Jet Potential Core Length Research Articles

Computational Study on the Effect of Vane Design in Enhancing the Mixing of Subsonic Jet and Sonic Jet

The purpose of this study is to numerically analyze the effect of vortex generators that are shaped like vanes in enhancing the mixing of subsonic and sonic jet and to determine the best design which yields maximum reduction in jet potential core length and minimum thrust loss at the nozzle exit. Four different nozzle designs namely, models A, B, C and D are designed and compared with a base nozzle which is a plain circular nozzle without any vanes. The simulation is performed in ANSYS Fluent using the S-A turbulence model. The centerline pressure decay and radial pressure decay from models A to D are compared with that of the base nozzle to determine the ability of the vane to enhance the jet mixing characteristics. To evaluate the thrust loss, the total pressure at the exit plane of models A to D is measured and compared with that of the base nozzle. When comparing all the designs, it is observed that Model B produces the highest reduction in potential core length which is 66.4% at Mach no. 1 and Model D produces minimum total pressure loss which is 0.47% at Mach no. 0.4. In contrast to the conventional method, this design introduces a novel approach by placing the vanes parallel to the flow instead of the usual perpendicular arrangement. This unique configuration allows the vanes to redirect the flow rather than hinder it, resulting in a total pressure loss of less than 3%.

Control of subsonic jets using vanes as vortex generators

Abstract The passive control of jets using vanes as vortex generators is studied by numerical simulation in this paper. The vanes are positioned inside the nozzle near the exit, inclined to the flow with the longitudinal direction of the jet. Two configurations namely, straight vanes (k = 0 mm−1) and curved vanes (k = 0.05 mm−1) are considered. Curvature k is defined as the reciprocal of the radius of the vanes. The blockage due to the presence of the vanes is 0.5%. The total pressure variation along the jet centreline and along the radial distance is determined from nozzle exit at a Mach number of 0.4, 0.6 and 0.8. It is found that the vanes cause faster decay of the jet, both in the near field and far field compared to the base nozzle (plain circular nozzle) and the curved vanes perform better than the straight vanes in promoting the jet mixing. A maximum of 54% reduction in jet potential core length is achieved by the curved vanes and the jet becomes asymmetrical due to the presence of the vanes inside the nozzle, as observed in the radial pressure decay plots and Mach number contours.

Numerical investigation of jet array impingement cooling with effusion holes

• Heat transfer characteristics of jet array impingement with effusion holes are examined. • A wide range of geometrical and flow parameters are numerically investigated. • The length of jet potential core has a significant effect on flow structure and heat transfer. • A new correlation for predicting the average Nusselt number is proposed. Although impingement/effusion cooling systems have been extensively studied over the past several decades, studies on flow and heat transfer characteristics over a wide range of geometrical and flow parameters are still lacking. Moreover, most studies have been related to gas-turbine blade cooling, and only a few studies have focused on gas-turbine casing cooling. In addition, studies that attempted to develop a Nusselt number correlation for jet array impingement with effusion holes––where these holes are installed in the impingement plate––are scarce. This study primarily aimed to explore the heat transfer characteristics of jet array impingement with effusion holes and to provide basic data for the design of a gas-turbine casing cooling system. Perforated circular holes in two parallel plates were arranged in a staggered configuration. The air injected through the jet holes impacts the impingement plate, and the spent air from the impinging jets is then discharged through the effusion holes in the impingement plate. A three-dimensional numerical unit cell model was constructed to investigate the effect of the hole pitch, distance between the jet plate and the impingement plate, Reynolds number based on the hole diameter, and hole diameter on the impingement heat transfer. The numerical model was validated by comparing its numerical predictions for a wide range of geometric parameters using previous experimental data. The local and average Nusselt numbers on the impingement plane were obtained for 74 simulation cases, and the results show that the jet potential core has a significant influence on the flow structure and heat transfer in a confined jet impingement. The accuracy of previous Nusselt number correlations for a jet array impingement was assessed, and we proposed a new correlation for predicting the average Nusselt number for a jet array impingement with effusion holes.

