Group Combustion Number Research Articles

EXPERIMENTAL INVESTIGATIONS ON THE ROLE OF GROUP COMBUSTION MODE IN LIFTED SPRAY FLAMES WITH KEROSENE FUEL

In the present work, detailed experimental investigations on lifted spray flames have been carried out to identify the various combustion modes using group combustion number analysis with kerosene fuel. Solid cone pressure swirl nozzle N1 ((m_f ) ̇ =1.7 kg/hr at ΔPinj = 9 bar) is considered and fuel injection pressure varied from 3 - 48 bar. Detailed droplet size and droplet velocity measurements using shadowgraphy and PIV helped obtain the group combustion number and dimensionless droplet separation distance. A Chiu (1977, 1982) criterion of group combustion number is used along with experimental observations to understand the spray combustion mode in lifted spray flames. Internal group combustion mode is observed at lower injection pressures (3 bar) in which individual droplet combustion is significant. At intermediate pressures, the combustion mode is defined by internal group combustion with a double reaction zone. The inner reaction zone burns with local extinction, and the outer reaction zone burns like a diffusion flame. A group combustion number range of 1 - 103 is investigated for various combustion modes in lifted spray flames without coflow air. For air coflow conditions (Vcoflow = 0.4 m/s), both individual droplet burning mode and double reaction zone flame structure disappear from the flame. At higher injection pressures, small fuel vapor cloud burning with external group combustion mode is observed in the G range of 103 - 106.

Optical characterization of droplet clusters and group combustion in spray diffusion flames

The clustering of fuel droplets and group combustion of droplet clusters are experimentally investigated in a spray burner. The burner was operated in the presence and absence of coflowing air around a pressure swirl injector which generates a kerosene spray. The Mie scattering images of the spray droplets were recorded by PIV technique, while planar measurement of droplet size was achieved by application of the ILIDS technique. The photographs of the spray flame and droplets gave evidence of distributed flame structure with the inner reaction zone containing several small diffusion flames indicating group burning of droplet clusters. The invariance of average droplet size downstream of the injector exit further supported the above observation and suggested significant reduction of gasification rate of droplets within the clusters. Application of Voronoi analysis to focused images of the spray droplets facilitated characterization of clusters of droplets. Accordingly, the Group combustion number was evaluated for each droplet cluster based on the measured cluster size and inter-droplet distance. The results reveal multi-scale droplet clustering and multi-mode combustion of the clusters during the spray burning process.

Fluid dynamic and autoignition characteristics of early fuel sprays using hybrid atomization LES

Early fuel spray development and subsequent autoignition are investigated using our turbulent atomization model that has enabled physically-closed primary atomization modeling. Thanks to its modeling methodology, no parameter tuning or adjustment is needed to simulate a turbulent spray. Due to turbulent atomization, a dense droplet layer is formed over the injected liquid core. With evaporation, this region has a triple layer structure, which is composed of the inner layer with many droplets, the vaporization layer and the outer layer exposed to the hot air. With combustion, hot spots are generated in the outer layer, while the inner layer is too cold and hinders the hot gas to penetrate inside. The group combustion number indicates that this is sheath evaporation/combustion mode and its structure is preserved down to the liquid core length. The liquid core thinning is determined by atomization characteristics and under evaporative/reactive conditions where the outer gas temperature is high and thus the kinematic viscosity is also high, the flow motion is larger and the core is unsteadily affected by this motion. Due to this, the Rayleigh–Taylor (RT) atomization mode is further excited and core breakup occurs relatively faster. It is also observed that the unsteady core head flapping motion leads to non-uniform distributions of droplets and vapor. Due to this non-uniformity, the initial autoignition in the outer layer does not occur uniformly. After core breakup, the droplet layer rolls up in the front region and the group combustion number becomes lower, where external group combustion behavior is observed. Multistep chemistry improves the ignition delay estimation compared with the global chemistry, but the spray dynamics is mostly determined by the above fluid dynamic effects. Therefore, the present study indicates that the accurate simulation capability of fluid dynamics, i.e. primary atomization and early spray development, is important in predicting spray combustion behavior.

