Axisymmetric Jet Research Articles

Large eddy simulations of single and multiple turbulent round jets

We present results from three large eddy simulations (LES). The first two were those of a single jet at a Reynolds number of 11000 with different cell density distributions. The simulation results are validated with earlier experimental and computational studies at the same Reynolds number and almost the same boundary conditions. We establish the repeatability and reproducibility of the characterisation of a single axisymmetric jet. Additionally, we performed an LES of five round jets using the same discretisation schemes and boundary conditions. The five jets were placed in a cross or plus configuration, with a central jet surrounded by four outer jets. The mass flux, momentum flux and the Reynolds number of the five jet configuration were set to be equal to those of the single jet. Further, we analyse the near-field development of the multiple jets, along with entrainment and symmetry characteristics as the jet evolves. LES's ability to provide information about large-scale motions was used to compute conditional statistics. We, then, present details of an initial attempt to characterise the turbulent non-turbulent interface boundary and the coherent structures in the core of the jet in a unified manner using helicity density as the detector variable.

Hybrid constraint multi-line absorption spectroscopy for non-uniform thermochemical measurements in axisymmetric laminar and jet flames

We propose a novel strategy for the accurate determination of non-uniform thermochemical distributions in axisymmetric flames using line-of-sight laser absorption spectroscopy with the constraints of hybrid information. In this method, the computational fluid dynamics (CFD) simulation or other diagnostic method (e.g., thermocouple measurement) is used to qualitatively characterize the thermochemical properties, whereas the kinetic modelling, chemical equilibrium calculation and thermocouple measurement are used to constrain the range of unknown parameters. Experimental demonstrations were firstly performed in a standard laminar CH4/air premixed flame stabilized on a McKenna burner at different equivalence ratios (Φ = 0.8-1.2). With proper hybrid constraints, multiple solutions are avoided, and the deviation and convergence time can be simultaneously reduced comparing to the conventional method. For further demonstration, this strategy was then applied to a turbulent jet flame to recover the significant thermochemical gradient without the need for tomographic reconstruction techniques.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.

Wave breaking and jet formation on axisymmetric surface gravity waves

Axisymmetric standing waves occur across a wide range of free surface flows. When these waves reach a critical height (steepness), wave breaking and jet formation occur. For travelling surface gravity waves, wave breaking is generally considered to limit wave height and reversible wave motion. In the ocean, the behaviour of directionally spread waves lies between the limits of purely travelling (two dimensions) and axisymmetric (three dimensions). Hence, understanding wave breaking and jet formation on axisymmetric surface gravity waves is an important step in understanding extreme and breaking waves in the ocean. We examine an example of axisymmetric wave breaking and jet formation colloquially known as the ‘spike wave’, created in the FloWave circular wave tank at the University of Edinburgh, UK. We generate this spike wave with maximum crest amplitudes of 0.15–6.0 m (0.024–0.98 when made non-dimensional by characteristic radius), with wave breaking occurring for crest amplitudes greater than 1.0 m (0.16 non-dimensionalised). Unlike two-dimensional travelling waves, wave breaking does not limit maximum crest amplitude, and our measurements approximately follow the jet height scaling proposed by Ghabache et al. (J. Fluid Mech., vol. 761, 2014, pp. 206–219) for cavity collapse. The spike wave is predominantly created by linear dispersive focusing. A trough forms, then collapses producing a jet, which is sensitive to the trough's shape. The evolution of the jets that form in our experiments is predicted well by the hyperbolic jet model proposed by Longuet–Higgins (J. Fluid Mech., vol. 127, 1983, pp. 103–121), previously applied to jets forming on bubbles.

Experimental validation of inviscid linear stability theory applied to an axisymmetric jet

We study the development of perturbations in a submerged air jet with a round cross-section and a long laminar region (five jet diameters) at a Reynolds number of 5400 by both inviscid linear stability theory and experiments. The theoretical analysis shows that there are two modes of growing axisymmetric perturbations, which are generated by three generalized inflection points of the jet's velocity profile. To validate the results of linear stability theory, we conduct experiments with controlled axisymmetric perturbations to the jet. The characteristics of growing waves are obtained by visualization, thermoanemometer measurements and correlation analysis. Experimentally measured wavelengths, growth rates and spatial distributions of velocity fluctuations for both growing modes are in good agreement with theoretical calculations. Therefore, it is demonstrated that small perturbations to the laminar jet closely follow the predictions of inviscid linear stability theory.

