Stagnation Region Research Articles

Heat propagation phenomenon of compressed natural gas /air premixed laminar flame impinging on a flat surface

In this paper the heat transfer characteristics have been determined theoretically using heat propagation phenomena of compressed natural gas (CNG)/air premixed laminar flames impinging on flat surfaces. Effects of varied equivalence ratios (Ф), Reynolds numbers (Re), separation distances (H/d) and burner diameters (d) on stagnation point heat flux have been investigated. The simulation findings were validated with the available experimental data. The flat plate with the maximum heat flux at the stagnation position is located close to the margin of the innermost premixed reaction zone (i.e., at H/d, Re and d are 5, 1000, 8 mm). The flow of heat steadily decreases in axial direction as the separating distance from the inner reaction zone's tip increases. Greater separation lengths at the stagnation region cause a subsequent increase in heat flux. The objective of this investigation is to get an idea about the impact of existence of the plate on the flame and temperature distribution and to get clarity about the reason behind higher amount of heat flux owing to tip of the innermost reaction area at stagnation region.

Multiphysics simulations reveal haemodynamic impacts of patient-derived fibrosis-related changes in left atrial tissue mechanics.

Stroke is a leading cause of death and disability worldwide. Atrial myopathy, including fibrosis, is associated with an increased risk of ischaemic stroke, but the mechanisms underlying this association are poorly understood. Fibrosis modifies myocardial structure, impairing electrical propagation and tissue biomechanics, and creating stagnant flow regions where clots could form. Fibrosis can be mapped non-invasively using late gadolinium enhancement magnetic resonance imaging (LGE-MRI). However, fibrosis maps are not currently incorporated into stroke risk calculations or computational electro-mechano-fluidic models. We present multiphysics simulations of left atrial (LA) myocardial motion and haemodynamics using patient-specific anatomies and fibrotic maps from LGE-MRI. We modify tissue stiffness and active tension generation in fibrotic regions and investigate how these changes affect LA flow for different fibrotic burdens. We find that fibrotic regions and, to a lesser extent, non-fibrotic regions experience reduced myocardial strain, resulting in decreased LA emptying fraction consistent with clinical observations. Both fibrotic tissue stiffening and hypocontractility independently reduce LA function, but, together, these two alterations cause more pronounced effects than either one alone. Fibrosis significantly alters flow patterns throughout the atrial chamber, and particularly, the filling and emptying jets of the left atrial appendage (LAA). The effects of fibrosis in LA flow are largely captured by the concomitant changes in LA emptying fraction except inside the LAA, where a multifactorial behaviour is observed. This work illustrates how high-fidelity, multiphysics models can be used to study thrombogenesis mechanisms in patient-specific anatomies, shedding light onto the links between atrial fibrosis and ischaemic stroke. KEY POINTS: Left atrial (LA) fibrosis is associated with arrhythmogenesis and increased risk of ischaemic stroke; its extent and pattern can be quantified on a patient-specific basis using late gadolinium enhancement magnetic resonance imaging. Current stroke risk prediction tools have limited personalization, and their accuracy could be improved by incorporating patient-specific information such as fibrotic maps and haemodynamic patterns. We present the first electro-mechano-fluidic multiphysics computational simulations of LA flow, including fibrosis and anatomies from medical imaging. Mechanical changes in fibrotic tissue impair global LA motion, decreasing LA and left atrial appendage (LAA) emptying fractions, especially in subjects with higher fibrosis burdens. Fibrotic-mediated LA motion impairment alters LA and LAA flow near the endocardium and the whole cavity, ultimately leading to more stagnant blood regions in the LAA.

Turkey’s higher education boom: the ‘one university in every city’ policy in small cities

ABSTRACT Over the past two decades, Turkey’s higher education system has undergone a massive and globally unprecedented transformation. Enrollment rates increased more than fivefold, driven by the government’s comprehensive policy of establishing ‘one university in every city.’ As a result, new universities were instituted in 41 small cities that previously lacked independent higher education infrastructure. The Justice and Development Party (AKP) governments framed this initiative as a key ideological distinction from their predecessors, emphasizing it as a cornerstone of their educational and social policy, particularly targeting small cities across the country as well as the country’s youth. While the ideological divergence between the AKP and prior governments played a critical role in justifying this expansion, the establishment of universities in small cities had both intended and unintended consequences. These institutions became place-making anchors in otherwise economically and demographically stagnant regions, fostering a new social life centered around the youth.

