Jet Momentum Flux Research Articles

The effects of room length on jet momentum flux

As to mixing ventilation in indoor environments, the turbulent jet plays a major role in driving the air movement, contaminant transport, and heat transfer. The main characteristic of a turbulent jet is its momentum flux. By entrainment of air, the flow of a jet increases and may enhance the flooding of contaminant. In investing the jet’s momentum flux, it is generally regarded that the supply jet collides with the opposing wall and the jet is transformed into a wall jet. However, this is not always true if a jet is not sufficiently strong, or the length of a room is large. Therefore, this study adopted computational fluid dynamics (CFD) to investigate the supply jet development and its momentum flux by varying the room length. Initially, the width of the air supply inlet was the same with that of the room. By defining n as the ratio of room length and height, when n = 3, there is a horizontal a vortex which is the normal behaviour. When the room length increased further, the supply jet was unable to collide with the opposing wall. This investigation got two vertical vortices at the room end which is new. The two new vertical vortices were most pronounced for n = 5. It is possible that increasing the length of the room introduces a gradual transition towards a flow in a rectangular duct. This flow is probably very much governed by the side walls. Therefore, this study reduced the width of the air supply inlet by half and maintained the same flow rate. However, a single vertical vortex was identified at the room end for n = 5. In both scenarios, the supply jet may create new vortices that would enhance the flooding of contaminants.

Development of wood-burning rocket cookstove with forced air-injection

Nearly 40% of the world's population still relies on biomass stoves as their primary cooking source. Indoor cooking with biomass stoves produces harmful emissions such as fine particulate matter (PM2.5) and can result in significant cardiovascular and respiratory disease. Air injection technology, in the forms of over-fire, under-fire, and staged (a combination of under- and over-fire), has been used for decades to reduce emissions in small-scale industrial biomass burners. Recently, researchers have begun integrating air-injection into biomass cookstoves, but the number of studies is limited, and improper air-injection can result in worsening stove performance. In this paper, we develop and analyze three wood-burning biomass rocket stove air-injection strategies using computational fluid dynamics and experiments. We consider over-fire and under-fire air-injection independently, and then develop a staged air-injection system that combines the optimal jet characteristics from the purely over-fire and under-fire systems. Results show that significant performance gains, both in terms of reduced emissions and increased burn-rate are achievable with forced air-injection. For a radial over-fire air injection system, jet momentum flux is found to control fuel and air mixing and is the driving metric for reducing particulate emissions. For the under-fire system, excess air ratio was found to control both the emissions rate and firepower. In a comparison of all three air-injection strategies, over-fire air injection was found to have the largest reduction in emissions, under-fire air injection had the largest increase in firepower and reduction in boiling times, and the staged air injection method achieved both improved emissions and burn-rate. Additionally, the combustion start-up phase was observed to account for the majority of emissions in all forced-draft stoves, and is most sensitive to under-fire injection, remaining an area ripe for improvement.

Effect of jet momentum flux and heat density on NOx emission in a flameless gas turbine combustor

Experimental testing and validated CFD models have been used to evaluate the performance of the FC4GT flameless gas turbine (GT) combustor operating at a global equivalence ratio of 0.44. The methane-fuelled combustor was tested at atmospheric pressure, modelled at elevated pressures up to 25 bar, and optimised for a combustor pressure of 15 bar. The combustor was analysed for firing rates between 15 kW and 16 MW, which correspond to heat density range between 0.15 MW/m3.bar and 150 MW/m3.bar, respectively. The performance of the combustor was also evaluated with pure hydrogen. Overall, the flow fields while using both fuels, are found to be similar; exhibiting highly uniform spatial (temperature and oxygen) distributions, and low exhaust NOx emissions. Volumetric histograms of temperature and oxygen in the combustor are identified as a powerful analysis tool for quantifying the level of uniformity of the scalar mixing and combustion field, NOx emission, and for determining the combustion regime.The momentum flux of the primary air jet has been identified as a key parameter in controlling NOx emission. Across the power generation mode for GT operation with a heat density of 5–150 MW/m3.bar, the predicted NOx emission (15% O2 dry) for methane was 34–6 ppmv, respectively, and for hydrogen 353–33 ppmv, respectively. At 50 MW/m3.bar, the predicted NOx emission for methane was 17 ppmv, and 146 ppmv for pure hydrogen. This is a significant outcome for a non-premixed combustor considering that the NOx emission of the GE6FA GT engine using partially premixed combustion at 50 MW/m3.bar, is 15 ppmv for methane, and 200 ppmv for 85% hydrogen. Overall, the low NOx emission, for methane and hydrogen, combined with the highly uniform spatial temperature and oxygen distributions, and supported by experimental visualisation provide irrefutable proof that a flameless combustion regime has been achieved. It also demonstrates the plausibility of a non-premixed flameless combustor achieving comparable NOx emission to premixed flames.

