Coflow Velocity Research Articles

Experimental investigations on the stabilization of lifted kerosene spray flames with coflow air

ABSTRACTIn the present work, detailed experimental investigations have been carried out to understand the effect of coflow air on the stabilization of lifted spray flames with kerosene fuel. Two full cone type pressure swirl nozzles N1 and N2 ( = 1.7 and 7.08 kg/h at ΔPinj = 9 bar) are considered for experiments with a moderate fuel injection pressure range (4–12 bar). The flame liftoff height is observed to increase with fuel injection pressure for lower coflow velocity range (≤ 0.2 m/s). At higher coflow velocities (0.2–0.4 m/s), the flame liftoff height decreases with an increase in fuel injection pressure. At lower injection pressures, the flame liftoff height has been observed to increase significantly for higher coflow velocities. Detailed droplet velocity measurements using PIV and droplet size measurement using the shadowgraphy technique in cold flow helped understand the stabilization of lifted spray flames in the domain. At higher coflow velocities and lower injection pressure, smaller droplet number density allows additional air entrainment and flame stabilizes at the downstream location, where SMD and droplet flow velocities are small. At higher injection pressure, high droplet number density reduces the additional air entrainment, which stabilizes lifted spray flame at a relatively downstream location. At lower coflow velocity and lower injection pressure, flame stabilizes upstream, where mixture burning velocity is nearly equal to flow velocity irrespective of higher SMD and droplet flow velocity. At higher injection pressure, flame stabilizes only after the end of high velocity zones, as observed from PIV measurements.

PM from the combustion of heavy fuel oils

This work presents an experimental study investigating the formation and oxidation of particulate matter from the combustion of heavy fuel oil, HFO, droplets. The study includes results from both a falling droplet in a drop tube furnace and a suspended droplet in a heated convective flow. The falling droplets in a heated coflow air with variable temperature path and velocity were combusted and the resulting particles, cenospheres, were collected. To characterize the microstructure of these particles, scanning electron microscopy (SEM), and energy dispersive X-Ray (EDX) analysis were used. The particles were found to have either a porous or a skeleton/membrane morphology. The percentage of particles of either type appears to be related to the thermal history, which was controlled by the heated co-flow velocity. In the suspended droplet experiments, by suspending the droplet on a thermocouple, the temperature inside the droplet was measured while simultaneously imaging the various burning phases. A number of specific phases were identified, from liquid to solid phase combustion are presented and discussed. The droplet ignition temperature was seen to be independent of the droplet size. However, the liquid phase ignition delay time and the droplet lifetime were directly proportional to the initial droplet diameter.

Effects of co-flow and equivalence ratio on flickering in partially premixed flame

The effects of air co-flow and equivalence ratio on flickering of a partially premixed flame of methane/air were experimentally studied for various combinations of burner diameters, fuel/co-flow velocities, and equivalence ratios of methane/air fuel. A scaling law was proposed by introducing the corrections made to the Froude number, equivalence ratio, and co-flow velocity ratio to describe the flickering characteristics of the flame in the co-flow. It is found that the corrected Strouhal number of the partially premixed flame increases with an increase in the co-flow at higher equivalence ratios with a frequency jump, whereas the value is still higher without the frequency jump at lower equivalence ratios. The oscillation amplitude weakens with an increase in the corrected co-flow velocity ratio at high equivalence ratios, which reduces at low equivalence ratios. The corrected critical co-flow velocity ratio used to suppress the flickering is found to be independent of the equivalence ratio in this scaling law. The proper orthogonal decomposition (POD) analysis of the flame indicates that the random flickering motion at high equivalence ratios is weakened and the fluctuating energy distribution of the lower POD modes is modified when the equivalence ratio is low. These changes in the flame structure are due to the disappearance of the clip-off flame in the partially premixed flickering flame at low equivalence ratios in the co-flow.

Plasma-Assisted Stabilization of Lifted Non-premixed Jet Flames

The present study experimentally investigates the effects of plasma discharges on the stabilization of lifted non-premixed jet flames in a stream of co-flow air. The plasma discharge is produced on the sharp edge of the fuel nozzle exit, facilitating its impact on flame stabilization. It is observed that the application of plasma discharges has an impact on the enhancement of flame lift-off velocity, lift-off height, and hysteresis phenomenon and leads to plasma-attached flames, plasma-enhanced lifted flames, and plasma-ineffective lifted flames, depending on flame lift-off conditions. A maximum enhancement of approximately 84% is observed for the flame lift-off velocity when the co-flow velocity is sufficiently low. As the co-flow velocity or the jet Reynolds number is low, the flame is anchored at the nozzle by the discharge. As the co-flow velocity or the jet Reynolds number is increased, the flame detaches but with a decrease in lift-off height compared to the flame without the discharge. If the co-fl...

