Arlon J. Hunt

Lawrence Berkeley National Laboratory, Berkeley, California, United States

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Publications (78)80.57 Total impact

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    ABSTRACT: We present a design space exploration of a 5 MWth Small Particle Solar Receiver for solar tower power plants. This new solar receiver, developed under the support of the U.S. Department of Energy's SunShot Program, aims to volumetrically absorb concentrated solar irradiation using an air-particle mixture to drive a gas turbine or a combined cycle at much higher temperature than the state-of-the-art molten salt receivers. Among other advantages, the thermodynamic efficiency of the power block and the overall efficiency of the plant would considerably increase with this technology. The design space consists of the wall angle of the receiver, the geometry of the window (necessary to allow the solar irradiation to enter into the receiver) and the radiative properties of the walls. The constraints are based on material limits, ensuring the mechanical integrity of the quartz window, and other technical issues; though some of them are imposed via a penalty method. The design space is explored through parametric studies and a multidisciplinary approach is adopted. The aluminum oxide walls, the 45° spherical-cap window and the 45° wall- angle receiver are preferred due to their best compromise between thermal efficiency and wall temperature.
    Full-text · Article · Dec 2014 · Energy Procedia
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    ABSTRACT: We present a design space exploration of a 5 MWth Small Particle Solar Receiver for solar tower power plants. This new solar receiver, developed under the support of the U.S. Department of Energy’s SunShot Program, aims to volumetrically absorb concentrated solar irradiation using an air-particle mixture to drive a gas turbine or a combined cycle at much higher temperature than the state-of-the-art molten salt receivers. Among other advantages, the thermodynamic efficiency of the power block and the overall efficiency of the plant would considerably increase with this technology. The design space consists of the wall angle of the receiver, the geometry of the window (necessary to allow the solar irradiation to enter into the receiver) and the radiative properties of the walls. The constraints are based on material limits, ensuring the mechanical integrity of the quartz window, and other technical issues; though some of them are imposed via a penalty method. The design space is explored through parametric studies and a multidisciplinary approach is adopted. The aluminum oxide walls, the 45º spherical-cap window and the 45º wall-angle receiver are preferred due to their best compromise between thermal efficiency and wall temperature.
    Full-text · Conference Paper · Sep 2013
  • Fletcher J. Miller · Arlon J. Hunt
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    ABSTRACT: The concept of absorbing concentrated solar radiation volumetrically, rather than on a surface, is being researched by several groups with differing designs for high temperature solar receivers. The Small Particle Heat Exchange Receiver (SPHER), one such design, is a gas-cooled central receiver capable of producing pressurized air in excess of 1100 C designed to be directly integrated into a Brayton-cycle power block to generate electricity from solar thermal power. The unique heat transfer fluid used in the SPHER is a low-density suspension of carbon nano-particles (diameter ∼ 200 nm) to absorb highly concentrated solar radiation directly in a gas stream, rather than on a fixed absorber like a tube or ceramic foam. The nano-particles are created on-demand by pyrolyzing a small flow of natural gas in an inert carrier gas just upstream of the receiver, and the particle stream is mixed with air prior to injection into the receiver. The receiver features a window (or multiple windows, depending on scale) on one end to allow concentrated sunlight into the receiver where it is absorbed by the gas-particle suspension prior to reaching the receiver walls. As they pass through the receiver the carbon nano-particles oxidize to CO2 resulting a clear gas stream ready to enter a downstream combustor or directly into the turbine. The amount of natural gas consumed or CO2 produced is miniscule (1–2%) compared to what would be produced if the natural gas were burned directly to power a gas turbine.The idea of a SPHER, first proposed many years ago, has been tested on a kW scale by two different groups. In the new work, the engineering for a multi-MW SPHER is reported. An in-house Monte Carlo model of the radiation heat transfer in the gas-particle mixture has been developed and is coupled to FLUENT to perform the fluid dynamic calculations in the receiver. Particle properties (size distribution and complex index of refraction) are obtained experimentally from angular scattering and extinction measurements of natural gas pyrolysis in a lab-scale generator, and these are corroborated using image analysis of Scanning Electron Microscope (SEM) pictures of particles captured on a filter. A numerical model of the particle generator has been created to allow for scale-up for a large receiver. We have also designed a new window for the receiver that will allow pressurized operation up to 10 bar with a 2 m diameter window. Recent progress on overcoming the engineering challenges in developing this receiver for a prototype test is reported.
    No preview · Conference Paper · Jul 2012
