Free Jet Configuration Research Articles

Laser-induced fluorescence measurements of spatially resolved OH densities in a helium plasma jet interacting with three different targets in the open air

The presented research work is devoted to the measurements of the absolute densities of ground state OH radicals in an atmospheric pressure helium plasma jet launched in free air onto three different targets. The OH density is measured by using laser induced fluorescence (LIF). The plasma is generated inside a glass tube by a dielectric barrier discharge powered by a mono-polar pulsed high voltage. Estimation of the absolute density is based on a quantitative analysis of the spectral and temporal component of the LIF signal and does not require any assumption on the densities of colliding species. Based on this analysis, the spatially-resolved density distributions of OH radicals are determined in front of three different targets (metallic, dielectric, and ultra-pure water) placed in the open air at 2 cm from the exit of the glass tube. In order to also investigate the temporal behaviour of the OH densities, and the influence of the gas flow rate and of the amplitude of the voltage-pulse, planar LIF has been performed investigating the fluorescence signal (i) during the intervals of the discharge pulses, (ii) for various gas flow rates, and (iii) as a function of the amplitude of the voltage pulse. In the free jet configuration without a target, the OH densities were determined to be in the order of , whereas they increased up to a factor of 10 in contact with a target. The spatial distributions were also influenced by the dielectric properties of the target: in the case of the metallic target, the maximum of the OH density was found to be in the center between the exit of the glass tube and the target, whereas in the case of the dielectric target, the maximum was found to be close to the surface of the target. In the case of ultra-pure water, the densities were equally distributed between the exit of the glass tube and the target. With this work, the influence of a target on the production of OH radicals was demonstrated, which enables the optimisation of the plasma parameters for an efficient production of OH radicals in front of a target with various dielectric properties.

ECN Spray D visualization of the spray interaction with a transparent wall under engine-like conditions. Part I: Non-reactive impinging spray

The possibility for a jet of fuel to collide against the surfaces of the combustion chamber or the piston bowl into a thermal engine, has a direct effect on the droplets breakup and the spray atomization. In order to study the spray/wall interaction phenomenon and its influence on the macroscopic spray behavior, a diesel spray was visualized when impinging on a flat wall made of quartz at various ambient, injection and wall position conditions. Two high-speed cameras were employed simultaneously to perform Schlieren imaging and diffuse back illumination diagnostics, in order to observe both spray vapor and liquid phases during their contact with the wall. The experiments were performed in a constant pressure-flow test rig, able to reproduce diesel-like thermodynamic conditions. Two different fuels (n-dodecane and diesel #2) were injected within the chamber by employing a single-hole injector referred to as Spray D into the framework of ECN research group. To place the wall in the spray path, a wall positioning structure capable of being fitted into the chamber and of support the wall at various inclination angles and distances from the injector tip, was designed. The test rig was filled with N2 to keep an inert atmosphere and isolate the spray development from possible alterations produced by combustion. Results provide the influence of the injected fuel, operating and wall conditions on the macroscopic features of the transient spray in terms of spreading onto the wall and spray thickness characterized both temporally and spatially. Spray spreading was evaluated by a novel penetration-based parameter. It showed to be proportional to the square root of time similarly as free-penetration and is largely affected by the wall inclination respect to the spray axis. Spray angle showed to be influenced by the morphology of the spray that affects its air entrainment capacity and that changes with a wall respect to a free-jet configuration. This variation in the morphology also has a remarkable effect on spray evaporation, characterized in terms of liquid length and liquid spreading onto the wall. Finally, the large amount of data was used to obtain correlations for all the studied parameters.

