Nozzle Aspect Ratio Research Articles

Effect of jet nozzle geometry on flow and heat transfer performance of vortex cooling for gas turbine blade leading edge

In this paper, 3D viscous steady Reynolds Averaged Navier–Stokes (RANS) equations are utilized to investigate the influence of jet nozzle geometry on flow and thermal behavior of vortex cooling for gas turbine blades. Comparison between calculation with different turbulence models and the experimental data is conducted, and results show that the standard k-ω model provides the best accuracy. The grid independence analysis is performed to obtain the proper mesh number. First, the mechanism of vortex cooling is further discussed, and the pronounced impact of kinetic turbulence intensity, thin thermal boundary layer, violent radial convection and complex vortices on enhanced heat transfer performance is confirmed. Then, seven jet nozzle aspect ratios and seven jet nozzle to chamber cross section area ratios are selected to research the flow field and thermal characteristics of vortex cooling focusing on the streamline, static pressure ratio, total pressure loss ratio and Nusselt number. It is presented that the jet nozzle aspect ratio and jet nozzle to chamber cross section area ratio both impose a significant effect on the flow and thermal parameters. The averaged Nusselt number decreases at first and then increases with the increasing jet nozzle aspect ratio, reaching highest when aspect ratio equals to 1. The effect of area ratio on averaged Nusselt number is complex. Finally, the heat transfer results in this study are compared with other previous works. Results indicate that good agreement with previous data is achieved, and the enhanced thermal behavior may be acquired by carefully designing and optimizing the vortex chamber geometry.

Numerical study on the effect of jet nozzle aspect ratio and jet angle on swirl cooling in a model of a turbine blade leading edge cooling passage

In this paper, a numerical simulation is performed to predict swirl cooling of an internal leading edge cooling passage model for a gas turbine blade. The swirling cooling performance and its effectiveness are investigated for cases of two rectangular section inlets that cause flow to impinge tangentially on the internal surface of the circular cooling passage. The effects of aspect ratio of the jet nozzle, jet angle and Reynolds numbers on the local and average flows and heat transfer are investigated. The results indicate that the pressure loss and global area weighted average Nusselt number on the swirl chamber increases with increases of Reynolds number, and increases with decreases of the jet nozzle aspect ratio. A correlation of the area weighted average Nusselt number over a range of parameters is suggested. The distribution of Nusselt number over the swirl chamber shows that the thermal stresses will be lowest with a 60° jet nozzle angle.

Entrainment of air into an infrared suppression (IRS) device using circular and non-circular multiple nozzles

The present investigation predicts the mass entrainment into an IRS device which is used in naval or merchant ships. A high velocity jet emanating from the nozzle exit entrains ambient air to cool the hot combustion products of the gas turbines. Experiments have been carried out on a laboratory scale IRS device to measure the entrainment ratio. An optimum funnel overlap height (Hoverlap/Deq,nz=0) has been found out from the experiments which shows the highest mass suction rate at that overlap height. Moreover, the entrainment is observed to increase with the inlet mass flow rate of the nozzle. For a full scale IRS device, the conservation equations for mass, momentum and turbulent kinetic energy as well as its dissipation rate are solved using finite volume techniques in a three-dimensional computational domain by employing eddy viscosity based k–ɛ turbulence model with standard wall function. The parameters such as the nozzle aspect ratio (AR), geometrical shape of the nozzles, pitch circle diameter (PCD) and multiple nozzles affect the entrainment ratio significantly. With five triangular nozzles (AR=1), the entrainment rate is enhanced by 15.7% as compared to a single triangular nozzle (AR=1).

Experimental investigation of nozzle aspect ratio effects on underexpanded hydrogen jet release characteristics

Most experimental investigations of underexpanded hydrogen jets have been limited to circular nozzles in an attempt to better understand the fundamental jet-exit flow physics and model this behaviour with pseudo source models. However, realistic compressed storage leak exit geometries are not always expected to be circular. In the present study, jet dispersion characteristics from rectangular slot nozzles with aspect ratios from 2 to 8 were investigated and compared with an equivalent circular nozzle. Schlieren imaging was used to observe the jet-exit shock structure while quantitative Planar Laser Rayleigh Scattering was used to measure downstream dispersion characteristics. These results provide physical insight and much needed model validation data for model development.

