Strong Recirculation Zone Research Articles

Turbulence model study for aerodynamic analysis of the leading edge tubercle wing for low Reynolds number flows

A turbulence model study was performed to analyze the flow around the Tubercle Leading Edge (TLE) wing. Five turbulence models were selected to evaluate aerodynamic force coefficients and flow mechanism by comparing with existing literature results. The selected models are realizable k-ε, k-ω Shear Stress Transport (SST), (γ−Reθ) SST model, Transition k-kl-ω model and Stress- ω Reynolds Stress Model (RSM). For that purpose, the TLE wing model was developed by using the NACA0021 airfoil profile. The wing model is designed with tubercle wavelength of 0.11c and amplitude of 0.03c. Numerical simulation was performed at chord-based Reynolds number of Rec = 120,000. The Computational Fluid Dynamic (CFD) simulation reveals that among the selected turbulence models, Stress- ω RSM estimated aerodynamic forces (i.e. lift and drag) coefficients closest to that of the experimental values followed by realizable k-ε, (γ−Reθ) SST model, k-ω SST model and k-kl-ω model. However, at a higher angle of attacks i.e. at 16° & 20° k-ω SST model predicted closest drag and lift coefficient to that of the experimental values. Additionally, the critical observation of pressure contour confirmed that at the lower angle of attack Stress- ω RSM predicted strong Leading Edge (LE) suction followed by realizable k-ε, (γ−Reθ)SST model, k-ω SST model and k-kl-ω model. Thus, the superiority of Stress- ω RSM in predicting the aerodynamic force coefficients is shown by the flow behavior. In addition to this pressure contours also confirmed that k-kl-ω model failed to predict tubercled wing aerodynamic performance. At higher angles of attacks k-ω SST model estimated aerodynamic force coefficients closest to that of the experimental values, thus k-ω SST model is used at 16° & 20° AoAs. The observed streamline behavior for different turbulence models showed that the Stress- ω RSM model and k-kl-ω model failed to model flow behavior at higher AoAs, whereas k-ω SST model is a better approach to model separated flows that experience strong flow recirculation zone.

Large-eddy simulation investigation on the effects of inlet swirl/Reynolds number and fuel heating value on a turbulent kerosene spray flame

To progress toward the high efficiency and low emission aviation engine technology, it is mandatory to understand the complex phenomena including kerosene spray atomization, evaporation, turbulent mixing and combustion in the engine combustor. Nowadays with the rapid growth of computational capacity over the world, large-eddy simulation (LES) performs a more and more important role in understanding the complex multi-phase combustion process of kerosene fuels in aviation engine configuration. In the present work, LES is employed to investigate a swirling kerosene spray jet flame in a model combustor, which has been studied experimentally. The turbulent combustion is modeled by the flamelet generated manifold (FGM) approach with a 3-component surrogate kerosene skeletal mechanism. The averaged gas velocity and temperature predicted by LES generally agrees well with the experimental measurements. Local extinction is observed at the inner shear layer of the swirling spray flame, due to droplet-turbulence-flame interactions. A parametric study with 11 cases is then performed to investigate the effects of inlet swirl/Reynolds number and fuel heating value on the kerosene flame. As the swirl number increases, the enhanced turbulent mixing between kerosene fuel and air facilitates the combustion process, but combustion instabilities may occur if the swirl number is too high. A higher Reynolds number can promote the turbulent combustion via a stronger recirculation zone, but it also has strong dilution effects on the combustion field. Variations in the fuel heating value directly impact the temperature field, but the flow field, major species concentrations and droplet phase statistics are only slightly affected. These LES results give clues to achieve optimal combustion performance in realistic aviation engine configurations via better arrangements of fuel and air injections.

