Increasing Jet Velocity Research Articles

Damage and fracture characteristics of rocks with different structures under high-velocity water jet impact

This paper aims to investigate the damage and fracture characteristics of rocks with different structures under water jet impact. Three types rocks including heterogeneous coal, homogeneous sandstone, and transversely isotropic shale, were selected as the impact target rock. Water-jet impingement experiments with jet velocities varying from 447 m/s to 774 m/s were conducted. The rock macro-breakage characteristics were obtained by describing the breakage patterns, measuring the erosion depth and area, and counting the cracks number. A 3D reconstruction method based on the computed tomography and digital image processing technology was proposed to visualize the internal breakage characteristics and quantitatively analyze the damage field distribution. Combined with fracture morphology from scanning electron microscope, the failure mechanisms of rocks with three structures under water jet impact were revealed. The results indicate that coal, sandstone and shale will experience different breakage patterns with the increasing jet velocity: layered transverse cracks → “十”-type crack network → splitting crack, only one spindle-shape erosion pit, surface debris spalling → layered transverse cracks → “T”-type crack network. The effects of jet velocity and rock structure on the evaluation indexes (such as erosion depth and damage area, the number and angle of transverse cracks) were investigated thoroughly. In addition, the relationships between total damage degree based on breakage volume and jet velocity, and between damage variable based on breakage area and erosion depth, were compared and discussed. The failure mechanisms of three rocks impacted by water jets are as follows: (i) the reflection and interference of stress wave combined with the pressured water wedge effect cause the layered breakage and longitudinal splitting breakage of heterogeneous coal; (ii) the grinding effect of back flow mainly accounts for the formation of spindle-shape erosion pit in homogeneous sandstone; (iii) the shock stress wave effect and bedding structure lead to the spalling of large shale block and layered breakage, and the reflected wave tensile accompanied with pressured water wedge effect cause the tensile splitting breakage.

Correlations for heat transfer characteristics of the onset of nucleate boiling in submerged jet impingement

The submerged jet impingement boiling is one of the most effective heat transfer methods in dissipating both transient and steady concentrated heat loads. However, studies on the submerged jet impingement boiling still lack experimental data, especially for the heat transfer characteristics of the onset of nucleate boiling (ONB). Therefore, an experimental study of a submerged circular jet impinging on a polished copper surface is conducted in the subcooled water. The experiments investigate the steady-state heat transfer characteristics at the stagnation point from single phase convection to partial nucleate boiling. The effects of jet Reynolds number, liquid subcooling, and nozzle diameter on the onset of nucleate boiling are systematically studied. The experimental results show that the single phase convective heat transfer can be enhanced by increasing jet velocity at a fixed nozzle diameter or decreasing nozzle diameter under the same jet Reynolds number. The liquid subcooling seems to have negligible effect on the single phase heat transfer coefficient. For the ONB point, it is found that the ONB wall superheat is not influenced by liquid subcooling or jet parameters. The ONB heat flux, on the other hand, is found to depend on the experimental parameters. Increasing the jet Reynolds number and liquid subcooling, or decreasing the nozzle diameter could broaden the single phase convective regime and delay the onset of nucleate boiling, in terms of heat flux. Empirical correlations are proposed for predicting the stagnation point heat transfer coefficient, heat flux, and wall superheat at the ONB point. The comparison between the predicted values and the experimental data indicates that the empirical correlations are able to predict the heat transfer characteristics of the onset of nucleate boiling within an acceptable accuracy under the applicable range.

Theoretical and experimental investigation on fracture response of coal impacted by high-velocity water jet

