Jet Exit Velocity Profile Research Articles

Non-isothermal mixing characteristics in the extreme near-field of turbulent jets in hot crossflow: Effects of jet exit turbulence and velocity profile

The non-reacting and reacting jets-in-crossflow (JICF) are important flow configurations for effective mixing and combustion in practical applications. Many studies in the literature examine the overall mixing characteristics of isothermal, unconfined, non-reacting JICF. This experimental study expands on our recently published work that examined mixing characteristics in the near-field of a non-reacting jet in a hot vitiated crossflow (1500 K) for the jet-to-crossflow density ratio between 3.2 and 7.8 issuing from a round jet with a fully developed turbulent pipe flow exit profile. In this study, effects of the changing jet exit velocity profile to top-hat as well as exit turbulence levels (28% and 40%) with parabolic profiles are examined. Temperature measurements were made using laser Rayleigh scattering. The jet trajectory, centerline concentration decay based on adiabatic mixing assumption, Favre-averaged scalar dissipation, and mixing time scales were compared with the previous study results. Center-plane mixing metrics indicated that top-hat and pipe flow jets behave similarly, with better near-field mixing at lower momentum flux ratios and higher density ratios. The elevated turbulence cases have a higher near-field mixing efficiency with rates that are nearly independent of momentum flux ratios above 9.3 at a constant density ratio. Scalar dissipation analysis showed that the elevated turbulence jets differ from the nominal turbulence top-hat and pipe exit jet cases with a lack of strong peaks and slightly higher upstream crossflow magnitudes. Reducing the density ratio resulted in a decrease in the windward and leeward dissipation region size and magnitude.

The Effect of Rib Turbulators on Film Cooling Effectiveness of Round Compound Angle Holes Fed by an Internal Cross-Flow

Early stage gas turbine blades feature complicated internal geometries in order to enhance internal heat transfer and to supply coolant for film cooling. Most film cooling experiments decouple the effect of internal coolant feed from external film cooling effectiveness, even though engine parts are commonly fed by cross-flow and feature internal rib turbulators which can affect film cooling. Experiments measuring adiabatic effectiveness were conducted to investigate the effects of turbulated perpendicular cross-flow on a row of 45 deg compound angle cylindrical film cooling holes for a total of eight internal rib configurations. The ribs were angled to the direction of prevailing internal cross-flow at two different angles: 45 deg or 135 deg. The ribs were also positioned at two different spanwise locations relative to the cooling holes: in the middle of the cooling hole pitch and slightly intersecting the holes. Experiments were conducted at a density ratio of DR = 1.5 for a range of blowing ratios including M = 0.5, 0.75, 1.0, 1.5, and 2.0. This study demonstrates that peak effectiveness can be attained through the optimization of cross-flow direction relative to the compound angle direction and rib configuration, verifying the importance of hole inlet conditions in film cooling experiments. It was found that ribs tend to reduce adiabatic effectiveness relative to a baseline, smooth-walled configuration. Rib configurations that directed the internal coolant forward in the direction of the mainstream resulted in higher peak adiabatic effectiveness. However, no other parameters could consistently be identified correlating to increased film cooling performance. It is likely that a combination of factors is responsible for influencing performance, including internal local pressure caused by the ribs, the internal channel flow field, in-hole vortices, and jet exit velocity profiles. This study also attempted to replicate the possibility that film cooling holes may intersect ribs and found that a hole which partially intersects a rib still maintains moderate levels of effectiveness.

