Central Nozzle Research Articles

An experimental study on two-phase characteristics of vertical injection of gas through a slot nozzle into a liquid bath

Two-phase flow characteristics of gas injection through a slot nozzle into a liquid bath were studied experimentally. A dual-sensor electrical probe measurement system was used to obtain the distributions of multiple two-phase flow parameters. Measurements were conducted under various gas flow rates, nozzle widths and at different planes vertical to the slot nozzle. It is found that the void fraction, interfacial area density and bubble frequency show peak value in the nozzle center and decrease rapidly away from the nozzle exit. Both the void fraction and interfacial area density increase drastically with increasing gas injection rates, while the bubble frequency remains almost unchanged. However, the effects of two different nozzle widths (0.1 mm and 0.5 mm) on the above three parameters are not significant. As the measurement plane deviates from the center plane vertical to the slot nozzle, all the above three parameters show slight decreasing trends indicating the bubble plume is contracting to the nozzle center downstream. For the measurement zone 15 mm above the nozzle exit, the apparent bubble diameter and velocity remain almost constant in the whole flow field for each flow configurations. Moreover, good self-similarities were demonstrated for void fraction, interfacial area density and bubble frequency, with their profiles being able to collapse into a Gaussian function.

Near-wall and Turbulence Behavior of Swirl Flows through an Aerodynamic Nozzle

It is often a challenge to achieve uniform flow in turbulent swirl flow and to predict the flow within the nozzle as measurement diagnostics face difficulty to capture both mean flow and turbulence. The purpose of this study is to numerically investigate the near wall flow characteristics and turbulent behavior for the effect of different tangential inlet numbers of an incompressible turbulent swirl air jet. In this regard, axial-plus-tangential flow based swirling nozzle is considered for the simulation using finite volume method, where turbulence is approximated by the Shear Stress Transport (SST) k-ω model. The results show that axial and tangential velocity at the wall vicinity response the most. Moreover, the turbulent flow characteristic for no swirl flow is nearly uniform, but for swirl flow it fluctuates abruptly near the inlet section where the swirl has introduced. The skin friction coefficient for 2TP is the maximum for swirl flow and for no swirl condition the skin friction coefficient is nearly uniform. Due to the swirl introduction the pressure drop characteristics near the nozzle center response quickly and near the wall vicinity this property changes slowly. The magnitude of swirl decay fluctuates before the nozzle converging section however after the nozzle converging section the swirl decay is nearly constant. The local swirl near the inlet is highly unpredictable although after the nozzle converging section the local swirl profile is nearly similar for 2TP, 3TP and 4TP.

Flow dynamics and wall-pressure signatures in a high-Reynolds-number overexpanded nozzle with free shock separation

A delayed detached eddy simulation of an overexpanded nozzle flow with shock-induced separation is carried out at a Reynolds number of , based on nozzle throat diameter and stagnation chamber properties. In this flow, self-sustained shock oscillations induce local unsteady loads on the nozzle wall as well as global off-axis forces. Despite several studies in the last few decades, a clear physical understanding of the factors driving this unsteadiness is still lacking. The geometry under investigation is a subscale truncated ideal contour nozzle, which was experimentally tested at the University of Texas at Austin at a nozzle pressure ratio of 30. Under these conditions, the nozzle operates in a highly overexpanded state and comprises a conical separation shock that merges to form a Mach disk at the nozzle centre. The delayed detached eddy simulation model agrees well with the experimental results in terms of mean and fluctuating wall-pressure statistics. Wall-pressure spectra reveal a large bump at low frequencies associated with an axisymmetric (piston-like) motion of the shock system, followed by a broad and high-amplitude peak at higher frequencies associated with the Mach waves produced by turbulent eddies convecting through the detached shear layer. Moreover, a distinct peak at an intermediate frequency ( ) persists in the wall-pressure spectra downstream of the separation shock. A Fourier-based analysis performed in both time and space (azimuthal wavenumber) reveals that this intermediate-frequency peak is associated with the (non-symmetric) pressure mode and is thus related to the generation of aerodynamic side loads. It is then shown how the unsteady Mach disk motion is characterized by an intense vortex shedding activity that, together with the vortical structures of the annular shear layer, contributes to the sustainment of an aeroacoustic feedback loop occurring within the nozzle.

