Liquid Jet Impingement Research Articles

Experimental and numerical investigation of liquid jet impingement on superhydrophobic and hydrophobic convex surfaces

Experiments and numerical simulations were carried out to examine the vertical impingement a round liquid jet on the edges of horizontal convex surfaces that were either superhydrophobic or hydrophobic. The experiments examine the effects on the flow behaviour of curvature, wettability, inertia of the jet, and the impingement rate. Three copper pipes with outer diameters of 15, 22, and 35 mm were investigated. The pipes were wrapped with a piece of a Brassica oleracea leaf or a smooth Teflon sheet, which have apparent contact angles of 160° and 113°. The Reynolds number ranged from 1000 to 4500, and the impingement rates of the liquid jets were varied. Numerical results show good agreement with the experimental results for explaining flow and provide detailed information about the impingement on the surfaces. The liquid jet reflected off the superhydrophobic surfaces for all conditions. However, the jet reflected or deflected off the hydrophobic surface, depending on the inertia of the jet, the curvature of the surface, and the impingement rate. The results suggest that pressure is not the main reason for the bending of the jet around the curved hydrophobic surface.

Flow in the thin film created by a coherent turbulent water jet impinging on a vertical wall

When a liquid jet impinges on a vertical wall it forms a thin film which flows radially away from the point of impingement until a point where the outward momentum is balanced by surface tension and a film jump is formed. The model for the location for the film jump presented by Wilson et al. (Chem. Eng. Sci, 2012, vol. 68, pp 449–460) is revised to include the development of laminar and turbulent boundary layers in the thin film. The criterion for film jump formation is also revisited, and the analysis explains why the location is insensitive to the nature of the wall material at high flow rates. The model is compared with published data for velocity profiles in the thin film, the transition to turbulence, and new experimental data where the average velocity in the thin film was estimated from the initial growth of the radial flow pattern for flow rates of 1.95–4.01dm3s−1, corresponding to jet Reynolds numbers of 15,500–32,000. Very good agreement with the published and measured data is obtained, with no adjustable parameters, for jets impinging perpendicularly as well as at an oblique angle. The model shows that the parabolic velocity profile assumed by Wilson et al. gives a reasonable estimate of the average velocity, but it is not able to predict phenomena such as the observed transition to turbulence.

Application of a two phase lattice Boltzmann model in simulation of free surface jet impingement heat transfer

In this paper a two-phase lattice Boltzmann model, capable of handling large density jumps, is used to study the free surface jet impingement cooling in the non-boiling regime. The multiple-relaxation-time (MRT) collision operator is employed to enhance the numerical stability of the model at high Reynolds numbers. The capability of the outlined two-phase LB scheme in accurate capturing of the interface shape is assessed through relevant test cases. These include the oscillating drop, Rayleigh–Taylor instability, and Kelvin–Helmholtz instability. In addition to demonstrate the validity of the model in simulation of heat transfer the predicted distribution of the Nu number on the impingement plate are compared with those of analytical results. The validated numerical solver then is employed to study a planar liquid jet emanating from a nozzle into a quiescent gas and impinging the opposite plate. The influence of the jet Re number and liquid-to-gas density ratio on the instantaneous flow field, interface shape, and heat transfer rate is also examined. This work reports for the first time the simulation of an impinging free surface liquid jet using a two phase lattice Boltzmann model.

Numerical analysis of steady state heat transfer for jet impingement on patterned surfaces

This analysis considers the steady state heating of a plate with a patterned surface under free liquid jet impingement. A constant heat flux was applied at the plate from the bottom while the top surface was cooled by liquid slot jet impinging perpendicular to the plate. Calculations were done for Reynolds number (Re) ranging from 500 to 1000 and indentation depths from 0.000125 to 0.0005m for two different surface configurations. The effect of using different plate materials was explored for the rectangular step case. The distributions of the local and average heat transfer coefficient and the local and average Nusselt number were calculated for each case. It is seen that increasing the Reynolds number (Re) increases the local Nusselt number for all cases. It is observed that increasing the indentation depth for the rectangular surfaces leads to a decrease in local heat transfer coefficient whereas for triangular patterns, a higher depth results in higher heat transfer coefficient.

