Swirling Jet Research Articles

Prediction of combustion instability by combining transfer functions in a model rocket combustor

The flame transfer function (FTF) and acoustic transfer function (ATF) are estimated experimentally and numerically, respectively, and both are combined to predict combustion instability of a model rocket combustor with a single coaxial jet–swirl injector. FTF of the injector is measured experimentally by imposing external velocity excitation on the injector. ATF, i.e., model-chamber response to heat-release fluctuation from flames, is difficult to measure, and alternatively, an idea of numerical evaluation method is suggested by applying an equivalence-ratio perturbation to flames. Then, ATF is calculated by numerical simulations with a three-dimensional model and compared with ATF from the one-dimensional analytic equation. Results show that ATF estimated by the 3-D numerical model improves accuracy in predicting resonant frequencies and growth rates of pressure oscillations in the model chamber. Finally, a closed-loop system transfer function describing dynamic behaviors of the model chamber is formulated by combining FTF with ATF and it is applied to predict combustion instability of the chamber. This prediction is compared with experimental observation, which shows good agreement between them.

Film cooling performance and flow structure of single-hole and double-holes with swirling jet

This paper presents the results of a numerical study of the effects of swirling flow in coolant jets on film cooling performance. Some combined-hole designs with swirling coolant flow entering the delivery hole are proposed and analyzed. Adiabatic film cooling effectiveness values for cases with various blowing ratios are compared. Detailed flow structures and underlying mechanisms are discussed. The results show that film cooling effectiveness is improved with jet swirl at high blowing ratios, and that swirl strength has significant influence on film cooling performance. Combined-hole designs can further improve film cooling performance using swirling jets due to mixing of coolant flows and interaction of vortices. The largest improvements of area-averaged film cooling effectiveness for a single-hole swirl case and a combined-hole swirl case over corresponding non-swirling case results are 157% and 173%, respectively.

Flow and heat transfer characteristics of turbulent swirling impinging jets

• PIV to resolve the time-averaged flow field of turbulent swirling jets. • Flow field of free and impinging swirl jets are compared. • Flow features correlated to heat transfer characteristics. • Vortex breakdown examined at Re = 11,600–35,000 at low and high swirl. This paper addresses both the upstream and downstream flow domain in turbulent impinging jets, which have widespread applications in cooling, heating, and drying. The role of impingement on swirl induced vortex breakdown as well as the interplay between heat transfer (at the impingement plane) and the downstream flow domain are analysed. Particle Image Velocimetry (PIV) is used to resolve the flow field in a range of jets, covering three Reynolds numbers (Re = 11,600, 24,600, and 35,000), two swirl numbers (S = 0.3 and 0.74), and two impingement distances (H/D = 2 and 4) as well as free jets. With regard to the upstream flow field at low swirl (S = 0.3), both free and impinging jets do not exhibit vortex breakdowns. However, for high swirl (S = 0.74), vortex breakdown in the upstream is observed at Re = 11,600 and Re = 24,600, but not Re = 35,000. Impingement distance affects the size, position, and strength of the recirculation bubble (vortex breakdown). The recirculation bubble is more spherical and stronger at Re = 11,600 in contrast to the Re = 24,600. Near-field impingement (H/D = 2) significantly affects the size and shape of the bubble. In free jets, the width of the vortex breakdown bubble at Re = 11,600 spans −0.4 < r/D < 0.4, but shrinks to −0.2 < r/D < 0.2 at Re = 24,600. Correlating the velocity field near the impingement plane with the heat transfer characteristics shows that at low swirl conditions (S = 0.3), that do not lead to upstream vortex breakdown, the spatial extent of a recirculation zone stabilised onto the impingement plane reduces in size at a higher Reynolds number for both H/D = 2 and 4. However, higher swirl conditions (S = 0.7) which typically lead to upstream vortex breakdown, do not coincide with the appearance of any downstream recirculation zones that stabilise onto the impingement plane.