Plane hydrogen jets

This study is focused on understanding the structure and behaviour of hydrogen under-expanded jets from plane nozzles and their differences with circular nozzle jets. Results of numerical simulations of hydrogen highly under-expanded jets from a storage vessel at pressure 40 MPa through a circular nozzle and two plane nozzles with aspect ratios 5.0 and 12.8 respectively, all of the same cross-section area, are presented. Two stages approach is applied to simulate under-expanded unignited jets and jet fires. At the first stage, the high Mach number flow in a near field to the nozzle is simulated by compressible flow solver. At the second stage, incompressible flow solver is applied to simulated either unignited or combusting jets in the far from the nozzle field with “inner” boundary conditions taken from the first stage. The structure and behaviour of hydrogen plane highly under-expanded jets is scrutinised, including the switch-of-axis phenomenon when the exiting jet expands in the vicinity of the nozzle only in the direction of the minor nozzle axis while it contracts in the major axis direction. Simulations demonstrated that plane jets may provide faster concentration decay compared to axisymmetric jets with the same mass flow rate due to the difference in air entrainment. The concentration decay rate is shown to be a function of the plane nozzle aspect ratio. The eddy break-up model is applied to simulate under-expanded hydrogen jet fires from the equipment at pressure of 40 MPa. The circular and plane nozzle jet fire simulations are validated against experiments by Mogi and Horiguchi (2009). The simulations are in a good agreement with the experiment.

Bending–torsional flutter of a cantilevered pipe conveying fluid with an inclined terminal nozzle

Stability analysis of a horizontal cantilevered pipe conveying fluid with an inclined terminal nozzle is considered in this paper. The pipe is modelled as a cantilevered Euler–Bernoulli beam, and the flow-induced inertia, Coriolis and centrifugal forces along the pipe as well as the follower force induced by the jet-flow are taken into account. The governing equations of the coupled bending–torsional vibrations of the pipe are obtained using extended Hamilton's principle and are then discretized via the Galerkin method. The resulting eigenvalue problem is then solved, and several cases are examined to determine the effect of nozzle inclination angle, nozzle aspect ratio, mass ratio and bending-to-torsional rigidity ratio on flutter speed of the system.

Prediction of Jet Noise for Realistic Flow Problems Using Large Eddy Simulation

A systematic procedure is described to predict the noise emissions from realistic aero-propulsive jets. Large Eddy Simulation (LES) is used to compute the jet flowfield and coupled with the Ffowcs Williams and Hawkings (FW-H) equation for far-field noise predictions. A low-dissipation fifth-order upwind biased finite-volume reconstruction procedure is used along with selective fourth order inviscid flux blending. Higher order explicit time integrators are used for enhanced wave propagation. Also, non-contiguous block interfacing is employed to eliminate the traditional limitations of structured grid topologies for complex geometries. For simple jet configurations the LES/FW-H method is validated with University of Mississippi's National Center for Physical Acoustics (NCPA) experimental measurements. Four unique and more realistic applications are then shown. The first is a hot faceted jet with lobed corrugations, followed by an over-expanded military gas turbine engine exhaust with and without chevrons. Then a twin jet impinging on a jet blast deflector is shown, and lastly are two high aspect-ratio nozzles, one with and one without a bevel. The LES/FW-H methodology is shown to produce reasonable agreement with experimental measurements at modest grid resolutions. Details are discussed about the selected example problems highlighting the challenges associated with applying the tools to realistic geometries and jet configurations.