A numerical study of shear layer characteristics of low-speed transverse jets

Direct numerical simulation (DNS) and dynamic mode decomposition (DMD) are used to study the shear layer characteristics of a jet in a crossflow. Experimental observations by Megerian et al. (J. Fluid Mech., vol. 593, 2007, pp. 93–129) at velocity ratios ($R=\overline{v}_{j}/u_{\infty }$) of 2 and 4 and Reynolds number ($Re=\overline{v}_{j}D/{\it\nu}$) of 2000 on the transition from absolute to convective instability of the upstream shear layer are reproduced. Point velocity spectra at different points along the shear layer show excellent agreement with experiments. The same frequency ($St=0.65$) is dominant along the length of the shear layer for $R=2$, whereas the dominant frequencies change along the shear layer for $R=4$. DMD of the full three-dimensional flow field is able to reproduce the dominant frequencies observed from DNS and shows that the shear layer modes are dominant for both the conditions simulated. The spatial modes obtained from DMD are used to study the nature of the shear layer instability. It is found that a counter-current mixing layer is obtained in the upstream shear layer. The corresponding mixing velocity ratio is obtained, and seen to delineate the two regimes of absolute or convective instability. The effect of the nozzle is evaluated by performing simulations without the nozzle while requiring the jet to have the same inlet velocity profile as that obtained at the nozzle exit in the simulations including the nozzle. The shear layer spectra show good agreement with the simulations including the nozzle. The effect of shear layer thickness is studied at a velocity ratio of 2 based on peak and mean jet velocity. The dominant frequencies and spatial shear layer modes from DNS/DMD are significantly altered by the jet exit velocity profile.

Heat Transfer From an Isothermally Heated Flat Surface Due to Confined Laminar Twin Oblique Slot-Jet Impingement

Convective heat transfer from a heated flat surface due to twin oblique laminar slot-jet impingement is investigated numerically. The flow domain is confined by an adiabatic surface parallel to the heated impingement surface. The twin slot jets are located on the confining surface. The flow and geometric parameters are the jet exit Reynolds number, distance between the two jets, distance between the jet exit and the impingement surface, and the inclination angle of the jet to the impingement surface. Numerical computations are done for various combinations of these parameters, and the results are presented in terms of the streamlines and isotherms in the flow domain, the distribution of the local Nusselt number along the heated surface, and the average Nusselt number at the heated surface. It is found that the peak and the average Nusselt number on the hot surface mildly decreases and the location of the stagnation point and the peak Nusselt number gradually moves downstream as the impingement angle is decreased from 90 deg. The heat transfer distribution from the impingement surface gets more uniform as the impingement angle is reduced to 45 deg and 30 deg at lager jet-to-plate distance (4–8) with a corresponding overall heat transfer reduction of about 40% compared to the normal impinging jet case. The specified jet exit velocity profile boundary condition has considerable effect on the predicted Nusselt number around the impingement location. Fully developed jet exit velocity profile correctly predicts the Nusselt number when compared to the experimental data.

Large-Eddy Simulation of Turbulent Mixing of a Jet in Cross-Flow

An Eulerian–Lagrangian mathematical/computational methodology is employed for large-eddy simulation (LES) and detailed study of turbulent mixing in jet in cross-flow (JICF) configuration. Accurate prediction of mixing in JICF is crucially important to the development of advanced combustion systems. A high-order multiblock finite difference (FD) computational algorithm is used to solve the Eulerian velocity and pressure equations in a generalized coordinate system. The composition field, describing the mixing, is obtained from the filtered mass density function (FMDF) and its stochastic Lagrangian Monte-Carlo (MC) solver. Our simulations are shown to accurately predict the important flow features present in JICF such as the counter-rotating vortex pair (CVP), horseshoe, shear layer, and wake vortices. The consistency of the FD and MC parts of the hybrid LES/FMDF model is established for the simulated JICF in various conditions, indicating the numerical accuracy of the model. The effects of parameters influencing the jet penetration, entrainment, and turbulent mixing such as the jet velocity profile, and jet pulsation are investigated. The results show that the jet exit velocity profile significantly changes the trajectory and mixing of injected fluid. The jet pulsation is also shown to enhance the mixing depending on the flow Strouhal number. The LES/FMDF results are shown to be in good agreement with the available experimental data, confirming the reliability of LES/FMDF method for numerical simulation of turbulent mixing in complex flow configurations.

