Large eddy simulation of swirling jet in a bluff-body burner

The large eddy simulation (LES) is applied to an unconfined swirling flow of an air surrounding a bluff-body having a central jet of air, and the complicated flowfield that involves the recirculation and vortex breakdown is investigated. The Smagorinsky model is used as the sub-grid scale model. The results of the present numerical simulation are compared with the experimental data of the mean and stochastic root mean square (RMS) variations of two velocity components. Although the inflow conditions are specified in a simple manner, the obtained numerical results are in reasonable agreement with the experiments, except for a part of RMS variation values near downstream of the bluff body. The present numerical calculations can successfully reproduce the two characteristics of the flow, i.e., an upstream recirculation zone established just downstream of the burner plane and the additional recirculation zone established at the more downstream location.     

Parametric Study of Turbulent Flow in a Three—Dimensional Side—Dump Combustor

Turbulent flows in a three-dimensional side-dump combustor under various side-inlet angles and head heights are presented in terms of the axial mean velocity,swirl intensity,size of the head recirculation zone,and fraction of the inlet mass flow rate transported into the head region.The turbulence model adopted is the algebraic Reynolds stress(κ-ε-A）model,The effect of the side-inlet angle and the head height on the characteristics of the confined impinging flows are documented and some improved explanations to the existence of the optimum inlet angle and head height are addressed in this study.     

Experimental investigation on ignition and lean blow-out performance of a multi-sector centrally staged combustor

Improvement on extinction and pollution emission have become one of the most prominent research topics in gas turbine.It is widely recognized that the fuel/air mixture distribution in the recirculation zone is a critical factor in improving lean blow-out（LBO） and ignition.This paper proposed a new low emission scheme with fuel staged centrally and hybrid injector to improve flameout and emission.A relative small amount of fuel enters into central pilot airblast atomizer burner and then atomized by inner swirl air.The remaining majority of fuel is directly injected into vane channels of the primary swirler through a series of holes located on the sidewall of the main stage.Only pilot stage is fueled under ignition and lean flameout condition.The uniformity of fuel/air mixture distribution in the primary zone of the new design decreases NOX emission,meanwhile the fuel air mixture in pilot recirculation zone is locally rich to improve flameout and ignition.Experimental investigation was conducted to compare the new scheme with baseline design of dual-swirler in terms of LBO and ignition characteristics under the same condition in a multi-sector combustor.It is found that the fuel-air ratio of ignition limit and LBO decrease with the reference velocity increasing.The experimental results also show that the new scheme successfully improve lean blow-out and broaden the operation range of the combustor.The experimental results indicated that the centrally staged scheme can widen the operation boundary of the combustor and can provide guidance for design and optimization of combustion chamber.     

Numerical and Experimental Modelling of Gas Flow and Heat Transfer in the Air Gap of an Electric Machine

This work deals with the cooling of high-speed electric machines, such as motors and generators, through an air gap. It consists of numerical and experimental modeling of gas flow and heat transfer in an annular channel. Velocity and temperature profiles are modeled in the air gap of a high-speed test machine. Friction and heat transfer coefficients are presented in a large velocity range. The goals are reached acceptably using numerical and experimental research. The velocity field by the numerical method does not match in every respect the estimated flow mode. The absence of secondary Taylor vortices is evident when using time averaged numerical simulation.     

Experimental Investigations of the Flow Field Structure and Interactions between Sectors of a Double-Swirl Low-Emission Combustor

In this paper,the flow field characteristics of a double-swirl low-emission combustor were analyzed by using Particle Imaging Velocimetry(PIV)technology in an optical three-sector combustor test rig.The interactions between sectors and the flow field structure were explained.The results illustrated that there was a big difference between the flow field structures of the middle sector and the side sector under the same pressure drop,which was mainly induced by the interactions between sectors.The interactions made the swirl intensity of the middle sector weaker than that of the side sector,which made the recirculation zone of the middle sector be smaller than that of the side sector.With the increase of swirler pressure drop,the jet velocity at the exit of the swirler,the jet expansion angle,the width of the recirculation zone and the recirculating speed of the central axis became larger,enhancing the interactions between air streams from middle sector and side sector.The flow velocity in the central plane between sectors was small,especially the radial velocity,mainly because of the loss of the swirl intensity by the interactions between flow field of adjacent sectors.The expansion angle determined the position of the vortex in the primary recirculation zone;the axial and radial position of the vortex move downstream and radial outward with the increase of the jet expansion angle.The results of the mechanism of flow field organization in this study can be used to support the design of new low-emission combustor.     