Confined, Milliscale Unsteady Laminar Impinging Slot Jets: Effects of Slot Width on Surface Stagnation Point Nusselt Numbers

The effects of slot width for confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements of spatially resolved distributions of local Nusselt numbers measured on a constant heat flux surface. The effects of Reynolds number, nozzle-to-plate distance, and dimensional slot width on the local Nusselt number are investigated for slot nozzle width B values of 0.5 mm, 1.0 mm, and 1.5 mm. Reynolds numbers Re range from 120 to 200, nozzle-to-plate distances H/B vary from 0.75 to 12.5, and the nozzle aspect ratio y/B is 50. Observed are different stagnation point Nusselt number Nuo variations with Re, H/B, and B, where the onset of unsteadiness, and the intermittent flapping motion of the jet column are both associated with important variations to local, stagnation region Nusselt numbers Nuo, as experimental configuration and condition change. The variations of these stagnation-point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/B, for different dimensional values of B. Also presented are stagnation region Nusselt number variations, for steady, impingement jets at values of H/B less than 4.6–7.8. These are characterized by three separate regimes of behavior, each of which shows significantly different Nuo dependence upon Re, H/B, and B.

The effects of nozzle aspect ratio and nozzle number on the performance of the Ranque–Hilsch vortex tube

An experimental study is carried out to investigate the effects of nozzle aspect ratio and nozzle number on the performance of a vortex tube. Two sets of vortex generator (a single nozzle set with aspect ratio of AR = 0.25, 0.44 and 0.69 and a multiple nozzle set with 2 and 3 nozzle number having the same total flow area) are tested under different inlet pressures. Dry air is used as the working fluid. The experimental results reveal that the nozzle aspect ratio has a great effect on the energy/temperature separation mechanism. The increase in the nozzle aspect ratio leads to the larger mixing zones, which, in turn, decreases the temperature difference between the cold and hot stream, the heating and the cooling performance. The results also showed that the vortex tube with a single nozzle yields better performance than the vortex tube with 2 and 3 nozzles.

Milliscale confined impinging slot jets: Laminar heat transfer characteristics for an isothermal flat plate

Heat transfer and overall visualized flow characteristics of confined, laminar milli-scale slot jets are investigated, as they impinge upon an isothermal flat target plate, with a fully-developed profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance ratio H/B are investigated for Re=120–200, H/B=2–10, and B=1.0mm, with a nozzle aspect ratio of y/B=50. Instantaneous visualizations of overall slot jet flow structure show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures which form as secondary flows develop in fluid which, initially, is just adjacent to the jet column. For each Reynolds number considered, local stagnation region Nusselt numbers Nuo decrease dramatically as H/B increases to become greater than 7.2–13.2, as the Reynolds number is maintained constant at a value from 200 to 120, changes which occur just as continuous sinusoidal oscillation of the jet column begins to develop. The further development of continuous sinusoidal oscillating motion results in an approximate collapse of stagnation region Nusselt numbers measured at different Re and H/B values. When surface thermal boundary condition data are compared, the constant surface temperature data are generally higher than the constant surface heat flux data near the stagnation location, and lower at locations where x/B is greater than 1–2. The constant surface temperature data also show relatively low values with only very small changes with x/B, for x/B values which are greater than about 5.0. As such, the results illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition, as well as the restrictions on near-wall temperature gradients which result from a constant surface temperature thermal boundary condition.

NUMERICAL SIMULATION OF A TURBULENT FREE JET ISSUING FROM A RECTANGULAR NOZZLE

The aim of this paper is to demonstrate that the simple Reynolds-averaged Navier-Stokes (RANS) two-equation standard k − ε turbulence model is capable of predicting the main characteristics of turbulent free jets issuing from three-dimensional complex rectangular nozzle geometry. This paper also investigates the main characteristics of turbulent rectangular free shear jets by varying the nozzle aspect ratio. The computations were performed by using RANS equations and the turbulence terms were handled by testing the standard two-equation k − ε or k −ω turbulence models. The solution of the governing equations was obtained by using the finite volume method (FVM), where the SIMPLEC algorithm was adopted with a staggered grid to prevent decoupling velocity and pressure fields. The inflow boundary conditions including the inflow velocity profile and the turbulence intensity level were adopted from published experimental data. The main results demonstrate that the standard k − ε model, when applied with appropriate inflow boundary conditions, provides successful predictions of the main features of turbulent free jets issuing from rectangular nozzles, including the decay rate in the near- and far-field region, the spreading rate except in the very near field, vena contracta, and axis switching. The numerical results show also that increasing the nozzle aspect ratio leads to an increase in fluid entrainment closer to the nozzle exit in the near field, which is in complete agreement with experimental observations.