Lumped element modelling of synthetic jet actuators

In this paper, a lumped element model of synthetic jet actuator is presented and used to predict the temporal variation of the jet velocity. Unlike the previous model reported in the literature, in this model the mechanical movement of the diaphragm is decoupled from the fluid phenomenon in the actuator cavity to allow the modelling of fluid mechanics aspect of the actuator to be investigated and validated separately. Furthermore, the “minor losses” occurring at the orifice exit is included and linked to the jet exit velocity profile. The results from this model are validated using the existing experimental data and CFD simulations. It is found that this model is capable of predicting the temporal variation of synthetic jets both in phase and magnitude when the actuator is operating away from the Helmholtz resonance frequency. Although the model fails to predict the correct phase information at the Helmholtz resonance frequency, it can still produce the jet peak velocity that is in good agreement with the experimental data. The lumped element model presented in this paper can be used to provide a reliable prediction of the jet peak velocity needed in the initial design of synthetic jet actuators for practical applications.

Experimental Study of Flow Structures of Circular Pulsating Air Jet

Problem Statement: Applications of impingement jets in industry for heating and cooling purposes requires a high convective heat transfer coefficient. Numerous studies have been conducted to improve the convective heat transfer coefficient for a steady impinging jet. A pulsating jet has a very high potential in replacing steady jet after it been found able to increase the heat transfer coefficients at certain pulsating frequencies. The objectives of this study were to; (i) determine the velocity profile of a circular pulsating air jet at different pulse frequencies and Reynolds Number using a rotating valve pulse jet system and (ii) to compare the normalized steady and pulsed jet velocity at highest Reynolds number of 32 000 and highest pulsating frequency of 80Hz. Approach: Pulsation of the air jet was produced by a rotating cylinder valve mechanism at frequencies between 10-80 Hz. Flow structures of the heated steady and pulse single circular axisymmetric air jet velocity were measured using a calibrated hot-wire anemometer and presented in non-dimensional form. The measurements were carried out at three different Reynolds numbers which was set at 16000, 23300 and 32000. The jet exit velocity profile for all the test frequency is determined by plotting the graph of radial distance against the non-dimensional jet exit velocity. Results: The corresponding Reynolds number in this test is based on time-averaged centerline velocity. The results of the velocity measurement were plotted side by side using non-dimensional parameters in order to make direct comparison of the velocity profile at different frequencies and Reynolds numbers. Stagnation point velocities are the same for steady and pulsating jet for all pulse frequencies. As the radial distance from the stagnation point increases, pulsating velocity increases between 20-30% from radial distance of 2-22 mm. Conclusion: Results of the flow structures plotted show a distinctive exit air jet profile which can affect the impingement heat transfer characteristics. This was the result of enhanced turbulence intensity due to pulsating jet produced by the rotating cylinder. From the jet exit velocity profile obtained, it is found that mass flow rate for different test frequencies are slightly different due to the difference in the local velocity measurement affected by the pulses. The jet exit velocity profile data will be used to form a correlation between the pulsating jet velocity and heat transfer data.

Synthetic Jet Flowfield Database for Computational Fluid Dynamics Validation

A workshop on computational fluid dynamics (CFD) validation of synthetic jets and turbulent separation control was held at the NASA Langley Research Center in 2004 to assess the current CFD capability to predict an unsteady flowfield. Three selected test cases were 1) a basic flowfield generated by a synthetic jet in a quiescent environment, 2) a round oscillatory jet in a crossflow, and 3) the control of separated flow over a wall-mounted hump model. This paper describes the flowfield database for case 1. Flowfield measurements were obtained using three different techniques: particle image velocimetry (PIV), laser Doppler velocimetry (LDV), and hot-wire anemometry. In addition, the actuator operating parameters, including diaphragm displacement, internal cavity pressure, and internal cavity temperature, were documented to provide boundary conditions for CFD modeling. Among activeflow control devices being developed, synthetic jets have demonstrated promising flow control applications [1]. Considerable studies have covered the development of the actuator and the general behavior of the synthetic jets. However, there has not been a complete data set to support the validation of CFD prediction of synthetic jets. Smith and Glezer [2] measured the velocity profile of plane jets from rectangular slots with hot-wire and flow visualization. Crook et al. [3] studied the development of a round jet using an analytical model and compared their results with hot-wire measurements. Mallinson et al. [4] studied a round jet with variable orifice diameters and cavity heights using a hot wire. Chen et al. [5] provided velocity profiles at the centerline of a plane jet, and jet exit velocity profiles with a hot wire. Muller et al. [6] used PIV to measure the detailed flowfield of a circular jet. Bera et al. [7] used PIV to map plane synthetic jet flows from a 2-D slot. Guy et al. [8] studied a plane synthetic jet with hot-wire measurements. This study was not intended to be a parametric survey or performance optimization of synthetic jets. Experiment Setup

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.

