Liquid Jet Impingement Research Articles

Liquid Sheet Formed by Impingement of Two Viscous Jets

Impinging-jet injectors are commonly used in liquid-propellant rocket engines. Previous theoretical studies on sheets formed by two impinging liquid jets have largely ignored the affect of liquid viscosity, however, several experimental results have illustrated that liquid viscosity could greatly influence sheet characteristics. This paper proposes an improved theoretical analysis model for two viscous liquid impinging jets, in which viscosity and energy dissipation are taken into account; experiments were also performed to validate the theory. Furthermore, based on this model, the effects of impingement angle, jet velocity profile, Weber number, and Reynolds number on sheet characteristics, such as sheet shape, thickness, and velocity distribution, were investigated.

Jet impingement and the hydraulic jump on horizontal surfaces with anisotropic slip

This paper presents an analysis that describes the dynamics of laminar liquid jet impingement on horizontal surfaces with anisotropic slip. Due to slip at the surface and the anisotropy of its magnitude, the overall behavior departs notably from classical results. For the scenario considered the slip length varies as a function of the azimuthal coordinate and describes superhydrophobic surfaces micropatterned with alternating ribs and cavities. The thin film dynamics are modeled by a radial momentum analysis for a given jet Reynolds number and specified slip length and the influence of slip on the entire flow field is significant. In an average sense the thin film dynamics exhibit similarities to behavior that exists for a surface with isotropic slip. However, there are also important deviations that are a direct result of the azimuthally varying slip and these become more pronounced at higher Reynolds numbers and at greater slip lengths. The analysis also allows determination of the azimuthally varying radial location of the hydraulic jump that forms due to an imposed downstream depth. Departure from the no slip case and from the scenario of isotropic slip is characterized over a range of jet Reynolds numbers and realistic slip length values. The results show that for all cases the hydraulic jump is elliptical, with eccentricity increasing as the Reynolds number or slip length increases, or as the downstream depth decreases. The radial location of the hydraulic jump is greatest in the direction of greatest slip (parallel to the microribs), while it is a minimum in the direction transverse to the rib/cavity structures. The model results for the hydraulic jump radial position are compared to experimental measurements with good agreement.

Cleaning of soft-solid soil layers on vertical and horizontal surfaces by stationary coherent impinging liquid jets

The cleaning action of stationary coherent liquid jets impinging (a) vertically downwards on horizontal plates, and (b) horizontally on vertical plates, was investigated using three soft-solid model soil layers: (i) PVA glue on glass and polymethylmethacrylate (Perspex) substrates; (ii) Xanthan gum on stainless steel; and (iii) petroleum jelly on glass. The liquid stream nozzle sizes, mass and volumetric flow rates and mean jet velocities investigated were: PVA, 2mm, 17–50gs−1 (0.06–0.139m3h−1), 5.3–15.9ms−1; Xanthan gum, 0.39–3.3mm, 2.1–148gs−1 (0.008–0.53m3h−1); 4.5–31.7ms−1; petroleum jelly, 2mm, 7.8–50gs−1 (0.06–0.139m3h−1); 2.5–15.9ms−1. For all three soils, rapid initial removal of soil from the jet footprint was followed by the growth of a nearly circular, clean region centred at the point of jet impingement. The rate of removal of soil decreased sharply when the cleaning front reached the hydraulic or film jump. The data for the radial growth removal stage were compared with a mathematical model describing removal of the adhesive soil layer, where the force on the cleaning front was evaluated using the result reported by Wilson et al. (2012): their theory gave the momentum of the liquid film; this momentum was balanced against the soil strength, giving a simple relation between the cleaned radius and time. All three soils showed reasonable agreement with the model, across the range of flow rates and temperatures studied. The kinetic constant in the model was sensitive to soil layer thickness and the nature of the soil. Cleaning tests on the petroleum jelly soils at different temperatures, and separate rheological measurements, showed that the kinetic time constant for coating removal was proportional to the (critical shear stress)−1.8. There was good agreement between results obtained with vertical and horizontal plates for the PVA and Xanthan gum soil layers. The petroleum jelly results differed, which is partly attributed to differences in preparing the layers of this rheologically complex material.

