Effect Of Bluff Body Research Articles

Effect of Bluff Body Attributes on Afterburner Combustion Characteristics of a Boron Solid Rocket Scramjet

Effect of Bluff Body Attributes on Afterburner Combustion Characteristics of a Boron Solid Rocket Scramjet

Effects of Central Split Bluff Body on Combustion Characteristics and NOx Emission with Non-Premixed H2/Air Flames

ABSTRACT Considering the safety and flashback problems of hydrogen-fueled micro power generation using premixed combustion for thermophotovoltaic applications, we focused on non-premixed hydrogen/air combustion characteristics. However, the fuel/air mixing is significant in the combustion chamber, so we proposed five bluff bodies and investigated the effects of inlet velocity of the fuel on non-premixed hydrogen/air combustion characteristics and flow patterns. A 21-species and 105-reactions hydrogen/air with NOx extended chemical mechanism is experimentally validated. The results show a significant influence of the inlet velocity and bluff body shape on temperature distribution, flame location, and residence time. All five combustors achieved a 99% energy conversion at the exit of the combustion chamber. Combustors B and D characterized by isosceles triangle-like and right triangle-like shapes excel because of improved hydrogen/air mixing behind the bluff body. Combustor D achieves the highest mean outer wall temperature of 992.03 K and lowest pressure loss of 155.56 Pa when the inlet velocity of fuel is 4 m/s. Moreover, the advanced mixing facilitated by the bluff bodies in Combustors B and D resulted in reduced NO emissions, with reductions of 16.8% and 20.3%, respectively, compared to Combustor C. These findings provide insights into combustion dynamics, leveraging bluff body-induced flow structures to enhance the micro combustor thermodynamics.

Effect of bluff body embedded in flow channel on power performance of microbial fuel cell

The microbial fuel cell (MFC) has emerged as an eco-friendly method for generating clean energy from wastewater. However, its current power performance falls short of meeting commercial application requirements, highlighting the urgent need for power enhancement. To address this, the implementation of bluff bodies within the MFC channel is a proof of concept to enhance nutrient mass transport without the need for external energy input. In this study, fluid simulation was first conducted to analyze the impact of different bluff body designs on flow regimes. The results showing bluff body having spread-out six circles arrays with a height of 0.5 mm (6C_H5) led to a wide velocity distribution within the flow channel, high velocity, and no solute accumulation on the electrode surface, thereby facilitating increased nutrient coverage during transport. Through experimental confirmation, MFC with 6C_H5 achieved an impressive 154 % increase in maximum power density compared to without a bluff body. The outcome can be attributed to sufficient nutrient acquisition for the bacteria to generate more power due to better nutrient transport and a shear environment. The significant finding of the bluff body’s impact on the fluid dynamic aspect has proven the feasibility of enhancing MFC power performance.

Experimental study on the effect of cone-shaped bluff body on lean premixed flames in a swirl burner

Experimental study on the effect of cone-shaped bluff body on lean premixed flames in a swirl burner

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.

Effect of bluff body addition in fuel-rich stream on reaction behaviours of large-proportion semicoke rich/lean blended combustion

Fuel-rich/lean combustion is considered as a novel combustion technology due to its enhanced ignition performance and low NOx emissions. To further improve the poor performance of large-proportion semicoke and bituminous coal blended combustion and reduce NOx emissions, under a bias concentration ratio (BCR) of 2:1, an addition of bluff bodies at the fuel-rich side is proposed to numerically and experimentally investigate the turbulence-chemistry interaction behaviours of the fuel-rich/lean combustion regime in a 0.3 kW pilot-scale bias combustion furnace for various bluff body blockage ratios (BR = 0%, 11% and 21%). The results show that, increasing the blockage ratio evidently shortens the ignition delay time and burnout time of bituminous coal and semicoke, and the promotive effect of triangular bluff-body (BR = 21%) is greater. The turbulent Damköhler number Dat maximums decrease from 194 to 113 with increasing BR, denoting the regime is traditional combustion mode controlled by mixing. On the fuel-rich side, a local homogeneous MILD combustion zone (Dat <10) extends with increasing BR. For the heterogeneous combustion, the char-O2 reaction determined by diffusion/kinetics and the char gasification reaction controlled by kinetics both show a kinetically controlled trend with increasing BR. Moreover, with increasing BR, a decreasing in the NO concentration in the main reaction zone can be attributed to an enhanced recirculation rate Kv. There is a strong correlation between the maximum NO and the local homogeneous/heterogeneous Damkhöler number on the fuel-rich/lean side in the plane of maximum Kv. Increasing BR can evidently decrease the maximum NO concentration on the fuel-rich side due to the enhanced local MILD combustion regime, and thus reducing NOx exit emissions. Considering temperature, ignition, burnout, turbulence-chemistry interactions and NOx emissions, a triangular-shaped bluff body on the fuel-rich side (BR = 21%) is recommended rather than a cubic bluff body in the fuel-rich/lean co-combustion of semicoke.