Aerodynamic study on supersonic flows in high-velocity oxy-fuel thermal spray process

To clarify the characteristics of gas flow in high velocity oxy-fuel (HVOF) thermal spray gun, aerodynamic research is performed using a special gun. The gun has rectangular cross-sectional area and sidewalls of optical glass to visualize the internal flow. The gun consists of a supersonic nozzle with the design Mach number of 2.0 followed by a straight passage called barrel. Compressed dry air up to 0.78 MPa is used as a process gas instead of combustion gas which is used in a commercial HVOF gun. The high-speed gas flows with shock waves in the gun and jets are visualized by schlieren technique. Complicated internal and external flow-fields containing various types of shock wave as well as expansion wave are visualized.     

Prediction of acoustic absorption performance of a perforated plate with air jets

The present study focuses on the prediction of acoustic absorption performance of a perforated plate with air jets by theoretical calculations.In addition,we experimentally measured the flow rate,internal pressure,acoustic pressure,and transfer function using an acoustic impedance tube.The normal incidence absorption coefficient was calculated from the measured transfer function using transfer function methods.We investigated the influences of background air space,flow velocity,thickness,aperture rate,and aperture diameter of a perforated plate on the acoustic absorption characteristics.The frequency characteristics of the acoustic absorption coefficient showed a maximum value at a local frequency.As the background air space increased,the peak frequency of acoustic absorption characteristics decreased.As the flow velocity passing through the apertures increased,the peak level of the acoustic absorption coefficient also increased.The theoretical results agreed well with the experimental ones qualitatively.     

Correlation analysis of transient heat transfer characteristics for air precooling aggregate

In dam works,air precooling of aggregate is a common and effective method to avoid temperature cracks in concrete structure.In order to offer a reliable design theory for the air precooling process to avoid unreasonable energy consumption,the transient heat transfer characteristics of the aggregate are intensively analyzed.The combined structure of the aggregate and the interstitial space in the hopper is treated as a porous structure,and the space-average method is used to simulate the transient heat transfer process.Simulation results show that size of the hopper and the average air velocity in the cross section have great influence on the transient heat transfer process of the aggregate,while the porosity in the range of 0.4 0.5 has little influence.Nomograms are abstracted from simulation results,and then correlations of the compared excess temperature are precisely fitted to predict the air precooling transient heat transfer process of the aggregate.     

Dynamic PIV Measurement of a Compressible Flow Issuing from an Airbag Inflator Nozzle

Among many equipment for passenger safety,the air bag system is the most fundamental and effective device foran automobile.The inflator housing is a main part of the curtain-type air bag system,which supplies high-pres-sure gases in pumping up the air bag-curtain which is increasingly being adapted in deluxe cars for protectingpassengers from the danger of side clash.However,flow information on the inflator housing is very limited.Inthis study,we measure the instantaneous velocity fields of a high-speed compressible flow issuing from the exitnozzle of an inflator housing using a dynamic PIV system.From the velocity field data measured at a highframe-rate,we evaluate the variation of the mass flow rate with time.The dynamic PIV system consists of ahigh-repetition Nd:YLF laser,a high-speed CMOS camera,and a delay generator.The flow images are taken at4000 fps with synchronization of the trigger signal for inflator ignition.From the instantaneous velocity field dataof flow ejecting from the airbag inflator housing at the initial stage,we can see a flow pattern of broken shockwave front and its downward propagation.The flow ejecting from the inflator housing is found to have very highvelocity fluctuations,with the maximum velocity at about 700 m/s.The time duration of the high-speed flow isvery short,and there is no perceptible flow after 100 ms.     