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.

Multidimensional Effects on Structure and Extinction Process of Counterflow Nonpremixed Hydrogen - Air Flames

The axisymmetric Navier-Stokes model together with detailed chemical kinetics and variable transport properties has been applied to analyze the effects of the multidimensional flow on the flame characteristics in the nitrogen-diluted hydrogen counterflow nonpremixed flame. Computations are carried out for the laminar counterflow hydrogen-air flames with two fuel dilutions. In order to investigate the effects of the jet exit velocity profiles on the hydrogen-air diffusion flame structure, computations are made for two boundary conditions simulating the plug-flow and parabolic-flow velocity profiles at nozzle exit. In case of the highly diluted hydrogen flames, the near-extinction flame structure and extinction flame process are numerically studied for two nozzle exit area-averaged velocities based on the plug-flow profile. Numerical results indicate that the jet exit profiles significantly influence the flame structure in terms of strain rate, flame thickness, peak temperature, overlap of fuel and air, and differential diffusion. Effects of multidimensional flow and strain rate on the near-extinction structure of the highly diluted hydrogen flames are also discussed in detail.

Influence of Geometry and Heat Release on Counterflow Diffusion Flames: A Navier-Stokes Model

A Navier-Stokes simulation of the opposed jet counterflow diffusion flame (CFDF) using a fuel-lean methane jet is described in this work. Plug flow (PLF) and parabolic flow (PAF) jet exit velocity profiles have been used in the simulation, and the results show that changing the velocity profile has a large influence on flame stretch. Flame stretch was also found to be strongly dependent on the jet separation distance. Flame stretch increased as jet separation decreased. Comparing non-reacting and reacting cases indicated that heat release in the flame zone modifies the velocity profile and hence the strain. The flame responded easily to changes in variables such as fuel/air jet velocity ratio, and the nitrogen coflow. The flame was also sensitive to the arrangement of the side walls that encloses the CFDF burner.

Experimental investigation of the unsteady structure of a transitional plane wall jet

A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional structure in the free shear layer initiates complete breakdown of the flow.

Near-field characteristics of a turbulent coflowing jet

Author(s): Gladnick, PG; Enotiadis, AC; Larue, JC; Samuelsen, GS | Abstract: The near-field evolution of the velocity and concentration fields for an axisymmetric jet flow of CFC-12 issuing into a coannular jet flow of air is presented. Results based on measurements of the time resolved velocity (two components) and (separately) the concentration, obtained using laser anemometry and Rayleigh scattering systems, show that the transport of momentum and mass in the near field depends on the large scale structure which forms in the shear layer at the edge of the jet. The type of instability and hence the flow development is shown to depend primarily on the ratio of the coflow to jet velocity (m), density ratio, and the jet exit velocity profile. Specifically, for velocity ratios less than and greater than unity, the statistical properties of the velocity and concentration fields are compatible with the existence of an annular vortex ring with positive vorticity for m l 1 and negative vorticity for m g 1. For a velocity ratio equal to unity, the results are consistent with the presence of pairs of counter-rotating vortices that are typical of a wake flow. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.

Center-Line Velocity Decay in Coaxial Free Jets

The effects of initial nonuniformities in the exit velocity profiles of incompressible coaxial air jets expelled into free air on the fully developed region of the jets is studied experimentally. The primary and annular jet exit velocities were nonuniform, and the average velocity of the primary jet was greater than that of the annular jet. The center-line velocity of the jet in the fully developed region is found to depend on the primary jet exit velocity profile and on the ratio of the central to annular jet average velocities.