Flow and aeroacoustic attributes of highly-heated transitional rectangular supersonic jets

Heated transitional supersonic jets exhausting from a rectangular nozzle at over-expanded conditions are investigated by Large Eddy Simulations and Ffowcs-Williams and Hawkings acoustic analogy. Four cases with a fixed nozzle pressure ratio but different temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0 are analyzed. Numerical results show that with the increasing temperature the jet velocity significantly increases, whereas its Reynolds number decreases by about one order of magnitude, which leads to a 30% decrease in the jet potential core length and reduction in the number of shock cells. The increasing temperatures also result in supersonic shear layer convection Mach numbers and consequently Mach wave radiations in the acoustic fields. Pressure skewness and kurtosis factors indicate crackle noise and non-linear propagation effects in high temperatures. For the most heated jet TR 7.0, the Mach wave radiation is identified radiating noise at about 120 degrees, while the large turbulence structure noise at about 150 degrees. Furthermore, the vortex sheet model analysis and the LES data detect the existence of upstream-propagating neutral waves inside jet TR 7.0. The observed screech frequency falls within the range of antisymmetric mode indicating that the highly-heated jet is characterized by an antisymmetric oscillation mode at the screech frequency.

Effects of fuel input on coherent jet length at various ambient temperatures

The electric arc furnace (EAF) is used to produce steel, primarily from scrap, using electricity and chemical energy. Typically, the EAF process consists of two stages: melting and refining. During the refining stage, high speed coherent gas jets are used to increase the depth of oxygen jet penetration for a better stirring of the liquid steel and to promote decarburization reactions. A coherent jet is defined when the supersonic gas jet is shrouded by a flame envelope, which leads to a higher potential core length (the length up to which the axial jet velocity equals the exit velocity at the nozzle). In this paper, a coherent jet computational fluid dynamics (CFD) model is developed with detailed consideration of compressible gas flow properties and oxy-fuel combustion effects. The model is validated by comparing the coherent jet axial velocity profiles with experimental measurements. The developed CFD model was used to investigate (1) the effect of ambient temperature on the coherent jet potential core length, and (2) at high-temperature steelmaking conditions, the effects of fuel composition on the coherent jet potential core length. The results indicate that the coherent jet potential core length increases with the ambient temperature in the range of 300–1922 K. At high-temperature steelmaking conditions, when the fuel input exceeds 6.5% of the primary oxygen volume flow rate, the potential core length of the coherent jet will not increase significantly. Three different types of fuel, blast furnace gas, natural gas, and coke oven gas were used to shroud a coherent jet, and the results indicate that a lower shrouding fuel molecular weight or gas density will increase coherent jet potential core length. This validated CFD model can be used to investigate the effects of burner operation parameters on coherent jet properties for industrial EAFs.

Investigation of Supersonic Oxygen Jet Potential Core Length at Various Ambient Temperatures

A computational fluid dynamics (CFD) model for a supersonic oxygen jet has been developed with detailed consideration of compressible flow properties and ambient temperature conditions. The model was validated by comparison of the axial velocity of the supersonic jet predicted by the CFD model with experimental measurements under different ambient temperatures. The effects of the turbulence model were studied. A modified standard k–epsilon model was used to consider turbulence mixing due to both compressibility and the temperature gradient. Overall, the predictions of the CFD model showed good agreement with experimental measurements for all temperature conditions. The potential core length of the supersonic oxygen jet was studied and compared with the results of empirical models, indicating good agreement with that used by Sumi et al. Parametric study on the ambient temperature was conducted, revealing that the potential core length of the supersonic oxygen jet increases linearly with the ambient temperature.

