Central Air Jet Research Articles

Experimental investigation of heat transfer characteristics of an inverse diffusion flame in a coaxial tube burner for with and without swirl

Heat transfer by flame jet impingement is widely used in many of the industrial and domestic applications like heating metal bars, scrap melting, shaping the glass, metal slab cutting, domestic cooking and others. Aim of the present experimental work is to study an effect of swirl on a local heat flux distribution of an inverse diffusion flame (IDF) jet impinging on a flat target surface in a coaxial tube burner. The twisted tape of twist ratio 3 (corresponding to the swirl number, S = 0.52) is used to create a swirl in the central air jet of the burner. The flame shapes and heat flux distributions are compared for the with and without swirling IDF under the different air jet Reynolds number (Rea) of 1000 to 2500, equivalence ratio (ϕ) of 0.4 to 1.3 and a burner-to-impingement plate distance (H) of 10 to 100mm. The distributions of heat fluxes are studied within a radius of 75 mm from the point of stagnation on an impingement plate. Results show that the swirling IDF helps in clean combustion of the fuel with much shorter flame height. Swirling in the flame jet enhances the peak heat flux for the higher air jet Reynolds number for slightly fuel rich conditions of ϕ = 1.1 at the optimal burner-to-target plate distance of 40 mm.

Experimental Investigation of Stability and Structure of Vertical LPG Inverse Diffusion Flames Issuing from an Elliptic Burner

The present paper experimentally examines the influence of the progressive variations of the central air jet velocity in a concentric circular-elliptical inverse diffusion flame (IDF) on the visual and thermal structure and stability of the developed flames. All experiments are conducted at a fixed fuel flow rate (liquefied petroleum gas) throughput that emerges from the annular elliptic passage having an aspect ratio of 2:1. The visual images are aided via a digital camera and shadowgraphs, while the thermal structure (axial and radial temperature profiles) is acquired using a bare fine wire (125 μm) thermocouple; rendering radiation loss insignificant. The visual images and shadowgraphs clearly indicate the existence of four regimes: (i) an annular partially premixed region at the burner rim, (ii) an inner central premixed blue flame, (iii) an outer luminous yellowish post combustion zone and (iv) the flame tip buoyant zone. The progressive increases of the central average air velocity (𝑉𝑎 = 7.4 m/s up to 31.3 m/s) result in shortening the visible flame length and narrowing the flame width. These are coupled with changing the flame appearance from a yellowish diffusion flame with a sooty core regime to an intense central premixed flame surrounded by a soot ring to blue flames exhibiting intense central radiation regime associated with soot oxidation. At extremely high air velocity > 21 m/s, locals flame extinction and re-ignition occurs at the boundaries of the ellipse minor axis and further increase of 𝑉𝑎 causes complete extinction of the main flame. These findings are very much supported by the mean gas temperature measurements that indicate steep temperature rise associated with the formation of a central premixed combustion within the flame core which is followed by the diffusion mode of combustion. A plateau of the axial temperature profiles is observed in the transition zone between the two regions whereby the rise in temperature due to soot oxidation is balanced by the radiation loss.

Design and investigation of a central air jet flameless combustor

Due to strict regulations on harmful emissions such as NOx and CO, many novel combustion techniques have been investigated in order to decrease emissions from gas turbines. One of these novel techniques is colorless distributed combustion (CDC) or flameless combustion (FC). CDC has been examined under reacting conditions using ANSYS Fluent at 27 different operating cases for the aim of developing design equations and a 3D flameless combustion map that includes the effect of air diameter, confinement size and excess air ratio on recirculation ratio to allocate flameless combustion regions. A simple lab scale combustor with central air jet arrangement is employed for the analysis. Eddy dissipation concept (EDC) either global or with detailed mechanism (DRM19) fails to predict the effect of excess air ratio in flameless combustion. A numerical approach for calculating the recirculation ratio is presented. As the air jet diameter decrease 40% there is about 54% increase in the recirculation ratio and about 90% decrease in the NOx emissions. It is found that above a recirculation ratio of 3 there is an insignificant effect on NOx emissions. Reducing the combustor diameter by a constant value for a given air diameter is found to result in a decrease in the recirculation ratio and the percentage of decrease is higher at high thermal intensities combustors. Moreover, it is observed that as the combustor diameter decrease to half of its value at constant air jet diameter and constant excess air ratio, there is an 80–90% increase in the NOx emissions inside the combustor. Furthermore, design equation is obtained to predict the appropriate air diameter that if combined with the available combustor diameter achieves a minimum recirculation ratio suitable to sustain flameless combustion.

