Co-flowing Air Stream Research Articles

Influence of Coflow on Buoyant Plume Puffing

Abstract This study investigates the influence of an annular coflowing air stream on the puffing behavior of a buoyant plume by employing the BiGlobal Linear Stability Analysis. An increase in the coflow is found to mitigate the puffing intensity and eventually stabilize the plumes. From the stability analysis, the critical coflow ratios, which represent the amount of coflow required to completely suppress the puffing, have been estimated for plumes spanning a wide range of nondimensional parameters. The analysis shows that the critical coflow ratio largely depends on the two buoyancy parameters, the Froude number, and the density ratio whereas it remains marginally affected by the plume Reynolds number. Plumes with higher buoyancy require larger coflow for suppressing puffing. From the instability analysis, we have obtained a correlation law for critical coflow ratios in buoyant plumes. Also, it is found that the plume puffing frequency increases with an increase in the coflow. We attempt to ascertain the reasons for instability mitigation and frequency increase in the puffing plumes because of coflow.

Mixing Enhancement by Pulsed Energy Deposition in Jet/Shock Wave Interaction

The mechanism of the supersonic round jet mixing enhancement by a pulsed energy deposition is studied numerically by solving the three-dimensional unsteady Reynolds-averaged Navier–Stokes equations with the second-order scheme in space and time. A round jet at different Mach numbers (, 1.76, 3.00) is parallelly injected into a coflowing air stream of Mach number 2.5, interacting with the oblique shock generated by a compression ramp of 20°. The density ratio between the jet and external flow is 0.5. The laser pulse with at 20 kHz is modeled as an ellipsoid for the energy addition, and proposed to put above or inside the jet for jet mixing control. It is found that the energy deposition inside the jet is much more effective for the jet mixing amplification compared with that above the jet. The jet mixing enhancement is achieved through the large-scale vortices, initiated by the energy deposition zone interacting with the oblique shock wave. It is shown that the jet cross-sectional area is doubled, and the turbulent kinetic energy is increased, even reaching 10 times the original level at the downstream location. However, a slight decrease about 0.3% of the total pressure recovery coefficient is observed far downstream.

Fractal structures arising from interfacial instabilities in bio-oil atomization

The intriguing multi-scale fractal patterns ubiquitously observed in nature similarly emerge as fascinating structures in two-phase fluid flows of bio-oil breakup and atomization processes. High-resolution microscopy of the two-phase flows under 15 flow conditions (cases of different flow rates of the liquid and co-flowing air streams as well as different degrees of liquid preheating) reveal that the geometrical complexities evolve under the competing/combined action of the instability mechanisms such as Kelvin–Helmholtz, Rayleigh–Taylor and Rayleigh–Plateau leading into the transition from break-up to atomization. A thorough analysis of the higher order moments of statistics evaluated based on the probability density functions from 15,000 fractal dimension samples suggest that a single-value analysis is not sufficient to describe the complex reshaping mechanisms in two-phase flows. Consistently positive skewness of the statistics reveal the role of abrupt two-phase mechanisms such as liquid column rupture, ligament disintegration, liquid sheet bursting and droplet distortions in a hierarchical geometrical entanglement. Further, large kurtosis values at increased flow inertia are found associated with turbulence-induced intermittent geometrical reshaping. Interestingly, the proposed power-law correlation reveals that the global droplet size obtained from laser-diffraction measurements declines as the two-phase geometrical complexity increases.

Dense Sprays with a Focus on Atomization and Turbulent Combustion

This paper presents a brief overview of recent research conducted by our group to advance current understanding of primary and secondary atomization as well as the turbulent combustion of sprays. The focus is on air-blast atomization. The burner employed exploits the benefit of recessing a liquid-injecting needle into the primary air-blast annulus to transition the spray exiting the burner from dilute to dense. A pilot is used to stabilize the flame to the burner which is sitting in a co-flowing stream of secondary air. Direct links are found between the wave instabilities and the size of the fragments shed from the liquid core. In the secondary atomization zone, the statistics of three types of fragments, namely droplets, ligaments and irregular fluid shapes are provided. This approach may provide a platform for the modelling of denser spray flames without the need to account for primary atomization. For reacting cases, mean temperature measurements are presented. This burner (referred to as the needle spray burner) is employed to stabilize dilute and dense spray flames.