This paper adopted theoretical derivation and experiments to investigate the stress wave propagation characteristics in rock under water jet impact and fracture response of coal. The evolution model of stress and displacement was presented to analyze the formation, propagation and attenuation of stress wave during rock fragmentation. The rock failure criterion for shear and tensile failure under dynamic impact loads, and crack propagation criterion under quasi-static pressure of water jet were established. And the broken pit range formed by shear component of stress wave and the damage range caused by tensile stress were obtained. Based on scanning electron microscopy and binarization method, experiments on water jet impinging coal under various velocities were conducted to study the failure patterns and fracture characteristics of coal. It is indicated that during the water-hammer pressure and its unloading stage, the radial stress and tangential stress decrease first and then increase, accompanying the occurrences of the radial and tangential tensile stresses peaks and the maximum compression displacement. In the subsequent stagnation pressure stage, the radial and tangential stresses trend to be the stable compressive stress and tensile stress, respectively. The peak values of radial stress and tangential stress at different stages all decay exponentially with the propagation distance. Moreover, the broken pit surrounded with radial and annular cracks is formed on coal when the jet velocity exceeds the threshold value, while the split fracture of coal will occur for the high impact velocity. The rock-breaking specific energy consumption decreases first and then increases with the increasing jet velocity. Besides, the theoretical damage scope is verified and close to the experimental value before the splitting fracture of coal. Combined with fracture morphology of coal debris with different sizes, two microscopic damage-failure modes of coal impacted by water jets are revealed, namely shear failure and tensile failure.

Dripping and jetting of semi-dilute polymer solutions co-flowing in co-axial capillaries

This work is focused on the mechanisms of the dripping and jetting flow modes of viscoelastic semi-dilute polyacrylamide aqueous solutions co-flowing with silicone oil in co-axial capillaries. A phase diagram of the dripping, jetting, and intermediate flow modes is established. It was found that in the dripping mode, the elongation velocity of the filament between the terminal droplet and the inner capillary is controlled solely by the continuous phase rate. At the same time, the decrease in the filament diameter is due to both stretching and outflow of the polymer solution into the terminal droplet. In the jetting mode, the thread diameter was found to evolve in three stages. In the first stage, the average jet velocity increases, whereas in the second and third stages, it becomes constant and corresponds to the velocity of the continuous phase. The transition from the second to the third stage is defined by the appearance of capillary waves resulting in the formation of the beads-on-string structure. In the third stage, the filament diameter between the neighbor beads decreases exponentially and is governed by the relaxation time, which strongly depends on polymer concentration, but does not depend on the continuous phase flow rate. A simple physical model was proposed for describing the evolution of dimensions of filaments and beads during development of jet capillary instability. The universal character of the evolution of filaments and beads sizes, which is independent of concentration of semi-diluted polymer solutions and flow rates of the continuous phase, is revealed.

Computational Analysis of Water-Submerged Jet Erosion

Erosion causes substantial damage in many industrial equipment such as pump components, valves, elbows, and plugged tees. In most cases, erosion is coupled with corrosion, resulting in major financial loss (nearly 3.4% of the global gross domestic product) as evidenced in oil and gas industries. In most cases, the erosion occurs in a submerged water medium. In this paper, erosion characteristics of stainless steel 316 were investigated computationally in a water-submerged jet impingement setup. The erosion profiles and patterns were obtained for various parameters over ranges of inlet velocities (3 to 16 m/s), nozzle diameters (5 to 10 mm), nozzle–target distances (5 to 20 mm), nozzle shapes (circular, elliptical, square, and rectangular), impingement angles (60° to 90°), and particle sizes (50 to 300 µm). The range of Reynolds number studied based on nozzle diameters is 21,000–120,000. The Eulerian–Lagrangian approach was used for flow field prediction and particle tracking considering one-way coupling for the particle–fluid interaction. The Finnie erosion model was implemented in ANSYS-Fluent 19.2 and used for erosion prediction. The computational model was validated against experimental data and the distributions of the erosion depth as well as the locations of the of maximum and minimum erosion points are well matched. As expected, the results indicate an increase in loss of material thickness with increasing jet velocity. Increasing the nozzle diameter caused a reduction in the maximum depth of eroded material due to decreasing the particle impact density. At a fixed fluid inlet velocity, the maximum thickness loss increases as the separation distance between the nozzle outlet and target increases, aspect ratio of nozzle shape decreases, and impingement angle increases. The erosion patterns showed that the region of substantial thickness loss increases as nozzle size/stand-off height increases and as particle size decreases. In addition, increasing the aspect ratio and impingement angle creates skewed erosion patterns.

