A. M. K. P. Taylor

Extinction of turbulent counterflow flames with reactants diluted by hot products

The effects of simultaneous dilution and preheat of reactants by mixing with hot combustion products are examined in terms of the stability of turbulent counterflow flames. Premixed flames were stabilized in the opposed flow of premixed natural gas/air mixtures within the flammability limits and an opposing jet composed of hot products at temperatures up to 1750 K and oxygen mole fractions down to 0.02. The gain in stability of the premixed flames was small for temperatures from 300 to 1400 K, but temperatures higher than 1550 K always ignited flames of equivalence ratio as lean as 0.2 and these could not be extinguished by straining, in agreement with expectations from laminar counterflow premixed flames. This critical temperature is close to that below which chemical reaction is not self-sustaining. Turbulent diffusion flames were stabilized in the same arrangement with the hot product stream as oxidizer and it was found that for every 0.02 of oxygen mole fraction lost to dilution, the temperature had to increase by 100 K for the same extinction strain rate and that there was no extinction at air temperatures higher than about 1700 K. Laminar counterflow flame predictions of extinction are shown to be in agreement with the measurements and also show that stability is improved in the special case of adiabatic mixing of the air with hot combustion products, so that the temperature rise and the oxygen content are related, and this explains why flames stabilized by recirculation zones, where hot products are recirculated to mix with the incoming reactants, can be stable with their high stretch rates.

Extinction and temperature characteristics of turbulent counterflow diffusion flames with partial premixing

Abstract A turbulent counterflow diffusion flame of natural gas stabilized between two opposed jets discharging from straight tubes of 25.4 mm diameter fitted with turbulence-promoting perforated plates has been examined in terms of its appearance, extinction limits, and mean and fluctuating temperatures as measured by numerically compensated fine-wire thermocouples. It was observed that the flame was flat, blue, and located around the symmetry plane, that its appearance did not change with initial premixing of the fuel stream with air, and that a premixed flame could also be stabilized provided the equivalence ratio was smaller than that of the rich flammability limit. The bulk velocity for extinction of the nonpremixed flame increased with tube separation and with initial premixing, but decreased with an increase in turbulent intensity. The extinction data collapsed to a single curve to within 20% if the total strain rate acting on the flame (bulk plus small-scale turbulent) was plotted as a function of the air volume fraction in the fuel stream, implying a critical total strain rate for extinction that depended only on the degree of partial premixing. Partial premixing increased the resistance of the flame to straining, from about 350 s −1 for pure fuel to about 600 s −1 for an air volume fraction of 0.8, consistent with experiments and predictions for laminar counterflow flames and with experimental data from piloted turbulent jet flames. The present results for the total strain rate at extinction provide a quantitative description of the effect of partial premixing on flame stability and can be used to predict the extinction of nonpremixed flames in other geometries. The maximum mean temperature of the flame did not change as extinction was approached and was about 1300 ± 50 K for all flow conditions measured, while the rms fluctuations were about 450 K; this insensitivity is attributed to a low-frequency (as indicated by high-pass filtering the temperature time series) flame motion, which also resulted in broad temperature probability density functions.

Velocity and size characteristics of liquid-fuelled flames stabilized by a swirl burner

Velocity and droplet size characteristics of an unconfined quarl burner, of 16 mm quarl inlet diameter, have been measured with a phase-Doppler anemometer at a swirl number of about 0.29: the Reynolds number of the flow was 30000, based on the cold bulk velocity of 30.4 m s-1 and the hydraulic diameter. The atomization was achieved by shear between the swirling air and six radial kerosene jets and the resulting Sauter and arithmetic mean diameters were about 70 and 50 μm respectively after injection: velocity characteristics are presented for three 5 μm-wide size classes, 10, 30 and 60 μm. The flows correspond to no combustion and combustion of natural gas with a heat release of 8 kW supplemented by liquid kerosene flow rates sufficient to generate 21.6 and 37.2 kW : the gas equivalence ratio was 0.45 and atomized kerosene at two flow rates increased the overall ratios to 1.64 and 2.53. In non-reacting flow, droplets 30 μm and smaller are sufficiently small to be entrained by the mean air velocity towards the central part of the flow and into the swirl-induced recirculating air bubble. The 60 μm droplets are able to travel through the bubble uninfluenced by turbulent fluctuations in the air and are ‘centrifuged’ away from the centreline, through acquisition of a mean swirl velocity component, so that a large proportion of the kerosene volume flow rate lies at the edge of the swirling jet. Because larger droplets are centrifuged to the outer part of the flow, whereas the smaller are entrained towards the centreline, the Sauter and arithmetic mean diameters are, by 1.22 quarl exit diameters downstream of the quarl, approximately 65 and 36 μm at the outer part of the flow and 35 and 12 μm near the centreline in the inert flow. In reacting flow, droplets evaporate rapidly in regions of elevated temperatures and hence no droplets are found within the flame brush and recirculation region. The aerodynamic response of each size class to the air velocity is similar to inert flow so that the majority of the kerosene flow is centrifuged away from the flame. On exit from the quarl, the evaporation and burning rates cause the Sauter and arithmetic mean diameters to be about 70 and 50 μm and 60 and 30 μm at the inner and outer edges of the spray respectively. By 1.22 quarl exit-diameters from the exit of the quarl, the air motion entrains droplets smaller than about 30 μm towards the flame, at the inner edge of the spray, so that the Sauter and arithmetic mean diameters are 60 and 40 μm at the outer edge of the jet. There is comparatively little effect of changing the flow rate of kerosene because the combustion is controlled by the low available number of smaller droplets, although the Group combustion number corresponds to ‘cloud’ burning. The relative response of droplets to the mean and turbulent components of air motion, including the ‘centrifuging’ effect, can be scaled to other flows through dimensionless numbers defined in the text.