Highly stabilized partially premixed flames of propane in a concentric flow conical nozzle burner with coflow

Partially premixed turbulent flames with non-homogeneous jet of propane were generated in a concentric flow conical nozzle burner in order to investigate the effect of the coflow on the stability and flame structure. The flame stability is first mapped and then high-speed stereoscopic particle image velocimetry, SPIV, plus OH planar laser-induced fluorescence, OH-PLIF, measurements were conducted on a subset of four flames. The jet equivalence ratio Φ = 2, Jet exit Reynolds number Re = 10,000, and degree of premixing are kept constant for the selected flames, while the coflow velocity, Uc, is progressively changed from 0 to 15 m/s. The results showed that the flame is stable between two extinction limits of mixture inhomogeneity, and the optimum stability is obtained at certain degree of mixture inhomogeneity. Increasing Φ, increases the span between these two extinction limits, while these limits converge to a single point (corresponding to optimum mixture inhomogeneity) with increasing Re. Regardless the value of Φ, increasing the coflow velocity improves the flame stability. The correlation between recessed distance of the burner tubes and the fluctuation of the mixture fraction, Δξ, shows that at Δξ around 40% of the flammability limits leads to optimum flame stability. The time averaged SPIV results show that the coflow induces a big annular recirculation zone surrounds the jet flames. The size and the location of this zone is seen to be sensitive to Uc. However, the instantaneous images show the existence of a small vortical structure close to the shear layer, where the flame resides there in the case of no-coflow. These small vertical structures are seen playing a vital role in the flame structure, and increasing the flame corrugation close to the nozzle exit. Increasing the coflow velocity expands the central jet at the expense of the jet velocity, and drags the flame in the early flame regions towards the recirculation zone, where the flame tracks and matches the spatial locations of low axial velocity fluctuations. At downstream, the flame is seen to conform to the passage of large scale structure. At Uc = 10 and 15 m/s, part of the primary reaction zone is rolled up towards upstream burner nozzle, anchoring the flame to the nozzle tip. This indicates that the stabilization of these flames in the presence of the coflow is controlled by the mutual interactions between the central jet and the coflow through the recirculation zone from one side, and the degree of the inhomogeneity of the central jet mixture from the other side.

NUMERICAL INVESTIGATION ON THE INTERACTION OF A PAIR OF HOT OFF-DESIGN SUPERSONIC JETS WITH A JET BLAST DEFLECTOR

The interaction of a pair of hot off-design jets from biconical supersonic nozzles is numerically investigated by the high-resolution RANS/ILES method. The distance between the jet axes is four nozzle exit diameters. To improve the computational accuracy, combined computations of the flows in the nozzles and in the jets escaping from these nozzles are performed. The computations are carried out for free jets with a co-flow and for airfield jets with a jet blast deflector. Distances where mixing layers interaction begins, where jets are merging and where the difference of flow characteristic between one jet and twin jets are observed were obtained. The influence of the pair of jets on the near acoustic field is considered. For airfield jets, the effects of the co-flow velocity and the distance from the nozzle exit to the jet deflector on the size and shape of dangerous areas in terms of temperature, pressure fluctuations, and axial velocity are detected. Generalizing dependencies are presented for evaluating the influence of these parameters on the typical sizes of the dangerous zones. The results of the simulations are compared with similar data for single jet impingement onto the jet deflector. Specific features of the effects of the co-flow velocity and the jet deflector position on pressure, temperature, and their fluctuations over the jet blast deflector height are found.