Investigation on Coherent Jet Potential Core Length in an Electric Arc Furnace

During the electric arc furnace (EAF) refining stage, coherent jets are used to increase the depth of oxygen jet penetration for better oxygen efficiency and stirring of the liquid steel. A coherent jet is defined when the supersonic jet is shrouded by a flame envelope, which leads to a higher potential core length (the length up to which the axial jet velocity equals the exit velocity at the nozzle). In this paper, a coherent jet computational fluid dynamics (CFD) model is developed with detailed consideration of compressible flow properties and combustion effects. The model is validated by comparing the coherent jet axial velocity profile with experimental measurements. Simulation results show that the combustion reaction mechanism used has a significant effect on prediction of flame temperature, which will affect potential core length. The current model uses a 28‐step reaction mechanism to achieve both computational efficiency and model accuracy. Turbulence model modification is necessary for accurately predicting the flow properties. The current proposed modifications are needed for accurate prediction of both compressibility and temperature gradient effects.

Numerical simulation of jet mixing enhancement using rectangular control jets

The effect of rectangular control jet on the mixing of round jet with ambient air was studied numerically, in the present work. The effect on the length of main jet potential core and the penetration depth of the control jet, due to variations in velocity and width of the control jet, was studied. The mechanism of jet mixing enhancement was also studied. The results indicated that due to the rectangle jets, the main core region was bifurcated. Thus, there were two definitions for the length of main jet core: the length of centerline potential core and the length of actual potential core. Both lengths decreased with increase in control jet velocity. Besides, the length of the actual potential core decreased with increase in control jet width. However, for the length of centerline potential core, there was a specific width, at which the length of centerline potential core reached a minimum value. Finally, the deformation of the main jet was a factor, more critical compared to the streamwise vortices, which had an effect on the control jet.

Flow characteristic of highly underexpanded jets from various nozzle geometries

Flow characteristics of highly underexpanded jets at the same nozzle pressure ratio of 5.60 but issuing from four different nozzles, i.e., the circular, elliptic, square, and rectangular nozzles, are studied using large eddy simulations. The results show that the square jet penetrates fastest, although the turbulence transition is similar for different jets. The penetration rates of different jets show the similar linear dependency on the square root of time, but the penetration constant Γ for the noncircular jets deviates more than 5% from the theoretical value of 3.0. The circular and square jets both correspond to a three-dimensional helical instability mode, while the elliptic and rectangular jets have a two-dimensional flapping instability in their minor axis planes. All the jets undergo a Mach reflection forming the Mach disk, but the Mach disk in the elliptic and rectangular jets is not easily visible. The intercepting shocks in the square jet originate at the four corners of the nozzle exit at first, while the formation of the intercepting shocks is only observed in the major axis planes for the elliptic and rectangular jets. In addition, great differences are observed on the mixing characteristics between different jets. In particular, the elliptic jet penetrates slowest, has the shortest length of jet potential core, and takes the largest mixing area.

An investigation on supersonic bevelled nozzle jets

This paper reports upon a numerical and experimental study on supersonic jets exhausting from bevelled nozzles with 30° and 60° exit inclination angles. To begin with, a simple but effective method to design a shock-free convergent–divergent circular jet nozzle to produce a well-conditioned supersonic Ma = 1.5 jet based on simple circular fillets is presented. Subsequently Reynolds-Averaged Navier–Stokes simulations of circular non-bevelled and bevelled nozzle jets were performed at over-expanded, perfectly expanded and under-expanded conditions. Lastly, these supersonic jets were visualized experimentally using a modified Z-type Schlieren system. Results show that the shock cell structure within the jet potential core changes from a diamond pattern to triangular and rectangular patterns as the nozzle pressure ratio and inclination angle are varied. Furthermore, jet plumes are deflected differently when the bevelled nozzles were operated at off-design conditions, with changes to the supersonic jet potential core length. Finally, quantitative analysis of the results reveals that bevelled nozzles are able to reduce the intensity of the supersonic jet shock cell structure considerably, which is potentially useful for broadband shock-associated noise mitigation purposes.

