Numerical solutions based on explicit filtered LES for computing turbulent flow field, of free round jets and impinging round jet on cuboidal cavities, are presented and discussed in this dissertation work. One-parameter fourth-order explicit filter is implemented to account for sub-grid scale effects. Compact difference schemes proposed by Hixon & Turkel involving only bidiagonal matrices is used to evaluate spatial derivatives. Compact schemes with overall fourth order accuracy and eight order accuracy are used in simulating free and impinging jets respectively. Simulations of free round jets are used for validating LES approach. 6 simulations of free round jet, in three levels of computational grids at three different Reynolds number, are performed to understand the effects of Reynolds number and turbulent length scales. Energy in the smaller length scales are found to be higher for higher Reynolds number. Potential core collapse is found to occur at shorter distance for high Reynolds number jets. Accurate computation of smaller length scales of turbulence is found to be essential for high Reynolds number flows. LES of subsonic impinging jets are performed on cuboidal cavities to understand the physical phenomenon. High intensity, low frequency sounds are captured, in the presence of cavity, as reported by other research works. Lip-thickness is found to have an effect on the intensity of sound produced. Matching of Jet shear layer roll up frequency with cavity’s natural frequency to produce resonance phenomenon is attempted and observations are presented.

Austenitization followed by quenching is a well-known conventional heat-treating procedure which is widely used on carbon steels with the aim to obtain high strength in as-quenched condition. Such quenching is usually done by immersing a steel product into the cooling medium which provides a uniform cooling of the surface. The cooling rate can be adjusted to a certain degree on a “component” length-scale by using different cooling mediums such as water, oil, polymer solution, etc. However, certain steel products such as beams, pillars in automobile industry or different machinery parts in agriculture require a proper and controllable cooling gradient and thus mechanical property gradient within the product. It is difficult to control the cooling rates locally on the length-scale smaller than the product only by replacing the quenching medium. In addition, quenching by immersing the product into the cooling medium is accompanied by thermal stresses due to the different cooling rates of the surface and the core, and also accompanied by transformation stresses due to the volume change during phase transformations. These stresses may lead to negative effects such as undesired residual stresses or even cracks. Therefore, cooling must be properly optimized and controlled to eliminate these drawbacks. Such a controllable cooling can be performed by several impingements of the water jets onto a hot austenitized surface at certain locations. By controlling the water flow, number of jets, their locations and other parameters, the global and the local cooling rates can be optimized for a specific industrial application later on.  This thesis demonstrates the potential and capability of the water Impinging Jet Quenching Technique (IJQT) to provide a flexible and controllable cooling for both differential and for uniform quenching cases. The test rig of IJQT was developed in the University of Gävle and was used to perform quenching experiments in this study: differential cooling of thick sheets and uniform quenching of bars to different depths. Differential cooling was performed on square-shaped carbon steel sheets with thickness of 15 mm, and the uniform quenching with different flow rates was performed on carbon steel cylindrical bars with 100 mm in diameter. Along with the physical experiments, Comsol Multiphysics 5.6 software was used to solve a 1D heat transfer problem to estimate the cooling rate profile along the radius of the bar. The experiments were verified by observations and characterization of the microstructure using light optical microscopy (LOM), and by examining the mechanical properties through tensile tests and hardness measurements. The results of the quenching experiments and verifications showed a high potential and flexibility of the IJQT in differential cooling case as well as in the uniform quenching case. / <p>Funding agencies: For financial support Sweden’s Innovation Agency Vinnova, SSAB,Väderstad Components, Swedish Knowledge Foundation and Universityof Gävle are acknowledged.</p>