A comparison of partially premixed methane/air combustion in confined vane-swirl and jet-swirl combustors

ABSTRACT The characteristics of partially premixed methane/air combustion in aconfined geometry are experimentally investigated in two types of swirling flows introduced by using avane swirl (VS) and tangential jet swirl (TJS), respectively. To make acomparison, acoaxial jet (CJ) burner is also employed. The fuel and air are individually injected through their own inlets, partially premixed in amixing chamber, and then burned in aconfined cylindrical combustor. In VS and CJ combustors, the fuel is axially injected from the center; while in the TJS combustor the fuel is injected through either acentral inlet or tangential inlet. The geometry swirl number varies from 0 to 3.38. The flame structures, extinction limits, temperature distribution, NOx, CO concentrations and flow velocities are systematically examined to compare the flame characteristics. The detailed observations show that the swirl plays asignificant role in the fuel mixing, flame stabilization, and flame structures. As fuel is axially injected, ayellow flame is established at low airflow rate regardless of the swirl number values. As the airflow rate increases, ablue flame is obtained; the flame extinguishes at avery low global equivalence ratio of 0.25 in the VS and TJS combustors. When fuel and air are tangentially injected, ablue flame similar to premixed one is established; asteady flame is obtained from lean to rich limits (equivalence ratio of 0.5 to 1.68). The temperature is distributed more uniformly in the high swirl TJS combustor, particularly for the one adopts pure tangential jets of fuel and air, in which the measured NOx and CO concentrations are the lowest. The differences were mainly attributed to different mixing modes.

Study on NO emissions during the coupling process of preheating-combustion of pulverized coal with multi-air staging

Controlling the NOx formation during the combustion process is the most direct and efficient way to minimize NOx emissions, so low NOx combustion technology is essential. In this work, in order to develop multi-air staging pulverized coal (PC) preheating-combustion technology and further optimize the preheating-combustion burner, a 35 kW PC preheating-combustion test system was built, and three types of burners with different outer secondary air (OSA) injection structures were designed. Using Huangling bituminous coal (HL coal) and Wuhai bituminous coal (WH coal), the coupling process of PC preheating-combustion with multi-air staging on NOx emissions was studied systematically. The results showed that the depth furnace air staging could further reduce NOx emissions. When close-coupled over-fire air (CCOFA) and separated over-fire air (SOFA) were applied simultaneously, a significant NOx reduction of 70.2% could be reached 217 mg m−3. Compared with the NOx emissions, the unburned carbon in fly ash presented opposite tendency. Compared to burner II (high-speed swirl jet) and III (low-speed axial jet), burner I (high-speed axial jet) showed the lowest NOx emissions under the multi-air staging preheating-combustion conditions. Furthermore, the depth furnace air staging had greater denitrification potential for high-quality bituminous coal.

Determination of single and double helical structures in a swirling jet by spectral proper orthogonal decomposition

This paper studies the coherent structures found in an annular swirling jet flow undergoing vortex breakdown with control parameters, the Reynolds number Re = 8500 and the swirl number Sw = 0.38. The flow field is simulated using the large eddy simulation method with a dynamic k model. The first- and second-order statistics of the velocity fields are compared to tomographic particle image velocimetry measurements of the same flow configuration to validate the numerical simulation. The fast Fourier transform of the sampled velocity and pressure signals indicates a precessing vortex core with a frequency of 22 Hz. This frequency is in line with the one detected by spectral proper orthogonal decomposition, which is utilized to identify the coherent structures in the near-field region of the swirling flow in the present work. In detail, apart from the single helical structure usually found in swirling flows, a double helix, rarely observed in turbulent swirling jets, is also identified. This structure is not a second-order harmonic mode of the single one, as shown by statistical analysis of the mode temporal coefficients. Moreover, the calculation of energy production shows that this coherent precessing motion extracts energy from the mean flow field in the wake behind the bluff-body and in the breakup region of the vortex.

