Nozzle Exit Reynolds Number Research Articles

Entrainment rate predictions of axis-symmetric non-swirling jets using free-jet-theory, Reynolds-averaged Navier-Stokes modelling and large-eddy-simulations resolved up to Kolmogorov scale

Jet entrainment - relevant to mixers, sprays and combustion technologies - has been the subject of this work. We limit our considerations to air jets issued from convergent nozzles of diameter smaller than 25.4 mm, the nozzle exit Reynolds number in the range 30,000 to 100,000 and Mach numbers not exceeding 0.4. The emphasis is on jet self-similarity region (60 < x/d0 < 210) and the key question is with which accuracy can Computational Fluid Dynamics (RANS and LES) and free-jet theory predict jet entrainment. Seven jets have been considered.The realizable k-є model has outperformed the other models and provides the entrainment predictions within ±6 % margin from the measured data. The standard k-є and the Shear-Stress Transport (SST) k-ω models deliver entrainment figures which are larger than the measured data by 22 % – 24 % whilst predictions of either the Reynolds Stress Model (RSM) or Re-Normalization Group (RNG) k-є models can be off (too large) by as much as 34 % and 40 %, respectively. Such a clarity in classification of turbulence models has been obtained after minimization of numerical related errors to a degree which was not achievable in the past. The Panchapakesan&Lumly's jet has been computed using the Large Eddy Simulations with the filter size of the order of Kolmogorov scale throughout the jet e.g. at the inlet, potential core and the far field. Excellent predictions of the jet spread rate, velocity profiles and the entrainment have been obtained at the expense of huge computational resources.The well-known engineering correlation m˙e/m0˙=0.32(x/d0) provides entrainment figures that are by 10 % or less larger than the measured values.

PARAMETRIC STUDY AND DEVELOPMENT OF A CORRELATION FOR NUSSELT NUMBER IN NANOFLUID JET IMPINGEMENT

Numerical simulations for nanofluid ( water with Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; nanoparticles) jet impinging perpendicularly on a flat circular heated p late have been performed. A heated p late is subjected to constant heat flux boundary condition. A two-phase modified mixture mo del was used for the prediction of heat transfer coefficient, and comparisons are made with standard mixture model. Present results for average Nusselt number are validated with experimental data available in the literature. Though a standard mixture model predicted heat transfer with accepted accuracy, it was found that accuracy of modified mixture model is better (around 5&amp;#37; improvement) compared to standard mixture model. Thereafter, parametric study was performed considering nozzle exit Reynolds number (Re), spacing ratio (&lt;i&gt;H/D&lt;/i&gt;), nanoparticle volume fraction (&amp;phi;), and nanoparticle diameter (dp) on heat transfer prediction. The results reveal that particle diameter in the range 10-100 nm has no effect on the Nusselt number, Furthermore, heat transfer increased with increasing Reynolds number and volume fraction. However, spacing ratio shows first increasing and then, decreasing trend (similar to a log-normal distribution curve) in the prediction of heat transfer. Finally, a new correlation was developed for Nusselt number using nonlinear regression analysis. In the correlation, a two-phase multiplier was used, which is the ratio between two-phase Nusselt number (Nu&lt;sub&gt;nf&lt;/sub&gt; ) and single-phase Nusselt number (Nu&lt;sub&gt;sp&lt;/sub&gt;). The simplified correlation is found to predict data with maximum error of 8.9&amp;#37;, average error of 2.74&amp;#37; and &lt;i&gt;R&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; &amp;#61; 0.955.

Experimental study of external lateral flow effects on turbulent isothermal upward/downward slot jets impinging inside an open cavity

In this work, the flow within an open cubical cavity with an inlet vertical planar jet exposed to an external lateral flow is studied experimentally using stereoscopic time-resolved particle image velocimetry (TR-PIV). The turbulent structure in the near field of the planar jet issuing perpendicularly from a rectangular slot nozzle is evaluated for both vertically upward and downward directions of the discharged jet. For both flow configurations, the jet discharge rectangular nozzle spans 30% of the cavity span leaving a clearance between the jet edge and sidewalls. The working fluid is water, and the complex dynamics of the three-dimensional (3D) flow field is investigated for fixed lateral flow Reynolds number of Re=3000 and three nozzle exit Reynolds numbers based on the mean streamwise exit jet velocity and nozzle hydraulic diameter of Rej=1500, 3000 and 5000. The spatial development and structure dynamics of the shear interface instability are analyzed in planes normal to the cavity roof. Flow visualization images of the mean and fluctuating velocity, vorticity and shear strain rate distributions are obtained for upward and downward directions of the slot jet, and spectral analysis of the oscillating instability is performed to describe and quantify the strong shear layer interactions that take place between the discharged eddies of the splitting jet and the external lateral flow.

