Potential Core Length Research Articles

Effect of pulsation on flow and thermal characteristics of a wall jet

In the current investigation, a computational analysis of the thermal and dynamic characteristics of a pulsed incompressible laminar wall jet is performed under a forced convection regime. The considered wall is isothermal with a temperature dissimilar from that of the surrounding fluid. The nozzle outlet is marked by a periodic and parabolic profile of the longitudinal velocity as well as a uniform temperature. The dimensionless transport equations governing the heat and momentum transfer phenomenon are solved using the finite difference method with irregular mesh. Our results are validated against those which emanate from the non-pulsed jet in its different regions. The spatial and temporal evolution of the wall jet characteristics such as the velocities, the friction coefficient, the jet spreading, and the Nusselt number are presented and analyzed. It was revealed that the inclusion of pulsation leads to the spreading of the jet and the curtailment of the potential core length relative to a steady jet. Moreover, the pulsation strengthens the entrainment and mixing of the ambient fluid and therefore improves the heat transfer in the potential cone area.

Simulation and application of post-combustion oxygen lance in a top-blown converter

ABSTRACT Flow field characteristics of conventional four-hole oxygen lance and single-flow post-combustion (PC) oxygen lance at steelmaking temperature are studied by numerical simulation, and the industrial trials are carried out in the 120 t top-blown converter to evaluate their metallurgical effects. The simulation results indicate that with the increase of ambient temperature, the potential core length of oxygen jet, jet velocity and radial width (jet diameter) increase. And the jet characteristics of PC oxygen lance are better than those of conventional lance in the radial and axial directions. The industrial trials show that, compared with the conventional lance, using the PC oxygen lance can shorten blowing time by 93 s, reduce actual oxygen consumption by 3.56 m3 t−1 and increase dephosphorization rate by 1.67%. Meanwhile, PC oxygen lance is better for slagging and solves the problem of splashing or drying caused by increasing the oxygen flow rate of the conventional lance.

Impingement cooling of an isoflux flat plate by blockage jet

Impingement cooling of an isoflux flat plate by a round jet, generated through a perforated blockage plate, is experimentally investigated at a fixed jet Reynolds number of Rej = 20,000. With the perforation diameter (Dj) fixed, varying the thickness (t) of blockage plate forms three distinct turbulent jets: (1) orifice jet (e.g., t/Dj ≤ 0.5), (2) blockage jet (e.g., 0.5 < t/Dj < 8.0), and (3) tube (or nozzle) jet (e.g., t/Dj ≥ 8.0). Thermofluidic characteristics of these jets in both free exit and impingement were measured and directly compared under consistent conditions. Particular focus was placed upon how these characteristics vary in the intermediate range of jet relative thickness t/Dj, with reference to the two limiting cases, namely, the orifice jet and the tube jet. It was demonstrated that the blockage jet behaves neither as an orifice jet, nor as a tube jet, and its properties cannot be considered interchangeable. Measurements of jet flow properties - exit velocity profile, potential core length and centerline turbulence - highlight the distinguishing features among the three jet types. While the blockage jet exhibits an intermediate thermal performance, the tube jet provides the poorest performance, and the orifice jet performs the best. Since it is not always possible to use an orifice jet in practice (e.g., due to structural constraints), this study provides evidence for the existence of optimal H/Dj for a given jet type (i.e., fixed t/Dj): correspondingly, both primary and secondary thermal peaks shift when t/Dj and H/Dj are varied.

Pressure Drop Model and Jet Features of Ultra High Pressure Water Jet for Downhole Intensifier

Abstract Existing models can hardly provide a reliable guidance in designing intensifier utilizing axial vibration energy of drill strings. The formation process of jet and the pulsation characteristics of flow field had not been reported. Therefore, a new model combined plunger movement and nozzle pressure drop is proposed. The jet formation process, characteristic of water jet distribution, pressurization performance, and impinging pressure under different parameters were studied based on a computational fluid dynamics (CFD) method. The results show that there is a good agreement between the model and the CFD. Increasing the plunger diameter can prolong the holding time of the high-pressure water jet and increase the peak pressure, but it does not affect the pressurization period, which is mainly influenced by the spring. There are three representative stages observed in the forming process of high-pressure water jet, and in stable potential core stage, the inlet pressure does not affect the length of potential core, which is about seven times the nozzle diameter. The impinging pressure decreases with the radial distance, the maximum of which is at the center of the jet. The stand-off distance should be less than 7d, otherwise the impinging pressure will decrease sharply. The research can provide a reference for the optimal design of intensifier and will be beneficial for downhole pressurized jet-assisted drilling techniques.