Bluff-body effect on thermal and NOx emission characteristics in a micro-planar combustor fueled with premixed ammonia-oxygen

In this work, we examine the bluff body effect on thermal and NOx emission performance in a micro-planar combustor fueled with premixed ammonia/oxygen using three-dimensional numerical simulations. For this, a three-dimensional model is developed with a detailed chemical kinetic mechanism. The effects of 1) the presence/absence of the bluff-body, 2) its dimensionless height h and 3) its dimensionless axial location l are examined. It is found that the implementation of a bluff-body can significantly affects the thermal and NO emission characteristics. Up to 39.1 % reduction of NO formation and 41.6 K increase in the outer wall temperature (OWT) can be achieved as the bluff-body is optimized, when compared to the case in the absence of the bluff-body. Furthermore, increasing h leads to OWT being increased, but it can suppress the NO generation in some cases. In addition, increasing l is shown to be involved with a high OWT and a low NO concentration, mainly due to the variation in the flow field and thus heat transfer characteristics. Finally, the dimensionless pressure loss is strongly affected by the NH3 flow rate, and the dimensionless parameters h and l. This work reveals that implementing a proper designed bluff-body is an effective way to enhance thermal performance and reduce NOx emission.

The Effect of Swirling Air-to-Liquid Momentum Ratio on the Spray and Droplet Characteristics

The effect of swirl flow on the spray characteristics (structure, droplet diameter and droplet velocity) is experimentally investigated for varying air-to-liquid momentum ratios in this work. The diagnostic techniques employed include high-speed shadowgraphy and 1D-PDPA. A commercial pressure swirl injector is mounted in a swirl stabilized model gas turbine burner to investigate the spray characteristics with and without the presence of swirling flowfield under isothermal conditions. In the absence of the injector flow the burner produced a converging-diverging flowfield at the burner exit, influenced by the bluffbody effect near to the exit and the swirling intensity farther downstream. The investigations reveal an unmistakable influence of the swirling flow on the droplet size, velocity and spatial distribution. Under the investigated momentum flux ratios the conical spray structure is altered and the droplets size and velocity at each location changed with the spatial variation in the magnitude and nature of the swirling flowfield. In general fine droplets are produced near to the high velocity air inflow, and coarser droplets in the recirculation zone owing to the longer residence time. The mean axial velocity of the droplet reduced in presence of swirling flow, with the droplets showing negative velocities at downstream locations.

Effects of bluff-body on the thermal performance of micro thermophotovoltaic system based on porous media combustion

To improve thermophotovoltaic system efficiency, a porous media combustor with a bluff-body as the flame holder was proposed. A two-dimensional numerical simulation of premixed hydrogen/air combustion inside a planar porous media channel with the local thermal non-equilibrium model was conducted. Effects of the bluff-body flame holder on mean wall temperature and system efficiency were examined. By inserting the bluff-body, the blowout limits of the planar porous media combustor for ϕ = 0.6, 0.8, 1.0, 1.2 are increased by 33%, 19%, 12%, 20%, respectively. Numerical results show that the flame stabilization is dominated by flow and heat recirculation. Further analysis reveals the mechanism of heat transfer on flame stabilization due to three heat recirculation paths. Considering the view factor and the external quantum efficiency, the system efficiency with the bluff-body is increased by 14.72% as compared to the one without the bluff-body at ϕ = 0.8, vin = 4 m/s.

