Jet Exit Velocity Research Articles

Numerical investigation of a transient free jet resembling a laser-produced vapor jet

In the present study, the transiently developing free jet emanating from a laser-impacted surface is considered. The jet velocity profiles are varied with time in connection with the vapor jet velocity profiles emanating from the laser-produced cavity. Consequently, jet expansion from the laser cavity situation is modelled in the simulations. The jet exiting profiles measured previously are employed in the present simulations. Since the thermophysical properties of the laser-produced vapor are unknown, air properties are used for the jet in the simulations. A numerical method employing control volume approach is introduced to discretize governing equations of flow and energy. It is found that in the early period, jet behavior is similar to slowly flowing jets as reported in the literature. The self-similar transient jet behavior occurs as the time progresses; in which case jet exit velocity profiles become similar.

Local jet impingement boiling heat transfer with R113

An experimental study was performed to characterize the boiling heat transfer of impinging circular submerged jets on simulated microelectronic chips with a nominal area of 5 mm × 5 mm. The heat transfer modes included natural convection, partially developed nucleate boiling, fully developed nucleate boiling and critical heat flux. The study included the effects of jet parameters and fluid subcooling on the nucleate boiling. The results showed that the nucleate boiling data varied only with fluid subcooling regardless of jet parameters and that both the pool and impingement nucleate boiling curves at the same subcooling condition were well correlated. The high heat flux portions of the boiling curves with jet exit velocities greater than 10 m/s were corrected for the elevated saturation temperature. A new expression was developed with an interpolation method to construct the partially developed nucleate boiling curve.

Characterisation of the 1997 Vulcanian explosions of Soufri?re Hills Volcano, Montserrat, by video analysis

The activity of Convention at Montserrat Soufriere Hills Volcano, Montserrat, during the period 1995–1999 included numerous violent explosions. Two major cycles of Vulcanian explosions occurred in 1997: a first of 13 explosions between 4 and 12 August and a second of 75 between 22 September and 21 October. The explosions were short-lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows, and buoyant plumes ascended 3–15 km into the atmosphere. Each explosion discharged on average 3×105 m3 (dense-rock equivalent, DRE) of magma, draining the conduit to depths of 1–2 km. The paper focuses on the first few seconds of three explosions of the 75 that occurred in September/October 1997: 6 October 1997 at 17:50, 7 October 1997 at 16:02 and 9 October 1997 at 12:32. Physical parameters such as exit velocities, magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum-dominated finger jets visible on videos during the initial stages of each explosion. The model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent, and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters. Each explosion reveals a progressive increase in eruptive intensity with time, jet exit velocities increasing from 40 m s−1 at the beginning of the explosion up to 140 m s−1 after a few seconds. Modelling suggests that the first magma to exit was largely degassed, whereas that discharged after a few seconds contained up to 2 wt% water. Magma overpressures up to ~10 MPa are estimated to have existed in the conduit immediately prior to each explosion. Progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre-eruptive gradients in water content and magma pressure in the upper reaches (probably 100–500 m) of the conduit. Fountain collapse occurred during the first 10–20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant. Such high eruptive densities were due to the presence of partially degassed magma in the conduit.

Confined swirling jet impingement onto an adiabatic wall

Impinging swirling jets generate interesting flow fields and depending on the magnitude of the swirl velocity, circulation cells develop in the region close to the solid wall. Moreover, axial momentum of the jet is influenced by the magnitude of the swirl velocity. This, in turn, results in considerable entropy generation in the flow field. In the present study, confined swirling jet impingement onto an adiabatic wall is investigated. The flow and temperature fields are computed numerically for various flow configurations. Different jet exit velocity profiles are considered and their effects on the flow field are examined. The entropy production due to different flow configurations is computed and the irreversibility ratios due to fluid friction and heat transfer are determined. It is found that the jet axis tilts towards the radial direction as swirl velocity increases and reducing the velocity profile number enhances the entropy generation due to heat transfer. The irreversibility ratio variation with the velocity profile number behaves opposite for the fluid friction and heat transfer.