Numerical Simulation of Supersonic Oxygen Jets at High Ambient Temperature

In this paper, the behavior of cold supersonic oxygen jets entering into high-temperature environment is predicted with four two-equation turbulence models and they are the standard k–ϵ model, the realizable k–ϵ model, the standard k–ω model, and the SST k–ω model. The comparison with experimental results in the literature shows that the standard k–ω turbulence model is superior to the other models when simulating the supersonic jet at high temperature. On this basis, the characteristics of supersonic oxygen jets at high temperature especially at steelmaking temperature are studied. The simulation results indicate that the attenuation of axial velocity of jets slows down and the length of jet potential core extends when increasing the ambient temperature. Meanwhile, the dynamic pressure of multiple jets near the lance centerline increases as the ambient temperature rises. In addition, the effects of ambient temperature on the dynamic pressure of the jet core and the coalescence of multiple jets are also discussed.

English

In the present work, the effect of nozzle angle (22.5º, 45º and 67.5º) on mixing time for jet mixing tanks with the various ratios of liquid height (H) to tank diameter (D), including 0.5, 1, and 1.5, are studied by using computational fluid dynamics (CFD). The results revealed that CFD model with standard k-epsilon is successfully employed to predict the concentration profiles and mixing time by using the fine mesh and second order upwind scheme. The simulated results showed that the different jet nozzle angles result in different flow patterns. The results also indicate that the mixing time is mainly a function of the jet potential core length. Moreover, the jet path length or jet centerline velocity (jet kinetic energy) is considered as the secondary effect on mixing time, which depends on the tank geometry. Key words: Computational fluid dynamics (CFD), jet, mixing, turbulence, k-epsilon model.

Influences of ambient pressure and nozzle-to-target distance on SC-CO2 jet impingement and perforation

It is reported that supercritical carbon dioxide (SC-CO2) jet can efficiently erode rocks at a comparably low threshold pressure with high penetration rate. However, the influences of bottom-hole pressure and borehole irregularities on SC-CO2 jet cutting efficiency have not been studied. Therefore, in this research, comprehensive methods of numerical simulations and lab experiments were carried out to investigate the influences of the ambient pressure and the nozzle-to-target distance on the jet impinging pressure and perforation performance. Results show that, both the effective jet impinging pressure and the eroded depth of perforation hole notably decrease with the increase of the ambient pressure, when jet inlet pressure is constant. When the pressure difference between inlet pressure and ambient pressure is kept constant, the effective impinging pressure hardly changes with ambient pressure, but eroded depth increases at first and then decreases, bounded by the critical pressure of CO2. As the nozzle-to-target distance extends, both the depth and volume of the perforation hole decrease, and diameter of the perforation hole increases at first and then decreases. Under the research condition of certain pressure and temperature, a distance of 8 times the nozzle diameter is the critical distance that clarifies the different effect of nozzle-to-target distance on the hole diameter. Different features between using SC-CO2 jet and water jet, including the optimal distance for rock-erosion efficiency, are attributed to the difference in fluid properties. Further simulation results show that, unlike water, SC-CO2 fluid is compressible and it leads to specific variations of jet structure, namely the increase of length of SC-CO2 jet potential core, and flow type from non-submerged jet to submerged jet. As a consequence, under the simulated bottom-hole conditions, SC-CO2 jet is proven to be able to acquire high rock-erosion efficiency and the jet-assisted drilling rate at a larger application ranges. This research can promote the future application of SC-CO2 jets.

Mixing Characteristics of Elliptical and Rectangular Subsonic Jets with Swirling Co-flow

AbstractThis paper presents a computational analysis of effects of swirling co-flow and non-circular subsonic compressible inner jets on centerline velocity decay, mass entrainment and jet spreading rate. Three different exit shapes of elliptical, rectangular and circular inner jets were compared for three different co-flow conditions such as no co-flow, straight co-flow and swirling co-flow. Co-flow is issuing from a circular annular duct. Swirling co-flow is created in the co-flow duct by introducing a swirler with stationary angular vanes of 50° oblique to the jet axis. Reynolds number of inner jet is calculated based on its equivalent diameter as 200342. It is found that the swirling co-flow has strong influence on the boundary condition of inner jet and alters the major features of the jet such as jet potential core length, centerline velocity decay rate and jet spread rate. Streamwise corner vortices of different jet conditions have been captured using velocity vector plot to show the effect of swirling co-flow on the jet flow field. Swirling co-flow with elliptical inner jet exhibits higher velocity decay rate and jet spreading rate than the equivalent area circular and rectangular jet.