The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow

Cavity flows occur in a wide range of low-speed applications (Mach number ≤0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 kg/ms2, 0.44 kg/m·s2, and 0.96 kg/m·s2). The investigation was performed for an open cavity with length to depth ratio of 4 at the Reynolds number based on a cavity depth of approximately 50,000. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed in this investigation. The aim of the jet blowing is to separate the cavity separated shear layer from the recirculation zone to reduce the cavity return flow, and hence stabilise the cavity separated shear layer. It was found that increasing the jet momentum flux causes an increase in the cavity return flow due to the increase in the thickness of the cavity separated shear layer. The study also found that the jet populates the separated shear layer with a large number of small-scale disturbances. These disturbances increase the broad band level of the pressure power spectra and Reynolds shear stress in the cavity separated shear layer. On the other hand, the jet disturbances make the shedding of the large vortical structures more intermittent.

River Jets Versus Wave-Driven Longshore Currents at River Mouths

At river mouths, fluvial jets and longshore currents (LSCs) generated by waves interact hydrodynamically. This idealized numerical modeling study simulates a large number of hydro-morphodynamic conditions (650) to explore the emergent hydrodynamics determined by different mouth bar volumes and geometries, river discharge, wave heights, and directions and their potential stress on river-mouth development. We find that in the absence of a river-mouth bar (RMB), interactions are driven by momentum balances, expressed either as the balance of wave momentum flux (Mw) and jet momentum flux (Mj), or the balance of river jet discharge (QJet) and longshore current discharge (QLSC). When a RMB is present, the topography modifies the structure of the jet by spreading it, and we quantify this mechanism through the lateral jet transfer rate (LJT). Secondly, topography generates complex longshore wave-driven circulation as a result of the protruding shoreface which serves as a platform on which counter LSCs develop. The balance in QJet/QLSC may be used as an indication of the type of circulation. High and oblique waves favor longshore circulation and RMB bypass, whereas low waves and normal-to-coast angles generate diverging LSCs on the mouth bar crest which interrupts the longshore circulation. A quantification of the dynamic diversion is proposed in the form of the non-dimensional Dynamic diversion index (DyD), which scales with the product of Mj and Mw, and can account for the absolute strength of hydrodynamic interactions occurring at river mouths. RMB morphology can affect DyD in multiple ways by strengthening or by weakening the interactions. The DyD effect seems to increase with increasing RMB size, indicating that the RMB scale regulates the interplay of the wave-driven circulation and the river jet which further controls the adjacent topography changes.

Efficiency of pulsed-jet propulsion via thrust-drag decomposition

By using an axisymmetric immersed-boundary model, fluid dynamics of a cephalopod-inspired propeller undergoing periodic inflation–deflation deformation in background flow is numerically studied in a low Reynolds number regime. A thrust-drag decoupling method based on physical analysis is proposed, in which the jet-related thrust is obtained as the summation of three parts: the jet momentum flux, the normal stress at the exit plane, and the flow acceleration inside the body. This method enables the calculation of the propulsive efficiency, especially the efficiency at the steady-swimming state. Systematic simulations are then conducted to study the effects of the Reynolds number and stroke ratio on force generation and efficiency. Two Reynolds numbers, the incoming-flow Reynolds number Re∞ and the jet-flow Reynolds number Rej, are involved. When Re∞ is fixed, the thrust generation is found to depend mostly on jet-flow velocity at high Rej, while the effect of incoming-flow velocity is pronounced at relatively low Rej, mostly through its influence on the excessive pressure at the nozzle. Within the range of incoming-flow Reynolds number considered in this study (40–150), our results show that the whole-cycle propulsive efficiency of the propeller lies in the range of 11%–30%.