Printing direction dependent microstructures in direct ink writing

Direct ink writing enables deposition of multiphase filaments with designed microstructures. Using acoustophoresis, we establish a narrow distribution of microparticles at the center of a direct-write nozzle. The distribution shifts and widens after deposition depending on the printing direction. We use particle image velocimetry and digital image analysis to identify flows transverse to the printing direction and characterize particle distributions in the printed filament. Sources of direction-dependent effects include square nozzles, co-deposition of support material, a rotationally asymmetric microstructure established in the nozzle, and speed inaccuracies that occur in 3-axis gantries. We propose an analytical model for predicting print direction-dependent flows and particle distributions as a function of anisotropy of the particle distribution in the nozzle, a disturbed zone near the nozzle, fluid reshaping of the print bead, uniform rotation of the print bead, calibration of the ink and support nozzle positions, and 3D printer motor error. Using the model, we propose strategies for controlling direction dependent microstructures in direct ink writing. The analytical model can be easily adapted to similar direct-write applications to diagnose sources of direction dependent microstructures.

Numerical Simulation and Experimental Study on Flow of Polymer Aqueous Solution in Porous Jet Nozzle

The addition of a polymer to the jet medium enhances its ability to break rock, and the structure of the nozzle plays a vital role in the full utilization of energy. In this paper, a self-propelled porous jet bit with a support plate is designed, which can prevent the drill bit from jamming due to the jet nozzle against the bottom of the well during the drilling process. And the structural design of the cone-converging nozzle is applied to the forward center nozzle. The polymer additive jet flow field and the pure water jet flow field were compared by numerical simulation and experimental investigation. The results show that the polymer additive jet has a longer isokinetic core, and the rock-breaking volume of the polymer additive jet is much larger than that of the pure water jet, and the optimal spray distance is increased. The forward central jet with the conical convergent nozzle structure has more efficient rock-breaking ability.

Justification the Number of Spray Tips of the Universal Fan Sprayer

The authors noted the disadvantages of fan sprayers such as OVH-600 and VP-1, associated with uneven application of pesticides due to the oscillatory motion of the working parts, especially in the junction of adjacent passages. (Research purpose) To increase the uniformity of the pesticide solution application over the entire width of the fan sprayer. (Materials and methods) The JV «Agriхim» developed a universal fan sprayer designed for the treatment of field crops, orchards and vineyards controlled by air-droplet flow. It was shown that the number of nozzles depends on the technological scheme of operation and the width of the sprayer. (Results and discussion) The authors determined the optimal number of injectors – 25 pieces. Depending on the flow rate of the working solution, three groups of tips were distinguished. (Conclusions) The authors revealed that the new design eliminates oscillatory movements of the working parts of the sprayer, a significant width (24 meters vs. 16 meters in production models) allows to increase dramatically the productivity of work, and the number of changeovers is minimized. They simplified and cheapened the design of the sprayer due to the rejection of a number of kinematic pairs and drives. They increased reliability of the unit, eliminating rubbing and moving parts and components. They presented the layout of 25 nozzles for high-quality pesticide treatment: 5 rear central nozzles were installed on the air ducts at an angle relative to the soil surface, and on the opposite direction of the air ducts placed 3 and 7 nozzles in each side of the axis of symmetry of the sprayer. It was calculated that the rate of flow of the working fluid in the range of 100-500 liters per hectare can be adjusted by changing the pressure in the system from 2 to 8 bar and the working speed from 3 to 7 kilometers per hour.