Effect of the size of air bubbles on enhancement of heat transfer in an impinging liquid jet

The numerical modeling of heat transfer in a bubbly impinging jet is carried out. The axisymmetric system of RANS equations that take into account the two-phase nature of the flow is resolved based on the Euler approach. The turbulence of the liquid phase is described by the Reynolds stress transport model with taking into account the effect of bubbles on modification of the turbulence. The effect of the gas volumetric flow rate ratio and the bubble size on the flow structure and the heat transfer in a gas–liquid impact stream is studied. It is shown that the addition of the gas phase in a turbulent fluid causes an increase up to 1.5-fold in heat transfer. The comparison of the simulation results with experimental data showed that the developed model enables the simulation of turbulent bubbly impinging jet with heat transfer with the pipe wall in a wide range of gas fraction.

Cleaning of complex soil layers on vertical walls by fixed and moving impinging liquid jets

Cleaning by a horizontal water jet, impinging onto a soiled Perspex vertical plate, is described. The plate, the substrate, was coated with PVA or petroleum jelly, the soil. The substrate was either.(i) fixed, for batch tests in which the cleaned area, roughly circular, grew with time, or(ii) the substrate moved vertically up or down in its own plane, the water jet remaining fixed; this reproduced the effect of a jet moving across a surface for cleaning, as found in real tank cleaning operations.In the batch experiments, growth of the radius a of the cleaning area is well described, at early times t, by a5 – ao5 = K5 (t – to), ao being the initial radius of the cleaned area at time to; K is a constant. At later times with petroleum jelly, the cleaning front reached a maximum value, when the outward momentum of the radially flowing water film balanced the strength of the soil. This maximum value is modelled as a ramp of viscoplastic soil inclined at angle χ to the substrate surface, where χ was found to vary from 7° to 25°.In the tests of continuous cleaning of petroleum jelly, a lengthening cleaned area, of width wc, was observed on the moving substrate. Near the jet was a stationary clean front, whose shape looked like half an ellipse. This shape, and the width wc, are well described by theory (Wilson et al., 2015, 123, 450–459) using parameters from the above-mentioned batch experiments. This establishes a good link between batch and continuous cleaning experiments.

Experimental and numerical investigations of the impingement of an oblique liquid jet onto a superhydrophobic surface: energy transformation

This study presents the theory of impinging an oblique liquid jet onto a vertical superhydrophobic surface based on both experimental and numerical results. A Brassica oleracea leaf with a 160° apparent contact angle was used for the superhydrophobic surface. Distilled water was sent onto the vertical superhydrophobic surface in the range of 1750–3050 Reynolds number, with an inclination angle of 20°−40°, using a circular glass tube with a 1.75 mm inner diameter. The impinging liquid jet spread onto the surface governed by the inertia of the liquid and then reflected off the superhydrophobic surface due to the surface energy of the spreading liquid. Two different energy approaches, which have time-scale and per-unit length, were performed to determine transformation of the energy. The kinetic energy of the impinging liquid jet was transformed into the surface energy with an increasing interfacial surface area between the liquid and air during spreading. Afterwards, this surface energy of the spreading liquid was transformed into the reflection kinetic energy.

The influence of nozzle diameter on the circular hydraulic jump of liquid jet impingement

In this study, the circular hydraulic jump of jet impingement cooling was experimentally investigated using water as the test fluid. The effects of nozzle diameter (0.381, 0.506, 1, 2, 3.9, 6.7, 8mm) on the hydraulic jump radius were considered. The results indicate that the dimensionless hydraulic jump radius (rhj/d) is independent of the nozzle diameter under fixed impingement power conditions, while the dimensionless hydraulic jump radius increases with decreasing nozzle diameter under fixed jet Reynolds number conditions. Based on the experimental results, a new correlation for the hydraulic jump radius is proposed as a function of the impingement power alone. It is shown that the proposed empirical correlation for the dimensionless hydraulic jump radius has the same form as that derived from a dimensional analysis of the conservation equations. In addition, the results clearly show that the dimensionless hydraulic jump radius depends on two dimensionless groups, jet Reynolds and Froude numbers, rather than just one, jet Reynolds number.

