Jet Height Research Articles

Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill

To achieve a comprehensive understanding of a three-dimensional (3D) wind field and the speed-up phenomenon in a downburst wind flow over a 3D hilly terrain, a succession of laboratory tests utilizing 12 hill models with cosine-squared cross-section was conducted using a physical downburst simulator with a jet diameter of 0.6 m. By placing the models in the strong horizontal wind region and the strong vertical wind region, the corresponding wind profiles for both the horizontal and vertical velocities were measured. It was found that the wind flowed predominantly over the crest of the hill in the case of low hills, whereas wind flow around the hill body became increasingly pronounced as the hill height increased. In addition, the speed-up region, where the horizontal wind velocity exceeds the impinging jet velocity, was identified, and found to move from the crest to the two sides of the hill as the hill height increased. Accordingly, the most significant topographic multipliers of all locations on the hill might appear at the crest, the hill foot, or elsewhere, depending largely on the hill height. Among all cases, the maximum topographic multiplier was 1.12, and occurred at the ridge, while the ratio of hill height to jet height was 5/12. Additionally, empirical equations are presented to facilitate the determination of wind loads induced by a downburst flow over an isolated hill.

Evaluation of maximum wall shear stress from air impingement to remove food deposits from stainless steel surfaces

The risk of cross-contamination during manufacturing of low water activity foods requires careful attention to cleaning and sanitation. Considering the challenges encountered in such manufacturing, air impingement can provide a unique approach to remove deposits from the food-contacted surface, particularly, the crevices or cracks. The objective of this study was to evaluate the shear stress-induced by air jet impingement for removal of food residues. The wall shear stress distribution on a surface below an air jet was measured using oil-film interferometry. The air impingement was operated at three level of air velocity (Reynolds number = 7756, 13,444, and 18,787) and three different nozzle height (24 (H/D = 6), 32 (H/D = 8), 40 mm (H/D = 10)). The shear stress was found to become higher along with the increase of air velocity and decrease of nozzle height. The observed distribution of wall shear stress on the surface was fitted using the Gaussian model and Polynomial model. The Gaussian model appeared to be preferable according to the statistical analysis, which aligned with previous literature. Based on the Gaussian model, the maximum wall shear stress was estimated. The maximum wall shear stress increased with Reynolds number ( Re ) and the ratio of jet height to jet nozzle diameter. The highest maximum shear stress (10.74 Pa) occurred at a Reynolds number of 18,787 and a height of 24 mm (H/D = 6). This trend was directly correlated with removal efficiency. The greatest removal of the treatment was achieved by 40% at maximum wall shear stress. A linear relationship between percent residue removed and maximum wall shear stress was obtained from the experimental results, and a threshold maximum wall shear stress of 2.73 Pa has been proposed for removal of peanut butter residue from a stainless-steel surface. The current study demonstrates the promising of the air impingement in the cleaning operation for low water activity food. • Wall shear stress increased with an increase in velocity and decrease in height. • Removal of food deposits revealed a similar trend to velocity and height. • Linear relation was found between maximum shear stress and food removal.

Numerical Simulation of Single‐Jet Impingement Cooling of a Seamless Steel Tube

Herein, numerical simulations for single‐jet impingement cooling of a heated seamless steel tube are carried out. The effect of water flow, jet height, and jet position are analyzed during numerical simulations. Water flow varies from 2 to 10 L min−1, jet height varies from 10 to 50 cm, and jet position gradually rotates from right above steel tube (α = 90°) to bottom (α = −90°). The distribution of pressure, turbulence kinetic energy, Nusselt number, and surface temperature of steel tube are obtained. This analysis reveals that water flow is an important factor affecting heat transfer efficiency. With the increase in water flow, the improvement of heat transfer efficiency gradually decreases. The cooling capacity of the steel tube in the circumferential direction is greater than that in the axial direction of the steel tube. In contrast, as the jet height gradually increases at α = 90°, the Nusselt number increases first and then decreases. In addition, the jet position also seriously affects the cooling capacity of the jet. When the jet is stationed at the top and bottom, the cooling capacity deviation between the two reaches the maximum.

