Bluff Body Diameter Research Articles

Experimental study on flow characteristics in a new separator

The confined multiphase swirling flow characteristics were studied in the presence of a new bluff body for gas/liquid separation. It is a hollow cylinder with a tapered front. The flow visualization was performed using High Speed Camera. The gas-to-liquid ratio was controlled by manipulating the air and water flow rates in the flow loop. Based on previous studies, the generated air core exhibits different regimes with stable and unstable regions along the pipe axis. The proposed bluff body should be placed with the stable region to avoid the loss of separation efficiency due to the oscillations of the air core. Three flow regimes were considered based on the water flow rate. At the low water flow rate, the air core is thinner and breaks down a few centimeters upstream of the bluff body. The generated bubbles are sucked inside the bluff body. Increasing the air flow rates causes the air core to get closer to the bluff body until reaching it completely and might enlarge exceeding the bluff body diameter. For all the cases considered, the air core is wavy with highly instable diameter and length. When increasing the water flow rate, a more stable air core is formed closer to the bluff body. The high-water flow rate appears to be necessary to stabilize the air core and reach flow regimes favorable for an efficient separation through both the annular space and the internal passage.

Wake-induced vibration and heat transfer characteristics of three tandem semi-circular cylinders

This paper studies the wake-induced vibration (WIV) and heat convection of the downstream tandem bluff bodies under the influence of an upstream bluff body at Re = 200 and Pr = 0.7. For the reduced fluidic velocity (Ur) in the range of 1–16 and different bluff body diameter ratios (d/D), three wake interference patterns, namely, the quasi-co-shedding (QCS) pattern, the co-shedding (CS) pattern, and the coupling between QCS and CS (QCS-CS) are formed due to the interference between the shear layer and vortex shedding process. The occurrence conditions of the three patterns are closely related to Ur and d/D. The vibration response of the midstream bluff body is roughly consistent with that of a heat-isolated semi-circular cylinder (HISC) if d/D is small. By increasing d/D, the vibration amplitude of the midstream bluff body first increases, then decreases with Ur. The vibration amplitude of the downstream bluff body starts to increase from small d/D and then can maintain a high value with the increase of Ur, even at large d/D. Compared with the HISC, the tandem configurations can help reduce the drag force. Because of the wake interference and the ‘blocking effect’ from the upstream bluff body, although the heat convection of the midstream and downstream bluff bodies can be effectively strengthened through WIV, the time-averaged Nusselt numbers (NuA) of the midstream and downstream bluff bodies are still lower than that of the HISC. Besides, heat transfer efficiency has been found to correlate with the transverse vibration amplitude of bluff bodies. Moreover, it is revealed that there is a trade-off between the heat transfer efficiency of the equipment and its service lifespan because increasing the vibration amplitude enhances convection but also increases the risk of potential damage to the structural integrity.

Control of the flame and flow characteristics of a non-premixed bluff body burner using dielectric barrier discharge plasma actuators

Reducing emissions and improving combustion are necessary steps to reach greener combustion systems. In this study, the effect of employing an annular plasma actuator mounted on the surface of a bluff body in a non-premixed burner is numerically investigated. Activating the plasma actuator induces an ionic wind near the surface of the bluff body that modifies the flow features. While the previous relevant studies were mainly focused on volume dielectric barrier plasma discharges, this study investigates the influences of surface plasma discharges. Also, the effect of plasma actuator position, fuel-to-air velocity ratio, and the size of the bluff body are investigated. The characteristics of the flow and temperature fields, the mass fraction of carbon monoxide and carbon dioxide inside the burner in the presence, and the absence of a plasma actuator are analyzed. Also, spatial mixing deficiency (SMD) is used to evaluate the plasma actuator's influence on the reactants' mixing. The results show that among various configurations of annular plasma actuators, positioning the plasma actuator on the bluff body surface acting upwards near the air duct, strengthens the external recirculation region which leads to a favorable change in the characteristics of the flame inside the burner. With the activation of the plasma actuator, the spatial mixing deficiency (SMD) values decrease by an average of 12.75%, showing improvement in mixing, and the flame becomes more uniform compared to the baseline case leading to the reduction of CO concentration. In addition, the root of the flame is strengthened and the height of the body of the flame measured from the bluff body surface is reduced by about 43%. For both low and high fuel-to-air velocity ratios, the presence of a plasma actuator leads to a positive effect on the flame structure. For a high fuel-to-air velocity ratio, the external recirculation area has a favorable size. The created flame is elongated and the root of the flame is entirely away from the burner. When the plasma actuator is active, it causes the flame to get closer to the burner tip. For various diameters of the bluff body, the plasma actuator showed reasonable capability in altering the flame behavior. For larger bluff body diameters, the plasma actuator strengthens both recirculation zones, especially the external recirculation zone, leading to a uniform and shorter flame.