Study on Collected Flow Rate of Compound Swirl Jet with Varying Input Energy

Study on Collected Flow Rate of Compound Swirl Jet with Varying Input Energy

Experimental Study of Mean Flow Characteristics in the Near Field of Wedge-Shaped Swirling Jets

In this investigation experiment was carried out in 80 mm diameter swirling pipe jet, where swirl was generated by attaching wedge-shaped helixes in the pipe. All measurements were taken at Re 5.3e4. In the plain pipe jet the potential core was found to exist up to x/D=5 but in the swirling jet there was no existence of potential core. The mean velocity profiles were found to be influenced by the presence of wedge-shaped helixes in the pipe. The velocity profiles indicated the presence of sinusoidal flow field in the radial direction existed only in the near field of the jet. This flow field died out after x/D=3 and the existence of jet flow diminished after x/D=5.

Experimental Study of Mean Flow Characteristics in the Near Field of Wedge-Shaped Swirling Jets

In this investigation experiment was carried out in 80 mm diameter swirling pipe jet, where swirl was generated by attaching wedge-shaped helixes in the pipe. All measurements were taken at Re 5.3e4. In the plain pipe jet the potential core was found to exist up to x/D=5 but in the swirling jet there was no existence of potential core. The mean velocity profiles were found to be influenced by the presence of wedge-shaped helixes in the pipe. The velocity profiles indicated the presence of sinusoidal flow field in the radial direction existed only in the near field of the jet. This flow field died out after x/D=3 and the existence of jet flow diminished after x/D=5.

Obtaining powders with particle shape near to spherical by water atomization of melts

Abstract. Problem. Recently, there is a tendency to increase volumes of use of spherical powders. But due to high cost of spherical powders obtained by conventional technologies, improvement of current more economical methods of its production is interesting. Goal. The goal is to substantiate formation of metal powders of spherical shape (or near to it) at high-pressure water atomization with use of swirl jet. Methodology. On the basis of results of atomization of medium-carbon ferromanganese with use of modified annular hydraulic nozzle and wellknown literature data, conditions for spheroidization of metal drops at atomization «in spout» are analyzed. By mathematical modelling method the semiempirical equation for definition of average median size of powder particles is deduced. Results. Using the example of atomization of ferromanganese, possibility of obtaining powders with particle shape near to spherical by water atomization is confirmed. The model of atomization is proposed. It is shown that impact is carried out by water, but through thin gas layer, without direct contact of water and metal. Thickness of gas layer is important. At optimal parameters of atomization solidification time of drops increases, that is favourable for obtaining particles of spherical shape or near to it. Originality. Model of water atomization of melts «in spout» is proposed. Conditions needed to form particles of spherical or spheroidal shape during water atomization are substantiated. Engineering equation to determine median size of powder particles is obtained. Practical value. Described technology is promising to produce metal powders with particles of spherical shape or near to it. Use of obtained powders of ferromanganese for coating of welding electrodes demonstrated the advantage over ground powders such as elimination of swelling coating during production of electrodes, better plastic strength of coating, and higher impact strength of welded seam at negative temperatures.

Modeling capabilities of unsteady RANS for the simulation of turbulent swirling flow in an annular bluff-body combustor geometry

In this paper, we investigate the capabilities of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations for capturing the helical structures found in a three-dimensional, incompressible and isothermal annular swirling jet undergoing vortex breakdown. The flow topology is representative of a bluff-body combustor operating at a Reynolds number Re = 8500 and swirl number S = 0.39. As a turbulence model, the SSG Reynolds stress model (RSM) was chosen. The numerical simulation is validated by means of tomographic particle image velocimetry measurements of the same flow configuration. To detect the coherent structures in the flow field, the recently introduced spectral proper orthogonal decomposition technique is adopted, which gives both temporal and spatial information on the large scale coherent structures. The spectral analysis of the sampled velocity signals identified a precessing vortex core with a frequency of 24.3 Hz. In particular, two different large-scale helical flow structures are identified: a single and a double helix. Both of them are wound in the counter-swirl direction and wrapped around the central breakdown bubble. Those findings are quite similar to recent experimental tomographic particle image velocimetry results (Vanierschot et al., Phys. Rev. Fluids (2018) & J. Fluid Mech. (2020)). This study demonstrates that the unsteady RANS approach with RSM is able to predict the coherent structures found in an annular swirling jet undergoing vortex breakdown both temporally and spatially with reasonable accuracy and this approach can hence be used in the design of bluff-body combustors, where a multi-dimensional parameter space may require many simulations to find the optimal design.