Flow-Field and Performance Study of Coaxial Supersonic Nozzles Operating in Hypersonic Environment

The integration of multiple propulsion systems in a coaxial configuration is one of the challenges to realize hypersonic passenger transportation. This study is an attempt to understand the performance of two co-axial jets exiting from the base of slender body, operating in single and dual operation mode in freestream hypersonic flow environment. In addition, the effects on performance by adding a different length common channel to both co-axial jets are studied. In the first part of this study, the experiments have been performed for small slender body kept in hypersonic Mach 7 flow, which consists of two inner high-pressure chambers and two co-axial nozzles at the base: central nozzle (Mach 4) and surrounding nozzle (Mach 2.8) along with extended common region, termed as common channel. Schlieren images have been captured for single and dual operation modes. Axisymmetric numerical simulations have been performed for further understanding of the flow interactions and have been qualitatively validated with experimental images. In the second part, the parametric study has been performed using numerical simulations for resized model with various exit Mach numbers for central and surrounding jets along with effect of no common channel and with common channel for various operation modes. One of the findings of the study is that dual jets should operate and exit at same plane (no common channel) with the same exit area (each nozzle with half of total available exit area) in order to have higher total thrust from both jets than the sum of individual jets operating in single operation mode. For higher central jet Mach numbers, the corresponding surrounding jet Mach number will be lower, and in dual operation mode (without common channel), the total thrust will be the same or lower than the sum of the individual jet operations. Regarding the effect of common channel, it has been found out that the introduction of the extended short or long common channel in dual mode operation does not have significant effect on thrust, while the jet flow field is strongly affected by the common channel presence. In single operation mode, for Mach 2 central-jet, the thrust performance decreases 12.2-14.6 % in presence of short and long (29.5 mm and 59 mm) common channel, while for Mach 2 surrounding jet, the thrust performance increases by 15-17.4 % in presence of common channel.

Two-fluid modeling of a wet spouted fluidized bed with wet restitution coefficient model

Hydrodynamics of wet fluidized beds is significantly affected by the liquid bridge force between solid particles. In this study, two-fluid modeling of wet fluidizations with wet restitution coefficient model was realized in Ansys Fluent® through its user-defined function portal. Wet restitution coefficient was calculated using the sub-model proposed by Davis et al. (Elastohydrodynamic rebound of spheres from coated surfaces, J. Fluid Mech., 2002, 468, 107–109.), which depends on liquid viscosity, liquid layer thickness, and impacting velocity. A spouted bed with central air nozzle at the bottom was chosen as the simulation system. The simulation was first validated against available experimental and CFD-DEM results. Then, effects of the liquid viscosity and liquid layer thickness on fluidization hydrodynamics and distributions of relevant quantities were investigated. The simulation results demonstrate that the effect of liquid viscosity is significant while the influence of liquid layer thickness can be ignored in the current study. Finally, some thoughts on future studies for two-fluid modeling of wet fluidizations with wet restitution coefficient model were discussed.

ПОРІВНЯЛЬНЕ ДОСЛІДЖЕННЯ НА ФІЗИЧНІЙ МОДЕЛІ ОСОБЛИВОСТЕЙ ВЕРХНЬОЇ ПРОДУВКИ ЧЕРЕЗ НАКОНЕЧНИК ЗІ СКЛАДЕНИМ СОПЛОМ ПРИ ПРОДУВАННІ ОДНОФАЗНОЇ РІДИНИ