Effect of Burner Geometry on Heat Transfer Characteristics of an Impinging Inverse Diffusion Flame Jet with Swirl

An experimental investigation is carried out on an inverse diffusion flame (IDF) burners having central air jet and being surrounded by fuel jets for an impingement heat transfer. A thorough investigation has been performed to study the effects of the diameter of air jet, orientation of fuel jets and swirl in the central air jet on an inverse diffusion flame shapes, mainly on heat transfer characteristics. The twisted tapes of twist ratios (TR) 2.5 and 3.75 (corresponding to the swirl numbers of 0.62 and 0.41) are used for two different IDF burners to create a swirling flame jet. A comparison is made between the two IDF burners for with and without swirl conditions operating at a fixed air jet Reynolds number (Rea) of 2000 and equivalence ratio (ϕ) varying from 0.4 to 1.4 for a fixed burner surface to impingement plate distance of 20 mm. It is observed that for the same operating conditions, diameter of an air jet and orientation of fuel jets impart a substantial change in an inverse diffusion flame behavior by enhancing air–fuel mixing intensity. The experimental results show that the burner with smaller air jet diameter along with swirl produces a higher heat transfer rate to an impingement plate. An experimental work provides a contribution to improve a combustion characteristic of IDF for impingement heating on a flat surface.

Micromechanical analysis of bubbles formed in fluidized beds operated with a continuous single jet

Advancement in the knowledge of bubble dynamics and properties can significantly aid to improve the design and operation of gas-solid fluidized beds. In this work, bubble properties in a single jet fluidized bed were studied by the combined approach of CFD and DEM. The results illustrated that the process of the continuous injection of a central air jet to the bed was successfully reproduced, with the formation of a series of bubbles which rise through the bed and burst at the bed top. During this process, the series of bubbles align vertically which show coalescence in three different levels of the bed. However, the coalescence patterns depend upon jet velocity and are fairly regular. The bubbles are categorized into two types: primary/leading bubbles and trailing bubbles. The size of primary bubbles keeps increasing with bed height while the size of trailing bubbles decreases since trailing bubbles coalesce with primary bubbles. Primary bubbles, although formed at different times, are of similar shape at different heights of the bed while trailing bubbles are generally elongated in shape. Trailing bubbles have a higher velocity than primary bubbles. The spatial analysis of pressure gradient, drag, and particle-fluid interaction force shows that the presence of bubbles leads to different particle flow zones where forces vary significantly. Moreover, the bubble passage can be delineated by the temporal variation of the particle-fluid interaction force.

Particle shape effect on bubble dynamics in central air jet pseudo-2D fluidized beds: A CFD-DEM study

Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.

Effect of air jet momentum on the topological features of turbulent CNG inverse jet flame

Inverse jet flame (IJF) configuration is a variant of coaxial nonpremixed jet flame which can produce a nonluminous compact blue flame with independent control of air and fuel jet momentum. In the present work, the effect of central air jet velocity on visible topological features such as, visible flame height and base flame height is investigated experimentally. Also the fluid dynamics behind the evolution of base flame in IJF configuration is unraveled in this study. Notably, buoyancy induced oscillations of IJF, which is relatively unexplored in open literature is studied in this work. Moreover, a systematic classification of IJF based on the evolution of its visible flame features with variation in the central air jet and annular fuel jet velocities is reported. In addition, a semi-empirical correlation for visible flame height of IJF with air-fuel momentum ratio is arrived in the present work which can be useful in the design of IJF based burners utilized for impingement heat transfer applications.

