bluuebunny/crossref_metadata_2025 huggingface dataset:
An Analysis of the Flow Fields Within Geometrically-Similar Miniature Scale Centrifugal Pumps
Authors: Kearney Daniel , Punch Jeff , Grimes Ronan
Published: July 2009

Thermal management has become a key constraint in the development of contemporary electronics systems. It is evident that heat fluxes are currently approaching the limits of conventional forced air cooling, and that liquid cooling technologies are now under consideration. As the space available to incorporate a pump is often limited, miniature-scale pumps are required. Because such pumps operate at low Reynolds numbers, their operation may deviate from that predicted from the conventional pump scaling laws, and their efficiencies reduced. This paper investigates such deviations, and reduced efficiency, through experimental measurements of the performance of two geometrically-similar pumps of – a fully scaled pump of diameter 34.3mm, and a half scale version of the same construction. A facility for the measurement of bulk pressure-flow performance characteristics is described. Particle Image Velocimetry (PIV) measured velocity profiles were extracted at varying radii in the blade passages, and at varying angular positions in the volutes. The absolute, relative, radial and whirl velocity vectors were evaluated for each flow field at three operating points and compared with conventional pump theory. The data was plotted non-dimensionally to investigate points of similitude. Fluidic phenomena occurring in the impeller passage at both pressure and suction sides of the impeller blades are addressed. The theoretical velocity triangles occurring at the impeller tip are compared with the experimental data. The blade angle at inlet and discharge are found to have a large bearing on the poor pumping performance. The quantitative velocity characteristics are discussed in the context of efficiency degradation at decreasing Reynolds numbers.
Thermal Modeling for Design Optimization of a Microfluidic Device for Continuous Flow Polymerase Chain Reaction (PCR)
Authors: Kumar Sumeet , Thorsen Todd , Das Sarit Kumar
Published: July 2009

Polymerase Chain Reaction (PCR) is a molecular biological method for the in vitro amplification of nucleic acid molecules which has wide applications in the area of genetics, medicine and biochemistry. The typical three step PCR cycle consists of heating the sample to 90–94 °C to denature double-stranded DNA, cooling down to 50–54 °C to anneal the specific primers to the single stranded DNA and finally increasing the temperature to 70–75 °C for extension of the primers with thermostable DNA polymerase. The temperature sensitivity of the reaction requires precise temperature control and proper thermal isolation of these three zones. In this paper we present the design of a continuous flow PCR microfluidic device with the channels fabricated in (poly) dimethylsiloxane (PDMS) and thin film Platinum Resistance Temperature Detector (RTD) elements fabricated on glass substrate to define the three different temperature zones. The fluidic arrangement has a water jacket layer to minimize evaporation from the porous PDMS walls. A detailed thermo fluidic model of the device is presented to predict the performance and efficacy of the proposed design. Numerical simulations are carried out to find the temperature distribution and temperature gradients in the device and a parametric study is done by varying flow rate, heat flux and channel dimensions in order to optimize the design for achieving temperature isolation and sharp temperature gradients between different zones.
Approximate Solution for the Heat Transfer Controlled Growth of a Steadily Translating Vapor Bubble
Authors: Shusser Michael
Published: July 2009

Existing analytical solution for the problem of the heat transfer controlled growth of a spherical vapor bubble moving with a constant velocity under the assumptions of a thin thermal boundary layer and potential flow results in a complicated integral equation for the bubble radius and is too unwieldy to be used in multiphase flow models. The goal of this work is to suggest an approximate solution for this problem that gives correct asymptotic behavior and yields a simpler expression for the bubble growth rate. Comparison with the exact solution showed that this way a good approximation can be obtained.
Infrared Quantum Dots for Liquid-Phase Thermometry in Silicon Microchannels
Authors: Kim Myeongsub , Yoda Minami
Published: July 2009

The exponential growth in the component density of integrated circuits has created huge thermal management challenges for next-generation microelectronics, with projected local heat fluxes approaching 1 kW/cm2 within five years. There is thus an urgent need to develop compact, high heat flux cooling methodologies. Developing and evaluating effective microelectronic single-phase liquid cooling systems, however, also requires measuring liquid-phase and wall surface temperatures to determine the local convective heat transfer coefficient in complex piping systems with dimensions comparable to the diameter of a human hair. Yet there are few if any practical thermometry techniques that can measure temperatures in such small geometries without disturbing the coolant flow and hence affecting cooling performance. Most optically based non-intrusive liquid-thermometry techniques exploit the temperature-sensitive spectral characteristics such as emission intensity and lifetime of the photoluminescence from various tracers at visible wavelengths. Applying such techniques to measure temperatures in silicon (Si) devices, however, has been hampered by a lack of suitable temperature indicators because Si is opaque at visible wavelengths. Silicon is, however, partially transparent in the near-infrared (IR), with absorption coefficients as high as 15 cm−1 at wavelengths of 1.2–1.6 μm. We have therefore investigated the temperature sensitivity of the photoluminescent emission from oleate-capped lead sulfide (PbS) quantum dots (QD) suspended in toluene that emit at a wavelength of 1.55 μm. These QD are found to have an emission intensity that decreases by as much as 0.5% per K for temperatures ranging from 293 K to 333 K. Results are presented for temperature measurements through Si surfaces using PbS QD. The accuracy and reproducibility of these temperature measurements are discussed.
Temperature Modeling for Carbon Nanofiber Breakdown
Authors: Fabris Drazen , Kitsuki Hirohiko , Yamada Toshishige , Sun Xuhui , Gonzalez Cruz Jorge , Yang Cary Y.
Published: July 2009