Numerical study on edge tone with compressible direct numerical simulation: Sound intensity and jet motion

A two-dimensional model of the edge tone is studied by a highly accurate and reliable method of direct numerical simulation of the compressible Navier-Stokes equations, and used to verify key features observed in previous experimental and numerical studies, and to discover new features related to the jet motion and the edge tone generation mechanism. The first and second modes of the edge tone that are numerically reproduced agree well with Brown’s equation. In the mode transition region, dynamical mode transition is observed at a fixed jet velocity. For both first and second modes, the pressure distributions are antisymmetric with respect to the edge plate, and the sound intensity is proportional to the fifth power of the jet velocity. These results are consistent with the edge tone being radiated from a dipole-like source. Spatial profiles of the velocity and the velocity variance of the oscillating jet are also investigated for each mode over a range of the jet velocity including the mode transition regime. The amplitude of the velocity oscillation becomes constant with increasing jet velocity, while a measure of the amplitude of the velocity variance profile, which is introduced to characterize the strength of the jet fluctuation and named the ’fluctuation strength’, is proportional to the third power of the jet velocity. Some properties of the fluctuation strength correspond to properties of the sound intensity, including the first mode having larger amplitude than the second mode, and the way of deviating from the power law at smaller values of jet velocity and in the mode transition region. It is proposed that the third-power law exhibited by behavior of the fluctuation strength could be related to the increase of the skewness observed in the velocity profile with increase of jet velocity, and a model calculation is used to support this proposal.

Numerical simulation on the oil capture performance of the oil scoop in the under-race lubrication system

The under-race lubrication system providing a moderate amount of oil to high-speed bearings is one of the key factors to ensure its good lubrication and cooling. To investigate the oil capture performance of the oil scoop, a complete numerical calculation model was established, and unsteady simulations of oil–gas two-phase flow inside the under-race lubrication system were carried out. The results indicate that the oil capture efficiency increases first and then decreases with increasing rotating speed at different oil jet velocities. The ratio of the rotating Reynolds number of the oil scoop to the Reynolds number of the oil jet corresponding to the maximum oil capture efficiency changes linearly with oil jet velocity. There are three variation trends of oil capture efficiency at different rotating speeds with the increase of oil jet velocity: monotonically decreasing, first increasing, and then decreasing and monotonically increasing. However, the amount of oil captured by the oil scoop increases monotonically under all operating conditions. The effect of oil temperature on oil capture efficiency is mainly due to the change of dynamic viscosity of the oil. The oil capture efficiency can be improved by appropriately increasing the oil temperature within a certain temperature range.

Drop sizes and its distribution in jet-driven liquid–liquid mixing column: substantial application for the liquid–liquid extraction process

The present work reports the study of drop size distribution, drop aspect ratio, and interfacial area in a liquid–liquid jet-driven mixing column. Four different liquids, i.e., kerosene, diesel, 1-decanol, and paraffin, are used as dispersed phases and water as the continuous phase. The effect of the jet velocity, the volume fraction of the dispersed phase, and the physical properties of a liquid mixture such as density, viscosity, and surface tension are studied on the drop size distribution and drop aspect ratio. The column diameter of 0.37 m and nozzle diameter of 0.005 m were used. The velocity of the jet is varied between 0.68 m/s–1.71 m/s. It is observed that the drop size decreases as the volume fraction of the dispersed phase and jet velocity increases. Drop size distribution is found to follow the Log-logistic drop size distribution function. The aspect ratio is close to unity for a small drop, and it decreases as the drop size increases. The increase in the Eötvös number leads to a decrease in the aspect ratio. The correlation for the drop aspect ratio is proposed in terms of the Reynolds and Eötvös numbers in the ranges of 0.061 <Re < 342.59, 0.004 <Eo < 1.166, and (−4.756) < log10 (Mo) < (−9.171), respectively. Interfacial area of drop enhances with an increase in the dispersed volume phase and jet velocity. Empirical correlation is developed to predict Sauter mean drop diameter, drop aspect ratio, distribution function parameters, and interfacial area based on operating variables and physical properties of the system.

Research on Drag Reduction Mechanism and Optimization Design of Bionic Jet Flow

In the process of sailing, the frictional resistance of underwater objects must be reduced, which is of great significance for improving speed, range and saving energy. Based on the inspiration of the shark breathing process, this paper simplified the water spray process from the external bronchial slit of the shark breathing to the cross-flow lateral jet process. By establishing a numerical model, the influence mechanism of the jet on the flow resistance was studied, and by analyzing different jet velocities, the change of viscous resistance reveals the influence of jet velocity on viscous resistance. The results show that the side jet can reduce the boundary layer on the surface of the object, thereby having a drag reduction effect, and the drag reduction rate can be about 10%. In addition, as the jet velocity increases, the drag reduction effect is enhanced, and at the back of the jet port the vortex is formed, the outlet pressure is reduced, and the drag reduction effect is strengthened. At the same time, the study found that the arc-shaped jet port is more conducive to the reduction of resistance.