Turbulent jets issuing from rectangular nozzle with a rectangular notch at the midspan

The turbulent flowfield of turbulent jet issuing from rectangular nozzle (Aspect Ratio=12.5) with a rectangular notch at the midspan, has been investigated experimentally. Four aspect ratios of rectangular notch (NAR: Notch Aspect Ratio) used in this experiment were 2.5, 7.5, 12.5 and 165. The Reynolds number based on the nozzle width d and the exit mean velocity Ue, was kept constant 30000 (NAR=2.5 and 7.5), 15000 (NAR=12.5) and 13000 (NAR=165), respectively. Longitudinal mean velocity and turbulent intensities were measured using an X-array Hot-Wire Probe (5 µm in diameter, 1 mm effective length) operated by the linearized constant temperature anemometers (DANTEC), and the spanwise and the lateral mean velocities were measured using a yaw meter. The signals from the anemometers were passed through the low-pass filters and sampled using A.D. converter. The processing of the signals was made by a personal computer. Acquisition time of the signals was usually 80 seconds. From this experiment, it was revealed that the attachment of a rectangular notch to the rectangular jet suppressed the development of the turbulent velocity scales near the jet centre in the upstream region for the cases of NAR≥7.5.

Numerical simulation of supersonic jet instability modes of plane and notched rectangular nozzles

The effect of exit geometry of a rectangular nozzle on the instability modes and mixing characteristics of under-expanded supersonic jets is computationally investigated. The unsteady three-dimensional viscous simulation is based on the Proteus code developed at NASA Glenn Research Center. A shock adaptive grid generator was developed to enhance the shock simulation. The nozzle aspect ratio for both plane and notched rectangular nozzles in this study is 5.0 and the fully expanded jet Mach number is 1.526. For the case of a plane rectangular nozzle, the ‘flapping’ oscillations, which are extensively observed in schlieren flow visualization and reported in acoustic measurements, are also captured in the presented computations. The flapping frequency in experimental measurements (7400 Hz) is closely predicted in the presented computations (7500 Hz). The symmetrical instability mode is also observed as viewed from the nozzle’s small and large dimensions at twice the flapping frequency. For the notched rectangular nozzle, the flapping oscillations ceased and instead a spanwise oscillation mode was observed as viewed from the nozzle’s large dimension. The instantaneous mass flowrate at nine jet widths downstream of the nozzle exit showed an increase of 8.5 per cent in mass flowrate in the jet emerging from the notched as compared to the plane rectangular nozzle.

Experimental investigation of the effect of endplates and sidewalls on the near field development of a smooth contraction rectangular jet

The effects of different boundary conditions on the near field development of a turbulent rectangular jet issuing from a smooth contraction nozzle of aspect ratio 15 have been studied experimentally with x -wire Hot-Wire Anemometry (HWA). Four different configurations resulting from the presence or absence of endplates–configuring a jet out of a wall–and sidewalls, confining the jet at the planes of the short side walls of the exit nozzle, under identical inlet conditions have been considered. The mean streamwise and lateral velocity and associated turbulent characteristics up to an axial distance of 35 nozzle widths, H , have been measured. Results comprise centreline data for three Reynolds numbers, based on mean exit velocity and nozzle width, Re = 10,000 , 20,000 and 30,000 as well as lateral distributions at 13 downstream locations for Re = 20,000 . In the absence of sidewalls the presence or absence of an endplate has little effect on the development of the flow field. Measurements made in the presence of sidewalls indicate that the presence of the endplate results in a jet with higher lateral outward velocities within the jet’s shear layers and smaller inward lateral velocities outside the jet’s edges, followed by higher turbulent intensities of both streamwise and lateral components in the early stages of development. Furthermore, in configurations comprising sidewalls, the presence of an endplate results in a faster spreading and decaying jet whereas a reduction of the momentum flow in the far field is also observed.

