Papers by Mohamed El-genk
Saturation boiling of PF-5060 on rough Cu surfaces: Bubbles transient growth, departure diameter and detachment frequency
International Journal of Heat and Mass Transfer, Dec 1, 2015
Abstract Investigated is the transient growth of discrete bubbles in saturation nucleate boiling ... more 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.
Effects of inclination angle and liquid subcooling on nucleate boiling on dimpled copper surfaces
International Journal of Heat and Mass Transfer, Apr 1, 2016
Abstract This paper investigated natural convection and nucleate boiling of PF-5060 dielectric li... more Abstract This paper investigated natural convection and nucleate boiling of PF-5060 dielectric liquid on 10 × 10 mm uniformly heated copper (Cu) surfaces with different diameter dimples, for potential application to immersion cooling of high power computer chips. The circular dimples, 300, 400, and 500 μm in diameter and 200 μm deep, have a triangular lattice with a pitch-to-diameter ratio of 2.0. The total number of dimples on the 10 × 10 mm surfaces increases with decreasing the dimple diameter. In addition to the dimple diameter, the performed experiments quantified the effects of the surface inclination angle and the liquid subcooling on the nucleate boiling heat transfer coefficient, hNB, and the critical heat flux (CHF). The inclination angle varied from 0° (upward facing) to 180° (downward facing) in increments of 30°, and the liquid subcooling includes 0 K (saturation), 10 K, 20 K and 30 K. In the upward facing orientation, the Cu surface with 400 μm dimples gives the highest hMNB (∼1.06 W/cm2 K) and CHF (∼19.3 W/cm2). The values for the surface with 500 μm dimples are ∼1.0 W/cm2 K and ∼18.7 W/cm2, respectively, and ∼0.7 W/cm2 K and ∼18 W/cm2 on the surface with 300 μm dimples. The CHF and hMNB on all dimpled Cu surfaces decrease with increased inclination angle to the lowest values in the downward facing orientation. These values are ∼36% and ∼33% of those in the upward facing orientation, respectively. In addition, the CHF in the upward facing orientation, increases linearly with increased liquid subcooling at a rate of 1.8%/K.
Thermally anisotropic composite heat spreaders for enhanced thermal management of high-performance microprocessors
International Journal of Thermal Sciences, Feb 1, 2016
Abstract Numerically investigated is the performance of thermally anisotropic composite spreaders... more Abstract Numerically investigated is the performance of thermally anisotropic composite spreaders for enhanced thermal management of high performance microprocessors. The spreaders are comprised of two 0.5 mm-thick Copper (Cu) laments separated by a thin (δ = 0.25–1.0 mm) layer of thermally anisotropic material, such as graphite or highly oriented pyrolytic graphite (HOPG). The exposed rough surface (Ra = 1.79 μm) of the top Cu lament is cooled by saturation nucleate boiling of PF-5060 dielectric liquid. The performed 3-D numerical analyses quantify the effect of the Figure-of-Merit ( FOM ) of the thermally anisotropic layer, on the total thermal power removed, the spreader's total thermal resistance, and the maximum temperature of the underlying 20 × 20 mm chip. The spreaders suppress the propagation of the chip hot spots, and increase the total power removed. They remove 160–317 W of the thermal power dissipated by the underlying chip, at a chip maximum surface temperature of 80–120 °C. Developed empirical correlations estimate the total thermal power removed and the surface area of the composite spreaders. Increasing the FOM from 0 (all Cu spreader) to 400 (highly anisotropic spreader) increases the total thermal power removed from ∼88 to ∼450 W and the spreader dimensions from ∼25 × 25 to ∼69 × 69 mm. The total thermal resistance of the spreaders ranges from 0.16 to 0.4 °C/W.
Authors
Nuclear Technology, Jun 1, 1981
AIP Conference Proceedings, 2008
Space nuclear power systems require a radiator to dissipate the waste heat generated during the t... more Space nuclear power systems require a radiator to dissipate the waste heat generated during the thermal-to-electric conversion process. A previously conducted radiator trade study showed that radiators with titanium/water Loop Heat Pipes (LHP) have the highest specific power (ratio of heat dissipation to radiator mass) in the temperature range from 300 K to 550 K. A prototype titanium/water LHP was designed and fabricated to operate within this temperature range. The LHP was all titanium, to eliminate incompatibility problems between water and dissimilar metals. The LHP had a 2.54 cm (1 inch) O.D., 20 cm (8 in.) long evaporator wick, and was designed to carry 500 W of heat load. The liquid and vapor lines were roughly 2 m long, typical of the requirements for a spacecraft radiator. The LHP was tested to more than 550 W, at an adverse elevation of 5 cm and an operating temperature of 413 K. This paper describes the details of the titanium/water LHP design, wick development, and titanium LHP fabrication and tests.