Erosion threshold of a liquid immersed granular bed by an impinging plane liquid jet

Erosion threshold of a model granular bed by a jet in a quasi bidimensional configuration has been studied experimentally in both laminar and turbulent regimes. The jet is a liquid sheet which impinges normally a packing of immersed beads monodisperse in size and density. The erosion threshold has been characterized at different impact distances of the jet on the sediment and for different grain size and fluid viscosity. In the explored range of parameters, we show that the erosion threshold is well described by a critical inertial Shields number based on the local flow velocity at the impinging point. This has been done by a careful analysis of the different jet flow regimes taking into account the position of the virtual origin of the jet.

Numerical Analysis of Film Boiling in Liquid Jet Impingement

A numerical approach is presented for computing film boiling in liquid jet impingement on a high-temperature plate. The conservation equations of mass, momentum and energy are numerically solved in the liquid, vapor, and air phases. The sharp-interface level-set formulation is employed to track the liquid-air interface, as well as the liquid-vapor interface with phase change. A simplified analytical model for a thin vapor film, whose thickness is several orders of magnitude smaller than the liquid layer, is incorporated into the level-set formulation. The multiscale approach is tested through the computations of film boiling in a circular water jet.

Effect of ambient air on liquid jet impingement on a moving substrate

An experimental investigation into the effect of surrounding air pressure on liquid jet impingement on a moving substrate was performed. The study was carried out with Newtonian liquids impacting dry substrates. A variety of jet speeds, substrate speeds, and liquid viscosities were studied. It was observed that, as is the case for Newtonian droplet impact, the surrounding air pressure plays a crucial role in the splashing behaviour of jet impingement. There exists a threshold pressure below which splash does not occur. It is proposed that for certain impingement conditions lamella detachment from the substrate occurs due to aerodynamic forces acting on the leading edge of the lamella, which destabilizes the balance between surface tension and fluid pressure forces.

Local two-phase heat transfer from arrays of confined and submerged impinging jets

Local heat transfer distributions at high spatial resolution are obtained under two-phase transport conditions in confined and submerged impingement from arrays of miniature jets. The dielectric liquid HFE-7100 is investigated to enable direct cooling of electronic components. Three round orifice geometries with the same total orifice open area are investigated, including a single orifice of 3.75mm diameter, a 3×3 array of 1.25mm diameter orifices, and a 5×5 array of 0.75mm diameter orifices. A thin-foil heat source backed by a magnesium-fluoride window is fabricated to allow detailed mapping of the heated surface temperature via infrared (IR) thermography. The rigorous experimental calibration procedures employed, and correction for heat spreading within the thermally conductive IR-transparent window, yield low-uncertainty local heat transfer coefficient distributions. Each of the three orifice geometries is characterized at volumetric flow rates of 450ml/min, 900ml/min, and 1800ml/min, resulting in a Reynolds number range of 1920–39400. Pressure drop across the confined jets is measured for all experimental cases. The test facility and measurement techniques employed are validated against heat transfer and pressure drop correlations in the literature for single-phase jet impingement from a single round orifice. Spatially resolved temperature contour maps, along with local heat transfer coefficient and boiling curves, are presented as a function of applied heat flux. Boiling is shown to coexist with single-phase convection under the impinging liquid jets. Two-phase enhancement is exhibited at large radial distances from the single jet axis, and in regions between neighboring jets within the arrays. The arrays of jets result in higher area-averaged heat transfer than a single jet at a fixed flow rate; however, the arrays display larger relative nonuniformity in local two-phase heat transfer coefficient and surface temperature. While the 5×5 array resulted in a higher (and the 3×3 a lower) pressure drop than the single jet, all orifices displayed pressure drop that is independent of the applied heat flux and vapor generation.

