Jet Deflection Research Articles

Enhancement of Impingement Cooling in a High Cross Flow Channel Using Shaped Impingement Cooling Holes

Impingement systems are common place in many turbine cooling applications. Generally these systems consist of a target plate that is cooled by the impingement of multiple orthogonal jets. While it is possible to achieve high target surface heat transfer with this configuration, the associated pressure drop is generally high and the cooling efficiency low. Furthermore, especially in large impingement arrays, the buildup of cross flow from upstream jets can be significant and results in deflection of downstream impingement jets reducing the resultant heat transfer coefficient distribution. This paper presents a computational and experimental investigation into the use of shaped elliptical or elongated circular impingement holes designed to improve the penetration of the impinging jet across the coolant passage. This is of particular interest where there is significant cross flow. Literature review and computational investigations are used to determine the optimum aspect ratio of the impingement jet. The improved heat transfer performance of the modified design is then tested in an experimental rig with varying degrees of cross flow at engine representative conditions. In all cases, a 16% increase in the Nusselt number on the impingement target surface in the downstream half of the cooling passage was achieved. Under the first four impingement holes, a Nusselt number enhancement of 28–77% was achieved, provided no additional cross flow was present in the passage. When appropriately aligned, a significant reduction in the stress concentration factor caused by the addition of a hole can be achieved using this design.

Theoretical analysis for the deflection of granular jets

We consider a dilute granular jet colliding with an oblique planar wall. Collisions between particles and collisions between particles and the wall are inelastic. We derive an exact solution for the mean force experienced by the wall for dilute jets. We show that the mean force on the wall can be a non-monotonic function of the angle between the wall and the jet. This occurs because particles that rebound from the wall can collide with incoming particles and be scattered.

Characteristics of flow from an oxy-fuel burner with separated jets: influence of jet injection angle

This paper presents an experimental study of flow development and structure on a separated jet burner in reacting and non-reacting flows. Effects of deflection jets in an aligned configuration of three round jets are emphasized. The idea is based on the confinement of a central jet of fuel by two side jets of oxygen to improve mixing, to control flame stability, and to reduce pollutant emissions. The fields of mean velocity and fluctuation intensity were measured using Particle Image Velocimetry. The deflection of jets has a considerable effect on the dynamic behavior and on the flame characteristics. Results showed that the deflection of jets favors mixing and accelerates merging and combining of jets to a single one. Measurements in reacting flow showed a high influence of combustion on dynamic fields. Compared to non-reactive case, in combustion, larger radial expansion and higher velocity were observed, particularly, above the stabilization point of the flame.

Structure of the Continuous Liquid Jet Core during Coaxial Air-Blast Atomisation

This paper investigates the structure of the continuous liquid jet of a coaxial air-blast atomiser over a range of Weber numbers 60-1040, Reynolds numbers of liquid jet 5400-21700 and air to liquid momentum ratios of the two streams of 1.7–335. A novel optical technique, based on internal illumination of the liquid jet through the jet nozzle by a laser pulse, which excites a fluorescing dye introduced in the atomizing liquid, was used to obtain instantaneous measurements of the breakup length and the three dimensional location of the liquid core of the continuous liquid jet. The latter was achieved by simultaneously imaging the liquid jet from two directions normal to each other. Such measurements are usually prevented by droplets surrounding the liquid jet at the dense spray near the nozzle exit. The measurements showed that the break-up length of the liquid jet scaled well with the air to liquid momentum ratio. The standard deviation of the temporal fluctuations of the break-up length was around 10% of the mean breakup length for each considered flow condition. The instantaneous jet surface does not develop axi-symmetric wave structures but the time-averaged liquid jet is axi-symmetric around the nozzle axis, while the maximum deflection of the liquid jet occurs close to the breaking point.

