| 1. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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A Condition Monitoring for Collapsing Bubble Mechanism for Sonoluminescence and Sonochemistry
- 2015
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Ingår i: Journal of Thermal Science and Engineering Apllications. - : ASME International. - 1948-5093 .- 1948-5085. ; 7:2, s. 8-021014
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- The acoustic cavitation phenomenon is a source of energy for a wide range of applications such as sonoluminescence and sonochemistry. The behavior of a single bubble in liquids is an essential study for acoustic cavitation. The bubbles react with the pressure forces in liquids and reveal their full potential when periodically driven by acoustic waves. As a result of extreme compression of the bubble oscillation in an acoustic field, the bubble produces a very high pressure and temperature during collapse. The temperature may increase many thousands of Kelvin, and the pressure may approach up to hundreds of bar. Subsequently, short flashes can be emitted (sonoluminescence) and the high local temperatures and pressures induce chemical reactions under extreme conditions (sonochemistry). Different models have been presented to describe the bubble dynamics in acoustic cavitation. These studies are done through full numerical simulation of the compressible Navier–Stokes equations. This task is very complex and consumes much computation time. Several features of the cavitation fields remain unexplained. In the current model, all hydrodynamics forces acting on the bubble are considered in the typical solution. Bubble oscillation and its characteristics under the action of a sound wave are presented in order to improve and give a more comprehensive understanding of the phenomenon, which is considered to have a significant role in different areas of science and technology.
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| 2. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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Assessing Learning in Higher Education
- 2012
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Rapport (populärvet., debatt m.m.)abstract
- Assessment is a key activity within the higher education system. From the student´s perspective, assessment shapes what is to be learned; from the perspective of the educator it gives clear inputs about the quality of learning. Thus, as defined by the term, ‘Constructive alignment’, by careful planning of the assessment strategy and wise selection of the assessment methods, the teaching-learning experience can be designed in an aligned manner towards the achievement of the learning outcomes. Although, defined in different ways, the basic purpose of assessment is to work as a feedback tool to both, students and teachers, to improve the student learning. In this work, we review and comment on theoretical aspects of assessment including its significance and characteristics. Moreover, some important considerations regarding the design of the assessment activities are presented, together with the description of some methods for assessing learning. With this work we would like to invite the reader to reevaluate the significance of assessment within the learning experience, and by this, hopefully the educator is “inspired” to improve the assessment activities in the classroom or another less conventional learning space.
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| 3. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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Bubble Power and Ultrasound
- 2013
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Ingår i: International Journal of Enhanced Research in Science Technology & Engineering. - 2319-7463. ; 2:11, s. 130-134
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- If anyone begins to think about the term ultrasound, the first thing that comes to the mind is the fact that it is used in animal communications (e.g., bat navigation and dog whistles). Also, one may start to think that this term is familiar in daily life and has been used many times in medicine applications for foetal imaging, (SONAR). However, many questions come to the mind, such as, what is happening when ultrasound is transmitted through the body? If it is sound, he asks himself why I cannot hear any sound? The interaction between bubbles and sound waves in liquids leads to the phenomenon of acoustic cavitation. The meaning of the word cavitation is not known or understandable for many people. This paper is to contribute the knowledge of bubble power under the action of ultrasound and to give a more comprehensive description of this phenomenon.
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| 4. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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Design Considerations for Solid Oxide Fuel Cell Auxiliary Power Units for Luxury Passenger Vehicle
- 2013
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Ingår i: The proceeding of 8th International Conference on Multiphase Flow, 2013.
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Konferensbidrag (refereegranskat)abstract
- SOFC systems, working at high temperatures of about 800°C, have recently attracted significant interests for application as automotive and stationary power supply systems. This paper explores the possibilities and limitations for achievement of solid oxide fuel cells (SOFCs) as 7 kW auxiliary power units on luxury passenger vehicles operating on diesel fuel. Various issues are discussed, e.g., the requirements and specifications for the unit as well as the advantages, challenges, and development issues for SOFCs in such applications. System design and analysis are carried out. The calculations are performed for two voltage systems 12/14 V and 36/42 V, respectively. The influences of others parameters like the mass flow rates of air and gases, water production and heat production are also evaluated for the two voltage systems.
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| 5. |
- Alhelfi, Ali Kadhim Hadi
(författare)
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Development and Assessment of Methods for the Prediction of Hydrodynamic and Ultrasound induced Cavitation
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The cavitation phenomenon has today become a keystone for different areas of science and technology including various industrial processes and medicine. Recently increasing interest and much attention have been paid to study this phenomenon. The last two decades have witnessed the development of numerous devices for different important applications of considerable practical interest, and the range of these applications is in continuous increase. This thesis aims to gain more knowledge about the mechanisms and dynamics of the cavitation phenomenon. In the present study, various techniques are developed and investigated. The major principal parameters associated with these phenomena are all presented in order to improve and give more comprehensive understanding of the phenomenon. The study presents enhanced numerical models to analyze the cavitation phenomenon and simulated results of formation and collapse of a single bubble in a liquid are provided. A convenient model is applied and its enhancements are examined numerically. The validity and comparison with the available experimental data were found favorable. The model is employed to deduce the bubble dynamics in flow domains for the following situations, 1. Bubble dynamics in an acoustic field. The sound waves create pressure variations through the medium and cause regions of rarefaction and compression. These pressure variations can set a tiny bubble into radial motions, i.e., expansion and compression. The bubble in an acoustic field grows as the pressure associated with the sound waves gives rarefaction. When the pressure turns to compression, the bubble is compressed and may reach an unstable size and then collapse violently. During this process, the nonlinear motion of the bubble is a complicated process. Despite many efforts, the demand of creating more suitable models with a realistic applicability calls for numerical calculations of the bubble motion in an acoustic field. This will provide improved knowledge about the real situation and the main results associated with this phenomenon. 2. Bubble dynamics close to a rigid boundary. In fact, the simulation of non-spherical bubble dynamics and its interaction with solid boundaries has received much less attention due to the complexity of the problem. A main reason of the structural damages in the cavitation phenomenon is due the formation of micro jets generated due to the bubble collapse and impingement on the solid surfaces or boundaries. The boundary integral method (BIM) is employed to compute the bubble motion and explosion of a cavitation bubble close to a rigid boundary. The liquid is considered to be incompressible, inviscid, and irrotational around the bubble. These assumptions satisfy the conditions for the Laplacian equation.