Quenching technique is widely used in industrial applications as it enhances the mechanical properties of metals such as hardness and tensile strength. This technique consists of a heating process followed by fast cooling which results in different microstructures that enhance the metal behavior. Current competitive market in metal field requires the implementation of advanced and optimizing techniques by means of efficient and sustainable quenching techniques. Furthermore, cooling by multiple array of water jets offers wide range of cooling rate control and consequently the achievement of the desired properties. Quenching cooling rate for a rotary cylinder by multiple impinging jets is investigated in this experimental study. A rotating steel cylinder is heated up to 700°C by an induction heater and cooled down in short time by an array of water impinging jets in order to study quenching process of the test specimen by the impinging jet technique. This fast cooling has been found to be a crucial parameter that enhances the characteristics of steel thoroughly. The magnitude of its influence has been previously studied in water pools cooling techniques. Consequently, a further understanding of quenching technique is aimed in this study by the variation of different parameters: the multiple jet’s pattern (inline and staggered), jet-to-jet spacing (S/d=4 and 6), rotational speed (10-70rpm) and water subcooling temperature (55-85K) that have been studied in 10 experiments. Running of the experiments have been done with the help of different programs such as LabVIEW and NiMAX. Measurements of the temperature along the cylinder has been carried out by using some embedded thermocouples that have been connected to the DAQ.  Results from the study revealed faster cooling with rotation speed 30rpm since the contact between hot surface and impinged water jet is improved for lower speeds. However, rotation speed10rpm results experienced negative effects. In addition, jet-to-jet spacing S/d = 4 caused higher cooling rate than S/d = 6 since the impinged water from neighbor jets lead to higher interaction between water fronts and consequently a more uniform cooling. Furthermore, significant differences have been found in temperature drop between points located closer to the center of the cylinder and the ones beneath the cooling surface. Regarding the multiple array configuration of nozzles, staggered configuration revealed more uniform cooling over the surface due to the fact that placement of the jets led to a better distribution of the impinged water in the measurement line. The effect of higher subcooling temperature in agreement with previous studies results in which higher cooling rate and more drastic temperature drop. The aim of this study is to make a better understanding of the multiple water impinging jets quenching technique in order to make further research in the area of enhancing the mechanical properties of steel by understanding effect of the quenching parameters and their characteristics in order to optimize the quenching technique for different applications.

Heat transfer brought about by impinging jets is common in several industrialapplications such as the cooling of heated objects. The purpose of this investigationis to create a numerical model of an impacting jet, then to use it in parametricresearch and in the cooling of an electronic device. In this work, COMSOL Multiphysics software has been used to carry out a numericalanalysis of a cooling jet impacting a vertical surface. Water and air are the two fluidscompared to each other, and three different materials were selected for modelling theheated object. The model created can be used in many areas. For example, the caseof the air jet hitting an aluminum object is commonly used in the cooling of electronicdevices while the water jet hitting a brick is used in building and construction. Inother words, the highlight of this project is to create a useful tool that can be easilymodified in order to investigate a specific area. To set an example, the cooling of aninsulated gate bipolar transistor (IGBT) has been investigated in this work. Equations related to the conservation of mass and momentum, coupled to the energyconservation equation, have been solved. Several assumptions were carefully andreasonably considered to simplify the simulation and ensure the accuracy andreliability of the results. The effect of the type of the heated material and the distancebetween the tube nozzle and the heated object have been investigated in the study.The results show that the jet is effective in cooling heated objects and can be used inparametric research. In fact, surface temperature and jet velocity have been displayedand discussed and it has been found that the most optimal combination for efficientheat transfer is a shorter distance between the tube nozzle and the heated object, amaterial with higher thermal conductivity, and water as the jet fluid. The impingingjet device has been tested in the cooling of Insulated Gate Bipolar Transistor (IGBT)and the results showed that the temperature of the IGBT drops by 14.57%. Despite the accuracy of the results, it is important to recognize the limits of thenumerical model such as the discretization of the physical domain, the resolution ofthe mesh, the assumptions, and the simplifications. In addition, this investigation waslimited to three variable factors. Therefore, further studies are recommended tofurther optimize the cooling effect of the jet, such as the study of nozzle shape, jetvelocity, jet mass flow rate and a 3D-simulation. / Värmeöverföring som orsakas av påverkande strålar är vanligt förekommandeinom flera industriella tillämpningar, såsom kylning av uppvärmda objekt. Syftetmed denna undersökning är att skapa en numerisk modell av en påverkande stråleoch sedan använda den i parametrisk forskning och för att kyla en elektroniskenhet. I detta arbete har COMSOL Multiphysics-programvaran använts för att genomföraen numerisk analys av en kylstråle som påverkar en vertikal yta. Vatten och luft ärde två vätskor som jämförs med varandra, och tre olika material valdes för attmodellera det uppvärmda objektet. Den skapade modellen kan användas inommånga områden. Till exempel används fallet med luftstrålen som träffar ettaluminiumobjekt vanligtvis vid kylning av elektroniska enheter, medanvattenstrålen som träffar en tegelsten används inom bygg- ochkonstruktionsbranschen. Med andra ord är höjdpunkten i detta projekt att skapa ettanvändbart verktyg som lätt kan modifieras för att undersöka ett specifikt område.Som ett exempel har kylningen av en isolerad gatebipolär transistor (IGBT) undersökts i detta arbete.  Ekvationer relaterade till mass- och rörelsebevarande, kopplade tillenergibevarelsesekvationen, har lösts. Flera antaganden beaktades noggrant ochrimligtvis för att förenkla simuleringen och säkerställa noggrannheten ochtillförlitligheten hos resultaten. Effekten av uppvärmt materials typ och avståndetmellan tubmunstycket och det uppvärmda objektet har undersökts i studien.Resultaten visar att strålen är effektiv för att kyla uppvärmda objekt och kananvändas inom parametrisk forskning. Faktum är att yttemperaturen och strålenshastighet har visats och diskuterats, och det har konstaterats att den mest optimalakombinationen för effektiv värmeöverföring är ett kortare avstånd mellantubmunstycket och det uppvärmda objektet, ett material med högre termiskledningsförmåga och vatten som strålens fluid. Den påverkande strålenheten hartestats för att kyla en isolerad gatebipolär transistor (IGBT) och resultaten visadeatt temperaturen på IGBT sjunker med 14,57%. Trots resultatens noggrannhet är det viktigt att erkänna begränsningarna hos dennumeriska modellen, såsom diskretiseringen av det fysiska området, upplösningenpå nätet, antaganden och förenklingar. Dessutom var denna undersökningbegränsad till tre variabla faktorer. Därför rekommenderas ytterligare studier för attytterligare optimera strålens kylningseffekt, såsom studiet av munstyckets form,strålens hastighet, strålens massflödesintensitet och en 3D-simulering.