Droplet/turbulence interaction and early flame kernel development in an autoigniting realistic dense spray

A very detailed evaporating/reacting spray simulation has been conducted extending our previous liquid atomization data in order to understand the physical processes leading to group combustion. The head region of a realistic dense Diesel jet spray is considered, and the droplets and diffusive layers are resolved. At this stage, the droplets have the relative velocity to the high-temperature air flow and this generates comparable-size turbulent eddies around the droplets. By these eddies, mixing is enhanced more strongly than molecular diffusion and merged vapor clouds are formed especially in the area of large droplet number density. This suggests that external group combustion is likely to occur and this is also confirmed by the estimation of group combustion number. This finding requires a modification to the point-source model so that the droplet-generated mixing enhancement effect should be more accurately included. A simple time scale analysis shows that final ignition occurs after the liquid jet head disappears and the resident time is sufficiently large enough.

Transient group combustion of the pulverized coal particles in spherical cloud

A transient group combustion model for the pulverized coal particles in a spherical cloud is developed to predict the transient group combustion characteristics. The submodels, which account for the detailed combustion process of both homogeneous and heterogeneous reactions, are also applied for the pulverized coal particles. The numerical simulation of the collective behaviors of ignition and its subsequent burning were carried out. The ignition mechanism is characterized by the heterogeneous process of the dilute cloud and the homogeneous process of the dense cloud. Two dominant flame structures are observed: one flame penetrating inside the cloud and the other moving outside the cloud. The effects of various parameters (i.e., radiation heat transfer, group combustion number, air temperature, air oxygen concentration, particle size distribution, and particle number density) on the particle mass burning rate and on the overall performance of the group combustion are examined. The results are in good agreement with existing experimental data.

Observation of Combustion Mechanisms of Acoustically Levitated Droplet Clusters

In order to elucidate the microscopic combustion behaviors of droplet clusters, we took advantage of droplet clusters suspended in the air almost at the same position using an ultrasonic levitator. We observed images of OH Laser Induced Predissociative Fluorescence (LIPF) and Mie-scattering of droplets simultaneously to examine relations between the combustion reaction zone and unburned droplet regions for confirming droplet group combustion mode of the clusters. In addition, we observed time-series images of Mie-scattering of burning droplets illuminated by a laser-sheet, and light emission in CH band at the same time. Simultaneously at a specific point in the imaged region, OH radical chemiluminescence was detected by specially designed light collection optics, Multi-color Integrated Receiving Optics (MICRO), and velocity and diameter of droplets were measured by Phase Doppler Analyzer (PDA) to obtain Characteristics of spray. We calculated the diameter of each droplet in a cluster based on the Mie-scattering theory, and derived droplet group combustion number of the droplet cluster in time-series. As a result, number density of droplets in the cluster decreased monotonically in case of the low number-density clusters. In contrast, number density of droplets in the cluster decreased first and kept constant, and then decreased again in case of the high number-density clusters. This is explained due to the fact that the droplet group combustion mode changed from the internal group combustion mode to the single droplet combustion mode.

Regime Variation of Spray Combustion on Homogeneous Turbulence

Numerical simulation of spray combustion in homogeneous isotropic turbulence was performed under realistic time and spatial scales. The simulation was realized by newly developed numerical methods which had a lot of advantage in simulation of unstable subjects. The method was preliminary examined in several situations to prove the method was able to be applied for direct numerical simulation. In non-combusting cases, we found liquid droplet followed typical “preferential effect” and number density fields of droplets showed homogeneous and heterogeneous distribution according to Stokes number. The effect appeared in both small and large structural turbulences. In combusting cases, flame formation also varied by the Stokes number in turbulence where large structure existed in flow field. On the other hand, in turbulence with small structure, the regimes became homogeneous regardless of the Stokes number. We defined “local group combustion number” which was different from existing one, however it was convenient to understand the combustion regime variation and it could explain how combustion was proceeded.

초음파를 이용한 부상유적군의 연소기구에 관한 기초연구

This paper describes to observe the combustion process of only one droplet cluster. In this study, liquid fuel was atomized by ultrasonic wave to form an acoustically levitated droplet cluster. In order to elucidate the detailed structure of burning process of the droplet cluster, laser tomography method was applied. Time-series planar images of fuel droplets were processed and diameter of the each droplet was calculated based on the Mie-scattering theory. Using these data, the modified droplet group combustion number was estimated in time-series. As the result, when the internal droplet group combustion occur, the modified group combustion number dose not decrease monotonically, but show a tow-staged decreasing process. In all case of combustion process, combustion reactions were measured two types that combustion speed was fast and slow. It was casued by difference of concentration degree and droplet size distribution.