Coherent Structure in a Two-Dimensional Jet Passively Controled by Wake of a Cylinder Installed in Nozzle Contraction.

It is possible to control the spread of a plane jet passively by the vortices from a two-dimensional cylinder installed in the nozzle contraction. In this study, the wake characteristics of a cylinder and the coherent structure of a jet by this passive control method were examined by flow visualization and quantitative velocity measurements. In wide range of strouhal number based on vortex shedding frequency and jet exit velocity, the spreads of the jet were found to be suppressed. This suppression is caused by the high frequency vortices from the control cylinder which inhibits growth and pairing of the low frequency vortices. On the other hand, in certain cases of the strouhal number being 0.05-0.10, the spread of the jet was remarkably enhanced associated with distinctive peaks of spectrum at higher order of vortex shedding frequency caused by the deformation of the vortices in flow contraction nozzle. In the case of enhanced mode, vortex from the cylinder moves to the opposite side shear layer with the same vorticity sign as the vortex, and it enhances roll up of the shear layer.

EXPERIMENTAL INVESTIGATION OF THE HEAT TRANSFER CHARACTERISTICS OF CONFINED IMPINGING SLOT JETS

This article presents an experimental investigation of the heat transfer characteristics between the confined impinging slot jets and a flat surface. During the experiments, single slot, multiple slots with cross-flow effects, and multiple slots with symmetrical exhaust ports between neighboring jets are tested. The exhaust ports are manufactured to eliminate the cross-flow effects from the upstream slot, which degrade the heat transfer of the downstream impinging jets.Liquid crystal thermography is employed to map the temperature distributions on the test plate. Local heat transfer coefficients are obtained, and then the average heat transfer coefficients are evaluated. The effects of the nozzle-to-plate spacing H/B, Reynolds number, and the presence of the exhaust ports on the local Nusselt number distributions are examined. The local distributions of the heat transfer coefficients of the single slot jet and the central slot of the three slot arrays are compared with each other. The spent fluid removal diminishes the influence of the cross flow, and the heat transfer rates are enhanced compared with the multiple impinging slot jets with cross-flow effects. The average heat transfer coefficients of two slot systems with different slot widths are compared with each other at the same nozzle-to-plate spacing, and the narrower one gives higher heat transfer rates at the same jet exit velocity.

Observations of Cavitation Bubbles in a Starting Submerged Water Jet.

Cavitation bubbles in a starting water jet discharged from a circular nozzle into still water are investigated by a conventional photography technique in a moderately low range of jet exit velocity. Most of bubbles are initially generated in starting vortices and connected with each other in the form similar to vortex rings. Nearly axisymmetric lumps of disconnected bubbles are also observed frequently. By analyzing photographic data acquired from the side-and end-view pictures of the ring-like bubbles, their average properties, such as trajectory, geometry and size, are evaluated.

Scaling parameters for underexpanded supersonic jets

An analysis and experiments were carried out to study the spreading and centerline property decay rates of underexpanded supersonic jets. The main purpose was to determine a suitable set of normalization parameters that would account for the initial expansion process and allow for a comparison of the asymptotic mixing rates of jets within a large range of exit-to-ambient pressure ratios. A set of expressions were developed for after-expansion equivalent jet exit diameter, velocity, temperature, and density to allow for a better collapse of jet spreading and centerline decay rates. Measurements were made for five different underexpanded sonic jets with jet exit-to-ambient pressure ratios of pe/pa=1, 2.5, 7.5, 15.5, and 20.3, corresponding to (isentropically) fully expanded jet Mach numbers of Mj=1, 1.68, 2.38, 2.85, and 3.03, respectively. For each jet, both centerline and profile measurements were made using special probes that simultaneously measured local total pressure, static pressure, total temperature in the jet, as well as ambient conditions. These measurements were made in the subsonic flow regime, in some cases, extending as far as 270 nozzle diameters from the exit plane. The experimental results were analyzed and the asymptotic jet properties were determined using the new renormalization parameters.