Carbon nanofibers (CNF) are proposed for electrical interconnect applications because of their relatively large current capacity and ability to form well-aligned one-dimensional structures. It is experimentally determined that nanofibers that are suspended between two electrodes breakdown at or near the nanofiber center. Based on published property values a simple model is used to calculate the temperature and quantify the effect of heat generation at the CNF/electrode interface on the nanofiber temperature. The model has the capability to separately account for the substrate temperature and the temperature at the CNF/electrode junction. It is determined that the CNF reaches a temperature at which carbon oxidation is likely to occur.
Simulation of a CO-H2-Air Turbulent Diffusion Flame by the Chemical Equilibrium Method With a Few Chemical Reactions
Authors: Fukumoto Kazui , Ogami Yoshifumi
Published: July 2009

The aim of our research is to build a model that can evaluate the amount of combustion products by using the chemical equilibrium method with a few chemical reactions. This paper presents an eddy dissipation concept/chemical equilibrium model (EDC/CE) and validates it by simulating a CO-H2 air turbulent diffusion flame. The obtained results were compared with Correa’s experimental data, Gran’s computational data, and the computational data obtained by using a chemical equilibrium model in FLUENT. An advantage of the EDC/CE model is that the amount of any combustion products are obtained without using detailed chemical mechanisms. The results obtained by the EDC/CE model are in good agreement with the reference data. With the combustion model that we have developed, the amount of combustion products can be calculated without detail chemical mechanisms, and the accuracy of this model is in the same order as that of the EDC model.
An Investigation of the Physical Mechanism of Heat Transfer Augmentation in Boundary Layer Flows Subject to Freestream Turbulence
Authors: Hubble David O. , Diller Thomas E. , Vlachos Pavlos P.
Published: July 2009

Water tunnel experiments have been performed to examine how large scale, high intensity freestream turbulence affects heat transfer through a laminar flat plate boundary layer. Time-Resolved Digital Particle Image Velocimetry (TRDPIV) was used to examine the flow field along with time-resolved heat transfer from the wall. The surface heat flux was measured with a newly developed thin-film sensor called the Heat Flux Array (HFA) capable of measuring heat flux at 10 locations at frequencies to 35 Hz. Freestream conditions were controlled using passive grids producing turbulence intensities of 5.5% with integral length scales of 2 and 3.5cm. This was shown to increase mean convective heat transfer coefficients by up to 15% with fluctuations to 40% above cases of very low freestream turbulence. It was also shown that fluctuations in heat flux convected at approximately half the freestream velocity.
Thermal Management in the Measurement of Metal Hydride Kinetics
Authors: Voskuilen Tyler G. , Zheng Yuan , Pourpoint Timothe´e
Published: July 2009

Many metal hydride nanopowders are currently being investigated as a potential hydrogen storage media. The kinetic properties of hydrogen absorption in TiCrMn, a metal hydride of interest, are largely unknown. This study will use coupled thermal and kinetic modeling to analyze a combination of novel and well-established techniques which can be used to experimentally determine these parameters. Since these measurements must be taken at isothermal conditions and the metal hydride absorption reaction is highly exothermal, specific thermal considerations must be made in these models. Typical instruments available for kinetics measurements suspend the samples in a small chamber, effectively thermally isolating them from the cooling or heating system designed to control sample temperature. The design modeled herein will eliminate that convective resistance layer, thereby increasing the amount of heat that can be rapidly diffused out of the sample. Additionally, an electronically controlled active temperature control system will be modeled as a method of maintaining “quasi-isothermal” conditions in the metal hydride during measurements.
Predicting System Performance and Estimating Parameters for Systems Burdened With Uncertainties and Noise Using Bayesian Inference
Authors: Emery A. F. , Bardot D.
Published: July 2009

The precision of estimates of system performance and parameter estimation is often based upon the standard deviation obtained from the usual equation for the propagation of variances derived from a Taylor series expansion [1]. With increasing computing power, it is often suggested that the more complex Bayesian inference approach may yield improved estimates of the precision. The Bayesian approach has not been widely used in the heat transfer and fluid mechanics communities. The paper develops the necessary equations and applies them to two typical heat transfer problems. It is shown that, even for the simple problem of heat loss from a fin, that the predicted performance can be a strong function of relatively minor changes in the heat transfer coefficients or the thermal conductivity and as a consequence that the form of the parameter variability has a substantial effect.
Curvature Effect on the Thermal Conductivity of Nanowires
Authors: Liu Liang-Chun , Huang Mei-Jiau , Yang Ronggui
Published: July 2009