Effect of heart rate on the hemodynamics in healthy and stenosed carotid arteries

Stenosis in the internal carotid arteries is a serious cardiovascular condition. It is well-reported that low and oscillatory wall shear stress enhances the risk of stenosis progression. However, the effects of increased heart rates in highly stenosed arteries are not well explored. A detailed understanding of the flow features and stress distribution in stenosed carotid arteries at different heart rates may help clinicians to prescribe better exercise schedules for patients. In this study, we probe the effects of elevated heart rates on the hemodynamics in healthy and stenosed carotid arterial geometries using an immersed boundary method-based computational framework. Our results reveal that a strong recirculation, secondary velocity, and oscillatory shear index (OSI) zone develop inside a severely stenosed carotid artery at normal heart rate. Higher heart rates may potentially improve arterial health by reducing OSI only for the healthy and mild stenosis carotid arteries. However, the increased heart rates worsen the arterial health of severely stenosed arteries by onsetting flow instabilities, enhancing the spread and severity of the recirculation zone and the magnitude of the secondary velocity, the pressure drops across the stenosis, and the spread of high OSI (≥0.2) zone downstream. Furthermore, in the case of severe stenosis, the wall shear stress at the stenosis throat rises significantly, which can contribute to plaque rupture and thrombus development. Here, we report in detail the behavior of stress levels and pressure fluctuations in the carotid artery model at different stenosis levels for normal and elevated heart rates.

Visualization of recirculation zones over a perforated plate: An optical flow technique for characterization of fluid dynamics in structured packing

In the context of increasing the performance of structured packing used in distillation columns, we develop a state-of-the-art flow diagnostic method to investigate the intricate fluid dynamics of a falling film interacting with a perforation. Perforations are one of the several key microstructure elements in industrial structured packing whose role in mass transfer enhancement and liquid redistribution is yet not well understood. We focus the present investigation on the liquid curtain, or "twin film", blanketing the perforation. Twin films are prone to strong capillary waves and recirculation zones, and experiments of fundamental nature are required to assess the fluid dynamics in the curtain formation in the perforations and ultimately uncover mechanisms that intensify interfacial mass transfer. Here, a three-dimensional two-component (3D-2C PTV) optical-flow-based technique using the concept of defocus is implemented on a test bench designed to study the effect of perforations on falling liquid films. This method provides an opportunity to evaluate the hydrodynamics of the liquid curtain through streamline patterns and dynamic modes and to describe key features that could promote mass transfer intensification in structured packing.

Secondary air induced flow structures and mixing in a fixed bed combustor

Air staging features widely in biomass combustion from small space heaters to industrial-scale moving grate systems. Whilst studies have been conducted into the impact of air staging on emissions and combustion performance, there is little or no insight into the exact flow dynamic and physical mechanisms that are induced by secondary air under such conditions. This paper uses experimental data to validate numerical predictions before investigating the flow field structures and mixing characteristics in a fixed bed air staged combustor. The study utilizes Constant Temperature Anemometry (CTA) experiments to establish boundary conditions for a lab-scale fixed bed combustor. Results from numerical simulations obtained using [Formula: see text] l- 𝜔 model were in good agreement with experimental data. Conditions tested cover five different secondary to total air ratios (Qs/Qt) and two different locations of secondary air injection. Results show that at Qs/Qt = 0.50, 0.25 and 0.18 one strong recirculation zone is induced upstream of the secondary air injection and one downstream. Varying the point of injection of secondary air from h/D = 0.64 to h/D = 0.40 also had an effect on the size of the upstream recirculation zone. Qs/Qt = 0.50 had the highest values of mixing index among all spatially located planes at the upstream of secondary air injection. The overall findings shed light on the possible flow interactions between secondary air and the top layers of fuel bed. They also highlight the significance of secondary air on inducing both upstream and downstream flow structures.