Numerical investigation of hydrodynamic performance and power consumption in stirred reactors with perforated Scaba 6SRGT impellers for non‐Newtonian fluid

AbstractThe present study aims to further understand the effect of perforated Scaba 6SRGT impellers on stirring performance and impeller power consumption by numerically investigating the hydrodynamic properties of non‐Newtonian fluids in a three‐dimensional stirred reactor. After model validation, the fluid velocity, stagnant region dimensionless volume, and impeller power consumption are quantitatively analyzed under the conditions of influencing parameters, including blade perforation pitch, ratio of distance between adjacent perforations, and perforation arrangement. The results reveal that the range of distribution of acceleration and peak velocity of the fluid along the radial appear in regions lower and larger than the dimensionless radius of the impeller (|r/R| = 0.45), respectively. Increasing the perforation pitch from 14 to 20 mm increases fluid velocity from 0.284 to 0.305 m/s and power consumption from 26.99 to 27.25 W, while reducing the stagnant region volume from 16.76% to 14.71%. The fluid velocity and impeller power consumption slightly increase with the ratio of distance between adjacent perforations. Circular‐shaped middle perforations yield higher fluid velocities and vorticity compared to lozenge‐shaped ones. The highest and lowest power consumptions are obtained by circular–circular–circular and lozenge–lozenge–lozenge perforation shape arrangements with values of 27.25 and 26.9 W, respectively.

Investigating the impact of vessel geometry on cerebral aneurysm formation using multi-phase blood flow models

Cerebral aneurysms represent a life-threatening condition associated with considerable morbidity and mortality rates. The formation of cerebral aneurysms is influenced by various factors, including vessel geometry, blood flow characteristics, and hemodynamic forces. In this study, we investigate the impact of vessel geometry on the formation of cerebral aneurysms utilizing computational fluid dynamics (CFD) simulations for multi-phase blood flow models.More precisely, we employ the Finite Volume Method to numerically solve the Navier-Stokes equations for simulating blood flow. To accurately capture the intricate nature of blood behavior, we utilize a multiphase blood flow model, as blood consists of red blood cells, white blood cells, and platelets suspended in the blood plasma.Our results demonstrate that the local curvature of the vessel has a pronounced effect on the blood flow patterns and hemodynamic forces within the vessel. Specifically, our simulations indicate that an increase in vessel curvature can lead to the formation of regions of high stress and flow stagnation, both of which are known to be associated with an increased risk of aneurysm formation.The current study provides significant insights into the impact of vessel geometry on the formation of cerebral aneurysms. The obtained results may aid in designing treatment and preventive strategies for cerebral aneurysms, while also contributing to the existing body of knowledge on the subject. Additionally, the approach developed in this study can be applied to investigate various other vascular pathologies, including arterial stenosis and atherosclerosis.

Multi-objective geometric optimization of protrusion, droplet ribs, and inclined upper plate of a slit jet impingement heat sink to enhance its thermal performance

To meet the needs of heat transfer and temperature uniformity in the heat dissipation of high-heat-flux electronic devices, a novel slit jet impingement heat sink with a protrusion in the stagnation region, droplet ribs in the wall jet region, and an inclined upper-plate (PDI-SJIHS) is proposed, the coolant is Al2O3-H2O nanofluid. The influences of the structural parameters of the above three components and the Reynolds number on the heat transfer and flow of the PDI-SJIHS are studied numerically. Using the comprehensive heat transfer and temperature standard deviation as objective functions, a multi-objective optimization of PDI-SJIHS is conducted using non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ). The results indicate that the combination of the three components enhances the thermal performance of the PDI-SJIHS, compared with the slit jet impingement heat sink with only one of the above components. Rising the Reynolds number can enhance the thermal characteristics of the PDI-SJIHS. For a Reynolds number of 8000 and nanofluids with a 3% volume fraction, the average convective heat transfer coefficient, friction coefficient, and comprehensive heat transfer coefficient of the optimized PDI-SJIHS are increased by 182%, 153%, and 108%, respectively, and the standard deviation of temperature is 91% less than that of F-SJIHS.