Effects of bluff bodies on the propagation behaviors of gaseous detonation

In this study, we report an experimental investigation of detonation propagation in a tube filled with bluff bodies. Experiments were carried out in a 6 m long rectangular cross-section (112 mm × 107 mm) tube. Firstly, the effect of an array of cube bodies (60 mm × 60 mm × 60 mm) on the detonation propagation characteristics were studied. Hydrogen, ethylene and acetylene mixed with air and hydrogen-oxygen diluted with argon were used as the test mixtures. Evenly spaced photodiodes were mounted on the top wall to recorded the optical signals, from which the detonation velocity could be determined. Soot foils were adopted in hydrogen-oxygen-argon mixture to record the evolution of the cellular structure. The results show that the flame accelerates rapidly in the obstructed tube. The critical conditions for deflagration to detonation transition (DDT) are found to be consistent with L/λ > 7, where L is the modified characteristic geometrical size for the tube with repeated cubes and λ is the detonation cell size. The long soot foils indicate that the detonation modes are influenced significantly by the bluff bodies. Near the limits, only some traces of transverse waves and shock waves can be observed, making the distinction between choked flame and quasi-detonation blurred. Secondly, the effect of a single bluff body on the detonation diffraction was investigated. Stoichiometric hydrogen-oxygen and those diluted with argon at sub-atmospheric pressures were used as the test mixtures. In the vicinity of the bluff body, a soot foil was used to record the detonation cellular structure evolution, from which the effect of the geometry of the bluff body on the diffraction and re-initiation processes was studied. Three transition regimes are observed: (1) a detonation propagates continuously over the bluff body; (2) the cellular structure initially fails and then recover due to the two symmetrical diffraction waves collision or interaction between shock waves and the bottom wall; (3) a detonation quenches and then re-initiate by flame acceleration. The re-initiation distance was found to be dependent on mixture sensitivity (chemical length scale) and the geometry of the bluff bodies (physical length scale).

Wind tunnel study of plume dispersion with varying source emission configurations

The concentration fields in the proximities of a local gas emission source are experimentally analyzed in several combinations of wind incidences and source emissions. These conditions are determined by the plume buoyancy, emission velocity and incident flow wind speed. Concentration measurements are performed by an aspirating probe in a boundary layer wind tunnel. The analysis included the mean concentration values and the intensity of concentration fluctuations in a neutral atmospheric boundary layer flow. Different configurations are tested: an isolated stack in a homogeneous terrain and a stack with a bluff body in close proximity, located windward and leeward from the emission source. The experimental mean concentration values are contrasted with Gaussian profiles and the dilution factor is analyzed with respect to the empirical curves of the minimum dilution. Finally, a study on the plume intermittency is performed in a cross-sectional plane near the emission source. It is possible to highlight the following observations: a) plume vertical asymmetry in the case of an isolated emission source, b) significant differences in the dispersion process related to the relative location of the emission source and bluff body effects, and c) different probabilistic behavior of the concentration fluctuation data in a cross-sectional measurement plane inside the plume.

Airflow Dynamics over a Beach and Foredune System with Large Woody Debris

Airflow dynamics over beach-foredune systems can be complex. Although a great deal is known about the effects of topographic forcing and vegetation cover on wind-field modification, the role of large woody debris (LWD) as a roughness element and modifier of boundary layer flow is relatively understudied. Individual pieces of LWD are non-porous elements that impose bluff body effects and induce secondary flow circulation that varies with size, density, and arrangement. Large assemblages of LWD are common on beaches near forested watersheds and collectively have a degree of porosity that increases aerodynamic roughness in ways that are not fully understood. A field study on a mesotidal sandy beach with a scarped foredune (Calvert Island, British Columbia, Canada) shows that LWD influences flow patterns and turbulence levels. Overall mean and fluctuating energy decline as flow transitions across LWD, while mean energy is converted to turbulent energy. Such flow alterations have implications for sand transport pathways and resulting sedimentation patterns, primarily by inducing deposition within the LWD matrix.