Investigation of a low emission peripheral vortex reverse flow (PVRF) combustor fuelled by LPG and ethylene

Peripheral Vortex Reverse Flow (PVRF) combustor is characterized by a strong peripheral recirculation zone resulting in stable combustion and low pollutant emissions. In this study, a PVRF combustor is investigated in the premixed and non-premixed modes with LPG and ethylene fuels at a maximum heat load of 6.25 kW and thermal intensity of 25 MW/m3-atm, relevant to gas turbine combustors. The fuel is injected co-axially, with respect to the air jet, using two different fuel injection diameters of 1 mm, and 2 mm. For both the fuels, the fuel injection velocity is higher as compared to the air injection velocity for fuel injection diameter of 1 mm, while it is lower for fuel injection diameter of 2 mm. CH* chemiluminescence images are acquired, and CO and NOx emissions are measured. Reynolds Averaged Navier Stokes (RANS) simulations are carried out at an equivalence ratio of 0.8 using the Eddy Dissipation Concept (EDC) model with a H2/CO/C1–C4 mechanism comprising of 50 species and 373 reactions. Delayed ignition and downstream shifting of the reaction zone facilitating higher product gas entrainment prior to reactions is observed in the non-premixed mode as compared to the premixed mode. NOx emissions were found to be lower in the non-premixed mode as compared to the premixed mode for both the fuels. This is possibly due to suppression of NO formation in the reaction zone via. prompt route which is also supported by lower CH* chemiluminescence intensity in the non-premixed mode. With LPG, the case with larger fuel injection diameter showed further delayed mixing as compared to the case with smaller fuel injection diameter, and resulted in even lower NOx emissions. For all the cases, CO emissions were of similar levels in the non-premixed and premixed modes suggesting that lower NOx emissions in the non-premixed mode is not at the expense of increase in CO emissions. Overall, the investigation shows that lower NOx emissions can be achieved in the non-premixed mode as compared to the premixed mode if more product gas entrainment is facilitated prior to the reactions by delayed mixing between fuel and oxidizer.

MODULATION OF RECIRCULATION ZONE BEHIND A CUBICAL OBSTRUCTION BY THE VERTICALLY PLACED TURBULENT MULTIJETS IN THE FORM OF SHOWER

The present numerical work reports the application of jets for modulating and eradicating the recirculation zone in the wake region behind a fully submerged two-dimensional square cross-section bluff body for high Reynolds number flow. The wake region of the bluff body is characterized by a strong recirculation zone and vortex shedding, leading to unwanted vortex-induced vibrations that may instigate structural damage. Jets of varying Reynolds number and center-to-center distance are explored to look into the most effective jet Reynolds number and jet spread in completely modulating the recirculation region. The numerical simulation is based on a two-phase volume-of-fluid model with open channel boundary conditions. The standard k-ω SST two-equation turbulence model is applied to close the time-averaged-momentum and continuity equations. The negative mean streamwise velocity signifies the recirculation region, and streamline contour plots are made to identify it. The variation in discharge velocity of the jet series is carried out for reallocation or complete disappearance of the recirculation region.

Inner and outer flow of an adhering droplet in shear flow

Adhering droplets in shear flow start to move downstream when a critical velocity ucrit is reached, i.e. the adhesion force is overcome by aerodynamic force. It is still not clear, whether drag-coefficient-estimation based on an approximation of the droplet as a rigid spherical cap is a valid approach. In contrast to rigid hemispheres there is a gas-liquid-interface, i.e. the velocity of the interface is not zero by all means. Therefore, there is a fluid transport inside the drop that depends on the outer flow structure. We report on a combined numerical and experimental investigation to find out the characteristic inner and outer flow structure of an adhering droplet in shear flow in order to evaluate the rigid-hemisphere-approach.The inner flow structure of the droplet is measured by particle-image velocimetry (PIV) while the wake flow of the droplet in the gas-phase is measured by a modified laser-doppler-velocimeter (laser-doppler profile sensor). Numerical simulations of a droplet have been carried out using a modified volume of fluid (VOF) model taking into account contact angle hysteresis effects in OpenFOAM®. The complete two-phase fluid motion of the liquid droplet and surrounding gas phase is simulated, i.e. the inner and outer flow of the droplet are captured with a three-dimensional simulation model.Experimental and numerical results agree very well concerning flow pattern inside and outside the droplet. There is a strong connection between the inner and outer flow of the droplet. For lower Reynolds numbers based on droplet diameter (Red < 800) the flow inside the droplet rotates in direction of the channel flow. On the leeward side of the droplet there is a small wake flow with slightly decreased flow velocity. At higher Reynolds numbers (Red ≥ 800) the flow pattern inside the droplet changes completely: A strong reverse flow close to the droplet surface appears. Concurrently a strong recirculation zone emerges in the wake of the droplet. This is due to the roll-up process of a separated shear layer originating from the droplet. Due to the coupling of the inner flow with the outer flow via the gas-liquid-interface our data show that at higher Reynolds numbers flow separation takes place on the windward half of the droplet. This is in contrast to the case of a rigid hemisphere, where the separation is located at approximately 90°, i.e. the top of the droplet, and does not move upstream with increasing Reynolds number. The height of the separation zone increases up to 1.29 times the droplet height, which is not observed for rigid hemispheres. Since the pressure field around an obstacle is governed by the flow separation location and the corresponding size of the recirculation zone, the coupling of the inner and outer flow changes the pressure distribution around the droplet with respect to a similar rigid structure. This will affect the aerodynamic forces acting on the droplet, which is why the approximation by a rigid spherical cap does not seem to be a suitable approach. Nevertheless, additional experimental and numerical efforts need to be done to assess the flow structures, pressure distributions and forces acting on the droplet in a suitable precision also considering possible aero-elastic effects.

