Measurements on supersonic free jets

Summary An experimental study of round supersonic air jets discharging into quiescent air is described. The initial stagnation enthalpy of the jets was equal to that of the atmosphere. Most of the experiments concerned a fully expanded jet with initial Mach number M _I =1.74. From the experimental results the turbulent Prandtl number and the turbulent coefficient of momentum transfer could be calculated in a large part of the mixing zone of the jets. A mixing parameter K has been introduced and calculated for the jets. The results of this investigation are compared with those obtained for low-speed jets.     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

On the dynamical equations for liquid jets

Quasi-one-dimensional equations for the three-dimensional motion of thin liquid jets have been derived by Entov and the present author [1, 2] from the balance integral equations for the mass, momentum, and angular momentum written down for a jet section. Simplified equations of this kind make it possible, in particular, to investigate with comparative ease the motion of bending jets and also the loss of stability of jets moving in air associated with the development of kinks, etc. It is of interest to obtain quasi-one-dimensional equations of jet motion by direct integration over the section of a thin jet of the three-dimensional differential equations of hydrodynamics. In the present note, this approach is illustrated by the example of bending of a jet in a plane.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 161–163, January–February, 1983.     

Flow and Heat Transfer in Underexpanded Air Jets Issuing from the Sonic Nozzle of a Plasma Generator

We present the results of an experimental investigation and numerical simulation of the gasdynamic structure of underexpanded dissociated-air jets and the heat transfer in these strongly nonequilibrium flows under the test conditions realized in the 100-kW electrodeless VGU-4 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS). The flow and heat transfer analysis is carried out on the basis of measurements of the static pressure in the plenum chamber, at the sonic nozzle exit, and on the low-pressure chamber wall, the stagnation pressure on the jet axis using a Pitot tube, and the heat transfer at the stagnation points of water-cooled models placed along the jet axis. The numerical simulation, based on complete Navier-Stokes equations, includes the calculation of (1) equilibrium air plasma flows in the discharge channel of the VGU-4 plasma generator; (2) underexpanded nonequilibrium dissociated-air jet outflow into the ambient space; and (3) axisymmetric jet flow past cylindrical models.     

Axisymmetric Coanda-assisted vectoring

An experimental demonstration of a jet vectoring technique used in our novel spray method called Coanda-assisted Spray Manipulation (CSM) is presented. CSM makes use of the Coanda effect on axisymmetric geometries through the interaction of two jets: a primary jet and a control jet. The primary jet has larger volume flow rate but generally a smaller momentum flux than the control jet. The primary jet flows through the center of a rounded collar. The control jet is parallel to the primary and is adjacent to the convex collar. The Reynolds number range for the primary jet at the exit plane was between 20,000 and 80,000. The flow was in the incompressible Mach number range (Mach < 0.3). The control jet attaches to the convex wall and vectors according to known Coanda effect principles, entraining and vectoring the primary jet, resulting in controllable r − θ directional spraying. Several annular control slots and collar radii were tested over a range of momentum flux ratios to determine the effects of these variables on the vectored jet angle and spreading. Two and Three-component Particle Image Velocimetry systems were used to determine the vectoring angle and the profile of the combined jet in each experiment. The experiments show that the control slot and expansion radius, along with the momentum ratios of the two jets predominantly affected the vectoring angle and profile of the combined jets.     

Mechanism of formation of cumulative jets in fluid expelled from a plasma accelerator

The results of an experimental investigation into the process of obtaining cumulative water jets from an electrical discharge in a plasma accelerator are presented. The physical mechanism of jet formation in the plasma accelerator and after the emergence of the fluid into the air is discussed. Nikolaev. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 39–44, Septeber–October, 1988.     

Theory of an Ideal Jet Thrust Augmentor

A theory of the ideal jet thrust augmentor is presented. The conditions of optimal outflow of the active (primary) and passive (secondary) jets from the device under consideration are obtained by solving the variational problem of maximum average thrust realization. The inlet values of the mass and total enthalpy fluxes for both flows, their entropies, and the inlet value of the passive-gas momentum component parallel to the flight velocity are preassigned. These conditions correspond to the use of jet engines, including a bypass turbojet gas generator, operating in steady mode, as well as a pulsed detonation rocket engine, as the high-pressure gas source. Along with the work done by the high-pressure gas on the low-pressure one, the gases may exchange heat. The possibility of heat transfer results in an appreciable increase in ideal thrust augmentor performance.     

Supersonic flow past a body with transverse jets

The results of an experimental study of the flow past a body with transverse jets are presented. The gas jets flowed out simultaneously from several nozzles arranged on the body's lateral surface. Various flow regimes were considered to such parameters as the relative jet momentum, the angle of incidence, etc. The experimental results were generalized on the basis of an approximate similarity law.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 75–80, July–August, 1995.     