Wave breaking and jet formation on axisymmetric surface gravity waves

Axisymmetric standing waves occur across a wide range of free surface flows. When these waves reach a critical height (steepness), wave breaking and jet formation occur. For travelling surface gravity waves, wave breaking is generally considered to limit wave height and reversible wave motion. In the ocean, the behaviour of directionally spread waves lies between the limits of purely travelling (two dimensions) and axisymmetric (three dimensions). Hence, understanding wave breaking and jet formation on axisymmetric surface gravity waves is an important step in understanding extreme and breaking waves in the ocean. We examine an example of axisymmetric wave breaking and jet formation colloquially known as the ‘spike wave’, created in the FloWave circular wave tank at the University of Edinburgh, UK. We generate this spike wave with maximum crest amplitudes of 0.15–6.0 m (0.024–0.98 when made non-dimensional by characteristic radius), with wave breaking occurring for crest amplitudes greater than 1.0 m (0.16 non-dimensionalised). Unlike two-dimensional travelling waves, wave breaking does not limit maximum crest amplitude, and our measurements approximately follow the jet height scaling proposed by Ghabache et al. (J. Fluid Mech., vol. 761, 2014, pp. 206–219) for cavity collapse. The spike wave is predominantly created by linear dispersive focusing. A trough forms, then collapses producing a jet, which is sensitive to the trough's shape. The evolution of the jets that form in our experiments is predicted well by the hyperbolic jet model proposed by Longuet–Higgins (J. Fluid Mech., vol. 127, 1983, pp. 103–121), previously applied to jets forming on bubbles.

Comparison of the mean skin friction with the heat transfer and the flow development in turbulent planar wall-attaching jet flows.

Measurements of the mean skin friction below turbulent planar wall-attaching offset jet flows were performed using oil-film interferometry for jets initially offset (Hs) 0.2 to 1 times the jet height (Hj) from the wall. The results were compared with turbulent flow field measurements and with previous heat transfer measurements. The profiles of the mean skin friction coefficient below the offset or preattachment region were similar when compared relative to the distance to the reattachment location (Xr). The initial recovery length of the mean skin friction downstream of the reattachment location was proportional to the reattachment length similar to the decrease in the wall pressure fluctuations in this region. The longer recovery and initial decay of the mean skin friction coefficient appeared to collapse in terms of (x-Xr)/Hj; as did the initial change in the mean static pressure, the measures of the mean flow development, and the Stanton number downstream of the reattachment point. The decay in the maximum of the turbulent stresses associated with the attached shear layer also collapsed in terms of (x-Xr)/Hj, unlike the fluctuating pressure and initial recovery of the mean skin friction. The change in the location of the minimum and zero in the turbulent shear stress as the flows transitioned to the wall jet region collapse in terms of x-Xr/Hj similar to the longer recovery and initial decay of the mean skin friction. The mean skin friction for flows with Hs of Hj and jet Reynolds numbers (Rej) of 21800 to 54500, varied as Rej-0.2 below the attaching jet, in the reattachment region, and in the downstream wall jet region.

Occurrence of Low-Level Jets over the Eastern U.S. Coastal Zone at Heights Relevant to Wind Energy

Two years of high-resolution simulations conducted with the Weather Research and Forecasting (WRF) model are used to characterize the frequency, intensity and height of low-level jets (LLJ) over the U.S. Atlantic coastal zone. Meteorological conditions and the occurrence and characteristics of LLJs are described for (i) the centroids of thirteen of the sixteen active offshore wind energy lease areas off the U.S. east coast and (ii) along two transects extending east from the U.S. coastline across the northern lease areas (LA). Flow close to the nominal hub-height of wind turbines is predominantly northwesterly and southwesterly and exhibits pronounced seasonality, with highest wind speeds in November, and lowest wind speeds in June. LLJs diagnosed using vertical profiles of modeled wind speeds from approximately 20 to 530 m above sea level exhibit highest frequency in LA south of Massachusetts, where LLJs are identified in up to 12% of hours in June. LLJs are considerably less frequent further south along the U.S. east coast and outside of the summer season. LLJs frequently occur at heights that intersect the wind turbine rotor plane, and at wind speeds within typical wind turbine operating ranges. LLJs are most frequent, intense and have lowest core heights under strong horizontal temperature gradients and lower planetary boundary layer heights.