Comparison between a Conventional and a New IRS Device in Terms of Air Entrainment: An Experimental and Numerical Analysis

An experimental study was conducted on a laboratory-scale new infrared suppression (IRS) device to assess the mass entrainment of ambient air into it under various operating conditions. A numerical analysis was also undertaken to assess the mass entrainment rate against pertinent input parameters independently. The numerical results were validated against the experimental data to ensure the reliability of the numerical analysis of the new IRS device at real scales. The numerical method solves the three-dimensional, incompressible Navier-Stokes equations; the mass continuity equation; and the two-equation-based eddy viscosity model for the turbulent k-epsilon equations in the flow field. Numerical assessment of the air entrainment was performed for the conventional and the newly proposed IRS devices. A number of experiments on the new IRS device were carried out under various operating conditions. From the numerical study, it was observed that the conventional IRS device performs better than the new IRS device up to a geometric ratio of 1.4 (which is the ratio of diameters of the successive funnels used in an IRS device). Beyond the geometric ratio of 1.4, the newly proposed IRS device outperforms the conventional one significantly. For the new IRS device, the maximum mass entrainment was found to occur when four funnels were used and the nozzle was kept in flush condition with the lower opening of the bottom-most funnel. Mass entrainment increases with the nozzle-exit Reynolds number for the range of values considered in the study.

Numerical analysis of air entrainment and exit temperature of a real scale conical infrared suppression (IRS) device

A numerical analysis has been performed on a real-scale infrared suppression device to estimate the air entrainment and corresponding temperature drop of the exhaust gases flowing through it. The analysis of the IRS device is done for the exhaust of a gas turbine plant installed on a naval warship. The operational data used in the analysis has been taken from LM2500 series gas turbine from GE. The numerical method solves the three dimensional, incompressible Navier-Stokes equations, the mass continuity equation and the two-equation based eddy viscosity model for the turbulent k-epsilon equations along with the energy equation in the flow field. The effect of using different number of nozzles at the gas turbine exit on the air entrainment and the exit plume temperature has been discussed. The results show that the maximum air entrainment and minimum plume temperature can be achieved by using four to six numbers of nozzles. The air entrainment and the exit plume temperature are also affected by the pitch circle diameter of the nozzle placement. The maximum air entrainment and minimum exit temperature is achieved at a pitch circle diameter of 1.6 m. The air entrainment considerably increases with the nozzle-exit Reynolds number (Re). The higher Reynolds number also ensures the minimum plume temperature at the exit of IRS device. Air entrainment gradually reduces with the nozzle protrusion. The highest entrainment is observed when the nozzles are flush with the bottom plane of the IRS device. However, the nozzle protrusion does not affect the exit plume temperature significantly. The funnel overlap height adversely affects the performance of the IRS device i.e. the air entrainment continuously decreases with increasing funnel overlap height. Moreover, increasing funnel overlap also causes the exit plume temperature to rise considerably.

The Impingement Heat Transfer Data of Inclined Jet in Cooling Applications: A Review

On the impingement heat transfer data, the experimental studies of air and liquid jets impingement to the flat surfaces were collected and critically reviewed. The oblique impingements of both single circular and planar slot jets were considered in particular. The review focused on the surface where the jet impingement cooling technique was utilized. The nozzle exit Reynolds numbers based on the hydraulic diameter varied in the range of 1,500–52,000. The oblique angles relative to the plane surface and the dimensionless jet-to-plate spacing vary in the range of 15°–90° and 2–12 respectively. The review suggested that the magnitude of maximum heat transfer shifted more for air jets compared with the liquid jets. The drop in the inclination angle and the jet-to-plate separation led to the increase in the asymmetry of heat transfer distribution. The displacement of maximum Nusselt number (heat transfer) locations was found to be sensitive to the inclination angle and the smaller jet-to-plate distance. Also, the Nusselt number correlations proposed by various researchers were discussed and compared with the results of the cited references.