The Impingement Heat Transfer Data of Inclined Jet in Cooling Applications: A Review

On the impingement heat transfer data,the experimental studies of air and liquid jets impingement to the flat surfaces were collected and critically reviewed.The oblique impingements of both single circular and planar slot jets were considered in particular.The review focused on the surface where the jet impingement cooling technique was utilized.The nozzle exit Reynolds numbers based on the hydraulic diameter varied in the range of 1,500–52,000.The oblique angles relative to the plane surface and the dimensionless jet-to-plate spacing vary in the range of 15°–90°and 2–12 respectively.The review suggested that the magnitude of maximum heat transfer shifted more for air jets compared with the liquid jets.The drop in the inclination angle and the jet-to-plate separation led to the increase in the asymmetry of heat transfer distribution.The displacement of maximum Nusselt number(heat transfer)locations was found to be sensitive to the inclination angle and the smaller jet-to-plate distance.Also,the Nusselt number correlations proposed by various researchers were discussed and compared with the results of the cited references.     

The jet mixing effect on reaction flow in a bluff-body burner

This study investigates the mixing effects of primary and secondary jets on flame stability as fuel/air is injected into a bluff-body burner. A two-dimensional spray combustion model, based upon a SIMPLER method, is used for numerical studies of combusting flow. The influence of the jet-to-air velocity ratio on the recirculation zone behind the bluff body, the center axial velocity and the temperature profiles is studied in detail. The results show that mixing between the two jets is controlled by two vortex eddies on the inside and outside of the bluff-body. With a proper bluff-body blockage ratio, cone angle and jet-to-air velocity ratio a more stable flame can be achieved.     

Mixing and Recirculation Characteristics of A double COncentric Burner with Bluff—Body

The concentric bluff-body jet burner is widely used in industrial combustion systems. This kind of burner often generates a considerably complex recirculation zone behind the bluff body. As a result, the fuel often remains in the recirculation zone, achieving stability of flame. This study investigates, by means of experiments, the variations of the aerodynamics as the fluid is injected into a combustion chamber through a double concentric burner with a bluff-body. The observation and measurement of the aerodynamics in our experiment are conducted under a cold flow. The controlled parameters in our experiment are: variations in the blockage ratio of the center bluff body, the cone angle of the bluff body, and the velocity ratio (U _s/U_p) of the secondary jet and primary jet; the injection of helium bubbles into the primary and secondary jets to observe the recirculation zone behind the bluff body; using Tufts for observing the characteristics of corner recirculation zone in a combustion chamber, measuring the average velocity of each point within the aerodynamics by the 5-hole pitot tube; measuring the distribution of static pressure of the combustion chamber walls with a static pressure tap.     

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

The premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two-dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.     

钝体形状对旋流燃烧器出口流场影响的数值研究

借助CFD软件,对配置中心钝体的双通道旋流燃烧器出口流场进行了冷态数值模拟,分别获得了锥型、楔型和抛物线型三种钝体后的流场分布、速度分布、中心回流区尺寸和中心回流量大小等结果.经对比分析发现：在其他条件相同的情况下,非流线形的锥型钝体和楔型钝体与接近流线形的抛物线型钝体相比,中心回流区的长度分别增加了24%和17%;中心最大回流量分别增加了18.3%和6.5%.在将抛物线型钝体燃烧器的旋流二次风量加大50%时,其中心回流区长度虽有所增加,但仍不能达到锥型钝体和楔型钝体的相当程度.研究结果表明：非流线型钝体比流线型钝体具有更大的中心回流区尺寸和回流量;钝体形状是影响出口流场特性的关键因素;与楔型钝体和抛物线型钝体相比,锥型钝体具有更好的稳燃效果.图7表1参7     

Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body

Combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body were numerically investigated. Results reveal that the blow-off limit of the micro-combustor with a regular triangular pyramid bluff body is 2.4 times of that in the micro-combustor without bluff body. With the increase of inlet velocity, the recirculation zone expands and preferential transport effect behind the bluff body is intensified. Therefore, the local equivalence ratio in the recirculation zone increases when Φ = 0.8, but the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds 10 m/s. When Φ < 1.0, adding small amount of hydrogen into gas mixture can speed up the significant elementary reaction, leading to an increase of methane conversion. It's found that both the methane conversion rate and the temperature behind the bluff body reaches the highest when blockage ratio increase to 0.22.     