Directional preference of the ballistic phonon transport plays an important role in the effective thermal conductivity of nanostructures. Curved nanowires can have very different thermal conductivities from straight ones. In this work, a Monte-Carlo simulator is developed and used to investigate the curvature effect on the phonon transport in silicon nanowires. The results show that the curvature of geometry does not alter the phonon transport efficiency in large wires but decreases the effective thermal conductivity in their nano-sized counterparts.
Two Phase Analysis of Heat Transfer and Dispersion of Nano Particles in a Microchannel
Authors: Nayinian Seyed Mojtaba Mousavi , Shams Mehrzad , Afshar Hossein , Ahmadi Goodarz
Published: July 2009

The effect of different parameters on dispersion of nanoparticles in a microchannel in slip flow regime is studied. The equations of particle motion and energy balance are solved numerically and the effect of particle diameter, starting position of particles in microchannel, and slip coefficient on dispersion of particles is discussed. Radiative heat flux in energy equation and drag force, Saffman lift force, Brownian force and gravitational force in momentum equation are included. The results show that the Brownian force has considerable effect on particle motion in microchannel. Particles temperature at the outlet can be controlled by variation of their diameter and starting position in microchannel.
Direct Numerical Simulation of EHD-Enhanced Film Boiling
Authors: Sharifi Payam , Esmaeeli Asghar
Published: July 2009

In applications involving boiling in micro-devices or under microgravity conditions it is extremely desirable to enhance the heat transfer rate to increase the efficiency of these systems. Here, a possible mechanism is to increase the convective effects by application of an electric field on the bulk of the fluid. While the enhancement of heat and mass transfer by electric field has been known for decades, a fundamental understanding of the problem is still lacking, primarily due to the difficulties in conduct of experimental studies. Direct Numerical Simulations (DNS) opens up enormous possibilities for detailed understanding of EHD-enhanced film boiling. Such simulations can make it possible to capture the dynamics of the boiling flows. Here, we present a front tracking/finite difference algorithm, in conjunction with a leaky-dielectric electrohydrodynamic (EHD) model, to study EHD-enhanced film boiling on horizontal surfaces. According to this study, the bubble shape and its frequency of release are highly dependent on the dielectric properties of fluid, and electric field strength. Our results show an improvement of about 50% in the Nu number over that of the regular boiling in the range of parameters that are explored here.
Effect of Polymer Chains on Heat Transfer on Nanofins
Authors: Singh Navdeep , Banerjee Debjyoti
Published: July 2009

Recent experimental results show that precipitation of nanoparticles on heater surfaces can result in heat flux enhancements observed in nanofluids. These precipitated nanoparticles can potentially act as nano-fins. Hence, in this study molecular dynamic simulations are performed to study the interfacial thermal resistance between a nanofin and a working fluid. A (5, 5) carbon nanotube (CNT) of diameter 6.78 Å and various lengths are immersed in various fluids in these analyses. For this simulation the total numbers of the fluid molecules, and the breath and the height of the cell are kept constant. So due to the different densities of the matrix, the length of the cell as well as the length of the nanotube is different for each matrix. In these simulations, the nanotube is placed at the centre of the cell and the fluid molecules surround the nanotube. Periodic boundary conditions are applied in all the directions. So the system under consideration is array of long nanotubes aligned in the horizontal direction. Simulation procedure consists of first minimizing the system. During the minimization the system is allowed to relax. During the simulations, nanotube and water molecules are allowed to move but the cell size remains constant. After minimization, NVT process is performed for 10ps to scale the velocities so that the average temperature of the cell is 300K. After the ensemble is equilibrated to the base temperature of 300K, the temperature of the nanotube is raised to 750K, by scaling the velocities of the carbon atoms. In the next step the system is allowed to relax under constant energy. This is done by performing the NVE equilibration for 10ps. The difference in the temperature of the carbon nanotube and the fluid is then calculated and plotted against the equilibration time. For the CNT-fluid system, the temperature decreases exponentially with time as predicted by various researchers in the literature. From the graphs the interfacial resistance for 1-Hexene, 1,7-Dodecene and 1,7,13-Octadecene is estimated and the effect of polymer chains is investigated.
Assessment of Heat Transfer Effectiveness: Application to Shell Bitumen Plant, Takoradi, Ghana
Authors: Simons Anthony , Nunoo Solomon
Published: July 2009