Confined, milliscale unsteady laminar impinging slot jets and surface Nusselt numbers

Different modes of unsteadiness which develop within confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements and numerical predictions of different flow characteristics, including spatially-resolved distributions of local Nusselt numbers measured on a constant heat flux surface. The effects of Reynolds number, and nozzle-to-plate distance on the local Nusselt number are investigated for a slot nozzle width B of 1.0 mm, Reynolds numbers Re from 120 to 200, nozzle-to-plate distances H/ B from 0.75 to 12.5, and a nozzle aspect ratio y/ B of 50. Observed are several different types of unsteady slot jet behavior, including: (i) a flow fluctuations/flapping motion mode of unsteadiness which is present for B = 1.0 mm, 4.75 ⩽ H/ B ⩽ 5.5, and 120 ⩽ Re ⩽ 140, (ii) an intermittent flapping motion of the jet column which is present for B = 1.0 mm, 9 ⩽ H/ B ⩽ 11.25, and 120 ⩽ Re ⩽ 200, and (iii) a continuous sinusoidal oscillation state, which is observed to be present for Re = 160 and H/ B = 10. The flow fluctuations/flapping motion mode of unsteadiness, and the intermittent flapping motion of the jet column are both associated with local maxima in local, stagnation point Nusselt number distributions. The variations of these stagnation point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/ B. Also described is the lateral variation of local Nusselt numbers for five nozzle-to-plate distances H/ B of 2, 6, 8, 10, and 12, and Reynolds numbers from 120 to 200.

Turbulent Jets Issuing from Rectangular Nozzle with a Rectangular Notch at the Midspan

The turbulent flowfield of turbulent jet issuing from rectangular nozzle (Aspect Ratio=12.5) with a rectangular notch at the midspan, has been investigated experimentally. Four aspect ratios of rectangular notch (NAR) used in this experiment were 2.5, 7.5, 12.5 and 165. The Reynolds number based on the nozzle width d and the exit mean velocity Ue, was kept constant 30000 (NAR=2.5 and 7.5), 15000 (NAR=12.5) and 13000 (NAR=165), respectively. From this experiment, it was revealed that the attachment of the rectangular notch to the rectangular jet restrained the increase of the turbulent velocity scales in the cases of NAR≥7.5. The principal strain rate Sxy on the y axis in the cases of NAR≤2.5 indicated the direct contribution to the production of the turbulent kinetic energy near the jet center in the upstream region, however, the development of the turbulent velocity in the cases of 7.5≤NAR≤12.5 was suppressed because of Sxy=0. Furthermore, the extra strain rate Szz caused by the secondary flow on the z axis contributed to the production of the turbulent kinetic energy near the jet center in the upstream region, and the other extra strain rate Syy on the y axis contributed to the suppression of the turbulent velocity.

Effects of notch geometry and sharpness on turbulent jets issuing from indeterminate-origin notched nozzles

An experimental investigation using hot-wire anemometry to identify the effects of notch configuration and relative sharpness on notched indeterminate-origin turbulent jets at R e = 20,500 was carried out. The notches are characterized by their V- and A-shaped cuts, as well as their relative sharpness due to the design aspect ratio ( A R = 2 and 4) of the notch. Regardless of exact notch geometry, high aspect-ratio nozzles are shown to produce significant flow differences between measurements taken along peak and trough planes, in contrast to low aspect-ratio nozzles. Half-jet width results demonstrate that as the aspect-ratio increases, smooth peaks result in a constant increase within the measurement range while sharp peaks produce an initial decrease till some downstream location before increasing thereafter. In contrast, smooth and sharp trough produces significant nonlinear increases in the half-jet widths with the aspect-ratio increment. Cross-over points linked to axis-switching behaviour are observed for all nozzles except A R = 4 A-notched nozzle, when half-jet widths along peak and trough planes for each nozzle are compared. Increasing the aspect ratio also results in a 50% reduction in the distance away from the nozzle origin where the cross-over point occurs for V-notched nozzles. On the other hand, similar increase in the aspect ratio produces no cross-over point for A-notched nozzles. These results indicate that suitably configured notched nozzles are able to impart significant control over circular jet flow behaviour by inducing and manipulating axis-switching behaviour typically found in noncircular jets.