LES predictions of confinement effect on swirling flow generated by single helical axial swirler

PurposeFor higher swirling flows (swirl > 0.5), flow confinement significantly impacts fluid flow, flame stability, flame length and heat transfer, especially when the confinement ratio is less than 9. Past numerical studies on helical axial swirler type systems are limited to non-reacting or reacting flows type Reynolds averaged Navier Stokes closure models, mostly are non-parametric studies. Effects of parametric studies like swirl angle and confinement on the unsteady flow field, either numerical or experimental, are very minimal. The purpose of this paper is to document modeling practices for a large eddy simulation (LES) type grid, predict the confinement effects of a single swirler lean direct injection (LDI) system and validate with literature data.Design/methodology/approachThe first part of the paper discusses the approach followed for numerical modelling of LES with the minimum number of cells required across critical sections to capture the spectrum of turbulent energy with good accuracy. The numerical model includes all flow developing sections of the LDI swirler, right from the axial setting chamber to the exit of the flame tube, and its length is effectively modelled to match the experimental data. The computational model predicts unsteady features like vortex breakdown bubble, represented by a strong recirculation zone anchored downstream of the fuel nozzle. It is interesting to note that the LES is effective in predicting the secondary recirculation zones in the divergent section as well as at the corners of the tube wall.FindingsThe predictions of a single helical axial swirler with a vane tip angle of 60°, with a duct size of 2 × 2 square inches, are compared with the experimental data at several axial locations as well as with centerline data. Both mean and unsteady turbulent quantities obtained through the numerical simulations are validated with the experimental data (Cai et al., 2005). The methodology is extended to the confinements effect on mean flow characteristics. The time scale and length scale are useful parameters to get the desired results. The results show that with an increase in the confinement ratio, the recirculation length increases proportionally. A sample of three cases has been documented in this paper.Originality/valueThe novelty of the paper is the modelling practices (grid/unsteady models) for a parametric study of LDI are established, and the mean confinement effects are validated with experimental data. The spectrum of turbulent energies is well captured by LES, and trends are aligned with experimental data. The methodology can be extended to reacting flows also to study the effect of swirl angle, fuel injection on aerodynamics, droplet characteristics and emissions.