Non-premixed flame characteristics and exhaust gas concentrations behind rifled bluff-body cones

Four bluff-body cones with/without rifled v-grooves were installed behind a non-premixed traditional combustion nozzle to intensify the bluff-body effects and swirl flow. The spiral rifles transformed axial momentum (or axial velocity) into tangential momentum (or tangential velocity). The interaction between the fuel tangential component and axial air flow increased turbulence intensity (T.I.). The Schlieren photography was utilized to visualize the flame structures and classify three flame patterns—jet flame, recirculation flame, and turbulence flame. The jet flame occurs when fuel-jet velocity is high and air-jet velocity (ua) is low. However, the turbulence flame exists at the high air-jet velocity. The flame lengths were measured using the direct photography scheme. The flame length at high ua is significantly shorter than that at low ua. Furthermore, the increase of rifle number (i.e., increasing T.I.) induces the high maximum temperature and low nitric-oxide concentration.

Effect of central cone-shaped bluff body on performance of premixed LPG burner

Improvement of air entrainment rate for liquefied petroleum gas (LPG) premixed burner was achieved by placing a needle rod inside a circular fuel jet nozzle. To investigate the effect of needle rod on flow entrainment, selection of experimental results together with investigation on jet characteristics using CFD are presented. The predictive quality was achieved after performing grid refinement and validation of turbulent model against existing experimental results. Two modified standard k-ε turbulent models were employed for prediction of the jets. The results indicated that the annular port created narrower jet pattern with higher degree of penetration. The momentum decay rate was significantly lower with an average of 48% at z/D=30, for instance, corresponding to lower Reynolds shearing stress associated with jet shape upstream. This corresponded to the vorticity magnitude of the inner core that caused transport of jet momentum into the central region of the jet near the exit plane. In contrast, the circular one performed more expanded pattern together with faster decay rate. This expanded jet created limitation of air entrainment when issuing into a confined duct, while deeper penetration of the annular jet created greater entrainment. An averaged improvement of air entrainment was found at 25% for the range of firing rate between 1.8 and 4.4kW.

Numerical investigations on effects of bluff body in flat plate micro thermo photovoltaic combustor with sudden expansion

In order to reveal combustion characteristics of H2/air mixture in a micro-combustor with and without bluff body, the effects of inlet velocities, equivalence ratios and bluff body’s blockage ratios on the temperature field, pressure of the combustor wall, combustion efficiency and blow-off limit were investigated. The numerical results indicate that the sudden expansion plate micro combustor with bluff body could enhance the turbulent disturbance of the mixed gas in the combustion chamber and the combustion condition is improved. Moreover, a low-speed and high temperature recirculation region was formed between the sudden expansion step and the bluff body so that the high and uniform wall temperature (>1000 K) could be gotten. As a result, it could strengthen the mixing process, prolong the residence time of gas, control the flame position effectively and widen the operation range by the synergistic effect of the bluff body and steps. When the blockage ratio ranged from 0.3 to 0.6, it could be found that the bluff body could play a stabilizing effect and expand combustion blow burning limit, and combustion efficiency firstly was increased with the inlet velocity and equivalence ratio, and then was decreased.

예혼합 화염 날림 메커니즘에 음향 가진이 미치는 영향에 대한 실험적 연구

In this study, blowoff was investigated in a ducted combustor with the bluff body when acoustic excitation was forced. To observe the flame structure, OH radical chemiluminescence was used and the image was analyzed by using POD (Proper Orthogonal Decomposition) algorithm. Natural gas mainly composed of methane was used as fuel. Blowoff occurred when the equivalence ratio was reduced. Equivalence ratio causing blowoff was measured by changing air flow rate, excitation frequency and sound pressure. Blowoff equivalence ratio was varied depending on the experimental conditions. Vortex frequency behind the bluff body and resonance effect in combustor are the main factors that affect the blowoff equivalence ratios with the excitation.

Arc-Groove를 가진 D-형상 물체 주위 유동에 관한 연구

Object of this study is to see whether the arc-groove on a D-shape bluff body effects the drag reduction or not. To this end, the changes of the boundary layer on the surface of the object, the downstream flow field and wake flow were found by experiments. The experiments are conducted by changing number and depth of the groove, angle of the first groove and Reynolds number(Re). Groove did not effect on the surface in the downstream boundary layer of the object and was minimal impact on the time mean velocity recovery of the wake flow. Also the effects of Groove did not have a significant impact on the structure of the wake and the wake frequency. Therefore it is found that the arc-groove of the drag reduction effect on the D-shape bluff body was smaller.