At Shell Bitumen Plant, Takoradi, Beverley Thermal Fluid Heater (BTFH) generates heat energy to heat thermal fluid (Thermia B) which flows through heat exchanger and then heats bitumen which is to be maintained at temperature range of 140 °C to 160 °C before it is discharged. High rate of heat losses have been observed and in order to maintain the temperature range of bitumen at (140 °C to 160 °C), a lot of heat energy is needed to be generated which means higher fuel consumption for BTFH. Industrial fuel oil is used to fuel the BTFH. This paper assesses the existing insulation system on the plant and seeks to improve on it so as to cut down heat losses. Consequently, the work looked at the estimation of heat losses, selection of materials for heat transfer and lagging purposes. In this wise, the existing laggings were modified by introducing fibreglass between the asbestos and masonry and thus reducing the current heat lost by 78%. Heat from the exhaust gas which would have otherwise, gone wasted, was utilised by redesigning the chimney and this yielded 0.868 kW of heat energy to aid the heating of the bitumen. In the face of rising cost of fuel and taking cognizance of the fact that cheaper natural gas and liquefied petroleum gas could be produced in Ghana, it is recommended that the heater should be fueled by either of these gases.
Transient Thermal Response of Skin Tissue
Authors: Pirtini Cetingul Muge , Herman Cila
Published: July 2009

The increased availability of thermal imaging cameras has led to a growing interest in the application of infrared imaging techniques to the detection and identification of subsurface structures. These imaging techniques are based on the following principle: when a surface is heated or cooled, variations in the thermal properties of a structure located underneath the surface result in identifiable temperature contours on it. These contours are characteristic of the structure’s shape, depth, and its thermal properties. We study the use of the transient thermal response of skin layers to determine to which extent the surface temperature distribution reflects the properties of subsurface structures, such as lesions. A numerical model using the finite element method is described to obtain this response and key results are reported in the paper. A sensitivity study is conducted first to better understand the thermal response of the system and the role of various system and model parameters. We explore the extent to which we are able to draw conclusions regarding the size, depth and nature of subsurface structures and accuracy of these conclusions based on the surface temperature response alone. This work validates the idea of examining the transient thermal response and using thermal imaging as a solution for lesion identification. A sensitivity study of surface temperature distribution to variations of thermophysical properties, blood perfusion rate, and thicknesses of skin layers is performed. It is observed that variations in these parameters have little impact on the surface temperature distribution. The work reported in the paper is a portion of a comprehensive research effort involving experiments on a phantom model as well as measurements on patients. Future work will focus on comparing the results of our 2D numerical modeling efforts with the experimental results using a skin tissue-mimicking phantom. Knowledge gained from the modeling and experimental efforts will be utilized in characterizing lesions in patient studies. The focus of this paper is the computational sensitivity analysis.
Natural Convection Heat Transfer From Vertical Square Ducts
Authors: Ali Mohamed E.
Published: July 2009

Experimental investigations have been reported on steady state natural convection from the outer surface of vertical square ducts in air. Five ducts have been used with cross section side length of 0.08, 0.07, 0.06, 0.04, and 0.02 m. The ducts are heated using internal constant heat flux heating elements. The temperatures along the vertical surface and the peripheral directions of the duct wall are measured. Longitudinal (circumference averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition regimes of natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (circumference averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for transition regime using the axial distance as a characteristic length. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers and the area ratio for the laminar regimes. The longitudinal or total averaged heat transfer coefficients are observed to decrease in the laminar region and increase in the transition region. Laminar regimes are obtained at the lower half of the ducts and its chance to appear decreases as the heat flux increases.
Formulation of Film Theory Equations for Modeling of Condensation of Steam-Air Mixtures in a Shell and Tube Condenser
Authors: Haseli Yousef , Dincer Ibrahim , Naterer Greg F.
Published: July 2009

Through development of the fundamental equations of Film Theory, condensation of steam in the presence of air in a horizontal counter-current shell and one-path tube condenser is modeled. The interaction between heat and mass transfer and hydrodynamics in the shell-side is taken into consideration. A comparison between the predictions of the model and a set of experimental data available in the archival literature indicates excellent accuracy of the new formulation. The accuracy of the method is further validated by generating profiles of the temperature and pressure drops of the gas flow through the baffles, at various air leakages. Additionally, the effects of air leakage and upstream cooling water temperature are investigated to determine how they influence the total condensation rate, shell-side gas temperature and pressure drops. The results show that the total condensation rate decreases 5% and 20.5% for an air leakage of 1% and 5%, respectively, compared to the situation of pure vapor. Also, increasing the inlet cooling water temperature from 46.5°C to 48.5°C leads to 16.2% reduction in the total condensation rate, i.e., 8.1% per °C. However, this ratio is higher at high temperatures. For example, as the cooling water temperature rises from 50°C to 51°C under identical process conditions, the total condensation rate decreases 11.7% (per °C).
Ignition and Combustion Characteristics of Liquid Fuel Droplets Containing Metal Nanoparticles
Authors: Lee Tae-Woo , Tyagi Himanshu , Sonenschein David , Phelan Patrick E. , Prasher Ravi , Peck Robert , Arentzen Paul
Published: July 2009