Air curtain performance: Introducing the adapted separation efficiency

Air curtains (ACs) are plane turbulent impinging jets that are used to separate two environments in terms of heat and mass transfer while still allowing traffic between these environments. The many applications of ACs across a wide variety of industries makes the evaluation of their performance an important but difficult task. The aim of this paper is to introduce a performance indicator, called the adapted separation efficiency, that is suitable for different types of systems that may involve different AC configurations (downward blowing, upward blowing, lateral blowing, multiple jets, etc.) at multiple scales, different transported quantities (heat, water vapor, particles, gases, etc.) subjected to various transport mechanisms (advection, molecular and turbulent diffusion) and varying environmental conditions (gradients in environmental pressure and/or density). It is defined using a conventional efficiency formula. The principle of this performance indicator is illustrated with a generic case study where the performance is evaluated for two basic AC configurations involving cross-jet pressure and density gradients, as well as different jet momentum fluxes. The case study is conducted based on computational fluid dynamics employing validated large eddy simulations.

A numerical study of a rectangular jet along a shear free boundary: Surface jet

A numerical simulation of a rectangular surface jet is performed at a Reynolds number of Rej=4400. The global parameters of the jet e.g. maximum velocity decay, jet surface normal and lateral spread rates, entrainment, jet momentum flux and turbulent momentum flux are in agreement with several other studies reported in the literature. It is shown that the mean velocity and Reynolds stress profiles scale with the maximum local streamwise velocity and jet half width in the surface normal and lateral directions. The current simulation provides balance, explicitly calculated budgets for the turbulence kinetic energy, Reynolds normal and shear stresses. The surface jet develops a thin layer of fast moving fluid in the lateral direction near the surface. This layer is called the ‘surface current’. It has been suggested that the surface current arises due to the Reynolds stress anisotropy in the near surface region. The current study shows that this explanation is incomplete. The turbulence production for the Reynolds stress in the lateral direction is negative, which can drive the mean flow in the lateral direction. The higher level of negative production in the near surface region is responsible for the development of the surface current.

Development of the impulse and thrust for laminar starting jets with finite discharged volume

Abstract

Pulsed-jet propulsion via shape deformation of an axisymmetric swimmer

By using an axisymmetric immersed-boundary model, we numerically investigate the thrust generation of a deformable body via pulsed jetting. We focus on a single discharging process resulting from the deflation of the body as inspired by the jetting mechanism of cephalopods, such as squids. We examine three jet velocity profiles, namely, impulsive, half-cosine, and cosine, in the relatively low Reynolds number regime. For the impulsive profile, we demonstrate via wake visualization that the leading vortex ring does not pinch off from the trailing jet although its circulation stops growing after a critical formation number (hereby, the formation number is defined as the ratio between the length and diameter of the jet plug) of 6–7. The exact value of the critical formation number depends on the jet velocity profile, suggesting that jet acceleration and viscous dissipation play significant roles in vortex ring evolution. In terms of thrust generation, our results indicate that besides the jet momentum flux, an important source of thrust generation is jet acceleration. The implication is that the jet velocity profile is a key factor in determining the propulsive performance.

On the origins of transverse jet shear layer instability transition

This experimental study explores the physical mechanisms by which a transverse jet’s upstream shear layer can transition from being a convective instability to an absolute/global instability as the jet-to-cross-flow momentum flux ratio is reduced. As first proposed in computational studies by Iyer & Mahesh (J. Fluid Mech., vol. 790, 2016, pp. 275–307), the upstream shear layer just beyond the jet injection may be analogous to a local counter-current shear layer, which is known for a planar geometry to become absolutely unstable at a large enough counter-current shear layer velocity ratio, . The present study explores this analogy for a range of transverse jet momentum flux ratios and jet-to-cross-flow density ratios , for jets containing differing species concentrations (nitrogen, helium and acetone vapour) at several different jet Reynolds numbers. These studies make use of experimental data extracted from stereo particle image velocimetry as well as simultaneous stereo particle image velocimetry and acetone planar laser-induced fluorescence imaging. They provide experimental evidence for the relevance of the counter-current shear layer analogy to upstream shear layer instability transition in a nozzle-generated transverse jet.

