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Now showing items 33 - 48 of 153

  • Performance analyses of VHTR plants with direct and indirect closed Brayton cycles and different working fluids

    Mohamed S. El-Genk   Jean-Michel Tournier  

    This paper investigated the performance of Very High Temperature Reactor (VHTR) power plants with helium working fluid and direct and indirect Closed Brayton Cycles (CBCs), and with binary mixture working fluids of He–Xe and He–N2 (molecular weight of 15 g/mole) and indirect CBCs. Also investigated are the effects of using low- and high-pressure compressors with intercooling, versus a single compressor, using bleed cooling the reactor pressure vessel in direct CBC helium plants, and varying the reactor exit temperature from 700 °C to 950 °C on the plant thermal efficiency, cycle pressure ratio and the size of and number of stages in the turbine and compressor. Analyses are performed for a shaft rotation speed of 3000 rpm, reactor thermal power of 600 MW and a temperature pinch of 50 °C in the intermediate heat exchanger (IHX) for the indirect CBCs.
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  • Estimates of helium gas release in 238PuO2 fuel particles for radioisotope heat sources and heater units

    Mohamed S. El-Genk   Jean-Michel Tournier  

    Release data of noble gases (Xe and Kr) from small-grain (7-40 mum), large-grain (ges300 mum), and monocrystal UO 2 fuel particles, during isothermal irradiation up to 6.4 at.% and 2030 K are reviewed and their applicability to estimate helium release from 238PuO 2 fuel particles (ges300 mum in diameter) is examined. Coated 238PuO 2 particles have recently been proposed for use in radioisotope power systems and heater units employed in planetary exploration missions. These fuel particles are intentionally sized and designed to prevent any adverse radiological effect and retain the helium gas generated by the radioactive decay of 238Pu, a desired feature for some planetary missions. Results suggest that helium release from large-grain (ges300 mum) particles of K could be <7% at 1723 K, <0.6% at 1042 K, and even less for polycrystalline particles fabricated using sol-gel processes. Results also suggest that helium release from small-grain plutonia particles at 1723 K could be >80% but less than 7% at 1042 K, which is in general agreement with the experiments conducted at Los Alamos National Laboratory more than two decades ago. In these experiments, the helium gas release from small-grain (7-40 mum) 238PuO 2 fuel pellets has been measured during steady-state heating at temperatures up to 1886 K and ramp heating to 1723 K
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  • An Investigation of the Breakup of an Evaporating Liquid Film, Falling Down a Vertical, Uniformly Heated Wall

    Mohamed S. El-Genk   Hamed H. Saber  

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  • Effects of working fluid and shaft rotation speed on the performance of HTR plants and the size of CBC turbo-machine

    Mohamed S. El-Genk   Jean-Michel Tournier  

    Investigated are the effects of the molecular weight of the working fluid, reactor exit temperature, and shaft rotation speed on the size and number of stages of the turbo-machine as well as the performance of high temperature reactor (HTR) plants with actively cooled reactor pressure vessel and direct or indirect closed Brayton cycles (CBCs). The present analyses for working fluids of helium (4 g/mol) and the 15 g/mol He-Xe and He-N 2 binary mixtures are performed for a reactor thermal power of 600 MW, shaft rotation speed of 3000-9000 rpm, and reactor exit temperature from 973 K to 1223 K. For the plants with indirect CBCs, the analyses assume a temperature pinch of 50 K in the IHX. Results show that the CBC compression ratio is relatively low (~2.6 for He and He-Xe and ~3.2 for He-N 2), increases very little with increasing the reactor exit temperature and near the maximum thermal efficiency of the plant. For the plants with a direct helium CBC, the thermal efficiency increases from 42% to 51% as the reactor exit temperature increases from 973 K to 1223 K, respectively, versus 37% to 47% for the plants with indirect He-CBC. The HTR plants with indirect He-Xe and He-N 2 CBCs and operating at a turbine inlet temperature of 1123 K have slightly higher thermal efficiencies (45.9% and 45.8%) than the He plant with indirect CBC (45.6%), generating ~1.6 MWe more electrical power. The molecular weight of the working fluid has a very small effect on the plant thermal efficiency, but significantly reduces the size and number of stages of the CBC turbo-machine. Increasing the shaft rotation speed also decreases the size and number of stages of the CBC turbo-machine. [All rights reserved Elsevier].
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  • CFD analyses and correlation of pressure losses on the shell-side of concentric, helically-coiled tubes heat exchangers