The interplay of plasticity and elasticity in elastoviscoplastic flows in wavy channels

Elastoviscoplastic (EVP) fluids, which exhibit both solid-like and liquid-like behaviors depending on the applied stress, are critical in industrial processes involving complex geometries such as porous media and wavy channels. In this study, we investigate how flow characteristics and channel design affect EVP fluid flow through a wavy channel, using numerical simulations supported by microfluidic experiments. Our results reveal that elasticity significantly influences flow dynamics, reducing pressure drops and expanding unyielded regions. Notably, we find that even minimal elasticity can shift the flow from steady to time-dependent regimes, a transition less pronounced in viscoelastic fluids. Additionally, we show that the development of stagnation regions can be prevented when using a modified EVP fluid with enhanced elasticity, thus providing a full global yielding of the material. This study elucidates the role of elasticity in modifying flow patterns and stress distribution within EVP fluids, offering insights into the optimization of industrial applications, such as the displacement of yield stress fluids in enhanced oil recovery, gas extraction, cementing, and other processes where flow efficiency is critical.

Effect of the Effective Metal Surface Area of Two Different Flow Diverter Stents on the Stagnation Region Formation Inside the Aneurysm Sac

Abstract Objective Flow diversion (FD) is a relatively new technique for treating large, wide-necked, or fusiform aneurysms. Although FD is a more preferred option than coiling or clipping techniques in neurosurgery and neuroradiology clinics, the blood flow mechanism inside the aneurysm sac is not fully understood after the treatment. Besides, effective metal surface area (EMSA), a property of an FD related to porosity, shows variation at the patient's aneurysm neck by providing more or less blood flow inside an aneurysm sac than planned, causing nonstagnant or stagnant fluid region formation in the sac, respectively. Thus, the change in FD's EMSA can significantly affect the treatment's effectiveness, making even operation unsuccessful when variation in FD's EMSA at the aneurysm neck is overlooked. Materials and Methods In this study, a large aneurysm of a 52-year-old female patient was numerically investigated by virtually placing two commercially available FDs with different EMSA values one by one into the aneurysm-carrying artery. Results While FD stents at the aneurysm site substantially reduced the blood flow into the aneurysm, an FD with a 15.6% EMSA caused blood to flow in the aneurysm sac to have six times more kinetic energy than that of FD with a 29.5% EMSA. Conclusion Although FD's EMSA value demonstrated nearly up to 20% reduction at the patient's aneurysm neck based on a product catalog value, numerical model results revealed that the stagnated region's formation inside the aneurysm sac could be determined within a 9% difference based on digital subtraction angiography reformat image.

Multi-physics simulations reveal hemodynamic impacts of patient-derived fibrosis-related changes in left atrial tissue mechanics.

Left atrial (LA) fibrosis is associated with arrhythmogenesis and increased risk of ischemic stroke; its extent and pattern can be quantified on a patient-specific basis using late gadolinium enhancement magnetic resonance imaging.Current stroke risk prediction tools have limited personalization, and their accuracy could be improved by incorporating patient-specific information like fibrotic maps and hemodynamic patterns.We present the first electro-mechano-fluidic multi-physics computational simulations of LA flow, including fibrosis and anatomies from medical imaging. Mechanical changes in fibrotic tissue impair global LA motion, decreasing LA and left atrial appendage (LAA) emptying fractions, especially in subjects with higher fibrosis burdens. Fibrotic-mediated LA motion impairment alters LA and LAA flow near the endocardium and the whole cavity, ultimately leading to more stagnant blood regions in the LAA.

