Circular Nozzle Research Articles

Detection algorithm for turbulent interfaces and large-scale structures in intermittent flows

A robust algorithm is introduced for detection of large-scale coherent structures in transitional and intermittent flows that feature turbulent/non-turbulent (T/NT) interfaces. The algorithm is applicable the instantaneous flow fields of wall-bounded and free shear flows, and can effectively identify coherent events in the velocity or vorticity fields, or sweep/ejection motions. A database from direct numerical simulation (DNS) of transitional boundary layer is used to develop and demonstrate the capabilities of the algorithm which consists of three steps. The first is identification of the T/NT interface by comparing the normalized vorticity magnitude to a threshold value that is independent of the Reynolds number. The vorticity normalization is specifically designed to be applicable in transitional flows, where regions of the flow can host juxtaposed regions of laminar and turbulent flow. With the definition of the T/NT interface, conditional statistics are computed and perturbation quantities are defined relative to their respective conditional means. Second, the influence of the small-scale turbulence is excluded by applying an anisotropic Gaussian filter. The filter size is determined from the spatial characteristics of the small-scale vortical motions. In the third step, one-dimensional cores and two-dimensional surfaces within the flow structures of interest are identified from local extrema in the fields, and are tracked as Lagrangian objects. Using the algorithm, the population trends and advection speeds of large-scale sweep/ejection events are computed in the transitional boundary layer. Two additional flow configurations are also considered: turbulent jet flow emerging from a circular nozzle and the turbulent flow in a channel with a wavy surface.

An experimental study on flame envelope morphologic characteristics of downward-orientated buoyant turbulent jet fires

This study experimentally investigates flame envelope morphologic characteristics of downward-orientated buoyant turbulent jet fires, where the initial jet momentum is opposite to flame buoyancy that an anti-buoyancy jet configuration is naturally formed. Such configuration exists practically in gas leakage jet fires, however, has not been quantified. Experiments were conducted employing four circular nozzles with propane as fuel. The flame downward distance from nozzle outlet, flame length and flame width were measured at various fuel velocities for such downward-orientated jets. The corresponding flame length and width for upward jets were also obtained for comparison. Results showed that the three flame dimensions increased with increasing fuel velocity or heat release rate. The ratio of flame downward distance to flame length correlated well to a dimensionless length scale, ξℓ, representing buoyancy to initial momentum of the jets. The flame width and flame downward distance normalized by nozzle diameter correlated well to non-dimensional heat release rate by 1/2 power and 3/5 power, respectively; and their ratio can be approached by a -1/10 power consequently. The ratio of flame width to flame length of downward jets increased with heat release rates, being much higher than that of upward jets which first increased then plateaued (0.145) finally deceased with increasing heat release rate. Both the flame downward distance and flame length of downward jet showed to be much smaller than flame length of corresponding upward jet. A formula was proposed to correlate flame downward distance as a function of flame Froude number (Frf). The flame downward distance was initially smaller then, however, increased faster and finally turned to be larger than flame length with increasing fuel velocity. The critical turning state as flame downward distance equaling flame length (namely flame upper tip descending to nozzle outlet level) of downward-orientated jet can be represented by ξℓ*=3.4 or Frf*=1.

Dimension reduction of compressible fluid models over product manifolds

In this paper we study the dimension reduction limits of the compressible Navier–Stokes equations over product Riemannian manifolds Oϵ≅M×ϵF, such that dim⁡(M)=n and dim⁡(F)=d are arbitrary. Using the method of relative entropies, we establish the convergence of the suitable weak solutions of the Navier–Stokes equations on Oϵ to the classical solution of the limiting equations on M as ϵ→0+, provided the latter exists. In addition, we also deduce the vanishing viscosity limit. The limiting equations identified through our analysis contain the weight function A:M→R+ as a parameter, where A(x) = area of the fibre Fx. Our work is based on and generalises the results in Bella et al. (2016) [3], Bella et al. (2014) [4] and Maltese and Novotný (2014) [19], and it contains as special cases the physical models of circular nozzles, thin plate limits and finite-length longitudinal nozzles.

