Drop evaporation in a turbulent gas jet

Evaporation of a semidispersive drop system in a turbulent gas jet is considered. A method for calculating drop evaporation in a turbulent gas jet is proposed based on a simplified solution of the scattering problem for an evaporating admixture. Evaporation of water as it is atomized in a turbulent air jet is experimentally studied. Approximate agreement is obtained between the results of the calculations and experiments.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 73–79, January–February 1976.     

Effects of Small- and Large-Scale Structures in a Piloted Jet Diffusion Flame

Laser Doppler Anemometry (LDA) and Planar Laser-Induced Fluorescence (PLIF) measurements have been performed in a turbulent nonpremixed jet flame. One of the features of this configuration is a central co-axial fuel jet surrounded by a turbulent annular air flow. The whole is placed within a low-speed coflowing air stream. This three-flow system with turbulent primary air differs from flow systems used for nonpremixed jet flames reported in the literature and is very useful for obtaining information on the mixing process between fuel and primary air. Next to the characterization of the velocity field, special attention has been paid to the conditional seeding of the central fuel jet and of the annular air flow. Together with visualizations of the OH radical, an important combustion intermediate which is formed during combustion, and the NO radical, which is seeded to the central jet flow, the resulting statistics reveal the properties of small- and large-scale structures in the flame.     

An experimental investigation of droplet evaporation and coalescence in a simple jet flow

An experimental study of a simple jet flow, which contains a dispersion of fine droplets, has been carried out in order to investigate the effect of turbulence, evaporation and coalescence on the droplet size distributions within the jet. Very little evaporation occurs in the potential core of the jet, while in the far-field, where the potential core has vanished and the droplets disperse more readily, evaporation occurs predominantly in the outer portions of the spray. Evidently, turbulence enhances the evaporation rate of droplets at the edges of the spray, and fresh air entrained from the outer regions increases the evaporative driving force. Coalescence has also been observed within the spray, although this effect is rather subtle compared to the evaporation effect in the dilute jets investigated here. Nevertheless, sufficient measurements have been taken to validate, at least partially, any coalescence models, in addition to any turbulence and evaporation models for dilute poly-disperse sprays.     

A detailed experimental investigation of well-defined,turbulent evaporating spray jets of acetone

This paper aims at investigating the detailed structure of turbulent non-reacting dilute spray flows using advanced laser diagnostics. A simple spray jet nozzle is designed to produce a two-phase slender shear flow in a co-flowing air stream with well-defined boundary conditions. The carrier flow is made intentionally simple and easy to model so that the focus can be placed on the important aspects of droplet dispersion and evaporation, as well as turbulence–droplet interactions. Phase Doppler interferometry is employed to record droplet quantities, while planar laser-induced fluorescence imaging is applied separately to obtain acetone vapour data. Measurements are conducted for four acetone spray jets in air at several axial stations starting from the nozzle exit. The combined liquid and vapour mass fluxes of acetone integrated across the jet at downstream locations agree satisfactorily with the total mass flow rate of acetone injected.     

Influence of vent positions on smoke clearance from a room

Results of solutions to the equations governing the clearance of smoke in laminar and in turbulent flow from a single rectilinear room are presented fof five different vent arrangements. These results show that the average smoke concentration in the room decreases in an approximately exponential manner with time at a rate that depdends much more on room geometry (vent positions in the present case) than on whether the flow is laminar or turbulent. The benefit of this observation is that, provided the clearance air jet is disrupted by impingement on a solid surface within the room, the analysis of smoke movement can for many practical purposes be limited to consideration of laminar flow only. An exceptional case is when the jet is allowed to take a straight line path between the inlet and outlet vents. Here in the absence of the large scale eddies caused by jet impingement, the smaller scale eddies of turbulent flow become relatively important as a mixing mechanism and give higher clearance rates than in laminar flow. Even so, this type of geometry is best avoided. The appearance of the jet is shown in a graphic presentation of the numerical results for laminar flow.     

Observations on Upstream Flame Propagation in the Ignition of Hydrocarbon Jets

A variety of investigators have attempted to characterize the mechanisms of how reaction zones stabilize, or propagate, against incoming reactants, particularly in stable lifted jet flames both laminar and turbulent. In this paper, experiments are described that investigate the characteristics of upstream flame propagation in turbulent hydrocarbon jet flames. An axisymmetric, gaseous turbulent jet mixing in air has been selectively ignited at downstream positions to assess the upstream propagation of the bulk reaction zone. The farthest axial position that permitted the reaction zone to propagate upstream after application of the ignition source, referred to as the “upper propagation limit”, or UPL, is determined for a variety of jet and air co-flow parameters. There is an inverse relationship between the upper propagation limit position and the jet Reynolds number. Conversely, there is a direct relationship between the upper propagation limit and the co-flow velocity. Interpretation of the results is related to the velocity at the stoichiometric surface. Global discussion is made as to what these results imply about the stabilization and propagation of turbulent lifted jet flames.     