Cold Flow Measurements of Supersonic Low Aspect Ratio Jet-Surface Interactions

In streamlined airplane configurations, additional noise sources can be created from interactions between the jet flow and surfaces on an aircraft’s body. During takeoff and landing procedures the ground is close enough to already cause jet-surface interference. To assess this interaction, the presence of a flat plate impinging on a supersonic jet of a low aspect ratio (2:1) rectangular nozzle of equivalent exit diameter, De = 20.65 mm, is studied from the minor and major axis orientation. The impact of the plate, 30De in length to resemble an aircraft carrier deck, is studied at supersonic nozzle pressure ratios (NPRs) of 2.5–4.5 for a low jet temperature ratio of TR = 1.1. Streamwise particle image velocimetry (PIV) data was taken to extract average velocity and turbulence kinetic energy (TKE) of the flow. Plate offset (h) distances of h/De = 0, 1, 2, and 3 from the nozzle lip are studied to assess trends related to shock cell spacing, potential core length, and shear layer development relative to a free jet configuration. The surface offset from the nozzle is shown to vary flow properties and even increase screech tones particularly at h/De = 1 and 3, while the in between distance of h/De = 2 is shown to reduce them. These off-design conditions causing screech are of particular interest in this study.

Analysis of conductive target influence in plasma jet experiments through helium metastable and electric field measurements

The use of cold atmospheric pressure plasma jets for in vivo treatments implies most of the time plasma interaction with conductive targets. The effect of conductive target contact on the discharge behavior is studied here for a grounded metallic target and compared to the free jet configuration. In this work, realized with a plasma gun, we measured helium metastable HeM (23S1) concentration (by laser absorption spectroscopy) and electric field (EF) longitudinal and radial components (by electro-optic probe). Both diagnostics were temporally and spatially resolved. Mechanisms after ionization front impact on the target surface have been identified. The remnant conductive ionized channel behind the ionization front electrically transiently connects the inner high voltage electrode to the target. Due to impedance mismatching between the ionized channel and the target, a secondary ionization front is initiated and rapidly propagates from the target surface to the inner electrode through this ionized channel. This leads to a greatly enhanced HeM production inside the plasma plume and the capillary. Forward and reverse dynamics occur with further multi reflections of more or less damped ionization fronts between the inner electrode and the target as long as the ionized channel is persisting. This phenomenon is very sensitive to parameters such as target distance and ionized channel conductivity affecting electrical coupling between these two and evidenced using positive or negative voltage polarity and nitrogen admixture. In typical operating conditions for the plasma gun used in this work, it has been found that after the secondary ionization front propagation, when the ionized channel is conductive enough, a glow like discharge occurs with strong conduction current. HeM production and all species excitation, especially reactive ones, are then driven by high voltage pulse evolution. The control of forward and reverse dynamics, impacting on the production of the glow like discharge, will be useful for biomedical applications on living tissues.

Icing test capabilities at DGA Aero-engine Testing

Since the 1960s, DGA Aero-engine Testing has built facilities to test and certify parts and equipment of aircraft and rotorcraft in icing conditions. For aerofoil or blade testing, a 2D test set-up has been developed. For engine tests, a free jet configuration is used. Until recently, the standard conditions for icing were associated with the presence of clouds composed of supercooled small droplets. Nevertheless, recent accidents and incidents have highlighted the danger of other icing conditions such as supercooled large drops (SLD), ice crystals and mixed phase which is composed of ice crystals and small droplets. So, current and future challenges include the ability to simulate these conditions. To generate droplets larger than 100 micrometres, a new atomiser has been developed and calibration methods have been modified in consequence. The next planned step is the generation of ice crystals and mixed phase.

Comparison of a Direct-Connect and Freejet Dual-Mode Scramjet

Testing of identical dual-mode scramjet flowpath geometries in the freejet and direct-connect configurations was conducted in two separate facilities. These tests enabled a comparison study between the two configurations, which was directed toward the determination of the effects of inlet distortion and backpressure on the performance and operability of a dual-mode scramjet. Bulk flow conditions were matched between the two facilities at the isolator entrance plane to simulate Mach 4.8 flight, and a series of metrics were established to quantify similarities and differences. The effects of flowpath backpressure in the direct-connect case were seen to be isolated to regions close to the exhaust. An approximate 10% decrease in combustor pressure rise and consequently integrated pressure force were observed in the freejet configuration; shock train length, however, remained the same. Mode transition was delayed from an equivalence ratio of in the direct-connect case to in the freejet configuration. Further, ignition difficulties were experienced in the freejet tests which were not encountered in those of direct-connect tests, limiting the scope of comparable test points. This work represents the first attempt at a quantitative comparison in the literature of an identical direct-connect and freejet dual-mode scramjet.

