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Abstract Natural gas hydrate solid fluidization mining technology is a new hydrate mining technology, but due to the lack of combined nozzle design theory and basis, the crushing efficiency of mining tools is low. To solve this problem, the characteristics of the combined jet flow fields of nozzles with diameters from 1 mm to 4 mm and combination angles from 10° to 20° were studied in this paper by means of experiments and simulations. The results show that the maximum velocity at the merging point increases negatively linearly with increases in the combination angle and positively linearly with increase in the nozzle diameter. The position of the merging point changes little with the nozzle diameter and increases exponentially with increases in the combination angle. The effective crushing lengths of nozzles with different diameters all turn (increase first and then decrease) when the combination angle is 12.5°. The optimal combination angle for nozzles with diameter less than 4.5 mm is 12.5°, and 15° is for nozzles with diameter more than 4.5 mm. This study reveals the flow field variation mechanism for the combined nozzle and provides guidance for the design of the combined nozzles of gas hydrate combined nozzles. [ABSTRACT FROM AUTHOR]

Abstract After shortly highlighting the main features of the classical hydrodynamic theory of pool boiling critical heat flux (CHF), the present paper describes a new approach to predict CHF in the presence of a submerged liquid jet impinging onto the heating surface in saturated conditions at both atmospheric and sub-atmospheric pressure. This approach, based on the instability of liquid-vapor interfaces, has proved to be able of fitting CHF data for submerged jets as well as for electrohydrodynamic (ionic) jets. The main dimensionless parameters governing the phenomenon have been disclosed, namely the Bond number and an equivalent Froude number. Two distinct fluid-dynamic regimes have been identified, herein named velocity-prevailing and gravity-prevailing , depending on the equivalent Froude number values and the subsequent effect of vapor buoyancy on liquid-solid contacts. The developed correlations providing CHF for both regimes have coefficients of determination higher than 0.9 with respect to the available experimental data. Further development may involve the identification of dimensionless parameters and specific correlations for CHF under free-surface jets. [ABSTRACT FROM AUTHOR]

The pressure oscillation is an important characteristic of direct contact condensation of steam in subcooled water. Many experimental works have been performed on pure steam submerged jet condensation. However, the effect of non-condensable gas content on the characteristics of the pressure oscillation is not yet fully understood. So, present paper aims to investigate the effect of air mass fraction on the characteristics of the pressure oscillation. Experimental results show that: for pure steam jets, the pressure oscillation dominant frequency decreases with the rise of water temperature and nozzle diameter. While it increases with the rise of steam mass velocity, which is consistent with the most of previous research results. The pressure oscillation intensity increases with the rise of water temperature, steam mass velocity and nozzle diameter. For air-steam mixture gas jets, the effect of water temperature and steam mass velocity on pressure oscillation characteristics is the same as pure steam jets. The pressure oscillation dominant frequency rapidly decreases with the rise of air mass fraction. However, air mass fraction has a complex effect on pressure oscillation intensity. As the air mass fraction increases, the pressure oscillation intensity rapidly increases at first, then slowly decreases and then slowly increases. In addition, new correlations for pressure oscillation dominant frequency and intensity are developed. The predicted results agree well with the experimental results. [ABSTRACT FROM AUTHOR]

Submerged gas-liquid mixture injection into water through a micro-channel shows unique fluid characteristics by various flow regimes in the micro-channel generating special jetting phenomena which have never been systematically studied. We injected air-water mixtures into water through submerged 1.0 and 1.5 mm diameter micro-channels. Bubbly, Taylor, bubble-train Taylor, churn and Taylor-annular flows were observed, and generated respective jetting flow regimes. Overall, two flow regimes namely “unstretched bubbles” and “stretched bubbles” were identified in the jet region. The former consists of three flow patterns, namely discrete bubbles, gathered bubbles and grown irregular bubbles. The latter occured when liquid slugs having strong enough momentum to stretch the bubble interface, from which single-bubble stretched, twin-bubbles stretched and even bubble rupture were further identified. We compared flow regime maps in micro-channels with those of submerged jets, and proposed the correspondence relationship, whose universality was confirmed by the consistency for different sized micro-channels. Transitions between two overall flow regimes of submerged jets, as well as flow patterns of unstretched bubbles were found determined by flow regime transitions in the micro-channel. Besides, transition laws between flow patterns of stretched bubbles were affected by the uniformity of slug length throughout the micro-channel, which were quantitatively analyzed. [ABSTRACT FROM AUTHOR]