EFFECT OF NOZZLE GEOMETRY ON THE ATOMIZATION AND SPRAY CHARACTERISTICS OF GELLED-PROPELLANT SIMULANTS FORMED BY TWO IMPINGING JETS

Near- and far-field atomization processes of impinging doublets are experimentally characterized using nongelled and gelled water as working fluids. The main emphasis of the study is on the effect of nozzle geometry, such as orifice inlet shape and aspect ratio, on jet stream surface dynamics and break-up processes before and after jet impingement, respectively. Gelled water jets (non-Newtonian), due to increased viscosity, display fewer surface disturbances than nongelled water jets (Newtonian). As a result, for a given flow rate the sheet formed by impinging jets is much more stable, and the corresponding break-up length is much greater for gelled water than for nongelled water jets. The nozzle aspect ratio has a more significant effect on the near-field jet stream characteristics for both fluids than the orifice inlet shape. Longer nozzles (l /d = 20) form more stable jet streams and delay the breakup of sheets, leading to greater breakup length than that attained with shorter nozzles (l /d = 5). For similar Reynolds numbers and hence, much higher respective flow rates, the droplet size for gelled water is much smaller than that for nongelled water. Also, impinging jets employing gelled water produce sprays with greater spatial distributions and wider ranges of droplet size than jets with nongelled water.

The effect of sidewalls on rectangular jets

An experimental study is presented regarding the influence of sidewalls on the turbulent free jet flow issuing from a smoothly contracting rectangular nozzle of aspect ratio 15. “Sidewalls” are two parallel plates, flush with each of the slots’ short sides, practically establishing bounding walls extending the nozzle sidewalls in the downstream direction. Measurements of the streamwise and lateral velocity mean and turbulent characteristics have been accomplished, with an x-sensor hot wire anemometer, up to an axial distance of 35 nozzle widths, for jets with identical inlet conditions with and without sidewalls. Centreline measurements for both configurations have been collected for three Reynolds numbers, Re D = 10,000, 20,000 and 30,000. For Re D = 20,000 measurements in the transverse direction were collected at 13 different downstream locations in the range, x = 0–35 nozzle widths, and in the spanwise direction at three different downstream locations, x = 2, 6 and 25 nozzle widths. Results indicate that, the two jet configurations (with and without sidewalls) produce statistically different flow fields. Sidewalls do not lead to the production of a 2D flow field as undulations in the spanwise mean velocity distribution indicate. They do increase the two-dimensionality of the jet increasing the longevity of 2D spanwise rollers structures formed in the initial stages of entrainment, which are responsible for the convection of longitudinal momentum towards the outer field, establishing larger streamwise mean velocities at the jet edges. In the near field, up to 25 nozzle widths, lower outward lateral velocities in the presence of the sidewalls are held responsible for the decrease of turbulent terms including rms of velocity fluctuations and Reynolds stresses. Skewness factors increase monotonically across the shear layers from negative values to positive forming sharp peaks at the outer edges of the jet, illustrative of the presence of well defined 2D roller structures in the jet with sidewalls.

S0504-3-4 ボルテックスカップに関する研究 : 噴流の流入条件の影響について(噴流,後流および剥離流れの流動解析と応用(その3))

In semiconductor manufacturing processes, a new pneumatic non-contact handling device is developed. This device named Vortex Cup makes use of a vacuum at a central area of swirling flow in order to hold the wafer against the device and to prevent significant wafer damage and foreign material contamination. Vortex Cup provides lower-cost, higher-performance than other pneumatic type devices. Experiments on Vortex Cup have turned out that the vacuum depends on the gap and supply flow rate, and swirl axis is inclined inside a cup. In order to improve performance of Vortex Cup, we focused on nozzle inlet height and aspect ratio of a cup. In this report, pressure distribution of various cup is measured. Furthermore, fluid motion in a cup is revealed by ink injection method.