Pulsed laser measurements of particle and vapour concentrations in a turbulent jet

A turbulent jet of air has been seeded with both particles and a vapour. An excimer pumped dye laser excited visible fluorescence from the biacetyl vapour and Mie scattering from the micron-size particles. It was possible to measure the simultaneous scattering from both phases by using interference filters to separate the signals. It has been found that the biacetyl vapour provides an adequate tracer for measurements of the concentration field in a turbulent flow. Furthermore, the feasibility of making simultanous concentration measurements of two phases in a turbulent flow has been demonstrated.     

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 10⁴), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

The near field behavior of turbulent gas jets in a long confinement

The present investigation reports on the near field behavior of gas jets in a long confinement and points out the differences between this type of jet flow and those of free jets and jets in a short confinement.The jet, with a diameter of 8.73 mm, is aligned concentrically with a tube of 125 mm diameter; thus giving a confinement area ratio of 205. The arrangement forms part of the test section of an open-jet wind tunnel and this gives a confinement length-to-jet diameter ratio of 1,700. Experiments are carried out with carbon dioxide, air and helium/air jets at different jet velocities. Mean velocity and turbulence measurements are made of the jet near field using a one-color, one-component laser doppler velocimeter operating in the forward scatter mode. In addition, the turbulent shear field of an air jet is examined in more detail using hot-wire anemometers.In view of the long confinement, the presence of the jet is not being felt immediately at the tunnel exit. Consequently, the air column inside the tunnel is first compressed by the jet and then slowly pushed out of the tunnel. This behavior causes the jet to spread rapidly and to decay quickly. As a result, an equilibrium turbulence field is established in the first two diameters of the jet. This equilibrium field bears striking similarity to that found in self-preserving, turbulent free jets and jets in short confinement and is independent of jet fluid densities and velocities. In terms of these characteristics, the near field of jets in a long confinement is very different from that found in free jets and jets in short confinements.     

Development of a two-dimensional turbulent jet in an infinite space

The second and third terms in the asymptotic expansion of the stream function in the nonsimilar problem of the development of a two-dimensional turbulent jet in an unbounded space are found in final form. Results of experimental investigations of free turbulent jets are cited, and the effect of the initial velocity profile on the aerodynamic characteristics of the jet is considered. The problem of the development of a two-dimensional turbulent jet in an unbounded space has been considered in [1–3]. The existing solution is similar, and is valid only at a sufficiently large distance from the slit. Allowance for the finite dimensions of the slit leads to a nonsimilar problem. The papers [4–6] are devoted to the experimental investigation of the free two-dimensional turbulent jet.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 137–142, July–August, 1971.     

Large-scale instability of a fine-grained turbulent jet

The initial growth of a large scale perturbation on a fine-grained turbulent jet is studied via linear stability analysis. The turbulent jet is assumed to be homogeneous and isotropic with zero mean shear, and the inviscid stream outside the jet has a uniform velocity profile. The incremental Reynolds stress caused by the large scale perturbation is modeled by a viscoelastic constitutive equation, following the analysis of Crow (1968). It is found that the jet is always unstable to both sinuous and varicose types of perturbation, with the sinuous mode having a larger growth rate. In particular, short waves are always amplified, in contrast to the short wave stabilization at low speed found by Townsend (1966) for a purely elastic jet. The growth rates of these short waves are finite, and are smaller than those for the classical Kelvin-Helmholtz instability of an inviscid jet, but larger than those for the Hooper-Boyd (1983) instability of a viscous jet with continuous velocity profile.     

Numerical Study of the Near-Wall Gas-Droplet Jet in a Tube with a Heat Flux on the Surface

A model for calculating the flow of a turbulent mixture of air and suspended liquid particles injected into the near-wall region is developed within a unified approach of mechanics of heterogeneous media in the two-velocity and two-temperature approximation of the Eulerian approach. The influence of droplet evaporation in the near-wall jet on heat transfer between the two-phase gas-droplet flow and the wall is studied in the case of heat addition to the latter. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 1, pp. 5–17, January–February, 2006.     

The mathematical simulation of various condensation regimes in turbulent isobaric jets

Mathematical models are considered and calculations made for flows in turbulent isobaric steam—air jets in the presence of condensation of the water vapor they contain. The models consist of gasdynamic equations for a turbulent jet, equations for a differential two-parameter model of turbulence, thermodynamic relations, and kinetic equations. A study is made of steam—air jets in a regime of condensation in equilibrium, when the flow region is broken down into zones of frozen flow and flow in equilibrium, described by the equations for a turbulent jet with the use of the traditional thermodynamic relations and of the thermodynamic relations for condensation in equilibrium. An analysis is made of the influence of pulsating motion on the kinetic parameters: rate of nucleation, the critical size of the nuclei, and rate of growth of the drops. It is shown that the rate of nucleation, determined from a quasilaminar averaging model, is several orders of magnitude less than the mean value obtained by averaging using the density distribution of the passive admixture concentration probability. A numerical study is made of the heterogeneous condensation in turbulent jets on extraneous particles entering from the nozzle. Kinetic equations are written down for the case when the rate of growth of the drops does not depend on their radius. A study is made of the dynamics of the transition of heterogeneous condensation from disequilibrium to equilibriumTranslated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 59–67, January–February, 1985.     