液体燃料粒子群の自発着火に及ぼす粒子間隔の影響

The present study aims to establish a new experimental methodology (Fine Wire Sustaining method, FWS) and to obtain the fundamental data of spray combustion, the combustion of the liquid fuel droplets cloud. To simulate the fuel droplet cloud, each droplet was suspended at points of intersection of fibers. The geometrically controlled droplets arrayed in two or three dimensions imitated the liquid fuel droplet cloud. The experiments were performed to investigate the flame shape and the ignition delay of the droplet cloud in high temperature. The influences of the surrounding droplets on the flame shape and on the ignition delay of the center droplet were investigated. The droplet distance was the experimental parameters. As a result of the experiments, the combustion characteristic of the droplet cloud could be investigated by using the new experimental methodology. The flame shape of the fuel droplet cloud could be classified by using the group combustion number G. The ignition delay of the center droplets arrayed in three dimensions decreased with the droplet distance to the extent that the flames are isolated.

Laser Diagnostics of Gun-Type Spray. Clustering and Compound-Cluster Combustion of Liquid Spray.

A concept of is proposed in order to investigate an applicability of group combustion theory to practical spray combustion, in which strong turbulent coherent vortex is formed and influenced spray behavior significantly. This turbulent coherent structure causes inhomogeneous droplet distribution and its population, and causes interphase exchange of properties and combustion under the effects of preferential interactions. In application of group combustion ; theory to practical spray combustion, these points were taken into account and phase Doppler measurement (PDA) data was analyzed : there are multiple clusters, characterized by their size and followabiliy (Stokes effect) ; clusters of different droplet sizes are produced at the same time in the flow field ; the turbulent scale on cluster formation should be considered by the droplet's inter-arrival time statistics. In this research, PDA measurement with high sampling-rates and high validation-rates was carried out by optimizing the PDA set-up parameters. The integral length scale of each size cluster and the inter-arrival time statistics of the droplets can be obtained, based on the instantaneous droplet velocities as well as, but also Stokes' number, cluster size and inter-arrival time. The size-classified group combustion number is evaluated by the experimental data to demonstrate the applicability of group combustion theory. The interaction between the turbulent coherent vortex and small droplets of less than 30μm generates essential clusters of the droplets and its combustion can be characterized by group combustion. The size of these clusters corresponds to the integral scale of the turbulent vortices. The effect of turbulent coherent structure on compound-cluster formation is examined and this effect is successfully evaluated by the Stokes effect.

1102 3次元的に配置された大規模液体燃料粒子群の火炎形態および群燃焼数の経時変化に関する研究(GS-6 燃焼(液体燃料))

1102 3次元的に配置された大規模液体燃料粒子群の火炎形態および群燃焼数の経時変化に関する研究(GS-6 燃焼(液体燃料))

Effect of Turbulence on Spatial Distribution and Group Behavior of Droplet in a Spray Flame

This paper presents results of the study on the effect of turbulence on droplet dispersion and group behavior in liquid-fuel spray flame. Insertion of grids with different mesh sizes in front of the nozzle resulted in change of turbulence intensity, while the change of turbulence integral scale was negligible. For smaller mesh and lower turbulence intensity total vaporization rate is rapidly increasing, while the droplet size distribution remains practically unchanged. Measurements of combustion group number showed that decrease of average group combustion number is well correlated with increase of vaporization rate. Therefore, the reason for observed vaporization enhancement, could be explained by more uniform spatial distribution and weaker inter-droplet interactions, which usually strongly reduce vaporization rate of droplets in dense sprays. Contrary to observed increase of vaporization rate, flame length is growing. This phenomenon is attributed to reduction of mixing of evaporated fuel and ambient air for lower turbulence intensity and poorer mixing between two gaseous phases compared to mixing between the air and droplets in liquid phase.