Experimental investigation of free-surface jet impingement quenching process

Liquid jet impingement cooling technique is critical in many industrial processes, such as fuel bundle cooling post-loss-of-coolant-accident in nuclear reactors, heat treatment of metal parts post-hot-processing, etc. The ability of liquid jets to extract high heat flux from metal parts, with temperatures as high as 800–1000°C, at moderate flow rates has made them indispensable in these applications. The complex mechanism of flow boiling heat transfer during jet impingement cooling is not been well understood. This study presents a systematic methodology for the measurement and estimation of the temporospatial variation of heat transfer on the impingement surface during jet impingement cooling of extremely hot steel plate. The effect of jet impingement velocity and subcooling variations from 2.5 to 10m/s and 60 to 87K, respectively, on the temporospatial heat transfer variation on the impingement surface is reported.A gradually growing circular wetted region, with its periphery named as the wetting front, forms soon after the cooling starts but its velocity decreases as it grows in diameter. A local maximum in the surface heat flux closely follows the wetting front, with the local maximum heat flux reducing with distance from the stagnation point. The wetting front velocity and local maximum heat flux increase with both the jet velocity and subcooling. The enhancement in the local film velocity and subcooling result in a strong suppression of boiling activity and, resultantly, high heat transfer rates at plate surface temperatures in much excess of the critical temperature of the coolant is achieved. This observation confirms the industrial practice of using impinging jets for accelerated cooling of hot steel plates.

Effect of heater size and Reynolds number on the partitioning of surface heat flux in subcooled jet impingement boiling

The overall heat transfer rate due to boiling of an impinging subcooled liquid jet is attributed to several simultaneous mechanisms including liquid and vapor phase convection, quenching (transient convection) and evaporation. In the present research, the Rensselaer-Polytechnic Institute (RPI) wall-boiling model is employed to carry out computational simulations on subcooled turbulent jet impingement boiling of water in a confined and submerged configuration. The effects of the controlling parameters, viz. heater-nozzle size ratios (wH/wN), degree of surface superheats (ΔTsat), jet-Reynolds numbers (Re) are studied on the partitioning of the total heat flux into liquid phase convection, quenching (transient convection) and evaporation. The effects of these controlling parameters were studied in the ranges 1⩽wH/wN⩽11, −5°C⩽ΔTsat⩽25°C (heat fluxes upto 200W/cm2) and Re=2500 and 3750, for isothermal and isoflux heaters. The present computational approach was validated by comparison of the local temperature distribution on the heater, and boiling curves against experimental data in literature for axisymmetric and slot jets. The liquid phase convective and quenching components of the total heat flux were found to be larger for relatively lower values of wH/wN, implying smaller heaters. On the contrary, the evaporative counterpart was found to be smaller for relatively smaller heaters upto about wH/wN=3, beyond which the change was negligible with increase in wH/wN. Beyond a threshold surface superheat, the heat flux due to quenching was found to be the largest contributor to the total heat flux; and this threshold value of surface superheat decreased almost exponentially with increase in heater size. It was also observed that irrespective of the heater size, the liquid phase convective component of the total heat flux for Re=3750 was consistently larger as compared to Re=2500, while the quenching and evaporative components of the total heat flux were nearly equal for both Reynolds numbers. An artificial neural network (ANN) was trained and tested with data sets generated from the present research, and ready-to-use weights are presented.

Rewetting of a hot horizontal surface through mist jet impingement cooling

An experimental investigation has been carried out to study the rewetting behaviour of a hot horizontal stainless steel surface during the mist jet impingement cooling. The experiments have been performed to study the rewetting behaviour for three different initial surface temperatures viz. 255, 355, 565°C. An axis-symmetric nozzle has been used to develop the mist jet of constant flow rate. The variation in surface temperature has been acquired up to 20mm downstream spatial locations away from the stagnation point. It has been observed that unlike liquid jet impingement cooling the rise in surface initial temperature increases the rewetting temperature and the wetting delay increase with rise in the initial surface temperature but the rewetting velocity reduces. Further, the maximum surface heat flux is the highest at 20mm spatial location for 565°C initial surface temperature. Whereas, at the stagnation point, the maximum surface heat flux is not affected by the change in surface initial temperature.