Modeling of transition boiling under an impinging water jet

Liquid jet impingement is known for its high capability of extracting heat from the surface it impinges on. Steady state subcooled boiling experiments were carried out in the stagnation region of a water planar jet of 0.6 and 0.75m/s jet velocity and 15°C of subcooling. The transition boiling regime is found to have a region of constant heat flux. After the critical heat flux (CHF) is reached, the heat flux decreases till a local minimum value is reached; called the first minimum. The heat flux then increases till it reaches a relatively constant value, even with the increase of the surface degree of superheat. The constant heat flux region is called the shoulder heat flux. With the aid of high speed imaging, Rayleigh–Taylor instability has been observed at the liquid–vapor interface which is accelerated by gravity and jet dynamic pressure. A modified expression for the interface acceleration is proposed. The vapor layer formed in the transition boiling is not stable; it follows periodic cycles of break up and formation. Two surface wetting mechanisms are proposed based on observations: (i) direct liquid touching the surface at moderate degrees of superheat and (ii) liquid columns intruding the vapor layer and touching the surface at discrete locations at high degrees of superheat. A new wall heat flux partitioning model is proposed. The proposed model divides the wall heat flux, based on the two observed wetting mechanisms, into two components: (i) quenching heat flux and (ii) intrusion heat flux. The current model estimates the wall heat flux during transition boiling covering the surface superheat range from the CHF till the Leidenfrost point with a normalized root mean square error (NRMSE) of 33%.

Numerical thermal analysis and optimization of a water jet impingement cooling with VOF two-phase approach

The fluid flow and heat transfer characteristics of a free-surface liquid jet impingement cooling have been investigated numerically. The slot jet with water impinging normally on a flat plate is employed. To describe the turbulent structure, the turbulent governing equations are solved by a control-volume-based finite-difference method with a power-law scheme and the well-known turbulence model, which are associated with wall function. Numerical computations have been conducted with variations of jet exit Reynolds number (11,000≤Red≤17,000), dimensionless jet-to-surface distance (3≤H/d0≤12), dimensionless jet width (1≤B/d0≤2), and the heat flux (140kW/m2≤q″≤280kW/m2). The theoretical model developed is validated by comparing the numerical predictions with available experimental data in the literature. Under the studied ranges, the variations of local Nusselt numbers by hydraulic diameter Nud of the dimensionless jet-to-surface distance 3≤H/d0≤12 along the flat plate decrease monotonically from its maximum value at the stagnation point. In addition, the shape of the inlet area and jet-to-surface distance are optimized by using the response surface methodology (RSM) and the genetic algorithm (GA) method after solutions are carefully validated with available experimental results in the literature. Based on the optimal results, the optimum condition is in H/d0=7.86 and B/d0=2 for this physical model.

Liquid Jet Impingement With an Angled Confining Wall for Spent Flow Management for Power Electronics Cooling With Local Thermal Measurements

The local surface temperature, heat flux, heat transfer coefficient, and Nusselt number were measured for an inline array of circular, normal jets of single-phase, liquid water impinging on a copper block with a common outlet for spent flow, and an experimental two-dimensional (2D) surface map was obtained by translating the jet array relative to the sensors. The effects of variation in jet height, jet pitch, confining wall angle, and average jet Reynolds number were investigated. A strong interaction between the effects of the geometric parameters was observed, and a 5 deg confining wall was seen to be an effective method of managing the spent flow for jet impingement cooling of power electronics. The maximum average heat transfer coefficient of 13,100 W/m2 K and average Nusselt number of 67.7 were measured at an average jet Reynolds number of 14,000.

Visualization of High Speed Liquid Jet Impaction on a Moving Surface

Two apparatuses for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device (for examining surface speeds between 0 and 25 m/sec) and a spinning disk device (for examining surface speeds between 15 and 100 m/sec). The air cannon linear traverse is a pneumatic energy-powered system that is designed to accelerate a metal rail surface mounted on top of a wooden projectile. A pressurized cylinder fitted with a solenoid valve rapidly releases pressurized air into the barrel, forcing the projectile down the cannon barrel. The projectile travels beneath a spray nozzle, which impinges a liquid jet onto its metal upper surface, and the projectile then hits a stopping mechanism. A camera records the jet impingement, and a pressure transducer records the spray nozzle backpressure. The spinning disk set-up consists of a steel disk that reaches speeds of 500 to 3,000 rpm via a variable frequency drive (VFD) motor. A spray system similar to that of the air cannon generates a liquid jet that impinges onto the spinning disc, and cameras placed at several optical access points record the jet impingement. Video recordings of jet impingement processes are recorded and examined to determine whether the outcome of impingement is splash, splatter, or deposition. The apparatuses are the first that involve the high speed impingement of low-Reynolds-number liquid jets on high speed moving surfaces. In addition to its rail industry applications, the described technique may be used for technical and industrial purposes such as steelmaking and may be relevant to high-speed 3D printing.

