Swirling Air Flow Research Articles

スワーラーを装着したブラフボディ型燃焼器における火炎の安定性(G.S. 反応・燃焼機構,OS5:宇宙環境の高度利用を目指して)

スワーラーを装着したブラフボディ型燃焼器における火炎の安定性(G.S. 反応・燃焼機構,OS5:宇宙環境の高度利用を目指して)

Dry Flowing Abrasive Decontamination Technique for Pipe Systems with Swirling Air Flow

A dry abrasive decontamination method was developed for removing radioactive corrosion products from surfaces of coolant pipe systems in decommissioning of a nuclear power plant. Erosion behavior of inside surfaces of stainless and carbon steel pipes by a swirling air flow containing alumina or cast-iron grit abrasive was studied. Erosion depths of the test pipes were approximately proportional to an abrasive concentration in air and an exponent of flow rate of airstream. The experimental results indicated that the present method could keep satisfactory erosion ability of abrasives even for a large-size pipe. The present method was successfully applied to 60Co-contaminated specimens sampled from a pipe of the water cleanup system of the Japan Power Demonstration Reactor.

P32-05 超小型ガスタービン用アニュラー型要素試験燃焼器の試作

We have proposed an annular-type hydrogen-fueled combustor of a micro gas turbine with a 3 kW output for distributed electrical power generators, and build its sector model test combustor to determine the combustion characteristics. Excellent performance concerning flame stability and heat release rate was attained, but the temperature distribution was not uniform even in the downstream of the combustor. The latter was found to be due to the combination and separation of the adjoined flames by flow field interaction between the swirling air flows with same direction. In addition, using the same test combustor, we preliminary studied the feasibility of a propane-fueled micro combustor. The results showed that the stability and length of the propane flame in the combustor remarkably lowered. Combustion technique improving the combustion characteristics was obtained by an additional experiment.

Lateral solids dispersion in a high-density riser with swirling air flow

Lateral mixing of solids in a high-density riser is investigated by a phosphor tracer method. The lateral solids dispersion is determined by measuring the solids RTD at different radii using point sources for tracer injection. A two-dimensional dispersion model describes the measured RTD curves satisfactorily. The cross-sectional average solids fraction has the dominant effect on the lateral solids mixing in the high-density riser. The lateral Peclet number increases exponentially with the solids fraction and is not sensitive to gas velocity and solids circulation rate. The lateral Peclet number in high-density riser is over 1200, indicating that the lateral mass transfer is a limitation factor for the application of high-density riser as a high efficient reactor. Correlation of the lateral Peclet numbers fits the experimental data well. When swirling air is introduced into the riser, the lateral Peclet number decreases and the lateral solids mixing is intensified by the swirling flow.

Effects of Fuel Nozzle Displacement on Pre-Filming Airblast Atomization

In gas turbine combustors, optimum arrangement between a fuel nozzle and a swirler/prefilmer module must be sought to achieve satisfactory ignition and stability characteristics in addition to reduced level of emissions. However, due to thermal expansion of the combustor or misalignment of the fuel nozzle, the location of a fuel nozzle may vary. Displacement of a fuel nozzle may change the amount of fuel injected to the pre-filming device (usually the inner swirler wall) and the location of attachment, which in turn affects the thickness of pre-filming liquid sheet on the wall. As a result, the spray structure formed by pre-filming airblast atomization may be significantly changed. An experimental investigation is carried out to study the effects of fuel nozzle displacement on the structure of a spray formed by a dual orifice pressure atomizer and a counter-rotating dual swirler. The inner wall of the swirler is designed to be used as a pre-filming device. The behavior of droplets, the flow characteristics of the swirling air flow, and the interaction between droplets and the flow are studied. Optical diagnostic methods including a flow visualization and an Adaptive Phase/Doppler technique are used. Distributions of droplet size, number density, and liquid phase volume flux are presented for various fuel nozzle displacements, in addition to gas phase velocity.

Discrete‐vortical elements simulation of air jet interaction with swirling air motion inside a 2D cylinder

A computational procedure based on a hybrid Lagrangian‐Eulerian discrete‐vortical element formulation and conformal transformation schemes are employed in this study to simulate the interaction of an air jet with swirling air flow inside a two‐dimensional cylinder. Such an investigation is of importance to many flow‐related industrial and environmental problems, such as mixing, cooling, combustion and dispersion of air‐borne or water‐borne contaminants because of the role of vortices in the global transport of matter and heat. The basis for the simulation is discussed and numerical results compared with theoretical results for the velocity field and streamfunction obtained by the method of images. The swirling air motion and the features of a real jet are well simulated and numerical results are validated by predictions of theory to within 20 per cent. To illustrate the merging and interaction processes of vortices and the formation of large eddies, velocity vectors, particle trajectories and streamline contours are presented.