Influence of a Central Jet on Isothermal and Reacting Swirling Flow in a Model Combustion Chamber

We studied flow dynamics in a model combustion chamber using Large-eddy simulations (LES) and Particle image velocimetry (PIV) at the Reynolds number Re of 15,000. The swirl is produced using a Turbomeca swirler and air flow, while combustion is supported by a central methane/air jet. We compared four flow regimes, assessing the effect of the central jet for isothermal and lean reacting conditions. A detailed comparison for isothermal and reactive cases without the central jet is described, validating the LES results against PIV. We observe that unsteady dynamics are governed by global instability in the form of a well-known precessing vortex core (PVC). The central jet slightly changes the dynamics of PVC in the isothermal case where a strong recirculation zone is still formed. However, for the reacting case, the bubble is completely destroyed with no signs of strong vortical structures in the inner shear layer. These observations are confirmed using spectral analysis and proper orthogonal decomposition, describing the contribution of different flow modes in terms of azimuthal harmonics.

Large Eddy Simulations of a Low-Swirl Gaseous Partially Premixed Lifted Flame in Presence of Wall Heat Losses

The use of lifted flames presents some very promising advantages in terms of pollutant emissions and flame stability. The focus here is on a specific low-swirl injection system operated with methane and derived from an air-blast atomizer for aero-engine applications, which is responsible for flame lift-off. The key feature of this concept is the interaction between the swirling jet and the confinement walls, leading to a strong outer recirculation zone and thus to an upstream transport of combustion products from the main reaction region to the flame base. Here, the representation of the physics involved is challenging, since finite-rate effects govern the lift-off occurrence, and only a few numerical studies have been carried out on this test case so far. The aim of the present work is therefore to understand the limits of some state-of-the-art combustion models within the context of LES. Considering this context, two different strategies are adopted: the Flamelet-Generated Manifold (FGM) approach and the Thickened Flame (TF) model. A modified version of the FGM model including stretch and heat loss effects is also applied as an improvement of the standard model. Numerical results are compared with the available experimental data in terms of temperature and chemical species concentration maps, showing that the TF model can better reproduce the lift-off than the FGM approach.

Numerical investigation on mixing improvement mechanism of transverse injection based scramjet combustor

The major hurdle in augmenting the performance of supersonic engines is the appropriate distribution of injected fuel within the supersonic chamber. That's why SSTk−ω model is used to explore the mixing characteristics of a transverse jet injection into a supersonic airstream. In this aspect, initially, the performance of the supersonic combustor is analysed using hydrogen and ethylene fuel in terms of mixing efficiency. Our results suggest that the flame holding capacity of hydrogen-fuelled combustor is better as than an ethylene-fuelled combustor. Additionally, to strengthen the mixing of fuel with supersonic air-flow, a novel shock generator is introduced and placed at different locations of the combustor. The flow-structure is studied comprehensively using two single shock generator and two dual shock generator to conclude the best possible geometry for improved performance. The results reveal that the fuel penetration depth of geometry 3 with dual shock generator has increased considerably. Moreover, geometry 3 offers enhanced mixing between the supersonic air and fuel compared to remaining models with the formation of a strong vortex pair and recirculation zone near the injection slot.

The effect of swirl burner design configuration on combustion and emission characteristics of lean pre-vaporized premixed flames

The present paper experimentally provides a quantitative comparison between the aerodynamic flow field at non-reaction conditions (cold flow) and the combustion and emission characteristics (hot conditions) of two swirl burners having the same swirl number of 0.55 but with two different configurations. The first burner features an annular outer swirler (swirl angle of 40°) and multiple central circular jets, leading to “low swirl combustion”, whereas the second burner has a blocked central passage and an outer swirler (swirl angle of 35°) resulting in “high swirl combustion”; featuring a central recirculation zone. The cold flow study is conducted using a calibrated five-hole probe while the combustion and emission characteristics are obtained via measurements of flame temperatures and the dry volumetric analysis of CO and NOx emissions. Each burner is coaxially mounted to a vertical cylindrical combustor; inside which a lean (equivalence ratio: ϕ = 0.75), pre-vaporized (preheated to a temperature of 523 K) partially premixed mixture of Jet A-1 fuel and air are admitted. The former low swirl combustion burner gives a bulk stable and uniform reaction zone (W-shape) close to the burner exit with a very slight lifting; confirming the concept of “stable lifting” while reducing the pressure drop across the burner exit and eliminating hot spots in the reaction zone. The high swirl combustion burner exhibits a strong central recirculation zone (better flame stability) that is surrounded by an outer V-shape reaction zone having comparatively higher peak levels of temperature, CO and NOx that would lead to greater liability of higher exhaust emission.