The structure of an acoustically forced, reacting two-phase jet

An experimental study of the structure of an acoustically forced, reacting two-phase jet was performed. The jet was acoustically forced to control the formation and evolution of large-scale structures in the near field of the jet. Phase-locked data acquisition techniques were used to correlate droplet statistics and dynamics with features of the large-scale structures. Phase Doppler interferometry was used to acquire droplet statistics. Planar imaging techniques were applied to document the distribution of droplets within the jet. The results show that the interaction between droplets and large-scale structures leads to a nonuniform distribution of droplets in the reacting jet. The combination of transport effects and droplet evaporation leads to the formation of droplet clusters. The group combustion behavior of the droplet field was evaluated by estimating the group combustion number from experimental data. External sheath burning is present in the early portion of the flame followed by a transition to external group combustion as clusters begin to be the dominant feature. Late in the cluster lifetime there is a shift to internal group combustion.

Mesoscale structures of turbulent sprays

Mesoscale structural phenomena, mechanisms, and the logarithmic law of decay of the group combustion modes of clusterlike structures in turbulent premixed sprays were examined analytically and the results compared with recent experimental results. A universal law of cluster gasification was obtained in the form of the normalized functions, which are dependent on the cluster's interior and exterior fluid flow structures, cluster parameters, and environmental conditions. The results were used to formulate the law of the decay of the probability, P I , of an internal group combustion mode of a cluster as the function of the geometrical group combustion number, G C , the gasification functions, and the non-dimensional parameters of the micro- and mesoscale aerothermochemical processes of droplets and clusters. The transition in the group combustion mode of a member cluster of iso- G C family, expressed self-consistently in the form of a logarithmic decaying law, ln P I =ln B −e ln G C , and alternatively by a power law, P I = B(G C ) −e , in the broad ranges of a cluster's group combustion number, G C , and other parameters of turbulent sprays, was found to be in agreement with the recent experimental results of turbulent sprays found by Akamatsu et al. The decay parameters, e and B , of clusters were also determined as the functions of mesoscopic structural partition functions. The decay parameters B and e of the mean cluster are estimated to be 0.152 and 0.278, respectively, from the experimental data. The group mode index, Λ, defined by the Damkohler number, gasification correction factor, and energy functions has been introduced to describe the departure of the probability of the group combustion of a member cluster from that of the mean cluster in an iso- G C family. Those clusters with Λ>1 exhibit high probability of internal group combustion, and clusters with Λ

Dilute laminar spray diffusion flames near the transition from group combustion to individual droplet burning

A dilute laminar spray flame of heptane was experimentally studied in a coflow configuration. The spray flame was characterized by using phase Doppler interferometric techniques and thermocouples. When burning in air, the flame had a candle-like appearance. It was naturally anchored a few millimeters above the burner through a stabilization annular flame. A droplet vaporization region could be visually identified above this annulus, as a relatively dark inner core. The spray burned in an internal or partial group combustion mode, with an inner core surrounded by a common flame and some droplets at the outer periphery burning individually. Thanks to the relatively narrow droplet size distribution in the spray, some characteristic parameters of group combustion theory could be computed from the experimental measurements reliably. In the region bound by the common flame, the ratio of interdroplet distance to droplet diameter ranged from 16 to an upper limit at least as large as 50, which shows the remarkable efficiency of droplet clouds at inhibiting oxidizer penetration within the core. A group combustion number, that was determined using the flame radius as cloud length scale, was found to be approximately constant in the lower part of the combustion region. This part of the flame was found to set the stage for the burning mode throughout the bulk of the spray flame. The value of this characteristic number was also consistent with predictions of group combustion theory for spherically symmetric spray flames burning in internal group combustion mode. Another aspect of the theory that found experimental confirmation was the transition from group combustion to individual droplet burning that was promoted via oxygen enrichment of the oxidizer stream. Such an enrichment caused two effects. The group combustion number decreased via a decrease of the flame radius. At the same time, the critical group combustion number G cr for the transition to individual droplet burning, which is a function of the oxygen concentration in the free stream, increased. At sufficiently large oxygen concentration, the condition G < G cr was eventually reached, resulting in individual droplet burning. Experimentally, as the oxygen concentration was raised, the percentage of droplets experiencing individual droplet combustion progressively increased. At a molar oxygen concentration of 0.62, the transition to individual droplet burning seemed complete, as manifested by the flame appearance as a collection of chemiluminescent streaks radiating from a common origin.