Measurements of admixture concentration fluctuations in a turbulent shear flow using an averaged Talbot image

The present study is concerned with adopting of a Talbot effect-based technique for analyzing flows with random phase inhomogeneities. It is shown that this method is a powerful tool for diagnostics of turbulent flows. The potential of the technique is illustrated by measuring mean and fluctuating values of admixture concentration of two-dimensional turbulent helium jet issuing into the ambient air. Averaged air and helium concentrations throughout the flow field are determined using local light refraction measurements with a high spatial resolution from a long-exposed Talbot image of the jet. The analysis of light intensity distributions in light spots of a Talbot-image shows that the jet turbulence is inhomogeneous and anisotropic. Quantitative information on rms fluctuations of concentration gradients throughout the flow field is obtained from local photometric measurements at the Talbot light spots.     

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k ^?1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.     

A numerical and experimental investigation of the interactions between a non‐uniform planar array of incompressible free jets

The interaction between multiple incompressible air jets has been studied numerically and experimentally. The numerical predictions have been first validated using experimental data for a single jet configuration. The spreading features of five unequal jets in the configuration of one larger central jet surrounded by four smaller equi‐distant jets, have been studied, for different lateral spacing ratios of 1.5, 2.0 and 2.5 and a central jet Reynolds number of 1.24×105 (corresponding to a Mach number of 0.16). Flow of five equal jets has also been simulated, for the sake of comparison. The jet interactions commence at an axial distance of about 3–4 diameters and complete by an axial distance of about 10 diameters for the lowest spacing ratio of 1.5. For larger spacing ratios, the length required for the start and completion of jet interaction increase. Peripheral jets bend more towards the central jet and merge at a smaller distance, when their sizes are smaller than that of the central jet. The entrainment ratio for multiple jets is higher than that for a single jet. Excellent agreement is observed between the experimental data and theoretical predictions for both mean flow field and turbulent quantities, at regions away from the jet inlet. The potential core length and initial jet development, however, are not predicted very accurately due to differences in the assumed and actual velocity profiles at the jet inlet. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigation of three dimensional air flow characteristic and effect of heated obstruction within room

This study reports the results of a numerical investigation of three-dimensional turbulent buoyant recirculating flow within rooms with heated obstruction. The study involves the solution of partial differential equations for the conservation of mass, momentum, energy, concentration, turbulent energy and its dissipation rate. These equations were solved together with algebraic expressions for the turbulent viscosity and heat diffusivity using k-ε turbulence model by performing simulations on FLUENT 6.3. The CFD method was validated via comparing with the available experimental data. A comparison with experimental results shows good agreement. This means that the present computer code has a good capability to simulate 3D airflow and effect of obstruction within room. The present study demonstrates the flow behavior, thermal distribution and CO₂ concentration inside the room in the presence of heat flux obstruction. Two different configurations of ventilation system have been studied. Mixing and Displacement ventilation system have been used in two geometries depending on location of opening inlet. The ventilation effectiveness for heat removal (E_T) is used to evaluate the indoor climate and average temperature is an important parameter in designs the ventilation systems. Two notable points are presented; first, mixing ventilation is depending on throw of jet. CO₂ concentration and temperature distribution have been effected in upper zone more than occupied zone with presence the obstruction. Second notable points are presented; in displacement ventilation buoyancy effect is considerable. Vertical temperature gradient above the obstruction implies that both fresh air and CO₂ concentration.     

On Turbulent Jets Issuing from Notched-Rectangular and Circular Orifice Plates

This paper reports an experimental investigation of the near-field flow characteristics of two air/air turbulent jets issuing respectively from notched-rectangular and circular orifice plates with identical opening areas or equivalent diameters (D _e ). Planar particle image velocimetry (PIV) was used to measure the velocity field at the same Reynolds number, based on D _e , of Re = 72,000. Consistent with previous work on other noncircular jets, the present study finds that the notched jet has a higher rate of mixing than does the circular counterpart. In particular, this jet in the very near field transfers its momentum to the surroundings at a greater rate, evidenced by a notably shorter unmixed core and faster turbulence intensity growth. The higher rates of overall decay and spread of the notched jet are maintained over the entire measurement region and likely beyond. In addition, the phenomenon of axis switching is also found to occur in this jet.     

The swirling radial jet

The similarity solution of the radial turbulent jet with weak swirl is discussed and a new solution of the radial turbulent jet with swirl is proposed without restrictions assumed in the weak swirl solution.