Function Of Swirl Number Research Articles

Sound Pressure Level Analysis of a Liquid-Fueled Lean Premixed Swirl Burner with Various Quarls

Swirl burners are widely used in numerous practical applications since they are characterized by low pollutant emission and a wide operating range. Besides reliable operation, a burner must fulfill noise emission regulations, which is often a sound pressure level in dB(A) when people are affected. Therefore, the present paper evaluates the overall sound pressure level (OASPL) variation of a 15-kW liquid-fueled turbulent atmospheric swirl burner at various setups. Firstly, the combustion air flow rate was adjusted, which induced a swirl number modification due to the fixed swirl vanes. Secondly, the atomizing pressure of the plain-jet airblast atomizer was modified, which also affected the swirl number. High atomizing air jets notably increased combustion noise by intensifying the shear layer. Thirdly, a geometrical modification was performed; 0°–60° half cone angle quarls in 15° steps were installed on the lip of the baseline burner for extended flame stability. By filtering the OASPL to the V-shaped flames, a linearly decreasing trend was observed as a function of swirl number. Their derivative also has a linearly decreasing characteristic as a function of the atomizing pressure.

Analysis of local entrainment rate in the initial region of isothermal free swirling jets by Stereo PIV

Entrainment is of crucial importance in many engineering applications, while the relevance of swirling flows is related to their ability to improve mixing in the initial region of a jet, as a consequence of a larger entrainment of ambient air. This is particularly effective at swirl levels above the critical value associated with the onset of vortex breakdown and the generation of the instability referred to as the precessing vortex core (PVC). Despite the importance of the entrainment process in swirling flows, there have been very few quantitative data in the literature. In view of the above observation, the initial region of a swirling gas jet was investigated experimentally using Stereo Particle Image Velocimetry (SPIV) in order to evaluate the local entrainment rate as a function of swirl number and axial distance from the nozzle. The experiments were carried out on a model burner used to generate isothermal swirling air jets at Reynolds number, Re = 21,800. This configuration was chosen due to its relevance to industrial burners and the ability to reproduce a round, turbulent gas jet issuing into ambient air, since this is the most investigated case and allows comparison with previously published results with and without swirl. Measurements were made for the region extending from the nozzle exit to 2.5 nozzle diameters downstream. Several swirl conditions were investigated, characterized by swirl numbers ranging from S = 0 to S = 2.2. Time-averaged SPIV allowed simultaneous evaluation of the swirl number and characterization of the structure of swirling flows in the near-field region before and after the development of the inner recirculation zone. The entrainment rate was evaluated by using the integral method applied to the measured SPIV velocity maps. The results provided quantitative evidence that the entrainment rate in the near field is enhanced considerably by the swirl, especially after the onset of vortex breakdown and PVC, reaching values significantly larger than those measured in non-swirling jets. The analysis of the average local entrainment rate revealed a large variability with both the swirl number and the axial distance from the nozzle, leading to the conclusion that the swirl affects the entrainment process in a complex way.The significance of the present results consists in a better understanding of the mixing in the initial region of a typical burner and the possible contribution to validation of numerical modelling of the combustion process.

Characterisation of acoustically linked oscillations in cyclone separators

The hydrodynamic oscillations of a cyclone separator – in particular the precessing vortex core (PVC) phenomena – are investigated by measuring their radiated sound spectra. Strong coherence was observed between internal flow oscillations measured via hot wire anemometry and the external acoustic field measured via microphone. This means that the oscillations can be characterised by using acoustics as a proxy. The oscillations cause narrow-band noise, referred to as cyclone hum. System characterisation by dimensional analysis used velocity and length scales of the vortex core region as scaling parameters. The relevant non-dimensional parameters are a Strouhal number for the cyclone hum centre frequency, a Reynolds number, a geometry based swirl number and numerous geometric scales defining the shape of the device. Cyclones with multiple sizes of inlets and outlets were tested at different flow rates using external microphones to detect the cyclone hum. The results produce an excellent collapse of the data, yielding a simple relationship for Strouhal number as a function of swirl number and the outlet diameter ratio. The non-invasive method of examining oscillations that is presented in this paper could be applied to other swirling systems.

