Convective heat transfer from a partially premixed impinging flame jet. Part I: Time-averaged results

Axial and radial profiles of time-averaged local heat fluxes of methane-air jet flames impinging normal to a cooled plate are reported, as functions of equivalence ratio, Reynolds number, and nozzle-plate spacing. Time-resolved behavior for these conditions is examined in the companion paper, Part II. Flame structure was studied visually and photographed. Both premixed and diffusion flame behavior was observed. Nozzle-stabilized flames revealed a stable, axisymmetric flame structure at nozzle-plate spacings less than 14 diameters. At greater nozzle-plate spacings, buoyancy-induced instabilities caused the flame to oscillate visibly. Lifted flames exhibited varied flame structures dependent upon the Reynolds number, equivalence ratio, and nozzle-plate spacing, stabilizing in the free jet, at the stagnation zone, or downstream in the wall jet. Local heat flux measurements made in the stagnation zone and along the plate adjacent to the wall jet flame revealed correlation of the local heat flux to the flame structure. Negative heat fluxes resulted from cool gases impinging on the hotter plate. The magnitude of positive heat fluxes depended on the proximity of the flame to the sensor surface, the rate of heat release, and the local molecular and turbulent transport.     

Experiments on the scalar structure of turbulent CO/H2/N2 jet flames

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H₂/N₂ (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneous measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separately. This paper focuses on the scalar results but includes some limited velocity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise distance is scaled by nozzle diameter. However, OH mole fractions are lower and NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residence times. Turbulent flame measurements are compared with steady strained laminar flame calculations. Laminar calculations show remarkably close agreement with measured conditional means of the major species when all diffusivities are set equal to the thermal diffusivity. In contrast, laminar flame calculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in determining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differential diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometric simplicity of the unpiloted jet flame in coflow, while including a chemical kinetic system of intermediate complexity between hydrogen flames and the simplest hydrocarbon flames. Aspects of the measurements, including Favre-averaged profiles, conditional statistics, mixture fraction pdf’s, and departures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The complete data are available on the World Wide Web for use in model validation studies.     

Investigation of local flame structures and statistics in partially premixed turbulent jet flames using simultaneous single-shot CH and OH planar laser-induced fluorescence imaging

We report on the application of simultaneous single-shot imaging of CH and OH radicals using planar laser-induced fluorescence (PLIF) to investigate partially premixed turbulent jet flames. Various flames have been stabilized on a coaxial jet flame burner consisting of an outer and an inner tube of diameter 22 and 2.2 mm, respectively. From the outer tube a rich methane/air mixture was supplied at a relatively low flow velocity, while a jet of pure air was introduced from the inner one, resulting in a turbulent jet flame on top of a laminar pilot flame. The turbulence intensity was controlled by varying the inner jet flow speed from 0 up to 120 m/s, corresponding to a maximal Reynolds number of the inner jet airflow of 13,200. The CH/OH PLIF imaging clearly revealed the local structure of the studied flames. In the proximity of the burner, a two-layer reaction zone structure was identified where an inner zone characterized by strong CH signals has a typical structure of rich premixed flames. An outer reaction zone characterized by strong OH signals has a typical structure of a diffusion flame that oxidizes the intermediate fuels formed in the inner rich premixed flame. In the moderate-turbulence flow, the CH layers were very thin closed surfaces in the entire flame, whereas the OH layers were much thicker. In the high-intensity-turbulence flame, the CH layer remained thin until it vanished in the upper part of the flame, showing local extinction and reignition behavior of the flame. The single-shot PLIF images have been utilized to determine the flame surface density (FSD). In low and moderate turbulence intensity cases the FSDs determined from CH and OH agreed with each other, while in the highly turbulent case a locally broken CH layer was observed, leading to a significant difference in the FSD results determined via the OH and CH radicals. Furthermore, the means and the standard deviations of CH and OH radicals were obtained to provide statistical information about the flames that may be used for validation of numerical calculations.     

Blow-off process of highly under-expanded hydrogen non-premixed jet flame

Relationships between flame lift-off heights and reservoir pressure were experimentally investigated in order to clarify blow-off process of hydrogen non-premixed jet flames with a highly under-expanded jet structure. In this study, straight nozzles with diameters of 0.34, 0.53, 0.75 and 1.12 mm were used with maximum reservoir pressure for spouting hydrogen of 13.2 MPa. Experimental results are shown that lift-off heights in stable under-expanded jet flames do not vary significantly and are independent of the reservoir pressure in the range of studied pressure. However, the lifted heights are affected by the nozzle diameters and become smaller as the nozzle diameters increase. From experimental results, the condition for the blow-off process of under-expanded subsonic jet flames was proposed. It was concluded that the under-expanded jet flame could be blown off when the maximum waistline position, where radial distance from the jet axis to an elliptic stoichiometric contour reaches its maximum comes closer to the nozzle exit than the edge of the jet flame base.     