Evaluation of the sensitivity and response of IR thermography from a transparent heater under liquid jet impingement

The feasibility of a visible/IR transparent heater and its suitability for IR thermography is experimentally examined. The most common transparent conductive coating, Indium Tin Oxide (ITO), is quite reflective and its optical properties depend on thickness and manufacturing process. Therefore, the optical properties of several thicknesses and types of ITO, coated on an IR window (BaF2), are examined. A highly transparent Cadmium Oxide (CdO) coating on a ZnS window, also examined, is found to be unusable. Transmissivity is found to increase with a decrease in coating thickness, and total emittance is relatively low. A thick ITO coating was examined for IR thermography in the challenging test case of submerged water jet impingement, where temperature differences were characteristically small and distributed. The measurements under steady state conditions were found to agree well with the literature, and the method was validated. Comparison of two IR cameras did not show the LWIR low-temperature advantage, up to the maximal acquisition rate examined, 1.3KHz. Rather the MWIR camera had a stronger signal to noise ratio, due to the higher emissivity of the heater in this range. The transient response of the transparent heater showed no time-delay, though the substrate dampens the thermal response significantly. Therefore, only qualitative transient measurements are shown for the case of pulsating free-surface jet impingement, showing that the motion of the hydraulic jump coincides with thermal measurements. From these results, recommendations are made for coating/window combination in IR thermography.

Flow Visualization and Heat Transfer Characteristics of Liquid Jet Impingement

Equipment used to cool horticultural produce often involves three-phase porous media. The flow field and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships amongst dimensionless groups. This work represents a first step towards the goal of harnessing the power of computational fluid dynamics (CFD) to better understand the heat transfer processes that occur in beds of irrigated horticultural produce. The primary objective of the present study is to use numerical predictions towards reducing the energy and cooling water requirement in cooling horticultural produce. In this paper, flow and heat transfer predictions are presented of a single slot liquid jet impinging on flat and curved surfaces using a CFD code (FLUENT) for 2D configurations. The effects of Reynolds number, nozzle to plate spacing, nozzle width, and target surface configuration have been studied. Reynolds numbers of 250, 375, 500, 700, 1000, 1500, 1800, and 1900 are studied where the liquid medium is water. Here, the Reynolds number is defined in terms of the hydraulic nozzle diameter, inlet jet velocity, and fluid kinematic viscosity. The results show that Reynolds numbers, nozzle to plate spacing, and nozzle width have a significant effect on the flow field and heat transfer characteristics, whereas the target surface configuration at the stagnation area has no substantial impact. The use of a numerical tool has resulted in a detailed investigation of these characteristics, which has not been available in the literature previously.

Hydrodynamic cavitation in microsystems. II. Simulations and optical observations

Numerical calculations in the single liquid phase and optical observations in the two-phase cavitating flow regime have been performed on microdiaphragms and microventuris fed with deionized water. Simulations have confirmed the influence of the shape of the shrinkage upon the contraction of the jet, and so on the localisation of possible cavitating area downstream. Observations of cavitating flow patterns through hybrid silicon–pyrex microdevices have been performed either via a laser excitation with a pulse duration of 6 ns, or with the help of a high-speed camera. Recorded snapshots and movies are presented. Concerning microdiaphragms, it is confirmed that very high shear rates downstream the diaphragms are the cause of bubbly flows. Concerning microventuris, a gaseous cavity forms on a boundary downstream the throat. As a consequence of a microsystem instability, the cavity displays a high frequency pulsation. Low values Strouhal numbers are associated to such a sheet cavitation. Moreover, when the intensity of the cavitating flow is reduced, there is a mismatch between the frequency of the pulsation of the cavity and the frequency of shedded clouds downstream the channel. That may be the consequence of viscous effects limiting the impingement of a re-entrant liquid jet on the attached cavity.