    Denise A. Haskins   Mohamed S. El-Genk  

    Highlights • CFD analyses investigated pressure losses on shell side of annular heat exchangers of concentric, helically coiled tubes. • Analyses performed for isothermal flows of liquid sodium, helium gas and water. • A continuous friction factor correlation is developed for Reynolds numbers from 5 to 10 7 . • Investigated is the effect of the numerical grid refinement on CFD results. • Flow filed images demonstrate the effects of Reynolds number and number of coiled tubes. Abstract This paper presents and discusses the results of Computational Fluid Dynamic (CFD) analyses for calculating the friction pressure losses of isothermal flows of liquid sodium, water, and helium gas, on the shell side of annular heat exchangers (HEXs) of concentric, helically coiled tubes. The analyses covering wide ranges of HEX geometrical parameters and flow Reynolds numbers, varied the number of HEX coils from 4 to 16, and examined the effect of refining the numerical mesh grids on the results. Expressing the friction factor in terms of an effective HEX porosity, a weighted average of the areal and volume porosities, collapses the friction factor results on a single curve for developing a continuous dimensionless correlation. The developed correlation, which agrees with the CFD results for 5.0 < Re < 10 7 to within ±6%, is given as: f = ( 58 / Re ) + ( 0.315 / Re 0.02 ) . For low Re flow in helically coiled tubes HEXs, the friction pressure losses increase proportional to the mass flux, G , but inversely proportional to D e 2 . For high Re flow, these losses increase proportional to the mass flux to the 1.98 power, G 1.98 , and inversely proportional to the equivalent hydraulics diameter to 1.02 power, D e 1.02 . For low Reynolds flows ( Re ⩽ 20), the HEX pressure losses are ∼10% lower than in open annuli, but become higher with increased Reynolds number to as much as 97% at Re = 10 7 .
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  • Saturation boiling of PF-5060 on rough Cu surfaces: Bubbles transient growth, departure diameter and detachment frequency

    Arthur Suszko   Mohamed S. El-Genk  

    Highlights • Experimental results show large enhancement in nucleate boiling with increasing surface roughness. • Recorded transient growth, departure diameter and detachment frequency of vapor bubbles on smooth and rough cu surfaces. • Growth rate and departure bubble diameter on smooth Cu are higher than on rough surfaces. • Estimated densities of active sites for bubble nucleation confirm the measured enhancements in nucleate boiling. Abstract Investigated is the transient growth of discrete bubbles in saturation nucleate boiling of PF-5060 liquid on rough Cu surfaces at an applied heat flux of ∼0.5 W/cm 2 . The uniformly heated 10 × 10 mm surfaces have an average roughness, Ra = 0.039 μm (smooth Cu), and 0.21–1.79 μm. The transient growth rate, departure diameter and detachment frequency of the discrete vapor bubbles are determined from images captured at 210 fps using high-speed video camera. On smooth Cu, the bubble departure diameter and detachment frequency are 655 ± 40 μm and 31 ± 4 Hz. On the other Cu surfaces (Ra = 0.21–1.79 μm), the measured values of 438 ± 17 μm and 38 ± 3 Hz, respectively, are independent of surface roughness. Results, in conjunction with the experimental nucleate boiling curves, estimate the surface average density of active nucleation sites as a function of wall superheat. For smooth Cu, the density of active sites ranges from 100 to 2000 cm −2 , compared to 650–10,000 cm −2 for the rough Cu surfaces. For all surfaces, the active nucleation sites density increases with increasing the wall superheat and/or surface roughness.
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  • Some improvements to the solution of Stefan-like problems

    Mohamed S. El-Genk   August W. Cronenberg  

    Two approximate analytical methods are developed for solving one-dimensional transient heat-conduction problems with phase transformation, where the growth rate of a frozen crust (layer) on a cold wall is sought. The two methods presented provide an accurate prediction of the instantaneous position of the moving boundary as applied to one-dimensional melting and freezing problems. The important feature of these methods is that the instantaneous frozen layer thickness is given in an integral form which is relatively insensitive to small variations in the Kernel function from the exact solution
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  • Molten fuel radial motion and cladding melting during a PCM event in LWRs

    Mohamed S. El-Genk  

    The thermal behavior of the fuel and cladding during off-normal operating conditions, generally termed power-cooling-mismatch (PCM), are of great interest to light water reactor (LWR) safety analysis. During a power-cooling-mismatch event, fuel melting may begin at the center of the rods propagating radially outward. The induced pressure at the center of the rod due to fuel melting, fission gas release, and UO 2 fuel vapor would tend to force such molten fuel to flow through radially open cracks in the outer unmelted portion of the pellet and relocate in the fuel-cladding gap. The zircaloy cladding, which is at high temperature during film boiling, may undergo melting at its inside surface upon being contacted by the extruded molten fuel, eventually resulting in a thermal failure of the cladding. Three topics of interest are analyzed. First, fuel conditions during a hypothesized PCM accident are assessed with regard to pellet cracking and central fuel melting. Secondly, the transient freezing of a superheated liquid penetrating an initially empty crack, maintained at constant subfreezing temperatures, is studied analytically. Thirdly, conditions for potential melting of zircaloy cladding upon being contacted by the extruded molten fuel are investigated analytically. The analytical predictions were consistent with the experimental results from PCM in-pile tests
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  • Permanent Revolution: Mohamed Bouazizi\"s Incendiary Ethics of Revolt

    Pugliese   J.  