Research on the rapid combustion process of butane under microwave discharge

To study the combustion process of fuel in the microwave plasma torch, we designed a butane microwave plasma device exploiting a tungsten rod as an electrode. Through analysis of the image record by high-speed camera, we found that the discharge of butane microwave plasma torch is a cyclic process at atmospheric pressure at a frequency of around 100 Hz. During the discharge, the active particles continuously diffuse from the electrode to the outside like the bloom of the flower. Then, the variation of plasma torch of jet height and temperature with microwave power is obtained. In addition, we studied the effects of different butane flow rates on the plasma torch. The results illustrate that excessive butane will lead to carbon deposition on the electrode. All in all, this work provides a new understanding of the combustion of the microwave plasma torch, which is conducive to the further development of microwave plasma in the fields of waste gas treatment, fuel combustion, and plasma engine.

Mixing characteristics of liquid jet injected behind a curved pylon in supersonic flow

Mixing of liquid jet in Mach 1.71 crossflow using a curved pylon configuration has been studied experimentally. The liquid acetone jet was injected at 60° against the free stream airflow behind the pylon. High-speed Schlieren and Mie scattering experiments were performed to evaluate the mixing characteristics of liquid jet injected behind the curved pylon. POD and SPOD analyses were employed on the high-speed Schlieren images to observe the coherent structures, their energy levels, and frequencies at which these structures occur. Results showed improvements in penetration height and the lateral spread of liquid jet for the curved pylon configuration. The penetration height of the liquid jet was 7.79% more for the curved pylon case as compared with the standard pylon case for the same momentum flux ratio. Further increase in the momentum flux ratio increased the penetration height to 15.53% more as compared with the standard pylon case. The spread area of the liquid jet was 17.42% more at x/dj=25 and 25.97% more at x/dj=40 for the curved pylon case than the standard pylon case for the same momentum flux ratio. The presence of counter-rotating vortices at the top of the liquid plume and in the near-wall region caused a higher spread area in curved pylon cases as compared with standard pylon cases.

Modeling and optimization of the thermal-hydraulic performance of direct contact heat exchanger using quasi-opposite Jaya algorithm

In this study, the thermal-hydraulic performance of a direct contact heat exchanger (DCHE) was explored and optimized by applying the multi-objective quasi-opposite Jaya (MOQO-Jaya) algorithm. Inlet temperature of the continuous phase, continuous phase flow rate, dispersed phase flow rate, evaporator height, nozzle diameter, and number of jets were considered as the design variables for the optimization. The optimization functions were the volumetric heat transfer coefficient (VHTC), entransy dissipation, and entropy production. Study results indicated that the entransy dissipation and entropy production were negatively correlated with the heat transfer capacity in a DCHE, with a Pearson correlation coefficient above 0.96; that is, the smaller the entransy dissipation and entropy production, the better the heat transfer performance in the DCHE. The sensitivity of design parameters demonstrated that temperature played an important role. The optimization results and performance of the MOQO-Jaya algorithm were compared with those of the non-dominated sorting genetic algorithm (NSGA-II). The convergence of MOQO-Jaya was 2.24 times quicker than that of NSGA-II in DCHE optimization. The correctness of numerical model for optimizing the DCHE was verified through experiments. The errors of the experimental and algorithm optimizations both were within ±10%. Results demonstrated that after multi-objective optimization, the VHTC increased by 33.89% and entransy dissipation reduced by 44%, compared with those of traditional experiments.