Subsonic Jet Mixing via Active Control Using Steady and Pulsed Control Jets

A comparison of the performance of steady and pulsed control jets in enhancing the mixing rate of a high Reynolds number (10 6 ) high subsonic Mach number (0.9) jet is reported. The key parameters affecting control jet performance are identified as control jet pressure ratio, Strouhal number (St), and mode number (m), and an existing nozzle test facility is modified to allow detailed investigation of these parameters. Optimization of the control jet design lead to increased mixing, such that a primary jet potential core length reduction of 50% was achieved. The increase in jet/ ambient entrainment interfacial area was seen as an important factor. Pulsation at St = 0.22 in an antisymmetric mode (m = 1) gave the best results. The data provided, based on extensive laser Doppler anemometry mean velocity and turbulence measurements, allowed an improved understanding of the physical processes that are crucial for enhanced mixing and contribute a useful new data set for computational fluid dynamics validation of rapid mixing jet flows.

オリフィス自由噴流の混合・拡散特性の改善・制御

Mixing and spreading characteristics of an orifice free jet are investigated changing the orifice plate thickness t/do, where do is the nozzle exit diameter, ranged from 0.01 to 1.5. Schlieren images show high mixing and spreading performances at t/do = 0.5 and Reynolds number Re = 1.5×104 because at the t/do = 0.5 the jet fluctuates by the separated flow from the inner edge of orifice alternates attachment and detachment to the outer edge. Hot-wire anemometry measurements are also performed to investigate the mean and fluctuating velocity profiles. It is found that the jet potential core length is insensitive to Re at Re > 7.0 × 103, but significantly influenced by the orifice plate thickness. The potential core length for t/do = 0.5 is the shortest reflecting the fluctuation at the nozzle exit.

Active control of high-speed and high-Reynolds-number jets using plasma actuators

Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

Development and use of localized arc filament plasma actuators for high-speed flow control

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0–200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10–20 µs of arc operation, from below 1000 °C to up to about 2000 °C. At longer discharge pulse durations, 20–100 µs, the plasma temperature levels off at approximately 2000 °C.Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

The influence of counterflow on the mixing and acoustic characteristics of a Mach 1.4 rectangular jet operated at on- and off-design conditions were studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear-layer mixing and reduces the jet potential core length under all operating conditions. Significant changes in both shock-cell spacing and strength were ohserved when counterflow was applied to nonideally expanded jets. Consequently, screech tones were either reduced or totally eliminated, and broadband shock-associated noise was shifted to higher frequencies. In the underexpanded mode, a Mach disk was formed at certain levels of counterflow, which substantially weakened the subsequent periodic shock-cell structure and reduced the broadband shock-associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Interestingly, it was also discovered that a jet operating at overexpanded conditions could be decelerated nearly isentropically by applying the proper amount of counterflow. This modification led to a 4 dB reduction in OASPL. Based on the present study, it is suggested that counterflow warrants further investigation as a potential noise reduction technique.

Heat Transfer Characteristics on Impingement Surface with Control of Axisymmetric Jet(I)

The present experiment is conducted to investigate heat transfer characteristics on the impinging surface with secondary flows around circular nozzle jets. The changed vortex pattern around jet affects significantly the flow characteristics and heat transfer coefficients on the impinging surface. The effects of the jet vortex control are also considered with jet nozzle-to-plate distances and main jet velocities. The vortex pattern around a jet is changed from a convective instability to an absolute instability with a velocity suction ratio of the main jet and the secondary counterflow. With the absolute instability condition, the jet potential core length increases and the heat transfer on the impinging surface is increased by small scale eddies. The region of high heat transfer coefficients is enlarged with the high Reynolds number due to increasing secondary peak values. The effect of suction flows is influenced largely with collars attached the exit of the jet nozzle because the attached collar guides well the counterflow around the main jet.