Browsing by Subject "Melting"

Now showing 1 - 4 of 4
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    The effect of water on partial melting in the upper mantle.
    (2010-06) Tenner, Travis Jay
    This thesis presents experimental constraints on incipient melting of mantle peridotite under hydrated conditions. High P-T experiments were performed at pressures of 3 to 13 GPa, and at temperatures of 1200-1450°C. These experiments measure mineral/melt H 2 O partitioning and storage capacity of peridotite components, as well as determine melting phase relations and the compositions of partial melts and residues of hydrated peridotite. Incipient melt H 2 O concentrations are estimated by peridotite/melt H 2 O partitioning ([Special characters omitted.] ). To parameterize [Special characters omitted.] , mineral/melt H 2 O partition coefficients were determined for all crystalline phases of the peridotite solidus assemblage (Chapter 2). Combining these [Special characters omitted.] values with corresponding modal abundances along the solidus yields a [Special characters omitted.] of 0.005-0.010 from 1 to 5 GPa, which is dependent on pressure due to varying garnet and pyroxene modal abundances, and to variable pyroxene Al content. This [Special characters omitted.] range predicts that incipient melts of MORB source (50-200 ppm H 2 O bulk ) and OIB source (300-1000 ppm H 2 O bulk ) upper mantle contain 0.5-3.8 wt.% and 3-20 wt.% dissolved H 2 O, respectively. The amount of dissolved H 2 O in incipient melt dictates hydrous solidus depression, Δ T , which ultimately controls the stability of hydrous melts at P and T . This Δ T -H 2 O melt relationship was investigated at 3.5 GPa by partially melting hydrated peridotite from 1200-1450°C (Chapter 3). Mass balance of phases allows for determination of melt fractions ( F ) from experiments, as well as estimation of H 2 O melt . Δ T values are quantified as the difference in melting temperature between dry and wet peridotite at a particular F . Parameterization of Δ T as a function of H 2 O melt predicts that solidus melts with 1.5, 5, 10, and 15 wt.% dissolved H 2 O generate Δ T values of 50, 150, 250, and 300°C, respectively. Combination of this paramterization with [Special characters omitted.] (Chapter 2) insinuates that 500 ppm H 2 O bulk is necessary to stabilize melt across the observed seismic low velocity zone (LVZ) beneath oceanic lithosphere, which is significantly greater than the MORB source upper mantle H 2 O bulk of 50-200 ppm. This observation argues against suggestions that hydrous melting is solely responsible for the LVZ. At higher pressures the aforementioned parameterizations are difficult to constrain experimentally, but the onset of hydrous melting can be determined by the peridotite H 2 O storage capacity, defined as the maximum H 2 O concentration that peridotite can store without stabilizing a hydrous fluid or melt. A new method of determining a minerals H 2 O storage capacity is employed, in which a hydrated monomineralic layer is equilibrated with a layer of hydrated peridotite and a small amount of melt (Chapter 4). Experiments were carried out at conditions near the 410 km transition zone (TZ) depth to investigate hydrous melting due to the H 2 O storage capacity contrast between the TZ and upper mantle. Measured olivine and orthopyroxene H 2 O storage capacities, combined with estimates of garnet H 2 O storage capacity, and P -dependent lherzolite modes, yields a peridotite H 2 O storage capacity of 700-1100 ppm directly above 410 km. This is not consistent with pervasive melting above 410 km, as this range is several times greater than MORB source upper mantle H 2 O bulk . However, regional melting in areas such as H 2 O-rich OIB source, or areas of recent subduction may likely occur, leaving residues with ∼1000 ppm H 2 O bulk .