Basic ignition, evaporation and combustion behavior of liquid fuel droplets containing metal nanoparticles is experimentally investigated. The addition of metal nanoparticles enhances the ignition and evaporation of the liquid fuel droplets, even at low volume fraction of the nanoparticles (0.1% to 0.5%). These enhancements were, however, independent of the amount, type and size of the nanoparticles and limited to ignition and low temperature evaporation conditions. At higher temperatures, evaporation and combustion rates of the liquid droplets were not significantly affected by addition of nanoparticles. The data suggest, therefore, that the primary role of nanoparticles is in enhancing the thermal diffusion during initiation phase (ignition and low-temperature evaporation) where even in very small amounts addition of nanoparticles increases the ignition probability and evaporation rates.
Thermal Radiation in a Packed Bed With Internal Heat Generation
Authors: Hoffmann Jaap E.
Published: July 2009

Thermal radiation and conduction are the main modes of heat transfer in a packed bed with internal heat generation during a loss of forced cooling incident. Due to the large number of spheres, the only feasible way of treating the bed in Computational Fluid Dynamics is through a porous medium approach. Traditionally, CFD treats a porous medium as a fluid zone, whilst fluid properties are adjusted to account for the presence of the solids. In this work, the bed is treated as a gray gass, emitting and absorbing thermal radiation at the solid temperature. A unit cell approach is used to determine the absorption and scattering coefficients. This approach prevents direct radiation between surfaces on opposite sides of the bed, but allows the free surface of the bed to exchange radiation with surrounding structures. Results generated with the proposed model were compared to modelling results using an effective conductivity (Zehner-Schlu¨nder model [1]) to combine the contributions of conduction and radiation.
Designing Shell-and-Coil Mixed Convection Heat Exchangers
Authors: Ghorbani Mianroudi Nasser , Taherian Hessam , Gorji-Bandpy Mofid
Published: July 2009

A design problem of a mixed convection heat exchanger can not be treated totally similar to the case of the regular heat exchangers due to extra unknown parameters. In this paper procedures for designing mixed convection shell-and-coil heat exchangers are proposed for both rating and sizing problems. Flowcharts of the procedures are presented and a number of examples of problems have been tested. The proposed procedures show reasonable accuracy (max. error of 11.57%) in determining the required heat transfer area and also in predicting the thermal performance of a heat exchanger with known geometry (max. error of 7.87%).
Advances in the CFD Modelling of the PBMR to Improve the Prediction of Bypass Flows
Authors: Janse van Rensburg J. J.
Published: July 2009

An area that has been identified as important in the development of High Temperature Reactors (HTRs) is the prediction of leakage and bypass flows through such a reactor. It is therefore essential to understand the causes of bypass flows and to determine the effect on the predicted fuel and component temperatures. This paper discusses the identification of leakage flows that are applicable to the PBMR design and the ranking of these leakage flows. The modeling methodology and results are also discussed. Similar to previous HTR’s, it was found that leakage and bypass flows are important to the operation of the PBMR. However, through a focused approach, it is shown that PBMR is able to improve the understanding of this phenomenon, to quantify the flows and the subsequent influence on the operation of the system. This has resulted in a reduction of leakage and bypass from approximately 46% to 20%. The improved understanding of leakage and bypass flows empowers PBMR to address this issue already during the design phase of the project, which subsequently results in a vast improvement over historical HTR designs. This gives PBMR a distinct advantage over previous High Temperature Reactors.
Rotor Scale Model Tests for Power Conversion Unit of GT-MHR
Authors: Baxi C. B. , Kodochigov N. G. , Belov S. E. , Borovkov M. N.
Published: July 2009

A power-generating unit with the high-temperature helium reactor (GT-MHR) has a turbomachine (TM) that is intended for both conversion of coolant thermal energy into electric power in the direct gas-turbine cycle, and provision of helium circulation in the primary circuit. The vertically oriented TM is placed in the central area of the power conversion unit (PCU). TM consists of a turbocompressor (TC) and a generator. Their rotors are joined with a diaphragm coupling and supported by electro-magnetic bearings (EMB). The complexity and novelty of the task of the full electromagnetic suspension system development requires thorough stepwise experimental work, from small-scale physical models to full-scale specimen. On this purpose, the following is planned within the framework of the GT-MHR Project: investigations of the “flexible” rotor small-scale mockup with electro-magnetic bearings (“Minimockup” test facility); tests of the radial EMB; tests of the position sensors; tests of the TM rotor scale model; tests of the TM catcher bearings (CB) friction pairs; tests of the CB mockups; tests of EMB and CB pilot samples and investigation of the full-scale electromagnetic suspension system as a part of full-scale turbocompressor tests. The rotor scale model (RSM) tests aim at investigation of dynamics of rotor supported by electromagnetic bearings to validate GT-MHR turbomachine serviceability. Like the full-scale turbomachine rotor, the RSM consist of two parts: the generator rotor model and the turbocompressor rotor model that are joined with a coupling. Both flexible and rigid coupling options are tested. Each rotor is supported by one axial and two radial EMBs. The rotor is arranged vertically. The RSM rotor length is 10.54 m, and mass is 1171 kg. The designs of physical model elements, namely of the turbine, compressors, generator and exciter, are simplified and performed with account of rigid characteristics, which are identical to those of the full-scale turbomachine elements.
Examination of Dust in AVR Pipe Components
Authors: Fachinger Johannes , Barnert Heiko , Kummer Alexander P. , Caspary Guido , Seubert Manuel , Koster Albert , Makumbe Munyaradzi , Naicker Lolan
Published: July 2009