Critical Heat Flux of Confined Round Single Jet and Jet Array Impingement Boiling

This study involves experimental investigation of key parameters influencing CHF for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Close examination of the data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressures combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions.

Research on the Increase of Mineral Dissociation Degree by High Pressure Water Jet

The traditional grinding technology of ball mill has many problems, such as high energy consumption, unable to maintain the original crystal shape, and it is difficult to obtain high quality and high purity ultrafine powder. The high-pressure water jet mineral dissociation technology is a new technology which is developed in response to these requirements. The experiment shows that the water jet technology has a strong ability of ultrafine grinding. With the increase of jet velocity, the crushing effect is more obvious. The high-pressure water jet pulverization is superior to ball milling for the separation of rare earth minerals, which has the characteristics of high dissociation degree and relatively uniform particle size. The water jet pulverization is more beneficial to the separation and recovery of rare earth minerals.

Investigation of cavitation bubble collapse in hydrophobic concave using the pseudopotential multi-relaxation-time lattice Boltzmann method* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11874140 and 11574072), the Fund from the State Key Laboratoryof Acoustics, Chinese Academy of Sciences (Grant No. SKLA201913), the National Key Research and Development Program of China (Grant No. 2016YFC0401600), and the Postgraduate

The interaction between cavitation bubble and solid surface is a fundamental topic which is deeply concerned for the utilization or avoidance of cavitation effect. The complexity of this topic is that the cavitation bubble collapse includes many extreme physical phenomena and variability of different solid surface properties. In the present work, the cavitation bubble collapse in hydrophobic concave is studied using the pseudopotential multi-relaxation-time lattice Boltzmann model (MRT-LB). The model is modified by involving the piecewise linear equation of state and improved forcing scheme. The fluid–solid interaction in the model is employed to adjust the wettability of solid surface. Moreover, the validity of the model is verified by comparison with experimental results and grid-independence verification. Finally, the cavitation bubble collapse in a hydrophobic concave is studied by investigating density field, pressure field, collapse time, and jet velocity. The superimposed effect of the surface hydrophobicity and concave geometry is analyzed and explained in the framework of the pseudopotential LBM. The study shows that the hydrophobic concave can enhance cavitation effect by decreasing cavitation threshold, accelerating collapse and increasing jet velocity.

Dynamics of laminar ethylene lifted flame with ozone addition

The effect of ozone (O3) addition on ethylene (C2H4) non-premixed jet flames is investigated. Stable C2H4 lifted flames are established with oxygen/nitrogen co-flow at reduced oxygen content conditions. It is observed in the experiments that after O3 addition, the flame liftoff height could either increase or decrease, depending on the initial value of the flame liftoff height before O3 is added. Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) measurement shows that prompt ozonolysis reaction between C2H4 and O3 produces large amounts of CH2O upstream of the flame. In contrast to previous studies of O3 addition on lifted flames—with saturated hydrocarbon fuels in which O3 decomposition dominates—the ozonolysis reaction between C2H4 and O3 changes the chemical composition of flow even at room temperature. Such chemical reaction causes the simultaneous increase of both the triple flame propagation speed of lifted flame and the axial jet velocity along the stoichiometric contour, which also therefore changes the dynamic balance between these two values to stabilize the flame. While the increase of triple flame propagation speed tends to decrease the flame liftoff height, the increase of axial jet velocity along the stoichiometric contour tends to have the opposite effect. A competing kinetic-dynamic process forms, and the final location of the flame depends on the degree of ozonolysis reaction, which is determined by the initial flame liftoff height before O3 is added.