No description available.

No description available.

<p>The impinging jets atomizer is widely used in engineering applications. As two liquid jetsimpinging to each other, a liquid sheet is first formed and then breaks up into small dropletsto comply the atomization. The features such as size, shape, velocity, thickness, etc., of thesheet/droplet are controlled by various impingement parameters such as impinging angle,jet velocity, and physical properties of the liquid. Since the sheet generation is prior to thedroplet, the modeling of the sheet is the premise of the droplet modeling. Therefore, to studythe atomization of the impinging jet atomizer, it is important to pay effort on the research ofimpinging sheet both experimentally and theoretically. In this research, the characterizationof the impinging sheet formed by two jets is given in two specific aspects, the thicknessand the velocity. A non-intrusive measurement technique, partial coherent interferometry(PCI) is developed and applied to measure the thickness of the impinging sheet dynamically.The PCI unitizes the calibrated linear relationship between the optical path difference andthe degree of coherence to measure the impinging sheet thickness. By placing the sheet inone of the two branches of the designed interferometer, the optical path is altered basedon the sheet thickness and shown as the change of the degree of coherence of interferencepattern recorded by the camera. With a calibration process, the thickness of the sheet is thencan be measured via a designed interferometer. The velocity measurement of the impingingsheet is implemented via particle tracking velocimetry (PTV) adopted with the shadowgraphtechnique. To implement the particle tracking velocimetry, seeding particles are added intothe fluid and with the aid of an imaging acquiring system and the post-processing algorithm,the locations of those particles in different frames are identified. Thus, the velocity of the fluidis estimated as the velocity of the particles calculated from the recorded images. However,while applying the PTV to investigate the impinging sheet studied in this research, theparticles can be recorded at a large field of view with insufficient magnification. This is ownedto the so-called "particle induced lens effect" found when applying the small particles to athin liquid sheet. When the seeding particles move to the region where the sheet thicknesshas a similar scale as the particle, the fluid will wrap around the particle and act as a positivelens. For shadowgraph imaging, the collimated light forms an enlarged shadow at the image plane by passing through such lens. Experimentally, the thickness measurements via PCIare implied to the impinging sheet generated under a range of Reynolds number between 269to 370 and velocity measurements via PTV are implied to the ones under Reynolds numberof 362 to 430. The measured results for both thickness and velocity are different from thetheoretical model of the impinging sheet which implies the need for a review of sheet model.Therefore, in this research, the author proposed a revised impinging sheet model considerthe friction effect due to the air over the sheet. A theoretical analysis is made base on theboundary layer equation under the cylindrical coordinate with unique boundary conditionsassumed for the impinging sheet. By introducing the unique similarity variable found byauthor, the equation could be transformed to an ordinary differential equation and solvednumerically. The revised model first predict the air boundary layer profile over the sheet,then, estimate the sheet velocity profile as a function of the distance to the impinging pointand the azimuth angle. As a parameter of the revised sheet model, the jet velocity profilebefore the impingement is also assumed as a free jet gradually developed from a Posieuilleflow and estimated in advance. The revised model is compared with the experimental resultsand some key parameters are identified empirically.</p> <p>Other than the thickness and velocity, this research is also interested in measuring thegeometry of the sheet and the detached droplets. Thus, a multi-view digital inline holography(DIH) technique is developed to capture the three-dimensional shape of the impinging sheetand the locations of the droplets. The DIH determines the shape and location of the targetin a detection volume base on the recorded hologram. The MvDIH, as the name suggested,combines the DIH results from multiple orientations to reconstruct the shape and the locationof the target. Two reconstruction ideologies, cross-section based one and the outline basedone, are proposed. The former estimates the target by finding the intersection of the recordedcross-sections of the target from different views. The latter estimates the target geometryby combining the outlines determined by DIH at different views. To evaluate the feasibilityof such technique, a test model which imitates the droplet and liquid ligament structure isapplied to the measurement in this research. Yet, the application on a real impinging sheetis not implemented.</p>

