With the increased utilization of multicomponent fuels, such as natural gas and biogas, in industrial applications, there is a need to be able to effectively model and predict the properties of jet flames for mixed fuels. In addition, the interaction of these diluted fuels with outside influences (such as differing levels of coflow air) is a primary consideration. Experiments were performed on methane jet flames under the influence of varying levels of nitrogen dilution, from low Reynolds number lifted regimes to blowout, observing the influence of the nitrogen on lifted flame height and flame chemiluminesence images. These findings were analyzed and compared with existing lifted jet flame relations, such as the flammable region approximation proposed by Tieszen et al., as well as to undiluted flames. The influence of nitrogen dilution was seen to have an effect on the liftoff height of the flame, as well as the blowout velocity of the flame, but was seen to have a less pronounced effect compared with flames with coflowing air.

Presented are the results of experiments designed to investigate flame lift-off behavior in the hysteresis regime for low Reynolds number turbulent flows. The hysteresis regime refers to the situation where the jet flame has dual positions favorable to flame stabilization: attached and lifted. Typically, a jet flame is lifted off of a burner and stabilized at some downstream location at a pair of fuel and coflow velocities that is unique to a flame at that position. Since the direction from which that condition is arrived at is important, there is an inherent hysteretic behavior. To supplement previous research on hysteretic behavior in the presence of no coflow and low coflow velocities, the current research focuses on flames that are lifted and reattached at higher coflow velocities, where the flame behavior includes an unexpected downstream recession at low fuel velocities. Observations on the flame behavior related to nozzle exit velocity and coflow velocity are made using video imaging of flame sequences. The results show that a flame can stabilize at a location downstream despite a decrease in the local excess jet velocity and assist in determining the effect of coflow velocity magnitude on hysteretic behavior. These observations are of utility in designing maximum turndown burners in air coflow, especially for determining stability criteria in low fuel-flow applications.

Studies are presented that examine the fluctuations in liftoff height of lifted methane flames in the presence of air coflow. At a certain jet exit velocity, a flame will lift from the burner exit and stabilize at some downstream position. The partially-premixed flame front of the lifted flame oscillates in the axial direction, with the fluctuations becoming greater in flames stabilized further downstream. These fluctuations are also observed in flames where blowout is imminent. This work investigates the role of fuel velocity and air co-flow on flame fluctuations in both stable and unstable regimes. The results of video imaging of a lifted methane-air diffusion flame are presented and discussed. Images are used to ascertain the changes in the reaction zone that influence these fluctuations and relate the movement to blowout.

Contemporary interest exists for understanding how reaction zones stabilize and counter-propagate against incoming reactants. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. Observations of the leading-edge flame behavior with respect to upstream propagation and recession downstream are made with sequential CH-PLIF imaging, and data have been revisited in light of the recent research of McCraw et al. (Flow Turbul Combust 70(1):83–97, 2007). It is found that in cases where a distinct branch of the outer (fuel-lean) edge of the reaction zone is present, the edge of the flame is either witnessed to propagate upstream or locally disappear. In cases where no distinct branch other than the main branch is observed, the flame is witnessed to either remain stationary or drop back downstream. These observations support the notion that structures in the low speed, outer edge of the reaction zone are involved in the upstream phase of the flame propagation.

The mechanisms that cause jet-flame blowout, particularly in the presence of air coflow, are not completely understood. This work examines the role of fuel velocity and air coflow in the blowout phenomenon by examining the transient behavior of the reaction zone at blowout . The results of video imaging of a lifted methane-air diffusion flame at near blowout conditions are presented. Two types of experiments are described. In the first investigation, a flame is established and stabilized at a known, predetermined downstream location with a constant coflow velocity, and then the fuel velocity is subsequently increased to cause blowout. In the other, an ignition source is used to maintain flame burning near blowout and the subsequent transient behavior to blowout upon removal of the ignition source is characterized. Data from both types of experiments are collected at various coflow and jet velocities. Images are used to ascertain the changes in the leading edge of the reaction zone prior to flame extinction that help to develop a physically-based model to describe jet-flame blowout. The data report that a consistent predictor of blowout is the prior disappearance of the axially oriented flame branch. This is witnessed despite a turbulent flames' inherent variable behavior. Interpretations are also made in the light of analytical mixture fraction expressions from the literature that support the notion that flame blowout occurs when the leading edge reaches the vicinity of the lean-limit contour, which coincides approximately with the conditions for loss of the axially oriented flame structure.

A variety of investigators have attempted to characterize the mechanisms of how reaction zones stabilize, or propagate, against incoming reactants, particularly in stable lifted jet flames both laminar and turbulent. In this paper, experiments are described that investigate the characteristics of upstream flame propagation in turbulent hydrocarbon jet flames. An axisymmetric, gaseous turbulent jet mixing in air has been selectively ignited at downstream positions to assess the upstream propagation of the bulk reaction zone. The farthest axial position that permitted the reaction zone to propagate upstream after application of the ignition source, referred to as the “upper propagation limit”, or UPL, is determined for a variety of jet and air co-flow parameters. There is an inverse relationship between the upper propagation limit position and the jet Reynolds number. Conversely, there is a direct relationship between the upper propagation limit and the co-flow velocity. Interpretation of the results is related to the velocity at the stoichiometric surface. Global discussion is made as to what these results imply about the stabilization and propagation of turbulent lifted jet flames.

The theoretical basis for the integration of direct methods into the single-wavelength anomalous dispersion technique is reexamined. The analysis shows that the approximations responsible for the ability to obtain unique estimates of the two- and three-phase structure invariants [Hauptman (1982). Acta Cryst. A38, 632-641; Giacovazzo (1983). Acta Cryst. A39, 585-592] or twofold estimates of the three-phase structure invariants [Kroon, Spek & Krabbendam (1977). Acta Cryst. A33, 382-385] are also responsible for the substantial errors observed in the applications. It is shown that, in the general case, the method of joint probability distributions leads to twofold estimates of the two-phase invariants and eightfold estimates of the three-phase invariants. Finally, it is shown that more accurately determined three-phase invariant estimates can be obtained by the use of anomalous scatterer substructure information, when available, and the use of a strategy that recognizes cases in which the eight estimates are clustered around one or two values. These cases are then distinguished from those where the eight estimates are widely scattered by a weighting function.

The probabilistic theory of the three-phase structure invariants for a pair of isomorphous structures [Hauptman (1982). Acta Cryst. A38, 289-294] is reexamined. The analysis leads to distributions capable of estimating cosine invariants in the full range of -1 to +1. In particular, it is shown that heavy-atom substructure information can be incorporated easily into the distributions. The initial applications, using calculated diffraction data from the protein cytochrome c550, MR ≃ 14 500, and its PtCl2-4 derivative show that a remarkable increase in accuracy results from the use of the revised distributions, particularly after the incorporation of heavy-atom substructure information. Finally, it is shown that in the individual phase determinations the redundant cosine invariants play a role identical to that of the multiple isomorphous derivatives and thus provide the basis for the solution of the phase problem in the single isomorphous replacement case.