Velocity measurements in a strongly swirling natural gas flame

The present study was focused on the turbulent velocity field of a central annular natural gas jet which penetrated a strongly swirling air flow. Due to the high swirl number S=0.95 and the high momentum ratio, the fuel jet was almost immediately integrated into the air stream. High rates of shear resulted in an intensive turbulent mixing process between natural gas and air. The central hub of the fuel exit annulus stabilized the reverse flow zone at a fixed location. The present nozzle configuration resulted in a very stable and symmetric flame.

Study of Flame Stability in a Step Swirl Combustor

A prime requirement in the design of a modern gas turbine combustor is good combustion stability, especially near lean blowout (LBO), to ensure an adequate stability margin. For an aeroengine, combustor blow-off limits are encountered during low engine speeds at high altitudes over a range of flight Mach numbers. For an industrial combustor, requirements of ultralow NOx emissions coupled with high combustion efficiency demand operation at or close to LBO. In this investigation, a step swirl combustor (SSC) was designed to reproduce the swirling flow pattern present in the vicinity of the fuel injector located in the primary zone of a gas turbine combustor. Different flame shapes, structure, and location were observed and detailed experimental measurements and numerical computations were performed. It was found that certain combinations of outer and inner swirling air flows produce multiple attached flames, aflame with a single attached structure just above the fuel injection tube, and finally for higher inner swirl velocity, the flame lifts from the fuel tube and is stabilized by the inner recirculation zone. The observed difference in LBO between co- and counterswirl configurations is primarily a function of how the flame stabilizes, i.e., attached versus lifted. A turbulent combustion model correctly predicts the attached flame location(s), development of inner recirculation zone, a dimple-shaped flame structure, the flame lift-off height, and radial profiles of mean temperature, axial velocity, and tangential velocity at different axial locations. Finally, the significance and applications of anchored and lifted flames to combustor stability and LBO in practical gas turbine combustors are discussed.

A differential-pressure probe for velocity measurements in swirling air flows

For measuring air velocities in swirling flows a differential pressure probe of small axial length was developed. The determination of the velocity is based on measuring the difference between the stagnation pressure and the base pressure of a circular disk whose surface is perpendicular to the main flow direction.

A differential-pressure probe for velocity measurements in swirling air flows

A differential-pressure probe for velocity measurements in swirling air flows

Development and experimental verification of momentum, heat and mass transfer model in spray drying

A mathematical model of momentum, heat and mass transfer in the atomization zone was proposed. Uneven distribution of particles and entrainment effects were taken into account in the model. To verify the model an extensive experimental investigation was performed on water evaporation at different initial air temperatures, feed rates, flow rates of the drying agent and different parameters of atomization: spray cone angle and initial particle size distribution. Changes in air temperature inside the stream of sprayed material, material temperature, evaporation rate, and changes in Sauter diameter and distribution of particle diameters vs. their distance from the atomizer were determined experimentally. Damping screens allowed a flat profile of air flow rate in the tunnel to be obtained. Good agreement between experimental results and theoretical data was achieved. In the paper an attempt was made to apply the model to the calculation of spray drying in a pilot plant dryer. The air supplied to the dryer tangentially to the axis was characterized by a high swirl. The model of air flow in the dryer was determined theoretically using the Computational Fluid Dynamics CFX program. Results obtained in this way were used in our own model to determine particle trajectories in the dryer and to solve heat and mass balances for the continuous and dispersed phases. In order to consider swirl air flow pattern in the dryer, changes in heat and mass balance were made for the particle—drying agent system. The model was verified experimentally on the basis of results of investigations on drying of a 20% solution of sodium chloride. Additionally, our own results were compared with those obtained from the particle source in the cell type model for the drying system being studied. It was found that our model well described the process of drying in the analyzed system. Results obtained from our model were also very similar to the results obtained from the particle source in the cell type model. An extensive literature survey on spray drying, including unsteady state phenomena, is presented in the paper. Main sources of errors occurring in modelling of the spray drying process were discussed. A critical estimation of the existing mathematical models of the process was made and some of them were described in detail.