Recent improvements to concentrating solar dish systems lead to further focus on hybridization systems for small-scale power generation applications. Variability of the solar load creates new requirements for combustion systems. This thesis presents a CFD simulation of the air flow inside a new combustor design for the combination of an impinging air solar receiver and a MGT. The system consists of a LPP tubular combustor with radial main swirler and central pilot burner. Focus is made on the pressure loss at the downstream impinging cooling wall for appropriate flow distribution between reacting and bypass air. Heat transfer is not studied due to lack of time. A fully-hexahedral multi-zones mesh of the system without fuel injection has been generated with Ansys ICEM software, making use of its O-grid capabilities. A realizable k-epsilon model is used for turbulence modelling. Several impinging hole’s diameters are studied to find the right balance between the two streams. Streamlines are also observed to confirm the location of recirculation zones and recommend design improvements. / Nya förbättringar av koncentrerade solskålssystem leder till ytterligare fokus på hybridsystem för småskaliga applikationer för elproduktion. Ojämn solstrålning skapar nya krav på förbränningssystem. Detta examensarbete presenterar en CFD-simulering av luftflödet i en ny förbränningsdesign för en kombination av en solfångare med forcerad konvektionskylning och en mikrogasturbin (MGT). Systemet består av en LPP-rörbrännare med radiellt virvelsystem och central pilotbrännare. Studien fokuserar på tryckförlusten vid slaghålsväggen, som används för kylning vid förbränning, och lämplig flödesfördelning mellan reagerande- och förbigående flöde. Värmeöverföring studeras inte på grund av tidsbrist. Ett helt sexkantigt nät med flera zoner i systemet utan bränsleinsprutning har genererats med Ansys ICEM-programvara som använder O-nätfunktioner. En realiserbar k-epsilon-modell används för turbulensmodellering. Flera slaghålsdiametrar studeras för att hitta rätt balans mellan de två strömmarna. Även strömlinjer observeras för att bekräfta placeringen av återcirkulationszoner och kunna rekommendera förbättringar av designen.

This numerical study presents investigation of impinging jets cooling effect on a hot flat plate. Different configuration of single jet, 5-cross and 9-square setups have been studied computationally in order to understand about their behaviour and differences behind their physics. Moreover, a specific confined wall was designed to increase two crucial parameters of the cooling effect of impinging jets; average heat transfer coefficient of impingement wall and average air temperature difference of outlet the domain and jet inlet. The 2-D simulation has been performed to design the confined wall to optimise the domain geometry  to achieve project goals contains highest average heat transfer coefficient of hot plate in parallel to highest average air temperature difference of outlet. Different effective parameters were chosen after 2-D simulation study and literature review; Jet to wall distance H/D = 5, Radial distance from centre of plate R/D = 20, jet diameter D = 10 mm. The 3-D computational study was performed on single jet, 5-cross and 9-square configurations to investigate the differences of results and find best setup for the specific boundary condition in this project. Single jet geometry reveals high temperature level in the outlet, but very low average heat transfer coefficient due to performance of a single jet in a domain (Re= 17,232). In the other side, 5-cross setup has been studied for Reynolds number of 9,828, 11,466, 17,232 and 20,000 and it was found that range of 11,466 to 17,232 performs very well to achieve the purposes in this study. Moreover, turbulence models of ,  and  have been used to verify the models (Re=17,232) with available experimental data for fully developed profile of the jets inlets and wall jet velocity and Reynolds stress components near the wall boundary condition. All three turbulence models predict well   the velocity components for jets fully developed profile and for wall boundary condition of the target plate. But since  model has been validated with the Reynolds stress components by experimental data, therefore is more reliable to continue the study with verified simulation. Finally 9-square configuration was investigated (Re=17,232) and the result compared with other setups. It was concluded that 5-cross multiple jets is best design for this project while 9-square multiple impinging jets also fulfils the project purpose, but for extended application in industry each setup is suitable for specific conditions. 5-cross multiple jets is good choice for large cooling area which can be used in number of packages to cover the area, while 9-square jets setup performs well where very high local heat transfer is needed in a limited area.