Implication of geometrical configuration of cavity on combustion performance in a strut-based scramjet combustor

The present work describes the numerical investigation on the implication of the geometrical configuration of a cavity with a strut-based scramjet combustor. Firstly, the combustion characteristics of a combustor with a cavity and the combustor without a cavity are compared and evaluated to portray the effect of a cavity. On the other hand, a separate study is performed to investigate the interplay of cavity parameters namely, length to depth (L/D) ratio of a cavity and angle of a cavity (α) on the performance of a strut-based scramjet combustor. Our study reveals the formation of a strong recirculation zone in the cavity region which improves the mixing and combustion efficiency in the case of a cavity-based combustor. It is observed that the combustion commences relatively earlier in the case of a cavity-based combustor and reaches to its peak value of around 80%. Further, the size of a recirculation region depends on the distance between the strut injector and the length of a cavity. Moreover, the shear layer is observed in the cavity region inducing a weak shock at the trailing edge of a cavity which deteriorates the performance of a scramjet combustor particularly for a higher angle of a cavity(α) and lowers L/D ratio of a cavity. Also, the combustion efficiency is found to be optimal with moderate pressure losses at L/D = 4 and α = 300.

Flow and Frequency Characterization of the Synchronized Stacked Sweeping Jets

This work characterizes a pair of feedback-type fluidic oscillators in a stacked arrangement. Quasi-rectangular synchronization channels were used to connect both feedback channels of each oscillator; thus communication between fluidic oscillators was established. Synchronization through these channels provided strong cross-correlation values () obtained with hot-wire probes for a total flow rate range of 10–30 standard liters per minute. The oscillation frequencies were varied from 263 to 672 Hz for a Reynolds number range from 5660 to 17,000, respectively. The resultant sweeping jets were also observed through water flow visualizations. Details of the internal flowfield leading to synchronized behavior were extracted employing numerical simulations that were validated with the frequency measurements. It was determined that the feedback flow in feedback channels occurs simultaneously in a periodic manner, and thus jet switching inside each oscillator is synchronized. It was also observed that a pair of strong recirculation zone forms for every period inside both synchronization channels, and this zone plays a major role in balancing the momentum in the feedback channels, thus creating simultaneous feedback flow. The overall switching mechanism of the fluidic oscillators seems to be intact, whereas the pair found to act as a single dynamic system.

Operability of a premixed combustor holding hydrogen‐enriched oxy‐methane flames: An experimental and numerical study

This work presents a detailed investigation on the stability, flame macrostructure and combustion characteristics of oxy-methane/hydrogen (H2CH4O2CO2) flames in a premixed swirl-stabilized model gas turbine combustor. The effects of equivalence ratio, inlet Reynolds number and inlet swirl number on stability and combustion characteristics were examined experimentally and numerically. A base operation case was selected as a reference to study the effects of the different parameters with equivalence ratio of 1.0, bulk mean throat velocity of 5.2 m/s, and swirl number of 0.98. Fixed compositions of the oxidizer (30%O2/70%CO2) and fuel (50%H2/50%CH4) streams were considered throughout the study. Large eddy simulation (LES) technique was adopted to compute the premixed turbulent flame regime. Experimentally captured flame images were compared with the flame contours obtained from the numerical simulations, and good conformations were observed. The results showed that near-stoichiometry flames exhibit strong inner recirculation zone (IRZ) and are stabilized between two shear layers. At lower equivalence ratios, the flames are corner-stabilized, and the flow is dominated by a single primary eddy in the IRZ. The calculated strain rates predict flame quenching at equivalence ratio of 0.5, which was also confirmed experimentally. It was observed that the effect of Reynolds number on the flame shape is not significant; however, the flow is dominated by larger primary eddy with strong IRZ as the Reynolds number increases. The IRZ also gets stronger with the increase in the flow swirl number. At higher swirl numbers, the flame stabilization occurs at the inner shear layer and the outer recirculation zone (ORZ) diminishes. It was found that the chemical time scale for all cases under investigation is shorter than the Kolmogorov time scale, signifying that the chemical reactions are dominated by the laminar flame speed.