Velocity Characteristics of a Confined Highly-Turbulent Swirling Flow Near a Swirl Plate (Data Bank Contribution)

Axial and tangential components of the velocity vector are measured using a Laser Doppler Velocimeter (LDV) system in a confined highly turbulent isothermal swirling flow near a swirl plate. The flow has essential features of swirl-stabilized flame combustors. Throughout this study, a constant “nominal” swirl number of 0.36 is generated by air jets from a set of slots in a swirl plate. A low-speed coflowing air, referred to as dilution air, is uniformly distributed around the swirling flow by use of an annular-shaped honeycomb. Three different swirling air flow rates with a fixed dilution flow rate are studied and results are discussed. Detailed mean axial and tangential velocity profiles at several axial locations show that the size and the strength of the central recirculation zone are strongly dependent on the swirling air flow rate. Increasing the swirl air flow rate increases both the radial extent and the axial length of the central recirculation zone. Mean total and reversed air flow rates are calculated by integrating the mean axial velocity profiles. In the setup used in this study and up to the axial positions investigated, the reversed flow rate as a percent of the total flow rate seems to be linearly proportional to the reversed-flow zone area, being independent of the swirl air flow rate at a fixed nominal swirl number value. As swirl air flow rate is increased, the root mean square (rms) of the axial and tangential velocity fluctuations increase monotonically at almost all radial positions except sufficiently away from the swirl plate and near the chamber axis. Several velocity biasing correction methods are reviewed. A simple velocity biasing correction scheme is applied in this study to investigate its effect on the conclusions reached in the study.

Experiments on Spray Combustion in a Gas Turbine Model Combustor

Abstract A hollow-cone kerosene spray with a nominal cone angle of 30 degrees from a pressure-swirl fuel atomizer was used in a swirl-stabilized combustor. Swirling air flow with a calculated swirl number of 0.36 is generated with a swirl plate having an exit air velocity vector of 30 degrees with respect to the chamber axis. Effects of swirl and dilution air flow rates on the shape and stability of the flame are investigated. A Phase Doppler Particle Analyzer (PDPA) is used to measure drop size, mean and rms values of axial drop velocity, fuel volume flux, drop velocity and size distributions, and size-classified drop velocity profiles for two cases of with and without combustion and at six different axial locations from the nozzle. A thermocouple is used to measure the uncorrected average temperature at these axial positions. For the no-combustion case all air and fuel flow rates were kept at the same values as the combusting spray condition. A Laser Doppler Velocimeter (LDV) is employed to measure the axial gas velocities and investigate the flew field in the combustor without fuel injection.

粗粉砕炭の旋回湿式燃焼のガス化燃焼特性

This paper describes the experimental results of crushed coal firing two-stage combustion with the primary stage of cycolone slagging combustion. Combustion experiments were carried out under various air equivalence ratios in primary combustion, in several experiment series with different coal type, coal feed rate and coal in-jection azimuth. The emphasis was directed toward the carbon conversion, ash retention and NOx formation in the primary stage combustion (partial gasification combustion).The optimum condition of primary combustion air equivalence ratio that gives the best carbon conversion efficiency depends upon coal type (stoichiomeric air volume), coal feed rate and coal injection azimuth. This suggests that in crushed coal cyclone slagging combustion the hydrodynamic mixing process between coal jet and swirling air flow (secondary air flow) plays a dominant role in determining the gasification combustion performance. In every experiment series, the best results of 95% both for carbon conversion and ash retention efficency were achieved. The measured product gas composition in the primary combustion shows good agreement with equilibirum calculation, suggesting that the gas phase reaction in the cyclone slagging combustion proceeds to attain chemical equilibrium. NOx emission of 100-150ppm (in flue gas, O2 6% base) was achieved under the condition of primary air equivalence ratio less than 0.8. The obtained results of NOx emission control represent the availability of the crushed coal burning two-stage combusion with the primary stage of cyclone slagging combustion to meet satisfactorily the NOx emission regulation.

Aerodynamic characteristics of swirling spray flames: Pressure-jet atomizer

The effect of swirl on droplet transport processes is examined in a pressure-atomized, hollow-cone kerosene spray, introduced into coflowing nonswirling and swirling air flow fields. An ensemble light scattering technique, based on measurement of the polarization ratio, provided spatially resolved measurements on the local values of droplet mean size and number density in dense regions of the nonburning spray. Laser velocimetry was employed to measure the axial, radial, and tangential velocity components of the droplets and combustion air stream. Droplet velocity distributions and time histories provided information on the transport of individual droplets under nonburning and burning conditions. High-speed cinematography, short-exposure photography, and video movies were also employed to observe the global features of the spray flame. The results reveal that the spray flame has a complex three-dimensional structure. The introduction of swirl to the combustion air modifies the droplet/air velocity field in addition to the spatial distribution of droplet size and number density. Larger droplets are transported downstream relatively unperturbed by the surrounding air stream while smaller droplets are entrained by the recirculating aerodynamic pattern. In addition, observed instabilities during liquid jet breakup appear to result in droplet clustering further downstream. The interaction between the fuel droplets and air flow field is therefore a process that significantly affects the overall characteristics of spray flames. Regulation of fuel/air interactions can mitigate fuel losses into the surrounding environment as well as control flame stability.