Measurements of heat transfer coefficients between turbulent liquid jets and a radially finned heated disk placed at the bottom of an open vertical circular cavity

Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric, turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface, is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤Dc≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Red≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance Rth increased with the confining wall diameter Dc and also with the nozzle diameter d. A minimum value of the thermal resistance of Rth,min=0.4 cm2 K/W was attained for a combination of d=4.42 mm, Dc=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of the thermal resistance and the Rth-predictions. The total fin effectiveness ɛf was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the heated round disk.

Supersonic liquid jets: Their generation and shock wave characteristics

The generation of high-speed liquid (water and diesel fuel) jets in the supersonic range using a vertical single-stage powder gun is described. The effect of projectile velocity and mass on the jet velocity is investigated experimentally. Jet exit velocities for a set of nozzle inner profiles (e.g. straight cone with different cone angles, exponential, hyperbolic etc.) are compared. The optimum condition to achieve the maximum jet velocity and hence better atomization and mixing is then determined. The visual images of supersonic diesel fuel jets (velocity about 2000 m/s) were obtained by the shadowgraph method. This provides better understanding of each stage of the generation of the jets and makes the study of their characteristics and the potential for auto-ignition possible. In the experiments, a pressure relief section has been used to minimize the compressed air wave ahead of the projectile. To clarify the processes inside the section, additional experiments have been performed with the use of the shadowgraph method, showing the projectile travelling inside and leaving the pressure relief section at a velocity of about 1100 m/s.

A Study on the Flow Field around a Circular Cylinder Placed in a Rectangular Orifice Jet.

A long circular cylinder with the diameter of 20 mm was placed in an air jet, issuing from a rectangular orifice with the short side, h, of 3 mm and the aspect ratio of 10. The cylinder was settled on the symmetrical plane of the jet flow along the long side of the exit. The impinging distances between the jet exit and the front surface of the cylinder, xi, were varied within fifty times of the short side length. The jet exit velocity was kept constant at 24 m/s and the Reynolds number, based on the short side length, was about 4 900. It was found that the mean and fluctuating(rms) pressure profiles around the circular cylinder showed remarkable changes with the impinging distance and the position along the cylinder axis, z. The value of fluctuating pressure at the stagnation, being different from that of fluctuating velocity of the free jet, was large even at the vicinity of the jet exit. The negative pressure region, formed on the front surface of the cylinder, alone diminished clearly at a position of z over the impinging distance of xi/h<5. The saddle-backed velocity profiles for the orifice jet as well as the diminshment of the negative pressure region along z-axis might result in the off-axis distributions of the drag per unit width. The drag per unit width at z/h=3.3 of xi/h=5 was increased up to 2.4 times of the drag at z/h=0. The coefficient of total drag on the cylinder was estimated 1.16 at the jet exit and 0.76 at xi/h=50.

Nonlinear scattering of crossed ultrasonic beams in the presence of turbulence: Experiments performed with continuous waves

Experiments are reported where two primary cw (f1=2.35 MHz and f2=1.65 MHz) beams are generated by 2.54-cm-diam transducer units (T1 and T2) of focal length 14 cm. The 4-MHz receiving unit (R) is a 2.54-cm-diam circular plane array. Turbulence is generated by a D=0.635-cm-diam submerged water jet (exit velocity 7 m/s) whose orifice is 56D from the interaction region. All transducer beam axes and jet axis form a common plane. The ensemble averaged spectrum (near 4 MHz) is measured (using a Tektronix digital oscilloscope) as a function of the ‘‘symmetry’’ scattering angle θ*. After transferring the data to a computer, ‘‘notebooks’’ in Mathematica are developed to curve fit the Doppler shift 〈fd〉 and the n=2nd, 3rd, and 4th spectral moments 〈(f−〈fd〉)n〉 vs θ* to the theory in an effort to measure the mean flow and turbulent velocity correlation functions of the turbulence [Korman and Parker, in 13th ISNA, Bergen 1993, Advances in Nonlinear Acoustics, edited by H. Hobaek (World Scientific, Singapore, 1993), pp. 650–655]. For example, for n=2, 2nd order correlations 〈v2x〉, 〈vxvy〉, and 〈v2y〉 can be measured.