Quantitative measurement of binary liquid distributions using multiple-tracer x-ray fluorescence and radiography.

The complex geometry and large index-of-refraction gradients that occur near the point of impingement of binary liquid jets present a challenging environment for optical interrogation. A simultaneous quadruple-tracer x-ray fluorescence and line-of-sight radiography technique is proposed as a means of distinguishing and quantifying individual liquid component distributions prior to, during, and after jet impact. Two different pairs of fluorescence tracers are seeded into each liquid stream to maximize their attenuation ratio for reabsorption correction and differentiation of the two fluids during mixing. This approach for instantaneous correction of x-ray fluorescence reabsorption is compared with a more time-intensive approach of using stereographic reconstruction of x-ray attenuation along multiple lines of sight. The proposed methodology addresses the need for a quantitative measurement technique capable of interrogating optically complex, near-field liquid distributions in many mixing systems of practical interest involving two or more liquid streams.

An Experimental Study on the Rewetting of a Hot Vertical Surface by Circular Water Jet Impingement

The present experimental study analyzes the rewetting behavior of a hot vertical stainless-steel foil by a circular impinging liquid jet. The transient temperature of the hot foil, recorded by an infrared camera, is used to evaluate the rewetting velocity and heat flux distribution. The rewetting velocity varies within 2.0–20.0 cm/sec. Maximum surface heat flux is highest at the stagnation point and reduces in the downstream direction. A correlation for the rewetting velocity is developed that predicts 85% of test data within an error band of ± 30%. Present results agree well with the available theoretical models and test data.

Simulation of the impinging liquid jet cooling process of a flat plate

Purpose – The purpose of this paper is to develop a numerical model to simulate the flow field as well as the conjugate heat transfer during unsteady cooling of a flat plate with a single submerged water jet. At wall temperatures above the liquid boiling point, the vapor formation process and the interaction of the vapor phase with the developing jet-flow field are included. Design/methodology/approach – The time-dependent flow and temperature distribution during all occurring boiling phases as well as the local and temporal distribution of the heat transfer coefficient on a flat plate can be simulated. Findings – The influence of the liquid jet flow rate (10,800=Re_d=32,400) and the nozzle distance to the plate (4=H/d=20) on the transient cooling process are analyzed. This includes the time-dependant positions of the transition regions between the boiling phases on the plate as well as the temperatures at these transition regions. Additionally, the local heat transfer rates are a direct result of the unsteady cooling simulation. Originality/value – A single model approach is developed and utilized to simulate the unsteady cooling process of a flat plate with an impinging water jet including all occurring boiling phases.

Computational Modeling and Simulation of Nonisothermal Free-surface Flow of a Liquid Jet Impinging on a Heated Surface

The study is aimed at formulating and validating the computational model for the nonisothermal two-phase flow with free-surfaces. The testing flow configuration consists of the liquid jet impacting on a heated wall generating simultaneous heat transfer from the wall surface. The interface capturing methodology implemented in the open-source software OpenFOAM® based on the volume-of-fluid (VOF) model in the framework of Computational Fluid Dynamics (CFD) is extended to incorporate heat transfer. The potential of the model is evaluated by contrasting the results of numerical simulations to the existing experimental and numerical results. The computational procedure based on the numerical model demonstrates good qualitative and quantitative predictive capabilities by reproducing correctly the studied flow and heat transfer characteristics.

Numerical simulation of the quenching process in liquid jet impingement

Direct numerical simulation is performed for quenching of a hot plate in liquid jet impingement. The flow and thermal characteristics associated with the quenching process, which includes film boiling in the fluid region as well as transient conduction in the solid region, are investigated by solving the conservation equations of mass, momentum and energy in the liquid, gas and solid phases. The liquid–vapor and liquid–air interfaces are tracked by the sharp-interface level-set method modified to treat the effect of phase change. The computations demonstrate that the boiling curve of wall heat flux versus temperature does not depend on the transient or steady-state heating conditions. The effects of initial solid temperature and solid properties on the quenching characteristics are quantified.