Effects of swirl intensity on heat transfer and entropy generation in turbulent decaying swirl flow

In the present work, numerical simulations of turbulent incompressible nonisothermal pipe flows were conducted to investigate the effect of inlet swirl intensity and streamwise swirl decay on heat transfer and local entropy generation in the flow. The RANS, energy and entropy equations along with Shih's realizable k–ε turbulence model were numerically solved using second order finite volume upwind discretization scheme. The CFD model results showed very good agreement with established LDV measurements. The streamwise trends of Nusselt and Stanton numbers were studied at different inlet swirl intensities. A new CFD-based empirical correlation for predicting the entropy augmentation number as a function of swirl number was proposed. It is shown that the swirl number radically affects the local entropy generation due to viscous dissipation in the inner core of the Rankine vortex structure.

Stability of a swirled liquid film entrained by a fast gas stream

We study the liquid flow inside a recessed gas-centered swirl coaxial injector, where a swirled liquid flowing against an outer wall is destabilized by a central fast gas stream. We present measurements of the liquid intact length inside the injector, as a function of swirl number and dynamic pressure ratio. We propose a simple model to account for the effect of these parameters. We next study the surface instability inside the injector: its frequency is measured for several swirl angles, and as a function of gas velocity. Results are first confronted to the predictions of an inviscid linear stability analysis including swirl, and second to the predictions of a viscous linear stability analysis where swirl is not included. The viscous analysis captures the experimental frequency.

Characterization of PF flames under different swirl conditions based on visualization systems

The complex phenomena underlying reacting flows are at present an active research topic for improving efficiency and control of industrial combustion processes. The present study assesses the use of a reliable and low cost visualization system based on CCD (charged couple device) technology for the identification of different flame states as a function of swirl number. The experiments were performed in a 500kWth pulverized coal (PF) swirl burner. The visualization system was previously used for similar analysis of an atmospheric gas swirl burner of 50kWth. Both flames were analyzed under low and high swirl conditions in order to identify possible common features concerning swirl flames. The analysis procedure proposed in this work extracts different statistical and spectral parameters from the intensity of radiation stored by each CCD element. The representation of flame features as two-dimensional distributions has led to the identification of typical structures in swirl flows and has allowed us to detect changes in flame structure as swirl number increases. Besides, the quantification of aforementioned parameters has led to relevant findings concerning frequency analysis and flame stability as a function of swirl. Specifically, results show that the characteristic frequency in terms of flame flicker is sensible to changes in swirl conditions.

모형 가스터빈 연소기에서 스월수에 따른 유동 특성에 관한 실험적 연구

희박예혼합 가스터빈 연소기에 적용되는 스월 유동은 연소효율증가와 배기가스저감을 목적으로 적용된 것로 연소기 내 유동장의 전단층생성에 의한 재순환영역이 생성되게 한다. 이러한 재순환영역은 연소가스 재순환에 의한 화염온도와 화염길이를 낮추는 효과를 가지고 있다. 또한 희박연소에서 연소불안정성 억제효과도 있다. 본 연구에서는 모형가스터빈연소기에서 스월러를 이용하여 스월유동을 유도하고, 연소기 노즐에서 평균속도가 일정 할 때의 스월수 증가에 따른 유동장의 속도분포 특성을 분석하였으며 이때 PIV 계측 실험과 난류통계기법을 이용하여 난류인자들을 도출하였다. 스월수의 증가는 연소기 내부 유동장의 형태가 바뀌게 되고, 재순환영역의 위치가 연소기 노즐방향으로 이동하게 되어 화염길이를 줄여 주며, 난류강도와 난류길이 스케일이 감소하게 되어 에디의 크기가 작아짐을 확인하였다. The swirl flow applied for high efficiency and reduction of emission such as NOx, CO in a gas turbine engine makes recirculation zone by shear layer in the combustion chamber. This recirculation zone influences a decreasing flame temperature and flame length by burned gas recirculation. Also it is able to suppress from instability in lean-premixed flame. In this study, it was found that the swirl flow field was characterized as function of swirl number using PIV measurement in dump combustor. As increasing swirl number, a change of flow field was presented and recirculation zone was shifted in the nozzle exit direction. Also turbulent intensity and turbulent length scale in combustor were decreased in combustion. It has shown reduction of eddies scale with swirl number increasing.