Pebble Bed HTGR’s like the AVR in Ju¨lich have the advantage of continuous fuelling. However the multiple passes of the fuel pebbles through the core have the disadvantage that the pebble’s movement through the fuelling system and the core produces graphite dust. This dust is transported from the core to other parts of the primary circuit and deposits on components. Although previous experiments performed during AVR operation have given some insight into the dust particle size and activity, there is little information on the behaviour of the dust that was deposited in the system. The decommissioning of the AVR has provided the opportunity to sample and characterise such dust from a number of components and gauge the adhesion strength. From the side of PBMR Pty Ltd this opportunity is considered important to enhance the knowledge about dust characteristics before the PBMR Demonstration Power Plant (DPP) is operational and able to produce specific plant information through sampling and analysis. AVR GmbH has provided a number of pipes and joints for investigation of loose and bound dust. Phase 1 of the analysis was used to determine the best techniques to be used on larger items. No measurable loose dust could be collected. Thereupon rings were cut from a T-section and subdivided into eight segments. The surface of the untreated segments were photographed and documented by optical microscopy, the dose rates were measured and gamma-spectrometry performed. Following this a mechanical or chemical decontamination was carried out to remove and isolate the bound dust. The average isolated dust amount was about 2 mg/cm2. Both decontamination processes indicates a strong bonding of the dust surface layer. In the case of mechanical decontamination about 60% and by chemical decontamination about 95% of the radionuclide inventory could be removed. The contribution of removed metal needs to be investigated in more detail. The median number related particle size measured by optical microscopy was found to be in the range of 0.2 to 0.7 μm whereas the median weight related size is in the range of 0.8 to 1.5 μm. The initial results indicate that this dust sticks very strongly to the pipe surface. Phase 2 will concentrate on longer pieces of piping where hopefully more loose dust can be obtained and analysed. If the same strong bonding is observed the reason for this phenomenon needs to be explained and perhaps tested with non-active dust.
Multi-Year Fuel Cycles in the MHR Core Using LEU/Th Fuel
Authors: Shayer Z. , Baxter A. , Shenoy A.
Published: July 2009

The current Modular Helium Reactor (MHR) fuel cycle uses fissile LEU (19.8 wt% U-235) and depleted uranium in separate TRISO particles, in a single fuel rod within a graphite matrix. The TRISO particle volume packing fraction (PF) in the fuel rods is 29%, of which the LEU particle PF is 62%. The lifetime between refuelings is about 476 effective full power days (EFPD). In this paper we assess the possibility of replacing the depleted uranium TRISO particles with thorium TRISO particles, and evaluate the impact of such replacement on fuel cycle length. A preliminary scoping study was performed to determine the most promising fuel rod/zoning configurations. The scoping study indicates that there is advantage to separating the thorium TRISO particles from the LEU particles at the fuel rod level instead of mixing them within a single rod. An axial checkerboard distribution of the fuel rods where all uranium and all thorium rods are interchangeable along the axial direction within the graphite block is the most promising configuration that was identified in this study and can be lead to a fuel cycle length extension of 50% to 80% relative to the current design, with only a modest increase in the fissile material loading (15%–20%). To this advantage can be added the benefit of a significant reduction in nuclear waste and in health risk. This study also lays the foundation for improving the fuel rod arrangement within the graphite block and the graphite blocks within the entire reactor core. The analysis is limited to a once-through fuel cycle based on in-situ fissioning of the U-233, without further separation and reprocessing.
Flow Visualization of Pebble Bed HTGR
Authors: Lee Jae-Young , Lee Sa-Ya
Published: July 2009

The nuclear core of High Temperature Gas Reactor (HTGR) with pebble bed type has been investigated intensively due to its benefits in management, but its complicated flow geometry requested the reliable analytical method. Recent studies have been made using the three dimensional computational methods but they need to be evaluated with the experimental data. Due to the complicated and narrow flow channel, the intrusive methods of flow measurement are not proper in the study. In the present study, we developed a wind tunnel for the pebble bed geometry in the structure of Face Centered Cubic (FCC) and measure the flow field using the Particle Image Velocimetry (PIV) directly. Due to the limitation of the image harnessing speed and accessibility of the light for particle identification, the system is scaled up to reduce the mean flow velocity by keeping the same Reynolds number of the HTGR. The velocity fields are successfully determined to identify the stagnation points suspected to produce hot spots on the surface of the pebble. It is expected that the present data is useful to evaluate the three dimensional Computational Fluid Dynamics (CFD) analysis. Furthermore, It would provide an insight of experimental method if the present results are compared by those of scaled down and liquid medium.
Mining Process and Product Information From Pressure Fluctuations Within a Fuel Particle Coater
Authors: Marshall Douglas W. , Barnes Charles M.
Published: July 2009