The aim of the work was to study the nature and peculiarities of the interaction of the lance of the converter lance flowing from the folded nozzles with a liquid bath of single-phase liquid. The study was conducted on the basis of a physical model of the converter with top purge. The design of a folded nozzle consisting of a central cylindrical nozzle and a slit annular nozzle located around it is investigated. A visual and video recording comparative study in comparable operating conditions of the folded nozzle with different design features, in particular, with an additionally deepened central nozzle in the folded nozzle and a protruding central nozzle; with operation of one cylindrical nozzle, four nozzles of identical section and an annular slot nozzle. It is established that the jet flowing from the folded nozzle differs in the possibility of more active interaction with the liquid bath than one cylindrical nozzle and four nozzles of equivalent plane. The depth of penetration of the jet into the bath liquid, the parameters of the formed hole of the jet penetration, the oscillations of the bath were studied. It is established that the peculiarities of crater formation in the liquid, characteristic when using a folded nozzle, are due to the use of a slotted annular part of the nozzle. The wider crater is due to the use of a slotted annular part of the nozzle. It is shown that the structural variants of the folded nozzle with a recessed or protruding central nozzle do not increase either the depth or the width of the formed crater. It is also established that the peculiarities of crater formation in the liquid when using a folded nozzle are due to the use of a slotted annular part of the nozzle. To form the best purge conditions, the slice of the slotted annular part should be flush with the slice of the central nozzle. With this design, the jet pulse is maintained for a longer period compared to the use of only one central nozzle, probably due to the protection of the central jet, and due to its deep penetration properties.

Spray Characteristics of Rotary Micro Sprinkler Nozzles Used in Orchard Pesticide Delivery

HighlightsDroplet sizes were determined for rotary micro sprinkler nozzles used in solid set canopy delivery systems.An empirical multiple-variable model was developed to predict volume median diameters in spray patterns.Sprinkler nozzles produced medium to coarse droplets to minimize pesticide drift in orchards and trellised systems.Droplet size information can be used to select optimal nozzles for either irrigation or pesticide delivery systems.Abstract. Rotary micro sprinkler nozzles can be used for both irrigation and pesticide applications in orchard systems, but little to no information is available on their droplet size distributions. In this study, the droplet size distributions were investigated and described for rotary micro sprinkler nozzles with five different orifice diameters. A particle/droplet laser image analysis system was used to measure droplet spectra at two pressures (207 and 310 kPa) and two radial distances (0.25 and 0.85 m) from the sprinkler nozzle center. Nozzle orifice sizes, rotational speeds, and flow rates were also measured. Droplet sizes varied with the orifice size, operating pressure, and sampling location. Spiral-shaped spray patterns formed due to the spinning discharge port, within which droplet densities varied with location, orifice diameter, and operating pressure. The volume medium diameters (Dv0.5) for green-black, orange-blue, black-black, blue-black, and red-gray nozzles were respectively 317, 338, 379, 352, and 218 µm at 207 kPa and 283, 250, 283, 270, and 222 µm at 310 kPa. An empirical multiple-variable regression model was developed to predict Dv0.5 in the spray patterns discharged from the nozzles. Test results demonstrated that the rotary micro sprinkler nozzles produced medium to coarse droplets that could be used to minimize spray drift while maintaining efficacy in orchard pesticide applications. Keywords: Chemical application, Droplet size, Irrigation, Rotary nozzle, Spray drift reduction.

Formation and optimization of deposition surface shape during fused silica glass synthesis by chemical vapor deposition

AbstractThis paper investigates the effects of deposition surface shape on temperature distribution of fused silica glass by SiCl4 chemical vapor deposition and performs the optimization of deposition surface shape to obtain the glass with a small temperature gradient. The computational model is first developed to describe the complicated physical and chemical phenomenon in the system. Numerical simulations are then performed to evaluate the effects of SiCl4 injection position on flame characteristics and SiO2 deposition rate. It is found that the SiO2 deposition rates with the unimodal, uniform, and bimodal distributions are obtained when SiCl4 is injected from the central, secondary layer and fourth layer nozzles, respectively. The deposition surface shape is mainly determined by the gas temperature gradient and particle number density near the deposition surface. Thereafter, the temperature distributions in synthetic silica glass are presented for the cases with convex, flat, and concave deposition surface shapes. Large temperature gradients exist at the side of glass if the convex and concave surfaces are formed, although a high temperature uniformity is obtained in the center of glass for the convex deposition surface. The optimal surface shape that has a very small temperature gradient in glass is obtained by the optimization of deposition surface depth.