Retracted: “Symmetric and Asymmetric Disturbances in the Rayleigh Zone of an Air-Assisted Liquid Sheet: Theoretical and Experimental Analysis,” [ASME Journal of Fluids Engineering, 2020, 142(7), p. 071302; DOI: 10.1115/1.4045998

AbstractThe temporal analysis of symmetric (dilatational) and asymmetric (sinusoidal) perturbations at the interface of a water sheet in a coflowing air stream focuses on low gas Weber number region (Weg < 0.4), namely, Rayleigh breakup zone. The motive for this investigation is to acquire a better insight of breakup phenomena involved, rather than technical relevance, by utilizing Kelvin–Helmholtz instability. Accordingly, perturbations are introduced on the basic flow whose stability is to be examined by the method of normal (Fourier) modes. The temporal growth-rate of perturbations is traced to extract the wavenumber associated with maximum growth-rate. Thus, the critical wavelength, in conjunction with the phase velocity of the disturbance will facilitate to obtain the corresponding breakup frequency of the liquid sheet. The analytical findings on liquid sheet breakup frequency with increasing Weber number ratio exhibit the dominance of symmetric wave over asymmetric wave. It also shows independent evolution of breakup frequency with respect to Weber number ratio for the respective perturbation modes, which appears to be a pointed profile. That is, the frequency contour for dilatational mode dips, whereas it rises for the sinusoidal mode and at the Weber number ratio of 0.518 the crossover occur. The theoretical results were substantiated by high-speed flow visualization studies that discern the coexistence of low-frequency (primary) and high-frequency (intermediate) breakup events. Furthermore, the empirical average frequency data track reasonably well with the dilatational instability.

Effects of bubbles in the liquid jet on the air-blast atomization

The air-blast atomization with bubbles in the liquid jet was investigated using the high-speed camera and Malvern laser particle size analyzer. The gas was aerated into the liquid jet to form the gas-liquid mixture, which would be atomized by the co-flowing air stream. Results show that droplet size D32 decreases at the beginning and then increases with the volumetric flow rate of the aerating gas. A formula was derived to delineate the nonmonotonic trend of the droplet size. The relative deviations between the predicted results and experimental results are less than ±5%. A contour map describing the dependence of the droplet size on the aerating gas, liquid jet, and co-flowing air stream was obtained, which could provide some guidance for the further investigations and industrial applications.

Изучение пределов устойчивого горения диффузионного пламени микроструи водорода, истекающей из круглого микросопла, при введении в водород или воздух инертных и реагирующих газов

The effect addition of inert (He, N2, Ar, CO2) and reacting (СН4, O2, CF3Br, (CH3O)PO) gases in a hydrogen or in coflowing air stream on lift-off diffusion flame conditions of hydrogen micro jet effluxed from the round micro nozzle was experimentally studied. Using the schlieren technique a critical hydrogen flow rate was established at which the flame of the hydrogen microjet detaches from the nozzle when introducing additives of the studied gases into both air and hydrogen. It has been established that the addition of all the studied gases to the hydrogen leads to a decrease in the velocity range of its microjet at which flame stabilization is possible, regardless of whether the gases introduced into hydrogen are inert or reactive. It is shown that in the case of the addition of various gases to the hydrogen, the main factor determining the critical flow rate at flame lift-off from the micro nozzle is the average molecular weight of the H2 gas mixture with additives. At addition of the studied gases into the coflowing air, the critical flow rate of H2 is determined by their affect on the chemical reactions of hydrogen oxidation (inhibition effectiveness), as well as by a decrease in oxygen concentration due to dilution of air by additives. The data obtained are of interest to hydrogen energy in terms of determining the limits of sustainable combustion of the hydrogen microjet, as well as determining the minimum phlegmatizing concentrations of additives of inhibitors and fire suppressant in the air, preventing the ignition and explosion of hydrogen in case of emergency at its leak.