Heat transfer correlation models for electrospray evaporative cooling chambers of different geometry types

Abstract Development of future electronics for high speed computing requires a silent thermal management method capable of dissipating a broad range of heat generated from application-specific integrated circuits, while keeping the skin temperature below 45 °C. Electrospray evaporative cooling (ESEC) chambers show promise because of their ability to dissipate a broad range of heat within a relatively small size. However, the development and the optimization of ESEC chambers are currently restricted, in part due to the lack of sufficient empirical heat transfer correlations. This paper investigates empirical heat transfer correlations for ESEC chambers with three different geometry types. Since the unstable multi-jet behavior of an ESEC chamber is similar to that of a free-surface traditional impinging liquid jet, these correlations are based on the traditional impinging liquid jet’s empirical correlations, yet are modified to factor in the electric field effect. The results show that the heat transfer enhancement ratio correlations and the Nusselt number correlations for different ESEC chambers cover more than 83% of the experimental data, within ±10% deviation. The sensitivity analysis results and experimental data prove that the variation in the enhancement ratio is sensitive to that of the potential and the flow rate. It is not sensitive to the geometric factor of the same ESEC type. This paper presents a natural convection correlation for chip-scale, heated, flat surfaces when the Rayleigh number is below 3000. Further investigation is necessary to extend these heat transfer correlations to cover additional parameters for different thermal management applications.

CONVECTIVE HEAT TRANSFER OF AN IMPINGING OIL JET BETWEEN A TWO-PHASE FLOW AND A HOT SURFACE

The present work aims to study numerically the cooling system in car engines to ensure the piston thermo-mechanical resistance. Most engines use an oil jet cooling system coupled with “cocktail shaking” to extract heat from the piston. This cooling method brings into play a two-phase incompressible turbulent flow in a mobile environment, because of motion of pistons in the cylinder. The need for a more effective cooling involves an accurate understanding of the physical mechanisms. The idea is to support the engine design process to account for advanced technologies to improve turbine or engine performances, burn less fuel and generate less green house gas. In the present work, a numerical model dedicated to the simulation at small scale of oil/air two-phase flows and related heat transfers is proposed to characterize the cooling of engine elements under fragmented jet impact. Large Eddy Simulations of an impinging liquid jet onto a heated semi-hemispherical target were run and the numerical results were compared with experimental data. The mixed-scale model (P. Sagaut, Large Eddy Simulation for Incompressible Flows, Springer, 2000) and the Wall Adapting Local Eddy (WALE) model (Nicoud and Ducros, Flow, Turbulence and Combustion, vol. 62, pp. 183–200, 1999) were evaluated. The best results were obtained using the WALE approach, the mixed-scale model being too dissipative near solid walls. Then cocktail shaking in simplified piston geometries is presented to illustrate the interest of the global simulation approach.

Surface flow and drainage films created by horizontal impinging liquid jets

The surface wetting behaviour of spray ball jets was investigated by considering the simple case of a single horizontal, coherent jet of liquid impinging normally on a vertical surface. The liquid flows radially outwards from the point of impingement until a feature resembling a hydraulic jump is formed. The liquid subsequently drains downwards to give a uniform falling film, here labelled gravity flow, or a thin strand labelled rivulet flow. A simple model of the radial flow region based on momentum balances gave excellent agreement with measurements of flow region radius and falling film width data reported previously by Morison and Thorpe (2002) for various aqueous solutions and surfaces under typical industrial flow rates. The model also gave good predictions of flow region dimensions in new, unreported bench scale tests performed with water on glass and Perspex at lower flow rates. The latter studies involved wetting rates lower than the stability criterion reported by Hartley and Murgatroyd (1964). A transition between gravity and rivulet flow was observed with water on glass between 40°C and 60°C: only rivulet flow was observed with water on Perspex, indicating the importance of contact angle in this region. The model is reported in dimensionless form, which allows the wetted width to be predicted directly from the mass flow rate.

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.