Turbulent hydrocarbon jet flames

To consider turbulent hydrocarbon jet flames as an ensemble of wrinkled laminar flames gives useful information about details of the combustion process by studying reactive diffusive interfaces. Combined with the conserved scalar approach, non-premixed turbulent jet flames can be computed with satisfactory accuracy.Due to heat release in the region of largest shear, orderly flow structures are by far more pronounced in hydrocarbon jet flames than in nonreacting jets. Heat release in the region of largest shear influences the development of turbulence in the initial flame region. However, the influence of density gradients on the turbulent flow field is negligible over the major part of diffusion flames.     

Basic considerations in the turbulent flame propagation in premixed gases

A general discussion is given of some fundamental problems of turbulent flame propagation in premixed gases. The following subjects are considered in greater detail: Stability of laminar flames in turbulent flow, shear wave-flame interaction, flame generated turbulence, influence of small scale turbulence on flame propagation and structure of turbulent flames at high Reynolds numbers. The principal object of this study is to describe the basic physical facts which have to be taken into consideration in the modeling of turbulent flames in gases without giving a detailed survey of all the research that has been carried out in the field.     

Characteristics of turbulent nonpremixed jet flames under normal- and low-gravity conditions

An experimental study was performed with the aim of investigating the structure of transitional and turbulent nonpremixed jet flames under different gravity conditions. Experiments were conducted under three gravity levels, viz., 1 g, 20 mg, and 100 μg. The milligravity and microgravity conditions were achieved by dropping a jet-flame rig in the University of Texas at Austin 1.25-s and NASA-Glenn Research Center 2.2-s drop towers, respectively. The flames studied were piloted nonpremixed propane, ethylene, and methane jet flames at source Reynolds numbers ranging from 2000 to 10,500. The principal diagnostic employed was time-resolved cinematographic imaging of the visible soot luminosity. Mean and root-mean-square (RMS) images were computed, and volume rendering of the image sequences was used to investigate the large-scale structure evolution and flame tip dynamics. The relative importance of buoyancy was quantified with the parameter, ξL, as defined by Becker and Yamazaki (Combust. Flame 33 (1978) 123-149). The results showed, in contrast to some previous microgravity studies, that the high-Reynolds-number flames have the same flame length irrespective of the gravity level. The mean and RMS luminosity images and the volume renderings indicate that the large-scale structure and flame tip dynamics are essentially identical to those of purely momentum-driven flames provided ξL is less than approximately 2-3. The volume renderings show that the luminous structure velocities (i.e., celerities) normalized by the jet exit velocity are approximately constant for ξL<6, but scale as for ξL>8. The flame length fluctuation measurements and volume renderings also indicate that the luminous structures are more organized in low gravity than in normal gravity. Finally, taken as a whole, this study shows that ξL is a sufficient parameter for quantifying the effects of buoyancy on the fluctuating and mean characteristics of turbulent jet flames.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

Numerical study on high-temperature diluted air combustion for the turbulent jet flame in crossflow using an unsteady flamelet model

The turbulent jet flame in a crossflow with highly preheated diluted air has been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the flamelet concept coupled with the standard k–ε turbulence model. The NO formation is estimated by using the Eulerian particle transport equations in a postprocessing mode. For methane and propane with various conditions of inlet air temperature and oxygen concentration, the three-dimensional characteristics of the flame are successfully captured. The jet-flame trajectory is in remarkably good agreement with the existing cold-flow correlations. When the oxygen concentration is high, the maximum flame temperature becomes high and the two fuels show quite different characteristics in the downstream region. On the other hand, for low oxygen concentrations, the temperature difference between the two fuels is relatively small and remains fairly constant throughout the combustion chamber. The propane gives a higher NO formation compared to the methane especially when the oxygen concentration is high. A higher temperature, longer residence time of the combustion gases may be responsible for the higher thermal NO formation.     

Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H₂/CO₂/air flames as a model of syngas/air combustion were investigated at pressure of 0.5 MPa and compared to that of CH₄/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH₄/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.     

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.     

Direct comparison of turbulent burning velocity and flame surface properties in turbulent premixed flames