On the effect of turbulence models on CFD simulations of a counter-current spray drying process

Accurate modeling of the flow field by means of capturing turbulence is crucial in CFD simulations. However, choosing the appropriate turbulence model remains quite challenging for simulating spray drying applications. Only a few studies have touched on this issue, although experimentally validated comparisons throughout the dryer are rare. This work aims to provide an assessment of five different turbulence models (RNG k−ε, standard, BSL and SST k−ω as well as transition SST) in terms of the predicted flow field throughout a lab-scale counter-current spray dryer. None of the tested models could initially provide a satisfactory match with locally measured temperatures within the chamber. The popular choice RNG k−ε model led to highest discrepancies, while the k−ω variants performed only slightly better. All these models under-predicted the dissipation of the central hot air jet. Modification to the k−ω variant’s characteristic constant to allow increased production of turbulence led to satisfactory agreement between the measurements and simulation results. Extended analysis revealed that different turbulence models produced significantly different drying histories. Only the k−ω SST variant with modified constant could provide predictions close to measured outlet particle moisture content and air conditions. The RNG model proved unsuitable due to unrealistic results with particle injection as well. The differences in predictions with injection among the models were attributed to different transient self-sustained air fluctuation behavior predicted within the chamber. This work will be useful in the selection of turbulence models which is fundamental to accurate CFD modeling of spray dryers.

Particle scale modelling of bubble properties in central air jet gas-solid fluidized beds

Bubbles properties play an important role in determining the characteristics of gas-solid fluidized beds and thus influence the bed performance significantly. In this work, the bubble properties such as bubble size and bubble shape are studied by the combined approach of CFD and DEM for bubbling gas-solid fluidized beds operated with a continuous central jet. The process of the continuous injection of a central air jet to the bed is successfully reproduced featured with the formation of series of bubbles which rise through the bed and burst at the bed top. The results show that the average initial bubble diameter increases with the increase of jet velocity, and can be well predicted by the leakage based empirical models. The bubble size distribution in the overall bed shows a decrease in frequency as the size of the bubble increases for all the jet velocities. At a higher jet velocity, the distribution is more scattered signifying the presence of small and large bubbles. The distributions of bubble aspect ratio and bubble shape factor are also examined in detail, showing a normal distribution. Specifically, the distribution of bubble aspect ratio exhibits a positively skewed distribution for all jet velocities while that of bubble shape factor shows a negatively skewed distribution. The study provides a useful basis for further work on the understanding of bubble dynamics.

Evaluation criteria for a flameless combustor based on recirculation and mixing - A CFD approach

In the present work, the effect of combustor geometry and inlet air velocity on flameless critical parameters such as recirculation ratio and mixing is investigated. The main objective is to develop evaluation criteria for flameless combustors design based on these critical parameters. A simple lab scale combustor with central air jet arrangement is employed for the analysis. Three-dimensional computations were performed under non-reacting conditions using ANSYS Fluent. Reducing the air jet diameter to half its original value leads to a 100% increase in the recirculation ratio, which promotes flameless operation. Moreover, it results in a slightly more than 200% increase in turbulence intensity, which encourages hot products entrainment and accelerates mixing with incoming reactants. From another side, scaling down the combustor to half its original size results in 66% reduction of recirculation ratio, which might suppress flameless operation. Besides, it accelerates incoming reactants mixing in the near burner region due to the increased jet spreading and decay rate. These results show the considerable influence of combustor geometry on recirculation ratio and mixing. Furthermore, the critical role of later parameters in the previously reported transition between conventional and flameless combustion modes for the present combustor is demonstrated. A threshold value of recirculation ratio, Kcritical, is found critical for flameless operation and is noticed to increase with increasing excess air ratio. Therefore, the two parameters are linked together through a developed mathematical correlation. Finally, combustor geometry is represented through the dimensionless parameters; air jet to combustor diameter ratio, and combustor scaling factor. Recirculation ratio is linked to these parameters through additional correlations. Through these developed correlations, geometry aspects for flameless combustors can be preliminary identified and/or evaluated.

Experimental Study on Bluff-Body Stabilized Premixed Flame with a Central Air/Fuel Jet