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied.

Fluid Flow and Heat Transfer Characteristics of Multiple Swirling Impinging Jets at Various Impingement Distances

This research investigates the fluid flow and heat transfer characteristics for the effect of multiple swirling jets impinging on a heated plate. In this regard, numerical simulations were performed for the inline-type jet arrangement using ANSYS v16.2. Governing equations for turbulent swirl flows were solved by coupled algorithm whereby turbulences are described by SST k-ω model. The analysis is studied for Reynolds number Re = 11600 and swirl number 0.74 at impingement distances equal to 1, 2, 3 and 4 times nozzle diameter. The numerical results showed that impinging distance has a significant effect on both heat transfer and fluid flow characteristics. In case of low impinging distance (H=1D) swirling effect was dominant and the strong recirculation zones resulted in a higher heat transfer from the heated surface. With the increase of impingement distance, the turbulent kinetic energy reduced significantly near the heated surface. It was evident that for higher impingement distance (H=4D) the effect of swirl was greatly reduced resulting in a lower heat transfer from the heated surface

The Influence of Air Flow Velocity and Particle Size on the Collection Efficiency of Passive Electrostatic Aerosol Samplers

ABSTRACT Electrostatic sampling is a promising method for the collection of bioaerosol particles. Although the underlying physics responsible for particle collection are well understood, the collection efficiency of simple passive electrostatic samplers is difficult to predict. Under these conditions, the collection efficiency becomes very sensitive to ambient air current and particle size, especially for submicron particles relevant for airborne virus transmission. In this paper, we compare two electrostatic aerosol sampler designs, a commercial product consisting of a flat collector plate located in the same plane as the charging needles and an axisymmetric design sampling directly to a liquid droplet. The aerosol particle collection efficiency of the samplers is investigated for particle size ranging from 0.25 to 2 µm while the air flow velocity surrounding the samplers is varied from 0.3 to 1 m s–1. For the planar design, at all ambient flow velocities, the submicron fraction of the particles captured originates in streamlines up to a maximum of 75 mm above the surface of the device collector, which greatly limits the volume of air being effectively sampled. The axisymmetric design features a non-monotonic capture efficiency as a function of particle size, with a minimum between 0.4 and 0.8 µm. The flow field in the inter-electrode region, captured using particle image velocimetry (PIV) reveals the presence of strong recirculation zones that can be responsible for the increased collection efficiency for very small particles.

Impact of co‐flow on the spray flame behaviour applied to nanoparticle synthesis

AbstractFlame spray pyrolysis (FSP) is an established process to synthesize nanoparticles of various metals and metal oxides. Applying open or enclosed configurations of the FSP reactor is an efficient tool to control the fuel‐oxidizer ratio in the reaction zone and, thus, the temperature distribution and the particle formation and growth process. In the present work, geometrical setups representing an open and an enclosed flame reactor are compared and their influence on the temperature, velocity, and particle characteristics is investigated. In addition, several distinct kinetic mechanisms for the combustion reactions are evaluated and their effects on the local reactor temperature and gas composition distribution are analyzed. An Eulerian‐Lagrangian approach is adopted to describe the multiphase turbulent gas‐droplet flow and a monodisperse approach based on the population balance equation (PBE) model is implemented to predict the particle formation and evolution. From the open reactor results, the air entrainment mass flow rate of gas into the flame is calculated. Several numerical experiments are performed with the enclosed setup. Supplying an appropriate co‐flow rate into the enclosed reactor results in similar flame behaviour as found for the open reactor configuration. By reducing the co‐flow gas, strong recirculation zones and particle deposition on the enclosure walls are observed. In this situation, the local temperature increases considerably, resulting in larger primary nanoparticle diameters.