Free-surface Jet Research Articles

Experimental investigation of circular free-surface jet impingement quenching: Transient hydrodynamics and heat transfer

An experimental study of heat transfer during quenching of a cylindrical stainless steel test specimen has been performed. A subcooled water jet is directed onto the upward facing flat face of the cylinder. The test specimen is heated to an initial temperature slightly above 900 °C and then quenched. The resulting boiling curves and heat transfer distributions are presented for impingement velocities of 2.85 and 6.4 m/s (Re = 7900 and 18,900). High-speed imaging shows that three distinct regions on the quenched surface can be identified: an expanding circular wetted region surrounding the impinging point, annular transition zone just outside the wetting front, and a unwetted region outside this zone. The free-surface of the liquid in the wetted region is smooth in the nucleate and transition boiling regimes. The annular transition zone or the wetting front region outside the wetted region is characterized by a highly disturbed liquid–gas interface, which can be attributed to intense vapor generation. At the outer edge of the transition zone, the liquid is deflected away from the surface. The velocity of the wetting front significantly increases with the jet impact velocity, which indicates that the wetting front position is governed by the ability of the flowing liquid to transport the bubbles radially outwards from the wetted region.

Free-surface liquid jet impingement on rib patterned superhydrophobic surfaces

We report experimental results characterizing the dynamics of a liquid jet impinging normally on hydrophilic, hydrophobic, and superhydrophobic surfaces spanning the Weber number (based on the jet velocity and diameter) range from 100 to 1900. The superhydrophobic surfaces are fabricated with both hydrophobically coated silicon and polydimethylsiloxane that exhibit alternating microribs and cavities. For all surfaces a transition from a thin radially moving liquid sheet occurs. This takes the form of the classical hydraulic jump for the hydrophilic surfaces but is markedly different for the hydrophobic and superhydrophobic surfaces, where the transition is significantly influenced by surface tension and a break-up into droplets is observed at high Weber number. For the superhydrophobic surfaces, the transition exhibits an elliptical shape with the major axis being aligned parallel to the ribs, concomitant with the frictional resistance being smaller in the parallel direction than in the transverse direction. However, the total projected area of the ellipse exhibits a nearly linear dependence on the jet Weber number, and was nominally invariant with varying hydrophobicity and relative size of the ribs and cavities. For the hydrophobic and superhydrophobic scenarios, the local Weber number based on the local radial velocity and local depth of the radially moving liquid sheet is observed to be of order unity at the transition location. The results also reveal that for increasing relative size of the cavities, the ratio of the ellipse axis (major-to-minor) increases.

Experimental investigation of a free-surface turbulent jet with Coanda effect

The deviation of a jet from the straight direction due to the presence of a lateral wall is investigated from the experimental point of view. This flow condition is known as Coanda jet (from the Romanian aerodynamicist Henry Marie Coanda who discovered and applied it at the beginning of XXth century) or offset jet. The objective of the work is to detail the underlying mechanisms of such a phenomenon aiming to use it as a flow control method at polluted river flows mouth. To do this, a large laboratory free-surface tank with an incoming channel has been set up and velocity field measurements are performed by Optical Flow methods (namely Feature Tracking). Preliminary tests on the well-known free jet configuration without any marine structure (i.e. lateral wall) are performed to allow comparison with free jet scaling and self-similar solutions. The presence of the free-surface gives rise to centerline velocity decay which is lower than in free unbounded plane or circular jets due to the vertically limited ambient fluid entrainment. In the second part of the paper, the effect of a lateral wall on the jet configuration is examined by placing it at different lateral distances from the jet outlet. The resulting velocity fields clearly show an inclined Coanda jet with details which seems to depend on the lateral wall distance itself. The analysis of self-similarity along the inclined jet direction reveals that for wall distances larger than 5 jet widths this dependence almost disappears.

Jet impingement quenching phenomena for hot surfaces well above the limiting temperature for solid–liquid contact

Experiments were conducted to understand the phenomena that happen just after a subcooled free-surface circular water jet impinges on a high temperature surface. A 2 mm-water-jet of 5–80 K subcooling and 3–15 m/s velocity was impinged on the flat surface of a cylindrical steel/brass block that was preheated to 500–600 °C. The transient temperature data were recorded and used to predict the surface temperature by an inverse heat conduction technique. A high-speed video camera was also employed to capture the flow condition. It is found that for a certain period of time the surface temperature remains well above the thermodynamic limiting temperature that allows stable solid–liquid contact. What happens during this period and what makes the surface temperature drop to the limiting temperature are important questions whose possible answers are given in this article. The cooling curves at the center of the impinging surface for different experimental conditions are also explained in relation with the limiting temperature and three characteristic regions having different types of flow patterns are identified.