Effects of swirl number and bluff body on swirling flow dynamics

Bluff-body swirling flows have been widely employed in gas-turbine combustors to achieve flame stabilization. Meanwhile, considerable efforts have been made to understand swirling flow dynamics, the effects of swirl number and bluff body on flow structure and dynamics are still not well understood. To this end, a series of direct numerical simulations of isothermal swirling flows have been conducted in this work in order to investigate the impact of swirl numbers and bluff-body diameters on the flow structure, Reynolds stresses, and turbulent kinetic energy (TKE) transport. It is found that a change in the swirl number can affect the inner recirculation zone (IRZ) and hence momentum transport. Specifically, as the swirl number increases, the vortex core formed at downstream locations can merge with the IRZ. Moreover, including the bluff body not only contributes to the formation of the IRZ but also serves as a disturbance source for the flow, which is favorable for the formation of large-scale vortex structures. Then, the impact of swirl number and bluff body on Reynolds shear stresses and anisotropy invariants is investigated to identify the locations of the inter shear layer (ISL), the outer shear layer (OSL), and the main swirling zone (MSZ). The results show that as the swirl number increases, both the ISL and MSZ shift to the wall, indicating a large IRZ. Furthermore, the analysis of TKE indicates that for cases with a bluff body, TKE mainly occurs in the ISL and OSL, featuring a dual peak distribution. However, for cases without a bluff body, the distribution of TKE is primarily concentrated in the ISL. These results suggest that both increasing the swirl number and/or including the bluff body could help with TKE transport, which can lead to a wide range of TKE distribution.

Experimental Research of Symmetrical Airfoil Piezoelectric Energy Harvester Excited by Vortex-Induced Flutter Coupling

In order to solve the problem of self-energy supply of vehicle-mounted micro-sensors, bridge detection and some other low-power electronic devices in their working state, a vortex-induced flutter composite nonlinear piezoelectric energy harvester (VFPEH) with symmetrical airfoils on both sides of a cylindrical bluff body is designed. The VFPEH consists of a cantilever beam, a cylindrical bluff body connected to the free end of the cantilever beam, and two airfoil components symmetrically fixed at both ends of the shaft, which enables coupling between vortex-induced vibration and flutter. The airfoil symmetrically arranged on both sides of the cylindrical bluff body induces the cantilever beam to produce bending and torsional composite vibrations at high wind velocities, realizing energy harvest in the two degrees of freedom motion direction, which can effectively improve the output power of the energy harvester. Based on a wind tunnel experimental platform, the effect of key parameters matching impedance and the diameter of the cylindrical bluff body on the output performance of the VFPEH is investigated, together with the output performance of the classical vortex-induced energy harvester (VEH), the flutter energy harvester (FEH) and the VFPEH. The experimental results show that for the VFPEH under a combination of vortex-induced vibrations and flutter vibrations has a better output performance than the VEH and the FEH when using the same size. The coupling of vortex-induced vibration and flutter can reduce the start-up wind velocity of the VFPEH and expand the wind velocity range of the high output power of the VFPEH. The VFPEH has a better output performance at the cylindrical bluff body diameter of 30 mm and a load resistance of 140 kΩ. When the wind velocity range is 2 m/s–15 m/s, the maximum output power of the VFPEH is 6.47 mW, which is 129.4 times and 24.9 times of the maximum output power of the VEH (0.05 mW) and FEH (0.26 mW), respectively.