The Next Generation Nuclear Power/Advanced Gas Reactor (NGNP/AGR) Fuel Development and Qualification Program included the design, installation, and testing of a 6-inch diameter nuclear fuel particle coater to demonstrate quality TRISO fuel production on a small industrial scale. Scale-up from the laboratory-scale coater faced challenges associated with an increase in the kernel charge mass, kernel diameter, and a redesign of the gas distributor to achieve adequate fluidization throughout the deposition of the four TRISO coating layers. TRISO coatings are applied at very high temperatures in atmospheres of dense particulate clouds, corrosive gases, and hydrogen concentrations over 45% by volume. The severe environment, stringent product and process requirements, and the fragility of partially-formed coatings limit the insertion of probes or instruments into the coater vessel during operation. Pressure instrumentation were installed on the gas inlet line and exhaust line of the 6-inch coater to monitor the bed differential pressure and internal pressure fluctuations emanating from the fuel bed as a result of bed and gas “bubble” movement. These instruments are external to the particle bed and provide a glimpse into the dynamics of fuel particle bed during the coating process and data that could be used to help ascertain the adequacy of fluidization and, potentially, the dominant fluidization regimes. Pressure fluctuation and differential pressure data are not presently useful as process control instruments, but data suggest a link between the pressure signal structure and some measurable product attributes that could be exploited to get an early estimate of the attribute values.
Optimization of a Radially Cooled Pebble Bed Reactor
Authors: Boer B. , Kloosterman J. L. , Lathouwers D. , van der Hagen T. H. J. J. , van Dam H.
Published: July 2009

By altering the coolant flow direction in a pebble bed reactor from axial to radial, the pressure drop can be reduced tremendously. In this case the coolant flows from the outer reflector through the pebble bed and finally to flow paths in the inner reflector. As a consequence, the fuel temperatures are elevated due to the reduced heat transfer of the coolant. However, the power profile and pebble size in a radially cooled pebble bed reactor can be optimized to achieve lower fuel temperatures than current axially cooled designs, while the low pressure drop can be maintained. The radial power profile in the core can be altered by adopting multi-pass fuel management using several radial fuel zones in the core. The optimal power profile yielding a flat temperature profile is derived analytically and is approximated by radial fuel zoning. In this case, the pebbles pass through the outer region of the core first and each consecutive pass is located in a fuel zone closer to the inner reflector. Thereby, the resulting radial distribution of the fissile material in the core is influenced and the temperature profile is close to optimal. The fuel temperature in the pebbles can be further reduced by reducing the standard pebble diameter from 6 cm to a value as low as 1 cm. An analytical investigation is used to demonstrate the effects on the fuel temperature and pressure drop for both radial and axial cooling. Finally, two-dimensional numerical calculations were performed, using codes for neutronics, thermal-hydraulics and fuel depletion analysis, in order to validate the results for the optimized design that were obtained from the analytical investigations. It was found that for a radially cooled design with an optimized power profile and reduced pebble diameter (below 3.5 cm) both a reduction in the pressure drop (Δp = −2.6 bar), which increases the reactor efficiency with several percent, and a reduction in the maximum fuel temperature (ΔT = −50 °C) can be achieved compared to present axially cooled designs.
Experimental and Simulation Insight on the Transport of Silver Fission Product in SiC
Authors: Gerczak Tyler J. , Tan Lizhen , Allen Todd R. , Khalil Sarah , Shrader David , Liu Yun , Morgan Dane , Szlufarska Izabela
Published: July 2009

Understanding of the fission product transport in TRISO fuel particles is fundamental to improving the safety and performance of high temperature gas cooled reactors. Previous experiments showing silver release from TRISO fuel have focused on release measurements and not direct observation of the fission product transport. The possible diffusion of Ag via a grain boundary diffusion mechanism is being examined. By characterizing the SiC grain boundary structure according the coincidence site lattice scheme and detecting diffusion along specific grain boundaries, insight into the relationship between SiC microstructure and Ag release may be obtained. In addition computer modeling is being used to investigate the diffusion of silver through SiC. We employ a multi-scale approach based on ab initio techniques, molecular dynamics, and continuum rate equations in order to establish relationships between complex microstructures and diffusion rates. Initial work has begun on transport through bulk SiC and on building realistic models of grain boundaries in SiC.
High Temperature Reactor: Driving Force to Convert CO2 to Fuel
Authors: McCormick John L.
Published: July 2009