Experimental and numerical study of air entrainment into a louvered conical IRS device and comparison with existing IRS devices

An experimental study was carried out on a laboratory-scale louvered conical infrared suppression (IRS) device to predict the air entrainment into it. A comparison has been made between the louvered conical, conical and the cylindrical IRS device based on the air entrainment ratio with increasing geometric ratio. A numerical study on the flow domain was also performed by solving the continuity and Navier-Stokes equations using the k-epsilon turbulence model. The air entrainment was compared with the experimental data and a good match was found. The results showed that the louvered conical IRS device outperforms the other two IRS devices significantly when the geometric ratio exceeds a value of 1.4. The mass entrainment of the ambient air was maximum when four louvered funnels at optimum overlap were used. Increasing or decreasing the number of funnels from this value lowers the air entrainment. In the analysis, it was also found that the nozzle protrusion adversely affects the air entrainment and hence for getting the highest entrainment, the nozzle was flush with the lower opening of the bottom most funnel. The funnel overlap also acts as an important parameter as far as the rate of air entrainment is concerned. The air entrainment through the IRS device was found to be maximum when the non-dimensional funnel overlap height (Hoverlap/Dnz) was kept at 8. The mass entrainment rate was found to increase linearly with the nozzle exit Reynolds number in the range of parameters for which the study was undertaken.

Application of various RANS based models towards predicting turbulent slot jet impingement

Numerical simulation near the stagnation zone for an impinging jet is challenging because of the presence of sharp gradient due to the change of velocity from zero to the system scale velocity. A comparative study of four Reynolds averaged Navier–Stokes (RANS) based turbulence models has been done by numerically simulating a two-dimensional turbulent slot jet impinging air normally on a flat plate. Nozzle-to-plate spacings equal to 4 and 9.2 have been taken as two test cases with nozzle exit Reynolds number of 2×104. The turbulence intensity at the exit of the nozzle is kept constant at 1%. The capability to predict the fluid flow of the standard k−ε model, the low Reynolds number (LRN) k−ε models proposed by Launder and Sharma (LS) and Yang and Shih (YS), and the standard k−ω model have been evaluated by comparing the computational results with the experimental data available in the literature. The numerically obtained results have been compared with the experimental results on impingement surface pressure, free jet mean velocity profiles, spanwise velocity profiles, spanwise distribution of root mean square velocity fluctuation, skin friction coefficient distribution, non-dimensional velocity profile in wall coordinates. The standard k−ε model predicted the largest recirculation loop among the turbulence models used. The skin friction coefficient distribution is reproduced by YS and k−ω model for both the spacings whereas the other LRN model LS and the standard k−ε model are unable to predict the results. The linear law of the wall is found to be valid up to Y+≃4.0 in the recirculation region. The velocity profiles in wall coordinates in the recirculation region are behaving similar to those in the wall jet region of an impinging jet and the developed region of a wall jet.

New correlation for prediction of air entrainment into an Infrared Suppression (IRS) device

In the present research, the conservation equations for mass, momentum, energy as well as the equations for k and ɛ have been solved numerically to compute the rate of air entrained into an Infrared Suppression (IRS) device using finite volume method. Hence, four different correlations (Eqs. (15)–(18)) have been developed to predict the mass suction over a wide range of operating parameters which have been used to design the IRS device for naval or merchant ship. The nozzle exit Reynolds number, diameter of bottom funnels, funnel overlap height, nozzle distance from the bottom funnel as well as nozzle fluid temperature are varied over a wide range of 5×103≤Re≤106, 1.42≤Dbf/Dnz≤1.77, −0.16≤Hov/Dnz≤0.16, −0.1633≤Hnz/Dnz≤0.1633, and 1≤Tnz/T∞≤2, respectively; so as to develop the above correlations. Furthermore, two correlation equations (Eqs. (19) and (20)) have been developed to predict the outlet temperature of the IRS device. The present computational results have also been validated with the results of existing literature as well as with some of our own experimental results. Correlation equations (Eqs. (15)–(20)) have been developed by carrying out a regression analysis through a computer program written in Engineering Equation Solver (EES).

Onset and influencing factors of deflecting oscillation in planar opposed jets

The oscillation behaviors of planar opposed jets are experimentally and numerically studied for a Reynolds number range of 16–5000. The flow of planar opposed jets is visualized by some white smoke, and the images of smoke-seeded flow are captured by a 2-D planar particle imaging velocimetry (PIV) and a high-speed camera. Effects of the nozzle separation, the exit Reynolds number, the confinement of boundary and the width-to-height ratio of exit on the oscillation behaviors of planar opposed jets are investigated and discussed. Different flow regimes have been identified at various nozzle separations and exit Reynolds numbers. The transition from the stable separated flow to the impingement plane flapping flow and the onset of deflecting oscillation are reported for the first time from experimental results. The map describing the flow regimes of planar opposed jets is presented. Results show that the confinement of boundary and the width-to-height ratio of exit have effects on the deflecting oscillation.