Direct Numerical Simulations of the Evaporation of Dilute Sprays in Turbulent Swirling Jets

The effects of swirled inflows on the evaporation of dilute acetone droplets dispersed in turbulent jets are investigated by means of direct numerical simulation. The numerical framework is based on a hybrid Eulerian–Lagrangian approach and the point-droplet approximation. Phenomenological and statistical analyses of both phases are presented. An enhancement of the droplet vaporization rate with increasing swirl velocities is observed and discussed. The key physical drivers of this augmented evaporation, namely dry air entrainment and swirl-induced centrifugal forces acting on the droplets, are isolated with the aid of additional simulations in which the inertial properties of the droplets are neglected. The correlation between swirl and dry air entrainment rate is found to be responsible for the increase of the global evaporation rate and the spray penetration length reduction, while swirl-induced centrifugal forces are found to be effective only in the jet shear layer, close to the injection orifice, for the analyzed cases.

A model for the constant-density boundary layer surrounding fire whirls

Abstract

Smoke based visualization of turbulent swirl jet flow

The present study deals with the visualization of turbulent jet flow from swirl nozzles. A low-cost turbulent jet facility developed was utilized for all the experiments. Two types of vanes are considered for the present study viz. flat vanes and airfoil-shaped vanes. All the vanes are fixed to a circular hollow hub at the center. The flow is visualized by seeding smoke from incense material as tracer particles. A solid-state laser with 405nm wavelength is used for the illumination via a glass rod thereby producing a laser sheet. Cross-sectional visualizations are made at various axial downstream locations. The effect of the vane angle has a greater influence on the production of jet flow vortices due to swirlers. The present study reveals that the increase in the vane angle increases the production of vortices.

Heat Transfer From a Heated Flat Surface Due to Swirling Coaxial Turbulent Jet Impingement

Abstract Heat transfer from an isothermally hot flat surface due to swirling coaxial turbulent jet impingement is investigated numerically. The coaxial jet construction consists of implanting a thin-walled round tube inside a coaxial outer pipe. Two different fluid streams or jets, having different average velocities, flow through the inner tube, and the annular space between the inner tube and the outer pipe. The ratio of the average velocities of the jets, the ratio of the pipe diameters, the jet exit Reynolds number, the strength of the swirl, and the separation distance from the jet exit to the impingement surface are the main parameters for this flow configuration. The effects of the swirl strength on the jet impingement heat transfer at the target surface are investigated by computing the flow and thermal fields for various combinations of the problem parameters. The presented results contain the plots of the flow streamlines, the contours of the temperature, the contours of the swirl velocity, as well as the distribution of the local and average Nusselt number on the impingement surface. It is found that, compared to the single round jet, the coaxial jet produces enhanced and more uniform heat transfer at the heated surface. The jet-spreading and mixing are affected by the imposed jet swirl which modifies the heat transfer process. Thus, the heat transfer compared to a non-swirling jet is either enhanced or diminished depending on the combination of the problem parameters.

Impingement of coaxial jet on convex element for confined and unconfined flow

Jet impingement is most effective and active method for cooling and heating of any surface or system. The ability of jet impingement is greatly influenced by nozzle configuration and other dimensional and non-dimensional parameters. Impinging coaxial swirl jet generates interesting flow filed on any test surface and influences both pressure and heat distribution on impinging surfaces. In present study, an experimental investigation is carried to analyze the effects of turbulent coaxial swirl jet on the pressure distribution (PC &amp; PCO) on convex element. For better and acceptable results, the desirable parameters are identified from previous research works. The present experimental result highlights the independency of pressure coefficient (PC) for jet-Reynolds number (Re=70000 to 45000), effect of circumferential angle (θ) or inclination of test element, effect of jet exit to test element distance (Z/dh) and effect of confinement on PC &amp; PCO pattern on a convex test element. The higher pressure coefficient value are obtained at lower Z/dh = 1 &amp; at θ = 15° to 12°and significant drop in the values are seen with increase in the Z/dh &amp; θ. At θ = 20° to 30° the value of PC &amp; PCO reaches to negative magnitude. The use of confinement tube enhancementthe pressure distribution (PC &amp; PCO) by 61% to 64% is seen for the same flow conditions.