Flapping Instability of Vertically Impinging Turbulent Plane Jets in Shallow Water

A submerged, vertical, turbulent plane jet impinging onto a free water surface will be self-excited into a flapping oscillation when the jet velocity, exiting the jet orifice, exceeds a critical value. The dependence of the critical velocity W0c and the flapping frequency f0 on the water depth H and the jet orifice width d was investigated in detail in this study. The jet flapping motion was visualized by a laser induced fluorescence technique and measured with a laser Doppler velocimeter; a supplemental measurement of the displacement of water surface by a surface wave gauge was made. The jet flapping characteristics are interpreted in terms of the effective water depth given by He = H-z0, where z0 is the virtual origin of the jet. The critical jet exit velocity was found to increase linearly with the effective water depth and to decrease with the square root of the jet orifice width. The flapping frequency decreased with the root square of the effective water depth and independent of the jet orifice width. These results led to a critical condition for the onset of instability as St = 0.929((H 2 z0)/d) 23/2 , where St is the critical Strouhal number defined by St = f 0 d/W0c.

K-1302 ノズル内の二次元物体によって励起された二次元噴流の組織構造(S15-1 噴流構造)(S15 噴流・はく離流れの計測と制御)

It is possible to control the spread of a plane jet passively by the vortices from a two-dimensional cylinder installed in the nozzle contraction. In this study, the wake characteristics of a cylinder and the coherent structure of a jet by this passive control method were examined. In wide range of strouhal number Sto based on vortex shedding frequency (fv) and jet exit velocity, the spreads of the jet were found to be suppressed. On the other hand, hi particular cases of Sto being 0.08-0.10, the spread of the jet was remarkably enhanced associated with distinctive peaks of spectrum which arose from the deformation of the vortices by flow contraction in nozzle.

LDA Measurements of Feed Annulus Effects on Combustor Liner Port Flows1

A detailed LDA experimental study is reported on the effect of variations in feed annulus conditions on port flow jet characteristics. The data are relevant to primary and dilution jet flows as found in gas-turbine combustor liners. Alteration of jet velocity ratios and associated bleed flows past the jet ports for a typical annulus height/port diameter configuration was observed to produce significant variations in separation regions in the annulus, distortions in downstream annulus profiles, and jet exit conditions. Profiles of jet exit velocity, flow angle, and turbulent kinetic energy distribution have been provided which should prove invaluable as boundary conditions for related CFD studies. For the first time the influence of swirl in the approach annulus was examined, at a level consistent with residual swirl passing down the annulus from compressor exit conditions. Noticeable deviations in jet characteristics were again observed, reducing jet entry angles by some 20 deg over the rear half of the port.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers are calculated on a nonreacting flow basis. Jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air are varied to obtain the local stoichiometric axial velocity, U st , and scalar dissipation rate, χ st , at points that are upstream of the flame stabilization locations by a redirection region length. It is found that U st decreases consistently with increasing χ st when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold at downstream locations, a rich-joined flame is formed with U st independent of the local scalar dissipation rate. Instead, U st decreases with increasing ξ st , the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing U st with increasing χ st is also found for numerical simulations with a unity Lewis number and for experimental data of lifted methane flames. These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependence on local scalar dissipation rate. In addition, stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Efficiencies of low-momentum jet diffusion flames in crosswinds