Impingement heat transfer to the synthetic jet issuing from a nozzle with an oscillating cross section

This experimental study is focused on a rectangular synthetic (zero-net-mass-flux) jet impinging on a wall. The working fluid is air. A novel variant of a synthetic jet actuator (SJA) is proposed. Following biomimetic principles, the actuator incorporates a nozzle in which the cross-sectional area periodically oscillates during the driven cycle. Two independent actuating systems are used to drive the main SJA diaphragm (as seen in common SJAs with rigid nozzles) and the oscillating nozzle walls (unique to this study). Namely, the diaphragm is driven electrodynamically, and the nozzle walls are driven using a pair of piezoelectric transducers. The operating frequency of 62 Hz is selected by tuning both systems at their resonances. The nozzle slot width oscillates around an average value of approximately 6 mm. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, direct measurement of the jet momentum flux using a precision scale, hot-wire anemometry, and an evaluation of the heat transfer coefficients on the exposed wall using heat flux sensors. The local distribution of the heat transfer coefficient on the wall (Nusselt number in a dimensionless form) indicated the three-dimensional character of the flow field with an axis-switching effect. It was concluded that the momentum flux and the heat transfer rate could be enhanced by the phase delay between the diaphragm and nozzle cycles. The reason is the fact that the nozzle cross-sectional area during the extrusion stroke is smaller than the area during the suction, thus the SJ velocity and momentum flux are promoted during extrusion while the losses are reduced during suction. The maximal effect was found at the phase shift of 270° for the ratio of the cross-section areas during the extrusion and suction of 0.74.

Non-isothermal mixing characteristics in the extreme near-field of a turbulent jet in hot crossflow

The nonreacting and reacting jet in crossflow (JICF) is an important flow configuration for effective mixing and combustion in practical applications. Many studies in the literature exist that examine the overall mixing characteristics of an isothermal, unconfined, nonreacting JICF. This experimental study examines the mixing characteristics in the very near field (s/d ≤ 3) of a nonreacting jet in a hot crossflow of combustion products (1500 K), a configuration relevant to gas turbine combustion. A range of jet-to-crossflow momentum flux ratios (5.2 ≤ J ≤ 24.2) and jet-to-crossflow density ratios (3.2 ≤ ρj/ρcf ≤ 7.8) was studied for a round jet with fully developed turbulent pipe flow and 4% mean turbulence intensity at the jet exit. Temperature measurements were made using planar laser Rayleigh scattering. Jet trajectory, jet centerline concentration decay based on adiabatic mixing assumption, Favre-averaged scalar dissipation, and scalar mixing time scales were determined as a function of the above-mentioned jet parameters. The observed center-plane mixing metrics indicated that better near field mixing was exhibited for lower values of the momentum flux ratio and larger values of density ratio in the extreme near field of the jet. As the momentum flux ratio was increased, windward and leeward mixing around the elongated potential core decreased, as indicated by the relative temperatures in these regions. The magnitude of scalar dissipation in the windward region decreased as the jet momentum flux increased, while the leeward dissipation region increased in size and magnitude as the momentum flux ratio increased. When the density ratio was decreased toward unity, both the windward and leeward dissipation regions reduced in size and magnitude.

Trajectory of a Liquid Jet in a High Temperature and Pressure Gaseous Cross Flow

An experimental study of liquid jet injection into subsonic air crossflow is presented. The aim of this study was to relate the jet trajectory to flow parameters, including jet and air velocities, pressure and temperature, as well as a set of nondimensional variables. For this purpose, an experimental setup was developed, which could withstand high temperatures and pressures. Images were captured using a laser-based shadowgraphy system. A total of 209 different conditions were tested and over 72,000 images were captured and processed. The crossflow air temperatures were 25 °C, 200 °C, and 300 °C; absolute crossflow air pressures were 2.1, 3.8, and 5.2 bars, and various liquid and gas velocities were tested for each given temperature and pressure. The results indicate that the trajectory and atomization change when the air and jet velocities are changed while keeping the momentum flux ratio constant. Therefore, it is beneficial to describe the trajectory based on air and jet Weber numbers or momentum flux ratio in combination with one of the Weber numbers. Also, examples are given where both Weber numbers are kept constant but the atomization is changed, and therefore, other terms beyond inertia terms are required to describe the spray behavior. It is also shown that the gas viscosity has to be considered when developing correlations. The correlations that include this term are generally better in predicting the trajectory. Therefore, Ohnesorge numbers in combination with the Weber numbers is used in the present correlations to describe the trajectories.