Computational study of buoyancy effects in a laminar starting jet

Vortical structures formed in evolving jets are important in applications such as fuel injection in diesel engines and fuel leaks. When the jet fluid is different from the ambient fluid, the buoyancy can play an important role in determining the jet flow structure, and hence, the entrainment and fluid mixing processes. In the present study, a jet of helium injected in air is investigated, with emphasis placed on delineating the buoyancy effects on vector–scalar fields during the starting phase. We utilize a computational model, previously validated to predict the flow field of low-density gas jets. The model incorporates finite volume approach to solve the transport equation of helium mass fraction coupled with conservation equations of mixture mass and momentum. Computations were performed for a laminar jet to characterize the advancing jet front, and to capture the formation and propagation of vortex rings and the related pinch-off process. Results show significant effects of buoyancy on jet advancement, as well as on vorticity and helium concentration in the core of the vortex rings.     

Experimental investigation on the similarity of a 3D rectangular turbulent jet

 The paper presents an experimental investigation of turbulent jets issuing from rectangular nozzles. Nozzles with aspect ratios between 3 and 10 were used. Eight different initial conditions were studied. The following jet parameters were measured and evaluated: mean velocity components, jet boundaries, jet momentum, jet entrainment, turbulence intensities and Reynolds stresses. A DISA 55M thermoanemometer and a data acquisition system BE256 were used. The influence of the initial conditions on the similarity of the flow was determined with respect to the mean axial velocity, turbulence intensity and the Reynolds stresses. A significant influence of the initial conditions on the flow structure was observed. The possibility for jet control is discussed and suggestions are given about the need to investigate different parameters. Received: 25 November 1996/Accepted: 30 October 1997     

Heat and mass transfer in a system of plane turbulent jets with diffusion combustion

We consider turbulent motion of premixed chemically active gases in an infinite system of plane turbulent jets in the presence of diffusive combustion. The proposed calculation method permits determining the distribution of all the parameters in the mixing zone, including the longitudinal pressure. Numerical examples of the calculation of hydrogen combustion in air are presented.The study of heat and mass transfer in jet flows presents major difficulties at the present time. Therefore all the existing methods for calculating jet flows with heat and mass transfer and chemical processes [1–5] are based on an extension of the known semiempirical theories of free turbulence to the more complex cases of flow with chemical reactions. The present study is no exception in this sense; it covers an investigation of the motion in an infinite system of plane turbulent jets with diffusive combustion.     

A gas jet superposition model for CFD modeling of group-hole nozzle sprays

In the present study, a jet superposition modeling approach is explored to model group-hole nozzle sprays, in which multiple spray jets interact with each other. An equation to estimate the merged jet velocity from each of the individual jets was derived based on momentum conservation for equivalent gas jets. Diverging and converging group-hole nozzles were also considered. The model was implemented as a sub-grid-scale submodel in a Lagrangian Drop–Eulerian Gas CFD model for spray predictions. Spray tip penetration predicted using the present superposition model was validated against experimental results for parallel, diverging and converging group-hole nozzles as a function of the angle between the two holes at various injection and ambient pressures. The results show that spray tip penetration decreases as the group hole diverging or converging angle increases. However, the spray penetration of the converging group-hole nozzle arrangement is more sensitive to the angle between the two holes compared to diverging nozzle because the radial momentum component is converted to axial momentum during the jet–jet impingement process in the converging group-hole nozzle case. The modeling results also indicate that for converging group-hole nozzles the merged sprays become ellipsoidal in cross-section far downstream of the nozzle exit with larger converging angles, indicating increased air entrainment.     

矩形喷嘴平行射流组流体动力学特性

依据动量和质量守恒原理,建立了这平行射流组流动行为的数学模型,并获得了模型的解析解研究了矩形喷嘴平行射流组的流动行为特性。最后,计算结果与实验结果进行了比较。     

高压水射流冲击煤体的力学特征

以质量守恒与动量守恒定律为基础, 建立了高压水射流冲击煤体的力学模型。运用此模型分析了高压水射流在冲击煤体的过程中, 未破水体、破碎水体、煤体的破碎区与扩孔区的力学特征, 利用严格的力学守恒关系得出高压水射流冲击煤体的简化常微分方程组。将理论计算结果与现场实验和数值模拟结果进行对比, 结果表明:理论计算结果与数值模拟结果和实验结果基本一致。此模型具有明确的力学意义, 且能够反映真实的冲击过程。     

Heat transfer characteristics from an array of thin strips pin fins due to their exposures to a single downward jet impingement

This paper investigates the heat transfer characteristics from thin strips pin fins due to their exposure to a single circular downward air jet impingement. Five aluminum specimens were considered; each one has a rectangular base of 84 mm × 78 mm and it has an array of about 300 thin strips pin fins. A test rig consists mainly of air compressor; nozzle and protractor mechanism was setup. Experiments were done to find out the effects of attack angle, Reynolds number, nozzle-to-target spacing, lateral pitch and parallel pitch among the fins on the heat transfer characteristics. Empirical correlations were deduced to describe the experimental data. A CFD-numerical model was introduced to monitor the flow characteristics on a scale of more details than that possible in the experimental work. The comparison among the results of the present work and those by the literature shows about 50% improvement in heat transfer characteristics rather than the single jet impingement onto flat plates, cylindrical surfaces, ribbed walls and multiple jets impingement onto flat plates.     