Low-momentum propane, natural gas, and propane/CO 2 diffusion flames in a crosswind were studied experimentally using a closed-loop wind tunnel. Flames were established at the exit of a burner tube mounted vertically in the wind tunnel and perpendicular to the airflow, a configuration that is relevant to continuous gas flaring in the atmosphere. Analysis of the products of combustion showed that inefficiencies result from fuel stripping, and photographic images link this process to changes in mean and instantaneous flame structure. Flame images also show qualitatively that a maximum mean flame length and the onset of downwash for different fuels are related to momentum flux ratio ( R) of the two streams. Other features of the flame, such as burning in detached pockets and the disappearance of the flame tail, do not coincide at fixed values of R for different fuels. The measured combustion efficiency data show that increased crosswind speed ( U ∞) adversely affects the efficiency, while increased jet exit velocity ( V j ) makes the flame less susceptible to the effects of crosswind. Consideration of buoyancy and momentum forces as defined by a Richardson number successfully predicted the velocity dependency of the combustion inefficiency as being U ∞/ V j 1/3 and not R.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers [28–30] are calculated on a nonreacting flow basis. The local stoichiometric axial velocity, U st , and scalar dissipation rate, χ st , are obtained at points that are upstream of the stabilization locations by a redirection region length. Variation is investigated with jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air. It is found that U st decreases consistently with increasing χ st when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold, a rich-joined flame is formed at downstream locations with U st independent of the local scalar dissipation rate. Instead, U st decreases with increasing ξ st , the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing U st with increasing χ st is also found for numerical simulations with a unity Lewis number [18] and for experimental data of lifted methane flames [19]. These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependency on local scalar dissipation rate. In addition, different flame stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Effects of fuel Lewis number on nitric oxide emission of diluted H 2 turbulent jet diffusion flames

Results are reported on the effect of fuel Lewis number ( Le F ) in turbulent jet diffusion flames of diluted H 2 fuels. The results include the flame temperature, the emission index of nitric oxides (EINO x ), and the Damköhler number ( Da) scaling of EINO x ( EINO x /[ L f 3/d F 2U F] = Da n, where L f , d F , and U F are flame length, jet diameter, and jet exit velocity, respectively), which has been previously established. The effect of Le F was found to exist, although diminished in turbulent flames. Depending on the level and the type of dilution, the diluent that resulted in a lower value of Le F also had a higher flame temperature and produced larger values of both EINO x and the normalized emission index EINO x /[ L f 3 /d F 2 U F ], which is also considered as the global NO formation rate of the flame. The exponent n = −1/2 was reproduced for the present He-diluted flames whose Le F ≈ 1.0. For Ar- and CO 2-diluted flames with Le F ≥ 1.0 (20% and 40% dilution), n = −1/2 was also preserved. For dilution with 60% Ar or CO 2, whose Le F are 0.68 and 0.528, respectively, the exponent n was found to be positive. These results suggest the significant effects of Le F on NO x emission from turbulent diffusion flames.

The effect of inclination on the heat transfer between a flat surface and an impinging two-dimensional air jet

Abstract An experimental study was performed to determine the effect of the inclination of an impinging two-dimensional air jet on the heat transfer from a uniformly heated flat plate. The impingement surface was a stainless steel plate of the same width as the jet nozzle. Local Nusselt numbers were determined as a function of three parameters: (a) inclination angle of the air jet relative to the plate in the range of 90–40°, (b) nozzle exit-to-plate spacing ( z / D ) in the range of 4–12 and (c) Reynolds number based on the hydraulic diameter of the slot nozzle in the range of 4000–12 000 (corresponding to an exit jet velocity from 6.3 to 18.7 m/s). The results are presented in the form of graphs showing the variation of the local Nusselt number as a function of these parameters. The region of maximum heat transfer shifts towards the uphill side of the plate and the maximum Nusselt number decreases as the inclination angle decreases. The location of the maximum heat transfer region appears to fall between 0 and 3 D uphill from the geometrical impingement point, and was found to be insensitive to the Reynolds number in the range used in this study. For low values of inclination angle, the local Nusselt number on the uphill side from the maximum heat transfer point was insensitive to jet exit-to-plate spacing. Correlations are proposed to predict the local Nusselt number as a function of x / D , z / D , θ and Re .