The Effects of High Centrifugal Acceleration on Bluff-Body Stabilized Premixed Flames

Abstract An experimental study is conducted on bluff-body stabilized premixed flames in a curved, square cross section duct. High flow velocities coupled with a small radius of curvature of the duct induce high centrifugal acceleration normal to the flame sheet. A cylindrical flame holder spans the width of the square cross section and is positioned at the channel midheight. Flame shear layers are stabilized on the radially inward (upper) and outward (lower) edges of the flame holder. Side-view high-speed Schlieren images and high-speed pressure measurements are captured. Static stability, overall pressure loss, and statistics and velocimetry results from the Schlieren images are reported, and results are compared to a straight configuration with no centrifugal acceleration. Two bluff-body diameters are studied to show the effect of flame holder diameter on static stability. For the curved configuration, blowout velocities are higher for the smaller bluff-body diameter, likely due to flow acceleration effects. Blowout velocities are lower for the curved configuration compared to the straight configuration which may be due to the destabilizing Rayleigh–Taylor (RT) effect on the upper flame layer. Overall pressure loss is slightly higher for the curved configuration than the straight configuration. High-speed Schlieren results show centrifugal acceleration causes significant structural and velocimetric asymmetry in the bluff-body wake. In the curved configuration, the upper flame layer displays destabilizing RT instabilities, and the lower flame layer displays stabilizing RT effects. The upper flame shows vigorous RT instabilities which broaden the flame brush and sustain a flame leading edge independent of inlet Reynolds number or velocity. Conversely, the lower flame exhibits suppression of Kelvin–Helmholtz and flame-generated instabilities in the wake, which confines the flame brush and significantly reduces transverse flame velocities. The lower flame edge profile moves toward the channel centerline with increasing inlet Reynolds number. The upper flame in the curved configuration shows higher flame edge velocities than the straight configuration while the lower flame shows velocities closer to zero. The empirical constant to the power law relation for upper flame edge velocities agrees with RT-dominated flame growth theory for this experimental scale and agrees with other RT-dominated flame studies.

Numerical analysis of influence of various bluff-body shapes on diffusion flame dynamics

In this paper, various bluff-body shapes (cylindrical, square, star) and two different surface topologies (smooth, wavy) are applied as passive tools for controlling a non-premixed hydrogen flame in a combustion chamber. We focus on the dynamics of the flame and its time-averaged characteristics in the close vicinity of an injection system within formed recirculation zones and also in a far-field. The research is performed with the help of large-eddy simulations (LES) method using the ANSYS Fluent software and a high-order academic code SAILOR. Flame behaviour is found to be strongly dependent on the geometry of the bluff-body whereas its wall topology affects the flame characteristics only slightly. In the cases with the square and star bluff-body, small vortical structures originating at the corners deform large vortical structures created by the Kelvin-Helmholtz instability mechanism. This intensifies the mixing and combustion process and, in the configuration with the square shape bluff-body, translates to the shortening of the recirculation zone by 15% of the equivalent bluff-body diameter and the flame, which in the axis develops closer to the bluff-body. The star shape leads to the most uniform flame at the radial border or the recirculation zone.

Vortex-induced piezoelectric cantilever beam vibration for ocean wave energy harvesting via airflow from the orifice of oscillation water column chamber

Micro piezoelectric generators have attracted immense interest to convert mechanical vibrations into electric energy for ultralow power wireless sensors. Herein, an ocean wave vortex-induced vibration piezoelectric energy harvester(Wavee-VIVPEH) is proposed to extract wave energy. The renewable and sustainable ocean environment energy harvesting device consists of an oscillating water column(OWC) air chamber, wind tunnel, and piezoelectric energy converter. A fixed bluff body in the vent tunnel induces airflow vortexes and excites the piezoelectric cantilever vibration converting up the ultralow frequency of ocean waves. The novel Wave-VIVPEH is a complex multi-physics system requiring feasible methodologies to enhance its performance. The theoretical models are derived to systematically investigate and optimize the operation. The flume and wind tunnel experiments and the related fluid-solid electric coupling simulations are carried out to verify the concept's feasibility. The aerodynamic kinetic performance in the OWC chamber is characterized by the models. The instantaneous maximum exhalation airspeed reaches 16.55 m/s with the air pressure of 349.97 Pa and the wave period of T = 2.5 s. In this wave condition, the results are well agreed with the simulation values of wind speed of 17.96 m/s, and air pressure of 382.55 Pa. The output power characteristics of the piezoelectric energy harvester are analyzed for the distance of the vortex wake between the piezoelectric cantilever beam and the bluff body. With the bluff body diameter of 25 mm and distance of d/dD= 4, the theoretical results on the maximum peak voltage and output power are 10.46 V, 3.26 mW agree with the simulation optimized values of the maximum peak voltage of 10.85 V and the output power of 3.55 mW. When the wind speed is 15 m/s, the theoretical voltage of 8.62 V approaches the experimental result of 8.47 V. Especially, the comparisons of the theoretical power and voltage to experimental and simulational results indicate that the ingenious Wave-VIVPEH has great potential in ocean wave energy harvesting applications to supply power for intelligent buoys.