Les instabilités aéroélastiques de bandes d’acier constituent aujourd’hui l’un des problèmes majeurs dans les sections de refroidissement par jets impactants des lignes de recuit continues.En effet, le traitement thermique des nouveaux aciers nécessite de très fortes pentes de température impliquant constamment des augmentations de vitesse de soufflage susceptibles de mettre en jeu des instabilités aéroélastiques. Des flottements ainsi que des divergences de bande ont déjà été constatées et identifiées. Ces deux instabilités impliquent dans la plupart des cas des chocs entre la bande et les buses de soufflage ce qui engendre des défauts de surface sur la bande.Un banc d’essai a été conçu et fabriqué dans le but d’analyser ces instabilités et d’anticiper leur apparition. A partir d’observations, la dynamique structurelle de la bande a été simplifiée à un mode de rotation rigide. Le banc comporte une plaque oscillante en mouvement forcé.Celle-ci est impactée par un dispositif de plusieurs jets axisymétriques turbulents ayant une disposition identique à celle des tours industrielles. Les efforts aérodynamiques stationnaires et instationnaires agissant sur la plaque sont mesurés au moyen de capteurs de pression.L’impact de plusieurs jets en interaction crée de très importants gradients de pression sur la plaque il est donc nécessaire que la grille de prises de pression soit très fine pour que l’estimation des efforts aérodynamiques soit correcte. La plaque est donc instrumentée de 91capteurs de pression sur une surface de 18 cm². Elle peut également être translatée dans les ois directions de l’espace, ce qui permet d’obtenir la distribution des efforts instationnaires ainsi que des coefficients aéroélastiques sur une grande surface de plaque et à différentes distances d’impact.Les mesures de pression stationnaires ont permis d’établir les courbes d’évolution des efforts d’impact des jets sur la plaque en fonction de la distance jet-plaque ainsi que de la géométrie des buses. Les résultats ont permis de déterminer la stabilité statique de la plaque en mouvement de pompage. Les mesures de vitesses des jets libres ont été effectuées paranémométrie à fil chaud et ont permis de déterminer leurs propriétés statistiques.Les mesures de coefficients aéroélastiques sur la plaque en rotation ont été effectuées surune seule géométrie de soufflage, pour différentes vitesses réduites. Les résultats mettent en évidence l’importance des effets de bords sur la stabilité de plaque. Des méthodes de post traitements ont proposées afin d’extrapoler les résultats à différentes largeurs de bande. Ils sont confrontés aux travaux de Regardin et al. (réf. [1]) et mettent en évidence des désaccords avec le cas réel. Des suggestions sont apportées afin d’améliorer la représentativité du banc vis-à-vis des bandes industrielles. / Aeroelastic instabilities of steel strips impinged by arrays of cooling gas jets have becomeone of the main issues in cooling sections of continuous annealing lines. Indeed, the new steeltreatments require very high temperature variation rates which involve increases in jetvelocities that are likely to onset some aeroelastic instabilities. Strip flutter and divergencehave already been observed and identified. These two aeroelastic instabilities imply a strongrisk of contact with the blowing boxes, which can seriously blemish the strip.An experimental test rig was designed and built in order to analyze and predict of theseinstabilities. From observations, the strip’s structural dynamics was simulated by a rigidrotation mode. The rig included a forced oscillating plate which is impinged by an array ofaxisymmetric jets having the exact industrial geometry. The plate was instrumented withpressure sensors to measure the steady and unsteady surface pressures. The impingement ofinteracting jets creates very large pressure gradients on the plate and therefore a tight mesh ofpressure taps (91 over an 18cm² jet impingement surface) was necessary to allow a goodestimation of the aerodynamic loads The plate could also be moved in the three coordinatedirections as to obtain surface mappings of the unsteady jet forces and aeroelastic coefficientscan be obtained over a wide area and different jet-to-plate distances.The variation of the impinging aerodynamic forces was established as a function of the jetto-plate distance for different nozzle geometries. These results were used to determine the jetstatic stability in plunging motion. Velocity and turbulence measurements in free jets werecarried out using hot wire anemometry in order to determine their statistical properties.Aeroelastic coefficient measurements were carried out on the oscillating plate with onlyone nozzle geometry and different reduced velocities. Results show that the plate’s stability ismainly dependent on the boundary effects. Post processing methods are suggested in order toapply the results to larger plates. Results are compared to the data of Regardin et al. (réf. [1])and emphasize some discrepancies with respect to the real case. Finally some improvementsto the test-rig are suggested for it to be more representative of the industrial situation.