Influence of nozzle geometry on underexpanded axisymmetric free jet characteristics

The effect of nozzle geometry on the structure of a supersonic free jet is investigated both experimentally and numerically for three simple nozzle geometries. In the experiments, the relation between the Mach disk height and diameter and the nozzle pressure ratio are investigated using the schlieren method. In contrast to results obtained in previous studies, our experimental results show that the Mach disk height changes depending on the nozzle geometry. Numerical investigations were conducted by introducing flows upstream of the nozzle and the influence of these upstream flows on free jet configuration has been discussed.

Eulerian Quadrature-Based Moment Models for Dilute Polydisperse Evaporating Sprays

Dilute liquid sprays can be modeled at the mesoscale using a kinetic equation, namely the Williams–Boltzmann equation, containing terms for spatial transport, evaporation and fluid drag. The most common method for simulating the Williams–Boltzmann equation uses Lagrangian particle tracking wherein a finite ensemble of numerical “parcels” provides a statistical estimate of the joint surface area, velocity number density function (NDF). An alternative approach is to discretize the NDF into droplet size intervals, called sections, and to neglect velocity fluctuations conditioned on droplet size, resulting in an Eulerian multi-fluid model. In comparison to Lagrangian particle tracking, multi-fluid models contain no statistical error (due to the finite number of parcels) but they cannot reproduce the particle trajectory crossings observed in Lagrangian simulations of non-collisional kinetic equations. Here, in order to overcome this limitation, a quadrature-based moment method is used to describe the velocity moments. When coupled with the sectional description of droplet sizes, the resulting Eulerian multi-fluid, multi-velocity model is shown to capture accurately both particle trajectory crossings and the size-dependent dynamics of evaporation and fluid drag. Model validation is carried out using direct comparisons between the Lagrangian and Eulerian models for an unsteady free-jet configuration with mono- and polydisperse droplets with and without evaporation. Comparisons between the Eulerian and Lagrangian instantaneous number density and gas-phase fuel mass fraction fields show excellent agreement, suggesting that the multi-fluid, multi-velocity model is well suited for describing spray combustion.

Experimental investigation of a free-surface turbulent jet with Coanda effect

The deviation of a jet from the straight direction due to the presence of a lateral wall is investigated from the experimental point of view. This flow condition is known as Coanda jet (from the Romanian aerodynamicist Henry Marie Coanda who discovered and applied it at the beginning of XXth century) or offset jet. The objective of the work is to detail the underlying mechanisms of such a phenomenon aiming to use it as a flow control method at polluted river flows mouth. To do this, a large laboratory free-surface tank with an incoming channel has been set up and velocity field measurements are performed by Optical Flow methods (namely Feature Tracking). Preliminary tests on the well-known free jet configuration without any marine structure (i.e. lateral wall) are performed to allow comparison with free jet scaling and self-similar solutions. The presence of the free-surface gives rise to centerline velocity decay which is lower than in free unbounded plane or circular jets due to the vertically limited ambient fluid entrainment. In the second part of the paper, the effect of a lateral wall on the jet configuration is examined by placing it at different lateral distances from the jet outlet. The resulting velocity fields clearly show an inclined Coanda jet with details which seems to depend on the lateral wall distance itself. The analysis of self-similarity along the inclined jet direction reveals that for wall distances larger than 5 jet widths this dependence almost disappears.