Baroclinicity in stable atmospheric boundary layers: Characterizing turbulence structures and collapsing wind profiles via reduced models and large‐eddy simulations

AbstractBaroclinicity is a common, while often overlooked, phenomenon in the atmospheric boundary layer (ABL) that significantly impacts mean wind and turbulence structures. It adds another layer of complexity to the much‐studied stable barotropic ABLs by modulating the pressure gradient in height. Despite the prevalence of baroclinic and stable conditions in nature, our knowledge of their interacting effects on stratified ABL features is limited. The article aims to bridge this knowledge gap by systematically varying baroclinicity and stability using the large‐eddy simulation (LES) technique. The results of 15 conducted LESs indicate that baroclinicity alters the Ekman spiral shape and the low‐level jet's height in stable ABLs. Friction velocity, Obukhov length, shear production and the ABL height are highly influenced by baroclinicity in weakly stable ABLs, whereas they converge to a constant asymptote as the stability increases, independent of the baroclinicity regime. This is attributed to the strong turbulence destruction in very stable ABLs that decouples the surface from higher elevations where the baroclinicity effect is more significant. A reduced model for baroclinic wind profiles is developed and an analytical estimation of that for weakly baroclinic regimes is tested against LES results. Two rescaling methods in the inner and outer layers of stable baroclinic ABLs are used in a novel way to non‐dimensionalize the wind profiles. The suggested approaches reasonably collapse baroclinic/barotropic stable wind profiles in all considered LES cases. This study's findings elucidate the underlying physics of baroclinic stable ABLs and are useful for characterizing the wind profiles in weather/climate models, field measurements, and various industrial applications.

Forced convective heat tranfer in a metallic foam cylinder cooled by a slot jet flow and comparison with a smooth cylinder and a full flow

The aim of the present paper is to study experimentally the enhancement of heat transfer, due to the presence of a layer of aluminum foam covering a smooth circular cylinder. The measurements are carried on with the impingement of a rectangular jet of air, which has a transversal dimension equal or smaller than the cylinder diameter, or jet flow, and full flow (or uniform flow, which has a greater dimension than the cylinder diameter). Literature does not report any paper dealing with this multiple problem. Experimental heat transfer and fluid dynamics measurements are carried out at Reynolds number ranging from ReD=2200 to ReD=24,750. The layer of the aluminum foam, 8 mm thick, is glued onto the smooth cylinder, with an external diameter of D = 10 mm. The height of the slot jet is H = 10 mm, with a width of W = 135 mm. Jet flow visualizations are performed for the smooth and the foam-covered cylinder to explain the heat transfer results from the fluid dynamics point of view. The flow visualizations show that the jet flow always penetrates into the foam, wetting it completely, while the reattachment never occurs downstream. In the smooth cylinder, the jet completely envelops the cylinder, at greater Reynolds numbers and a distance of x/H = 7, with a small recirculation zone, showing that the rear surface also participates in the heat transfer. The heat transfer on the foam-covered cylinder increases, mainly, because of the large increase in the exchange surface, while it is somewhat penalized, at the higher Reynolds values and at the greater distance from the slot exit, because the rear surface is little involved in the heat transfer.