Adsorption Science & Technology, 2010
A novel surface-imprinting polymer (MIP/K 2 Ti 6 O 13 ) was prepared using potassium hexatitanate... more A novel surface-imprinting polymer (MIP/K 2 Ti 6 O 13 ) was prepared using potassium hexatitanate whiskers (K 2 Ti 6 O 13 ) as the carrier employing electrostatic interaction and π-π interaction between the template molecule [dibenzothiophene (DBT)] and the functional monomer ]. The synthetic polymer was characterized via Fourier-transform infrared (FT-IR) spectroscopy, surface area and porosity analysis, and X-ray diffraction (XRD). Static adsorption experiments were conducted to evaluate the adsorption kinetics, adsorption isotherms and adsorption selectivity of MIP/K 2 Ti 6 O 13 . Adsorption kinetic studies revealed that a contact time of 300 min was necessary to attain equilibrium. Adsorption isotherm studies indicated that the capacity of MIP/K 2 Ti 6 O 13 towards the adsorption of DBT at 298 K, 308 K and 318 K was 16.18 mg/g, 22.74 mg/g and 29.89 mg/g, respectively. Selectivity experiments showed that the adsorption capacity of DBT on MIP/K 2 Ti 6 O 13 was greater than for the other three analogues, while the imprinting factor towards DBT (α) was 2.06.
AIP Conference Proceedings, 1994
The present political and environmental climate may slow the inevitable direct utilization of nuc... more The present political and environmental climate may slow the inevitable direct utilization of nuclear power in space. In the meantime, there is another approach for using nuclear energy for space power. That approach is to let nuclear energy generate a laser beam in a ground-based nuclear reactor-pumped laser (RPL), and then beam the optical energy: into space. Potential space applications for a ground-based RPL include (1) illuminating geosynchronous communication satellites in the earth's shadow to exlend their lives, (2) beaming power to orbital transfer vehicles, (3) providing power (from earth) to a lunar base during the long lunar night, and (4) removing space debris. FALCON is a high-power, steady-state, nuclear reactor-pumped laser (RPL) concept that is being developed by the Department of Energy with Sandia National Laboratories as the lead laboratory. The FALCON program has experimentally demonstrated reactor-pumped lasing in various mixtures of xenon, argon, neon, and helium at wavelengths of 0.585, 0.703, 0.725, 1.271, 1.733, 1.792, 2.032, 2.63, 2.65, and 3.37 ttm with intrinsic efficiency as high as 2.5%. Frequency-doubling the 1.733_m line would yield a good match for photovoltaic arrays at 0.867 i.tm. Preliminary designs of an RPL suitable for power beaming have been "completed. The MWclass laser is fairly simple in construction, self-powered, closed--cycle (no exhaust gases), and modular. This paper describes the FALCON program accomplishments and power-beaming applications.
Criticality Calculations for Step-2 GPHS Modules
Nucleation and Atmospheric Aerosols, 2008
The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) will use an improved version of t... more The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) will use an improved version of the General Purpose Heat Source (GPHS) module as its source of thermal power. This new version, referred to as the Step-2 GPHS Module, has additional and thicker layers of carbon fiber material (Fine Weaved Pierced Fabric) for increased strength over the original GPHS module. The GPHS uses alpha decay of {sup 238}Pu in the oxide form as the primary source of heat, and small amounts of other actinides are also present in the oxide fuel. Criticality calculations have been performed by previous researchers on the original version of the GPHS module (Step 0). This paper presents criticality calculations for the present Step-2 version. The Monte Carlo N-Particle eXtended code (MCNPX) was used for these calculations. Numerous configurations of GPHS module arrays surrounded by wet sand and other materials (to reflect the neutrons back into the stack with minimal absorption) were modeled. For geometries with eight GPHS modules (from a single MMRTG) surrounded by wet sand, the configuration is extremely sub-critical; k{sub eff} is about 0.3. It requires about 1000 GPHS modules (from 125 MMRTGs) in a close-spaced stack to approach criticality (k{sub eff} = 1.0) when surrounded bymore » wet sand. The effect of beryllium in the MMRTG was found to be relatively small.« less
Space nuclear power systems, 1985 : proceedings of the Second Symposium on Space Nuclear Power Systems held in Albuquerque, New Mexico, January 14-16, 1985
Neptunium Oxide Target Fabrication Facility Considerations for the Production of Plutonium-238
AIP Conference Proceedings, 1994
This work was conducted to identify manufacturing advantages and disadvantages for three target p... more This work was conducted to identify manufacturing advantages and disadvantages for three target pin designs, then determine the facility suitability for production of each design. The three conceptual target designs considered in this evaluation included a duplex pin, a wafer pin, and a sphere pack type pin. The approach was to produce process flow sheets and conceptual process lines for the target pins' manufacture and subsequent grouping into target assemblies. The process flow and equipment designs then were evaluated for personnel exposure considerations (the amount of nuclear radiation that the manufacturing personnel would be exposed to); waste minimization (reducing material waste, especially nuclear waste, for the entire target cycle is necessary); manpower requirements; and post-irradiation operations (work that would be conducted on the assemblies and target pins after the conversion in the reactor of some of the neptunium into plutonium-238). Each target style has inherent strengths and weaknesses associated with its design; however, the results indicate that the proposed facility is adequate to manufacture and assemble any of the proposed target designs. Other valuable major conclusions were made during this study and are reported in the text.