Large eddy simulation of soot evolution in turbulent nonpremixed bluff body flames

Large Eddy Simulation (LES) is utilized to investigate soot evolution in a series of turbulent nonpremixed bluff body flames featuring different bluff body diameters. The modeling framework relies on recent development in the soot subfilter Probability Density Function (PDF) model that can correctly account for the distribution of soot with respect to mixture fraction, correcting errors in previous soot subfilter PDF models that significantly overpredict soot oxidation. With the previous soot subfilter PDF model, no soot was observed outside of the recirculation zone in past studies on similar bluff body flames. Results obtained with the current LES modeling approach compare favorably with the experimental measurements of the flow field and the soot volume fraction. Notably, the current LES modeling approach correctly predicts large soot volume fractions in the recirculation zone, a decrease in the soot volume fraction through the high-strain neck region, and then an increase again in the downstream jet-like region. Consistent with the experimental measurements, the larger bluff body diameter, with its larger recirculation zone with longer residence times, generates more soot in the recirculation zone and also more soot in the high-strain neck region. Analysis of the soot volume fraction source terms lead to mechanistic understanding of soot evolution in the entirety of the bluff body flames. Most of the soot generated in the recirculation zone is oxidized but some escapes unoxidized and is passively transported through the neck region. Virtually no new soot forms in the downstream jet-like region, and the increase in the soot volume fraction in the jet-like region is due to acetylene-based surface growth of the soot transported through the neck region. This mechanism could not be predicted with the previous soot subfilter PDF model, with the recent soot subfilter PDF model being critical in the understanding of this basic mechanism.

Toluene addition to turbulent H2/natural gas flames in bluff-body burners

A key challenge in the transition towards using hydrogen as an alternative carbon-free fuel is the reduced thermal radiation due to the absence of soot. A novel solution to this may be doping with highly sooting bio-oils. This study investigates the efficacy of toluene as a prevapourised dopant in turbulent pure hydrogen and blended hydrogen/natural gas flames as a means of improving soot loading and radiant heat transfer. All flames are stabilised on bluff-body burners to emulate the recirculation component of many industrial combustors. Total heat flux and illuminance increase non-linearly with toluene concentration for fuel blends and bluff-body diameters. By reducing the bluff-body diameter from 64 mm to 50 mm, a 20/80 (vol%) H2/natural gas mixture produces a more radiative flame than a 10/90H2/natural gas mixture in the smaller bluff-body. Opposed-flow flame simulations of soot precursors indicate that as strain rate increases, although overall soot precursor concentration decreases, a 20 vol% hydrogen mixture will produce more soot than a 10 vol% mixture. This suggests the addition of hydrogen up to 20 vol% may be beneficial for soot production in high strain environments.

T-type Piezoelectric Cantilever-Beam Energy Harvester based on Vortex-Induced Vibration

A new piezoelectric cantilever energy harvester based on vortex-induced vibration was proposed. The influence of some parameters was by mathematical modeling, numerical simulation and experimental testing. The results showed that the voltage value increased with the increase of wind speed within the test range. When the bluff body was a cylinder with a diameter of 0.02 m, a height of 0.05 m, and the ratio of the distance between bluff body and harvester and bluff body diameter was 2, the maximum voltage of the T-type piezoelectric cantilever beam energy harvester was 1.02 V. It provided theoretical and experimental references for energy harvesting in the wind.