Plasma arc cutting: speed and cut quality

When cutting metal with plasma arc cutting, the walls of the cut are narrower at the bottom than at the top. This lack of squareness increases as the cutting speed increases. A model of this phenomenon, affecting cut quality, is suggested. A thin liquid layer, which separates the plasma from the solid metal to be melted, plays a key role in the suggested model. This layer decreases heat transfer from the plasma to the solid metal; the decrease is more pronounced the higher the speed and the thicker the liquid metal layer. Since the layer is thicker at the bottom of the cut, the heat transfer effectiveness is lower at the bottom.The decrease in heat transfer effectiveness is compensated by the narrowness of the cut. The suggested model allows one to calculate the profile of the cut. The result of the calculations of the cutting speeds for plates of various thicknesses, at which the squareness of the cut is acceptable, agrees well with the speeds recommended by manufacturers.The second effect considered in the paper is the deflection of the plasma jet from the vertical at a high cutting speed. A qualitative explanation of this phenomenon is given.We believe the considerations of this paper are pertinent to other types of cutting with moving heat sources.

Propulsion and aerodynamic performance evaluation of jet-wing distributed propulsion

Distributed propulsion is the idea of redistributing the thrust across the drag producing elements of a vehicle. Our configuration has a modest number of engines with part of the exhaust flow vented from thick trailing edges of the wings to cancel the local profile drag and the rest of the exhaust flow providing thrust to cancel the induced drag and drag of the fuselage and tails. CFD studies were performed on two-dimensional wing sections in transonic, viscous flow to (1) investigate the effect of jet-wing on propulsion efficiency and the flow field (2) determine design changes for achieving efficient distributed propulsion, and (3) investigate the effect of jet-flaps with small jet deflection angles on aerodynamic parameters. The jet-wing distributed propulsion can give propulsive efficiencies on the order of turbofan engine aircraft and if the trailing edge of a conventional Outboard airfoil is expanded, efficiency can be increased by 7.5%. An increase in propulsion efficiency was achieved without expanding the trailing edge for a thicker Inboard airfoil. The results of the Outboard airfoil jet-flap cases support the idea of using deflected exhaust jets from trailing edges as control surfaces.

The effect of density ratio on the near field of a naturally occurring oscillating jet

The triangular oscillating jet nozzle generates a triangular jet partially confined within an axi-symmetric chamber to produce a large scale flow oscillation that has application in thermal processes. Particle image velocimetry and oscillation frequency measurements were conducted to investigate the influence of the jet fluid to ambient fluid density ratio on the resulting oscillating flow. The investigation was conducted with a jet momentum flux of 0.06 kg m s−2 (Re = 7.3−47.2 × 103) and density ratios ranging from 0.2 to 5.0. The initial spread and decay of the emerging jet was found to depend upon the density ratio but in a more complex way than does an unconfined jet. Both the spread and decay are strongly influenced by the instantaneous angle of jet deflection, with greater deflection leading to increased spreading and decay of the jet. Decreasing the density ratio below unity results in a rapid decrease in the deflection angle, while increasing the density ratio above unity results in an increase in the deflection angle, albeit with less sensitivity. The frequency of oscillation was also shown to depend on the density ratio with an increase in the density ratio causing a decrease in the dominant oscillation frequency.

Hemodynamics of the Mitral Valve Under Edge-to-Edge Repair: An In Vitro Steady Flow Study

Edge-to-edge repair (ETER) is a mitral valve repair technique that restores valvular competence by suturing together the free edges of two leaflets. This repair technique alters mitral valve inflow and thus left ventricle hemodynamics during diastole. Our objective was to investigate fluid mechanics immediately downstream of the mitral valve under ETER during diastole. Fresh porcine mitral valves of the annulus size M32 with chordae removed were installed into a steady flow loop simulating a peak diastolic inflow through the mitral valve. Digital particle image velocimetry was used to measure the velocity field immediately downstream of the mitral valve under normal and ETER conditions. First, to study the suture length effect, suture was applied in the central position of the leaflet edge with suture lengths of 3 mm, 6 mm, and 9 mm, respectively. Then, 3 mm suture was set in the central, lateral, and commissural positions of the leaflet edge to study the suture position effect. Flow rate was 15 l/min. Velocity, Reynolds shear stress (RSS), and effective orifice area were assessed. A total of five mitral valves were tested. The normal mitral valve without the ETER had one jet downstream of the valve, but the mitral valve with the central or lateral sutures under the ETER had two jets downstream of the valve with a recirculation region downstream of the suture. The maximum velocity, the maximum RSS in the jets, the pressure drop across the mitral valve, and the jet deflection angle increased with the increase in suture length in the central position. When the suture position effect was investigated with the 3 mm suture, the maximum velocity, the maximum RSS, and the pressure drop across the valve in the central suture position were greater than those of the lateral and the commissural suture positions. The lateral suture demonstrated major and minor jets with the greater maximum velocity and maximum RSS in the major jet. When the suture was in the commissural position, the flow field downstream of the mitral valve was similar to that of the normal mitral valve without the ETER. The effective orifice area was smallest when the suture was applied in the central position as compared with other suture positions. Both suture length and position have an important impact on fluid mechanics downstream of the mitral valve under the ETER in terms of flow pattern, maximum velocity, and RSS distribution. The altered hemodynamics of the mitral valve and thus of the left ventricle by the ETER may change mitral valve and left ventricle function.