Progress in Nuclear Energy, Apr 1, 2018
Space nuclear reactors usually use high enrichment (> 93%) fuel in order to reduce the launch mas... more Space nuclear reactors usually use high enrichment (> 93%) fuel in order to reduce the launch mass and cost. In addition to nuclear safeguard and non-proliferation concerns, high enrichment limits the fuel loading to ensure sufficient subcriticality, in case of a launch abort accident resulting in the bare reactor being flooded with seawater and submerged in wet sand. The core of the fast neutron spectrum PeBR is divided into three equal sectors that are neutronically and thermally coupled during reactor operation, but hydraulically decoupled. The PeBR design for lunar outposts, avoids launch criticality concern by being launched unfueled and loaded, after emplaced below grade on the Moon, with 1.0 cm diameter fuel pellets of ZrC-coated UC fuel particles dispersed in a ZrC matrix. The pellets are launched separately in specially designed canisters that satisfy launch subcriticality requirements. This paper investigates the effects of decreasing the fuel enrichment on the size, mass, and the full power operation life of the PeBR and on the design, dimensions, and mass of the fuel pellets launch canisters. Decreasing the fuel enrichment from 93.96% in the base design, to 70%, 50%, 40%, 30% and 25% increases the total mass of the reactor and the fuel pellets by 48%, 133%, 229%, 443%, and 697%, respectively,
Gas-lift enhanced natural circulation of alkali and heavy liquid metals for passive cooling of nuclear reactors
International Journal of Multiphase Flow, Oct 1, 2021
Abstract Gas-lift enhanced natural circulation of alkali and heavy liquid metals is a desirable o... more Abstract Gas-lift enhanced natural circulation of alkali and heavy liquid metals is a desirable option for passive cooling of advanced large, small, and micro nuclear reactors, accelerator driven systems, thermal solar plants, and experimental facilities and test loops. The injected inert gas into the riser or the chimney above the reactor core needs to be in the bubbly flow regime for best performance. Gas-heavy liquid metals flow maps are developed based on a well-established theoretical knowledge and the reported experimental data for various test loops. In addition, a thermal-hydraulics model for enhanced natural circulation of heavy liquid metals is developed and validated using the reported experimental data for LBE-N2 and LBE-Ar flow loops. The developed and validated thermal-hydraulics model and two-phase flow maps are used to parametrically investigate natural circulation enhancement of liquid sodium, molten lead, and liquid LBE in a representative loop with Ar gas injection. For enhanced natural circulation of alkali and heavy liquids metals of Na, Pb and LBE, the determined average gas void fraction in the bubbly flow regime is
Miniature Dc Electromagnetic Pumps of Molten Lead and Sodium to Support Development of Gen-Iv Nuclear Reactors
Social Science Research Network, 2022
Steam Generator Model and Controller for Cybersecurity Analyses of Digital I&C Systems in PWR Plants
A Composite Cu/HOPG Heat Spreader for Immersion Cooling of High Power Chips
Presented are the results of a 3-D numerical analysis of a composite heat spreader for immersion ... more Presented are the results of a 3-D numerical analysis of a composite heat spreader for immersion cooling of a 20 × 20 mm microprocessor. The spreader is comprised of two 0.5 mm thick Copper (Cu) laments separated by a layer of highly ordered pyrolytic graphite (HOPG), 0.25–1.0 mm thick. The exposed surface of the top Cu lament has an average roughness, Ra = 1.79 μm and is cooled by saturation nucleate pool boiling of PF-5060 dielectric liquid. Investigate is the impact of δHOPG on the total power removed, the maximum temperature of the underlying chip, Tmax, and mitigating the chip hot spots. Increasing δHOPG increases the total power removed, but also increases Tmax. The spreader with a 1.0 mm-thick δHOPG is capable of removing 318 W, without exceeding 90% of the critical heat flux (CHF), at Tmax = 120°C. This power removal is significantly higher than that with an all Cu spreader of the same thickness of 90 W, but at much lower Tmax of 67°C. Composite spreaders with δHOPG = 0.25, 0.5, and 0.75 mm are capable of removing up to 160 W at Tmax = 85°C, 228 W at 100°C, and 292W at 115°C, respectively. The HOPG suppresses the transmission of hot spots to the spreader surface and increasing δHOPG does not mitigate the hot spots.