Influence of viscosity on the impingement of laminar liquid jets

The impingement of low-viscosity liquid jets has been studied extensively for over a century due to their fundamental scientific interest and their practical importance in spray and atomization technologies. However, the role of the fluid viscosity in the impingement dynamics is largely unknown despite the fact that viscous liquids are common in spray and atomization processes ranging from spray drying in the food industry to the atomization of gelled propellants in rocket engines. Here, we report direct numerical simulations that enable a detailed analysis of the influence of viscosity on the impingement dynamics. The simulations solve the complete Navier–Stokes system governing the free-surface dynamics, and so fully account for the interplay of inertia, viscous and capillary forces. Results show that the liquid viscosity profoundly affects the impingement dynamics. The collision of viscous liquid jets generates a fluid sheet that thins at a rate r−1 with the distance r from the impact point at intermediate viscosities, in contrast to the inertial case in which the sheet thins at a faster rate r−2. As the viscosity increases, the fluid sheets become thicker and more uniform, and contrary to the inertial case, the velocity of the sheets is lower than the velocity of the jets. Results further reveal that due to viscous stresses the impact pressure generated by the collision of viscous liquid jets scales as Re−1, where Re is the jet Reynolds number.

Experimental characterization of hydraulic jump caused by jet impingement on micro-patterned surfaces exhibiting ribs and cavities

This paper reports experimental results characterizing hydraulic jumps that form due to liquid jet impingement on patterned surfaces with alternating microscale-ribs and cavities. The surfaces are tested in both hydrophilic (wetted cavity regions, Wenzel state) and superhydrophobic (unwetted cavity regions, Cassie–Baxter state) states. Similar to jet impingement on a smooth surface, when a liquid jet strikes a ribbed surface it moves radially outward in a thin film and eventually experiences a hydraulic jump, where the thickness of the film increases by an order of magnitude, and the velocity decreases accordingly. However, the anisotropy of the patterned surface causes a disparity in frictional resistance resulting in a hydraulic jump which is elliptical rather than circular in shape. Both the major and minor axes radii of the hydraulic jump are a function of the jet Reynolds number and imposed downstream water depth. We obtain results for cavity fractions (relative width of cavity to total rib/cavity spacing) of 0.5, 0.8, and 0.93 and compare these to results obtained on a smooth surface. The jet issued from a nozzle with a radius of 0.6mm, the Reynolds number ranged from 1.2×104 to 2.1×104, and a range of downstream depths were explored. The parameter ranges explored result in small scale hydraulic jumps, where the jump radii are generally smaller than a few centimeters. Results show both major and minor axis jump radii decrease with increasing water depth or decreasing jet Reynolds number, following classical hydraulic jump behavior. When the Wenzel state exists, the major axis is nominally the same for both the smooth and structured surfaces at the same Reynolds number and downstream depth. For both the Wenzel and Cassie–Baxter states, as the cavity fraction of the superhydrophobic surface increases, the major axis of the hydraulic jump radius increases modestly and the minor axis decreases. These changes are exaggerated when the Cassie–Baxter state prevails. Also, as the cavity fraction increases the eccentricity of the jump increases, and this is more pronounced when the Cassie–Baxter state exists.

Simultaneous Estimation of Boundary Heat Flux and Convective Heat Transfer Coefficient of a Curved Plate Subjected to a Slot Liquid Jet Impingement Cooling

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to simultaneously estimate the unknown boundary heat flux and convective heat transfer coefficient in a curved plate cooled by an impinging slot jet from the knowledge of temperature measurements taken within the plate. Subsequently, the distributions of temperature in the plate can be determined. It is assumed that no prior information is available on the functional forms of the heat flux and convective heat transfer coefficient; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements, and the effect of the errors and locations in these measurements upon the precision of the estimated results is also considered. Results show that an excellent estimation on the heat flux and convective heat transfer coefficient and temperature distributions can be obtained for the two test cases considered in this study.