Characterization of Impingement Heat/Mass Transfer to the Synthetic Jet Generated by a Biomimetic Actuator

Two biomimetic synthetic jet (SJ) actuators were designed, manufactured, and tested under conditions of a jet impingement onto a wall. Nozzles of the actuators were formed by a flexible diaphragm rim, the working fluid was air, and the operating frequencies were chosen near the resonance at 65 Hz and 69 Hz. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, jet momentum flux measurement using a precision scale, hot-wire anemometry, and mass transfer measurement using the naphthalene sublimation technique. The results demonstrated possibilities of the proposed actuators to cause a desired heat/mass transfer distribution on the exposed wall. It was concluded that the heat/mass transfer rate was commensurable with a conventional continuous impinging jets (IJs) at the same Reynolds numbers.

The effects of cross-flow fuel injection on the reacting jet in vitiated cross-flow

The effects of cross-flow fuel injection on a slotted jet flame consisting of an ethylene-air premixture are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to closely simulate the environment of a secondary combustor in a staged combustion system. It was found that increasing the cross-flow equivalence ratio (Φ∞) requires a consequent reduction in the jet equivalence ratio (Φj) for jet flame stabilization in order to avoid the formation of locally rich mixtures beyond the flammability limits of the flame. Stable flames were achieved for a low cross-flow equivalence ratio of Φ∞ = 0.4 across a range of jet equivalence ratio values and momentum flux ratios, demonstrating the ability of the transverse jet to extend the flammability limits of the cross-flow mixture. Significantly, as Φ∞ is increased beyond a certain point, no ethylene is required to be present in the jet mixture, and a jet consisting only of air is able to stabilize the flame. Due to the fluidic nature of the flame stabilization mechanism of these flames, they are dubbed fluidically stabilized flames (FSF). OH* chemiluminescence and high-speed particle image velocimetry were utilized to gain deeper understanding of the flame behavior and flow field features of the FSF. In contrast to bluff-body stabilized flames, it was found that the FSF provides increased control of the flame shape, with increasing flame width and penetration for higher jet momentum flux ratios (J). The FSF was also demonstrated to be a highly dynamical phenomenon, characterized by a dominant peak frequency that is dependent on both Φj and Φ∞. Proper orthogonal decomposition of the time-resolved velocity fields shows that heat release affects the dynamics of the FSF in a similar manner to the reacting jet in cross-flow (RJICF), as demonstrated in a previous study. Finally, the flame behavior was found to be highly dependent on the cross-flow fueling mechanism, with coherent flame oscillations present when the fuel injection point is closely-coupled to the flame stabilization location.

Vortex formation mechanism within fuel streams in laminar nonpremixed jet flames

A vortical structure occurring at the fuel stream in laminar nonpremixed jet flames was recently found and shown to have both a fluid-dynamic impact on the flow field and a possible influence on the flame stability and soot formation. We designed a systematic experiment and numerical simulation to investigate the physical mechanisms of this recirculation phenomenon in a coflow system. We hypothesized that a negative buoyancy, caused by the fuel jet being heavier than the ambient air, may play a significant role in the recirculation. Therefore, we experimentally varied the density of the fuel jet using a binary mixture of methane and n-butane, and tested the density of the coflow oxidizer by replacing nitrogen with carbon dioxide. Several fuel jet velocities, flame temperatures, and nozzle diameters were also studied to thoroughly investigate all parameters that might possibly affect the recirculation. As a result, we found that our modified Richardson number, which is based on the cold density difference between the fuel and the coflow, the flame length, and the jet momentum flux, explained the physical mechanism of the recirculation well, with Ri ∼60 being the critical value for formation of the recirculation. The negative buoyancy was the primary driving force behind the recirculation, while the jet momentum mitigated its formation.