Five kHz thermometry in turbulent spray flames using chirped-probe-pulse femtosecond CARS, part II: Structure of reaction zones

Temperature was measured in turbulent spray flames of ethanol and acetone stabilized on the piloted Sydney Needle Spray Burner (SYNSBURNTM) using single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS) with a repetition rate of 5 kHz. The burner features air-blast atomization of liquid injected from a needle that can be translated by a length Lr within a co-flowing air stream so that piloted spray flames ranging from dilute to dense can be studied. Part I of these investigations has reported on the CPP-fs-CARS technique and extensive details of data processing methodology. Part II is concerned with the structure of the reaction zones at different spray loadings and for different departures from blow-off. While not performed simultaneously, measurements of the size distribution of liquid fragments are also reported and discussed in conjunction with the measured temperature. Measured probability density functions of temperature show that for flames with the same liquid loading but different recess lengths, Lr, the near-field spray structure that forms upstream of x/D = 10 affects flame structure and stability further downstream. As the spray exiting the burner becomes denser, with a higher proportion of ligaments and ‘irregular’ shaped objects, the entrainment of hot pilot gases into the spray envelope is affected, hence changing the rates of vaporization and subsequent combustion. The reported results will also form a useful platform for validating sub-models of atomization and combustion in turbulent, dilute to dense spray flames.

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURNTM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH4/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.

Numerical study of local entropy generation in a heated turbulent plane jet developing in a co-flowing stream

• Effect of co-flow and directed co-flow with positive and negative angles is studied. • Numerical results are found to be in good agreement with experimental data. • High entropy generation corresponds to lower co-flow. • High values of entropy generation are near the nozzle exit. • The Merit number increases with the co-flow velocity. The present paper numerically investigates the effects of a co-flowing stream on the mean and turbulent flow properties, air entrainment, mixing efficiency, entropy generation rate, and Merit and Bejan numbers of a heated turbulent plane jet emerging in a co-flowing stream. The angle of moving external stream (α) is varied between −10° and + 10°.The first order k-ε turbulence model was used and compared to the existing experimental data. The Finite Volume Method (FVM) was used to discretize the governing equations. The predicted results were consistent with the experimental data. The obtained results showed that the alfa angle directed away from the centerline enhances the mixing. Moreover, the entrainment hypothesis was introduced to describe the development of turbulent jets issuing in a co-flowing air stream. This hypothesis is related to the mass flow rate of the surrounding fluid entrained into the jet. So far, it is obvious that the co-flow has considerably decreased air entrainment.

Turbulent spray flames of intermediate density: Stability and near-field structure

This paper presents an experimental study of turbulent spray jets and flames where the liquid loading is intermediate between the two extremes of dense and dilute; a region of significant importance in practical applications. A new piloted burner is introduced for this purpose featuring air-blast atomization, with liquid injection from a needle that can be translated within a co-flowing air stream. The stability characteristics are presented for three recess lengths using acetone as fuel. High-speed shadowgraph imaging is performed in the atomization region of jets and flames having different liquid loading and recess distances. Three types of fluid fragments are used to map the evolution of these sprays: droplets, ligaments and ‘irregular’ shapes. Statistics for each class of these fluid shapes are presented to map the boundary conditions at the jet exit plane and to track their evolution with downstream distance along the jet. These show clearly how ligaments and irregular shapes break down to supply more droplets further downstream. Measurements of liquid surface area show a peak in the mean area and its rms of fluctuations around two jet diameters from the point of injection marking the region of maximum atomization. It is also evident that the process of primary and secondary atomization is completed by about seven jet diameters from the liquid injection point. This burner provides a versatile platform for studying these flows and the resulting information is already proving to be extremely useful in the development and validation of related models for turbulent spray jets and flames.

Temperature measurement for hydrogen gas in turbulent jet diffusion flame with swirler of angle 45°

Temperature measurement of H 2 gas was investigated experimentally in turbulent diffusion flame with co-flowing air stream with swirler. The air velocity changes from slow to high flow rate. Reynolds number is varying throughout the process from laminar to turbulent region. Results showed that the temperature decreases with increase the air flow rate and also temperature decreases as decrease in M.F.R. and equivalence ratio. Flame stability increases with increasing the hydrogen content in the hybrid fuel mixture.

FLAME BEHAVIOR OF BIFURCATED JETS IN A V-SHAPED BLUFF-BODY BURNER

A plane fuel jet bifurcated into two streams of jets by a V-shaped bluff-body burner with co-flowing air streams was studied. The flame behavior, length, and temperature distributions of the bifurcated jets were experimentally examined. Flame behavior was characterized by flame visualization and instantaneous photograph. The flame length characteristics was obtained by using the long-exposure flame images. The temperatures distributions were measured by using a finewire thermocouple. Three characteristic flame modes were revealed-＂attached flame, transitional flame,＂ and ＂lifted flame＂. The flame length and width of the bifurcated jets increase with the increase of the Re_c in fixed Re_a. The length of the bifurcated flame was significantly shorter than that of the plane jet flame. The reduction was induced by the enhancement in the mixing and entrainment effects of the bifurcated flame. The maximum temperature of the bifurcated-jets flame decreases with the increase of the Re_c. The bifurcated jets enhanced the mixing of the fuel and the air and improved the combustion efficiency. The flame behavior and combustion characteristics of the bifurcated jets changed drastically due to the effects of entrainment and recirculation.