Measurement of the local group combustion number of droplet clusters in a premixed spray stream

We have succeeled previously in the experimental observation of the group combustion behavior of spray flames by statistically and spectrally processing the light-emission signals from radicals together with the Mie-scattering signal from droplet clusters in a premixed spray flame. It was interesting that even uniformly premixed droplet streams burn in groups due to their eddy motion and preferential flame propagation, hence generating temporal and spatial variation in the scale of droplet clusters. In the present study, time-series signals of the light emission in OH and CH bands, the Mie scattering from droplets. and the size and velocity of droplets with a phase Doppler anemometer (PDA) were monitored simultaneously. The high data rate and high validation rate of PDA measurement accomplished by the optimized design of optics enabled us to distinguish instaneous charateristics of individual droplet clusters in the flame. Thus, we estimated the modified group combustion number Gc for individual droplet cluster using the measured mean separation distance between droplet centers and the total number of droplets for each droplet cluster. Note that Chiu et al. defined their original group combustion number for a quasi-steady laminar combustion of a cloud of equally spaced drops based on the theory of isolated droplet combustion. We examined the transition of group combustion mode of droplet clusters as a function of the distance from the burner port using Gc. It was found that the derived Gc values were concentrated within the range of the original diagram predicted by Chiu et al. The Gc value averaged over all droplet clusters passing a monitoring spot. c, fell within the range of external group combustion mode at the inital stage of combustion and moved to the range of internal group combustion mode as the evaporation and combustion of the cluster proceeded, which was in good accordance with the previous experimental observations.

Anomalous group combustion of premixed clusters

Transient group combustion of a polydisperse octane fuel droplet cluster that is suddenly exposed to a hot environment is studied to examine transient combustion behavior and cluster structure and to identify the mechanism responsible for the recently observed anomalous group combustion phenomenon in a premixed spray. Mathematical theory adopts the updated laws of an interacting droplet in a nondilute spray, the criteria of droplet ignition, and the droplet state modulation. Numerical analysis reveals that a cluster ignition that may occur either in the interior or exterior space of the cluster, depending on the group combustion number and environment temperature, is followed by the spreading and splitting of a developing flame into dual flame modes: the inner and outer flame, respectively. The inner flame, which burns with the captured air and he fuel vapor, extiinguishes upon the depletion of the captured air whereas the outer flame (i.e., the cluster-bound group flame) burns largely with the air transported by diffusion from the environment to the cluster. It is proposed here that the transient dual combustion that occurs simultaneously inside and outside the cluster is one of the mechanism responsible for the experimentally observed anomalous group combustion in premixed sprays. Generally speaking dense clusters with high cluster equivalence ratios have little or no probability of exhibiting anomalous group combustion because the sheath phenomenon prohibits internal group combustion. Intermediately dense to moderately dilute clusters, however, exhibit anomalous group combustion with a probability approximately equal to the ratio of the depletion time of captured air to the combustion time of the droplet. Very dilute clusters exhibit no distinct dual combustion phenomena and therefore do not contribute to the anomalous combustion of cluster groups.

Group Combustion Behavior of Premixed Spray Streams and Group Combustion Number.

Spray flames inherently show complicated behaviors due to the group-forming character of droplets and the preferential flame propagation through the most favorable routes. In order to observe the detailed structure of spray flames, the light emission signals in OH- and CH-bands, the Mie scattering signals from droplets, and the size and velocity of droplets with a phase Doppler anemometer (PDA) were monitored simultaneously in a premixed spray flame stabilized by an annular hydrogen pilot flame. It was experimentally confirmed that the homogeneous premixed spray was divided into a number of droplet clusters due to the preferential flame propagation in the process of combustion, while there existed no droplet clusters in an approaching nonburning premixed spray flow. The group combustion number for each droplet cluster Gc was estimated from the present experimental data. It was found that the mean value of Gc fell within the range of the external and internal group combustion mode according to the definition of Chiu et al., and that the observed burning behavior of clusters corresponded well to their classification of group combustion mode.

Significance of Local Group Combustion Number Introduced.

For sprays of practical interest, the region where vigorous vaporization occurs is confined to a thin layer on their outer edges. The cubical array of droplets occupying a semi-infinite space provides a microscopic prototype to investigate this vaporization layer structure, upon which interactive droplet vaporization/combustion characteristics are examined to derive rational boundary conditions to be imposed at the spray edge when the spray is regarded as a continuum medium on large scales. It is found that not only the local droplet number density but also the gas-to-liquid density ratio plays an important role in excitation of prevailing vaporization/combustion modes, leading to their criteria in terms of the droplet separation distance as a function of the gas-to-liquid density ratio.

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.