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    Patricia Hull · Ian Shepherd · Arlon Hunt
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    ABSTRACT: The Mie model is widely used to analyze light scattering from particulate aerosols. The Diesel particle scatterometer, for example, determines the size and optical properties of Diesel exhaust particles that are characterized by the measurement of three angle-dependent elements of the Mueller scattering matrix. These elements are then fitted by Mie calculations with a Levenburg-Marquardt optimization program. This approach has achieved good fits for most experimental data. However, in many cases, the predicted complex index of refraction was smaller than that for solid carbon. To understand this result and explain the experimental data, we present an assessment of the Mie model by use of a light-scattering model based on the coupled-dipole approximation. The results indicate that the Mie calculation can be used to determine the largest dimension of irregularly shaped particles at sizes characteristic of Diesel soot and, for particles of known refractive index, tables can be constructed to determine the average porosity of the particles from the predicted index of refraction.
    Preview · Article · Jul 2004 · Applied Optics
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    ABSTRACT: Aerogels containing both Al2O3 and Cr2O3 were prepared by the reduction, by alcohols, of a precursor salt solution derived from Al(OH)3 and CrO3, followed by supercritical drying in either CO2 or ethanol. TEM analyses showed a microstructure typical of aerogels, with a connected matrix of ∼10-nm diameter particles and an open pore network. Subsequent thermal processing converts the initial aerogels to a high surface-area material comprised of Al2O3 and Cr2C3. Addition of ∼6% SiO2, relative to Al results in an increased retention of surface area at high temperatures. Surface areas of the aerogels after supercritical drying ranged from 240 to 700 m2/g, while after treatment at 1000 °C values ranged from 110 to 170 m2/g. The composition which showed the greatest temperature stability was 2(0.94Al2O3 · 0.06SiO2)Cr2O3. After treatment at 1000 °C, all a samples contained a large number of crystallites of the Cr2O3 phase, eskolaite, with diameters ranging from 0.5 to 1.0 μm. An additional unidentified phase may also be present. The presence of these larger crystallites leads to a lower transmittance in the near-IR due to increased scattering.
    Preview · Article · Dec 2003 · Journal of Non-Crystalline Solids
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    Michael R Ayers · Arlon J Hunt
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    ABSTRACT: We report on a new optical method to observe the onset of aggregation in alcoholic tetraethoxysilane (TEOS) sols using laser speckle contrast measurements. The contrast in a speckle image produced by coherent light provides information about the internal contrast of the medium being studied. For silica sols, changing the amount of acid or base catalyst was the most important factor in determining the aggregation behavior of the sol. We investigated this effect by varying the TEOS/base ratio by a factor of 6. This shifted the onset of aggregation as determined by speckle contrast from ∼46% of the gel time for the lowest amount of base to ∼74% for the highest. Conversely, varying TEOS/acid ratio by a factor of 3, shifted the onset of aggregation from ∼74% of the gel time for the lowest amount of acid to ∼64% for the highest. Measurements of this type provide information that can be used to test models of sol aggregation and gel formation.
    Preview · Article · Sep 2001 · Journal of Non-Crystalline Solids
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    Michael R. Ayers · Arlon J. Hunt
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    ABSTRACT: Chitosan, a polymer that is soluble in dilute aqueous acid, is derived from chitin, a natural polyglucosamide. Aquagels, where the solid phase consists of both chitosan and silica, can be easily prepared by using an acidic solution of chitosan to catalyze the hydrolysis and condensation of tetraethylorthosilicate. Gels with chitosan/tetraethoxysilane (TEOS) mass ratios of 0.1–1.1 have been prepared by this method. Standard drying processes using CO2 give the corresponding aerogels. The amount of chitosan in the gel plays a role in the shrinkage of the aerogel during drying. Gels with the lowest chitosan/silica ratios show the most linear shrinkage, up to 24%, while those with the highest ratios show only a 7% linear shrinkage. Pyrolysis at 700°C under nitrogen produces a darkened aerogel due to the thermal decomposition of the chitosan, however, the aerogel retains its monolithic form. The pyrolyzed aerogels absorb slightly more infrared radiation in the 2–5 μm region than the original aerogels. B.E.T. surface areas of these aerogels range from 470 to 750 m2/g. Biocompatibility screening of this material shows a very high value for hemolysis, but a low value for cytotoxicity.
    Preview · Article · Jun 2001 · Journal of Non-Crystalline Solids
  • Arlon J. Hunt · Michael R. Ayers