The current study investigates the global flow field characteristics of a submerged annular viscoplastic jet. The mass and momentum conservation equations, governing the steady laminar flow field, along with the Bingham rheological model, are numerically solved using a finite-difference scheme. Central and outer recirculation regions typically characterize the flow of a Newtonian annular jet. However, the current visualizations demonstrate the existence of new and unique-to-viscoplastic-fluids flow features. When the yield numbers are small, central and outer recirculation regions exist. However, the extent of the outer region and recirculation intensity of the outer and central regions are found to substantially diminish with the yield number. At intermediate yield numbers, a stagnant, attached-to-the-wall region replaces the outer recirculation while a central, yet weaker one, exists. At high yield numbers, stagnant regions replace the recirculating ones, i.e., flow recirculation throughout the whole flow field is eliminated once a critical yield number is exceeded. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: In hydrothermal spallation deep drilling a high-velocity, hot, supercritical water jet is directed towards the rock to induce fragmentation. One major challenge in the realization of this novel technique is the entrainment of comparatively cool, aqueous drilling fluid by the hot water jet, which can lead to significant heat losses before the hot jet''s energy can be transferred to the rock. The present work quantifies such entrainment effects by determining penetration lengths of supercritical water jets injected into a cool, subcritical environment using a novel optical Schlieren method. Penetration lengths of supercritical jets were found to be equal to the injector''s nozzle diameter and almost independent of the jet''s temperature at the nozzle exit and the jet''s mass flow under almost all experimental conditions investigated. A semi-empirical model adapted from steam jet studies confirmed these findings and indicates that heat and mass transfer are primarily controlled by turbulent mixing. [Copyright &y& Elsevier]

The characteristics of a turbulent jet issuing from a square cross section nozzle in the vicinity of a free surface are presented. Two-component laser Doppler measurements were obtained at eight stations downstream of the nozzle exit, up to a distance of 27 nozzle widths. The Reynolds number based on the exit condition was 40,000. The proximity of the free surface influence the turbulence levels in the jet. Both confinement effects and axis switching influence the evolution of the velocity profiles downstream of the nozzle. The shear stress profiles indicate the formation of a wider mixing zone in the bottom portion of the jet in regions closer to the nozzle exit. Through quadrant decomposition, ejection, and entraining type events were identified. The magnitudes of the peak shear stress in the various quadrants indicate differences in the turbulence characteristics on the top and bottom portions of the jet. As distinguished in the extreme event plots, there are differences in the magnitudes of the peaks and also in their locations. [ABSTRACT FROM AUTHOR]

This paper reports an experimental investigation on the velocity and turbulence characteristics in an evolving scour hole downstream of an apron due to submerged jets issuing from a sluice opening detected by an acoustic Doppler velocimeter. Experiments were carried out for the conditions of submerged jets, having submergence factors from 0.96 to 1.85 and jet Froude numbers from 2.58 to 4.87, over sediment beds downstream of a rigid apron. The distributions of time-averaged velocity vectors, turbulence intensities, and Reynolds stress at different streamwise distances are plotted for the conditions of initial flat bed, intermediate scour holes, and equilibrium scour hole downstream of an apron. Vector plots of the flow field show that the rate of decay of the submerged jet velocity increases with an increase in scour hole dimension. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics in evolving scour holes are analyzed in the context of self-preservation, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The most significant observation is that the flow in the scour holes (intermediate and equilibrium) is found to be plausibly self-preserving. [ABSTRACT FROM AUTHOR]