Bluff-body flame holders are commonly employed in many industrial applications. A bluff-body is usually adopted to enhance the downstream mixing of the combustion products and the fresh fuel-air mixtures, thus to improve the flame stability and to control the combustion process. In the present paper, flames stabilized by a conical-shape bluff-body flame holder with a central air/fuel jet were studied. Effects of both a central air jet and a central fuel jet on the structures and lean blowout limits of the premixed annular flames, and on the temperature on the upper surface of the bluff-body were investigated and presented. It was revealed that a central jet led to a considerable reduction of the temperature on the upper surface of the bluff-body. It was proposed to be caused by the alternation of flow structures (in the case with a central air jet) altogether with the flame lifting from the burner (in the case with a central fuel jet). Thus, it might be used to solve the problem of the bluff-body with high heat loads in practical applications. The flame stability characteristics, for example the unstable flame dynamics and the lean blowout limits, varied with the injection of an air or fuel jet through the central pipe. Different blowout behaviors, being with or without the occurrence of flame split and flashing, caused by a central air jet were presented in the paper. In addition, when a small amount of central fuel jet (i.e., Uf/Ua = 0.045) was injected into the flow fields, an unsteady circular motion of the flame tip along the outer edge of the bluff-body was observed as well. Whereas, with an increase in the amount of the central fuel jet, the flame detached from the outer edge of the bluff-body and then became much more unstable. With a central air or fuel jet injecting into the flow field, premixed flames stabilized by the bluff-body became more unstable and easier to blowout.

Experimental Investigation on Effects of Central Air Jet on the Bluff-body Stabilized Premixed Methane-air Flame

Flame stabilized by a bluff-body is a common scene in many engineering applications due to the enhanced mixing characteristics, improved flame stability, and ease of combustion control. We recently designed a burner which has a conical bluff body with a central air injector. In the current work, effects of the central air jet on the heat load of the bluff body, the flame structures and the flame blowoff limits were investigated. It was found that the central air jet can significantly reduce the heat load to the bluff body. It is a considerable solution to the problem caused by the high heat load in practical applications. The flame structures and blowout limits were altered with the addition of central air jet as well. Different blowout behaviors caused by the air jet were observed and reported. The bluff-body could be cooled down by the center air injection but then it seems not to stabilize the flame any more.

Experimental study of the inverse diffusion flame using high repetition rate OH/acetone PLIF and PIV

Most previous work on inverse diffusion flames (IDFs) has focused on laminar IDF emissions and the soot formation characteristics. Here, we investigate the characteristics and structure of methane IDFs using high speed planar laser-induced fluorescence (PLIF) images of OH, particle image velocimetry (PIV), and acetone PLIF imaging for non-reacting cases. First, the flame appearance was investigated with fixed methane loading (mass flux) but with varying airflow rates, yielding a central air jet Reynolds number (Re) of 1,000 to 6,000 (when blow-off occurs). Next, it was investigated a fixed central air jet Re of 4500, but with varied methane mass flux such that the global equivalence ratio spanned 0.5 to 4. It was observed that at Re smaller than 2000, the inner air jet promotes the establishment of an inverse diffusion flame surrounded by a normal diffusion flame. However, when the Re was increased to 2500, two distinct zones became apparent in the flame, a lower entrainment zone and an upper mixing and combustion zone. 10kHz OH-PLIF images, and 2D PIV allow the identification of the fate and spatial flame structure. Many flame features were identified and further analyzed using simple but effective image processing methods, where three types of structure in all the flames investigated here: flame holes or breaks; closures; and growing kernels. Insights about the rate of evolution of these features, the dynamics of local extinction, and the sequence of events that lead to re-ignition are reported here. In the lower entrainment zone, the occurrence of the flame break events is counterbalanced by closure events, and the edge propagation appears to control the rate at which the flame holes and closures propagate. The rate of propagation of holes was found to be statistically faster than the rate of closure. As the flames approach blow-off, flame kernels become the main mechanism for flame re-ignition further downstream. The simultaneous OH-PLIF/Stereo PIV measurements indicate that the individual breaks events could be correlated to local vortical flow structure and strain rates fields. The detailed measurements provide a more complete understanding of IDF flame characteristics and structure than was previously possible.

Flame stability limits and near blowout characteristics of CNG inverse jet flame

This paper reports the stability limits and near blowout characteristics of unrecessed and recessed compressed natural gas (CNG) inverse jet flames (IJFs) by varying the central air jet velocity for a constant fuel jet velocity. The experimental results indicate that unrecessed IJF has higher flame stability as compared to the recessed case for the same fuel jet velocity. In order to understand the reason for the enhanced flame stability in unrecessed IJF as compared to the recessed IJF, the visible appearance of these two inverse jet flame configurations are examined for understanding the various transient events taking place towards blowout. In the case of unrecessed IJF, initiation of local extinction and reignition phenomena on the flame surface ushers the onset of blowout. In contrast, this phenomenon is not observed in recessed IJF, rather the flame gets partially lifted from the burner rim prior to blowout. The enhanced flame stability of unrecessed IJF is attributed to the presence of base flame near the burner rim that acts as a pilot flame. On the other hand, the absence of base flame in recessed IJF is the reason for its lower flame stability limit as compared to the unrecessed IJF. Based on the nature of blowout events, blowout mechanisms for both unrecessed and recessed IJFs are proposed in the present work.