Heat transfer characteristics of a turbulent swirl jet issuing from a circular nozzle

Experimental and numerical studies on the flow field and cooling performance of a swirling jet, impinging on a flat surface is presented. A new nozzle that resembles the swirl injector of a liquid propellant rocket engine is used. This study considers different parameters including swirl number(S), Reynolds number(Re), normalised orifice to target spacing (Z/D) and confinement. The performance of various RANS and LES turbulence models is assessed using the data obtained from present experimental studies and that reported in literature. RNG k-ϵ turbulent model is successful in predicting heat transfer in non swirling flow. In the case of swirling flow, LES WALEM(Wall Adapting Local Eddy viscosity Model) predicts the heat transfer better than the other models. The performance of Swirling Impinging Jets(SIJ) and Cylindrical Impinging Jets (CIJ) is compared. At higher Z/D, introduction of swirl results in reduction in heat transfer and this is because of the increased spreading of the jet and reduction in velocity of the jet impinging on the target. Better performance is achieved at lower Z/D when a cylindrical jet is introduced with a small amount of swirl. For Z/D = 2 and S = 0.2, the peak Nu increases by 10 % and average Nu in the stagnation region increases by 6 % in comparison to CIJ. The position of the peak Nu moves away from the axis of the jet and there is better uniformity in Nu in the stagnation region. For Z/D = 2 swirling flow creates secondary peak(which usually occurs at high Re)even at low Re. At very low Z/D, top wall confinement causes increase in both peak heat transfer and average heat transfer in the stagnation region.

Pulsating Heat Pipe Performance Modeling with Liquid Metal Coolants Under Hypersonic Aerothermal Heating

Hypersonic heating loads concentrate at leading-edge compression areas to create excessively high local temperatures and thermally driven stresses. The fast and reliable thermal dispersion of heat pipes with significantly high thermal conductance can alleviate these localized thermal stiffness problems. The pulsating heat pipe (PHP) holds numerous advantages when compared to capillary, constant-conductance heat pipes for hypersonic thermal management applications, primarily because they lack a wicking structure. This paper numerically investigates thermal performances of a four-turn C103 niobium alloy PHP operating with lithium, potassium, sodium, and a eutectic sodium–potassium alloy (NaK-78) when exposed to heating conditions relevant to the hypersonic environment, investigating flight Mach numbers ranging from 6 to 8 and dynamic pressures ranging from 40 to 44 kPa. The robust thermofluidic properties of liquid metals, along with the powerful fluid pulsation induced in the PHP, can provide significant thermal transport from hot stagnant regions of the leading edge to the cooler trailing surfaces. Potassium showed superior thermal performance when compared to other liquid metal coolants under the presently tested conditions, with overall PHP thermal conductance as high as 34 W/K predicted for Mach 8 flight conditions. In contrast, sodium was associated with startup difficulties in the PHP; this paper attributes this to its significantly larger thermal conductivity, which can limit the vapor pressure difference over the liquid slug lengths. These predictions indicate an overall latent heat transfer dominance of around 70–95% in liquid metal PHPs.

Mixed convection flow and heat-mass transfer of cross liquid near a stagnation region with Dufour, Soret effects and chemical reaction

Heat-mass transfer in a liquid flow over a stretching surface at a stagnation regime is significant in optimal manufacturing and quality assurance depending on the rheological behavior of viscoelastic fluids in process machinery. Stretching sheet flow at a stagnation point is a standard procedure used in the metallurgical industries for manufacturing of different equipment and cooling systems. Due to its various applications in industrial operations, the current study aims to explore the heat-mass transfer of buoyancy-driven flow of cross liquid over a stretching sheet at a stagnation point. The heat and mass transfer under the framework of Dufour and Soret effects, radiant heat, and chemical reactions are assessed. Furthermore, the analysis also takes into account the linear and nonlinear convection-diffusion phenomena. The model findings are obtained by merging the Bvp4c in MATLAB using a local similarity technique up to the fourth truncation threshold. The findings illustrate that the action of magnetic field can be used to control the flow across the sheet. Temperature increases with thermal radiation, Biot number and Dufour number. The action of buoyancy ratio parameter result in improvement in fluid flow. With increasing trend of magnetic and Weissenberg number, the flow field and skin friction coefficient drops.