• Submerged jets are generated during laser-induced subcooled boiling • Velocity and direction of the jets can be controlled • Control of jet parameters is important for laser surgical treatment • Control parameters are laser power, boundary conditions, degree of subcooling, optical fiber radius • Introduction of a second fiber make it possible to change the jet direction The mechanism of submerged jets generation during subcooled surface and volumetric water boiling is studied experimentally and numerically. The boiling take place near the tip of the optical fiber through which the laser radiation is supplied. It is shown that the interaction of a collapsing bubble with the end face of the optical fiber leads to the symmetry breaking of the bubble, following by the water jet generation. The jet velocity depends on the collapse rate as well as on the ratio of the maximum bubble radius reached during the inflation stage to the fiber radius and, therefore, on the laser radiation intensity. Other problem parameters affecting characteristics of the generated jets are the temperature of the surrounding liquid and the fiber radius. Experiments with two perpendicular optical fibers demonstrate vector summation of the velocities of the jets generated by each individual fiber. Presented results might contribute to the development of new operating medical technologies for laser surgical treatment of a wide range of common diseases. [ABSTRACT FROM AUTHOR]

Sediment management in reservoirs with the help of water jets has motivated this work. Erosion caused by single and multiple submerged circular turbulent wall jets on a noncohesive sediment bed of finite thickness lying on a fixed boundary was studied with the help of laboratory experiments. Different combinations of jet diameter, jet separation, and sediment thickness to jet diameter ratio were tested. Results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle Froude number F0 given by the velocity at the nozzle and the effective diameter and submerged specific density of the sediment. Evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time t*. This time, made dimensionless with a time scale tc involving the volume of sediment scoured and the rate of erosion, was also related to the densimetric Froude number. A comparison with studies regarding erosion of a semiinfinite layer of sediment is also presented. [ABSTRACT FROM AUTHOR]

The upward seepage through the bed sediment downstream of an apron of a sluice gate structure is a common occurrence due to afflux of the flow level between the upstream and downstream reaches of a sluice gate. The result of an experimental investigation on the characteristics of the scour hole and the flow-field downstream of an apron due to submerged jets under the influence of upward seepage through the bed sediment is presented. Experiments were run for the conditions of submerged jets, having submergence factors from 0.99 to 1.72 and jet Froude numbers from 3.15 to 4.87, over beds of sediments (median sizes=0.8, 1.86, and 3 mm) downstream of an apron under upward seepage velocities. The characteristic lengths of the scour hole determined from the scour profiles are: the maximum equilibrium scour depth, the horizontal distance of the location of maximum scour depth from the edge of the apron, the horizontal extent of the scour hole from the edge of the apron, the dune height, and the horizontal distance of the dune crest from the edge of the apron, all of which were found to increase with an increase in the seepage velocity. Using experimental results, the time variation of the scour depth is scaled by an exponential law, where the nondimensional time scale decreases linearly with an increase in the ratio of the seepage velocity to the issuing jet velocity. The flow field in the submerged jets over both the apron and within the scour hole was detected using an acoustic Doppler velocimeter. The vertical distributions of time-averaged velocities, turbulence intensities and Reynolds stress at different streamwise distances, and the horizontal distribution of bed-shear stress are plotted for the conditions of scour holes with and without upward seepage. Vector plots of the flow field show that the rate of decay of the submerged jet decreases with an increase in the seepage velocity. The flow characteristics in the scour holes are analyzed in the context of the influence of upward seepage velocity on the decay of the velocity and turbulence intensities and the growth of the boundary layer. [ABSTRACT FROM AUTHOR]

The results of an experimental investigation on scour of noncohesive sediment beds (uniform and nonuniform sediments) downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented. Attempts are made to explain the similarity existing in the scour process and profiles (including dune in the downstream of the scour hole). The scour profiles at different times follow a particular geometrical similarity and can be expressed by the combination of two polynomials. Using experimental scour depth at different times, the time variation of scour depth is scaled by an exponential law, where time scale increases linearly with densimetric Froude number. The equilibrium scour depth, related to the sediment size relative to the sluice opening, decreases with increase in sediment size and sluice opening. On the other hand, the equilibrium scour depth increases with increase in densimetric Froude number. The variation of equilibrium scour depth with tailwater depth indicates a critical tailwater depth corresponding to a minimum equilibrium scour depth. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to formation of an armor layer, and therefore prompted study of the reduction of scour depth by a launching apron placed downstream of the rigid apron. The results show that the average reduction of scour depth by placing a launching apron was 39%, having a maximum of 57.3% and a minimum of 16.2%. The characteristic parameters affecting maximum equilibrium scour depth are identified based on the physical reasoning and dimensional analysis. Equation of maximum equilibrium scour depth obtained empirically agrees well with the experimental data. [ABSTRACT FROM AUTHOR]