CFD simulation of vortex flashing R134a flow expanded through convergent-divergent nozzles

Vortex control is a novel two-phase convergent-divergent nozzle restrictiveness control mechanism. The flow control is achieved by adjustable nozzle inlet vortex strength. The underlying mechanism behind vortex control is still unclear. In this study, 3D CFD simulations of vortex flashing R134a flows in convergent-divergent nozzles have been conducted. Good agreement was found between the simulation and experimental results. When there is no vortex applied, the void fraction at the nozzle center remains low. When the vortex is applied, vapor bubbles are driven towards the nozzle center. The applied vortex significantly increases the interphase mass transfer near the nozzle outlet with more uniform interfacial area per unit volume and better utilization of liquid superheats at the nozzle center for evaporation. Thus, after the introduction of inlet vortex, more vapor is generated in the divergent part of the nozzle. As a result, the pressure drop across the divergent part of the nozzle is increased. There is negligible vapor content upstream of the throat even though the pressure is already below saturation pressure. When the inlet vortex is applied, with elevated pressure at the nozzle throat and constant inlet conditions, the pressure difference across the nozzle convergent part decreases and therefore the nozzle mass flow rate is reduced. The overall nozzle outflow mass flow averaged axial velocity can be increased by 30.1%, nozzle isentropic efficiency increases from 37.7% to 63.8% and nozzle mass flow rate drops from 16.0 g s−1 to 12.6 g s−1 when inlet vortex is introduced.

Design and Characteristic Analysis of a MEMS Piezo-Driven Recirculating Inkjet Printhead Using Lumped Element Modeling.

The recirculation of ink in an inkjet printhead system keeps the ink temperature and viscosity constant, and leads to the development of a high-performance device. Herein, we propose a recirculating piezo-driven micro-electro-mechanical system (MEMS)-based inkjet printhead that has a pressure chamber, a nozzle, and double restrictors. The design and characteristic analysis are performed using a two-port lumped element model (LEM) to investigate the effect of design parameters on the system responses. Using LEM, the jetting pressure at the pressure chamber, velocity at the nozzle inlet, meniscus pressure, and Helmholtz resonance frequency are predicted and the comparative analysis of the jetting pressure and velocity between LEM and the finite element method (FEM) simulation is conducted to validate our proposed LEM method. Furthermore, the effect of a change in major design parameters on the jetting pressure, velocity, and Helmholtz resonance frequency is analyzed. On the basis of this analysis, the optimized device dimensions are finalized. From our analysis, it is also concluded that the restrictor is more sensitive than the pressure chamber in terms of their variations in depth. As the cross-talk effect can occur due to an array of hundreds or thousands of nozzles, we investigated the effect of a single activated nozzle on the non-activated neighboring nozzles, as well as the effect of multi-activated nozzles on a single central nozzle using our proposed LEM.

Effect of Nozzle Angle, Size and Pressure on Spray Distribution based on Laboratory Conditions

The objective of this study is to investigate spray parameters for different 40°– 95° even flat fan nozzle angles for banding spraying application, such as spray pattern width and spray volumetric distribution at pressures of 2 and 3bar using different nozzle sizes (0.15, 0.2, 0.3, 0.4 gpm). Spray distribution data was extracted from spray analyzer system or patternator. The results showed that nozzle angle and pressure significantly affected the spray pattern width. In addition, as the nozzle size increased, the liquid volume and the peak height under the nozzle center increased. These results suggest that the use of bigger nozzle angles improved the spray volumetric distribution.