Partial gravity flammability of cast PMMA rods in concurrent axial stagnation flow

Testing was performed in a partial gravity centrifuge drop vehicle in the Zero Gravity Research Facility with cast PMMA rods in concurrent axial buoyant stagnation flow to determine flammability limits as a function of partial gravity level. The partial gravity levels studied varied from 0.04 g to a simulated 1g for ambient oxygen concentrations from 13.2 % to 15.2 % O2 by volume at an average 57.6 kPa (8.4 PSIA) ambient pressure, which is very close to the anticipated Lunar habitat pressure. The PMMA flammability boundary as a function of oxygen concentration and gravity level has been determined at five gravity levels. Coriolis effects appear to be minimal in the stagnation region where the flame is stabilized while the tips of the flame do show some bending at the higher gravity levels. Lunar gravity levels are near the minimum in the oxygen - gravity level flammability boundary. This suggests that fire is a significant safety risk for future exploration missions to the Moon since materials are screened in normal gravity to evaluate their safe use in space. If normal gravity screening is not conservative, a material derating method will need to be applied to ensure the material is not flammable on the Moon. Since the blowoff boundary appears to be linear with forced flow velocity, it may be possible to conduct elevated forced flow blowoff testing that could then be extrapolated down to effective Lunar gravity levels to provide an oxygen delta between 1g and Lunar flammability limits to derate the material.

Flow around a slender body with sharp edges

AbstractThe behavior of harmonic functions near corners of the domain boundary has a universal character that may be applied to the study of the flow around slender bodies with sharp edges. Depending of the energy of the potential function near the tip of the corner, this may yield a fast rotating flow or a vortex in its vicinity. For internal angles, such as occur in grooves of the body, a stagnation region or an eddy may occur. Lateral forces are also affected by the presence of corners, yielding configurations that resemble either a circulation lift or a vortex lift.

A numerical investigation to assess changes to displacement front and by-passed zones employing kinetic interface-sensitive tracer

An important aspect in groundwater remediation is to understand changes of multiphase fluid front morphology and stagnant regions on macro scale. However, the prediction of those changes during two-phase flow remains a challenging task due to the interplay of various physical factors. Recent laboratory experiments have demonstrated tracers' ability to predict deformation in the front of a two-phase flow system by utilizing a new reactive tracer known as, the kinetic interface sensitive tracer (KIS). This research employs a reactive transport model coupled with a macro-scale two-phase flow model to numerically analyse how viscosity ratio, capillary number, and heterogeneities on the tracer's signal and its impact the frontal deformation. One homogeneous and two heterogeneous types of porous media are considered. The background porous medium is a fine-grained, low-permeability medium, with a coarser, high-permeability lenses, generating heterogeneous material properties. The high-permeability lenses account for 25 % of the total model area and are arranged in either periodic or random patterns. The findings are evaluated using four parameters (effective front length, swept area, front roughness, and transition zone length). The flow patterns dominating the shape of the front are characterized by the viscous and capillary forces i.e. capillary number and the viscosity ratio between the two fluids. The results show that changes in flow regimes can be quantified using effective front length, thus employing the effective front length the viscous fingering regions can be quantified. Furthermore, front roughness and transition zone length are extracted and their relevance to the by-passed zones is presented. The slope of the reactive KIS tracer breakthrough curve, plotted on a phase diagram, can also be used to predict the existence of the by-passed zones for a low viscosity ratio. Finally, changes in front roughness and transition zone length induced by the inclusions are correlated to the slope of the KIS tracer BTC. The findings of this study can contribute to a better understanding of the impact of different flow regimes on the KIS tracer breakthrough signals and the linkages between the tracer signals and the front sizes.