Separation of fetal and maternal circulations using multi-modal MRI

The stability limits, the stabilization mechanism, and the flow field structure of highly stabilized partially premixed methane flames in a concentric flow conical nozzle burner with air co-flow have been investigated and presented in this work. The stability map of partial premixed flames illustrates that the flames are stable between two extinction limits. A low extinction limit when partial premixed flames approach non-premixed flame conditions, and a high extinction limit, with the partial premixed flames approach fully premixed flame conditions. These two limits showed that the most stable flame conditions are achieved at a certain degree of partial premixed. The stability is improved by adding air co-flow. As the air co-flow velocity increases the most stable flames are those that approach fully premixed. The turbulent flow field of three flames at 0, 5, 10 m/s co-flow velocity are investigated using Stereo Particle Image Velocimetry (SPIV) in order to explore the improvement of the flame stability due to the use of air co-flow. The three flames are all at a jet equivalence ratio (Φj) of 2, fixed level of partial premixing and jet Reynolds number (Rej) of 10,000. The use of co-flow results in the formation of two vortices at the cone exit. These vortices act like stabilization anchors for the flames to the nozzle tip. With these vortices in the flow field, the reaction zone shifts toward the reduced turbulence intensity at the nozzle rim of the cone. Interesting information about the structure of the flow field with and without co-flow are identified and reported in this work.

An interband cascade laser-based in situ absorption sensor for nitric oxide in combustion exhaust gases

A direct absorption nitric oxide sensor for combustion exhaust gas measurements, based on an interband cascade laser operating at 5.2 µm, is presented. The sensor was applied to the hot air co-flow of an auto-ignition test rig (800–1300 K), which contains nitric oxide mole fractions of the order of 1 mol%, due to prior microwave plasma heating. The effect of non-uniform temperature along the beam path, on both absorption line strength and gas density, was included in mole fraction measurements at various co-flow temperatures and velocities. At an absorption length of only 82 mm, a noise-limited detection limit of 30 ppm with a 10 ms observation time was achieved at 800 K. The results were compared in detail to previously measured mole fractions, using a sampling gas analyzer.

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

ABSTRACTIn the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–ϵ turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.

Large Eddy Simulation of CO2 diluted oxy-fuel spray flames

We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.

Magnetic Effects on Flickering Methane/Air Laminar Jet Diffusion Flames

ABSTRACTThe article investigates the responses of buoyant jet diffusion flames to the application of magnetic field gradients. In a magnetic gradient, the paramagnetic oxygen is submitted to a magnetic force of attraction directed to the center of the magnet. Positive and negative magnetic gradients effects were compared to the case with no applied magnetic field. Measurements in methane/air flames from a coaxial injector show that over a range of air coflow velocity, the magnetic gradients affect both the oxygen supply at the flame edge and the displacement of the vortices in the air side of the high temperature reaction zone. Upward increasing magnetic field attracts paramagnetic oxygen upwards leading to variations of the lift height and the flame length and counteracts the gravity convective motion attested by a noticeable decrease of the flickering frequency, whereas the upward decreasing magnetic field generates a downward magnetic force on oxygen, depriving the flame edge of oxygen attested by a higher lift height and enhancing the gravity convection in air along the flame evidenced by an increase of the flickering frequency. The flame visible luminosity is shown to be impacted by the magnetic field gradient effect that is related to the soot production through modifications of local temperature, stoichiometry, and residence time.

An experimental study of ignition and combustion of single biomass pellets in air and oxy-fuel

This study presents an experimental investigation of the ignition and combustion characteristics of single biomass pellets in air and O2/CO2 atmospheres containing 21%, 30%, 40%, 50% and 100% oxygen mole fractions. In this experiment, the temperature of the gas surrounding the biomass pellets was set respectively at 873K, 973K and 1073K. The rate of co-flow gas was set at 10L/min, 15L/min and 20L/min. Single pellets of Pinusbungeana and rice husk were fixed on a thermocouple and burned in a vertical heating tube furnace. High-speed photography was used to record images of the combustion process of biomass pellets. The resulting images exhibit two ignition behaviors: (1) homogeneous ignition of volatile, and (2) heterogeneous ignition on the particles’ surface. After ignition, the combustion was no longer a homogeneous combustion of volatile. When the oxygen concentration exceeded 50%, the biomass showed a hetero-homogeneous combustion. Similarly, when the concentration of O2 increased, the flame became shorter and more stable. With the same oxygen concentration, and once the N2 was replaced by CO2, the ignition delay, internal ignition temperature and the volatile combustion time increased. Inversely, when the oxygen concentration exceeded 21% (i.e. under the conditions of oxy-fuel), these parameters were correspondingly reduced. In other words, the ignition and combustion intensity were increased. The effects of oxygen concentration and co-flow temperature on ignition and combustion of biomass pellets were greater than that of co-flow velocity. The study’s results show that the biomass pellets did not break during combustion.