    This essay stages a reflection on the complex relation that Levinas’ philosophy has to violence and revolution. Confining my commentary largely to one pivotal essay, “Ideology and Idealism,” I contend that Levinas must be seen as an advocate of revolution unconditionally oriented by ethics. He must be seen, moreover, as an advocate of nothing less than, in his own words, “permanent revolution.” Rather than offer a purely abstracted commentary on Levinas’ concept of permanent revolution, I stage a posthumous face-to-face of Levinas with Mohamed Bouazizi, the Tunisian street vendor who is seen as the catalyst who proceeded, after his self-immolation, to inspire both the Tunisian revolution and the revolutions of the Arab Spring that swept across North Africa and the Middle East. By interlacing Levinas’ profound meditations on violence, revolution and the ideality of justice with Mohamed Bouazizi’s suicide and the revolutions he catalyzed, I propose to illuminate the ethical dimensions of revolt, rebellion and revolution in the context of state violence.
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  • Mohamed S Fofana: Bank of Sierra Leone Charity Trust Fund

    Mohamed S Fofana  

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  • Mohamed S Fofana: Benefits of microfinance in Sierra Leone

    Mohamed S Fofana  

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  • United States Denounces Scotland\"s Release of Abdel Basset Mohamed al-Megrahi

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  • Mohamed S Fofana: Tackling money laundering in Sierra Leone

    Mohamed S Fofana  

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  • Experimental studies of critical heat flux for low flow of water in vertical annuli at near atmospheric pressure

    Mohamed S. El-Genk   Stanley J. Haynes   Kim Sung-Ho  

    Experiments are performed to measure the critical heat flux (CHF) for low flow of water in vertical annuli near atmospheric pressure (0.118 MPa). CHF data are collected for water inlet subcooling ranging from 44 K (or 182 kJ kg -1) to 75 K (or 312 kJ kg -1), and mass fluxes from 0 to 260 kg m -2 s -1. The CHF values vary from 155 kW m -2 for zero inlet flow to a maximum of 1418 kW m -2 for a water mass flux of 260 kg m -2 s -1. The data are classified and correlated according to the flow transition which occurred at CHF. Two CHF correlations are developed, one for the slug-churn and the churn-annular flow transitions and the other for the annular-annular mist flow transition. The heat flux at incipient flow chugging is also measured and found to vary from a minimum of about 40% of the CHF values in the smallest annulus (annulus ratio=1.575) to a maximum of approximately 88% of CHF in the other larger annuli (annulus ratio=1.72 and 2.0). Results also show that the heat flux at incipient chugging increases as either the water mass flux or inlet subcooling increases. The CHF values for zero inlet flow are predicted within plusmn25% using Block and Wallis' flooding correlation (1978)
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  • A review of refractory metal alloys and mechanically alloyed-oxide dispersion strengthened steels for space nuclear power systems

    Mohamed S. El-Genk   Jean-Michel Tournier  

    Mechanical and thermo-physical properties of refractory metal alloys and mechanically alloyed (MA)-oxide dispersion strengthened (ODS) steels are reviewed and their potential for use in space nuclear reactors is examined. Preferable refractory alloys for use in liquid metal and gas-cooled space reactors include Nb-1%Zr, PWC-11, Mo-TZM, Mo- xRe where x varies from 7% to 44.5%, T-111 and ASTAR-811C. These alloys are heavy, difficult to fabricate, and are not readily available. The advantages of the MA-ODS alloys are: (a) their strength at high temperatures (>1000K), which decreases slower with temperature than those of niobium and molybdenum alloys; (b) relatively lightweight and less expensive; (c) low swelling and no embrittlement with exposure to high-energy neutrons (>0.1 MeV) up to 10 27 n/m 2; and (d) high resistance to oxidation and nitration. The few data available on compatibility of MA-ODS alloys with alkali liquid metals up to 1100 K are encouraging, however, additional tests at typical temperatures (1000-1400 K) in space nuclear reactors are needed. The anisotropy of MA-ODS alloys when cold worked, and particularly rolled into tubes, should not hinder their use in space nuclear power systems, in which operation pressure is either near atmospheric or as high as 2 MPa, but joints weldability is an issue. [All rights reserved Elsevier]
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  • Buoyancy induced instability of laminar flows in vertical annuli—I. Flow visualization and heat transfer experiments

    Mohamed S. El-Genk   Dasari V. Rao  

    Experiments encompassing both flow visualization and heat transfer measurements (125< Rein<1700 and 2.5times10 5< Gr1<5times10 7 ) are performed to investigate buoyancy induced instability of developing laminar upflow and downflow of water in a vertical annulus having a diameter ratio of 2.0. The inner wall of the annulus is uniformly heated and the outer wall is insulated. Incipient flow instability is detected from the video images of the dyed flow field, the simultaneous decrease in the temperature difference between the heated wall and the water bulk from its laminar value and the fluctuations in the coolant temperature. The measured values of xfr, based on the video images of the dyed flow fields, are correlated for both buoyancy assisted and opposed flows as functions of local Grashof number, Grq, and local Reynolds number, Re. The Nusselt numbers for buoyancy induced turbulent flows, occurring downstream of the location of incipient instability, are as much as three times higher than those for stable laminar flows at the same inlet Reynolds number, but at lower Grq
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