Mixing enhancement using chevron nozzle: studies on free jets and confined jets

This paper reports an experimental study focused on the impact of chevrons (serrations on the trailing edge of the nozzle) on the mixing process of an incompressible jet issuing from a convergent nozzle. The study also explores enhancement of the mixing performance by a novel approach to geometry modification. Profiles of mean velocity were used to characterize the extent of mixing. For a comparative assessment, studies were carried out with a base line circular nozzle, a conventional chevron nozzle and an improvised tabbed chevron nozzle. Flow visualization studies were carried out for jets issuing from chevron nozzles and the results corroborate well with quantitative measurements. The impact of confinement on mixing of jets issuing from chevron nozzles is also studied. The results show that the proposed geometry modification can significantly improve the rate of mixing in the range of Reynolds numbers considered in the study. In confined jets, presence of chevrons was found to accelerate the process of jet break-down.

Vortex ring propagation and interactions studies

This paper experimentally investigates the vortex ring propagation and interactions with thin cylindrical and flat surfaces. Dye-based visualization technique is adopted for the interaction studies. Vortex rings are generated from a circular nozzle of 19 mm diameter with the stroke length ratios (length of the fluid slug to nozzle diameter, LN/DN) of 1 to 5, and ejection velocities in the range of 0.05 to 0.2 m/s. Vortex interaction studies are carried out with two different bodies; firstly, with the circular cylinders having the diameters of 0.2, 0.6, 1.5 and 2.5 mm, and secondly with a flat solid surface. Results indicate that the trails in the vortex ring start following at LN/DN = 4. The influence of the initial velocity is found to be insignificant on the vortex ring diameter, however, found to depend on stroke length ratio. Vortex-cylinder interaction studies indicated that vortex velocity decreases with increase in cylinder diameter after the interaction. Reconnections of vortex rings are observed in lower cylinder diameter cases. In case of vortex ring interaction with the flat surface, stretching of the vortex core is observed leading to a considerable increase in the vortex ring diameter.

Droplet generation during liquid jet impingement onto a horizontal plate

In sodium-cooled fast reactors, liquid sodium leakage from piping may lead to fire accident. In the case that the liquid sodium is discharged as a liquid jet, a number of droplets are produced during liquid jet impingement on the structures; the surfaces of splashed droplets serve as a main reaction field of sodium combustion. In the present work, a liquid jet was emanated vertically downward from a circular nozzle hole onto a horizontal disk to measure the total amount and the maximum size of splashed droplets. It was found that the splashing rate was negligibly small when the liquid jet impinged as a continuous jet whilst a significant amount of liquid was splashed when the liquid jet impinged as a broken jet. Thus, a prediction method was first developed for the impact frequency of the primary droplets produced due to the jet breakup. It was then shown that a phenomenological model using the impact frequency and the impact Weber number as the important variables can predict the splashing rate well. It was also indicated that the size of the maximum splashed droplets was fairly proportional to the size of primary droplets.

Measurements of concentration decays in underexpanded jets through rectangular slot nozzles

Previously studies of hydrogen releases have focused on hydrogen jets through round nozzles, but leaks are more likely to result in jets through slots. In this study, the concentration decays along the jet centerline were measured for underexpanded jets from rectangular slot nozzles with aspect ratios (AR) of 1–12.6 for storage pressures up to 3.7 MPa. The mass fractions along the jet centerlines decay inversely with distance from the nozzle as with axisymmetric jets but with a faster decay rate. All the data collapses onto a single curve when normalized by the proper factor of the downstream location, except for the jets from square nozzles (AR1). The decay rates for jets from rectangular nozzles with AR greater than unity are significantly larger than for the jets from square nozzles. The jets from square nozzles behave somewhat like the circular jets from circular nozzles due to the symmetric geometry, so the decay rate is closer to that of circular jets relative to the jets from the other rectangular nozzles. This database will be useful for model validations for the modeling of jets from rectangular nozzles.