Study on the Characteristics of Coherent Supersonic Jet with Superheated Steam

By establishing a mathematical model to simulate a mixed jet of oxygen and superheated steam from a coherent supersonic jet oxygen lance, we studied the effect of superheated steam on the fluid characteristics of the mixed jet. The model was initially verified through laboratory experiments prior to analyzing the fluid characteristics of the mixed jet in detail. These characteristics included the jet velocity, the temperature, the turbulent kinetic energy (TKE), and the mass distribution. The results showed that, at an ambient temperature of 1700 K, the jet velocity measured in the laboratory experiment was consistent with the fluid velocity obtained by numerical simulations, with an error of only 2.7%. In a high-temperature environment, the jet velocity of the mixed oxygen and superheated steam jet was increased, the TKE around the center jet was enhanced, the superheated steam exhibited an inhibitory effect on the combustion reaction of annular methane, and the potential core length of the coherent supersonic jet was reduced, which was conducive to methane combustion and delayed the reduction in the central jet velocity.

Effects of circular and non-circular nozzle exit geometries on subsonic and supersonic jet propagations

The mixing enhancement and core length reduction of a jet without significant loss of thrust are essential for reducing infrared radiation, mitigating aeroacoustic noise, improving combustion characteristics, and thrust vectoring. The jet mixing can be improved by manipulating the flow behavior. In subsonic and sonic jets, the flow manipulation may be achieved by utilizing nozzles with non-circular geometries that shed vortices of varying size due to their non-uniform azimuth curvatures. Non-uniform vortices generate differential spreading along the nozzle’s perimeter, causing axis switching and improving entrainment characteristics. Therefore, the present study examines the effects of two non-circular nozzle exit shapes (elliptic and square) on the mixing augmenting efficacy at subsonic and sonic flow conditions. The circular nozzle is tested for comparison. Both quantitative and qualitative analyses evaluate the efficacy of nozzles with non-circular exit geometries. Among the configurations investigated, the elliptic nozzle is superior in shortening the potential core length and enhancing the jet spread. A maximum reduction of 18.75% in core length with rapid jet decay was accomplished with the elliptic nozzle. The measurement of pressure profiles at different streamwise locations reveals that the spread rate is greater for elliptic and square jets than their circular counterpart. The elliptic jet exhibits the highest spread along the minor-axis direction compared to the major-axis direction. The differential jet spread rate in the elliptical jet causes an early axis-switching––direct evidence of mixing augmentation. Shadowgraph images show the asymmetric pattern of shock cell structures and differential spreading in elliptic and square jets.

Effect of effusion hole arrangement on jet array impingement heat transfer

Impingement/effusion cooling systems have been studied extensively over the past few decades to constantly improve cooling performance as the turbine inlet temperature of modern gas turbine engines has increased. However, a Nusselt number correlation is less developed for an impingement/effusion cooling system with effusion holes in its impingement plate than for jet array impingement without effusion holes. Moreover, Nusselt number correlations, available for a wide range of geometric and flow parameters with various hole arrangements, are sparse for gas-turbine casing cooling systems. Therefore, this study investigated the effects of these parameters on the flow and heat transfer characteristics of an impingement/effusion cooling system that uses a jet array of effusion holes to cool a gas-turbine casing. The circular holes in the parallel jet and impingement plates were arranged in five different hole arrangements. Air forced through holes in the jet plate struck the impingement plate then discharged through the effusion holes of the impingement plate. Three-dimensional simulations were performed to examine the effects of hole pitch, distance between the jet and impingement plates, ratio of total jet area to heat transfer area, and Reynolds number based on the jet hole diameter on impingement heat transfer. A total of 300 simulations were run for the five hole arrangements to obtain the local and average Nusselt numbers on the impingement plate. Our results show that the radial acceleration and turbulence near the impingement plate, and the length of the jet potential core strongly affect the impingement heat transfer. This study also evaluated previous Nusselt number correlations for jet array impingement. This paper presents a new correlation to predict the average Nusselt number of jet array impingement applicable to the five hole arrangements.

