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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.

A theoretical model has been developed to predict the thermal performance of inert, direct-fired, woven-metal fiber-matrix porous radiant burner. The local chemical heat release was modeled by a detailed mechanism, and convection heat transfer between the gas and the solid phases in the burner was described by an empirical heat transfer coefficient. The solid matrix was modeled as a gray medium, and the discrete ordinates method was used to solve the radiative transfer equation to calculate the local radiation source/sink in the energy equation for the solid phase. The fully coupled nature of the calculations without external specification of flame location represents a key advance over past efforts towards modeling of porous radiant burners, because for a given mass flow rate the actual heat loss from the flame determines its position and is not a free parameter. The calculated results for the burner surface temperature, the gas exhaust temperature and the radiation efficiency for a single layer Fecralloy burner were compared with experimental data from this laboratory and reasonable agreement was obtained for a range of operating conditions.

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 NOx 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.

This study was directed to understand the coupling effects of the noncircular geometry of the burner and a crossflow on the combustion of gas jets. This paper compares the characteristics of turbulent propane jet flames from circular (diameter=0.45 cm) and elliptic (major axis/minor axis=3) burners of equivalent exit area in a crossflow. The elliptic burner was oriented with its major axis or minor axis aligned with the crossflow. Experiments were conducted in a wind tunnel provided with optical and probe access and capable of wind speeds up to 12.5 m/s. The burners were fabricated with metal tubes. Instrumentation included a Pt-Pt/13% Rh thermocouple, a quartz-probe gas sampling system, chemiluminescent and nondispersive infrared analyzers, a video-recorder, and a computer data acquisition system. The measurements consisted of the upper and lower limits of jet velocity for a stable flame, flame configuration, and visible length. Flame structure data including temperature profiles and concentration profiles of CO2,O2, CO, and NO were obtained in a two-zone flame configuration (at jet to crossflow momentum flux ratio=0.11), where a planar recirculation exists in the wake of the burner tube followed by an axisymmetric tail. The relative emission indicators of CO and NO were estimated from the composition data. Results show that the upper and lower limits of the fuel jet velocity increase with the crossflow velocity for all burners, and the rate of increase is highest for the elliptic burner with its minor axis aligned with the crossflow. That burner configuration also produces the longest flame. The relative emission indicators show that the CO production is lower and NO production is higher with elliptic burners than with circular burners in crossflow.

Flame anchoring in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown (VBD). The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling with downstream pressure pulsations in the combustor affects the VBD process. The present paper describes combustion instability that is associated with vortex breakdown. The mechanism of the onset of this instability is discussed. Passive control of the instability was achieved by stabilizing the location of vortex breakdown using an extended lance. The reduction of pressure pulsations for different operating conditions and the effect on emissions in a laboratory scale model atmospheric combustor, in a high pressure combustor facility, and in a full scale land-based gas-turbine are described. The flashback safety, one of the most important features of a reliable gas turbine burner, was assessed by CFD, water tests, and combustion tests. In addition to the passive stabilization by the extended lance it enabled injection of secondary fuel directly into the recirculation zone where the flame is stabilized. Tests were conducted with and without secondary fuel injection. Measurements and computations optimized the location of the extended lance in the mixing chamber. The effect of variation of the amount of secondary fuel injection at different equivalence ratios and output powers was determined. Flow visualizations showed that stabilization of the recirculation zone was achieved. Following the present research, the VBD stabilization method has been successfully implemented in engines with sufficient stability margins and good operational flexibility. This paper shows the development process from lab scale tests to full scale engine tests until the implementation into field engines.