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    ABSTRACT: Light scattering methods have previously been used to monitor the formation of gels. In this report we present new light scattering techniques to study the properties of silica alcogels during the aging process. Monitoring one particular polarization transformation of scattered light with time reveals a clear increase in internal strain in standing alcogels with time. The stress birefringence coefficient of an acid-catalyzed SiO2 gel was found to be 1.3 Brewsters. Additionally, the evolution of the stiffness of alcogels was investigated using laser speckle methods. Specifically, image analysis of specklegrams obtained during multi-frequency acoustic excitation of aging gels was used to non-destructively measure the hardening of alcogels. For an acid-catalyzed gel with a theoretical density of ∼0.05 g/cm3 SiO2, the rate of hardening is found to be greatest between gelation and 2×the gel time, and drops considerably thereafter. The Young's modulus of the gel can be monitored over time with this method and was found to range from 6.2×103N/m2 after 6 h to 2.2×105N/m2 after 24 h for acid-catalyzed silica gels.
    No preview · Article · Jun 2001 · Journal of Non-Crystalline Solids
  • A. J. Hunt · M. R. Ayers · L. Sibille · D. D. Smith
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    ABSTRACT: The transition from sol to gel is a process that is critical to the properties of engineered nanomaterials, but one with few available techniques for observing the dynamic processes occurring during the evolution of the gel network. Specifically, the observation of various cluster aggregation models, such as diffusion-limited and reaction-limited cluster growth can be quite difficult. This can be rather important as the actual aggregation model can dramatically influence the mechanical properties of gels, and is significantly affected by the presence of convective flows, or their absence in microgravity. We have developed two new non-intrusive optical methods for observing the aggregation processes within gels in real time. These make use of the dynamic behavior of laser speckle patterns produced when an intense laser source is passed through a gelling sol. The first method is a simplified time-correlation measurement, where the speckle pattern is observed using a CCD camera and information on the movement of the scattering objects is readily apparent. This approach is extremely sensitive to minute variations in the flow field as the observed speckle pattern is a diffraction-based image, and is therefore sensitive to motions within the sol on the order of the wavelength of the probing light. Additionally, this method has proven useful in determining a precise time for the gel-point, an event often difficult to measure. Monitoring the evolution of contrast within the speckle field is another method that has proven useful for studying aeration. In this case, speckle contrast is dependent upon the size (correlation length) and number of scattering centers, increasing with increasing size, and decreasing with increasing numbers. The dynamic behavior of cluster growth in gels causes both of these to change simultaneously with time, the exact rate of which is determined by the specific aggregation model involved. Actual growth processes can now be observed, and the effects of varying gravity fields on the growth processes qualitatively described. Results on preliminary ground-based measurements have been obtained.
    No preview · Article · Apr 2001
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    ABSTRACT: We discuss space-flight experiments involving the growth of silica particles and gels. The effect of microgravity on the growth of silica particles via the sol-gel route is profound. In four different recipes spanning a large range of the parameter space that typically produces silica nanoparticles in unit-gravity, low-density gel structures were instead formed in microgravity. The particles that did form were generally smaller and more polydisperse than those grown on the ground. These observations suggest that microgravity reduces the particle growth rate, allowing unincorporated species to form aggregates and ultimately gel. Hence microgravity favors the formation of more rarefied structures, providing a bias towards diffusion-limited cluster-cluster aggregation. These results further suggest that in unit gravity, fluid flows and sedimentation can significantly perturb sol-gel substructures prior to gelation and these deleterious perturbations may be "frozen" into the resulting microstructure. Hence, sol-gel pores may be expected to be smaller, more uniform, and less rough when formed in microgravity.
    No preview · Article · Feb 2000
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    ABSTRACT: It has been hypothesized that gravity has an effect on the formation and resulting microstructure of sol-gels. In order to more clearly resolve the effect of gravity, pores may be non-destructively analyzed in the wet gel, circumventing the shrinkage and coarsening associated with the drying procedure. We discuss the development of an electrophoretic technique, analogous to affinity chromatography, for the determination of pore size distribution and its application to silica gels. Specifically a monodisperse charged dye is monitored by an optical densitometer as it moves through the wet gel under the influence of an electric field. The transmittance data (output) represents the convolution of the dye concentration profile at the beginning of the run (input) with the pore size distribution (transfer function), i.e. linear systems theory applies. Because of the practical difficulty in producing a delta function input dye profile we prefer instead to use a step function. Average pore size is then related to the velocity of this dye front, while the pore size distribution is related to the spreading of the front. Preliminary results of this electrophoretic porosimetry and its application to ground and space-grown samples will be discussed.
    No preview · Article · Feb 2000

