Free Water Jet Research Articles

On the Water Jet Quality at Part-Load Operation of Pelton turbines

Abstract Pelton turbines with multiple nozzles are characterised by their excellent performance over a wide operating range of mass flow rates. Thus, Pelton turbine manifolds must also be designed to deliver good flow quality to the nozzles at part-load operating conditions to accomplish the expectations. A reduced number of nozzles are opened at low mass flow rate operating conditions, where the choice of the open nozzles depends on the water jet quality generated at the individual nozzles. We investigate the flow phenomena occurring in the manifold with the operation of a reduced nozzle number employing numerical simulations to analyse beneficial nozzle opening selections. The turbulent two-phase flow is predicted by the Volume-Of-Fluid (VOF) approach and the k–ω SST turbulence closure model. Different nozzle operating scenarios are simulated, and the implications on the water jet quality, i.e. deviation and shape deformation, are reported. The results show that the free water jet quality criteria can change drastically at individual nozzle operating conditions because of the velocity changes at the branch line junctions.

Effect of inflow disturbances in Pelton turbine distributor lines on the water jet quality

The proper design of the manifold, distributing the flow to the individual nozzles, is crucial for the Pelton turbine performance. The generation of both energy losses and secondary flows must be reduced to a minimum to provide optimal water jet quality and, hence, efficiency. However, the inflow state due to the penstock is commonly disregarded at the design stage. By numerical flow simulation, we show how such penstock perturbations affect the water jet cross-section and axis deviation by considering different pipe bends upstream of the manifold. The two-phase flows resulting in a typical Pelton turbine manifold are predicted using the Volume-Of-Fluid (VOF) approach in conjunction with the k−ω SST turbulence closure model. Pipe bends are attached five manifold inlet diameters upstream of the manifold itself. The consequences on the secondary flows in the nozzles and the free water jet quality are reported quantitatively. The analysis shows that even small inflow perturbations into the manifold due to penstock piping significantly affect the water jet quality. A yet unreported mechanism is revealed, where the secondary flows mitigate the impact of the flow rate imbalance across the nozzle on the water jet deviation.

CFD analysis of turbulent free high-speed water jet issuing from circular and non-circular nozzle geometry in context of abrasive slurry jet machining

CFD analysis of turbulent free high-speed water jet issuing from circular and non-circular nozzle geometry in context of abrasive slurry jet machining

Shaping of a Free Jet of Overheated Water at Different Distances from a Cylindrical Channel

Shaping of a Free Jet of Overheated Water at Different Distances from a Cylindrical Channel

Assessment of CFD Solvers and Turbulent Models for Water Free Jets in Spillways

The capability of two different OpenFOAM® solvers, namely interFoam and twoPhaseEulerFoam, in reproducing the behavior of a free water jet was investigated. Numerical simulations were performed in order to obtain the velocity and air concentration profiles along the jet. The turbulence intensity was also analyzed. The obtained results were compared with published experimental data and, in general, similar velocity and air concentration profiles were found. InterFoam solver is able to reproduce the velocity field of the free jet but has limitations in the simulation of the air concentration. TwoPhaseEulerFoam performs better in reproducing the air concentration along the jet, the results being in agreement with the experimental data, although the computational runs are less stable and more time consuming. The sensitivity analysis of the inlet turbulent intensity showed that it has no influence in the characteristics of the jet core. With this research it is possible to conclude that: interFoam with k-Epsilon (k-ε) turbulence model is the best choice if the goal of the numerical simulations is the simulation of the velocity field of the jet. Meanwhile, twoPhaseEulerFoam with mixturek-Epsilon (mk-ε) shall be considered if the objective is the simulation of the velocity field and the air concentration.

Numerical Investigation of the Effect of CEDEX Profile on the Hydraulic Parameters in the Stepped Spillway and the Performance of This Profile in Various Chute Slopes

An important part in designing stepped spillways is making the spillway in order to direct the flow appropriately on the first step. In low discharges, the water flow moves over the spillway and arrives at the first step, and if the first step is high enough, the flow may leave the first step horizontally, skip some steps and land as a free jet of water on the lower steps, which might cause some damages to the structure. One of the solutions to this problem is to use CEDEX profile, transition steps between the ogee crest and the stepped chute. The present research carries out a numerical study of the effects of CEDEX profile on various hydraulic parameters and also the effects of the chute slope on the performance of this profile in preventing the flow jump. For this purpose, four models were created with different geometries and were studied in various discharges with FLOW-3D software. The results show that deploying CEDEX profile in the initial parts of the spillway leads to lower velocity and less risk of cavitation. Also, the results illustrate that CEDEX profile can shift the inception point of aeration to downstream. Furthermore, findings show that CEDEX profile, in the slope of less than 50°, can prevent flow jump in the first step. In steeper slopes, however, it fails to achieve the optimal performance.