Heat/mass transfer in rotating impingement/effusion cooling with rib turbulators

A detailed measurement of the heat/mass transfer coefficients on the ribbed surfaces for the rotating impingement/effusion cooling system has been conducted. Three different jet orientations (front, leading, and trailing) were investigated at the same rotating speed and impinging jet Reynolds number of 3000. A naphthalene sublimation method was used to obtain local heat/mass transfer coefficients. Regardless of rib turbulators, the leading and trailing orientations lead to totally changed heat/mass transfer distributions due to the jet deflection, while the Sh distributions of the front orientation were similar to those of the stationary case. For leading and trailing orientations, the influence of crossflow, which deflected wall jets, decreased due to the blockage effect of the rib turbulators. Therefore, the wall jets spread more widely and the interaction between adjacent wall jets along spanwise direction became stronger, enhancing the heat/mass transfer compared to that on smooth surface.

Effect of jet direction on heat/mass transfer of rotating impingement jet

The objective of this study is to investigate the heat/mass transfer characteristics on various impinging jets under rotating condition. Two cooling schemes related to impingement jet are considered; array impingement jet cooling and impingement/effusion cooling. The test duct rotates at Ro = 0.075 with two different jet orientations and the jet Reynolds number is fixed at 5000. Two H/ d configurations of 2.0 and 6.0 are conducted. The detailed heat/mass transfer coefficients on the target plate are measured by a naphthalene sublimation technique. The rotation changes the local heat/mass transfer characteristics due to the jet deflection and spreading phenomenon. For H/ d = 6.0, the jet is strongly deflected at the leading orientation, resulting in the significant decrease in heat/mass transfer. At the axial orientation, the momentum of jet core decreases slightly due to jet spreading into the radial direction and consequently, the value of stagnation peak is a little lower than that of the stationary case. However, reduction of heat/mass transfer due to rotation disappears at a low H/ d of 2.0. In the averaged Sh, the leading orientation with H/ d = 6.0 shows 35% lower value than that of the stationary case whereas the other rotating cases lead to a similar value of the stationary case.

An analytical model for predicting depth of cut in abrasive waterjet cutting of ductile materials considering the deflection of jet in lateral direction

This paper presents a modified model to predict the total depth of cut in abrasive waterjet cutting (AWJC). For predicting the depth of cut in the cutting wear region, the interaction of cylindrical waterjet with the target material and the deflection of jet in lateral direction perpendicular to the direction of traverse of jet in cutting are considered. Several researchers have suggested neglecting the threshold velocity, since its effect on material removal is negligible due to high velocity of jet. Therefore, the depth of cut in the deformation wear region is predicted with and without considering the threshold velocity to show that the effect of threshold velocity on material removal is negligible. The effectiveness of the model is demonstrated with the experimental data.

Improved Model for the Penetration of Liquid Jets in Subsonic Crossflows

A theoretical model for the penetration of a liquid jet in subsonic gaseous crossflow is developed. The model allows for the deformation of the jet cross section from circular to elliptic shapes along its path. A force balance analysis on an elliptical liquid element is performed. Aerodynamic, viscous, and surface tension forces are considered counting for the nonlinear terms at large deformations. The effect of mass shedding is also included in the model. This effect changes the jet trajectory and deformation at higher Weber numbers. In addition, the drag coefficients of elliptical cylindrical elements with different aspect ratios are calculated numerically for a range of Reynolds numbers. It is observed that the drag coefficient of the cylindrical element changes considerably with Reynolds number and the jet deformation. The change in the drag force considerably affects the jet deflection in the gas stream. Results show that the liquid-to-gas momentum ratio is not the only governing parameter in predicting the jet trajectory. Gas Weber number, rate of mass shedding from the jet, jet cross-sectional deformation, variation in the drag coefficient, and variation in the liquid and gas properties all affect the jet penetration.