Trout-like multifunctional piezoelectric robotic fish and energy harvester

This work presents our experimental studies on a trout-inspired multifunctional robotic fish as an underwater swimmer and energy harvester. Fiber-based flexible piezoelectric composites with interdigitated electrodes, specifically macro-fiber composite (MFC) structures, strike a balance between the deformation and actuation force capabilities to generate hydrodynamic propulsion without requiring additional mechanisms for motion amplification. A pair of MFC laminates bracketing a passive fin functions like artificial muscle when driven out of phase to expand and contract on each side to create bending. The trout-like robotic fish design explored in this work was tested for both unconstrained swimming in a quiescent water tank and under imposed flow in a water tunnel to estimate the maximum swimming speed, which exceeded 0.25 m s−1, i.e., 0.8 body lengths per second. Hydrodynamic thrust characterization was also performed in a quiescent water setting, revealing that the fin can easily produce tens of mN of thrust, similar to its biological counterpart for comparable swimming speeds. Overall, the prototype presented here generates thrust levels higher than other smart material-based concepts (such as soft polymeric material-based actuators which provide large deformation but low force), while offering simple design, geometric scalability, and silent operation unlike motor-based robotic fish (which often use bulky actuators and complex mechanisms). Additionally, energy harvesting experiments were performed to convert flow-induced vibrations in the wake of a cylindrical bluff body (for different diameters) in a water tunnel. The shed vortex frequency range for a set of bluff body diameters covered the first vibration mode of the tail, yielding an average electrical power of 120 μW at resonance for a flow speed around 0.3 m s−1 and a bluff body diameter of 28.6 mm. Such low-power electricity can find applications to power small sensors of the robotic fish in scenarios such as ecological monitoring, among others.

A numerical study on the flow-induced vibrations of flexible cylinders attached with fixed short fairings

Fairings have been used to suppress the vortex-induced vibrations(VIV) of deep water cylinders in the offshore and marine engineering applications. When properly functioning, they can reduce both the cylinder oscillation amplitude in the cross-flow direction and the fluid drag force in the in-line direction. Plenty research reports can be found on the rigid cylinder VIV suppression performances of fairings while very few research work discuss their VIV suppression performances on flexible cylinders. In the present work, a self-developed high-resolution fluid–structure interaction code based on the computational fluid dynamics is used to simulate the flexible cylinder responses attached with differently covered fixed short fairings. The Reynolds number based on the incoming velocity and the bluff body diameter is 1.68×104. The short fairings are found not to induce strong galloping oscillations in the flexible cylinders while the galloping is prominent in the rigid cylinder cases at large reduced velocities. The different short fairing setups are found to influence the local fluid–structure energy transport distributions and the travelling wave directions in the flexible cylinder responses, which eventually influences the flexible cylinder oscillation amplitude distributions, response frequencies and mode orders.

A numerical study on the vortex-induced vibration of flexible cylinders covered with differently placed buoyancy modules

Buoyancy modules are essential parts in the offshore oil and gas drilling and production practices. They provide weight compensation for the riser and help avoid excessive top tension exerted by the floating platform. However, the deployment of buoyancy modules with larger diameter than the bare riser may alter the geometric shape of the riser system and greatly influence the hydrodynamic performance. Engineering designs with buoyancy modules are mostly based on experience and rigid cylinder forced vibration experimental data. Since it is difficult if not impossible to perform real sea experiments, numerical simulation becomes important in aiding the buoyancy module design and setup. In the present work, a fluid–structure interaction code based on the computational fluid dynamic model is firstly validated with the riser experimental data from MARINTEK (the Norwegian Marine Technology Research Institute) and then used to simulate the riser responses with different continuous buoyancy module setups. The Reynolds number based on the incoming velocity and the bluff body diameter ranges from 4×103 to 1.68×104. The simulation results suggest that the shape alteration from the buoyancy deployment will change the vortex-induced vibration(VIV) response mode along with the vibration frequency. The cylinder fatigue damage, travelling wave response and the fluid–structure energy transfer characters are discussed under different buoyancy module setups.

Blow-off mechanisms of turbulent premixed bluff-body stabilised flames operated with vapourised kerosene fuels

The lean blow-off (LBO) behaviour of unconfined lean premixed bluff-body stabilised flames with various fuels was investigated. Methane and vapourised ethanol, heptane, Jet-A, and an alternative alcohol-derived kerosene (Gevo) were used. OH* chemiluminescence (5 kHz), OH- and Fuel-PLIF (5 kHz), and CH2O-PLIF (10 Hz) were deployed. For all fuels, as the flame approached LBO fragmentation was observed downstream, the two sides of the flame merged at the axis, pockets of OH and CH2O were found in the recirculation zone (RZ), and eventually the individual fragments extinguished. The CH2O seemed to enter into the RZ from downstream early in the LBO process, with reactants following suit at times closer to LBO. During LBO, the integrated OH* signal decreased slowly to zero and the duration of this transition was ∼ 25 (d/UBO) in the methane and ethanol flames and ∼ 60 (d/UBO) in flames operated with heptane and the two kerosenes (where d is the bluff-body diameter and UBO the LBO velocity). This large difference could be due to re-ignitions of partially-quenched fluid inside the RZ during the LBO event. Additionally, for the same bulk velocity, the kerosene flames blow-off at higher equivalence ratios than the single-component fuelled flames, which is possibly due to the higher Lewis number and lower extinction strain rates of these fuels. The results suggest that the blow-off mechanism is qualitatively similar for each of the fuels; however, the complex chemistry associated with heavy hydrocarbons appears to result in a prolonged LBO event.