Eulerian models for turbulent spray combustion with polydispersity and droplet crossing

The accurate simulation of the dynamics of polydisperse evaporating sprays in unsteady gaseous flows with large scale vortical structures is both a crucial issue for industrial applications and a challenge for modeling and scientific computing. The difficulties encountered by the usual Lagrangian approaches make the use of Eulerian models attractive, aiming at a lower cost and an easier coupling with the carrier gaseous phase. Among these models, the multi-fluid model allows the detailed description of polydispersity and size/velocity correlations for droplets of various sizes. The purpose of the present study is two-fold. First, we extend the multi-fluid model in order to cope with droplet trajectory crossings by using the quadrature method of moments in velocity phase space conditioned by size. We identify the numerical difficulties and provide dedicated numerical schemes in order to preserve the velocity moment space. Second, we conduct a comparison study and demonstrate the capability of such an approach to capture the dynamics of an evaporating polydisperse spray in a two-dimensional free jet configuration. We evaluate the accuracy and computational cost of Eulerian models and related discretization schemes versus Lagrangian solvers. It shows that, even for finite Stokes number, the standard Eulerian multi-fluid model is accurate at reasonable cost. To cite this article: S. de Chaisemartin et al., C. R. Mecanique 337 (2009).

Development of Optical Diagnostic Techniques to Correlate Mixing and Auto-Ignition Processes in High Pressure Diesel Jets

A tracer laser-induced fluorescence (LIF) technique for the visualization of fuel distribution in the presence of oxygen was developed and then used sequentially with high speed chemiluminescence imaging to study the correlation between the mixing and auto-ignition processes of high pressure Diesel jets. A single hole common rail Diesel injector allowing high injection pressures up to 150 MPa was used. The reacting fuel spray was observed in a high pressure, high temperature cell that reproduces the thermodynamic conditions which exist in the combustion chamber of a Diesel engine during injection. Both free jet and flat wall impinging jet configurations were studied. Several tracers were first considered with the objective of developing a tracer-LIF technique in the presence of oxygen. 5-nonanone was selected for its higher fluorescence efficiency. This technique was subsequently combined with high speed chemiluminescence imaging to study the correlation between mixing and auto-ignition. In the free jet configuration and for the parameter range studied, it was found that auto-ignition is preferentially located in rich regions of the upstream mixing zone. Also, in the jet wall configuration, auto-ignition appears in the centre of the jet and propagates towards the periphery, in the vicinity of the wall.

Diesel jet impingement on small cylindrical obstacles for mixture homogenization by late injection strategy

The paper deals with ‘destroying’ a high-velocity diesel fuel jet by impingement on small cylindrical obstacles. This process permits very quick jet distribution in the volume. Essential is a process of impingement on a first obstacle: its diameter, distance from the nozzle outlet, and injection pressure play a dominant role in characterizing jet distribution in space. This spatial distribution is based on a multijet model, initially observed by diesel jet impingement on a porous structure. In diesel jet impingement on a single obstacle with a smaller diameter (d=1 mm), the distribution angle decreases with an increase of injection pressure. For bigger obstacle diameters (d = 2 mm) the angle between both jets increases almost independently with regard to injection pressure. This angle does not change with time after initial impact. It decreases, however, with an increasing distance between nozzle and obstacle. For diesel jet impingement on more than one single obstacle, jet distribution depends on the obstacle's geometrical configuration but not necessarily on the number of obstacles. Quite similar distribution has been obtained for configurations with four and eight obstacles. It is possible to design a configuration of small obstacles such as to permit very good charge homogenization within a defined space (combustion chamber). Such an example of five-obstacle configuration has also been investigated in the paper. A multijet structure also permits higher air entrainment as compared with a free jet configuration. All these effects would promote fast spatial distribution of diesel jets and, under hot conditions, faster vaporization and better mixing with air.

Experimental investigation of sonic jet flows for wing/nacelle integration

An experimental study of compressible jet flows has been undertaken in a small transonic wind tunnel. The aim of this investigation was to realize a jet simulator in the framework of wing/nacelle integration research and to characterize the jet flow behavior. First, free jet configuration, and subsequently jet flow in co-flowing air stream configuration were analyzed. Flow conditions were those encountered in a typical flight condition of a generic transport aircraft, i.e. fully expanded sonic jet flows interacting with a compressible external flow field. Conventional experimental techniques were used to investigate the jet flows-Schlieren visualization and two-component Laser Doppler Velocimetry (LDV). The mean and fluctuating properties were measured along the jet centerline and in the symmetric plane at various downstream locations. The results of two configurations show remarkable differences in the mean and fluctuating components and agree well with the trend observed by other investigators. Moreover, these experiments enrich the database for such flow conditions and verify the feasibility of its application in future aerodynamic research of wing/nacelle interactions.