The Impact Force Acting on a Normal Flat Plate Due to Non-Continuum Flow Issuing from Planner Exit with Different Speed Ratio

This paper analysed the impact forces acting on a flat plate exposed normally to a rarefied plume issuing from a nozzle with a planner exit. The plate is assumed to be completely diffuse and the gaseous plume flow out of the exit is modelled with a Maxwellian distribution function, which was characterized with known number density, nonzero exit velocity, and temperature. The analytical collisionless flow results for drag force and shear force due to the impingement on the plate were derived. Effects of speed ratio, wall temperature, exit plume temperature, plate length and vertical distance between nozzle exit and plate center in the impact forces were studied. The results showed that the drag or shear force was directly proportional to the surface area of the flat plate. The drag and shear force increased as the speed ratio increased up to plate semi width of 0.7 m, then both forces decreased as the speed ratio increased up to the end of the plate length. The drag or shear force increased as the distance between the plate and nozzle exit decreased. In addition, the drag force increased as the plume exit temperature or plate temperature increased, while the shear force increased as the plume exit temperature increased. In considering the reflected particles from the plate or neglecting these particles; shear force was not affected, while drag force decreased when the reflected particles were neglected from the computation. Numerical simulation results obtained with the direct simulation Monte Carlo method validate the analytical collisionless flow solution for the surface pressure, shear stress distributions, and the impact forces.

Numerical study on the multiphase flow characteristics of gas curtain launch for underwater gun

A new launching method which can reduce the water resistance in the barrel and realize continuous launching by forming gas curtain without any complex device is proposed for underwater gun. A simulation-experimental platform is set up for the underwater gas-curtain launch. Based on the theory of gas jets in water and interior ballistics, a mathematical physical model for underwater gas curtain launch is established to predict the launching process of the underwater gun. The calculation results show that when the gun is launched in air and underwater, the muzzle velocity of the gas curtain launch reaches 90.9% of that launched in air under the conditions that the maximum pressure is equivalent. However, the muzzle velocity of underwater full-submerged launch is only 66.7% of that launched in air. The gas curtain can be effectively formed with the center nozzle, providing a gas path for the interior ballistics motion of the projectile. During the formation and expansion process of the gas curtain, the projectile head pressure fluctuates between 3 MPa and 6 MPa before the projectile starts to move. It can be seen that the underwater gas curtain launch can effectively form the gas curtain and realize the underwater low-resistance launch, gaining interior ballistics performance comparable to that of air launch. Compared with the conventional underwater full-submerged launch, underwater gas curtain launch can achieve preferable interior ballistics performance.

Large-eddy simulation of low-swirl multi-nozzle combustion with co- and counter-swirling arrays

Large-eddy simulation (LES) of CH4-air low-swirl flame was carried out in a multi-nozzle combustor with two burner configurations by using a premixed flamelet model. The multi-nozzle burner includes a co-swirling array where all five nozzles act in the same direction and a counter-swirling array where the center nozzle is in the opposite swirling direction to the outer nozzles. LES results are in good agreement with OH-planar laser-induced florescence data in terms of OH concentrations and combustion progress variables. Numerical results show that the flow of each nozzle is constant before merging. The neighboring flows interact with each other and generate a highvelocity zone with intensive turbulence. The kinetic energy in the interacting region for the co-swirling array is larger than that for the counter-swirling array. After neighboring flow combining, the flow develops into a unified swirling motion similar to a single swirling flow for the co-swirling array, whereas the flow maintains the individual swirling structures for the counter-swirling arrangement. However, the swirling array exerts minimal effect on multi-nozzle combustion in terms of the temperature distributions and combustion progress of premixed low-swirl multi-nozzle flames.