Flow evolution in the near field of a turbulent annular jet

The results of an experimental study of the flow structure in the near zone of an annular swirling jet are presented. The data were obtained using a two-component laser-Doppler anemometer for the annular jets with a swirl angle of 0–60°. The results of measurements of the average velocity and pulsations in the near jet region (up to 5 diameters of the outer ring) are presented. Based on the analysis performed, it is shown that formation of an internal toroidal vortex near the nozzle plays an important role in the flow dynamics and the process of annular jet mixing. Formation of a stationary toroidal vortex outside the ring is also noted. These structures determine the flow in the near zone of the jet. With the studied Reynolds numbers, this character of the flow in the near zone leads to an increase in the mixing rate, high degree of turbulence, and an increase in the jet ejection capacity.

Turbulent swirling impinging jet arrays: A numerical study on fluid flow and heat transfer

Impinging jets are generally utilized as an effective medium of heat and mass transfer. Various stream orientations and swirl effect may be a potential candidate to further improve the heat transfer characteristics. This paper numerically examines two different types of arrays of circular multiple jets and the effect of swirl on jets that impinges vertically onto a flat surface situated at a fixed vertical separation distance H = 2D, D the nozzle diameter and at Reynolds number equal to 11,600. In this regard, three-dimensional simulation was performed using finite volume method whereas turbulence was described by SST k-ω model. The results demonstrate that staggered arrangement ensures better mixing and higher velocity as well as heat transfer. The strength of recirculation for swirl jet is larger than that of non-swirl jet. Maximum pressure coefficient occurs at individual stagnation point of each jet, with a secondary peak between two neighboring jets. Strong heat transfer is predicted to occur at around the stagnation region for each non-swirling jet and between the neighboring jets for swirling jets. Turbulences are intense around the periphery of each jet and they coalesce as it moves downward.

Jet absorption and desulfurization technology of sulfur waste gas in the acrylonitrile apparatus

Considering the sulfuric acid-containing gas (such as H2S and SO2) in the waste gas of the Shanghai Petrochemical Acrylonitrile Apparatus, an injection, absorption, and desulfurization technology is developed for the pilot test of sulfur-containing waste gas in the acrylonitrile apparatus, under the operating conditions of the apparatus and the characteristics of the waste gas. A single hydraulic jet swirl core tube is adopted to study the effects of H2S gas concentration, SO2 gas concentration, NaOH absorption concentration, and intake air amount on desulfurization efficiency. Results show that under the smooth operation of the apparatus, the absorption efficiency of H2S gas and SO2 gas can continuously reach 85 % and 70 % respectively. The absorption efficiency is increased with the increased concentrations of H2S gas and SO2 gas as well as the increase of the liquid volume of the absorption liquid. However, the absorption efficiency decreased as the amount of air intake increases. The findings can serve as valuable reference for the use of injection, absorption, and desulfurization technology in engineering applications.

Transient modes in a swirl jet of superheated water

Experimental studies of boiling-up in a jet of superheated water have been carried out in the presence of a passive swirler during a discharge through a short cylindrical nozzle. The evolution of the jet disintegration of a boiling-up liquid has been traced under changes of thermodynamic parameters (temperature, pressure) along the saturation line. The characteristic structures and shapes of the jet corresponding to different modes of boiling-up have been identified. In a swirl jet 1 / f - fluctuations have been detected at both for low and for high superheats. The loss of the jet stability of superheated water under conditions of explosive boiling-up has been noted.