The dynamics of an axisymmetric turbulent jet in ambient turbulence interpreted from the passive scalar field statistics

The effect of approximately homogeneous isotropic turbulence on the dynamics of an axisymmetric turbulent jet (Re = 10 600 and 5800) in an ambient with a negligible mean flow is interpreted from the statistics of the passive scalar field. The ambient turbulence is generated by a random jet array and scalar concentrations are measured in orthogonal cross sections of the jet using planar laser-induced fluorescence. Statistics of the scalar field of the jet in a turbulent ambient are compared to those in a quiescent ambient, using classical Eulerian averages and those conditioned on the jet centroid. A two-region model for the jet structure in ambient turbulence is proposed based on the centroidal statistics. Following the developing region of the jet, the ambient turbulence disrupts the jet structure, due to modulation of the jet interface, meandering of the jet by large eddies, and entrainment of the turbulent ambient fluid, resulting in a faster concentration decay and reduced entrainment compared to the quiescent ambient. Further downstream, once the ambient turbulence has destroyed the jet, only molecular and turbulent diffusion modify the scalar concentrations. The regions' relative lengths depend primarily on the relative turbulence intensity (ξ) between the ambient and the jet, as assessed using centroidal analysis, which removes the effect of the relative length scale (l) on the jet behavior in the turbulent ambient. The centroidal scalar statistics reveal self-similarity and self-preservation in mean scalar properties before jet breakup, which occurs abruptly once ξ> 0.5. The smaller scales of ambient turbulence modulate the jet boundary and, when entrained, lead to a wider range of centerline concentrations and root mean square concentrations, which are hypothesized to increase local concentration gradients within the jet and reduce jet mixing.

Wetting layer evolution and interfacial heat transfer in water-air spray cooling process of hot metallic surface

In order to respond to the increased demand for clean energy without harming the atmosphere through polluting emissions, energy production from the hydrogen combustion become largely used. This work presents a numerical study of the injection conditions effect on the structure of the H2-air diffusion flame. The aim is to reproduce a practical case of non-polluting combustion and resulting in very high temperatures. The configuration is composed of two axisymmetric coaxial jets, as can be found in the diffusion burners. A presumed probability density function approach is used to describe the chemistry-turbulence interaction. The k-? model of turbulence is used. Particular attention is given to phenomena anchoring or blowout of the flame.

Numerical study of the injection conditions effect on the behavior of hydrogen-air diffusion flame

In order to respond to the increased demand for clean energy without harming the atmosphere through polluting emissions, Energy production from the hydrogen combustion become largely used. This work presents a numerical study of the injection conditions effect on the structure of the H2-Air diffusion flame. The aim is to reproduce a practical case of non-polluting combustion and resulting in very high temperatures. The configuration is composed of two axisymmetric coaxial jets, as can be found in the diffusion burners. A presumed probability density function (PDF) approach is used to describe the chemistry-turbulence interaction. K-epsilon model of turbulence is used. Particular attention is given to phenomena anchoring or blowout of the flame.

Design and optimization of aerospike nozzle for rotating detonation engine

Rotating detonation engine (RDE) is increasingly viewed as a viable technology to improve the performance of propulsion devices. At now, the design of RDE faces challenges when integrated with aerospike nozzle due to the large pressure fluctuations of rotating detonation waves. To optimize the nozzle configuration for thrust generation, the current work investigates the performance of RDE combined with various types of aerospike nozzles. The method of designing optimal aerospike nozzle for RDE with annulus combustion chamber is proposed. A series of three-dimensional numerical simulations are conducted to obtain the thrust performance of nozzles based on open-source software OpenFOAM. The flow-field structure, pressure gain in combustor and thrust performance of RDE are investigated. The results suggest that although the instantaneous flow-field in RDE is time-varying, the averaged plume field can be viewed as axisymmetric under-expanded supersonic jets. It's reasonable to use the time-averaged value of physical quantities for nozzle design. For nozzle with inward-bend cowl lip and isentropic ramp surface, the supersonic flow in nozzle undergoes approximate isentropic expansion with minimum loss. The flow at nozzle exit is basically parallel to chamber axis, generating the optimal thrust. This indicates that the aerospike nozzle is a good match for efficient expansion of rotating detonation products for propulsion. Moreover, the stagnation pressure gain in combustor has been greatly improved with the constriction nozzle throat. This suggests that in order to elevate the chamber pressure, the convergence section with nozzle throat should be adopted in RDE just as in traditional rocket engines.