Soot-flowfield interactions in turbulent non-premixed bluff-body flames of ethylene/nitrogen

Simultaneous measurements of soot concentrations and the velocity flowfields are used to better understand soot evolution and its correlation with the strain rate and residence time in a series of turbulent non-premixed bluff-body flames. Laser-induced incandescence (LII) and planar Particle image velocimetry (PIV) were applied simultaneously to measure the soot volume fraction (SVF) and the velocity field, respectively. Three flames were stabilised on axisymmetric bluff-body burners with different bluff-body diameters (38, 50, and 64 mm) but which are otherwise identical in dimension. A mixture of ethylene/nitrogen (4:1 by volume) was issued from a 4.6 mm central round jet at a bulk Reynolds number of 15,000. The annular co-flowing air velocity was kept constant at 20 m/s for all cases. In agreement with previous work, the highest SVF was found in the recirculation zone within the outer vortex, adjacent to the co-flowing air. The maximum SVF almost doubled, from 140 ppb to 250 ppb, when using the 64 mm burner, as compared with the 38 mm burner. Relatively small amounts of soot, around 30 ppb, were observed in the highly-strained neck zone. This was deduced from the instantaneous images as having been transported there from the recirculation zone, mostly from the inner vortex. The SVF in the jet region decreased with the increase in bluff-body diameter, which was found to be related to the decrease in the estimated total volume of the flame of almost 9%. The instantaneous images reveal the roller vortices between the co-flow and the recirculation zone supress soot and cause it to oxidise.

Effect of spark location and laminar flame speed on the ignition transient of a premixed annular combustor

The flame expansion process (“light-round”) during the ignition transient in annular combustors depends on a number of parameters such as equivalence ratio (and hence laminar burning velocity, SL, of the mixture), turbulent intensity, mean flow magnitude and direction, geometry, and spark location. Here, an experimental study on a fully premixed, swirled, bluff body stabilised annular combustor is carried out to identify the sensitivity of the light-round to these parameters. A wide range of conditions were assessed: two inter-burner spacing distances, two fuels (methane and ethylene), bulk velocities from 10 to 30 m/s, and ϕ between 0.75 and 1 for methane and 0.58 and 0.9 for ethylene. The spark location was varied longitudinally (x/D = 0.5 and x/D = 5, where D is the bluff body diameter, expected to lie inside and downstream of the inner recirculation zone of a single burner, respectively) and azimuthally. The propagation of the flame during the ignition transient was investigated via high speed (10 kHz) OH* chemiluminescence using two cameras to simultaneously image the annular chamber from axially downstream and from the side of the combustor. The pattern of flame propagation depended on the initial longitudinal spark location and comprised of burner-to-burner propagation close to the bluff bodies and upstream propagation of the flame front. The spark azimuthal position, in this horizontal configuration, had a negligible impact on the light-round time (τLR), thus buoyancy plays a minor role in the process. In contrast, sparking at x/D = 5 resulted in an increase in τLR by ~ 30–40% for all the conditions examined. The inter-burner spacing had a negligible effect on τLR. When increasing bulk velocity, τLR decreased. For a constant bulk velocity, τLR depended strongly on SL and it was found that mixtures with the same SL from different fuels resulted in the same τLR. Further, the observed propagation speed, corrected for dilatation, was approximately proportional to SL and was within 30% of estimates of the turbulent flame speed at the same conditions. These findings suggest that SL is one of the controlling parameters of the light-round process; hence turbulent flame propagation has a major role in the light-round process, in addition to dilatation and flame advection by the mean flow. The results reported in the study help explain the mechanism of light-round and can assist the development of efficient ignition procedures in aviation gas turbines.