Flow characterization in the uphill region of pulsed oblique round jet

An experimental study to investigate the effect of jet pulsations on the wall jet development in the uphill region of an obliquely inclined round water jet has been performed using particle image velocimetry technique. The study has been performed at a constant nozzle to target wall distance, L/D = 4 (D is the diameter of the nozzle) by varying the jet impingement angle θ (=60o, 45o, and 30o), Reynolds numbers (ReD = 1900 and 3280; based on nozzle diameter and average nozzle exit velocity Uavg), and Strouhal number (0 ≤ St ≤ 0.9; St=fDUavg, where f is the frequency of external pulsation). It is observed that the pulsations have no significant effect on the jet in the free jet region when the target plate is kept at a distance less than the potential core length (the potential core extends up to 4D–6D from the nozzle exit toward the impinging plate), and the jet impingement region extends up to 1D from the plate. The location of the stagnation point is observed to depend on all three parameters: the jet pulsation, the Reynolds number, and the jet impingement angle. An increase in Reynolds number creates an adverse pressure gradient toward the downstream direction in the uphill region, resulting in an intrusion of ambient fluid toward the wall jet. The distance between the geometric center and the stagnation point is observed to be minimum for St = 0.44 at both the Reynolds numbers. The wall jet that develops in the uphill region exhibits a maximum velocity decay rate and a jet half width growth rate corresponding to St = 0.44. These parameters are also observed to increase with the increase in the Reynolds number and decrease in the jet impingement angle. The velocity fields reconstructed using proper orthogonal decomposition reveal the dominant modes in the upstream location for St = 0.44 than the other pulsations. Furthermore, we observed that the jet after impingement deviates entirely in the downhill region for θ=30o irrespective of the jet pulsation, suggesting a non-dependence of the critical angle of inclination on jet pulsations for L/D = 4.

Numerical simulation of reynolds number effect of a free incompressible isothermal turbulent coaxial jet

This paper studies the effect of Reynolds number on a two-dimensional free incompressible isothermal coaxial turbulent jet over a range of high Reynolds numbers. This is necessary because of its application in noise control and mixing. The Reynolds numbers at the nozzle exit were 9824, 19648, 29472, 39296 and 49120. The models were designed in ANSYS Design Modeler and the numerical simulation was done using a finite volume based Computational Fluid Dynamics (CFD) in ANSYS FLUENT using the two-dimensional Realizable turbulence model. The Governing equations were discretized using the finite volume method with the solution based on the PISO algorithm. The decay of centerline velocity, turbulent kinetic energy profile, the radial profile of axial velocity and similarity profile were investigated along the flow direction. Contour plot indicates that the velocity is high at the jet exit and decreases downstream due to the rapid mixing of the inner and outer jet and the surrounding fluid. It is found generally that Reynolds number plays significant role especially before self-similarity region. The result shows that increasing the Reynolds number give rise to more turbulence which in turn decreases the potential core length, turbulent kinetic energy and enhances the mixing of the fluid. However, at the jet exit, the flow with the lowest Reynolds number has the highest turbulent kinetic energy because it suffers the greater shear. The spreading of the jet was more or less independent of the Reynolds number beyond the self-similarity region. It is also found that the velocity profile is brought to congruence at about z/D=25 for the Reynolds numbers considered

Flow characteristics of a rectangular jet with DBD plasma actuators at nozzle exit

The objective of this study is to establish an active flow control method for a 2:1 rectangular jet using a dielectric barrier discharge plasma actuator (DBD-PA) installed at the exit of a nozzle. These additional devices are able to change the velocity distribution of a jet stream by adding an induced flow to the airflow ejected from a rectangular nozzle. These DBD-PA were placed on the short and long sides, and on the four corners of the nozzle exit. The rectangular nozzle had a height of 30 mm and a width of 60 mm. The Reynolds number of the jet was 9,000. The flow field was measured by particle image velocimetry (PIV). The flow field was visualized by tracer injection method. The DBD-PA has an influence on the formation of the large-scale vortical structure in the share layers of the rectangular jet. For the jet with DBD-PA on the long sides of the nozzle, the length of potential core increases more than that with DBD-PA on the short sides and the four corners of the nozzle. The spread of a rectangular jet in the x-z cross-section is significantly affected by the DBD-PA installed at the short sides and the four corners of the nozzle.

Examination of Reynolds number effect on the development of round jet flow

In this study, the Reynolds number effect on the development of round jet flow is presented. The jet is produced from a smoothly contracting round nozzle and the flow structure is controlled by varying the air blower speed in order to obtain various Reynolds numbers (Re). The flow Reynolds number considered varies between 1140 and 9117. Mean velocity measurements were taken using hot-wire probe at different axial and lateral distances (0≤x/d≤50, where x is the downstream distance and d is the nozzle diameter) for the jet flow and at for 0≤x/d≤30 in long pipe attached to the nozzle. Measurements reveal that Reynolds number dictate the potential core length such that the higher the Reynolds number, the lower the potential core which is a measure of mixing of jet and ambient fluid. It shows that further away from the jet exit section, potential core decreases as Reynolds number increases, the velocity profile has a top hat shape very close to the nozzle exit and the shape is independent of Reynolds number. It is found that potential core extends up to x/d=8 for Reynolds number of 1140 as against conventional near field 0≤x/d≤6. This may suggest effect of very low Reynolds number. However, further investigation is required to ascertain this at extremely low Reynolds numbers. It is also observed that further away from the jet exit section, the higher the downstream distance, the higher the jet half-width (R1/2). Furthermore, the flow in the pipe shows almost constant value of normalised axial centerline velocity for a longer distance and this clearly indicates that there is mass redistribution rather than entrainment of ambient fluid. Overall, the Reynolds number controls the magnitude rather than the wavelength of the oscillation

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.