Single-phase convection and boiling heat transfer: Confined single and array-circular impinging jets

In the present study, the effects of diverse situations of confinement on heat transfer from single and array-circular jet impingements are carefully investigated over various heat transfer regimes of single-phase convection and fully developed nucleate boiling. For the single, circular, unconfined free-surface jet, the transition to turbulence was observed to start around x/ d = 5.5 and end around x/ d = 9. For the array-circular jet, however, the wall jet structure yielded no transition to turbulence for all the tested cases, instead monotonically decreasing the convection coefficient. Conversely, the single-circular jet experienced the transition for V ⩾ 6.1 m/s. For the confined submerged jet, the transition length was very short due to the vigorous mixing driven by lateral velocity components, and the locus of the secondary peak moved downstream as velocity increased. The temperature distributions of the confined array-circular jet were fairly uniform over the whole heated surface. The averaged single-phase convection coefficients indicated that the confined jet provided the most uniform convection in the lateral direction.

An experimental investigation of free and submerged miniature liquid jet array impingement heat transfer

Liquid water jet impingement cooling was investigated experimentally for both free-surface jet arrays and confined submerged jet arrays. The jet arrays consisted of straight holes of 1.0 mm diameter arranged in rectangular arrays with spacings of 3, 5 and 7 jet diameters between adjacent jets. For the impingement surface area of 780 mm 2, these jet array configurations can be considered well populated, with a total of 21, 45 and 121 jets impinging on the surface. Average heat transfer and pressure drop measurements are presented for volumetric flow rates in the range of 2 L / min ⩽ V ˙ ⩽ 9 L / min and dimensionless jet-to-target spacings between 2 ⩽ H/ d n ⩽ 30. For the submerged jet arrays a strong dependence on both jet-to-target and jet-to-jet spacing is observed and correlations are presented that adequately predict the experimental measurements. The free-surface jets show a non-monotonic change with jet-to-target spacing with a local minimum in the heat transfer coefficient at approximately H/ d n = 10. Here a transition from a submerged to a free jet flow configuration occurs. Once again, a correlating equation is presented that adequately predicts the free-surface jet array heat transfer data. The pumping power required to form the submerged and free jet flows show a different relationship to the heat transfer coefficient. Generally, submerged jets have a higher heat transfer coefficient for a given pumping power requirement.

On the near-wall structure in reverse-flow and post-reattachment recovery regions of separated flow and its equivalence to the structure in wall and free-surface jets

The statistical and structural properties of two wall jets, one developing along a real wall and the other along a shear-free wall, are contrasted with the equivalent properties in the post-reattachment-recovery and reverse-flow regions of a separated backward-facing step flow, respectively. The study was motivated, principally, by the wish to isolate and understand the mechanisms responsible for the poor representation of the post-reattachment recovery returned by most RANS closures. It demonstrates a substantive commonality in the turbulent processes between, on the one hand, the real wall jet and post-reattachment recovery, and on the other hand, between the zero-wall-shear jet and the reattachment zone, as well as the reverse-flowing near-wall layer in the backward-facing-step flow. All regions under consideration are found to be characterised by an influential interaction between an outer shear layer and the wall, but an important distinction arises from the presence or absence of strong near-wall shear which introduces significant changes to the nature of this interaction: in the absence or near-absence of wall shear, the interaction is largely inviscid, associated with wall-blocking, while strong shear tends to shield the wall from the outer flow. The commonality between the two sets of flow conditions is explored by way of mean-flow properties, budgets, anisotropy maps, departures from local equilibrium, non-dimensional strain parameter, length-scale variations and structural properties. The study shows that the post-reattachment-recovery region, like the real wall jet, is far from the state of local equilibrium found in a standard boundary layer, that the recovery of the flow towards a state of equilibrium is slow, and that the near-wall region is strongly affected by turbulent diffusion associated with the migration of large-scale structures towards the wall. In statistical terms, this last process is represented by the importance of turbulent transport by third moments and pressure–velocity correlations. Within the reattachment and reverse–flow layer, near-wall shear is weak, and so is the shielding of the wall from the outer layer. In these regions, the flow shares many of the properties of the zero-wall-shear jet.

Zero gravity of free-surface jet flow

Zero gravity of free-surface jet flow

Free Surface Jet Flow Interaction with a Pelton Bucket

The various flow phenomena in a Pelton turbine are quite complex and include the interaction of free surface jets with buckets in a rotating frame of reference. As the use of CFD for such flows is rather new it was decided to carry out individual validations of the flow codes for the different flow features. The main focus is the interaction of the jet with the bucket. The flow structures upstream of the jet have a strong influence on the jet shape and direction as well as on the secondary flow structure in the jet. Careful validations proofed that the selected CFD codes are able to compute these flows accurately. Based on this experience the flow of the free surface jet interacting with the buckets has been computed and the results have been compared to experimental data. These comparisons are in good agreement and provide the designer of Pelton turbines with a new insight and data previously not available. Inclusion of CFD flow predictions into the design process of Pelton turbines reduces development time and opens up new opportunities for optimization.