An integrated experimental-numerical procedure has been developed for fuel-air mixing prediction in a heavy-duty gas turbine burner. Optical measurements of the degree of mixing have been performed in a full-scale test rig operating with cold flow. Experimental data have been utilized to validate a CFD RANS numerical model. In fact, it is recognized that the turbulence behavior of jets in swirling air-flow stream is not accurately described by standard k-ε turbulence models; therefore advanced turbulence models have been assessed by means of experimental data. The degree of mixing between simulated fuel and air streams has been evaluated at the burner exit section by means of a planar Mie scattering technique. The experimental apparatus consists of a pulsed Nd:YAG laser and a high resolution CCD video camera connected to a frame grabber. The acquired instantaneous images have been processed through specific procedures that also take into account the laser beam spatial non-uniformity. A second order discretization scheme with a RSM turbulence model gives the best accordance with the experimental data. Such CFD model will be part of a more general method addressed to numerical prediction of turbulent combustion flames in LP technology.

Methane-fueled low NOx combustor research had been conducted under the Japanese supersonic/hypersonic propulsion research programme. A unique form of premixture jet swirl combustor (PJSC) was proposed for the ultra low NOx combustor of a Mach 3 turbojet engine. Fuel-air mixing tests and fundamental combustion tests were conducted to obtain the design data and combustion characteristics in the first phase of the research. A single can type combustor was fabricated and high temperature and high pressure combustion tests were carried out for the evaluation on NOx emission reduction capability of the PJSC concept in the second phase. In the final phase of research, a multi-sector combustor was fabricated and the performance demonstration test was conducted for the final evaluation of the pollutant exhaust emission goals and the combustor performance goals set in the HYPR project. The sequential three phased program was completed successfully, and the project goals of NOx emission, combustion efficiency, pressure loss and exit gas temperature pattern factor at the Mach 3 cruise condition, together with the ICAO regulatory levels for supersonic aircraft at LTO conditions, were all achieved in the performance demonstration test.

This paper describes a model used for the prediction of the formation of nitrogen oxides in modifications of an industrial diffusion flame, natural gas fueled can combustor. The flowfield inside the modified combustors is calculated using a Navier-Stokes solver. A fast chemistry assumption is used for modeling the heat release. Calculated turbulence parameters are then used for the calculation of the NOx formation rate in the post-processing mode with the aid of a flamelet model. The flamelet model permits the use of detailed kinetics with only minimal computational expense. The dependence of the NOx formation rate on the mixture fraction and scalar dissipation is calculated separately for each given condition. The validation of the model predictions is based on field test data taken earlier on several low NOx modifications recently applied to an industrial, reverse flow can type combustor. The reduced level of NOx emissions was achieved in these modifications by changes in the air distribution within the combustor liner. A comparison of the predicted and measured NOx emission levels shows good potential of the flamelet model.

This paper reports results of experimental and numerical investigations on ethane-air two-stage combustion in a counterflow burner where the fuel stream, which is partially premixed with air for equivalence ratios from 1.6 to 3.0, flows against a pure air stream. Similar to methane, the two-stage ethane combustion exhibits a green fuel-rich premixed flame and a blue diffusion flame. Flame structures, including concentration profiles of stable intermediate species such as C2H4,C2H2 and CH4, are measured by a gas chromatography and are calculated by numerical integrations of the conservation equations employing an updated elementary chemical-kinetic data base. The implications of the results from these experimental measurements and numerical predictions are summarized, the flame chemistry of ethane two-stage combustion at different degrees of premixing (or equivalence ratio) is discussed, and the relationship between NOx formation and the degree of premixing is established. The present work helps to increase understanding of flame chemistry of hydrocarbon fuels, identify important reactions for pollutant formation and suggest means to reduce emissions. [S0742-4795(00)01303-X]

The blowout limit of a methane jet diffusion flame is examined in the presence of a number of much smaller pilot jet flames of different fuels arranged within an experimental burner assembly in a co-flowing stream of air. It is shown that the blowout limit of the central jet flame can be extended very appreciably by increasing the flow rate through the smaller pilot jets. The basis for this extension to the blowout limit and the role of some changes in the operating parameters are discussed. It is suggested that the extension to the blowout limit observed is due mainly to the thermal contribution of the pilot jet flames.