Pulsating water jet erosion effect on a brass flat solid surface

The present study is focused on the disintegration effect of ultrasound-enhanced pulsating water jet (PWJ) technology on brass CW614N. The first part of the study discusses the effect of a combination of factors based on the full factorial design of experiments (DoE) 33. Traverse speed v (mm s−1), circular nozzle orifice diameter d (mm), and hydraulic power P h (kW) are selected as the disintegration variable factors. Mass material removal Δm (mg s−1) is evaluated based on the change in these variable factors. In the next part, a verification experiment is performed with by varying the traverse speed between 0.2 and 1.4 mm s−1. The mathematical model calculated in DoE is confirmed. Moreover, the significant effect of hydraulic power P h (kW) on the efficiency of the PWJ disintegration is demonstrated. The last part of the study discusses the surface and subsurface effects on a PWJ after brass CW614A erosion. A sample disintegration with hydraulic power P h = 13 kW and circular nozzle diameter d = 1.321 mm is observed. Optical profilometry and scanning electron microscopy are performed to visualise the surface erosion of a selected groove. A significant mass material removal is observed from the groove surface, and the disintegrated surface is characterised by erosion and crater formation. A slight cold deformation with a maximum depth of 200 μm is detected in the subsurface layer. The experiment and results present a part of an extensive research focused on describing the PWJ disintegration efficiency for metallic materials.

Large-eddy simulation of circular jet mixing: Lip- and inner-ribbed nozzles

Jet mixing of a circular nozzle in two different configurations is extensively investigated using large-eddy simulation (LES), i.e., the lip-ribbed one with an annular rib at the nozzle exit, and the inner-ribbed one with a rib placed 1.0D upstream of the nozzle exit; the latter one is particularly selected to generate unsteady reattachment of the separated shear layer near the nozzle exit. A jet issuing from a hydrodynamically smooth pipe with fully developed turbulent flow into a larger cylindrical domain (a ‘pipe jet’) is used as the baseline configuration for comparison. The Reynolds number based on the mean (bulk) velocity in the pipe and the pipe diameter is 6000. Validation of the LES approach is performed on the baseline pipe jet using particle image velocimetry (PIV) and laser-induced fluorescence (LIF) measurements; perfect agreement is obtained in terms of the mean streamwise velocity and concentration and their fluctuation parts. The LES results show that both ribbed jet configurations exhibit higher levels of mixing than the pipe jet; the most rapid decay of the centreline concentration and the largest entrainment are observed in the inner-ribbed jet. The inner-ribbed nozzle induced high flow fluctuations behind the rib due to frequent reattachment of flow to the pipe wall that are similar to what is known as fluid flapping motion. These flapping motions give rise to the energetic large-scale structures at the pipe exit and serve as an effective control imposed on the jet. The ‘slice’ proper orthogonal decomposition analysis performed on the flow helps to identify the jet's ‘preferred modes’ by selecting the most energetic structures with a specified azimuthal wavenumber. In the lip-ribbed jet, the separated shear layer modified the frequencies of the axisymmetric mode but made a limited contribution to the energy level of the jet preferred mode. In the inner-ribbed jet, however, the jet preferred modes were extensively altered. The single- and double-helical modes (m=1 and 2) were substantially intensified, with increasing energy levels up to 58%, giving rise to the significant enhancement of jet mixing, especially in the upstream regions.

Fabrication of Micro Diamond-Like Carbon-Nozzles by Plasma Oxidation Printing

The plasma oxidation printing (POP) was proposed as a digital manufacturing to shape the diamond-like carbon (DLC) thick film into a micronozzle on the tool steel substrate. Its head shape was first designed by computer-aided design (CAD) and printed onto DLC film by the maskless lithography. The unprinted DLC was removed in depth by the anisotropic etching. Fine circular nozzle with the inner diameter of 10 μm and the height of 10 μm was fabricated during the duration time of 3.6 ks. Different from the micromilling or the micro-electric discharging, the micronozzle geometry is directly fabricated from its CAD data. The micronozzle with the cross-star outlet was constructed without the change of processing procedure and without use of tooling.