Direct comparison of the turbulent burning velocity (obtained from flame speeds) to the flame perimeter ratio has been made in turbulent premixed flames propagating freely downward for propane/air mixtures at various equivalence ratios, with u′/S_L of ranging from 1.4 to 5.3. The turbulent flame speed ranged from 2.6 to about 7 times the laminar flame speed at high turbulence intensities, while the flame perimeter ratio ranges from 1.4 to 3.3. In the current freely propagating flames, the global flame curvature can lead to an enhancement of the flame speed by a factor of up to 3.5. This global flame curvature is attributable to the wall heat loss in the current burner configuration, and flame brush thickness has been used as a measure of the global flame curvature. For flames involving coupling of the globally curved flame geometry with flow divergence or any flow non-uniformity, correcting for this geometrical effect requires a careful consideration of the flame topology and flow field. The difference between the observed flame speed and the 2-D flame perimeter ratio, after correcting for the global flame curvature effect, is attributed to the fact that the flame wrinkles in three-dimensions are associated with a larger flame surface area than that determined from the flame perimeter ratio data. This also points to a need to better understand the 3-D geometrical effects including the global flame curvature and the local flame wrinkle structure in turbulent premixed flames. The observed turbulent flame speed data for the most part follow the flame speed models of Bray and Damkohler, wherein the flame surface area increase is modeled as a function of turbulence and thermochemical properties. The above results, taken together, indicate that the fundamental assumption that the turbulent flame speed depends primarily on the increased flame surface area is valid. This concept can be used to estimate the turbulent flame speed within reasonable accuracy provided that the 3-D flame effects associated with the global flame curvature and local flame wrinkle structure are considered.Keywords: Turbulent premixed flames, Flame speed, Flame surface, Burning velocity     

Lean premixed opposed jet flames in fractal grid generated multiscale turbulence

The opposed jet configuration presents an attractive canonical geometry for the evaluation of burning properties of turbulent flames with past studies typically limited to low Reynolds numbers. Fractal grid generated turbulence was used to remove the low turbulence level limitations associated with conventional perforated plate generators with the turbulent Reynolds number range moved from 50–120 to 130–318. Optimal grid configurations were determined with particular emphasis on reducing the impact of the flow upstream of the turbulence generators in order to facilitate simpler boundary conditions for computational studies. The resulting flow structures were analysed using proper orthogonal decomposition and conditional proper orthogonal decomposition. Velocity and reaction progress variable statistics, including conditional velocities and scalar fluxes, are reported for fuel lean methane, ethylene and propane flames approaching extinction. The instrumentation comprised particle image velocimetry with the flows to both nozzles seeded with 1 μm silicon oil droplets or 3 μm Al₂O₃ particles. Probability density functions were determined for the instantaneous location of the stagnation point to eliminate the possibility of low frequency bulk motion distorting velocity statistics. Probability density functions of flame curvature were determined using a multi-step flame front detection algorithm with estimates of the turbulent burning velocity provided along with a discussion of alternative determination methods. The data sets show that fractal grids generate multi-scale broadband turbulence and present an opportunity for a systematic evaluation of calculation methods for premixed turbulent flames that undergo a transition from non-gradient to gradient turbulent transport while approaching extinction.     

Experimental investigation of three-dimensional flame-front structure in premixed turbulent combustion—I: hydrocarbon/air bunsen flames

OH concentrations and three-dimensional gradients of the reaction progress variable have been measured in turbulent liquefied petroleum gas/air and compressed natural gas/air premixed flames stabilized on a Bunsen-type burner with a combined two-sheet Rayleigh scattering and planar LIF-OH imaging technique. The progress variable is observed to undergo a transition from lamella-like to non-flamelet front structure with increasing turbulence. This is consistent with the recently proposed change of the combustion regimes from complex-strain to turbulent flame front regime on a recently proposed premixed combustion diagram. The anisotropy of local flame-front orientation in three-dimensional space is explained by the forward propagation ability of the planar turbulent flame brush. Weighting functions have thus been derived for the isotropic pdf distributions of the in-plane and out-of-plane orientation angles to agree better with the experimental data. A linear scaling is found between the overall flame surface area and the turbulence intensity normalized by the laminar burning velocity. However, flames with excess backward-facing flame fronts do not comply with this linear relationship, showing enhanced flame surface folding. The thin-flame assumption breaks down when non-flamelet broadening effects become important, although the pdf’s of the progress variable are still bimodal-like. Non-unity Lewis-number combined curvature effects are evident for LPG/air flames of weak turbulence, in that the conditional mean scalar dissipation increases steadily from the unburnt to burnt side across the flame brush. A consistent correlation exists between the Favre-averaged scalar dissipation and progress variable variance. This implies that small-scale scalar dissipation of local flame-fronts is linked to large-scale scalar fluctuations. Sub- or super-flamelet OH concentration is found in lean LPG/air or CNG/air premixed flames, respectively, and occurs in line with a positive or negative correlation between OH concentrations and magnitudes of the progress variable gradient.     