Influence of Hydrodynamic Conditions on Micromixing in Microreactors with Free Impinging Jets

An experimental study and mathematical modeling of micromixing in a microreactor with free impinging jets (MRFIJ) with a diameter of 1 mm was carried out. In the experimental part, the iodide-iodate technique was used (involving parallel competing Villermaux–Dushman reactions with the formation of I3−). Theoretical assessment revealed that more than 50% of the introduced energy is dissipated in the jets collision region. Through the use of differentiated sampling, an uneven quality distribution of micro mixing in the central and peripheral zones of the reactor was found: at moderate flow rates (700–1000 mL/min, jets velocity of 15–21 m/s) the micromixing in the central part of reactor is up to 12 times better than that in the periphery. Furthermore, the weight fraction of the probes in the central zones of MRFIJ is reduced with increasing jet velocity; this effect is attributed to a more intense formation of ligaments and droplets upon collision of jets and their secondary mixing on the walls of the apparatus. In terms of the weighted average concentration, the best quality of micromixing in the samples is achieved at a flow rate of 300 mL/min. With an increase in the flow rate (and velocity) of the jets, the dependence of the I3− concentration on the flow rate has a nonmonotonic character, which is explained by a change in the nature of the flow in the collision zone of the jets: the transition from the formation of a liquid sheet to the intensive formation of ligaments and drops and secondary mixing of the liquid film formed on the walls of the reactor. The effect of “freshness” of solutions on the concentration of reaction products was studied.

Characteristics of clusters in the jet influence zone of riser

By analyzing solid holdup signals in the jet influence zone of risers, the characteristics of clusters were studied. The solid holdups inside clusters and their distribution ranges were calculated under the cases of both upward and downward feed jets. Moreover, the effects of the jet velocity on the cluster characteristics were analyzed. The solid holdups inside clusters have higher values and wider distribution ranges in the upward feed injection zone than in the downward zone, implying that the number of individual particles in a cluster is unstable under the influence of upward jets. For the case of downward jets, cluster formations in the jet influence zone of risers can more easily maintain stability, offering a better mixing environment for jets and particles. As the jet velocity increases, the solid holdups inside clusters in the riser wall of the upward injection zone increase accordingly, while the distributions of solid holdups inside clusters have no significant changes if the feed jets are downward. This phenomenon confirms that improved operational flexibility can be obtained if downward jets are mixed with the prelift gas–solid flow in risers.

EXPERIMENTAL BREAKUP CHARACTERISTICS OF ROUND LIQUID JETS OF A DILUTE POLYMER SOLUTION INTO QUIESCENT AIR

Experiments are performed on dilute polymer solutions jetting into quiescent air with a jet velocity range of 30–87 m/s. Polymer solutions are those with several hundred ppm concentrations of long-chain polyoxyethylene (millions of molecular weight) dissolved in purified water. The breakup characteristics of the jets were recorded by high-speed photography from four observation windows along the jet direction. The breakup morphologies of dilute polymer solution jets are significantly different from that of Newtonian fluids. Small-scale interfacial disturbances are suppressed, while a twisted liquid column waving with large amplitudes is observed. When the jet velocity exceeds a critical value, liquid films and filaments are peeled off successively from the liquid column. Four different breakup patterns are distinguished. The diameters of the central liquid column and the principal liquid filament are observed to decrease with increasing jet velocity.

Short cylindrical nozzles in a jet-driven Helmholtz oscillator

The present paper elaborates on the experimental study of jet-excited pressure fluctuations in a Helmholtz oscillator model with two openings in a cylindrical cavity. The length of the cylindrical nozzle in the front cover ℓN normalized by the nozzle diameter dN was ℓN/dN = 0.125, 0.33, 0.47, and 0.67. The diameter of the outlet opening in the back cover dOUT was in the range dOUT/dN = 1–2.5. The length of the cylindrical cavity LCH determined the jet length LJET in the spacing between the covers, LCH/dN = 0.5–3.5. The amplitude–frequency spectra were studied when the oscillator configuration was changed in the indicated intervals. From the generation amplitude, the best ratio of the sizes of the nozzle, chamber, and outlet was determined. The appearance of the jet tone of the hole and the alternation of acoustic modes were observed with a smooth increase in the Reynolds number to ∼8 · 104. The measurements showed very high amplitude of pressure fluctuations in an oscillator with a short nozzle and a short chamber at a significant jet velocity. A slight increase in the length of the chamber led to a rapid decrease in the generation amplitude. It is determined that the tone frequency is usually much lower than the resonance frequency in the chamber. Moreover, the tone frequency gradually increases with increasing jet velocity, while the resonance frequency remains unchanged, close to the natural frequency of the cavity chamber.