ЕСТЕСТВЕННАЯ ВЕНТИЛЯЦИЯ ГАЗОВОГО ПРОСТРАНСТВА В РЕЗЕРВУАРЕ С ПОНТОНОМ

The article deals with safe operation issues of vertical steel reservoirs with an internal floating roof when storing volatile oil products. The purpose of the work is to study the influence of ventilation openings area and wind speed on the duration of explosive state of vertical reservoirs with an internal floating roof. The influence of ventilation pipes' dimensions and the wind speed on the duration of explosive state of the reservoir has been studied. Method for calculating this time is proposed. It is shown that natural ventilation of the reservoir gas space is caused by the effect of two forces, which are formed due to: 1) the density difference between the vapor-air mixture in the reservoir and outside air; 2) wind pressure occurring on the roof of the reservoir. An algorithm for calculating the duration of reservoir being in an explosive state with wind pressure and no wind is obtained. The greater the difference in geodetic marks of the central and peripheral nozzles, the more efficient the ventilation. This distance will be greatest if the lower ventilation pipes are located on the upper belt of the reservoir or the reservoir is equipped with an air drain. Increase in wind speed of more than 10 m/s does not significantly affect the duration of the reservoir being in an explosive state. Increasing the diameter of the central nozzle from 200 to 500 mm can significantly reduce the duration of the reservoir degassing in windless weather.

Blowoff and Reattachment Dynamics of a Linear Multinozzle Combustor

This paper describes the coupled flow and flame dynamics during blowoff and reattachment events in a combustor consisting of a linear array of five interacting nozzles using 10 kHz repetition-rate OH planar laser-induced fluorescence and stereoscopic particle image velocimetry (S-PIV). Steady operating conditions were studied at which the three central flames randomly blew-off and subsequently reattached to the bluff-bodies. Transition of the flame from one nozzle was rapidly followed by transition of the other nozzles, indicating cross-nozzle coupling. Blow-off transitions were preferentially initiated in one of the off-center nozzles, with the transition of subsequent nozzles occurring in a random order. Similarly, the center nozzle tended to be the last nozzle to reattach. The blow-off process of any individual nozzle was similar to that for a single bluff-body stabilized flame, though with cross-flame interactions providing additional means of restabilizing a partially extinguished flame. Subsequent to blowoff of the first nozzle, the other nozzles underwent similar blow-off processes. Flame reattachment was initiated by entrainment of a burning pocket into a recirculation zone, followed by transport to the bluff-body; the other nozzles subsequently underwent similar reattachment processes. Several forms of cross-nozzle interaction that can promote or prevent transition are identified. Furthermore, the velocity measurements indicated that blowoff or reattachment of the first nozzle during a multinozzle transition causes significant changes to the flow fields of the other nozzles. It is proposed that a single-nozzle transition redistributes the flow to the other nozzles in a manner that promotes their transition.

Effect of fuel nozzle geometry and airflow swirl on the coherent structures of partially premixed methane flame under flashback conditions

The effect of fuel nozzle geometry and swirling airflow on the flashback and its relationship with the coherent structures of partially premixed methane flame is investigated experimentally. Particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are used to document flow characteristics, and Schlieren imaging technique is used to study flame appearance and vortex shedding frequency downstream of the burner exit. Proper orthogonal decomposition (POD) technique is applied to capture the coherent structures, along with phase averaging of the linear superposition of the first four POD modes. Seven different fuel nozzle geometries and two swirl number (S = 0.79 and 1.15) are tested. The nozzles are categorized into three groups, with each has similar equivalent diameter; namely, a symmetric nozzle (used as a reference), nozzles with polygonal orifices (group A), and angled multi-orifice nozzles (group B). The results of the flow field inside the mixing tube show that the strength of coherent structures and flashback propensity increase with the swirling airflow Reynolds number, swirl number, nozzle bluff body area, and the number of the peripheral angled orifices of the fuel (central) nozzle. On the other hand, the results of flame appearance outside of the mixing tube indicate that methane flame experiences symmetric vortex shedding at high swirl number and low Reynolds number, while it experiences PVC near blowout conditions at low swirl number and high Reynolds number. Furthermore, the frequency of coherent structures is found to depend on the swirling airflow Reynolds number, swirl number, and fuel nozzle geometry. Additionally, the flashback’s mean region inside the mixing tube is found directly proportional to the strength and frequency of the coherent structures.