AN EXPERIMENTAL AND NUMERICAL STUDY OF FACTORS INFLUENCING THE SCOUR DUE TO THE COLLISION OF WATER JET IN STILLING BASIN FLOOR

This study based on the laboratory experiments and numerical analysis to study the configuration of the local scour downstream of hydraulic structure result by free water jet. This jet will have a high kinetic energy making it able to transport the sediment particles downstream of the impinging area, thereby forming a scour hole. The purpose of conducting this study was to evaluate the minimum distance of scour hole from the nozzle jet. One hundred and thirty-eight runs were carried out to investigate the local scour downstream the hydraulic structure under clear-water conditions with different discharges, tail water depths, and jet angles. Two types of jets were used, normal jet and dispersion jet. Results indicated that minimum distance of scour increase with increasing densimetric Froude number and jet velocity. The increase in tail-water depth decrease the distance to the scour hole until access to a critical depth at which the scour hole is largest possible after that the distance back to become large when the depth is overstepping the critical value. The optimum jet angle depends on the jet velocity, with a value of 20° for jet discharges 60 and 80 L/min and 40° for jet discharges 100 and 120 L/min. The dispersion jet technique where used to investigate its effects on the scour processes where its observed that the scour hole resulted small than the scour hole from case of normal jet, Also, the numerical simulation was used by (FLOW3D) software program to investigated from the result of the experimental work, the results showed an acceptable agreement with the experimental results with percentage of error equal to (% 6.28).

Self-oscillating modes of penetration of free or submerged jets through the surface of a liquid

A brief review was conducted of the most significant results of studies performed at the Institute of Mechanics of Moscow State University regarding a new class of unsteady flow of incompressible fluid. Examples include such fluids arising upon the penetration of vertical submerged or free flat and axially symmetric water jets through the surface of water in relatively narrow channels, such as when free thin-walled water jets flowing from a conical slit nozzle with a vertical axis form in a rectangular vessel. A broad range of values of characteristic parameters was found to be generated by the previously unknown self-oscillating flow modes formed in the liquid. Different mechanisms for the occurrence of such modes and characteristics of the dependencies of the self-oscillation period on the main determining parameters were uncovered through empirical experiment and numerical modeling. These mechanisms and characteristics as well as possible applications of the obtained results are presented.

Self-Oscillation Regimes of Penetration of Free or Plunging Jets through Liquid Surface

A brief review is given for the most significant results obtained in studies carried out in the Institute of Mechanics, Moscow State University, and focused on a new class of transient flows of an incompressible liquid that form when vertical plunging or free flat or axially symmetrical water jets penetrate through the surface of water placed in relatively narrow channels or when free thin-wall water jet, flowing from a conical slotted nozzle with a vertical axis penetrates through water surface in a rectangular vessel. Wide ranges of the values of key parameters were found to exist at which self-oscillation flow regimes unknown before form in the liquid. Different formation mechanisms of such regimes and the characteristic features of the experimentally and numerically determined dependencies of self-oscillation periods on the key parameters are discribed. Possible applications of the obtained results are given.

Numerical and experimental investigations on a circular hydraulic jump due to normal impinging free liquid jet on a flat horizontal target plate

Circular hydraulic jump (CHJ) due to normal impingement of a free liquid jet of water and ethylene glycol on stationary horizontal plate is numerically simulated for the prediction of radius of the CHJ and for the film thickness of the radially spreading flow by solving continuity and momentum equation using volume-of-fluid method. The Froude number measured just after the jump is observed to be constant and independent of the flow rate and kinematic viscosity of the liquid. CHJ is also investigated both experimentally and numerically by making a small hole in the impingement region of the target plate such that the central core of the impinging jet drains out through the hole. The effect of this modified impingement region flow is observed to be insignificant as far as the radius of the jump and outer Froude number is concerned.