Cold flow characteristics of a novel bluff body hydrogen burner

The cold flow characteristics of a novel partial premixed bluff body (PPBB) low NO_x burner, capable of operating with hydrogen as well as methane-hydrogen blends, were investigated numerically. The PPBB burner features a frustum shaped conical bluff body generating a flame stabilizing recirculation zone. Fuel is partially premixed via jets in an accelerating cross-flow. Steady-state and transient non-reacting simulations using five different turbulence models, i.e. standard k-ε, realizable k-ε, shear stress transport (SST) k-ω, stress-blended eddy simulation (SBES) and large eddy simulation (LES), were conducted. The simulations were validated against particle image velocimetry (PIV) measurements of an unconfined non-reacting flow. All turbulent models were able to predict the recirculation zone length in good agreement with the experimental data. However, only scale resolving simulations could reproduce velocity magnitudes with sufficient accuracy. Time averaged and instantaneous results from the scale resolving simulation were analysed in order to investigate flow characteristics that are special about the PPBB burner design and of relevance for the combustion process. Two different burner configurations were studied and their effects on the flow field were examined. The recirculation zone volume as well as the entrainment into the wall jet around the bluff body were found to correlate with the elevation of the bluff body relative to the burner throat. Both of these parameters are expected to have a strong impact on the overall NO_x emission, since the near burner region is typically one of the main contributors to the NO_x formation.     

Study on strengthening combustion and NOx formation properties of rough‐surfaced bluff‐bodies

A new type of pulverized coal burner combining a bend with a rough surface taper 60° bluff‐body has been designed. The cold state test showed that behind the rough‐surfaced bluff‐body there exists a larger recirculation zone, recirculation ratio and stronger turbulence, but its pressure drop increases by about 15–20 per cent more than that of the smooth‐surfaced bluff‐body. The hot state test in a one‐dimensional furnace indicates that the rough‐surfaced bluff‐body has much faster ignition, higher flame temperature and higher burnout ratio. A heat balance model was established using a lumped parameter method for the recirculation zone. Through it, the condition of the primary air was derived for flame stability and the strengthening combustion ability of rough surface in bluff‐body was proved. On the basis of a hot state test, numerical stimulation was performed on NO_x formation. This suggests that rough‐surfaced bluff‐body only increases the production of NO_x a little. Copyright © 2001 John Wiley & Sons, Ltd.     

Annular Impinging Jet Controlled by Radial Synthetic Jets

This experimental study focuses on generation and control of annular impinging jets. The annular nozzle used in the investigations was designed with an active flow control system using 12 synthetic jets issuing radially from the central nozzle body. Measurements of the control effects were made on the impingement wall. The data acquisition involved wall pressure and wall mass transfer (by the naphthalene sublimation technique) using air as the working fluid. Also measured was time-mean flow velocity (by a Pitot probe) in the jet flow field. Moreover, flow visualization was carried out. Two main flow-field patterns (A and B) were identified. The patterns differ in the size of the separated-flow recirculation regions that develop attached to the nozzle central body: While pattern A is characterized by a quite small recirculation region (bubble) extending not far from the nozzle exit, pattern B exhibits a large recirculation region, reaching up to the impingement wall, on which it forms a stagnation circle. The control action modifies the flow field, resulting in changes of the corresponding heat/mass transfer distributions. The convective transfer rate on the stagnation circle can be demonstrably enhanced by 20% at a moderate nozzle-to-wall distance, equal to 0.6 of the nozzle outer diameter.     

Numerical study on premixed hydrogen/air combustion characteristics in micro–combustor with slits on both sides of the bluff body

An approach to improve premixed hydrogen/air combustion in micro combustor is numerically studied in this paper. The micro–combustor with slits on both sides of the bluff body shows better combustion efficiency and uniformity of temperature distribution. The effects of the controllable flow ratio (γ) and the angle of bluff body (θ) on combustion characteristics are investigated by using a two–dimensional model with the H₂/O₂ reaction mechanism. The results show that the increase of controllable flow ratio and angle of bluff body can improve combustion efficiency and decrease velocity extinction limit. However, at higher θ, increasing γ do not play an important role in improving combustion efficiency. In addition, at higher inlet velocity, combustion efficiency do not increase dramatically with the increase of θ. Moreover, at high inlet velocity, a special phenomenon of temperature ‘waist’ is observed in the micro–combustor with slits on both sides of the bluff body, which has a huge impact on combustion characteristics. Therefore, controllable flow ratio and angle of bluff body should be reasonably chosen to improve combustion characteristics.