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| 6. |
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| 7. |
- Alhelfi, Ali Kadhim Hadi
(författare)
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Heat Transfer in Liquid Film over the Surface of a Rotating Disk Subjected to Impinging Jet
- 2012
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Rapport (populärvet., debatt m.m.)abstract
- This work is theoretical analyses of heat transfer across film a flowing over the surface of a rotating disk. This field has wide industrial applications in Turbomachinery, chemical, and food industries. An analytical study is developed to investigate the heat transfer characteristics in the liquid film due to jet impinging at the center of the disk. A theoretical model is based on heat balance and energy equations developed for a rotating disk heated by a constant heat flux and the free surface are subjected to the surrounding. Local Nusselt number and temperature were found for different selected parameters such as Prandtl number and Reynolds number. Nusselt number obtained from the present model was compared with that obtained by Thomas et al., and the agreement between the results was acceptable.
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| 8. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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Modeling of Spherical Gas Bubble Oscillation in Acoustic Pressure Field
- 2013
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Ingår i: Proceedings Of The 8th International Conference on Multiphase Flow, 2013.
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Konferensbidrag (refereegranskat)abstract
- Different applications of high power ultrasound in industry and medical applications are based on the propagation and collapse of acoustic cavitation bubbles. This paper presents an enhanced numerical model and simulated results of formation and collapse of a single bubble in a liquid for two cases: bubble with uniform pressure and a bubble with variable pressure inside. For the two cases, both heat transfer inside the bubble and heat transfer to the surrounding liquid are taken into account. Various fundamental properties of oscillating bubbles in ultrasonic acoustic field, such as pressure, temperature and velocity fields inside the bubble under the influence of time-dependent acoustic pressure, have been investigated and the results show that neglecting the pressure gradient inside the bubble affects considerably the bubble dynamics.
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| 9. |
- Alhelfi, Ali Kadhim Hadi, et al.
(författare)
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NUMERICAL INVESTIGATION OF AN OSCILLATING GAS BUBBLE IN AN ULTRASONIC FIELD
- 2014
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Konferensbidrag (refereegranskat)abstract
- The discovery of acoustic cavitation phenomenon is an important role in the design of a wide range of devices handling liquids and it has led to a renewed interest in the bubble dynamics in a sound field.In this study, the nonlinear behaviour of individual gas bubble in liquid under the action of ultrasound fields has been analysed, and simulated results of formation and collapse of a bubble have been provided. The characterization of acoustic cavitation bubbles under theinfluence of periodic pressure field, e.g., the motion of the bubble surface, pressure, temperature and density fields inside the bubble have been investigated and the results are compared with experimental data. The numerically calculated results reveal that the assumption of polytropic approximation inside the bubble predicts that a radius-time curve does not fit to the observed data. Also, the results indicate that the pressure gradient and the heat transfer inside the bubble and across the bubble surface play a major role to predict the extreme conditions associated with the bubble collapse.
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| 10. |
- Alhelfi, Ali Kadhim Hadi
(författare)
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On Bubble Dynamics in Acoustic Cavitation
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nyligen har mycket uppmärksamhet ägnats åt studier rörande ickelinjära oscillationer av sfäriska gasbubblor i pulserande tryckfält. Denna lic-avhandlning syftar till att utöka kunskapen kring kavitationsfenomenet. I detta arbete undersöks olika modeller och en förbättrad numerisk modell har utvecklats för att analysera akustiska kavitationsfenomen. Resultaten från den utvecklade modellen innefattar bildande och kollaps av enstaka gasbubblor i en vätska under inverkan av ultraljud som en funktion av tiden. Modellen valideras och jämförs för tre huvudfall. Dessa fall är: enhetligt tryck och temperatur inne i bubblan, enhetligt tryck och varierande temperatur inne i bubblan, samt varierande tryck och temperatur inne i bubblan. För fall 2 och fall 3 tas hänsyn både till värmeöverföringen inne i bubblan och till den omgivande vätskan. Olika grundläggande egenskaper hos oscillerande bubblor i ett ultraljud akustiskt fält såsom tryck, temperatur, densitet, hastighet och acceleration inne i bubblan under påverkan av tidsberoende ljudtryck har undersökts och resultaten för varje enskilt fall jämförs med motsvarande experimentella data. De numeriskt beräknade resultaten visar att antagandet om ett enhetligt tryck och temperatur inuti bubblan predikterar ett samband mellan radien och tiden som avviker starkt från de uppmätta värdena. Vidare visades det att för att prediktera de extrema tillstånd som förknippas med en kollaps måste bubblans dynamik ta hänsyn både till tryckgradienten och värmeöverföringen inne i bubblan samt över bubblans yta.
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