Analysis of submerged single and twin gas jet injection using URANS-CFD simulations

This paper addresses the analysis and characteristics of submerged underexpanded air jets injected into water by means of single and twin tuyeres (i.e. straight tubes). First, a Computational Fluid Dynamics simulation of a single tuyere is compared with available experimental data found in the literature. In the twin tuyere configuration, spacing is varied and the resultant flow is compared to that obtained with the single tuyere configuration. When two tuyeres are closely placed, compared to a single tuyere injecting the same mass flow, an increase in the potential core of each jet and a combined core are found, along with an increase in mixing levels. With larger tuyere spacing, individual cores, shorter potential core length and a significant increase in mixing levels are found. In addition, a general selection guideline is presented to help the design of a process involving gas being injected into a liquid.

The Behavior of Supersonic Jets Generated by Combination Gas in the Steelmaking Process.

In the duplex steelmaking process, the oxygen flow rate is suppressed to reduce the increasing rate of the temperature in the molten bath, resulting in severe dynamic conditions. To improve the mixing effect of the molten bath, a Laval nozzle structure designed for combination gas has been proposed. In this research, five types of Laval nozzle structure have been built based on the combination gas content, and both numerical simulations and experiments are performed to analyze the flow field of the supersonic jet. The axial velocity and oxygen concentration were measured in the experiment, which agreed well with the numerically simulated data. The results show that both initial axial velocity and potential core length increase with the flow rate of combination gas. Further, applying a higher N2 flow rate could improve the oxygen utilization rate at different ambient temperatures, but this issue increases the oxygen utilization rate; however, the latter can be reduced at higher ambient temperatures.

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.

PIV investigation of high Reynolds number submerged water jets at high-pressure ambient conditions

High-pressure water jets bear a great technological potential to enhance geothermal deep drilling. Compared to existing water cutting technologies, significantly different operation conditions are encountered under deep-drilling conditions, such as high ambient pressures. The fundamental fluid mechanics are significantly affected by those operation conditions. In this work we examine the influence of increasing ambient pressure of up to 12.0 MPa on the water jet characteristics under submerged drilling conditions. PIV measurements of the jet flow field at changing cavitation numbers reveal two characteristic regimes, which are distinguished by a critical cavitation number. In the cavitating regime, the jet decays considerably faster with increasing distance to the nozzle than in the non-cavitating regime. In addition to that, an increasing cavitation intensity shortens the potential core length of the water jet and increases the jet spreading angle and with this has a similar effect on the jet as increasing turbulence intensity in single-phase flows. Related to the decreasing kinetic energy of the jet in the cavitating regime, the resulting impact force of the water jet on the specimen surface decreases with increasing cavitation intensity. Our investigations indicate that a technology transfer from water jet cutting to submerged jet drilling requires adjustments of both nozzle geometries and jet operation conditions.Graphic abstract

The near-field aerodynamic characteristics of hot high-speed jets

Abstract

Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing

This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and major axis is explained and its growth downstream the throat has a significant role in nozzle exit flow characteristics. The loss mechanism throughout the flow is shown as the entropy generated due to viscous dissipation and accounts for supersonic jet mixing. Axis switching phenomenon is also addressed by analyzing the streamwise vorticity fields at various locations downstream from the nozzle exit.

Response of supercritical round jets to various excitation modes

Abstract

Jet mixing enhancement of a supersonic twin jet nozzle using rectangular tabs

Computational study on the development of mean flow and mixing capability of a rectangular tab placed at the exit of a Mach 1.6 single and twin jet nozzles has been presented. Placing two identical conical nozzles side by side separated by a distance of 1.5 times exit diameters makes the twin jet configuration. The mitigation technique for twin jet cross coupling effects by rectangular tabs for the enhancement of aerodynamic and acoustic characteristics is validating. The results are relevant to situations wherever shock cell structures, potential core length, jet mixing related noise are of concern. Centre line pressure decay characteristics shows that there is an abrupt reduction in the core length and suppression of shock cell structure at off design conditions. The results are in good agreement with the experimental results.