In many industrial thermal applications, such as gas turbines and furnaces, a fuel jet is burned in the presence of a co-flowing oxidizing stream. This not only enhances the efficiency of the process by maintaining an uninterrupted supply of thermal energy, but it reduces the flame length and NO{sub x} emissions. The effects of changes in the jet nozzle geometry, i.e., nozzle shape and lip thickness, on the blowout limits of jet diffusion flames in a co-flowing air stream were experimentally investigated for a range of co-flow air stream velocities. Circular and elongated nozzles of different axes ratios were employed. Preliminary results showed that nozzles with low major-to-minor axes ratios improved, while high ratios reduced the blowout limit of attached flames compared with that for an equivalent circular nozzle. The nozzle shape had no apparent influence on the blowout limits lifted flames and the limiting stream velocity. The experimental blowout limits of lifted flames were found to be a function of the co-flowing stream velocity and jet discharge area. On the other hand, the stability of attached flames was a function of the co-flowing stream velocity, jet discharge area as well as the nozzle shape. The effect of premixing amore » fuel with the surrounding air was also studied. Generally, the introduction of auxiliary fuel into the surrounding stream either increased or decreased the blowout limit depending on the type of flame stabilization mechanism prior to blowout. The stability mechanism of the flame was found to be a function of the co-flow stream velocity and the auxiliary fuel employed. Methane was employed as the jet fuel, while methane, propane, ethylene, and hydrogen were used as auxiliary fuels in the co-flowing stream.« less

Experiments with an inshot burner used in residential natural gas furnaces are presented. The concentrations of NOx, NO, and CO in the combustion products of partially aerated natural gas flames were measured in a laboratory combustion chamber. When the conventional circular venturi inlet of the inshot burner was replaced by elliptic venturi inlets, an increase of up to 30 percent in the primary-air entrainment and a decrease of up to 20 percent in the NOx emission index were observed. Temperature field measurements in the flames were in conformity with the emission index measurements.

Background. Storage technology based on solid media heated in direct contact – so called regenerators – are well suited to promote the market introduction of solar central receiver plants with air receivers. However, starting from existing technologies, several design issues need to be addressed. Method of Approach. A test campaign was performed at the Solar Power Tower Julich, an experimental central receiver plant, to experimentally verify the functionality and to quantify the performance of the plant’s storage subsystem. To this end, a gas burner used during commissioning of the plant, was reactivated and used to run a series of operation sequences. Computer simulations have been set up and applied to retrace the storage behaviour to confirm the validity of the underlying models and to gain further insight into the relevant phenomena. Results. The test results confirm the full functionality of the storage subsystem, the ability to perform cycling at high discharge heat rates and relatively low heat losses, supporting the view that the technology represents a promising basis for up-scaled implementations. Conclusions. Measurement data and simulation results are in good agreement, confirming the maturity of existing design tools.

A numerical investigation of premixed combustion within a highly porous inert medium is reported. Specifically, results of a numerical model using detailed chemical kinetics and energy exchange between the flowing gas and the porous solid are presented. The current formulation differs from prior models of this type of combustion in that multistep kinetics is used and a better description of the thermophysical properties of the solid is applied in the present model. It was found that the preheating effect increases strongly with increasing convective heat transfer and with increasing effective thermal conductivity of the solid. The convective heat transfer is expected to increase with increasing number of cells per unit length of porous matrix but the absorption coefficient decreases with increasing cell size and decreasing cell density. Numerical simulations using baseline properties indicate that the lean limit can be extended to an equivalence ratio of about 0.36 for a methane–air flame and that the peak flame temperature is generally higher than the adiabatic flame temperature. The latter effect is predicted to be more pronounced at lower equivalence ratios.