Evaluation of parallel injector configurations for Mach 2 combustion

An experimental study of wall-mounted parallel injector ramps has been conducted to explore techniques to enhance mixing in a scramjet combustor. Downstream parallel injection may be useful at high speeds to extract thrust from hydrogen that has been used to cool the engine and the airframe. The swept ramp fuel injector employed here should produce vortex shedding and local separation (like a rearward-facing step), which should enhance mixing. Perpendicular fuel injectors were added downstream of the swept ramps to determine if the vortical wake flow generated by the parallel injectors—with no fuel injection—is effective in enhancing the mixing of the transverse fuel jet. For performance comparisons, an unswept, but otherwise identical, parallel injector was also tested. The injector ramps were designed to yield a reflected shock wave from the duct top wall such that it passed just downstream of the barrel shock of the fuel injectors. The Mach number at the exit of the fuel injector was designed to be 1.7 to produce an underexpanded fuel flow under almost all operating conditions. Direct-connect tests were conducted with a vitiated heater over a total temperature range. Flow visualization tests with an unducted freejet configuration were conducted using shadowgraph and ultraviolet television camera systems for OH radical visualization. Three duct configurations were tested. Tests of the swept ramp with a 100-mm constant-area duct downstream of the injector block resulted in upstream interaction at almost any equivalence ratio. Increasing the duct expansion rate allowed the equivalence ratio to be increased beyond 1.5 with rapid mixing and combustion over a wide total temperature range. The unswept ramp design resulted in lower combustion efficiency and a sensitivity of efficiency to heater total temperature, or duct velocity.

Prediction of secondary vortex flowfields generated by an interacting multiple free-jet configuration

APENALTY finite element algorithm for solution of the three-dimensional parabolic Navier-Stokes equations for subsonic turbulent flows is applied to the prediction of secondary vortex flowfields induced by a multiple free-jet configuration. The combined action of rapid decay of the initial high-speed parallel jets, turbulence level, induced entrainment from the far field, and geometric discreteness of a four-jet configuration, is predicted to produce a persistent system of eight counterrotating vortex pairs in the plane normal to the initial jet axis. Numerical predictions are interpreted and compared with available experimental data. Contents As detailed in the complete paper,1 a penalty finite element numerical algorithm is well suited to the solution of subsonic, three-dimensional multiple free-jet problem definitions. The abiding predominance of a preferred (axial) flow direction permits an order-of-magn itude analysis confirming that associated diffusion effects are insignificant. Discarding these terms yields the thin-layer Navier-Stokes (TLNS) approximation; provided a suitable pressure interaction procedure exists, solution of the TLNS system can be cast into the efficient space-marching, initial-value, parabolic NavierStokes (PNS) equation set. The ordering analysis also confirms that the transverse plane momentum equations govern pressure modifications to first order, while the continuity equation is the principal (divergence-free) momentum constraint statement. The key features of the PNS penalty finite element algorithm, as applied to the free-jet problem class, directly address these fundamental issues by 1) establishment of a quasilinear pressure Poisson equation, with complementary and particular solutions that readily admit interaction and far-field boundary condition requirements; and 2) a specific continuity equation constraint yielding efficient and accurate global communication of the local momentum defect relaxa

Turbulent Mixing in Inert and Combusting Flows Visualized by HF Tracer Absorption

Abstract A noninterfering diagnostic technique that employs monomeric hydrogen fluoride, HF, as a passive tracer gas has been developed. Infrared absorption by HF is used to infer turbulent mixing rates in inert and combusting flows. Data are presented on the turbulent diffusion of HF seeded streams of air, heated air, H2, and CO2 into an outer coaxial air stream. Combustion of C2H2 and air in a free jet configuration has also been investigated with the H F tracer. In most of these flow situations, the HF exhibited apparent rotational nonequilibrium. Development of the experimental technique and methods for data acquisition and analysis are discussed.