Experimental and numerical investigations in the near-burner region of a versatile multi-fuel burner

An experimental and numerical investigation on the effect of swirl on the motion of coal particles in the near-burner region of a multi-fuel swirl-stabilised laboratory burner of total thermal input of 100 kW, has been performed. The burner was designed as a scale model of a 10 MW coal burner operating in a cement rotary kiln, produced flames of different aerodynamic characteristics and was able to burn a combination of gaseous, liquid and pulverised solid fuels. Temperature and Laser Doppler measurements of all three velocity components obtained in isothermal and reacting single and multiphase flows as a function of swirl number in the range of 0.45 to 0.9 confirmed the ability of the burner to produce close-to-industrial conditions making it a powerful tool in the field of combustion research. In detail, the velocity measurements showed that the flow field was axisymmetric and an internal recirculation zone (IRZ) in the shape of a toroidal vortex was formed around the centreline for swirl numbers of at least 0.65. A 60% increase in the swirl number, from 0.65 to 0.9, resulted in a 30% widening of the IRZ. Solid particle measurements in the same configuration revealed that the width of the zone where coal particles recirculate is by 20% larger than that formed in the single phase case and that most of the coal particles are centrifuged away from the IRZ, particularly at high swirl numbers. A numerical investigation on the effect of the swirl number on the fluid and particle motion has also been performed. The flow field was modeled as 2D axisymmetric and results were obtained with both the RNG and k-e turbulence models and results were shown to be in good agreement with the measurements even at the high swirl number, when the RMG model was used. Lagrangian tracking of coal particles in the range of 1 to 150 μm was also performed by as a function of swirl number. The calculations revealed that particles of diameter larger than about 20 μm are centrifuged away from the IRZ in accordance to the measurements while particles larger than 100 μm, due to their high inertia remain on the IRZ boundary and are neither centrifuged nor entrained inside the recirculation zone, In accordance with the measurements, the calculations also showed that the effect of centrifuging is decreased when the swirl number of reduced.

PRECESSION AND RECIRCULATION IN TURBULENT SWIRLING ISOTHERMAL JETS

A burner with a hybrid bluff body swirl design, incorporating a central jet surrounded by a swirling annulus, is applied to the investigation of turbulent isothermal swirling jets. Nonswirling flows stabilized on this burner form only one stagnation zone on the face of the bluff body. Such a far upstream recirculation zone is typical of those formed behind solid obstructions placed in strong axial streams. The addition of swirl leads to more complex flow patterns, which may include a second recirculation zone (in more downstream axial locations), flow instabilities, and precession. Both of these effects are investigated. Laser Doppler Velocimetry is used to ascertain the presence of downstream recirculation. The seed- (alumina-) laden central jets in these flows are visualized using a combination of laser sheet forming optics and high-speed CCD camera. Postprocessing of the imaging data succeeds in resolving the precession frequency of the jet despite the variable seed density in the experiments. Precession frequency appears to be a function of swirl number as well as the Reynolds number of both the central jet and the swirling annulus.

The influence of swirl on oscillations in ducted premixed flames

Combustion oscillations in premixed flames stabilized behind a disk, rearward-facing steps, and annular rings without and with bypass flow in round ducts have been examined as a function of swirl number, upstream and downstream duct lengths, exit constriction, and equivalence ratio. With disk stabilization, rough combustion was associated with a quarter-wave in the upstream duct and swirl extended the length over which heat release occurred, thereby decreasing the susceptibility to rough combustion. In contrast, swirl shortened the length over which heat release occurred downstream of the rearward-facing steps and increased the susceptibility to rough combustion associated with quarter-waves in long downstream ducts without an exit nozzle. Since swirl improved flame stabilization, bulk-mode oscillations associated with exit constriction were experienced only for exit area ratios less than 0.25. The amplitude of oscillations in flames stabilized behind multiple-step expansions was small for shorter ducts without an exit nozzle, due to the flame stabilizing behind the second step, but longer ducts without and with constriction and swirl led to higher amplitudes. Annular rings, like disks, were associated with quarter-wave oscillations in the upstream duct, but swirl appeared to have no influence on the amplitude and frequency of oscillations. When the clearance between the outer diameter of the ring and the pipe was small, the ring acted like short rearward-facing steps and gave rise to rough combustion associated with a three-quarter wave in the entire duct with swirl increasing susceptibility to rough combustion. Bypass flow arrangements led to lower flammability limits and smaller amplitudes of oscillation due to poor mixing between the main and bypass flows, and swirl improved mixing between the flows and led to results similar to those in premixed flows.