Influence of corrugated jet plates on internal heat transfer and flow dynamics of double-wall cooling: experimental-numerical approach

In the present study, combined experimental and numerical investigations were carried out to analyze the fluid flow dynamics and heat transfer of impinging jets in a corrugated double-wall structure for exhaust nozzle cooling operating at high temperatures. The sinusoidal wavy plate, with multi-jet holes positioned at its trough, served as the jet plate, creating a staggered configuration with respect to the effusion holes. The transient thermochromic liquid crystal (TLC) method was employed to determine the local distribution of the Nusslet number on the flat effusion plate in a wind tunnel at small Reynolds numbers (ranging from 450 to 2700 based on the jet hole diameter (d)). In the numerical part of the study, the k-ω SST turbulence model verified with the TLC data was used to solve the RANS equations. The effects of jet-to-plate spacing (Have/d = 1.6–3.6), corrugated plate amplitude (A/d = 0.2–0.8), effusion hole diameter (de/d = 0.6–1.5), and Reynolds numbers were examined in detail. Our results showed that the introduction of the corrugated structure in the jet plate altered the high-velocity region pattern within the jet hole, leading to a 16 %-81 % reduction in jet core length compared to the flat jet plate. At a relatively high wave amplitude (A/d ≥ 0.5), the vortex structure underwent significant changes, inducing a higher level of turbulence kinetic energy in the near-wall region of the target chamber. This led to enhanced heat transfer near the stagnation and effusion hole regions compared to the flat double-wall cooling. The effusion hole diameter exerted a minimal effect on the overall Nusselt number of the corrugated double-wall cooling. At very low Reynolds numbers (Rej 〈900), the flow and heat transfer characteristics in corrugated double-wall cooling resembled those observed in flat double-wall cooling.

A Review of Flow Field and Heat Transfer Characteristics of Jet Impingement from Special-Shaped Holes

The jet impingement cooling technique is regarded as one of the most effective enhanced heat transfer techniques with a single-phase medium. However, in order to facilitate manufacturing, impingement with a large number of smooth circular hole jets is used in engineering. With the increasing maturity of additive technology, some new special-shaped holes (SSHs) may be used to further improve the cooling efficiency of jet impingement. Secondly, the heat transfer coefficient of the whole jet varies greatly on the impact target surface. The experiments with a large number of single smooth circular hole jets show that the heat transfer coefficient of the impact target surface will form a bell distribution—that is, the Nusselt number has a maximum value near the stagnation region, and then rapidly decreases exponentially in the radial direction away from the stagnation region. The overall surface temperature distribution is very uneven, and the target surface will form an array of cold spots, resulting in a high level of thermal stress, which will greatly weaken the structural strength and life of the equipment. Establishing how to ensure the uniformity of jet impingement cooling has become a new problem to be solved. In order to achieve uniform cooling, special-shaped holes that generate a swirling flow may be a solution. This paper presents a summary of the effects of holes with different geometrical features on the flow field and heat transfer characteristics of jet impingement cooling. In addition, the effect of jet impingement cooling with SSHs in different array methods is compared. The current challenges of jet impingement cooling technology with SSHs are discussed, as well as the prospects for possible future advances.

Winkler Titration Assessment of Biochemical Oxygen Demand (BOD5) in Oka Creek, Toru-Orua, Bayelsa State

Aim: The aim of study was to investigate the Biochemical Oxygen Demand (BOD) levels in water samples from the Oka Creek. Study Design: Qualitative study design. Place and Duration of Study: Water samples were collected from the Oka Creek, located in Toru-Orua in Sagbama Local Government Area, Bayelsa State, Nigeria. The study lasted for twenty days. Methods: Water samples were collected from four different locations and depths in the Oka Creek, Toru-Orua, Sagbama Local Government Area of Bayelsa State, Nigeria, at various times. Each sample was transferred into 250 ml bottles labeled L1, L2, L3, and L4. The temperatures of the samples were recorded before transporting them to the chemistry laboratory for treatment. Winkler titration method was employed in analyzing the samples. Results: Location L1 recorded the highest BOD value, 28.38 ppm, measured at sunset (25 °C), from a relatively stagnant region. The high BOD at this location is attributed to minimal photosynthetic activity and significant oxygen consumption by microorganisms. In contrast, L2, collected from the same region at 35 °C during peak sunlight, exhibited a BOD value of 21.76 ppm. The lower BOD at L2 is due to increased photosynthetic activity. These high values suggest significant sludge deposits, domestic sewage and agricultural runoffs, which could lead to oxygen depletion and negatively impact aquatic life. Sample L3, taken from a deeper, stagnant region at sunset (25 °C), recorded a BOD value of 4.50 ppm. This moderate BOD level suggests the presence of moderate sludge deposits and agricultural runoff, but higher water flow speed helped mitigate these effects. Location L4, with the lowest BOD value of 3.74 ppm, was collected during peak sunlight (35 °C). Deeper location and high water flow speed contributed to reduced BOD levels, indicating better water quality and a healthier aquatic environment. Conclusion: The BOD values at L1 and L2 (28.38 ppm and 21.76 ppm) exceed acceptable limits for fish growth. Conversely, BOD values at L3 and L4 (4.50 ppm and 3.74 ppm) fall within acceptable limits, suggesting healthier aquatic environment, as evidenced by the yearly bountiful fish harvests in these sections of the Oka Creek.