Performance enhancement of inverse diffusion flame burners with distributed ports

An experimental study regarding the influence of the air port design on the inverse diffusion flame characteristics was performed for two different burner configurations, wherein the combustion/heat transfer performance was optimized. In the first configuration that comprised a single air port and distributed fuel ports, decreasing the air jet diameter enhanced the combustion efficiency by 3.6% as testified by recording a maximum decrease of 81.3% in the CO emissions. As the central air jet Reynolds number thus reached 18038, the stability limit was extended by 181% in terms of the air jet velocity; while the flame length was reduced by 38.6%. Inasmuch as the flow residence time across the high-temperature zone thus decreased, NO x emissions as low as 6 ppm were obtained. In the other configuration that had multiple air ports, there was a decrease in the CO, HC, and NO x emissions respectively by 69%, 58%, and 31% in comparison to those of the single port having the same port area. It was found that the smallest jet diameter was accompanied by the highest peak temperature as well as the maximum heat flux to a plate on which the flame impinged, respectively, having the values of 1761℃ and 11.4 kW/m2. In this regard, increasing the air jet Reynolds number by 30% increased the peak flame temperature by 8.3%. In the primary equivalence ratio range between 0.8 and 2.0, the impingement plate optimum spacing for the maximum heat flux was found to correlate well with Reynolds number. Combining the nonunity Lewis number and the Gas Research Institute-3 (GRI-3) kinetics with the k − ɛ model verified the fuel jet deflection and soot development, but overpredicted the NO x emissions by 11.5%. Increasing the center-to-center distance between the central air jet and the distributed fuel ports reduced the flame length by 19%.

Experimental Characterization of Methane Inverse Diffusion Flame

This article presents 10-kHz images of OH-PLIF simultaneously with 2-D PIV measurements in an inverse methane diffusion flame. Under a constant fuel flow rate, the central air jet Re was varied, leading to air to fuel velocity ratio, Vr, to vary from 8.3 to 66.5. Starting from Vr = 20.7, the flame is commonly characterized by three distinct zones. The length of the lower fuel entrainment region is inversely proportional to Vr. The flames investigated resemble a string shear layer confining this zone, and converging into the second distinct region, the flame neck zone. The third region is the rest of the flame, which spreads in a jet-like manner. The inverse diffusion flames exhibit varying degrees of partial premixing, depending upon on the velocity ratio Vr, and this region of partial premixing evolves into a well-mixed reaction zone along the flame centerline. The OH distribution correlated with the changes in the mean characteristics of the flow through reduction in the local Reynolds number due to heat release. The existence of a flame suppresses or laminarizes the turbulence at early axial locations and promotes fluctuations at the flame tip for flames with Vr < 49.8. In addition, the flame jet width can be correlated to the OH distribution. In upstream regions of the flames, the breaks in OH are counterbalanced by flame closures and are governed by edge flame propagation. These local extinctions were found to occur at locations where large flow structures were impinging on the flame and are associated with a locally higher strain rate or correlated to the local high strain rates at the flame hole edges without this flow impinging. Another contributor to re-ignition was found to be growing flame kernels. As the flames approach global blow-off, these kernels become the main mechanism for re-ignition further downstream of the flames. At low Vr, laminarization within the early regions of the flame provides an effective shield, preventing the jet flow from penetrating through the flame.

Bubbling feature of an annular granular jet dispersed by a central air jet with acoustic excitation

An annular granular jet surrounding an excited air jet is studied experimentally by high-speed digital photography. Particle bubbles shaped more uniformly are periodically formed due to the addition of acoustic excitation to the central air jet. Two bubbling regimes, namely acoustic excitation regime and convergent nozzle regime, are considered to coexist in the bubble-formation process. Acoustic excitation regime plays a dominant role in the bubbling process at lower superficial air jet velocity. However, for larger superficial air jet velocity, convergent nozzle regime instead of acoustic excitation regime has absolute predominance in the bubbling process. The competition between the two regimes results in a non-monotonic variation of axial bubble size. In addition, the increases of excitation frequency and SPL can both encourage the growth of bubbles in the radial direction.