Gas velocimetry based on infrared laser-induced fluorescence

A novel method for gas velocity field measurements by means of infrared molecular tagging velocimetry is reported with proof-of-principle demonstration in a carbon dioxide (CO2) axisymmetric turbulent jet. Infrared laser-induced fluorescence utilizes the resonant vibrational energy level transitions of small gas molecules, such as CO2, to “tag” and trace the flow of the molecules by taking subsequent images of the infrared emission. Spectroscopic model of the molecular vibrational energy transfer processes is taken into account to design and optimize the measurement scheme. The infrared images are then analyzed, with detailed consideration of molecular diffusion, lateral velocity, and fluorescence lifetime, to yield quantitative velocity field distribution. The radial velocity distributions in the jet main region, with velocities ranging from 7 to 50 m/s, are obtained and shown to be in excellent agreement with theoretical predication and previous experimental works. Velocity uncertainties are discussed and estimated to be 7.7%, 6.7%, 6.1% for Re = 104, 2×104, 3×104 (maximum velocity uc=18.3,34.6,50.5 m/s), respectively. Spatial resolution along the laser beam is estimated to be 107 μm. To the best of the authors' knowledge, this is the first work of infrared molecular tagging velocimetry. With powerful excitation lasers targeting strong infrared molecular absorption transitions, this technique presents great potential for simultaneous flow-scalar field measurements at much-improved accuracy, spatial and temporal resolution, that can be used for the study of low-speed micro-flows, or instantaneous snapshots of turbulent flows.

Experimental study of heat transfer of impinging swirling jets and jets with chevrons

The aim of this work is to study the effect of different forms of passive change in the shape of the flow on the intensity of heat transfer in the impact jet. In this work, a cycle of experiments was performed to investigate an axisymmetric jet flowing normally onto a heated surface. The jet was located both in natural conditions and during swirling of the flow (S = 0.4; 0.7; 1.0). It is shown that the intensity of heat transfer on a heated target in the case of a chevrons jet has little effect on the character, but significantly intensifies heat transfer. In the case of a swirling jet, the intensity distribution on the wall changes its character and locally increases at small distances between the nozzle and the heater.

Effect of Application of Electric Force to Diesel Fuel Droplets Charged Through Millikan Oil-Drop Experiment on Physical and Thermal Characteristics of Flame

Abstract This paper proposes a new feasible method to allow continuous change in the primary injection spray cone angle of liquid fuel droplets, which are injected from nozzles in liquid fuel combustion systems, to control the flame shape and thermal characteristics of the flame. The method is based on electric force applied to fuel droplets charged through frictional effects between the internal surface of the nozzle and the fuel flow as the liquid fuel is sprayed (based on the Millikan oil-drop experiment). A sprint computational fluid dynamics code was developed to investigate the effect of application of electric force to charged diesel fuel droplets, which were injected from a pressure swirl atomizer, on physical and thermal characteristics of a two-dimensional axisymmetric turbulent jet diffusion flame. The results show that an electric field applied to charged fuel droplets (electric force) changes the spatial distribution of the liquid fuel droplets in the flame reaction zone. An applied electric force in (−y) direction diverts the fuel droplets towards the axis centerline of the furnace and, consequently, decreases the primary injection cone angle and increases the concentration of the evaporated droplets around the axis centerline, which enhances the fuel-oxidant mixing rate and raises the flame temperature. Unlike an applied electric force in (−y) direction, an applied electric force in (+y) direction decreases the flame temperature. However, as the primary injection cone angle is decreased, an applied electric force in (+y) direction increases the flame temperature.