Mode transition of a flue organ pipe

A flue organ pipe can be excited in various acoustic modes by changing the air pressure supplied to it. This research aims to reconstruct this behavior from the result of numerical flow simulation of a jet deflected by sound and from physical modeling simulation of the total sound production system. In the numerical flow simulation, motion of the jet in the pipe mouth was replicated: The jet emerges from a flue and travels in a space where the air oscillates laterally to the jet direction. As a result, the jet oscillates with the same frequency as the oscillation of the air i.e., sound. From the flow simulation, a model of the jet deflection was developed. This model was then used as a model of the sound source in the physical modeling simulation where not only the sound source but the resonance of the pipe is also modeled in a set of differential equations with delayed feedback. The mode transition observed in the physical modeling simulation was discussed by comparing with that experimentally observed.

Full-Coverage Film Cooling for a Turbine Blade with Axial-Shaped Holes

[Abstract] The film cooling effectiveness on a fully film cooled high pressure turbine blade is investigated using the Pressure Sensitive Paint technique. Three rows of radial angled cylindrical holes are arranged in the leading edge region, while axial laidback fanshaped holes are provided on the blade surfaces with four rows on the pressure side and two rows on the suction side. The shaped holes are featured with 10° lateral expansion from the hole centerline and additional 10° forward expansion to the blade surface. The coolant ejects through all the film cooling holes at four average blowing ratios ranging from 0.3 to 1.2. The influence of wake from upstream vane is simulated by placing a periodic set of rods upstream of the test blade. The freestream Reynolds number, based on the axial chord length and the exit velocity, is 750,000. The Mach numbers at the inlet and the exit are 0.27 and 0.44, respectively, resulting in a blade pressure ratio of 1.14. Results show the film cooling effectiveness increases with increasing of average blowing ratio. The presence of upstream wake rods can be very detrimental to the film effectiveness on the blade surface depending on the wake rod phase location. Compared with the case of no showerhead injection, the spanwsie averaged film cooling effectiveness in the downstream region of the pressure side and suction side rows increases with the showerhead injection. The film effectiveness on the pressure side is comparable for compound angle shaped holes and axial shaped holes; while on the suction side, the compound angle shaped holes provide better effectiveness than the axial shaped holes due to jet deflection and expanded hole breakout area..

Aeration Characteristics of Ski Jump Jets

Scour downstream of ski jumps may be avoided by jet deflection to an area where the energy dissipation is accomplished. The main purpose of this experimental study was the analysis of the jet air entrainment downstream of a ski jump, both for pure water and preaerated approach flow conditions. A systematic variation of the Froude number and the flow depth in the approach flow channel resulted in a range of discharge characteristics, whereas the geometry of the ski jump was maintained for all tests. The air concentration profile was measured at different locations downstream from the ski jump to evaluate the: (1) jet air concentration distribution; (2) location of minimum air concentration along the mixture flow jet and development of the minimum and the cross-sectional average air concentrations; (3) jet trajectories; and (4) process of air entrainment characteristics and jet disintegration. The results demonstrate the significant effect of the approach flow Froude number, the approach flow depth, and of preaeration on jet disintegration.

Effect of seepage on scour due to submerged jets and resulting flow field

Experiments were conducted to investigate the effect of upward seepage on the characteristics of scour hole due to horizontal submerged jets and the resulting flow field within the scour hole. In the experiments, the submerged jets had a range of submergence factors from 3.22 to 26.5 and jet Froude numbers from 0.82 to 1.48 over beds of sediments (median sizes = 0.8 mm, 1.86mm and 3 mm) downstream of a sluice gate under different upward seepage velocities. The characteristic lengths of the scour hole determined from the equilibrium scour profiles are: The maximum equilibrium scour depth, the horizontal distance of maximum scour depth from the sluice gate, the horizontal extension of scour hole from the sluice gate, the dune height and the horizontal distance of dune crest from the sluice gate. The experimental results show that all the characteristic lengths decrease with increase in seepage velocity, as a result of jet deflection by the upward seepage velocity. Using the experimental data, the time-variation of scour depth is scaled by an exponential law, where the time scale decreases linearly with increase in the ratio of seepage velocity to issuing jet velocity. The flow field within scour hole under different upward seepage velocities was detected by the acoustic Doppler velocimeter. Vector plots of the flow field show that the decay rate of submerged jet increases with increase in seepage velocity. The vertical profiles of turbulent kinetic energy and Reynolds stress at different streamwise distances and the horizontal distributions of bed shear stress acting on the scoured beds are plotted for different (with and without) upward seepage velocities.