A swirling double concentric jet is commonly used for nonpremixed gas burner application for safety reasons and to improve the combustion performance. Fuel is generally spurted at the central jet while the annular coflowing air is swirled. They are normally separated by a blockage disk where the bluff-body effects further enhance the recirculation of hot gas at the reaction zone. This paper aims to experimentally investigate the behavior of flame and flow in a double concentric jet combustor when the fuel supply is acoustically driven. Laser-light sheet assisted Mie scattering method has been used to visualize the flow, while the flame lengths were measured by a conventional photography technique. The fluctuating velocity at the jet exit was measured by a two-component laser Doppler velocimeter. Flammability and stability at first fuel tube resonant frequency are reported and discussed. The evolution of flame profile with excitation level is presented and discussed, together with the reduction in flame length. The flame in the unforced reacting axisymmetric wake is classified into three characteristic modes, which are weak swirling flame, lifted flame, and transitional reattached flame. These terms reflect their primary features of flame appearances, and when the acoustic excitation is applied, the flame behaviors change with the excitation frequency and amplitude. Four additional characteristic modes are identified; e.g., at low excitation amplitudes, wrinkling flame with a blue annular film is observed because the excitation induces vortices in the central fuel jet and hence gives rise to the wrinkling of flame. The central jet vortices become larger with the increase in excitation amplitude and thus lead to a wider and shorter flame. If the excitation amplitude is increased above a certain value, the central jet vortices change the rotation direction and pacing with the annular jet vortices. These changes in the flow field induce large turbulent intensity and mixing and therefore make the flame looks blue and short. Further increase in the excitation amplitude would lift the flame because the flow field would be dramatically modified.

A stagnation-flow burner facility was used to study the catalytic surface reactions of premixed combustion systems at atmospheric pressure. The configuration serves as an important platform to investigate the interaction between homogeneous and heterogeneous reactions with independent control of the characteristic chemical and physical residence time scales. Methane/oxygen/nitrogen and propane/oxygen/nitrogen mixtures were examined with and without the presence of a platinum catalyst located at the stagnation surface. The effects of oxidizer composition and nitrogen dilution were investigated. Lean flame extinction limits were determined for the two fuels and were found to be unaffected by the presence of the catalytic surface. The flame extinction data indicated that the systems were controlled by gas phase combustion with negligible contributions from heterogeneous reactions. The catalytic activity of the heated surface in response to the direct impingement of fuel/air mixtures onto the stagnation surface, without the presence of a flame, was quantified by the increase in the surface temperature. The methane/air mixtures demonstrated no catalytic activity for these conditions, whereas propane/air mixtures demonstrated temperature increases of over 100 K. The data indicate that the surface reaction was transport limited for the propane/air system.

This article aims to study the effect of pressure drop on the thermal behavior of porous burners. Since the reticulated ceramics are used in the burners’ construction, in the previous researches pressure drop arising from flow velocity was ignored. This research has showed that due to the increase of speed resulting from combustion, the consequence pressure drop creates considerable effects on the thermal performance of porous burners. To study this subject, the temperature of a point on the burner axis has been taken to be constant. The burned gas and exit surface temperature were obtained almost the same for two conditions, one with the pressure held constant and the other with a pressure drop. Results show that the firing rate was decreased up to 18%, compared to the constant pressure case. The thermal radiative efficiency of radiant porous burners, in which the pressure drop has been considered, was increased about 3–5% for the studied equivalence ratio of methane-air combustion.

Flame jet impingement heat transfer for a pair of Radial Jet Reattachment Combustion (RJRC) nozzles has been studied for flames which were highly, moderately, and weakly interactive. The most uniform heat flux and temperature distributions occurred at the closest between-nozzle spacing, when the flames were highly interacting, while the highest heat flux and surface temperatures were measured when the two flame jets were moderately interacting at intermediate between-nozzle spacings. The optimal spacing for two nozzles was determined based on maximum heat flux and surface temperature. In addition, the percent overall heat transfer to the impingement surface decreased with increasing between-nozzle spacing. The results of this study provide valuable information for applying RJRC nozzles to industrial flame jet impingement heat-treatment processes.

An analytical model of the deterministic properties of turbulent diffusion flames is derived using a similarity analysis. Some parameters appearing in the model are quantified by experiments using porous round burners with propane as the fuel. The validity of the model is verified by experiments as well as by comparing the theory with previous experimental work. The model shows that the deterministic properties are insensitive to material properties.