Large-eddy simulation and experimental study of deflecting oscillation of planar opposed jets

The deflecting oscillation of planar opposed jets is experimentally studied and numerically simulated by large-eddy simulation (LES) at 25 ≤ Re ≤ 10000 (Re= U0hρ/μ, where U0 is the bulk velocity of the jet at the nozzle exit, h is the height of the slit of the planar nozzle, and ρ and μ are the density and dynamic viscosity of fluid, respectively) and 4h≤ L ≤ 40h, where L is the nozzle separation. The numerical results are validated by comparing with the experimental results of planar opposed jets. Maps of parameter space describing the deflecting oscillation of planar opposed jets at various nozzle separations and exit Reynolds numbers are presented. And the variation features of deflecting oscillation periods and velocity-pressure of turbulent planar opposed jets are primarily investigated. The results of the study show that the LES can effectively predict the deflecting oscillation of planar opposed jets. The velocity and pressure at specific points vary periodically while the deflecting oscillation of planar opposed jets happens. Furthermore, the variation periods of velocity and pressure are in accordance with the periods of the deflecting oscillation. In essence, the deflecting oscillation of planar opposed jets is caused by periodical variation and transformation of the velocity and pressure.

Investigation of a continuous adjoint-based optimization procedure for aeroacoustic control of plane jets

A control optimization technique using the continuous adjoint of the compressible Navier–Stokes equations is implemented for aeroacoustic optimization of plane jet flows. The purpose of the adjoint equations is to provide sensitivity information, which is afterwards used in a gradient-based minimization of a prescribed cost functional, designed to describe the far-field sound pressure level (SPL). The objective of the present paper is to demonstrate the ability to reduce the sound in the near far-field of plane jets. Furthermore, as the continuous adjoint approach can become inaccurate, due to inconsistencies between the continuous and the discretized system, the accuracy of the continuous adjoint approach is investigated. The considered cases exhibit a nozzle exit Reynolds number of Rejet=ρujetD/μ=2000 and a Mach number of Mjet=0.9, performed using two-dimensional direct numerical simulation and three-dimensional large-eddy simulation, respectively. A comparison of the obtained gradient via adjoint and finite differences is presented and it is shown, that in order to obtain reliable gradient directions, the length of the optimization time needs to be restricted. Furthermore, a receding horizon optimization for the two-dimensional plane jet simulation is used to obtain a sound reduction over much longer time intervals. The influence of different formulations of the viscosity in the adjoint equations is finally investigated.

Heat Transfer Characteristics of an Array of Impinging Gaseous Slot Jets

A comprehensive experiment was conducted for heat transfer characteristics for an array of impinging gaseous slot jets to a flat plate with strong turbulence (nozzle exit Reynolds number Re=22500~64700).Find that turbulence intensity of flow has an important influence on local forced convective heat transfer coefficient. Meanwhile, the nozzle-to-plate spacing and nozzle exit Reynolds number Re would affect the mean forced convective heat transfer coefficient of the slot jets. And heat transfer efficiency of slot jets has been set to show the relation between ability of the jets and energy consumption of gas supply.

Experimental investigation of flow regimes of axisymmetric and planar opposed jets

AbstractDynamic behaviors of axisymmetric and planar opposed jets have been experimentally studied at 786 &lt; Re &lt; 6288. The flow patterns were investigated by a smoke‐wire technique, and the smoke‐wire photos were recorded by a high‐speed camera. Different flow regimes of axisymmetric and planar opposed jets have been identified. Axisymmetric opposed jets exhibit axial quasi‐periodic oscillations, stagnation point offsets, and steady states, while planar opposed jets exhibit both horizontal instabilities and deflecting oscillations. Effects of the nozzle separation and the exit Reynolds number on the dynamic characteristics of axisymmetric and planar opposed jets have been investigated and discussed. Maps of parameter spaces describing the flow regimes of axisymmetric and planar opposed jets at various nozzle separations and exit Reynolds numbers have been presented. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers ( Re = 1 × 10 4, 2 × 10 4 and 3 × 10 4) and four different inclination angles between the wall and the flow velocity at the nozzle ( β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.

Numerical analysis of choked converging nozzle flows with surface roughness and heat flux conditions

Choked converging nozzle flow and heat transfer characteristics are numerically investigated by means of a recent computational model that integrates the axisymmetric continuity, state, momentum and energy equations. To predict the combined effects of nozzle geometry, friction and heat transfer rates, analyses are conducted with sufficiently wide ranges of covergence half angle, surface roughness and heat flux conditions. Numerical findings show that inlet Mach and Nusselt numbers decrease up to 23.1% and 15.8% with surface heat flux and by 15.13% and 4.8% due to surface roughness. Considering each convergence half angle case individually results in a linear relation between nozzle discharge coefficients and exit Reynolds numbers with similar slopes. Heat flux implementation, by decreasing the shear stress values, lowers the risks due to wear hazards at upstream sections of flow walls; however the final 10% downstream nozzle portion is determined to be quite critical, where shear stress attains the highest magnitudes. Heat transfer rates are seen to increase in the streamwise direction up to 2.7 times; however high convergence half angles, heat flux and surface roughness conditions lower inlet Nusselt numbers by 70%, 15.8% and 4.8% respectively.