Numerical study of jet flow generated by impact on weakly compressible liquid

The early stage of two-dimensional jet flow generated by an impulsive start of a wedge, initially floating on a free surface otherwise flat, is investigated taking into account liquid compressibility. During this short initial stage the flow close to the intersection points is self-similar. Velocity of the body is assumed much smaller than the sound speed in the liquid at rest. The acoustic solution of this problem reveals that the flow velocity is singular about the intersection points. In order to obtain a uniformly valid description of the flow, an inner solution is derived by using stretched variables, which are dependent on the Mach number of the problem. It is shown that, for small Mach numbers, the inner flow is approximately potential and is governed by the Laplace equation. The solution of the boundary-value problem is achieved numerically through an iterative procedure. A modified velocity potential, which significantly simplifies the boundary conditions on the free surface, is introduced. Accurate solutions are presented in terms of free surface shapes and jet lengths for different dead-rise angles of the wedge. The analogy between this problem and that of jetting flow caused by shock wave impact on a wedge-shaped cavity is discussed as well.

Numerical study of MHD effect on liquid metal free jet under complex magnetic fields

In this paper, we present numerical studies of liquid metal free surface jet characteristic behavior under the influence of transverse fields and field gradients. The goal of the numerical simulation study is to evaluate the usefulness of a proposed numerical scheme for MHD analysis by comparing the numerical results with experimental observations. A 3D liquid metal MHD code based on an induced magnetic field formulation was developed and a penalty factor numerical method was introduced in order to force the local divergence-free condition of the magnetic field. The proposed numerical techniques give reasonable results as compared to experimental findings at a high Hartmann number.

Radiation protection aspects of a 4 MW target

The CERN Superconducting Proton Linac (SPL) is expected to provide a 2.2 GeV, 4 MW proton beam to feed facilities such as, for example, a neutrino factory or a neutrino superbeam. Material activation in such facilities is an important aspect that has to be taken into account at an early stage in designing it. In particular, the choice of the target has consequences on the induced radioactivity and dose rates in the target station and its surroundings. In the present work, the radiological aspects of a stationary target made up of tantalum pellets are compared with those of a free-surface jet of mercury. An estimation of the hadronic inelastic interactions and the production of residual nuclei in the target, the two concentric magnetic horns, the decay tunnel, the surrounding rock and a downstream dump were performed for both targets using the Monte Carlo code FLUKA. The aim was to assess the dose-equivalent rate that is to be expected during maintenance work and to evaluate the amount of residual radioactivity, which will have to be disposed of after the facility has ceased operation. The problem of after-heat in the tantalum target and the consequences of raising the proton beam energy from 2.2 to 4 GeV were also investigated.

평면 충돌제트에서 노즐 깃이 단상 및 비등 열전달에 미치는 영향

The water jet impingement cooling is one of the techniques to remove the heat from high heat flux equipments. Local heat transfer of the confined water impinging jet and the effect of nozzle collar to enhance the heat transfer are investigated in the fee surface jet and submerged jet. Boiling is initiated from the farthest downstream and increase of the wall temperature is reduced with developing boiling, forming the flat temperature distributions. The reduction in the nozzle-to-surface distance fur H/W1 causes significant increases and distribution changes of heat transfer. Developed boiling reduces the differences of heat transfer for various conditions. The nozzle collar is employed at the nozzle exit. The distances from heated surface to nozzle collar, Hc are 0.25W, 0.5W and 1.0W. The liquid film thickness is reduced and the velocity of wall jet increases as decreased spacing of collar to heated surface. Heat transfer is enhanced fur region from the stagnation to x/W8 in the free surface jet and to x/W5 in the submerged jet. For nucleate boiling region of further downstream, the heat transfer by the nozzle collar is decreased in submerged jet comparing with higher velocity condition. It is because the increased velocity by collar is de-accelerated downstream.

Theoretical study on impingement heat transfer with single-phase free-surface slot jets

A theoretical analysis was conducted to characterize heat transfers from horizontal surfaces to normally impinging slot jets under arbitrary-heat-flux condition. The thermal and hydraulic boundary layers of laminar flow were divided into four regions of flow along heat transfer surfaces including a stagnation zone and three wall jet zones, from which general expressions of heat transfer coefficients were obtained. Furthermore, these results were compared with experimental and analytical data available in published literature. Good agreements were observed from the comparisons.