Aerodynamic Studies on Non-Premixed Oxy-Methane Flames and Separated Oxy-Methane Cold Jets

Both cold and flame jets find numerous applications in different fields, ranging from domestic applications to aerospace and space technology. Indeed, the applications of isothermal and non-isothermal jets in the flame heating industry fascinated the researchers to gain an in-depth understanding. Nevertheless, these benefits are not standalone, rather, they are associated with major disadvantages such as improper jet mixing and flame instabilities that require careful remedies. In the present investigation, three-inline jets, having methane jet at the center and two oxygen jets at the periphery, are studied computationally in a three-dimensional domain, with and without considering the effects of combustion. To study the mixing characteristics of cold jets, the radial velocity distributions at different streamwise locations have been analyzed at the jet inlet velocity of 27 m/s. However, for oxygen and methane flame jets, inlet velocities are varied as 27 m/s and 54 m/s. Moreover, to investigate the effects of temperature variation on mixing characteristics at a particular jet velocity, the inlet temperatures of reactants are varied as 300 K, 500 K, and 700 K, at the jet inlet velocity of 27 m/s. Combustion is found to have a profound impact on the mixing characteristics. At the inlet temperature of 300 K, a higher centerline velocity decay is observed for non-reactive jets as compared to reactive flame jets. Moreover, the turbulent kinetic energy distribution is relatively higher in the case of non-reactive jets, which is the direct evidence of an augmented mixing. As is obvious, the turbulent kinetic energy at the jet shear layer is maximum due to the formation of large-scale coherent eddies. The decay in centerline velocity is found to be increasing with an increase of inlet temperature. Additionally, with an increase of jet velocity, the radial velocity profiles become more asymmetrical, consequently yielding an unstable flame.

Experimental analysis of pulsatile flow characteristics in prosthetic aortic valve models with stenosis

Bioprosthetic valves are widely used for aortic valve replacements for patients with severe aortic diseases. However, tissue-engineered leaflets normally deteriorate over time due to calcification, leading to life-threatening conditions that would require re-operation. The hemodynamics induced by a prosthetic stenosis is complicated and not fully understood. This in vitro experimental study focuses on the fluid dynamics of two aortic valve models with different prosthetic stenosis conditions. An in vitro cardiovascular flow simulator was utilized to provide the pulsatile physiological flow conditions. Phase-locked particle image velocimetry (PIV) and high-frequency pressure sensors were employed to measure the flow fields and pressure waveforms. Pressure data were evaluated for the two models representing moderate and severe stenosis conditions, respectively. The severe prosthetic stenosis induced a prolonged ejection period and increased acceleration time ratio. PIV results suggest the severe prosthetic stenosis resulted in a two-fold increase in peak jet velocity and a three-fold increase in peak turbulence kinetic energy compared to the moderate stenosis case. The severe stenosis also caused rapid expansion of the jet downstream of the valve orifice and increased eccentricity of the jet flow. The maximum Reynolds shear stress in the severe stenosis case was found similar to the bileaflet mechanical valve reported by previous literature, which was below the risk threshold of blood cell damage but could potentially increase the risks of platelet activation and aggregation.

Data centre floor tiles influence on server fan power consumption

This study presents the flow of cooling air in a single rack data centre test bench with 40 servers with cold/hot aisle containment and a raised floor with a floor tile. The inhomogeneous pressure and velocity field in the cold aisle and the effect on the local server fan power were assessed. Computational fluid dynamic simulations as well as measurements were performed. Due to the cold/hot aisle containment, the floor jet caused a low pressure region for the lowest rack positions 1–10. In the simulation this led to a predicted increase in server fan power consumption of up to 6% for the lower rack positions. In the experiments, an increase of the server fan power for lower rack positions was likewise noticeable but was blurred due to the variance between individual servers. The influence of the vertical pressure distribution on the total server fan power consumption was minor (below 2%) if leakage is neglected. Six partially covered floor tile configurations were investigated. Partially blocking the floor tile to redistribute the flow showed no beneficial effect in homogenising the vertical pressure distribution because of the resulting increase in floor jet velocity. The simulations over-predicted the vertical pressure inhomogeneity in the contained cold aisle because leakage was not taken into account.