for all geometries and under “real” conditions (sequential injection) for one situation. For each of the geometries the influences of the start of injection, the air/fuel equivalence ratio, injection pressure, and ignition timing on the power output and efficiency of the engine are analyzed. A comparison and discussion is given for all results. It is clearly shown that the start of injection for a certain engine speed and inlet geometry influences the volumetric efficiency and thus the power output of the engine due to the interaction between the injected hydrogen and the inlet pressure waves. Furthermore, the small influence of the injection pressure and the contradictory benefits of the different junctions between power output and fuel efficiency are measured. With retarded injection, so that cool air decreases the temperature of the “hot-spots” in the combustion chamber before the fuel is injected, backfire safe operation is possible.

A stochastic model for the HCCI engine is presented. The model is based on the PaSPFR-IEM model and accounts for inhomogeneities in the combustion chamber while including a detailed chemical model for natural gas combustion, consisting of 53 chemical species and 590 elementary chemical reactions. The model is able to take any type of inhomogeneities in the initial gas composition into account, such as inhomogeneities in the temperature field, in the air-fuel ratio or in the concentration of the recirculated exhaust gas. With this model the effect of temperature differences caused by the thermal boundary layer and crevices in the cylinder for a particular engine speed and fuel to air ratio is studied. The boundary layer is divided into a viscous sublayer and a turbulent buffer zone. There are also colder zones due to crevices. All zones are modeled by a characteristic temperature distribution. The simulation results are compared with experiments and a previous numerical study employing a PFR model. In all cases the PaSPFR-IEM model leads to a better agreement between simulations and experiment for temperature and pressure. In addition a sensitivity study on the effect of different intensities of turbulent mixing on the combustion is performed. This study reveals that the ignition delay is a function of turbulent mixing of the hot bulk and the colder boundary layer.

The development of integrated coal gasification combined cycle (IGCC) systems ensures higher thermal efficiency and environmentally sound options for supplying future coal utilizing power generation needs. The Japanese government and electric power industries in Japan promoted research and development of an IGCC system using an air-blown entrained-flow coal gasifier. On the other hand, Europe and the United States are now developing the oxygen-blown IGCC demonstration plants. Gasified coal fuel produced in an oxygen-blown entrained-flow coal gasifier, has a calorific value of 8–13 MJ/m 3 which is only 1/5–1/3 that of natural gas. However, the flame temperature of medium-Btu gasified coal fuel is higher than that of natural gas and so NO x production from nitrogen fixation is expected to increase significantly. In the oxygen-blown IGCC, a surplus nitrogen produced in the air-separation unit (ASU) is premixed with gasified coal fuel (medium-Btu fuel) and injected into the combustor, to reduce thermal- NO x production and to recover the power used for the ASU. In this case, the power to compress nitrogen increases. Low NO x emission technology which is capable of decreasing the power to compress nitrogen is a significant advance in gas turbine development with an oxygen-blown IGCC system. Analyses confirmed that the thermal efficiency of the plant improved by approximately 0.3% (absolute) by means of nitrogen direct injection into the combustor, compared with a case where nitrogen is premixed with gasified coal fuel before injection into the combustor. In this study, based on the fundamental test results using a small diffusion burner and a model combustor, we designed the combustor in which the nitrogen injection nozzles arranged on the burner were combined with the lean combustion technique for low- NO x emission. In this way, we could reduce the high-temperature region, where originated the thermal- NO x production, near the burner positively. And then, a combustor with a swirling nitrogen injection function used for a gas turbine, was designed and constructed, and its performance was evaluated under pressurized conditions of actual operations using a simulated gasified coal fuel. From the combustion test results, the thermal- NO x emission decreased under 11 ppm (corrected at 16% O 2 ), combustion efficiency was higher than 99.9% at any gas turbine load. Moreover, there was different effects of pressure on thermal- NO x emission in medium-Btu fuel fired combustor from the case of a natural gas fired combustor.

Effects of combustion chamber geometry and initial mixture distribution on the combustion process were investigated in a direct-injection diesel engine. In the engine experiment, a high squish combustion chamber with a squish lip could reduce both NO x and particulate emissions with retarded injection timing. According to the results of CFD computation and phenomenological modeling, the high squish combustion chamber with a central pip is effective to keep the combusting mixture under the squish lip until the end of combustion and the combustion region forms rich and highly turbulent atmosphere. This kind of mixture distribution tends to reduce initial burning, resulting in restraint of NO x emission while keeping low particulate emission.

The potential performance gain of utilizing pulse detonation combustion in the bypass duct of a turbofan engine for possible elimination of the traditional afterburner was investigated in this study. A pulse detonation turbofan engine concept without an afterburner was studied and its performance was assessed. The thrust, specific fuel consumption (SFC), and specific thrust of a conventional turbofan with an afterburner and the new pulse detonation turbofan engine concept were calculated and compared. The pulse detonation device performance in the bypass duct was obtained by using multidimensional CFD analysis. The results showed that significant performance gains can be obtained by using the pulse detonation turbofan engine concept as compared to the conventional afterburning turbofan engine. In particular, it was demonstrated that for a pulse detonation bypass duct operating at a frequency of 100 Hz and higher, the thrust and specific thrust of a pulse-detonation turbofan engine can nearly be twice as much as those of the conventional afterburning turbofan engine. SFC was also shown to be reduced. The effects of fuel-air mixture equivalence ratio and partial filling on performance were also predicted. However, the interaction between pulse detonation combustion in the bypass duct and the engine fan, for potential fan stall, and engine nozzle have not been investigated in this study.