The basic goal of this study is to present a numerical simulation model for turbulent water flow issued on frozen scoured beds. The model uses a finite volume method to solve the equations of motion and transport equations for two dimensions on a transformed rectangular domain using boundary-fitted coordinates. The internal characteristics of the mean flow of submerged horizontal jets including surface profiles on frozen scoured beds are computed by a two-dimensional k–ε turbulence model. Computations are carried out at different frozen-scoured bed profiles. A staggered grid system is adapted for variable arrangements to avoid the well-known checkerboard oscillations in pressure and velocity. The SIMPLE algorithm is adapted for the computation. No experimental studies were performed during this investigation. The diffusion characteristics of the submerged jet, growth of boundary layer thickness, velocity distribution within the boundary layer, and shear stress at the scour are investigated and compared with the results of others. [ABSTRACT FROM AUTHOR]

In this work experimental data on the geometry of dense inclined jets issuing in a lab-scale glass rectangular tank are presented. The surrounding fluid was always tap water at room temperature while the dense jets were water solutions of NaCl. Four parameters were changed in the experiments, namely nozzle diameter and inclination, and jet density and flow rate. Jet trajectories were revealed by a colored tracer. Images of the jet were recorded by a digital camera and then further digitally processed, eventually resulting in a time-averaged tracer intensity field. All the jet geometrical parameters, once normalized, were found to be very well correlated to the densimetric Froude number. Moderate jet viscosity variations were found to not significantly affect jet behavior. The reported data allow a quick and easy estimation of maximum rise level, position of the trajectory maximum, and impact point distance of dense jets issued at different angles above the horizontal. [ABSTRACT FROM AUTHOR]

The energy-momentum (E-M) method for calibrating submerged radial gates was refined using a large laboratory data set collected at the Bureau of Reclamation hydraulics laboratory in the 1970s. The original E-M method was accurate in free flow, and when the gate significantly controls submerged flow, but for large gate openings with low head loss through the gate, discharge prediction errors were sometimes large (approaching 70%). Several empirical factors were investigated with the laboratory data, including the combined upstream energy loss and velocity distribution factor and the submerged flow energy correction. The utility of the existing upstream energy loss and velocity distribution factor relation was extended to larger Reynolds numbers. The relation between the relative energy correction and the relative submergence of the vena contracta was shown to be sensitive to the relative jet thickness. A refined energy correction model was developed, which significantly improved the accuracy of submerged flow discharge predictions. Although the focus of this work was radial gates, the energy correction concept and these refinements potentially have application to all submerged sluice gates. [ABSTRACT FROM AUTHOR]

Combining the results of a laboratory study of plane turbulent wall jets on rough boundaries with shallow tailwater, with the results of an earlier work of Rajaratnam on wall jets on rough boundaries with deep tailwater, this paper attempts to describe the effects of boundary roughness and tailwater depth on the characteristics of plane turbulent wall jets on rough beds, which are important in the field of hydraulic engineering. The time-averaged axial velocity profiles at different sections in the wall jet were found to be similar, with some difference from the profile of the classical plane wall jet. The normalized boundary layer thickness δ/b, where b is the length scale of the velocity profile, was equal to 0.35 for wall jets on rough boundaries compared to 0.16 for the classic wall jet. Two stages were seen to exist in the decay of the maximum velocity um as well as in the growth of the length scale, with the first stage corresponding to that of deep tailwater and the second stage to shallow tailwater. In the first stage, the decay of the maximum velocity um at any section in terms of the velocity u0 at the slot, with the longitudinal distance x in terms of L which is the distance where um=0.5U0, was described by one general function, for smooth as well as rough boundaries. The length scale L in terms of slot width decreases as the relative roughness of the boundary increases. The onset of the second stage was not affected significantly by the bed roughness. The growth rate of the length scale b of the wall jet increased from 0.076 for a smooth boundary to about 0.125 for a relative roughness ks/b0 in the range of 0.25 to 0.50, where ks is the equivalent sand roughness and b0 is the thickness of the jet at the slot. [ABSTRACT FROM AUTHOR]