Effects of Effusion Cooling Pattern Near the Dilution Hole for a Double-Walled Combustor Liner—Part 1: Overall Effectiveness Measurements

The complex flow field in a gas turbine combustor makes cooling the liner walls a challenge. In particular, this paper is primarily focused on the region surrounding the dilution holes, which is especially challenging to cool due to the interaction between the effusion cooling jets and high-momentum dilution jets. This study presents overall effectiveness measurements for three different cooling hole patterns of a double-walled combustor liner. Only effusion hole patterns near the dilution holes were varied, which included: no effusion cooling; effusion holes pointed radially outward from the dilution hole; and effusion holes pointed radially inward toward the dilution hole. The double-walled liner contained both impingement and effusion plates as well as a row of dilution jets. Infrared thermography was used to measure the surface temperature of the combustor liners at multiple dilution jet momentum flux ratios and approaching freestream turbulence intensities of 0.5% and 13%. Results showed that the outward and inward geometries were able to more effectively cool the region surrounding the dilution hole compared to the closed case. A significant amount of the cooling enhancement in the outward and inward cases came from in-hole convection. Downstream of the dilution hole, the interactions between the inward effusion holes and the dilution jet led to lower levels of effectiveness compared to the other two geometries. High freestream turbulence caused a small decrease in overall effectiveness over the entire liner and was most impactful in the first three rows of effusion holes.

Autoignition-controlled flame initiation and flame stabilization in a reacting jet in crossflow

This paper describes an analysis of the mechanisms of autoignition-controlled flame initiation and flame stabilization in a nonpremixed jet in crossflows, using simultaneous high-speed (10 kHz) tomographic particle image velocimetry, OH-PLIF and line-of-sight flame emissions. Measurements are conducted on a turbulent, transverse, reacting propane jet issued into a crossflow generated by combustion of natural gas at an equivalence ratio of 0.4 with the crossflow velocity of 10 m/s, the crossflow temperature of 1350 K and the jet momentum flux ratio of 41. While several prior studies have analyzed the lifted character of the flame in similar configurations, we show that several dynamic processes precede the leading edge of the lifted diffusion flame, including formation and evolution of “autoignition kernels”, “flame kernels” and “flame fragments”. “Autoignition kernels”, i.e., discrete compact reaction zones with the peak hydroxyl (OH) fluorescence intensity below that of the diffusion flame, initiate preferably at bulges along the jet periphery where the strain rates and the scalar dissipation rates are lower. The autoignition kernel grows in both size and the OH-fluorescence intensity as it convects downstream. An autoignition kernel transitions into a propagating flame kernel, which quickly gets distorted and elongated in the direction of the principal expansion strain rate to form a flame fragment. Neighboring flame fragments merge with each other and with the downstream diffusion flame via edge-flame propagation. Merging of upstream flame fragments with the downstream diffusion flame results in an upstream advancement of the diffusion-flame front. The diffusion flame front is intrinsically unsteady because of the rather random formation and evolution of autoignition kernels, flame kernels and flame fragments, presumably due to the stochastic velocity, the strain rate and mixture-fraction oscillations.

Liquid jet breakup unsteadiness in a coaxial air-blast atomizer

The aim of this paper is to experimentally characterize the liquid jet breakup unsteadiness in a coaxial air-blast atomizer. The current research focuses on the measurement of the fluctuations of the jet breakup length and the flapping instability of the liquid jet, which contribute to the downstream fluctuations of the spray characteristics. The optical connectivity technique was used to measure the instantaneous breakup length of the water jet. Also, time resolved shadowgraph images of the primary jet breakup process were captured by high-speed imaging to characterize the jet instabilities at different axial locations from the atomizer exit. Experiments were performed for a wide range of air-to-liquid momentum flux ratio ( M) and aerodynamic Weber number ( Weg) corresponding to membrane- and/or fiber breakup mode of the jet disintegration process. The mean jet breakup length was found to vary inversely with M through a power law relation in agreement with the literature, while the breakup length fluctuations were found to first decrease and then increase with M. In order to capture the unsteady dynamics of the jet breakup process, the proper orthogonal decomposition analysis of the optical connectivity images was performed. The jet flapping and the fluctuations of the jet breakup length were identified as the second and the third spatial proper orthogonal decomposition modes, respectively, for all operating conditions of the atomizer. The amplitude and the frequency of the instabilities were measured by temporal tracking of the liquid–air interface on the shadowgraph images. The disturbance close to the injector exit corresponds to the Kelvin–Helmholtz instability, while close to the jet breakup point the jet exhibits the flapping instability, which is characterized by lateral oscillation of the jet about the atomizer axis. The influence of the liquid jet Reynolds number and momentum flux ratio on the KH and the flapping instabilities are examined.