Jet fires involving releases of gas and solid particle

The high-pressure leakage of underground pipeline could entrain the nearby sands and soils, and thus cause the jet fire of gas-solid mixtures. In addition, the thermal runaway of Lithium ion batteries could also cause a typical gas-solid jet diffusion flame. In comparison with the gaseous jet diffusion flame and the gas-solid-air jet premixed flame, the gas-solid jet diffusion flame lacks of exploration. Thus, a new facility consisting of a jet fire apparatus and a sand feeder, was designed to explore the gas-solid jet fire. The test repeatability was justified for the proposed facility. The dynamical differences of gas-solid and gaseous jet fires are clarified. It is found that the gas-solid jet fire holds a larger lift-off height, a less visible flame height and a less radiative fraction than the purely gaseous jet fire. In particular, for gas-solid jet fires at high exit velocities, the lift-off height increases, but the visible flame height and radiative fraction seem to decrease, as the particle size of sand decreases to increase the jet height of sand. (1) Inside the flame volume, the sand plays a role of heat sink to decrease the flame temperature and thus the flame burning velocity. Moreover, in the lift-off region, the sand dilutes the gas fuel and isolates the surrounding oxygen to inhibit the combustion. Both effects of sand cause a large lift-off height, especially for the sand of little particle size. (2) A large lift-off height means the increase of the air mass entrained from flame base or the portion of the premixed flame to the whole flame. In addition, the sand particle-laden flow would significantly increase the ambient air entrainment rate of fire plume. Both effects result in a less visible flame height. (3) The portion of the premixed flame increases in the bottom, which reduces the total soot amount inside the whole flame volume. The decrease of the flame temperature and the total soot amount reduces the radiative fraction. (4) The sand significantly affects the flame geometry and radiative fraction, which alters the radiant heat flux distribution and challenges the development of thermal radiation model for gas-solid jet fires. The gas-solid jet fire holds a less peak radiant heat flux but a higher positon of peak radiant heat flux than the gaseous jet fire. In addition, the line source radiation model available in literature is revised to well predict the radiant heat flux distribution of the gas-solid jet fires.

Simultaneous impact of twin drops on a semi-infinite liquid target

We take the enduring topic of drop impact on a deep pool of similar liquid further by allowing twin drops to impact simultaneously. Impacts are sufficiently proximal that impact crowns and craters interact, distorting and merging craters, and creating previously undocumented supersurface fluid interactions. The unique features of twin impacts occur when crowns collide to create a central veil that bifurcates the two craters and the expulsion of jet-like features atop colliding crowns. The emergence of a plethora of splash features is dependent on the Froude number (Fr=30−200) and drop separation distance. We analyze proximal crater evolution using theory developed for singular drops and develop scaling relations to predict crown and jet height. Crater and jet energies are compared for various impact velocities and drop separation distances. We find that craters close enough to merge produce thicker, but not higher, rebound jets.

Utilizing jet impingement on protrusion/dimple heated plate to improve the performance of double pass solar heat collector

The effect of geometrical parameters of jet impingement on indented protrusion absorber in double-pass solar heat collector (DPSHC) was investigated. For fixed optimum values of protrusion parameters namely arc angle (αp), relative height ratio (ep/Dh), and relative pitch ratio (p/Dh), four different jet height ratios (Hji/Dh) ranging from 0.11 to 0.44 have been used with varying streamwise pitch (Xji/Dh) and spanwise pitch ratio (Yji/Dh). At a constant jet diameter ratio (Dji/Dh), the Reynolds number (Re) of the airflow fluctuated between 2500 and 22500. It has been found that the Hji/Dh plays an important role in the performance of DPSHC and that the maximum value of the Nusselt number (Nudp) = 544 with friction factor (ffdp) = 0.206 was obtained at Xji/Dh=1.32, Hji/Dh=0.22, and Yji/Dh=1.32. The investigation reveals that the thermohydraulic performance (ηdp) of the DPSHC increases as the Re increases from 2500 to 15000 and after that from 15000 to 22500 it starts to decline. The optimum value of ηdp = 3.86 has been found corresponding to Re = 15000, Hji/Dh=0.22, Xji/Dh=1.32, and Yji/Dh=1.32. Nudp and ffdp correlations were developed in terms of jet impingement parameters and the selected range of Re which can be useful for estimating the performance of a DPSHC with the combined technique. The optimum deviation in the Nudp and ffdp values was ±10%, which correlated well with the experimental results.