Experimental Investigation of Soot Production and Oxidation in a Lab-Scale Rich\u2013Quench\u2013Lean (RQL) Burner

Swirl-stabilized, turbulent, non-premixed ethylene–air flames at atmospheric pressure with downstream radially-injected dilution air were investigated from the perspective of soot emissions. The velocity and location of the dilution air jets were systematically varied while the global equivalence ratio was kept constant at 0.3. The employed laser diagnostics included 5 kHz planar laser-induced fluorescence (PLIF) of OH, 10 Hz PAH-PLIF, and 10 Hz laser-induced incandescence (LII) imaging of soot particles. OH-PLIF images showed that the reaction zone widens with dilution, and that regions with high OH-LIF signal shift from the shear layer to the axis of the burner as dilution increases. Dilution is effective at mitigating soot formation within the central recirculation zone (CRZ), as evident by the smaller PAH-containing regions and the much weaker LII signal. Dilution is also effective at halting PAH and soot propagation downstream of the dilution air injection point. The high momentum dilution air circulates upstream to the root of the flame and reduces fuel penetration lengths, induces fast mixing, and increases velocities within the CRZ. Soot intermittency increased with high dilution velocities and dilution jet distances up to two bluff body diameters from the burner inlet, with detection probabilities of < 5% compared to 50% without dilution. These results reveal that soot formation and oxidation within the RQL are dependant on the amount and location of dilution air injected. This data can be used to validate turbulent combustion models for soot.

Effects of the cone angle on the stability of turbulent nonpremixed flames downstream of a conical bluff body

The effects of a stabilizer and the annular co-flow air speed on turbulent nonpremixed methane flames stabilized downstream of a conical bluff body were investigated. Four bluff body variants were designed by changing the outer diameter of a conically shaped object. The co-flow velocity was varied from zero to 7.4 m/s, while the fuel velocity was kept constant at 15 m/s. Radial distributions of temperature and velocity were measured in detail in the recirculation zone at vertical locations of 0.5D, 1D and 1.5D. Measurements also included the CO2, CO, NOx and O2 emissions at points downstream of the recirculation region. Flames were visualized under 20 different conditions, revealing various modes of combustion. The results evidenced that not only the co-flow velocity but also the bluff body diameter play important roles in the structure of the recirculation zone and, hence, the flame behavior.

Effects of the Bluff-Body Diameter on the Flow-Field Characteristics of Non-Premixed Turbulent Highly-Sooting Flames

ABSTRACT This paper presents a joint experimental and computational study of the effect of the bluff-body diameter on the flow field and residence time distribution (RTD) in a set of turbulent non-premixed ethylene/nitrogen flames with a high soot load. A novel optical design has been developed to undertake the PIV measurements successfully in highly-sooting turbulent flames, using polarizing filters. The mean velocity components and turbulent intensity are reported for three bluff-body burners with different bluff-body diameters (38, 50, and 64 mm), but which are otherwise identical in all other dimensions. The central jet diameter of 4.6 mm was supplied with a mixture of ethylene and nitrogen (4:1, by volume) to achieve a bulk Reynolds number of 15,000 for the reacting cases. Isothermal cases were also investigated to isolate the effect of heat release on the flow fiel. Pure nitrogen was utilized in the isothermal cases where the Reynolds number was kept the same as the reacting cases. The annular bulk velocity of the co-flowing air was kept constant at 20 m/s for all experiments. Computationally, a 2-D RANS model was developed, validated against the experimental data and was mainly used to investigate the effect of the bluff body diameter on the residence time in the recirculation zone. The flow structure for both isothermal and reacting cases was found to be consistent with the literature, exhibiting similar vortical structures of the recirculating zone and mixture fraction distribution, for similar momentum flux ratios. The flame length and volume were found to decrease by 20% and 9%, respectively, as the bluff diameter was increased from 38mm to 64mm. The length of the recirculation zone for the isothermal cases was found to be ~1.2DBB, while for the reacting cases it was ~1.5–1.75DBB. A stochastic tracking model was employed to estimate the pseudo-particles’ residence time distribution in the recirculation region. The model revealed that an increase in the bluff body diameter from 38mm to 64 mm leads to tripling of the mean residence time within the recirculation zone. Thermal radiation measurements from the recirculation zone show a 35% increase as the bluff body diameter is increased from 38 mm to 64 mm, whilst the total radiation from the whole flame drops by 15%, which is deduced to be due mostly to the decrease in flame volume. The effect of these differences on soot propensity and transport are described briefly and will be the subject of future investigations.