Study of heat transfer for a pair of rectangular jets impinging on an inclined surface

Critical design parameters in jet impingement heat transfer like nozzle hydraulic diameter, jet angle and velocity, physical properties of the fluid, and nozzle-to-target plane spacing are the subject. This paper identifies the dominant fluid-thermal characteristics of a pair of rectangular air jets impinging on an inclined surface. Heat transfer modes and flow characteristics are studied with eight different Reynolds numbers ranging from 500 to 20 000. Local and average Nusselt numbers are evaluated with two different boundary conditions on three specified lines located on the inclined surface. The correlation between stagnation Nusselt number and Reynolds number is presented. Turbulent intensity and wall y + distributions are compared on three lines parallel to the incline. The effect of jet impingement angle on local and average Nusselt number is also documented. Finally, a correlation between the average Nusselt number, nozzle exit Reynolds number and the jet angle is documented.

An inclined wall jet: Mean flow characteristics and effects of acoustic excitation

The mean velocity field of a 30° inclined wall jet has been investigated using both hot-wire and laser Doppler anemometry (LDA). Provided that the nozzle aspect ratio is greater than 30 and the inclined wall angle (β) is less than 50°, LDA measurements for various β show that the reattachment length is independent of the nozzle aspect ratio and the nozzle exit Reynolds number (in the range 6670–13,340). There is general agreement between the reattachment lengths determined by LDA and those determined using wall surface oil film visualisation technique. The role of coherent structures arising from initial instabilities of a 30° wall jet has been explored by hot-wire spectra measurements. Results indicate that the fundamental vortex roll-up frequency in both the inner and outer shear layer corresponds to a Strouhal number (based on nozzle exit momentum thickness and velocity) of 0.012. The spatial development of instabilities in the jet has been studied by introducing acoustic excitation at a frequency corresponding to the shear layer mode. The formation of the fundamental and its first subharmonic has been identified in the outer shear layer. However, the development of the first subharmonic in the inner shear layer has been severely suppressed. Distributions of mean velocities, turbulence intensities and Reynolds shear stress indicate that controlled acoustic excitation enhances the development of instabilities and promotes jet reattachment to the wall, resulting in a substantially reduced recirculation flow region.

An experimental study of slot jet impingement cooling on concave surface: effects of nozzle configuration and curvature

An experimental study has been carried out for jet impingement cooling on a semi-circular concave surface when jet flows were ejected from three different slot nozzles—round shaped nozzle, rectangular shaped nozzle and 2D contoured nozzle. Experiments have been conducted with variations of nozzle exit Reynolds number (Re2B) in the range of 5920 ⩽ Re2B ⩽ 25500 and nozzle-to-surface distance (zn) in the range of 1/2 ⩽ zn/B ⩽ 20 to determine the heat transfer coefficients under a constant heat flux condition. The developing structures of free jets were measured by hot-wire anemometer to understand the characteristics of heat transfer in conjunction with measured jet flows. Markedly different flow and heat transfer characteristics have been observed depending on different nozzle shapes. Average heat transfer rates for impingement on the concave surface are found to be more enhanced than the flat plate results due to the effect of curvature. Comparisons between the present results and the existing experimental results have also been made.

Effect of wall inclination on the mean flow and turbulence characteristics in a two-dimensional wall jet

Mean velocity and turbulence measurements, using hot-wire anemometry, were made up to 20 nozzle widths downstream of a nozzle in a two-dimensional (2-D) wall jet facility in which the angle of inclination of the wall to the nozzle was varied from 0 to 90° for a nozzle exit Reynolds number of 10,000. The effect of the wall angle (β) on the spatial development of the velocity field of the jet has been documented for β = 0°, 15°, 30°, 45°, and 90°. Results indicate that in the range 0° ⩽ β 5 45°, as the wall angle β increases, the centreline velocity decays faster, and the jet spreads faster, resulting in a shorter potential core and increase in jet volume entrainment. The positions of reattachment of the jet to the wall for various β have been deduced from surface pressure measurements and flow visualisation techniques.