Investigating Torricelli’s Law (and More) with a 19th-Century Bottle

The phenomenon of emerging water jets from the holes of containers has been extensively studied; nevertheless, it continues to inspire researchers and teachers. The main aspects of the recent studies concern: the range of the jets trajectories; the extension of Bernoulli’s equation, of which Torricelli’s expression represents an ideal situation, to real fluids; and the measure of the jets’ speed and the drainage time in dependence on the fluid system geometry and wetting conditions. The focus of this paper is to discuss Torricelli’s relation between jet speed and hole depth in the case of a 19th-century glass bottle featuring three lateral holes. Measurements of the free water level displacements and jet speed from the holes have been carried out with 11th-grade students.

Heat transfer and hydrodynamics of air assisted free water jet impingement at low nozzle-to-surface distances

In this study, heat transfer and hydrodynamics of air-assisted free water jet impinging a flat plate surface are experimentally investigated. The effects of the nozzle-to-surface distance (H/d = 0.02–0.51) and volumetric quality (β = 0.3–0.8) on the stagnation Nusselt number, stagnation pressure, and hydraulic jump diameter are considered. The results show that the normalized stagnation Nusselt number and hydraulic jump diameter drastically increase with decreasing the nozzle-to-surface distance, since the stagnation pressure increases due to the jet deflection effect. Based on the experimental results, new correlations for the stagnation Nusselt number and pressure are developed as a function of the nozzle-to-surface distance.

Effect of wire mesh on the heat transfer from a flat plate under free water jet impingement quenching

Experiments were conducted on a flat plate to investigate the influence of the wire mesh on the jet impingement cooling rates. Three different mesh sizes were used to be compared to a bare surface. Inverse heat conduction technique was used to estimate the surface heat flux and temperature without interfering with the flow/boiling phenomena occurring on the surface. The heat flux ratio between the meshed and the bare surface was found to be always less than unity; from 0.6 up to 1.0, which indicated heat transfer deterioration. The bare surface was found to have the lowest temperature gradients due to the fast wetting and uniform heat transfer induced by the impingement process; the increase was found to be 5–25% when compared to bare surface. For the meshed surface, higher temperature gradients were observed due to the localized boiling phenomenon, resulting in non-uniform heat transfer on the surface. Three mesh numbers were Mdw = 0.22, 0.28 and 0.34. At Mdw = 0.28, better cooling rates (closer to the case with bare surface) than the other two cases. An optimum Mdw at which the augmentation of heat transfer due to the wire mesh is not offsetted by the wetting resistance effects was noticed.

An Experimental Study on the Heat Transfer of Impinging Free Water Jet on Flat Plate

The heat transfer of Jet impingement is a very effective technique for exchanging high heat fluxs between a heated plate and a fluid. The purpose of current investigation is to carry out the experiment in order to study heat transfer characteristics between a vertical round water jet and a horizontal surface for different flow rates and geometric conditions. The effect of flow rates on heat transfer were investigated. The data obtained in this study are represented in terms of Nusselt number as a function of Reynolds and Peclet numbers. The correlation for the Nusselts number in terms of the Peclet number and L/Di was obtained. The proposed correlation predicts the current data of heat transfer very well.

Heat transfer and hydrodynamics of free water jet impingement at low nozzle-to-plate spacings

In this study, heat transfer and hydrodynamics of a free water jet impinging a flat plate surface are experimentally investigated. The effects of the nozzle-to-plate spacing, which is equal to or less than one nozzle diameter (H/d=0.08–1), on the Nusselt number, hydraulic jump diameter, and pressure at the stagnation point are considered. The results show that the normalized stagnation Nusselt number, pressure, and hydraulic jump diameter are divided into two regions: Region (I) jet deflection region (H/d⩽0.4) and Region (II) inertia dominant region (0.4<H/d⩽1). In region I, the normalized stagnation Nusselt number and hydraulic jump diameter drastically increase with decreasing the nozzle-to-plate spacing, since the stagnation pressure increases due to the jet deflection effect. In region II, the effect of the nozzle-to-plate spacing is negligible on the normalized stagnation Nusselt number and hydraulic jump diameter since the average velocity of the jet is constant, which means the jet deflection effect disappears. Based on the experimental results, new correlations for the normalized hydraulic jump diameter, stagnation Nusselt number, and pressure are developed as a function of the nozzle-to-plate spacing alone.