Radiological considerations on multi-MW targets. Part I: Induced radioactivity

CERN is designing a Superconducting Proton Linac (SPL) to provide a 2.2 GeV, 4 MW proton beam to feed facilities like, for example, a future Neutrino Factory or a Neutrino SuperBeam. The material activation in such facilities is an important aspect that has to be taken into account at an early design stage. In particular, the choice of the target has consequences on the induced radioactivity and dose rates in the target itself and in its surroundings. In the present work, the radiological aspects of a stationary target made up of small tantalum pellets are compared to those of a free-surface jet of mercury. An estimation of the hadronic inelastic interactions and the production of residual nuclei in the target, the magnetic horn, the decay tunnel, the surrounding rock and a downstream dump were performed for both targets by the Monte Carlo hadronic cascade code FLUKA. The aim was to assess the dose equivalent rate to be expected during maintenance work and to evaluate the amount of residual radioactivity, which will have to be disposed of after the facility has ceased operation.

Observations of high temperature impinging-jet boiling phenomena

A high-speed video camera and microphone were used to capture the flow behavior and boiling sound of a free-surface water jet impinging on a high temperature surface during quench cooling. It was found that depending on the superheat of the surface considerably different flow patterns appeared. For cases where the initial surface temperature was above about 300 °C an almost explosive pattern appeared. This was in contrast to slightly lower temperatures where a liquid sheet flow structure was apparent. The change in phenomena was accompanied by a sudden change in the boiling sound and an increase in the heat transfer rate.

Three-dimensional modeling of slab liquid jets used for heavy-ion fusion for beam line protection

IFE designs for thick liquid protection of heavy-ion inertial fusion reactors utilize banks of liquid jets to protect sensitive beam line components from neutrons and debris following target explosions. IFE designers must have knowledge of the surface quality of these jets in order to determine the distance between the jets and the ion beams that must propagate through the void spaces between them. Here, numerical simulations of such jet flows performed with the customized Flow3D solver are reported. These numerical simulations predicted no significant jet breakup in the region of interest, but did show surface and shape deformation that may end up determining the minimum standoff distance between jets and driver beams. The simulations also show small-droplet ejection that may adversely affect beam propagation characteristics. The intrusion distance of liquid into the beam lines was determined to be below 10% of the original jet thickness throughout the computational domain (up to 1 m downstream from the nozzle). Recommendations on how to avoid or minimize unwanted hydrodynamic phenomena (surface rippling and droplet ejection) by upstream conditioning and nozzle design are developed for free surface jets in vacuum in the context of a qualitative understanding of the physical mechanisms at play.

Experimental Observation and Theoretic Analysis MHD Effects of a Liquid Metal Jet in a Gradient Magnetic Field

The advanced limiter-divertor plasma-facing system (ALPS) has been studied for several years, but the magnetohydrodynamic (MHD) stability of free surface jet flow in a gradient transverse magnetic field is one of the key remaining issues. Recently, some experiments on jet flow were performed with a 0.2- to 1.95-T gradient magnetic field and 2.9, 3.24, and 4.10 m/s velocities for a flow diameter of 6 mm. The results indicated that the transverse gradient magnetic field strongly shortens the jet flow range and the shape of the cross section of the jet flow deforms from round to elliptical and finally becomes a bowed-down shape in the jet flow downstream under these experimental conditions. This paper includes simple modeling of jet flow MHD stability in a gradient transverse magnetic field, which derives the velocity and the area of the cross section of the jet flow along the flow path. The theoretical expected values are in good agreement with experimental results.

Analysis on MHD Stability of Free Surface Jet flow in a Gradient Magnetic Fields

The simplified modeling for analysis on MHD stability of free surface jet flow in a gradient magnetic fields is based on the theoretical and experimental results on channel liquid metal MHD flow, especially, the results of MHD flow velocity distribution in cross-section of channels (rectangular duct and circular pipe), and the expected results from the modeling are well agreed with the recent experimental data obtained. It is the first modeling which can efficiently explain the experimental results of liquid-metal free surface jet flow.

Experimental study of boiling heat transfer during subcooled water jet impingement on flat steel surface

The growth in demand for high quality metal alloys has placed considerable emphasis on the type of cooling methods used in manufacturing processes, in particular, the production of highly tailored steel through controlled cooling on the runout table. The present study focuses on the heat transfer (cooling of hot rolled steel strips) on a runout table. The purpose of the study was to develop an efficient experimental method and collect temperature data under conditions similar to those that occur during industrial runout table conditions in a steelmill. Surface and internal temperatures were measured during transient cooling of a flat, upward facing fixed steel plate cooled by a highly subcooled single, circular, free surface jet of water. Measurements were made at stagnation and several streamwise distances from the stagnation point. A numerical, finite difference model was applied to calculate the surface heat flux using measured temperatures. The effect of water flowrate and subcooling on the overall heat transfer with emphasis on the maximum heat flux is discussed.