B. Lenze

Experimental investigation on the stabilization mechanism of jet diffusion flames

Two contrary concepts have been suggested in order to explain the mechanism of flame stabilization based on premixed and diffusion flamelet combustion respectively. To contribute to the understanding of which model represents the true stabilization mechanism, two different natural gas jet diffusion flames, at exit velocities between flame detachment and blow-off, were investigated. The measured profiles of gas composition and velocity around the stabilization zone were used to derive the rate of mixing and fuel burnout. The results show that, for the flames investigated, about forty to fifty percent of the total fuel flow is already mixed at a molecular level upstream of the flame stabilization zone. This mixture then reacts over a very short distance, supporting the concept of premixed combustion in lifted jet diffusion flames.

Turbulent swirling flames: Experimental investigation of the flow field and formation of nitrogen oxide

The NO emissions of a 150 kW natural gas (with and without fuel N) burner were shown to be dependent on the geometry of the gas nozzle and the swirl intensity of the combustion air. Numerous detailed inflame measurements were performed in order to understand the local conditions that influence the formation of NO in highly turbulent swirling flames. It was found, that by controlling the axial fuel gas momentum in combination with the swirl number, mixing of fuel and air can be influenced, thus reducing NO formation. Using fuels with chemically fixed nitrogen requires flames with the main heat release under fuel-rich conditions, leading to type I flames. These flames have the potential to reduce the NO emissions up to 70% compared with premixed flames. To minimize thermal NO formation, high-temperature zones with near-stoichiometric mixtures must be avoided, which may be realized by type II flames. Moreover, swirl turned out to be a strong tool for minimizing thermal NO. Under overall fuel-lean conditions, higher swirl led to faster mixing of fuel and air, thus lowering the temperature level of the flame and reducing thermal NO emissions. The experiments were performed in cooperation with the German Technische Flammen (TECFLAM) joint research program, using a standardized combustion chamber and burner. By applying identical experimental setups and different measurement techniques by the TECFLAM partners, accurate and reliable measuring data were produced on the time-mean and on the turbulent fluctuation levels.

Development and assessment of correlations for predicting stability limits of swirling flames

Abstract Swirling flow systems are being used in many industrial processes like separation of particles in gas flows (cyclones), atomisation and spreading of liquids (water, oil) and fast mixing and high-intensity reaction in reactor systems as, e.g. stable and intense flames of swirl burners. The widespread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NOx-emissions (ultralean premixed combustion) raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation. Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operated near to the extinction limit, different methods for predicting lean blow-off limits have been developed and tested for different burner sizes and fuel gas compositions. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for expensive and time-consuming measurements in those flames. Several similarity criteria based on volumetric flowrates, burner size and material parameters of the cold gases, were found to be capable to predict stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,e-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model for the determination of the time mean reaction rates in turbulent flows was also tested for its potential for stability prediction. All the methods presented have specific advantages and limitations: the similarity criteria are restricted to geometrically similar systems, but they are easy to use, fairly precise and can take the detailled chemistry into account in an integral manner. The numerical field model necessitates large computational effort and is limited to very much simplified, global reaction mechanisms, but it offers the opportunity to make predictions for different burner geometries and swirl conditions.

Flame stabilization and turbulent exchange in strongly swirling natural gas flames

An experimental study has been made in the near field of a variable swirl burner under isothermal and reacting conditions in order to quantify the effect of combustion on the isothermal flow field, and to explain the mechanism of flame stabilization due to internal recirculation zones. A two colour Laser-Doppler velocimeter was used to determine turbulence properties in-/and outside of the recirculation zone. Temperature and species concentrations were measured by conventional measuring techniques. Turbulent exchange coefficients for momentum, matter and heat have been obtained by measurements and by calculations from time mean value distributions, respectively. The results show that, with strong swirl, the intensity of recirculation is reduced by the presence of combustion due to a marked decrease of the effective swirl number. Divergent burner nozzles at the burner exit lead to a radial extension of the reverse flow zone, but do not affect the reverse flow density. Reynolds number similarity of the diffusion flame is confirmed as long as reaction kinetic effects are negligible. On the other hand, blow-off occurs as a result of a reduction of local residence time in the ignition zone when increasing burner load. LDA-measurements exhibit high turbulence intensities and strong turbulent fluxes in the region of the stabilization zone. All turbulent fluxes agree with the gradient law, so that no countergradient diffusion exists under the conditions studied. The presence of combusion leads to a damping of turbulent exchange as compared with the isothermal flow of same swirl intensity.