Extinguishment of propane/air co-flowing diffusion flames by fine water droplets

In the present study, extinguishment of propane/air co-flowing diffusion flame by fine water droplets was investigated experimentally. Water droplets are generated by piezoelectric atomizers with the maximum droplets flow rate of 1500ml/h. When the fuel injection velocity Uf is low, an attached laminar diffusion flame with a premixed flame at the base is stabilized. At some distance from the burner rim, a transition from laminar to turbulent diffusion flame occurs, and a turbulent diffusion flame is formed in the downstream region. When the fuel injector rim is thin (δ=0.5mm), the flame stability deteriorates with increase of the co-flowing air stream velocity Ua and the water droplets flow rate Qm. The stability mechanism can be explained by the balance of the gas velocity and the burning velocity of premixed flame formed at the base. However, when the injector rim is thick (δ=5mm), a recirculation zone is produced downstream of the injector rim. The dependence of the quenching distance Hq on Uf and Qm is relatively weak, and the stability diagram shows curious features. It was shown that Ua is crucially important since it determines flow residence time; if Ua<0.4m/s, water droplets can evaporate when they go by the recirculation zone, and the water vapor can diffuse into the recirculation zone. However, if Ua>0.4m/s, the water droplets should pass by the recirculation zone without sufficiently evaporated and are not so effective to extinguish the flame. The supply velocity of droplet-laden air should be low enough so that water droplets can evaporate and water vapor can diffuse into the premixed region at the base to obtain sufficient effectiveness of water droplets for fire suppression.

Prediction of the Blowout Limits of Turbulent Nonpremixed Jet Flames Using the Premixed Combustion Theory

A method is proposed for predicting the blowout limits of lifted nonpremixed jet flames in a coflowing air stream. Based on the premixed combustion theory, the velocity at the base of a lifted flame is balanced by the turbulent flame speed. From this simple concept, Kalghatgi derived a universal formula for calculating the blowout limits of free jet flames. The authors aimed to extend the same concept to lifted flames in a coflowing stream. Local flow parameters needed to estimate the turbulent burning velocity were determined using a RANS model. Results show that at blowout conditions, the local jet velocity is approximately equal to the turbulent burning velocity estimated using Bray's model at the blowout location. Furthermore, this method allows the blowout limits to be predicted through an iterative procedure for a variety of operating conditions. It was shown that there is good agreement between calculated and experimental blowout limits.

Photographic Investigations of Jet Disintegration in Airblast Sprays

In this paper a comprehensive review of flow visualization techniques used for the analysis of spray characteristics is presented and an experimental demonstration of different lighting techniques has been made. Still photographs of sprays from an airblast atomizer and a swirl atomizer are taken under backlighting, slit lighting, incident lighting (front flash) and a new method of capturing scattered light image at 1200 to the incident light is introduced in the present work. Photographs taken by these methods are shown to be useful for deriving useful observations such as jet breakup phenomenon, the internal details and the external appearance of the spray. It is demonstrated that the information obtainable from each technique is different and that a combination of different techniques may be necessary for proper diagnosis. The article also includes a discussion on the jet breakup phenomenon as observed in the photographs and in addition a new concept of shear formation of smaller droplets from the surface of a liquid jet by the co-flowing air stream in an airblast atomizer has been proposed.

