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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.

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

Summary Various extractants used in current analytical procedures for the fractionation of sulphur (S) in soils were compared with respect to their ability to dissolve Al hydroxy sulphate minerals of defined composition (basaluminite, K alunite, mixtures of basaluminite and Na alunite). The minerals were synthesized and aged in the laboratory at 20°C and 50°C. The dissolution kinetics at 20°C of these Al hydroxy sulphates in deionized water, 0.02M HCl and 0.02M NaOH were also investigated. The dissolution stability of the Al hydroxy sulphate minerals increased in the order basaluminite < K alunite < Na alunite. The dissolving power of the reagents used increased in the order H2O ≤ 0.016M KH2PO4≪ 0.02M HCl ≈ acidic NH4 oxalate ≈ 0.5M NaHCO3 < Na2 CO3/NaHCO3 < 0.1M NH4F < 0.5M NH4F < 0.05M NaOH ≤ 0.05M LiOH. In Al hydroxy sulphate‐containing soils, inorganic S is probably understimated and ester sulphate S overestimated, if the inorganic S pool is assessed by extraction with KH2PO4, Ca(H2PO4)2 or NaHCO3 solutions. The dissolution of all studied Al hydroxy sulphates, particularly that of K and Na alunite, in H2O and 0.02 M HCl is strongly delayed by kinetic restraints. Thus it seems unlikely that Al3+ or SO2−4 activites in soil solutions are strictly regulated by precipitation/dissolution equilibria of these minerals except for horizons with extraordinarily slow seepage water movement.

A novel approach to the description of flame transfer functions of premixed swirling flames is presented based on a simplified analysis of the vorticity transport equation. With this model it is possible to reproduce transfer function amplitude in excess of unity observed for swirling flames as well as to explain the differences seen in flame transfer functions for different swirler types. Measurements of flame transfer functions were performed on perfectly premixed flames generated by an axial and a radial swirler with an annular exit. Flames were adjusted as to achieve respective dynamic similarity. The comparison of the flame transfer functions shows the influence of the swirler type. Good agreement with the model validates the theoretical approach and allows to attribute the specific differences seen between the two swirler types with the generation of a swirl number modulation induced by the convective time delay between radial swirler and burner exit. The model also allows to explain the change of flame transfer function amplitude with pre-heating observed in previous publications.

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Summary Current methods for determining inorganic sulphur (S) in aerated mineral soil horizons often result in underestimates. To overcome this defect we developed a new method combining a batch extraction with 0.5 m NH4F solution at a soil:solution ratio of 1:5 with a subsequent analysis of the mobilized SO42– by ion chromatography. The ammonium fluoride extraction enables us to characterize inorganic sulphate in non‐calcareous forest soils. It is more efficient than conventional procedures in which inorganic S is extracted with phosphate or bicarbonate solution. In contrast to the extraction with strongly alkaline reagents (NaOH, KOH, LiOH), the NH4+–NH3 buffer system in NH4F prevents the pH of the suspension from exceeding 9.0 and thus the undesired conversion of organic S into SO42– by auto‐oxidation and hydrolysis of ester sulphate. In a comparison we demonstrated that the inorganic S in six German forest soils is underestimated by up to 50% or 200 kg S ha−1 in the uppermost 60 cm, if it is assessed by extraction with 0.016 m KH2PO4 or 0.5 m NaHCO3 instead of 0.5 m NH4F. Conversely, the pool of ester sulphate is overestimated almost threefold.