Flame structure of methane inverse diffusion flame

This paper presents high speed images of OH-PLIF at 10kHz simultaneously with 2D PIV (particle image velocimetry) measurements collected along the entire length of an inverse diffusion flame with circumferentially arranged methane fuel jets. For a fixed fuel flow rate, the central air jet Re was varied, leading to four air to fuel velocity ratios, namely Vr=20.7, 29, 37.4 and 49.8. A double flame structure could be observed composed of a lower fuel entrainment region and an upper mixing and intense combustion region. The entrainment region was enveloped by an early OH layer, and then merged through a very thin OH neck to an annular OH layer located at the shear layer of the air jet. The two branches of this annular OH layer broaden as they moved downstream and eventfully merged together. Three types of events were observed common to all flames: breaks, closures and growing kernels. In upstream regions of the flames, the breaks were counterbalanced by flame closures. These breaks in OH signal were found to occur at locations where locally high velocity flows were impinging on the flame. As the Vr increased to 37.4, the OH layers became discontinuous over the downstream region of the flame, and these regions of low or no OH moved upstream. With further increases in Vr, these OH pockets act as flame kernels, growing as they moved downstream, and became the main mechanism for flame re-ignition. Along the flame length, the direction of the two dimensional principle compressive strain rate axis exhibited a preferred orientation of approximately 45° with respect to the flow direction. Moreover, the OH zones were associated with elongated regions of high vorticity.

English

The effect of the geometric parameters and jet dynamics of the port array inverse diffusion flame (IDF) burner on the flame characteristics were investigated experimentally usingliquefied petroleum gas&nbsp;(LPG) fuel. The geometric parameters are the central air flow area, the fuel jets flow area, the number of the fuel jets and the radius of the pitch circle around which the fuel ports are arranged. The jet dynamics are the air and the fuel flow rates and their momentum flux. Three burners were used for this purpose. The analysis of the results of the IDF jet dynamics showed that two flame fronts formed in the entrainment zone of the IDF, one of them in the co-flowing jet formed by the fuel and the central air jet and the other is in the peripheral submerged jet formed by the fuel jets and the ambient air. Beyond the neck, the central air jet, the fuel jets and the two flame fronts flow as one flame torch. At high air, Reynolds number and low fuel Reynolds number, corresponding to the primary equivalence ratios near unity, the IDF produced by the burner of pitch circle radius 10 mm are short blue color sootless flames. As the air Reynolds number was gradually decreased to a low value corresponding to a primary equivalence ratio 4.5, the flame was changed to yellow diffusion flame with high soot concentrations. The results also showed that the flame characteristics were highly affected by the number of the fuel ports. The flame became more bluish and shorter as the number of the fuel ports increases to 36 ports. &nbsp; Key words:&nbsp;&nbsp;Inverse diffusion flame (IDF), diffusion flame, burner configuration.

Noise generation by open inverse diffusion flames

The acoustic spectra of noise radiation from a turbulent non-premixed inverse diffusion flame were measured and the effect of the point of observation, air jet Reynolds number and overall equivalence ratio on the spectra were examined. The tests were conducted in a wide range of air and fuel flow rates and both the non-reacting and reacting cases were considered and discussed. For the non-reacting case, the noise emitted from the cold flow is mainly generated by the central air jet, with only a small role played by the fuel jets. The dominance of the noise produced by the air jet is confirmed by the observation that the cold flow noise is a strong function of the air jet flow rate or air jet Reynolds number. The spectral features of the noise from the combusting flames differ significantly from those of the cold flow noise due to the chemical reactions. Upon combustion, the noise radiated from the flames significantly overwhelms the corresponding cold flow noise in the range of frequency under consideration (80–3000 Hz). The distance of the point of observation only affects the magnitude of the sound pressure level while both the shape and magnitude of the sound pressure level are influenced by the azimuth of the point observation. The total sound pressure level increases with Re, while the effect of Ф on the total sound radiation level indicates that the highest level of noise occurs as complete combustion is approached.