Application of spectral proper orthogonal decomposition and resolvent analysis to periodically forced supersonic jets

The mechanisms by which near-nozzle forcing alters the turbulence structure and far-field sound of turbulent jets are not well understood. We perform large-eddy simulations of subsonic and supersonic axisymmetric isothermal turbulent jets subjected to an axisymmetric periodic forcing. The triple decomposition framework and spectral proper orthogonal decomposition are used to study the effect of the forcing on the underlying turbulence spectrum and determine how the periodic forcing alters coherent non-phase-locked structures. For subsonic jets, high-amplitude (1% of the jet velocity) low-frequency, Stf = 0.3, forcing is required to achieve a small change to the underlying turbulence spectrum and most energetic modes of the subsonic jet, despite producing a very energetic, phase-locked (tonal) response. The changes in the spectrum and the phase-locked structures are predicted well via resolvent analysis and linearized Navier-Stokes computations performed on the new turbulent mean, respectively. This suggests that there is little nonlinear interaction between the phase-locked structures and natural turbulence. High-frequency forcing, Stf = 1.5, shows similarly linear behavior except for around St≈0.75, which appears to be associated with vortex pairings triggered by the natural turbulence. Results of a similar analysis performed on a Mach 1.5 ideally expanded isothermal jet will also be presented.

Распространение осесимметричной турбулентной струи метана в спутном потоке воздуха при горении с конечной скоростью

The paper introduces a numerical method for solving the problem of the axisymmetric methane jet propagation in an infinite co-current air flow. For modeling, we used the dimensionless equations of the turbulent boundary layer of reacting gases in the Mises coordinates. To close the Reynolds equation, a modified k - ε turbulence model was used. The k - ε model is considered a low Rhine turbulence model. Assuming that the intensities of convective and turbulent transfers of components are the same and using the stoichiometric ratios of the concentrations of components during combustion, we reduced five equations for the transfer and conservation of the mass of components to two equations for the relative excess concentration of the combustible gas. The concentrations of the components were determined from the solutions of these equations. By using relatively excessive velocities and total enthalpy, we reduced the boundary conditions for the three equations to a general form. To solve the problem in the Mises coordinates, we used a two-layer, six-point implicit finite-difference scheme, which provides the second order of accuracy of approximation in coordinates. The equations for the conservation and transfer of substances being non-linear, an iterative process was implemented. The influence of the radius of the fuel nozzle on the indices of the turbulent jet and flame was investigated. Findings of research show that in an endless co-current flow of fuel with a decrease in the radius of the nozzle, the rate of the chemical reaction and the maximum temperature in the calculation area decrease, and the amount of unburned part of the combustible gas increases

Experimental investigation of the effect of cross wire on the flow field of elliptic jet

The effectiveness of cross wire in controlling the mixing characteristics of a circular and an equivalent elliptic jet is investigated experimentally. While circular jets are conventional, elliptic jets have gained attention due to their better mixing characteristics and faster decay. To further explore and augment the capabilities of elliptic jets for practical utility, it is investigated whether using an elliptic jet with cross wire control gives additional benefit in terms of mixing enhancement over an axisymmetric jet. Experiments are performed for subsonic and choked flow conditions with nozzle pressure ratios ranging from 1.2 to 7.0. Time-averaged pitot pressures and schlieren visualization is used for diagnosis. The jet bifurcation can be seen in controlled elliptical jets at all nozzle pressure ratios (NPRs). Core length is reduced to as much as 70% in the elliptical jet and 84% in the case of the circular jet. The core length values estimated from the present data are compared with the previous investigations.

Particle image velocimetry and constant temperature anemometer measurements of the jet produced by a centrifugal fan

This article presents a detailed investigation of the three-dimensional flow field produced by a small centrifugal fan used for cooling of electronic components, for instance, in the automotive industry. A particle image velocimetry (PIV) system captured numerous cross sections of the fan jet for analysis of velocity distribution and jet development. General parameters, such as the spreading rate, and flow specifics, such as the full width at half maximum contours in planes parallel to the fan outlet and the jet's rotation rate, were thus evaluated. In addition, constant temperature anemometry enabled a thorough investigation of the flow field at the fan's outflow port. Hot-wire measurements complement the PIV results by providing the power spectral density of the turbulent kinetic energy at several locations. Our results demonstrate that the cross-flow profile at the fan outlet consists of two counter-rotating vortices. This leads to a jet with a nearly elliptical, rotating cross section, which does not propagate in a direction perpendicular to the outlet plane. Several aspects of these distinctive flow features are evaluated and presented to provide both fluid mechanics and heat transfer engineers with (i) data on the air jet produced by a small centrifugal fan and (ii) reference data for computational fluid dynamics simulations. Despite complex flow development in the near field of the jet and the nonaxisymmetric jet contour in the intermediate field, our results are in good agreement with published data on axisymmetric jets in terms of spreading rate and the development of turbulence spectra observed in the intermediate field.