Slot/slots air jet impinging cooling of a cylinder for different jets–cylinder configurations

Heat transfer characteristics of a slot/slots jet air impinging on a cylinder have been experimentally investigated for two different orientations of slot/slots jet plan with respect to cylinder axis. The experiments were carried out to study the effects of orientation of slot/slots jet plan with respect to cylinder axis on the rate of heat transfer from the cylinder. Two different jet–cylinder configurations were studied: (1) single slot jet aligned with cylinder axis (slot length = cylinder length), and (2) multiple slot jets equally spaces distributed orthogonal to cylinder axis (each slot length = cylinder diameter and sum of slots lengths = cylinder length). For each configuration, parametric effects of Reynolds numbers (Re) ranging from 1,000 to 10,000, dimensionless slot widths (W/d) ranging from 0.125 to 0.5, and dimensionless slot orifice-to-cylinder spacing (Z/W) ranging from 1 to 12 on local and average Nusselt numbers around cylinder surface have been investigated. The results showed that: (1) cooling the cylinder by multiple slots jets situated orthogonal to cylinder axis gave more uniform surface temperature distributions and higher heat transfer rate than the case of cooling the cylinder by single slot jet aligned with cylinder axis, (2) for both configurations the Nusselt number around the cylinder increased with increasing Re and W/d, and (3) for both configurations there was a certain Z/W in the range 4<Z/W<6 at which the stagnation and mean Nusselt number were maximum. Correlations for the mean Nusselt numbers around cylinder surface have been presented for both configurations. Comparisons between the correlations predictions and the present and other previous experimental data have been conducted.     

Nonlinear stability of two-phase jets

The effect of solid particles on the flow stability and secondary regime branching in plane submerged jets is studied. The presence of the particles has an important influence on the macrostructure and microstructure of the jet flows, modifying the rate of turbulent momentum, heat and mass transfer [1,2].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 50–55, May–June, 1985.The authors are grateful to V. N. Shtern for useful comments.     

Motion and heat and mass transfer in a disperse admixture in turbulent nonisothermal jets of a gas and a low-temperature plasma

The motion and heat and mass transfer of particles of a disperse admixture in nonisothermal jets of a gas and a low-temperature plasma are simulated with allowance for the migration mechanism of particle motion actuated by the turbophoresis force and the influence of turbulent fluctuations of the jet flow velocity on heat and mass transfer of the particle. The temperature distribution inside the particle at each time step is found by solving the equation of unsteady heat conduction. The laws of scattering of the admixture and the laws of melting and evaporation of an individual particle are studied, depending on the injection velocity and on the method of particle insertion into the jet flow. The calculated results are compared with data obtained with ignored influence of turbulent fluctuations on the motion and heat and mass transfer of the disperse phase. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 3, pp. 95–108, May–June, 2008.     

Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Q_m = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works.     

Flow and breakup characteristics of elliptical liquid jets

This paper presents the results of an experimental study on liquid jets discharging from elliptical orifices into still ambient air. The experiments were conducted with a set of elliptical orifices of approximately same area of cross section but varying orifice aspect ratio using water and water–glycerol mixture as experimental fluids. The flow behavior of liquid jets was analyzed using their photographs captured by an imaging system. The measurements obtained for the elliptical liquid jets were compared with the circular liquid jets discharging from a circular orifice of the same area of cross section. Elliptical geometry of the orifice results in a flow process by which the emanating liquid jet periodically switches its major and minor axes as it flows downstream of the orifice. In this paper, we attempt to characterize the axis-switching process through its wavelength and amplitude. For a given elliptical orifice, the axis-switching process is dominantly seen in a particular range of flow conditions. The effects of the orifice aspect ratio and liquid viscosity on the axis-switching process are revealed through this study. The experimental results on jet breakup show that axis-switching process has a destabilizing effect on elliptical liquid jets within a particular range of flow conditions and it results in shorter breakup lengths compared to the circular jet. The extent to which axis-switching destabilizes the jet is dictated by the viscosity of liquid. An increase in orifice aspect ratio destabilizes elliptical liquid jets with low viscosity like water; however, this behavior seems to get obscured in water–glycerol mixture elliptical jets due to high viscosity.