Liftoff of turbulent jet flames—assessment of edge flame and other concepts using cinema-PIV

Three theories of the liftoff of a turbulent jet flame were assessed using cinema-particle imaging velocimetry movies recorded at 8000 images/s. The images visualize the time histories of the eddies, the flame motion, the turbulence intensity, and streamline divergence. The first theory assumes that the flame base has a propagation speed that is controlled by the turbulence intensity. Results conflict with this idea; measured propagation speeds remains close to the laminar burning velocity and are not correlated with the turbulence levels. Even when the turbulence intensity increases by a factor of 3, there is no increase in the propagation speed. The second theory assumes that large eddies stabilize the flame; results also conflict with this idea since there is no significant correlation between propagation speed and the passage of large eddies. The data do support the “edge flame” concept. Even though the turbulence level and the mean velocity in the undisturbed jet are large (at jet Reynolds numbers of 4300 and 8500), the edge flame creates its own local low-velocity, low-turbulence-level region due to streamline divergence caused by heat release. The edge flame has two propagation velocities. The actual velocity of the flame base with respect to the disturbed local flow is found to be nearly equal to the laminar burning velocity; however, the effective propagation velocity of the entire edge flame with respect to the upstream (undisturbed) flow exceeds the laminar burning velocity. A simple model is proposed which simulates the divergence of the streamlines by considering the potential flow over a source. It predicts the well-established empirical formula for liftoff height, and it agrees with experiment in that the controlling factor is streamline divergence, and not turbulence intensity or large eddy passage. The results apply only to jet flames for Re<8500; for other geometries the role of turbulence could be larger.     

Spatial and radiative properties of an open-flame hydrogen plume

Considerable effort is being directed toward updating safety codes and standards in preparation for production, distribution, and retail of hydrogen as a consumer energy source. In the present study, measurements were performed in large-scale, vertical flames to characterize the dimensional and radiative properties of an ignited hydrogen jet. These data are relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. Specifically, the data will provide a technological basis for determining hazardous length scales associated with unintended releases at hydrogen storage and distribution centers. Visible and infrared video and ultraviolet flame luminescence imaging were used to evaluate flame length, diameter and structure. Radiometer measurements allowed determination of the radiant heat flux from the flame. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames show good agreement with non-dimensional correlations and scaling laws developed for a range of fuels and flame conditions. This result verifies that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Characterization of high-pressure,underexpanded hydrogen-jet flames

Measurements were performed to characterize the dimensional and radiative properties of large-scale, vertical hydrogen-jet flames. This data is relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel and will provide a technological basis for determining hazardous length scales associated with unintended hydrogen releases at storage and distribution centers. Jet flames originating from high-pressure sources up to 413 bar (6000 psi) were studied to verify the application of correlations and scaling laws based on lower-pressure subsonic and choked-flow jet flames. These higher pressures are expected to be typical of the pressure ranges in future hydrogen storage vessels. At these pressures the flows exiting the jet nozzle are categorized as underexpanded jets in which the flow is choked at the jet exit. Additionally, the gas behavior departs from that of an ideal-gas and alternate formulations for non-ideal gas must be introduced. Visible flame emission was recorded on video to evaluate flame length and structure. Radiometer measurements allowed determination of the radiant heat flux characteristics. The flame length results show that lower-pressure engineering correlations, based on the Froude number and a non-dimensional flame length, also apply to releases up to 413 bar (6000 psi). Similarly, radiative heat flux characteristics of these high-pressure jet flames obey scaling laws developed for low-pressure, smaller-scale flames and a wide variety of fuels. The results verify that such correlations can be used to a priori predict dimensional characteristics and radiative heat flux from a wide variety of hydrogen-jet flames resulting from accidental releases.     

Lifted flames on fuel jets in co-flowing air

The three principal theories for the stabilization of lifted flames on turbulent jets of fuel are reviewed in the light of the most recent flame imaging experiments in the literature. Most of these experiments have been conducted with a small co-flow of air, but the observations are relevant to lift-off with higher ratios of co-flowing air to fuel jet velocity. The similarity solutions for jets in co-flow are developed, and data from a variety of fluid dynamic sources are assessed to yield the governing parameters for mean flow, turbulence and mixture fraction. New data for lifted flames on a methane jet in diffusing streams of co-flowing air are then presented. These data provide essential information on the intermittency, and on the properties of the jet conditioned on the presence of turbulent fluid. However, the co-flow lifts the flame to stabilize in better-mixed regions than in its absence. The ‘premixture’ model is confirmed for this situation, in which the lift-off heights were more than 20 jet diameters and where there is little intermittency at the stabilization radius. Nevertheless, mixing data for this geometry in the absence of a flame show that, with lift-off heights less than 20 jet diameters, the base of the flame would have been in the outer regions of the jet where the mixture of fuel in air only reaches stoichiometric proportions intermittently, with the passage of large eddies. Trading on many papers from the recent literature where this was the case, both experimental and computational insights as to the processes in this region are reviewed. A question remains about how ignition is maintained in these experiments with low turbulent lift-off. It is hypothesized that the mechanism is the diffusive heating of the slowly moving surrounding air which then provides an energy store for the incoming eddies. Further time-resolved observations of reaction zone and high temperature gas structure are required to test this model.