Large Eddy Simulation of evaporating kerosene and acetone sprays

Large Eddy Simulation (LES) is applied to turbulent kerosene and acetone sprays and the effect of turbulence and temperature fluctuations on the structure of the sprays under cold and evaporating conditions is discussed. Two different fuels were selected; kerosene which has a relatively high boiling point (480 K) at atmospheric pressure, and acetone, which has a boiling point of 329 K and thus evaporates at room temperature. In both cases liquid fuel is injected into a co-flowing air stream, which in the kerosene case is preheated at 450 K. A Lagrangian formulation is adopted for the particle dispersion coupled with an Eulerian description for the continuous phase. The Ranz–Marshall correlations are used for the Nusselt and Sherwood numbers. A stochastic model has been used for the representation of the sub-grid scale velocity fluctuations on the particle motion and temperature. The results show good agreement when compared with the experimental findings. The mean velocities of the liquid phase are found to follow the behaviour of a conventional jet flow and the profiles of the rms fluctuations are characterised by local maxima and minima, that indicate the location of the shear layers. The spreading rate is inversely related to the droplet size; as the size of the dispersed phase increases, the spreading rate is reduced. The maximum liquid flux decreases with the downstream distance, due to the evaporation of the liquid phase. Smaller particles were found to be dragged in the core jet region and droplet evaporation, was shown to be a size-selective process, enhanced for smaller particles.

Combustion characteristics of natural gas–hydrogen hybrid fuel turbulent diffusion flame

Combustion characteristics of natural gas – hydrogen hybrid fuel were investigated experimentally in a free jet turbulent diffusion flame flowing into a slow co-flowing air stream. Experiments were carried out at a constant jet exit Reynolds number of 4000 and with a wide range of NG–H 2 mixture concentrations, varied from 100%NG to 50%NG-50% H 2 by volume. The effect of hydrogen addition on flame stability, flame length, flame structure, exhaust species concentration and pollutant emissions was conducted. Results showed that, hydrogen addition sustains a progressive improvement in flame stability and reduction in flame length, especially for relatively high hydrogen concentrations. Hydrogen-enriched flames found to have a higher combustion temperatures and reactivity than natural gas flame. Also, it was found that hydrogen addition to natural gas is an ineffective strategy for NO and CO reduction in the studied range, while a significant reduction in the %CO 2 molar concentration by about 30% was achieved.

Simulations of Evaporating Spray Jet in a Uniform Co-Flowing Turbulent Air Stream

Numerical simulations are performed on the flow of fine, polydisperse droplets of acetone ejected from a round jet of air into an ambient turbulent, uniform co-flowing air stream. The objective is to validate the numerical model by comparing the predictions with experimental measurements of a well defined evaporating spray configuration (Chen et al., Int. J. Multiphase Flow 32(2006), 389–412). The carrier-phase is considered in the Eulerian context, while the dispersed phase is tracked in the Lagrangian framework. Various interactions between the two phases are taken into account by means of a two-way coupling. The stochastic separated flow (SSF) model is adopted for the spray calculations. The gas-phase turbulence terms are closed using the standard k-ε model. The spray evaporation is described using a thermal model with an infinite-conductivity. Overall, very good agreement is observed in the comparisons of the computational predictions and experimental measurements. The predicted droplet number-mean axial velocity, r.m.s. of fluctuating velocity for the various droplet classes at different downstream locations exhibit a self-similarity downstream of the nozzle exit. Near the nozzle-exit (around z/djet = 5), the r.m.s. of droplet number mean axial fluctuating velocity attains a maximum within the shear layer near r/djet = 0.4–0.6 for different droplet classes. Further downstream, the peak shifts towards the axis. A similar variation is noticed in the Sauter mean diameter (SMD) distribution of the droplets. It is concluded that, a higher level of turbulence leads to a faster depletion of the smaller droplets, resulting in an increase in the local droplet-SMD in those regions.

The effect of co-flow stream velocity on turbulent non-premixed jet flame stability

The stability behaviour of non-premixed jet flames in a co-flowing air stream was investigated experimentally. The experimental data obtained indicate that there exists a range of co-flow velocity where two distinctly different extinction limits can occur at the same co-flow velocity depending on whether the flame is lifted or attached at ignition. Results show that co-flow velocity has a much greater effect on the blowout limits of lifted flames than on the blowoff limits of attached flames. The blowout limit of lifted flames initially increase linearly with co-flow velocity independent of nozzle diameter until a peak value is reached, after which it decreases rapidly with increasing co-flow velocity. Such behaviour appears to be governed by two different mechanisms. A model for predicting lifted flame blowout limits has been developed. It is based on the ratio of the Kolmogorov time scale and the chemical time scale as a function of a jet similarity parameter. The model was used to predict the blowout limits for methane as well as the effect of diluents in either fuel or co-flow stream. Results show very good agreement with experimental data in the current investigation.