Basic considerations concerning the construction and usage of multiple hot-wire probes for highly turbulent three-dimensional flows

The authors discuss the use of multiple hot-wire probes in turbulent three-dimensional flows with the aid of a graphical analysis. It is shown that hot-wire sensors can be used to measure all three components of the velocity vector as long as its direction lies within the acceptability range, which can be extended to a hemisphere if a probe with at least four wires is used. The detection of flow reversal is, however, not possible with any number of simple wire sensors. A computer-aided calibration and data reduction method for a four-wire probe is described. Test measurements in an axisymmetric free jet prove the applicability of the method.

Assessment of a quintuple hotwire measurement technique for highly turbulent flows

A five-wire calibration and measurement technique is presented, which is an extension of the four-wire method developed by Döbbeling et al. (1990a, b). From numerical simulations of the uniqueness domain and the angular resolution it is concluded that the uniqueness domain of the quintuple technique can be expanded to a hemisphere as opposed to the four-wire techniques which are restricted to a conical domain of about 40° half angle. Measurements of the mean velocities and Reynolds stresses in a low-turbulence jet and in grid turbulence confirm and complement the results of the numerical simulations. It is thus shown that the quintuple method achieves increased accuracy in an expanded measurement range.

Experimental and numerical study concerning stabilization of strongly swirling premixed and nonpremixed flames

The wide-spread application of swirl-stabilized flames in industrial combustion devices necessitates an improved understanding of the stabilization mechanisms and stability limits, particularly for the design of modern combustors with high volumetric heat release. Due to the complex mixture and reaction structures in these flames, the predictions of simplified stability models are not satisfactory. We have carried out measurements on unconfined strongly swirling premixed and nonpremixed flames supplying detailed temperature and velocity data as well as mean concentration values of most stable species. Furthermore, significant modifications of the turbulent reaction structure with increasing flowrate are observed. A numerical model for the prediction of the relevant aerodynamic and chemical processes has been developed. A k ,e-model which was corrected for turbulent exchange in swirling flows provides improved prediction of flowfield and mixing, which is crucial for reaction modeling. The mean reaction rates are determined by integration of an “assumed shape joint-PDF,” which is based on experimental findings concerning the joint probability betwen temperature and mixture fraction and on a global 2-step reaction mechanism. Numerical results agree fairly well with experiments and indicate that combustion takes place mainly at low turbulent Damkohler numbers. It can be stated that the stability limits of strongly swirling flames are controlled by finite kinetic rates associated with residence times dependent on flowrate rather than by turbulent mixing rates; these interactions between reaction kinetics and turbulent mixing are correctly reflected by the numerical model.

Investigation on the Combustion-Turbulence Interaction in Premixed Stagnation Flames of H2-CH4 Mixtures

In the present paper measurements on both the influence of turbulence on premixed combustion and the effect of the flame front on the upstream turbulence conditions in the approach flow, using premixed stagnation flames as the experimental system, are presented.

Computer-aided calibration and measurements with a quadruple hotwire probe

A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like Kings law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.

Results of Experiments and Models for Predicting Stability Limits of Turbulent Swirling Flames

Swirling flames are used in many industrial applications like process furnaces, boilers and gas turbines due to their excellent mixing, stability, emission and burnout characteristics. The wide-spread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NOx-emissions raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation.Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operating near the extinction limit, different methods for predicting lean blow-off limits have been developed and tested. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for measurements in those flames.Several similarity criteria based on volumetric flow rates, burner size and material parameters of the cold gases, were found to be capable of predicting stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,ϵ-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model was also tested for its potential for stability prediction.© 1997 ASME