Measurements of thermal conductivities of alumina powders and Min-K in vacuum [AMTEC applications]
Thermal conductivities of two types of alumina powders, having average particle sizes of 1.0 /spl... more Thermal conductivities of two types of alumina powders, having average particle sizes of 1.0 /spl mu/m and 0.1 /spl mu/m and effective porosities of 91.4% and 96.2%, respectively, and of Min-K AMTEC cell insulation materials were measured in vacuum. The thermal conductivities of all three materials increased with temperature, but at varying slopes. The developed thermal conductivity correlations were within /spl plusmn/10% and /spl plusmn/5% of the data of the alumina powders and the Min-K, respectively. The Min-K thermal conductivity was the lowest and least dependent on temperature, increasing from 0.03 to 0.042 W/mK as the temperature increased from 400 K to 800 K, respectively. Analysis showed that the thermal conductance of the Min-K could be an order of magnitude lower in the axial direction, but as much as 2 to 3 orders of magnitude higher that those of 60 and 240 Molybdenum foils insulation in the perpendicular direction, respectively.
Thermal science and engineering progress, Mar 1, 2018
Nusselt number (Nu) data and correlations Central subchannel and bundle average Convection heat t... more Nusselt number (Nu) data and correlations Central subchannel and bundle average Convection heat transfer Hexagonal bundles ww 20.6 4 14.03 0.85 Both correlations agree with the majority of the compiled experimental and numerical data to within ± 15%. Nu ww is consistently lower than Nu b,ww , and the difference is independent of P/D, but increases from 38% at low Pe (< 100) to as much as 52% at Pe = 1100. Nu ww , is also higher than that reported earlier by the authors for bare heated tubes, Nu s , with the difference increasing from 42% to 111% with increased Pe from 300 to 1100, and decreased P/D from 1.3 to 1.1.
Thermal science and engineering progress, Mar 1, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Application of a model-reference adaptive controller to a space nuclear power system
Journal of Propulsion and Power, Sep 1, 1992
ABSTRACT
Bubbles Transient Growth in Saturation Boiling of PF-5060 Dielectric Liquid on Dimpled Cu Surfaces
Journal of Thermal Science and Engineering Applications, Feb 3, 2016
Investigated is the transient growth of vapor bubbles in saturation boiling of PF-5060 dielectric... more Investigated is the transient growth of vapor bubbles in saturation boiling of PF-5060 dielectric liquid on 10 × 10 mm, uniformly heated Cu surfaces with circular dimples, at an applied heat flux of 0.5 W/cm2. At such low heat flux, the surfaces are populated with growing discrete bubbles, emanating mostly from the manufactured dimples. The 300, 400, and 500 μm diameter and 200 μm deep dimples are manufactured in a triangular lattice with a pitch-to-diameter ratio of 2.0; thus, the total number of dimples increases with decreasing the dimple diameter. Captured video images of growing discrete bubbles at a speed of 210 frames per second (fps) confirm that the bubble diameter increases proportional to the square root of the growth time, and the bubble departure diameter and detachment frequency increase with increasing the dimple diameter. The total volumetric growth rate and diameter of the bubbles at departure increase with increasing the dimple diameter, ∼1.81, ∼4.75, and ∼8.2 mm3/s and ∼738 μm, ∼963 μm, and ∼1051 μm for the 300, 400, and 500 μm diameter dimples, respectively. The corresponding bubble detachment frequency is ∼8.6 Hz, ∼10.2 Hz, and ∼13.5 Hz, respectively. The fraction of the active dimples for bubble nucleation on the surfaces with 300, 400, and 500 μm dimples, at an applied heat flux of 0.5 W/cm2, is ∼0.85, ∼0.64, and ∼0.53, respectively. On these surfaces, the estimated bubble volume at departure is ∼0.21 mm3, ∼0.47 mm3, and ∼0.61 mm3, and the corresponding rate of energy removed by a single bubble is ∼1.99 mW, ∼5.24 mW, and ∼9.02 mW, respectively. These results help explain the measured enhancements in nucleate boiling and the critical heat flux (CHF) on the dimpled Cu surfaces.
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Papers by Mohamed El-genk