Flame morphology of horizontal jets under sub-atmospheric pressures: Experiment, dimensional analysis and an integral model

This paper investigated the flame morphology including horizontal projection length and vertical height of horizontal turbulent jets under various sub-atmospheric pressures, for which both data and model are few. Experiments are conducted for various nozzle diameters in the range of 3 mm to 7 mm. Propane is used as the fuel in this study. The sub-atmospheric pressures, created by a reduced pressure chamber, are varied from 40 kPa to 100 kPa. A digital video camera is used to measure the flame morphologic characteristics. The experimental results show that the flame horizontal projection length increases with the increasing of heat release rate or the decreasing of chamber pressure. Meanwhile, the flame vertical height first increases then decreases with the increasing of heat release rate or the decreasing of chamber pressure, which shows a non-monotonic behavior. A characteristic length scale is proposed by dimensional analysis considering jet initial momentum, flame buoyancy and ambient air density to illustrate the controlling mechanisms in the flow development. And the characteristic air entrainment flow rate ρ∞Lm2g′Lm is used to normalize the stoichiometric mass flow rate at the flame tip, which determines the total length of the trajectory. Non-dimensional formulas are developed based on the derived length scale and characteristic air entrainment flow rate to describe the flame horizontal projection length and vertical height of horizontal jets under various sub-atmospheric pressures. Finally, an integral model considering air entrainment mechanism, top hat profiles and strong plume theory for the conservation of mass and momentum in horizontal and vertical directions is developed, which shows to describe the flame morphologic characteristics well.

Experimental Study of the Sidewall Effect on Three-Dimensional Turbulent Wall Jet

Abstract An experimental study on the effect of sidewalls on the flow characteristics of a three-dimensional turbulent square wall jet is carried out at a Reynolds number of 25,000. The sidewalls are defined as the two parallel plates along the vertical jet centerline. Four different sizes of sidewall enclosure (here after referred to as ‘SWE’) are placed at the lateral positions (z) of ±3.5h, ±4h,±4.5h, and ±5h from the vertical jet centerline plane, where h is the height of square jet. The mean characteristics of fluid flow in wall normal (y) and lateral (z) directions at different downstream locations (x/h=0.2−45) are measured using a hotwire anemometer. The velocity measurements are also performed in the z – y lateral plane at four downstream locations (x/h=30, 35, 40, and 45). Results indicate that the mean velocity profile in lateral and wall normal directions behaves differently depending on the size of SWEs. The decay rate of the maximum mean velocity increases with decrease in size of SWEs after the downstream location (x/h≥20). The decay rate of the maximum mean velocity increases about 5.6% in 140 mm SWE as compared to 200 mm SWE. It is noted that spread of the jet in wall normal and lateral directions increases with decrease in size of SWEs after the attachment of the flow stream on the sidewalls. In the far-field location of the present case, the smaller size of SWE (140 mm SWE) has 10.7% and 23.1% higher spread rate as compared to larger size of SWE (200 mm SWE) in wall-normal and lateral directions, respectively. It is also seen that the self-similar profile gets delayed in wall-normal direction as compared to lateral direction for all the cases. The wall-normal self-similar profile is obtained early with increase in the size of SWEs and it is obtained at x/h=30, 27, 24, and 20 for 140 mm, 160 mm, 180 mm, and 200 mm SWEs, respectively. The flow stream seems to climb the sidewall and this tendency increases with increase in size of SWEs.

Explosive caldera-forming eruptions and debris-filled vents: Gargle dynamics

AbstractLarge explosive volcanic eruptions are commonly associated with caldera subsidence and ignimbrites deposited by pyroclastic currents. Volumes and thicknesses of intracaldera and outflow ignimbrites at 76 explosive calderas around the world indicate that subsidence is commonly simultaneous with eruption, such that large proportions of the pyroclastic currents are trapped within the developing basins. As a result, much of an eruption must penetrate its own deposits, a process that also occurs in large, debris-filled vent structures even in the absence of caldera formation and that has been termed “gargling eruption.” Numerical modeling of the resulting dynamics shows that the interaction of preexisting deposits (fill) with an erupting (juvenile) mixture causes a dense sheath of fill material to be lifted along the margins of the erupting jet. This can cause an eruption that would otherwise produce a buoyant plume and fallout deposits to instead form pyroclastic currents as the dense sheath drives pulsing jet behavior. Increasing thickness of fill amplifies the time variation in jet height. Increasing the fill grain size relative to that of the juvenile particles can result in a much higher jet due to poorer mixing between the dense sheath and the dilute jet core. In all cases, material collapses along the entire height of the dense sheath rather than from the top of a simple fountain. These gargle dynamics provide strong backing for processes that have been inferred to result in intraplinian ignimbrites and simultaneous deposition from high- and low-energy pyroclastic currents.