The rapidly increasing cost of petroleum products and uncertainty of long-term supply have prompted the U.S. military to aggressively pursue production of alternative fuels (synfuels) such as coal-to-liquids (CTL). U.S. Air Force is particularly active in this effort while the entire military is involved in simultaneously developing fuel specifications for alternative fuels that enable a single fuel for the entire battle space; all ground vehicles, aircraft and fuel cells. By limiting its focus on coal, tar sands and oil shale resources, the military risks violating federal law which requires the use of synfuels that have lifecycle greenhouse gas emissions less than or equal to emissions from conventional petroleum fuels. A climate-friendly option would use a high temperature nuclear reactor to split water. The hydrogen (H2) would be used in the reverse water gas shift (RWGS) to react with carbon dioxide (CO2) to produce carbon monoxide (CO) and water. The oxygen (O2) would be fed into a supercritical (SC) coal furnace. The flue gas CO2 emissions would be stripped of impurities before reacting with H2 in a RWGS process. Resultant carbon monoxide (CO) is fed, with additional H2, (extra H2 needed to adjust the stoichiometry: 2 moles H2 to one mole CO) into a conventional Fischer-Tropsch synthesis (FTS) to produce a heavy wax which is cracked and isomerized and refined to Jet Propulsion 8 (JP-8) and Jet Propulsion 5 (JP-5) fuels. The entire process offers valuable carbon-offsets and multiple products that contribute to lower synfuel costs and to comply with the federal limitation imposed on synfuel purchases. While the entire process is not commercially available, component parts are being researched; their physical and chemical properties understood and some are state-of-the-art technologies. An international consortium should complete physical, chemical and economic flow sheets to determine the feasibility of this concept that, if pursued, has broad applications to military and civilian aviation fleets and freight-hauling diesel engines.
Air Ingress and Corrosion Potential for PBMR Direct Cycle
Authors: Schmitz Walter , Koster Albert
Published: July 2009

The possibility of fuel and graphite degradation due to chemical attack is a perennial issue for HTR’s. For the direct cycle used in the PBMR design, only air ingress is a problem that merits serious attention. Initially, and as reported at a previous conference, investigation of the problem was tackled by assuming worst case conditions for a break at the core outlet pipe to determine what the grace time would be, before counter measures need be taken. The current work identified worst case break positions, quantifying air ingress rates, assuming a Guillotine break. These calculations include first order corrosion reactions in the bottom reflector and the core. Taking the worst possible large break location and the maximum initial air ingress as a determinant, a period of 24 hours was determined to be sufficient to prevent both serious fuel and core structure degradation. The acceptability of the extent of corrosion will be determined by the Safety Analysis Report (SAR), which is under preparation. However, it was realized that a more realistic specification and analysis of the problem was required to enable design decisions to be made, and a more detailed model of the break and the Main Power System (MPS) cavities was developed. This includes the maximum movement of large piping postulating a Double Ended Guillotine Break (DEGB) at worst possible locations. Further calculations on the improved model are described that investigate the influence of various pipe separations i.e. 50 mm and 500 mm at the turbine inlet. A strong correlation between the opening size and total core corrosion rate was confirmed. The simulation also established an approximate duration for air to be expelled to stop further ingress and the volume flow requirements for the inert gas system using helium or nitrogen.
Design and Analysis of Hot Internals for the Reactivity Control and Reserve Shutdown Units Under Test at the Helium Test Facility
Authors: Beyer E. J. J. , Craig K. J.
Published: July 2009

This paper describes the design process followed by Westinghouse Electric South Africa for the insertion of hot internals into the Reactivity Control System (RCS) and Reserve Shutdown System (RSS) Units Under Test (UUTs) at the Helium Test Facility (HTF) at Pelindaba, South Africa. The aim of the UUTs is to allow the validation of the high temperature operation of the RCS and RSS systems for implementation into the proposed Demonstration Power Plant of the PBMR. The units use electrical heaters to obtain pebble-bed reactor thermal conditions for both the control rods and small absorber spheres (SAS) under a pressurized helium environment. Design challenges include providing for strength under elevated temperatures (900°C maximum); pressure boundary integrity (9MPa maximum); separation of different volumes (representing core barrel, reactor citadel and other Reactor Pressure Vessel (RPV) volumes); thermal protection of carbon steel vessels by using thermal insulation; allowing for diverse thermal expansion coefficients of different materials; allowing for depressurization events within the insulation and internals; having access for temperature, pressure, stress and proximity sensors and electrical wiring through high pressure penetrations; and provision for assembly of the hot internals both on and off-site. Several thermal analyses using Computational Fluid Dynamics (CFD) were performed to evaluate both worst-case and operational conditions of the UUTs. Factors that were considered include thermal insulation properties, heat transfer modes (internal radiation, external radiation and natural convection, forced internal convection for cooling) and operating pressure (ranging from 1 to 9MPa). The thermal design uses elements originally proposed for hot gas duct design. The results obtained show that the proposed design satisfies ASME VIII requirements of the pressure boundary and that all challenges are successfully met.