Modeling of Bubble Growth Under an Impinging Free Planar Water Jet

ABSTRACTDeveloping a wall heat flux-partitioning model (WHFP) would reduce the empiricism commonly encountered in modeling of jet impinging boiling (JIB). An integral and necessary part of any WHFP model is a method to predict the area fraction on the heated surface influenced by bubble generation. Addressing such need, a scenario identification procedure (SIP) has been developed to predict bubble growth termination (BGT) during JIB. Two conditions have been considered in determining the most probable BGT scenario for JIB configuration: (1) the thermal equilibrium of the bubble with the surrounding liquid bulk, and (2) the dynamic equilibrium of forces acting on the bubble. If the bubble diameter corresponding to thermal equilibrium is reached first, then most probably the bubble will locally collapse. If the bubble diameter associated with dynamic equilibrium is reached first then bubble departure and sliding along the heated surface are expected to take place. The BGT predictions obtained using the developed SIP has been validated using visual observations of nucleate JIB on a horizontal flat surface. The planar free water jet velocity, degree of water subcooling, and surface superheat were varied between 0.4 m/s and 1.7 m/s, 10°C and 28°C, and 0°C and 30°C, respectively. Bubble dynamics was observed over a span of the heated surface from jet stagnation 10 ten jet widths. Experimental results show that the proposed SIP is capable of predicting both the prevailing BGT scenario and the maximum bubble diameter reasonably well. As mentioned before, these are very important elements in developing a WHFP model for JIB.

Flow visualization by mobile phone cameras

Mobile smart phones were completely changing people’s communication within the last ten years. However, these devices do not only offer communication through different channels but also devices and applications for fun and recreation. In this respect, mobile phone cameras include now relatively fast (up to 240 Hz) cameras to capture high-speed videos of sport events or other fast processes. The article therefore explores the possibility to make use of this development and the wide spread availability of these cameras in the terms of velocity measurements for industrial or technical applications and fluid dynamics education in high schools and at universities. The requirements for a simplistic PIV (particle image velocimetry) system are discussed. A model experiment of a free water jet was used to prove the concept and shed some light on the achievable quality and determine bottle necks by comparing the results obtained with a mobile phone camera with data taken by a high-speed camera suited for scientific experiments.

Self-oscillation periods of conical jet aerators with various apex angles

The penetration of free hollow thin-walled turbulent water jets into water is considered. These jets are generated in conical jet aerators with various apex angles. Stable regular self-oscillation modes of this penetration process are studied experimentally. A dependence of self-oscillation periods on the apex angle α, where 45 ≤ α ≤ 80°, is analyzed for the jet discharge range 150 ≤ Q ≤ 550 cm3/s when the height H of the annular nozzle above the water surface belongs to the range 1 ≤ H ≤ 28 cm.

Archerfish Actively Control the Hydrodynamics of Their Jets

Among tool-using animals [1-4], none are known to adaptively change the hydrodynamic properties of a free jet of water--a task considered difficult in human technology [5-7]. Hunting archerfish can strike their targets with precisely aimed water jets (e.g., [8, 9]), but they are also presently thought to be unable to actively control the hydrodynamics of their jets [8-13]. By using specifically trained fish, we were able to monitor several aspects of jet production and propagation as the fish fired at targets over a much wider range of distances than previously explored [10, 13]. We show that jets that have to travel farther also live longer. Furthermore, the time needed until water assembles at the jet tip is not fixed. Rather, it is adjusted so that maximum focusing occurs just before impact. Surprisingly, the fish achieve this by modulating the dynamics of changes in the cross-section of their mouth opening, a mechanism that seems to not have been applied yet in human-built nozzles. The timing adjustments archerfish make in order to powerfully hit targets over an extended range strikingly parallel the situation in the "uniquely human" ability of powerful throwing [14-18]. Based on the key role throwing played in human encephalization and cognitive evolution [14-20], skillfully "throwing" water should similarly have led to the correlated rapid evolution of cognitive skills in this animal.

Dependence of the self-oscillation period for a conical jet aerator cap on the jet width in the nozzle outlet

The penetration of free hollow thin-walled turbulent water jets into water is considered. These jets are generated in a conical jet aerator whose apex angle is 60°. The periods of steady regular self-oscillations appearing during the process of penetration are studied experimentally. A dependence of these periods on the annular nozzle gap width δ, 0.07 ≤ δ ≤ 0.12 cm, is analyzed for the jet discharge range 160 ≤ Q ≤ 550 cm3/s when the height H of the annular nozzle above the water surface belongs to the range 1 ≤ H ≤ 28 cm.