The interaction of turbulence and pressure gradients in a baffle-stabilized premixed flame

Simultaneous measurements of time-resolved velocity and temperature have been obtained by laser-Doppler anemometry and numerically compensated fine-wire thermocouples in the near wake of a premixed flame stabilized on a disk baffle located on the axis, and at the exit, of a confining pipe. The diameter of the disk was 0.056 m, the diameter of the pipe was 0.080 m, the volumetric equivalence ratio with natural gas as the fuel was 0.79 and the Reynolds number, based on pipe diameter and upstream pipe bulk velocity of 9 m/s, was 46 800. The purpose of the measurements is to quantify the relative magnitudes of terms involving the mean pressure gradient and Reynolds stresses in the balance of turbulent kinetic energy and heat flux in a strongly sheared, high-Reynolds-number, reacting flow. The latter term has been associated with non-gradient diffusion in other flows. Source terms involving the mean pressure gradient are large in the conservation of turbulent heat flux but not in the conservation of Reynolds stress. The ‘thin-flame’ model of burning suggests that the sign and magnitude of the heat flux is closely related to the conditioned mean velocities. The mean axial velocity of the reactants is larger (by up to 0.27 of the reference velocity) than that of the products on the low-velocity side of the shear layer that surrounds the recirculation bubble but the reverse is true on the high-velocity side. These observations are related to the sign of the axial pressure gradient, which is associated with the streamline curvature, and the consequent preferential acceleration of the low-density products. Generally, the Reynolds stresses of the products are higher than those of the reactants and, in contrast to previously reported measurements, the contribution to the unconditioned stresses by the difference in the mean velocity between products and reactants, the so-called ‘intermittent’ contribution, is small. This is a consequence of the high Reynolds number of our flow.

Scalar dissipation rate at the extinction of turbulent counterflow nonpremixed flames

Abstract An analysis based on the modeled governing equations for an axisymmetric turbulent isothermal opposed jet flow has been made with the purpose of facilitating estimates of the scalar dissipation rate in turbulent counterflow nonpremixed flames. The equation for the mean mixture fraction defined as unity in one stream and zero in the other has a solution identical to that for laminar opposed jet flow but with a turbulent diffusion coefficient instead of a laminar one and the only parameter that appears in the modeled equation for the rms mixture fraction fluctuations is the ratio of the turbulent to the bulk flow strain rate. Numerical solutions showed that the mixture fraction fluctuations increase as the bulk strain rate decreases and as the turbulent strain rate increases and these solutions, together with experimentally obtained values for the turbulent intensity at the extinction of counterflow nonpremixed flames with and without partial premixing, resulted in estimates of the mean scalar dissipation rates. The estimated scalar dissipation rate at extinction of flames at different bulk and turbulent strain rates had a constant value that increased with the volume fraction of air in the fuel stream. The present results provide support to the flamelet extinction theory of Peters and Williams [AIAA J. 21:423 (1983)].

Shadow Doppler technique for sizing particles of arbitrary shape

The output from a linear diode array is used in a modified laser Doppler velocimeter to measure the size and shape of irregular particles. The sizing accuracy for transparent and opaque particles between 30 and 140 µm is better than 10%. The inaccuracy caused by trajectories that lay at angles of less than 24° to the axis of the array was less than +5%, and a further inaccuracy of +5% was caused by defocusing of the particle from the center of the velocimeter measuring volume by up to ±500 µm. The advantages of the shadow Doppler technique over other techniques for sizing irregular particles, such as amplitude systems with pointer volumes, are that the shadow Doppler technique records shape, the optical arrangement is more robust, less precise alignment is required, and the equipment can be constructed at low cost.

Extinction of turbulent counterflow flames under periodic strain

The effects of imposed oscillations on the extinction of turbulent nonpremixed and premixed counterflow flames have been quantified as a function of amplitude and frequency of the oscillation. Forced flame extinction was shown to depend on the total duration of pulsation and ranged from a few milliseconds to almost a second, depending on the amplitude and the frequency of the oscillation. Thus extinction times increased quasi-exponentially with decreasing amplitude and increasing frequency of oscillation for nonpremixed flames but were a nonmonotonic function of frequency in premixed flames, with the longest extinction time corresponding to higher frequencies as the flame tended to stoichiometric. Premixing of the fuel or the air stream increased the stability of forced flames, with extinction times reaching their maximum values for twin stoichiometric flames and decreasing towards leaner or richer mixtures. Velocity measurements revealed that the rms of the velocity fluctuations due to the imposed forcing was comparable to, or larger than, the bulk velocity so that oscillated flames were subjected to and survived instantaneous strain rates which were higher than those which caused extinction of the corresponding unforced flames. Visualization showed that the instantaneous strain rate was up four times that of the bulk flow for about half the oscillation period and promoted flame extinction during this part of the oscillation cycle. However, if the duration of the oscillation was smaller than a critical time scale, extinction did not occur, revealing that there was weakening of the flame over several cycles during which the temperature was reduced.