Numerical study on mean flow field of turbulent dual offset jet

A numerical investigation on the mean flow and turbulence characteristics of dual offset jet for various separation distances between the two jets with a fixed offset height of the lower jet from the bottom wall is reported in this study. The numerical simulations have been performed by solving the Reynolds-averaged Navier-Stokes equations (RANS) with two-equation standard [Formula: see text] turbulence model. The Reynolds number based on the jet width and the inlet turbulence intensity are considered as 15,000 and 5%, respectively. The computational results for the mean flow reveal that after issuing from the nozzles, the adjacent shear layers of the offset jets meet together at the merging point and then the merged jets reattaches on the bottom wall at the reattachment point before they combine together at the combined point forming a single jet flow. In the far downstream, the flow field behaves like a classical single wall jet flow. The self-similarity of mean flow field is achieved at far down stream of combined point. An increase in separation distance between the two jets [Formula: see text] results in a decrease in magnitude of the streamwise maximum velocity of the combined jet but with same rate of decay. The converging region of the jets has depicted considerable growth of turbulence as the jet centrelines bend towards the merging point. According to the mean flow results, the distances of the reattachment point and the combined point from the nozzle exit gradually increase with the progressive increase in separation distance between the two jets within the range d/ w = 3–8.

Jet Dynamics Associated with Drop Impact on Micropillared Substrate

The jetting phenomenon associated with droplet impact upon a hydrophilic micropillared substrate was analyzed in detail using a high-speed camera. Viscosities of the fluids were varied using differing concentrations of glycerol in deionized water. This paper aims to connect similarities between this form of capillary jetting and another well-known jetting phenomenon from the bubble bursting. Both experience a cavity collapse when opposing fluid fronts collide which causes a singularity at the liquid surface, thus leading to the occurrence of jetting. Following processes used to define scaling laws for bubble bursting, a similar approach was taken to derive scaling laws for the dimensionless jet height, jet radius, base height, and radius of the jet base with respect to dimensionless time for the jetting phenomenon associated with the droplet impact. The development of a top droplet before the breakup of the jet also allows the examination of a scaling law for the necking diameter. We find that with the proper scaling factors, the evolution of the jet profile can collapse into a master profile for different fluids and impact velocities. The time dependence of the necking diameter before the jet breakup follows the power law with an exponent of ~2/3. Contrastingly, for other jet parameters such as the radius and height, the power law relationship with time dependence was not found to have a clear pattern that emerged from these studies.

Heat transfer characteristics for double water‐jets on thick plates with various jet velocities and heights

During cooling, the array jet impact can achieve a rapid and uniform cooling of the high-temperature plate. Previous research results mostly focused on single-jet cooling of thin plates, but the complex flow and heat transfer characteristics between multiple jets could not be obtained. To clarify the heat transfer mechanisms and cooling speed fields characteristics in different flow regions, double water-jet impingement experiments were carried out on an AISI 304 austenitic stainless-steel plate with a thickness of 50 mm. The jet exit velocity was set to 2.95, 5.90, 8.06, and 11.80 m/s, while the jet height was set to 50, 150, 250, 350, and 450 mm. Pure water was selected as the working fluid with a constant temperature of 12.8 °C. The results show that a complex trend with sequential increasing, decreasing, and increasing behaviors appears in the wetting front width, upon increasing the wetting region diameter. The confluence fluid was found to reduce the influence caused by different jet velocities and to increase the influence caused by different jet heights. In addition, it was found that an excessive amount of accumulated fluid reduces the heat transfer efficiency during transition boiling. Finally, the correlations between maximum heat flux and average maximum cooling speed were established, which provide useful data for optimizing the cooling technology.