A unified model of gaseous fuel and air mixing is applied here to study the use of shock waves for enhancement of mixing between methane and air. The model uses fuel mass fraction within infinitesimal fluid elements and the total derivative of this fraction with respect to time to measure the degree and rate of mixing, respectively. The model is accurate only for low-pressure combustors since it is based on the ideal gas law. The model is also limited to gaseous fuels that contain single chemical specie, or those that behave like single specie. The model presented here can be applied to any combustor geometry or operational conditions. Results show that mixing can be completed within the narrow region of the shock wave and therefore in a negligibly short time, if pressure, temperature, and velocity distributions within this region are optimized. Furthermore, the combined effects of air preheat and shock waves can enhance both mixing mechanisms with air penetration into the fuel and with fuel dispersion into the surrounding air. These results provide important guidelines for the mixing in supersonic combustors that are required to provide high efficiency and high intensity, while maintaining low levels of pollutants emission.

Five different flame states are identified in a compact combustion chamber that is fired by a 30 kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure. These flame states include a nozzle-attached tulip shaped flame, a nonattached torroidal-ring shaped flame (SSF) suitable for very low NO x emission in a gas turbine combustor and a Coanda flame (CSF) that clings to the bottom wall of the combustion chamber. Flame state transition is generated by changing the swirl number and by premixing the combustion air with 70% of the natural gas flow. The flame state transition pathways reveal strong hysteresis and bifurcation phenomena. The paper also presents major species concentrations, temperature and velocity profiles of the lifted flame state and the Coanda flame and discusses the mechanisms of flame transition and stabilization.

To support the development of a rich quench lean pilot zone for a staged aeroengine combustor, two rectangular rich quench lean (RQL) combustor sectors have been investigated under atmospheric conditions. Two advanced cooling mixing concepts, effusion and impingement cooling, with one and two rows of secondary air inlet holes in the mixing zone, have been measured using intrusive and noninstrusive measurement techniques. The results elucidate the interrelations between the cooling concepts and the respective mixing and emissions performances. The measurements were accompanied by numerical calculations supporting the interpretations of the measured data. Drawbacks were observed for the near stoichiometric conditions of the effusion cooling concept near the wall, however, the quench zone design with two rows of staggered holes performs well. On the contrary, the impingement cooling system shows good results for the homogeneity of the primary zone, but since less quench air is available with impingement cooling, optimum mixing is more difficult to achieve.

Detailed chemical kinetics was used in an engine CFD code to study the combustion process in HCCI engines. The CHEMKIN code was implemented in KIVA such that the chemistry and flow solutions were coupled. The reaction mechanism consists of hundreds of reactions and species and is derived from fundamental flame chemistry. Effects of turbulent mixing on the reaction rates were also considered. The results show that the present KIVA/CHEMKIN model is able to simulate the ignition and combustion process in three different HCCI engines including a CFR engine and two modified heavy-duty diesel engines. Ignition timings were predicted correctly over a wide range of engine conditions without the need to adjust any kinetic constants. However, it was found that the use of chemical kinetics alone was not sufficient to accurately simulate the overall combustion rate. The effects of turbulent mixing on the reaction rates need to be considered to correctly simulate the combustion and heat release rates.

The authority of an active combustion instability control system was improved by increasing the degree of mixing between a modulated gaseous fuel source and the remainder of the premixed reactants in a low-emissions combustor. Nonreacting acetone PLIF measurements were used to assess the mixedness of various fuel injection configurations, in both time-averaged and phase-locked modes. These configurations were also evaluated in combustion tests in which the authority of the actuator and the ability of the control system to attenuate the instability were measured. The results indicated that both control authority and emissions performance are tied directly to the ability to achieve temporal control over the spatially averaged fuel/air ratio leaving the premixer at any point in time while simultaneously maintaining the high spatial uniformity of this mixture. The cold-flow diagnostic techniques were proven to be an effective and low-cost method for screening fuel injection concepts.

Results of an ongoing collaboration between the engine manufacturer MTU and the German aerospace research center DLR on the NO x reduction potential of conventional combustors are reported. A program comprising optical sector combustor measurements at 1, 6, and 15 bars and CFD calculations is carried out. The aims are to gather information in the combustor at realistic operating conditions, to understand the differences between the sector flow field and data from tubular combustors, to verify the used CFD, and to discover the benefits and limitations of the applied optical diagnostics. Selected results of measurements and calculations of the isothermal flow and of measurements at 6 bars and 700 K at a rich-lean and overall lean AFR are reported. The used measurement techniques were LDA, PDA, Mie scattering on kerosene, quantitative light scattering, OH * chemiluminescence, and LIF on OH. The measurements were able to confirm the intended quick and homogeneous mixing of the three staggered rows of secondary air jets.