Torricelli's curtain: Morphology of horizontal laminar jets under gravity

Viscous fluid exiting a long horizontal circular pipe develops a complex structure comprising a primary jet above and a smaller secondary jet below with a thin fluid curtain connecting them. We present here a combined experimental, theoretical, and numerical study of this “Torricelli's curtain” phenomenon, focusing on the factors that control its morphology. The dimensional parameters that define the problem are the pipe radius a; the mean exit velocity U of the fluid; the gravitational acceleration g; and the fluid's density ρ, kinematic viscosity ν, and coefficient of surface tension γ. Rescaling of experimentally measured trajectories of the primary and secondary jets using a for the vertical coordinate and LD=U(a/g)1/2 for the horizontal coordinate x collapses the data onto universal curves for x<10LD. We propose a theoretical model for the curtain in which particle trajectories result from the composition of two motions: a horizontal component corresponding to the evolving axial velocity profile of an axisymmetric viscous jet and a vertical component due to free fall under gravity. The model predicts well the trajectory of the primary jet, but somewhat less well that of the secondary jet. We suggest that the remaining discrepancy may be explained by surface tension-driven (Taylor–Culick) retraction of the secondary jet. Finally, direct numerical simulation reveals recirculating “Dean” vortices in vertical sections of the primary jet, placing Torricelli's curtain firmly within the context of flow in curved pipes.

Lagrangian diffusion properties of a free shear turbulent jet

Abstract

Transient heat transfer of impinging jets on superheated wetting and non-wetting surfaces

Superhydrophobic (SH) surfaces possess desirable anti-fouling properties due to low wettability, but have also been shown to reduce heat transfer to subcooled water in impinging jet scenarios. In this work, superheated silicon substrates with varying wettability (hydrophilic or HPi, hydrophobic or HPo, SH) are quenched by an impinging water jet, where the substrate temperature is above the saturation temperature. Silicon wafers are either oxidized to create HPi surfaces, coated with Teflon to make the surface HPo, or plasma-etched and coated to create the necessary micro-texture for SH conditions. All wafers are integrated with an electric resistance heater and then heated to temperatures of 200–320 ∘C before impingement with an axisymmetric room temperature water jet of varying specified flow rates yielding jet Reynolds numbers between 6000 and 18,000. High-speed visual data is collected, showing how the lamellar liquid contact region, limited by thermal breakup due to boiling, grows radially as the surface cools to temperatures below saturation. This data is correlated to temperature data recorded on the back side of the wafer using a thermal camera. Results of this study confirm previous conjecture that surface wettability can alter maximum heat flux, which is quantified here for the described scenario by up to 40%, and can also affect jet thin film spreading by up to 50%. Increasing initial surface temperature decreases thin film spreading rate on all surfaces, and increases heat transfer on all but the SH surfaces. Increasing Reynolds number yields an increase in heat flux, and affects both the thin film spreading rate as well as the maximum radius of the thin film region.

Shooting method for linear inviscid bi-global stability analysis of non-axisymmetric jets

The shooting method is commonly used to solve the linear parallel-flow stability problem for axisymmetric jets, i.e., a flow having one inhomogeneous direction. The present extension to two inhomogeneous directions – i.e., a bi-global stability problem – is motivated by inviscid non-axisymmetric jets. The azimuthal direction is Fourier transformed to obtain a set of coupled one-dimensional shooting problems that are solved by two-way integration from both radial boundaries – centreline and far field. The overall problem is formulated as one of iterative root-finding to match the solutions from the two integrations. The approach is validated against results from the well-established matrix method that discretizes the domain to obtain a matrix eigenvalue problem. We demonstrate very good agreement in two jet problems – an offset dual-stream jet, and a jet exiting from a nozzle with chevrons. A disadvantage of the shooting method is its sensitivity to the initial guess of the solution; however, this becomes an advantage when the need arises to track an eigensolution in a sweep over a problem parameter – say with increasing offset in the dual-stream jet, or with downstream distance from the nozzle exit. We demonstrate the performance of the shooting method in such tracking tasks.