Low NO x emission from radially stratified natural gas-air turbulent diffusion flames

Research oriented to establish principles of design and operation of an ultra-low-NO x burner is discussed. Gradual admixing of the burner air with the centrally injected fuel is achieved by radial stratification of the flame. This radial fuel stratification is brought about by a combination of the swirling air flow and strong positive radial density gradients in the flame. A burner capable of varying the radial distributions of the burner airflow and recirculated flue gas, and of their swirl velocities has been used to realize the idea of radial fuel stratification in experimental flames using the flame tunnel of the MIT Combustion Research Facility. Distributions of gas velocities, temperatures and major chemical species concentrations determined in one megawatt (thermal) input natural gas-air turbulent diffusion flames show high fuel concentrations on the axis of the flames and demonstrate that fuel stratification is effective in reducing NO x emission without increasing trace emissions of combustibles. The effects of stratification are manifested in two ways: firstly, they improve combustion air staging by increasing fuel residence time in the pyrolysis flame zone, and secondly, permit increased amounts of flue gas to be recirculated through the burner without an impairment of the flame stability. The NO x emission at 3% O 2 reached with radial flame stratification but without flue gas recirculation was 70 ppm (56 ppm CO), and 15 ppm (<10 ppm CO) was achieved when 32% of the flue gas was recirculated through the burner.

Numerical prediction of turbulent mixing in a variable-density swirling pipe flow

Numerical predictions are compared with experiments on turbulent mixing in variable density swirling flows, where He or CO 2 is coaxially injected in turbulent swirling air flows in a pipe. The calculation employs two kinds of stress/flux equation model, i.e. the density-weighted Favre-averaged form and the density-unweighted conventional-averaged form. The calculation based on the former model can predict the phenomenon that the mixing of He is strongly retarded due to swirl than that of CO 2, whereas the latter model fails to predict this. Comprehensible interpretations of the interrelation between turbulent mixing, density non-homogeneity and swirl-induced pressure gradient are presented.

Investigation of the Effects of Flow Swirl on Heat Transfer Inside a Cylindrical Cavity

Experiments were conducted to determine local heat transfer coefficients on the inside surfaces of a cylindrical cavity that is cooled by a swirling air flow. Temperature-sensitive liquid crystals were used as temperature sensors. Five blowing (cooling) modes were tested: three with swirl numbers of 0.36, 0.84, and 1.73; a fourth with no swirl (axial flow), and a fifth that was similar to the fourth but had the flow direction reversed. Flow visualization and static pressure measurements were performed to improve understanding of the situation. The smoke-wire technique was successfully used to picture the flow patterns. Plots of local Nusselt number along the cavity surfaces were obtained for the five blowing modes and for three different Reynolds numbers. The swirling cases had similar flow fields with higher heat transfer rates near the cavity top and lower rates near the cavity bottom (the opposite of the nonswirling cases). A tornadolike structure on the cavity bottom was observed in the swirling cases. This structure became stronger and more violent as the degree of swirl and the Reynolds number were increased. The Nusselt number curves for the two nonswirling cases were of similar shape, although the flow direction was reversed.

Velocity measurements and energy distribution for isothermal, suddenlyexpanding, swirling flow in an industrial burner with bluff-body

This paper deals with two-dimensional, back-scatter, laser-Doppler anemometer measurements in confined, isothermal, nonreacting, suddenly-expanding swirling air flow. Simultaneous axial and azimuthal, mean and fluctuating velocity measurements were made by using a laser-Doppler anemometer (LDA) system. It is found that the maximum total reverse-flow rate occurs at about 0.50 furnace dia, with 60% of the inlet mass flow recirculated. Some quantitative information, such as the size, shape and intensity of the central recirculation zone, as well as the axial distribution of the energy flux, are provided.

Swirl effects on variable-density jet mixing in confined flows

An experimental investigation on swirl effects on inhomogeneous confined jet mixing in a combustor configuration is reported. The confined swirling flow was simulated by a swirler with a central jet mounted in a cyclindrical tube. Helium and air jets set at different velocities were injected into the confined swirling air flow. The resulting flow fields due to two vane swirlers with constant vane angles of 35° and 66° were compared. Results show that the 35° vane swirler produces a solid-body rotation core with a slope about twice that created by the 66° vane swirler. It is the behavior of this solid-body rotation core that determines jet mixing rather than the swirler vane angle. Consequently, the coaxial jet decays much faster, the mixing is more intense, and the turbulence intensities are higher for the 35° vane swirler. In view of these results, combustor designers should be more concerned with behavior of the solid-body rotation core produced by the swirler, instead of the swirler vane angle.