Ultrasonically generated pulsed water jet peening of austenitic stainless-steel surfaces

In this study, the effects of pulsating water jets were investigated as a surface treatment process using circular and flat nozzles by considering the integrity of a stainless steel (AISI 304) surface. The local energy distribution was controlled by changing the traverse speed and the pulsating water jet (PWJ) effects were assessed in terms of the residual stress and strengthening effect. The strengthening effect of the process was evaluated by measuring the micro-hardness of the treated surface and by studying the impact of the treatment on the surface based on micro-structural analyses using scanning electron microscope (SEM). The residual stress of the subjected area was evaluated using X-ray diffraction technique. Based on the results from the studied samples, it was found that the initial tensile residual stress was relieved and converted to a compressive residual stress. An increase in the hardness of the treated samples was also observed as compared to the untreated samples up to certain depth along the cross-section of the treated region. The micro-structural examination of the samples revealed the plastic deformation that occurred during the treatment process. Additionally, the acoustic emission (AE) generated during the impact was used as an online monitoring tool for observing the behaviour of the elicited signals under different parametric conditions, and as a control mechanism for obtaining better results. The experimental results show that the pulsating water jet constitutes a new potential technology for surface treatment processes.

Heat transfer investigations on methane-air premixed flame jet exiting from a circular nozzle and impinging over semi-cylindrical surfaces

The flame jet impingement on curved surfaces finds applications in various direct flame heating situations. The design of heating equipment demands the estimation of the surface temperature and of the convective heat flux to the target surface at steady state for various impingement parameters. The characterisation of flame jets in terms of Nusselt number and effectiveness aids the designer to numerically estimate the steady state surface temperature and heat flux by specifying the convective boundary condition with spatial distribution of reference temperature. The present study reports the heat transfer characteristics of single flame jet impinging on convex and concave surfaces of a semi-cylindrical target plate. The effects of mixture Reynolds number, equivalence ratio, curvature ratio and burner to plate spacing on the heat transfer behavior are investigated. The uniformity of the heat flux distribution and the overall thermal efficiency are determined from the local heat flux distributions. The flow fields along the curvilinear axes influence the overall thermal performance. The concave surface produces higher thermal efficiency and better uniformity of heat flux over the impingement surface as compared with the convex surface.

Nozzle Shape Guided Filler Orientation in 3D Printed Photo-curable Nanocomposites

Here, we report guided orientation of silver nanowires (AgNWs) in extruded patterns with photo-curable 3D printing technology. A printable conductive composite material composed of polymer matrix and silver nanowires shows significantly varied electrical properties depending on the cross-sectional shape of printing nozzles: flat or circular. The composite is designed to have highly conductive AgNWs and a dielectric polymer matrix like photo-curable methacrylate resin. The dielectric permittivity of photo-curable composite resin with 1.6 vol. % of AgNWs printed through a circular nozzle showed 27. However, the same resin showed much lower permittivity with 20 when it is printed with a flat nozzle. The cross-sectional sample morphology shows that AgNWs printed with a circular nozzle are aligned, and AgNWs printed with a flat nozzle are randomly distributed. A computational simulation of paste extrusion with two different nozzle shapes showed clearly different fluidic velocities at the nozzle exit, which contributes to different fiber orientation in printed samples. A radio frequency identification sensor is fabricated with 3D printed composite using a flat nozzle for the demonstration of AgNW based 3D printed conductor.