  • No preview · Article · Jan 2000
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    ABSTRACT: A chromatographic method for determining relative pore size distribution in sol-gels using electrophoresis is developed. Results from the Stober particles as well as the gels containing the smallest amounts of prehydrolyzed tetraethylorthosilicate (H5) show an effect of gravity on the resulting microstructure of sol-gels.
    No preview · Article · Jan 2000

  • No preview · Conference Paper · Jan 1999
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    ABSTRACT: Polarized light scattering was used to determine the size distribution and composition of the undiluted exhaust particulate stream from a one-cylinder, direct-injection diesel generator set. To do this an angle-scanning polarization-modulated nephelometer was used to completely characterize the scattered light. The size distribution and complex index of refraction of the particles were determined from the data by simultaneously fitting measured the angle-dependence of the four scattering transformation matrix elements with Mie scattering calculations. The no-load exhaust was composed of small non-absorbing particles (mean diameter 0.04 μm) and the particles at full load conditions were larger (0.12 μm) and more highly absorbing.
    No preview · Article · Oct 1998 · SAE Technical Papers
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    ABSTRACT: This paper describes research to investigate fundamental aspects of the effects of microgravity on the formation of the microstructure of metal oxide alcogels and aerogels. We are studying the role of gravity on pore structure and gel uniformity in collaboration with Marshall Space Flight Center (MSFC) on gelling systems under microgravity conditions. While this project was just initiated in May 1998, related research performed earlier is described along with the plans and rationale for the current micro gravity investigation to provide background and describe newly developing techniques that should be useful for the current gellation studies
    Full-text · Conference Paper · Jul 1998
  • M.R. Ayers · A.J. Hunt
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    ABSTRACT: Photoluminescent silica aerogel acts as the active element of an optical sensor for molecular oxygen. The luminescent aerogel is prepared by the action of energized reducing gases on a standard silica aerogel. Intensity of aerogel photoluminescence decreases as the collision frequency between oxygen molecules and the luminescent carriers in the aerogel matrix increases. This behavior is a characteristic of many photoluminescent materials and arises from a transfer of energy from the aerogel to surrounding oxygen molecules. A sensor for oxygen concentration or air pressure can therefore be simply constructed utilizing an ultraviolet source for excitation and a suitable detector for the emitted visible signal. Stern-Volmer quenching constants for the aerogel sensing element are 1.55 × 10-2 Torr-1 for hydrophilic aerogel and 2.4 × 10-3 Torr-1 for hydrophobic aerogel.
    No preview · Article · Apr 1998 · Journal of Non-Crystalline Solids
  • Arlon J Hunt