Characteristics of the spray from a diesel injector

Abstract Spatial and temporal profiles of the velocity of the entrained air and 60 and 30 μm droplets, together with the associated fluxes, from a 5-hole diesel spray exhausting into atmosphere at a repetition rate of 10 Hz have been measured with a phase-Doppler anemometer. The nozzle diameters, fuel charge per hole, injection duration and the area-averaged spray velocity during this duration were 0.18 mm, 2.35 mm3, 0.7 ms and U0 = 132 m/s, respectively. The Sauter mean diameter of the fuel droplets decreased from a maximum centreline value of around 80 μm at 100 diameters from the nozzle to 38 μm at 780 diameters, and a similar decrease was observed between 1 and 2 ms after the start of injection at the upstream location. The flux carried by the 30 μm droplets was up to twice that associated with the 60 μm droplets, 2 ms after injection, although the velocities of the larger droplets were consistently higher than those of the smaller droplets. The maximum measured ensemble-averaged relative velocity was 0.45 U0 for 60 μm droplets just after the arrival of the spray at 550 diameters from the nozzle. The magnitudes of the Weber number imply that droplet breakup was always confined to the leading edge of the spray and was limited to, at most, the initial 1 2 ms of the passage of the spray past a given point. Breakup was mostly complete by 550 diameters from the nozzle. Thus, the measured decrease in the mean diameter was due to small droplets, generated by breakup at the leading edge of the spray, losing velocity due to aerodynamic drag and falling behind the leading edge. Droplets generated late in the injection schedule were likely to overtake those generated earlier and together with the fan-spreading effect, which arises from the combination of the root mean square (RMS) droplet radial velocity and the radial profile of the ensemble-averaged droplet axial velocity, led to RMS velocities in the axial component of the droplets that were not associated with the transfer of turbulent motion from the air.

On the measurement of particle concentration near a stagnation point

Conclusions(1)This work has evaluated the particle number density measured by a single particle counting instrument, based on either the particle mean velocity or on the particle residence time in the measuring volume.(2a)In regions where the mean velocity of a size class is near zero the number density should be based on the residence time of the particle, Eq. (3).(2b)Equation (3) can be used for two-dimensional flow and removes the need to measure the magnitude of the velocity vector, as is the case with the definition of Eq. (1), provided that V(di) — and hence A(di) — is known. A twochannel laser-Doppler anemometer, however, permits the direct, “on-line” measurement of V(di) and A(di).(3)In regions where the mean velocity of all size classes is non-zero, there is little difference in the values of the Sauter mean diameter returned by the two equations.(4)In instruments which do not have the facility for measuring the residence time, it is suggested that the Sauter mean diameter should be evaluated directly from the measured value of ni.(5)For instruments based on laser-Doppler anemometry, the correction for the effect of frequency shifting on the cross-sectional area, A(di) and volume, V(di), of the anemometer is of the order of 25% for small particles and for Nf/N0 = 1.4.(6)Further work is required to establish the theoretical foundation of Eq. (3) in relation to the work of Buchhave et al. (1979). The accuracy with which C(di) can be measured is determined by the tolerances on A(di), V(di) and zp.Further experimental work is also required to determine the accuracy with which these quantities are known.

Mass flux, mass fraction and concentration of liquid fuel in a swirl-stabilized flame

Abstract Mass flux, mass fraction and concentration of kerosene droplets have been measured by a phase-Doppler instrument in a swirl-stabilized burner during inert and reacting flow. In the reacting flow, the flame was supported by natural gas added through the fuel stalk, while the kerosene was added to and atomized by the combusting air. The bulk mean velocity of the combustion air was 30.4 m/s, corresponding to a Reynolds number of 30,000, and the swirl numbers were 0.48 and 0.29, respectively, upstream and downstream of the kerosene injection. The equivalence ratios were 0.45 and 2.11 for the natural gas and the kerosene, respectively. The results show that the minimum swirl necessary for flame stability caused fuel to centrifuge from the region of combustion, so that only 8 kW of energy could be released out of the available 37.4 kW of the kerosene fuel. An important conclusion is that an optimum swirl number will exist with every atomization and burner arrangement of a liquid-fuelled flame and will be different from that associated with the corresponding gas-fuelled flame. The measurement techniques appropriate to regions where the flow instantaneously reverses are described and the existence of large droplets, which moved towards the injector inside the recirculation zone and supplied fuel to the base of the flame, are explained in terms of a “fountain effect” based on the mean drag between the gas phase and the droplets. Sources of uncertainties of the mass flux and concentration measurements with the phase Doppler instrument are considered in detail.