Experimental study on the spray characteristics discharging from elliptical diesel nozzle at typical diesel engine conditions

The elliptical orifice diesel nozzle has potential beneficial for improving spray and air-fuel mixing quality, which has great impact on combustion and emission performances for diesel engine. This paper presents experimental studies on the macro spray characteristics and the mechanism of axis-switching for elliptical and circular orifice diesel nozzles with same cross section area, with biodiesel fuel by using shadowgraph technique, under various injection pressures (50 MPa, 70 MPa, 90 MPa) and backpressures (1MPa, 2 MPa, 3 MPa). Two CCD cameras were adopted to acquire the macro spray images at the minor and major view planessimultaneously. Macroscopic spray characteristics from different view planes of spray were processed by MATLAB. The results indicated that the spray width of the elliptical spray was greater than that of the circular spray in all the view planes. Elliptical orifice spray exhibited axis-switching phenomenon during the injection process even at high injection pressure and backpressure. The increasing of backpressure enhances the aerodynamic effects, resulting in suppressing axis-switching of elliptical spray. However, increasing the injection pressure is beneficial for improving the axis-switching frequency, because the higher spray inertia resulting from higher injection pressure can cause greater spray surface-instabilities, which are conductive to enhance the axis-switching. Moreover, the spray width at the downstream increases as the backpressure increases, and for all geometrical orifices exhibited greater spray width under the higher injection pressure. In addition, the spray tip penetration of circular orifice was longer than that of elliptical orifice during the whole injection process. This is due to the larger spray surface of elliptical spray resulting in larger aerodynamic drag. Furthermore, the spray cone angle of the circular orifice is consistently smaller than that of elliptical orifice under typical diesel engine conditions. Because the axis-switching phenomenon of elliptical spray is helpful to improve the air entrainment ratio and thus resulting in larger spray cone angle. Finally, the present experiment results indicate that elliptical sprays are conductive to increase air entrainment ratio and air–fuel mixing quality as compared to circular sprays, due to the axis-switching phenomenon, larger spray cone angle and width.

The effects of nozzle geometry and equivalence ratio on a premixed reacting jet in vitiated cross-flow

The effects of nozzle geometry, jet equivalence ratio (ϕj), and momentum flux ratio (J) on the flow field, stability characteristics, and flame topology of a premixed ethylene–air jet injected transverse to a vitiated cross-flow are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to simulate the environment of a secondary combustor in a staged combustion system. The dependence of the flame liftoff height on J suggests that at these conditions the flame stabilization process is flame-propagation controlled rather than autoignition assisted. A circular nozzle and high aspect ratio slotted nozzle of identical exit area were investigated for jet to cross-flow momentum flux ratios ranging from 5 to 65 for jet equivalence ratios of up to ϕj = 5.0. High-speed particle image velocimetry was utilized to study the time-averaged flow field and OH* chemiluminescence was used to capture time-averaged and instantaneous features of the flame behavior. The nozzle geometry was determined to have a significant effect on RJICF flame stability, with substantially expanded blow-out limits for the slotted nozzle. Enhanced operability of the high aspect ratio slotted nozzle was shown to be attributable to the substantially larger and stronger recirculation zone on the leeward side of the jet when compared to the circular nozzle. This area is characterized by a more disperse region of elevated vorticity levels, resulting in the entrainment of more hot combustion products with a longer residence time in the recirculation zone, which in turn provides a stronger and more stable ignition source to the oncoming, unburned reactants. A correlation for the isothermal JICF trajectory was modified to account for gas expansion effects and found to satisfactorily capture the jet trajectory for both the non-reacting and reacting slotted nozzle. The jet trajectory was demonstrated to be independent of ϕj, whereas the jet flame penetration decreases as ϕj increases, indicating that where the flame situates itself is determined by the local mixture concentration rather than changes in the flow field. Nevertheless, ϕj was also found to have an effect on the mean flow field, with slightly higher magnitudes of reversed flow velocity and an increase in mean recirculation zone length observed as the fuel content of the jet is increased.