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    ABSTRACT: Light scattering is a useful tool to evaluate aerogel clarity, study its structure, pore size, mechanical strain, and examine the modes of sol–gel evolution that determine its microstructure. Ultraviolet–visible transmission spectroscopy can be used to study the wavelength dependent scattering to readily compare aerogels of differing origins, thickness, and to evaluate effects of residual contaminants. Infrared reflectance measurements can be used to determine the effective real and imaginary indices of refraction of porous aerogel materials for material property and radiant heat transfer studies. Measurements of scattering at a fixed angle can be used for quality control, to evaluate sources of scattering, and study inhomogeneities. Measurement of the Mueller matrix (describing the 16-element angle-dependent transformation of intensity and polarization of incident to scattered light) provides information about the anisotropy, large pore fraction, induced stresses, microstructure and inhomogeneities in the aerogel. The time evolution of scattering before and after gel formation gives information.
    No preview · Article · Apr 1998 · Journal of Non-Crystalline Solids
  • Michael R Ayers · Arlon J Hunt
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    ABSTRACT: Photoluminescent silica aerogel acts as the active element of an optical sensor for molecular oxygen. The luminescent aerogel is prepared by the action of energized reducing gases on a standard silica aerogel. Intensity of aerogel photoluminescence decreases as the collision frequency between oxygen molecules and the luminescent carriers in the aerogel matrix increases. This behavior is a characteristic of many photoluminescent materials and arises from a transfer of energy from the aerogel to surrounding oxygen molecules. A sensor for oxygen concentration or air pressure can therefore be simply constructed utilizing an ultraviolet source for excitation and a suitable detector for the emitted visible signal. Stern–Volmer quenching constants for the aerogel sensing element are 1.55×10−2 Torr−1 for hydrophilic aerogel and 2.4×10−3 Torr−1 for hydrophobic aerogel.
    No preview · Article · Apr 1998 · Journal of Non-Crystalline Solids
  • Arlon J. Hunt · Michael R. Ayers
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    ABSTRACT: The skeletal structure of aerogel is determined before, during, and after the gel is formed. Supercritical drying of aerogel largely preserves the pore structure that is determined near the time of gelation. To better understand these gel formation mechanisms we carried out measurements of the time evolution of light scattering in a series of gels prepared without conventional acid or base catalysis. Instead, ultraviolet light was used to catalyze the formation of silica gels made from the hydrolysis of tetraethylorthosilicate and partly prehydrolyzed tetraethylorthosilicate in ethanol. Time evolution of light scattering provides information regarding the rate and geometrical nature of the assembly of the primary silica particles formed in the sol. UV-catalyzed gels show volumetric growth typical of acid-catalyzed gels, except when UV exposure is discontinued at the gel point, where gels then show linear chain formation typical of base-catalyzed gels. Long term UV exposure leads to coarsening of the pore network, a decrease in the clarity of the aerogel, and an increase in the surface area of the aerogel. Additionally, UV exposure up to the gel point leads to increased crystallinity in the final aerogel.
    No preview · Article · Apr 1998 · Journal of Non-Crystalline Solids

Publication Stats

1k Citations
80.57 Total Impact Points

Institutions

  • 1986-2012
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 1983-1998
    • University of California, Berkeley
      • • Lawrence Berkeley Laboratory
      • • Department of Physics
      Berkeley, California, United States