Investigation of bluff-body micro-flameless combustion

Characteristics of lean premixed conventional micro-combustion and lean non-premixed flameless regime of methane/air are investigated in this paper by solving three-dimensional governing equations. At moderate equivalence ratio (∅=0.5), standard k–ε and the eddy-dissipation concept are employed to simulate temperature distribution and combustion stability of these models. The effect of bluff-body on the temperature distribution of both conventional and flameless mode is developed. The results show that in the premixed conventional micro-combustion the stability of the flame is increased when a triangular bluff-body is applied. Moreover, micro-flameless combustion is more stable when bluff-body is used. Micro-flameless mode with bluff-body and 7% O2 concentration (when N2 is used as diluent) illustrated better performance than other cases. The maximum temperature in premixed conventional micro-combustion and micro-flameless combustion was recorded 2200K and 1520K respectively. Indeed, the flue gas temperature of conventional mode and flameless combustion was 1300K and 1500K respectively. The fluctuation of temperature in the conventional micro-combustor wall has negative effects on the combustor and reduces the lifetime of micro-combustor. However, in the micro-flameless mode, the wall temperature is moderate and uniform. The rate of fuel–oxidizer consumption in micro-flameless mode takes longer time and the period of cylinders recharging is prolonged.

Improving Combustion Intensity and Modulating Flame Behaviors Using Helical-Grooved Cones

AbstractFour helical-grooved cones were installed behind an unconfined combustion nozzle to increase the bluff-body effect and turbulence intensity (T.I.). The cone configurations included a smooth cone and the other three cones cut with 1, 2 and 3 helical v-grooves. Experimental results showed that the helical v-grooves transformed the axial momentum (or the axial velocity) to the angular momentum (or the angular velocity). TheT.I.was enhanced by increasing the tangential component of fuel-jet momentum. The direct photography and thermocouple were utilized to observe the flame structures and to delineate the characteristic flame modes, flame length, temperature distribution, and combustion intensity. The flame modes were classified as jet flame, flickering flame, bubble flame, recirculation flame, lifted flame and ring flame. The flame length decreases as the groove number increases. The increasedT.I.and groove number (or bluff-body effect) improve the fuel-air mixing. The total combustion intensity increases with annular-air jet and with the groove number.

Effects of Acoustic Excitation on a Swirling Diffusion Flame

A swirling double concentric jet is commonly used for nonpremixed gas burner application for safety reasons and to improve the combustion performance. Fuel is generally spurted at the central jet while the annular coflowing air is swirled. They are normally separated by a blockage disk where the bluff-body effects further enhance the recirculation of hot gas at the reaction zone. This paper aims to experimentally investigate the behavior of flame and flow in a double concentric jet combustor when the fuel supply is acoustically driven. Laser-light sheet assisted Mie scattering method has been used to visualize the flow, while the flame lengths were measured by a conventional photography technique. The fluctuating velocity at the jet exit was measured by a two-component laser Doppler velocimeter. Flammability and stability at first fuel tube resonant frequency are reported and discussed. The evolution of flame profile with excitation level is presented and discussed, together with the reduction in flame length. The flame in the unforced reacting axisymmetric wake is classified into three characteristic modes, which are weak swirling flame, lifted flame, and transitional reattached flame. These terms reflect their primary features of flame appearances, and when the acoustic excitation is applied, the flame behaviors change with the excitation frequency and amplitude. Four additional characteristic modes are identified; e.g., at low excitation amplitudes, wrinkling flame with a blue annular film is observed because the excitation induces vortices in the central fuel jet and hence gives rise to the wrinkling of flame. The central jet vortices become larger with the increase in excitation amplitude and thus lead to a wider and shorter flame. If the excitation amplitude is increased above a certain value, the central jet vortices change the rotation direction and pacing with the annular jet vortices. These changes in the flow field induce large turbulent intensity and mixing and therefore make the flame looks blue and short. Further increase in the excitation amplitude would lift the flame because the flow field would be dramatically modified.