Pressure drop in dairy evaporators: Experimental study and friction factor modelling

The pressure drop that occurs during the falling film evaporation of dairy products is related to the downward co-flowing vapour and directly affects the saturation temperature of the product, resulting in a non-constant thermal driving force of the process along the evaporator. Frictional forces are the major contributor to the pressure drop, and the friction factor is a crucial parameter in reliably estimating the pressure drop. In the present study, the absolute vapour flow pressure losses during falling film evaporation of dairy products were measured using an experimental internal tube evaporator set-up. To simulate real industrial conditions, experiments were conducted by varying the co-flow inlet velocity (from 0 to 37 ms−1) over a wide range of product dry solid content DC (from DC = 0%–50%). Results obtained at DC = 0% show that the experimental approach is consistent when measurements are compared to the well-known Wallis correlation. A large number of experimental data has been obtained for DC between 13 and 40% for which the pressure losses have a strong dependence from the co-flowing vapour rate; in some specific cases, the influence of the surface bubbling phenomena has been put in evidence. The presented results, have been used to calibrate a new accurate correlation for the friction factor (4flv) coefficient showing a squared residual of R2 = 0.93.

Stabilization of lifted hydrogen jet diffusion flame in a vitiated co-flow: Effects of jet and coflow velocities, coflow temperature and mixing

The present paper reports on the numerical investigation of lifted turbulent jet flames with H2/N2 fuel issuing into a vitiated coflow of lean combustion products of H2/air using conditional moment closure method (CMC). A 2D axisymmetric formulation has been used for the predictions of fluid flow, while CMC equations are solved with detailed chemistry to represent the turbulence–chemistry interaction. Simulations are carried out for different coflow temperatures, jet and coflow velocities in order to investigate the impact on the flame lift-off height as well as on the flame stabilization. Furthermore, the role of conditional velocity models on the flame has also been investigated. In addition, the effect of mixing is investigated over a range of coflow temperatures and the stabilization mechanism is determined from the analysis of the transport budgets. It is found that the lift-off height is highly sensitive to the coflow temperature, while the predicted lift-off height using the mixing model constant, i.e., CΦ = 4, is found to be the closest to the experimental results. For all the coflow temperatures, the balance is found between the chemical, axial convection and molecular diffusion terms while the contribution from axial and radial diffusion is negligible, thus indicating auto-ignition as the flame stabilization mechanism.

Computational study of a turbulent gas-solid confined jet flow

In the present work, numerical simulation of a confined two-phase particulate jet emanating from a circular nozzle has been performed using Euler-Euler approach and incorporating four-way coupling. The effects of particle size (in the range 50–200μm) and solid loading ratio (in the range 1–5) on the gas phase flow field as well as turbulence have been studied at different co-flow velocities (i.e. 2m/s, 5m/s and 10m/s). It has been observed that the particles have a strong effect on the flow field and the turbulence of the gas phase. As a consequence, gas phase velocity decay rate has been reduced along the center line, and turbulence has been attenuated in the near field region due to the presence of the particles. For a single-phase confined jet, the radial velocity decay is observed in the near-field region (X/D<10). In the presence of a co-flow, the radial velocity has increased beyond X/D=10. In the two-phase jet, the gas phase velocity is decayed in the radial direction throughout the domain. Also, jet half width is reduced with decreasing particle diameter and increasing solid loading ratio (SLR).

Influence of Co-flow on Flickering Diffusion Flame

The influence of air co-flow on flickering methane diffusion flame was studied experimentally using the image processing technique and the proper orthogonal decomposition (POD) analysis. The flickering of the flame is characterized by the mean height, the oscillation amplitude and the Strouhal number, which are measured by the digital image analysis of the diffusion flame. The experiments are carried out for various combinations of burner diameters, fuel velocities and co-flow velocities. With increasing the velocity ratio of the co-flow to the fuel flow, the oscillation amplitude is decreased and the Strouhal number is increased slightly in proportional to the inverse Froude number, while the frequency jump occurs in the low co-flow velocity ratio. These results are commonly observed in all the burners of different diameters, while the critical co-flow velocity ratio to suppress the flickering is found to be increased with increasing the burner diameters due to the influence of Froude number. The POD analysis of the flickering flame shows that the flickering energy is dominant in the first two POD modes and they are axisymmetric except for the zero co-flow velocity case and fully suppressed case. The correlation of POD coefficients in the first two fluctuating POD modes suggests the suppression of large-scale structure of flickering due to the influence of co-flow.