A hydrodynamic model for subcooled liquid jet impingement at the Leidenfrost condition

Stable film boiling occurs in the stagnation region of an impinging subcooled liquid jet during quenching of very hot steel plates. During film boiling the liquid is separated from the surface of the plate by a continuous vapor layer. The minimum surface temperature required to support film boiling is referred to as the Leidenfrost temperature. The present work is devoted to the development of a theoretical model for determination of vapor layer thickness, wall heat flux and wall superheat at the Leidenfrost condition. The model is developed for the jet impingement region with a strong transverse pressure gradient, i.e. within the stagnation and the acceleration regions. For convenience, the entire stagnation and acceleration region is referred to as the stagnation region in this article. Due to the pressure gradient in the impingement region, both the vapor and the liquid flow outwards from the stagnation point. In the current model, it is assumed that the Leidenfrost condition corresponds to zero shear stress at the vapor–liquid interface in the entire stagnation region. Our analytical model is developed for both planar and circular jets, assuming that the entire stagnation region satisfies this condition. The model is based on a solution of the momentum equation in the vapor layer, and the energy equation in the liquid. For a planar jet, the predicted vapor layer thickness is in good agreement with the experimental data of Bogdanic et al. [1]. A vapor film thickness of 8 ± 2 μm for stable film boiling close to the Leidenfrost state has been measured, while the current model predicts a film thickness of 6.31 μm at the stagnation point for the same conditions. The wall heat flux is under-predicted by about 5–47% compared to the experimental data available in the literature, while the wall superheat is under-predicted by up to 70%. In the present analysis, the entire stagnation region has been considered to be at the Leidenfrost condition, which is unrealistic for experiments. In the published experiments, no spatially resolved heat transfer and wall temperature measurements were performed. It is possible that in the experiments transition and film boiling might occur simultaneously in the stagnation region at the minimum heat flux condition. Hence, the wall superheat estimations in this study deviate more than the heat flux estimations. Accurate experimental data are required to validate the model over a wider range of parameters.

Transient conjugate heat transfer from a hemispherical plate during free liquid jet impingement on the convex surface

This paper considers the analysis of transient heating of a hemispherical solid plate of finite thickness during impingement of a free liquid jet. A constant heat flux was imposed at the inner surface of the hemispherical plate at t = 0 and heat transfer was monitored for the entire duration of the transient until a steady state condition was reached. Calculations were done for Reynolds number (Re) ranging from 500 to 1,500 and dimensionless plate thicknesses to nozzle diameter ratio (b/d n) from 0.083 to 1.5. Results are presented for local and average Nusselt number using water as the coolant and various solid materials such as silicon, constantan, and copper. It was detected that increasing the Reynolds number decreases the time for the plate to achieve the steady-state condition. Also, a higher Reynolds number increases the Nusselt number. Hemispherical plate materials with higher thermal conductivity maintain lower temperature non-uniformity at the solid–fluid interface. Increasing the plate thickness decreases the maximum temperature in the solid and increases the time to reach the steady-state condition.

Impingement of liquid jets at atmospheric and elevated pressures: an observational study using paired water jets or water and methylcyclohexane jets

We have observed the impingement of two cylindrical liquid jets of either the same liquid, water, or two mutually immiscible liquids, water and methylcyclohexane (MCH), in either air under normal pressure (0.101 MPa) or nitrogen gas under elevated pressures up to 4.0 MPa. The flow rates of the two jets were adjusted such that they had equal axial momentum. Irrespective of the system pressure, we distinguished two characteristic regimes: the lower flow-rate regime, in which the jet impingement formed a regularly shaped planar sheet, and a higher flow-rate regime, in which a wrinkled sheet repeated azimuthal breakup. The transition from the former to the latter regime occurred at a lower flow rate for the water–MCH impingement than for the water–water impingement. An increase in the system pressure tended to shrink the liquid sheets, to promote the transition to the sheet-breakup regime and to intensify the liquid atomization. The formation of water–MCH compound droplets by the water–MCH impingement was confirmed visually.