Partial admission effect on the performance and vibration of a supersonic impulse turbine

This study experimentally investigates the effects of partial admission on the performance and vibration outcomes of a supersonic impulse turbine with circular nozzles.The turbine of a turbopump for a gas-generator-type liquid rocket engine in the Korea Space Launch Vehicle-II is of the supersonic impulse type with the partial admission configuration for obtaining a high specific power.Partial admission turbines with a low-flow-rate working gas exhibit benefits over turbines with full admission, such as loss reduction, ease of controllability of the turbine power output, and simple turbine configurations with separate starting sections. However, the radial force of the turbine rotor due to the partial admission causes an increase in turbine vibration.Few experimental studies have previously been conducted regarding the partial admission effects on supersonic impulse turbines with circular nozzles.In the present study, performance tests of supersonic impulse turbines with circular nozzles were conducted for various partial admission ratios using a turbine test facility with high-pressure air in order to investigate the resulting aerodynamic performance and vibration. Four types of turbines with partial admission ratios of 0.17, 0.42, 0.75 and 0.83 were tested.Results show that the efficiencies at the design point increase linearly as the partial admission ratios increase. Moreover, as the velocity ratios increase, the difference in efficiency from the reference turbine with a partial admission ratio of 0.83 becomes increasingly significant, and the magnitudes of these differences are proportional to the square of the velocity ratios.Likewise, the decrease in the partial admission ratio results in an increase in the turbine vibration level owing to the increase in the radial force.

High-Speed Additive Manufacturing Through High-Aspect-Ratio Nozzles

The feasibility of layer-by-layer manufacturing through high-aspect-ratio (HAR) nozzles for microextrusion of paste to deposit planes has been investigated. Various conditions for paste extrusion, including nozzle moving speed, piston speed, extrusion rate, and distance between the nozzle tip and substrate, have been evaluated. By linking various microextrusion parameters together with the aid of a critical distance concept derived from microextrusion using circular nozzles and addressing the extrusion delay in response to the change of the piston speed and air pocket problems properly, we successfully microextruded single planes, multilayer objects, and larger planes made of multiple smaller planes side by side through HAR nozzles. It is further demonstrated that the X–Y dimensions of an extruded plane in the steady-state extrusion stage are determined by the nozzle travel distance and the length of the HAR nozzle opening if microextrusion is conducted with proper conditions. However, the height of the extruded plane is not only determined by the microextrusion conditions, but also affected by the drying shrinkage of the paste after microextrusion. This demonstration of the feasibility of using a HAR nozzle machine opens the door to manufacture of multimaterial, multilayer devices with high productivity in the near future.

Influence of Nozzle Expansion Ratio of Circular and Rectangular Cross-Section Nozzle on Velocity and Temperature of Copper Particle with HP Cold Spraying by CFD and experimental analysis

Influence of Nozzle Expansion Ratio of Circular and Rectangular Cross-Section Nozzle on Velocity and Temperature of Copper Particle with HP Cold Spraying by CFD and experimental analysis

Experimental and Parametric Optimization of Micro Holes on ANSI 304 Using Micro Abrasive Jet Machining (MAJM)

Micro Abrasive jet machining (MAJM) is one of the most convenient micromachining technologies for hard and brittle materials. Now a day’s mostly ceramics and semiconductor materials are used for manufacturing electronic devices and micro fluidic channels. This process is having various distinct advantages, as compare to other micro-fabrication techniques this method is represented a very high erosion rate. In this paper attempt has been made to develop MAJM for difficult to machine materials such as stainless steel ANSI 304 plate. In order to raise the machining productivity of micromachining, the new design and fabrication of Laval nozzle has been first reported. Circular cross sectional nozzle is developed for high velocity, precise etching and patterning on difficult to machine material such as stainless steel ANSI 304 Plate. The present study focus the effect of different parameters like air pressure, abrasive material, abrasive mesh size, nozzle diameter, guiding pressure and Standoff distance on machining performance is investigated using Taguchi’s Design of Experiment methodology. Analysis of variance (ANOVA) is implementing to find out the significant influence of each factor.

Experimental Investigation of Heat Transfer Characteristics on a Flat Surface by Circular Air Jet Impingement

An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length to diameter ratio(l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet to plate spacing between 0.5 and 8 nozzle diameter. The local het transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6.