The effect of coflows on a turbulent jet impacting on a plate

The study examines the dynamic and the turbulent behavior of a plane jet impinging normally on a flat target plate in a flow at rest or in a coflow stream. First, numerical and experimental investigations of the characteristics of a turbulent plane jet impinging on a flat surface in still air are depicted. The measurements are conducted using Particle Image Velocimetry (PIV). A comparison has been carried out between numerical results of three turbulence closure models (k–ε standard model, RNG k–ε model and a second-order model RSM) and experimental data. It has been found that the second order turbulence model (RSM) reasonably predicts the mean flow properties of the flow field. Next, a numerical study of the flow field of an impinging jet in a coflow stream is conducted. The study of the jet in a no-directed coflow stream shows the presence of a phenomenon of recirculation near the flat plate. In addition, the study focuses on the influence of the coflow velocity ratio on the behavior of an impinging jet and the influence of the directed co-flow on various physical parameters such as the mean and the turbulent quantities. The results clearly show that the coflow stream imposed noticeable restrictions on the spreading of the impinging jet and decreases considerably the entrainment of air jet.

1514 フリッカリング火炎の変動特性に及ぼす当量比と周囲流の影響(OS2-3 熱流体の可視化と計測)

1514 フリッカリング火炎の変動特性に及ぼす当量比と周囲流の影響(OS2-3 熱流体の可視化と計測)

On predicting the length, width, and volume of the jet diffusion flame

In this paper, the flame length (Lf), width (Wf), and volume (Vf) behaviors of the jet diffusion flame were experimentally and theoretically studied. Various fuel nozzle diameters (df), fuel jet velocities (uf), and air co-flow velocities (uco) were involved in the experiments to investigate their impacts on flame characteristics systematically. Additionally, the analytical solutions for the flow field, fuel concentration, Lf, Wf, and Vf of the jet diffusion flame were derived by theoretical analysis. The results show that flame shape and dimensions of the jet diffusion flame depended on fuel jet Reynolds number (Ref) inherently. At the laminar regime, the dimensionless flame length normalized by df (Lf*) was proportional to Ref. At the laminar–turbulent transitional regime and fully-turbulent regime, Lf* kept nearly constant. The dimensionless flame width (Wf*) remained basically unchanged at the laminar or fully-turbulent regime; but it increased fairly quickly with the increment in Ref at the laminar–turbulent transitional regime. The dimensionless flame volume (Vf*) increased linearly with Ref at the laminar regime, and parabolically with Ref at the laminar–turbulent transitional regime. At the fully-turbulent regime, Vf* kept constant. Besides, with the increase in uco, Lf* maintained nearly unchanged, and Wf* decreased exponentially. Based on the above behaviors, new forms of correlations for Lf*, Wf*, and Vf* of the jet diffusion flame were proposed presently. Comparisons of these correlations with the existing ones in the literature were also conducted in this paper. Moreover, Lf*, Wf*, and Vf* correlations for the dimethyl ether/air jet diffusion flames were developed herein.

The flow field structure of highly stabilized partially premixed flames in a concentric flow conical nozzle burner with coflow

The stability limits, the stabilization mechanism, and the flow field structure of highly stabilized partially premixed methane flames in a concentric flow conical nozzle burner with air co-flow have been investigated and presented in this work. The stability map of partial premixed flames illustrates that the flames are stable between two extinction limits. A low extinction limit when partial premixed flames approach non-premixed flame conditions, and a high extinction limit, with the partial premixed flames approach fully premixed flame conditions. These two limits showed that the most stable flame conditions are achieved at a certain degree of partial premixed. The stability is improved by adding air co-flow. As the air co-flow velocity increases the most stable flames are those that approach fully premixed. The turbulent flow field of three flames at 0, 5, 10m/s co-flow velocity are investigated using Stereo Particle Image Velocimetry (SPIV) in order to explore the improvement of the flame stability due to the use of air co-flow. The three flames are all at a jet equivalence ratio (Φj) of 2, fixed level of partial premixing and jet Reynolds number (Rej) of 10,000. The use of co-flow results in the formation of two vortices at the cone exit. These vortices act like stabilization anchors for the flames to the nozzle tip. With these vortices in the flow field, the reaction zone shifts toward the reduced turbulence intensity at the nozzle rim of the cone. Interesting information about the structure of the flow field with and without co-flow are identified and reported in this work.