Quasi-steady and unsteady actuation by surface non-thermal plasma discharge for control of a turbulent round air jet

The dielectric barrier discharge (DBD) is a nonmechanical device able to generate electrohydrodynamic forces close to a dielectric wall. The generated “electric wind” is actually used to control airflow over various bluff bodies. In this study, a DBD actuator is investigated for the control of a round turbulent air jet. The separation along the bevel of a small angle (12°) axisymmetric diffuser is studied using LDV measurements for a 20 m s− 1 airjet velocity. The results demonstrate that a DBD actuator is suitable to separate a turbulent airflow naturally attached. Under actuation, a jet deflection of 13° is observed and the coherent structures are energized. Quasi-steady and unsteady actuations are also compared for excitation frequencies up to 400 Hz and the analysis of the turbulent spectra demonstrates that a vortex shedding could be forced at the excitation frequency (up to 160 Hz). The investigation of different duty-cycle regimes allows us to define a threshold value inducing similar flow modifications than those obtain with quasi-steady actuation but with a significant power consumption reduction of 20%. The knowledge of the separation dynamics is essential for future reactive control devices. The last part of this study is dedicated to this point. The response times of the forced separation and the natural reattachment are evaluated. The results indicate that separation is achieved in 34.5 ms and that the natural reattachment time is about 45 ms. In conclusion, this study highlights the dynamics of the separation using a single DBD actuator, and demonstrates that a jet deflection and a mixing enhancement can be performed.

Jet Deflection by a Quasi-Steady-State Side Wind in the Laboratory

We present experimental data on the steady state deflection of a highly supersonic jet by a side-wind in the laboratory. The use of a long interaction region enables internal shocks to fully cross the jet, leading to the development of significantly more structure in the jet than in previous work with a similar setup (Lebedev et al., 2004). The ability to control the length of the interaction region in the laboratory allows the switch between a regime representing a clumpy jet or wind and a regime similar to a slowly varying mass loss rate. The results indicate that multiple internal oblique shocks develop in the jet and the possible formation of a second working surface as the jet attempts to tunnel through the ambient medium.

The behaviour of lifted oxy-fuel flames in burners with separated jets

Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.

An investigation of bimodal jet trajectory in flow through scaled models of the human vocal tract

Pulsatile two-dimensional flow through static divergent models of the human vocal folds is investigated. Although the motivation for this study is speech production, the results are generally applicable to a variety of engineering flows involving pulsatile flow through diffusers. Model glottal divergence angles of 10, 20, and 40° represent various geometries encountered in one phonation cycle. Frequency and amplitude of the flow oscillations are scaled with physiological Reynolds and Strouhal numbers typical of human phonation. Glottal velocity trajectories are measured along the anterior–posterior midline by using phase-averaged particle image velocimetry to acquire 1,000 realizations at ten discrete instances in the phonation cycle. The angular deflection of the glottal jet from the streamwise direction (symmetric configuration) is quantified for each realization. A bimodal flow configuration is observed for divergence angles of 10 and 20°, with the flow eventually skewing and attaching to the vocal fold walls. The deflection of the flow toward the vocal fold walls occurs when the forcing function reaches maximum velocity and zero acceleration. For a divergence angle of 40°, the flow never attaches to the vocal fold walls; however, there is increased variability in the glottal jet after the forcing function reaches maximum velocity and zero acceleration. The variation in the jet trajectory as a function of divergence angle is explained by performance maps of diffuser flow regimes. The smaller angle cases are in the unstable transitory stall regime while the 40° divergent case is in the fully developed two-dimensional stall regime. Very small geometric variations in model size and surface finish significantly affect the flow behavior. The bimodal, or flip-flopping, glottal jet behavior is expected to influence the dipole contribution to sound production.