Jet array impingement heat transfer in a rectangular cavity with effusion holes

Various researchers have studied jet array impingement heat transfer in impingement/effusion cooling systems. However, there is a lack of research on impingement/effusion cooling systems installed within rectangular cavities that focus on the impact of the proximity of the jet hole to the cavity sidewalls on cooling performance. The main objective of this study is to investigate the flow and heat transfer characteristics of jet array impingement with effusion holes in a rectangular cavity, considering various spacings between the cavity sidewalls and the outermost jet hole. The design parameters in this study include the ratio of the jet hole pitch to jet hole diameter of 7.1, 10.0, and 16.7, and the ratio of the distance between the jet and impingement plates to jet hole diameter of 2, 6, and 10, with the Reynolds number based on the jet hole diameter ranging from 2500 to 15,000. Heat transfer characteristics in the stagnation region and wall jet region were examined using local Nusselt number distributions on the impingement surface, measured by liquid crystal thermography. The local Nusselt number was high in the stagnation region and decreased radially from the stagnation region as the wall jet region formed. The closer the outermost jet hole is to the sidewall, the higher the Nusselt number on the impingement surface near the sidewall. Moreover, the flow structure in the rectangular cavity was numerically investigated, and the velocity vectors and streamlines showed that primary and secondary vortices were generated in the middle of two neighboring jets and near the sidewall, respectively. This study also assessed previous average Nusselt number correlations. Based on experimentally determined average Nusselt number data with 54 center unit cells and 1296 side unit cells, new correlations to predict the average Nusselt number on the impingement surface in a rectangular cavity with effusion holes were developed.

Large eddy simulation of various EMBr effects on the fluid flow, heat transfer and solidification process in an ultra-high speed thin slab casting mould with multi-port SEN

Effective control of intense turbulence is a crucial challenge for achieving steady production with ultra-high casting speed in thin slab continuous casting. In thin slab casting process, specially designed multi-port submerged entry nozzle (SEN) with four outlets is utilised to ensure an ample supply of molten steel, necessitating the selection and optimisation of suitable Electromagnetic Braking (EMBr) equipment for steel jet control. This study established a comprehensive three-dimensional model of a funnel-type mould, employing a combined experimental-numerical approach to validate and investigate the flow, heat transfer, solidification and electromagnetic behaviour in the mould. The steel grade studied in the simulation is Q235B, and its physical properties were calculated based on its composition with a temperature range of 1450–1826 K. To analyse the influence of high casting speed on the flow and solidification behaviour in the mould, three casting speeds were selected for the study, which were 6, 7 and 8 m/min. The results indicate that the novel Bowl EMBr significantly suppressed the penetration of steel jet and thus enhanced the thickness and uniformity of the solidified shell. As the casting speed increases from 6 to 8 m/min, the solidified shell thickness at the mould exit decreases from 7.91 to 5.94 mm, with the stagnant growth region approaching the mould exit. This highlights the requirement to correspondingly increase EMBr strength under ultra-high casting speed condition to avoid remelting of the shell and the risk of molten steel leakage. The coupled mathematical model established in this study provides guidance for optimising EMBr structures and casting speed under special multi-port SEN conditions, offering recommendations for the rational control of flow, heat transfer and solidification process in the mould.