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    Heat and mass transfer during the melting process of a porous frost layer on a vertical surface
    (2013-05) Mohs, William Francis
    An important problem in the refrigeration industry is the formation and removal of frost layers on sub-freezing air coolers. The frost layer, a porous structure of ice and air, directly diminishes the performance and efficiency of the entire cooling system by presenting resistances to air flow and heat transfer in the air cooler. To return the system to pre-frosted performance the layer must be removed through a defrost cycle. The most common defrost cycle uses heat applied at the heat exchanger surfaces to melt the frost. Current methods of defrosting are inherently inefficient, with the majority of the heat being lost to the surrounding environment. Most studies have concentrated on the formation of the frost layer, and not the melting phenomena during the defrost cycle. In this study, direct measurements and a fundamental model to describe the melting process of a frost layer on a vertical heated surface are presented. The experimental facility provides the first direct measurements of heat and mass transfer during defrost. The measurements confirmed the multistage nature of defrost. The multistage model characterized the different thermal and mass transport processes that dominate each stage. The first stage is dominated by sensible heating of the frost layer. Both the experiment and model showed that heat and mass transfer through sublimation during the initial stages are insignificant, accounting for less than 1% of the total energy transfer. The second stage of defrost is dominated by the melting of the frost layer. The melt rate model generally predicts the front velocity within 25% of the velocity determined using the digital image analysis technique. Higher heat transfer rates resulted in faster melt velocity, and thus shortened defrost times. Evaporation of the melt liquid from the surface dominates the final stage. The heat transfer model for this stage predicts the heat transfer coefficient within ±25% of the experiment. The overall defrost efficiency was found to be primarily dependent on the initial frost thickness, with thicker layer having less heat lost to the ambient space and a higher efficiency.
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    Iron-Nickel-Sulfur-Carbon System Under High Pressure, With Implications To Earth’S Mantle
    (2016-10) Zhang, Zhou
    Fe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications. According to their chemical behavior, Fe-Ni-S-C phases preferentially store siderophile and chalcophile elements (and potentially noble gases). Physically, Fe-Ni-S-C phases have distinctly higher densities, surface tensions, and electrical conductivities, and lower melting points than mantle silicates. Understanding the geochemical and geophysical impacts caused by Fe-Ni-S-C phases requires accurate quantification of the basic properties of Fe-Ni-S-C phases under mantle conditions. This PhD thesis uses both high-pressure experiments and thermodynamic calculations to constrain the melting temperatures and compositions of Fe-Ni-S-C phases in the Earth’s upper mantle mantle, and their potential for deep carbon storage. This study suggests that monosulfides in the upper mantle are mostly molten, even in significant portions of cratonic roots under continental geotherms. Incorporation of carbon depresses the monosulfide solidus by 50-100˚C. Experiments and calculations of reactions between Fe-Ni-S melts and silicates at mantle conditions suggest that Fe-Ni-S melts are Ni-rich (Ni/(Ni+Fe)~0.6) monosulfides ((Fe+Ni)/S~1 or Xs~0.5) under oxidized (FMQ -2 to FMQ) conditions at 2 GPa. With increasing depth in the mantle (thus decreasing fO2), Fe-Ni-S melts become increasingly Ni- and S-poor, characterized by Ni/(Ni+Fe)~0.4, (Fe+Ni)/S~3, and Xs~0。4 at 8 GPa, and Ni/(Ni+Fe)~0.2, Xs~0.05 and (Fe+Ni)/S~10 at 12 GPa. Carbon solubility in Fe-Ni-S melts determined by high-pressure experiments suggests that carbon solubility decreases exponentially with increasing Xs. Based on mantle Fe-Ni-S melt compositions, C-S relations in carbon-saturated melts, and the typical mantle P-T-fO2 profile and sulfur abundance (200 ppm), it is suggested that significant amounts (40-100%) of deep carbon could potentially be stored in Fe-Ni-S melts in the Earth’s reduced deep upper mantle.
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    Numerical Modeling of the Metal Cutting Process in the Plasma Arc Cutting
    (2015-11) Park, Hunkwan
    The process of cutting metal with a plasma arc cutting tool is investigated and discussed. Focus is on the metal cutting process at the inside surface of the kerf. This is an important region that is not well documented due to the difficulty of experiments and the complexity of computation needed to characterize this process. In the present work, a three-dimensional numerical simulation using a plasma model combined with a melting process model is conducted and results are discussed, leading to a better understanding of the physical phenomena within the kerf region of a commercial plasma arc cutting tool. The modeling includes three different phenomena: 1) the plasma jet flow, 2) the Volume of Fluid (VoF) method in identify the gas to molten metal interface, and 3) the phase change model for computing the melting process. The model is implemented in the open source CFD software, OpenFOAM. Thermodynamic and transport properties, calculated by kinetic theory of gases and statistical mechanics, are implemented for accurate simulation in the high temperature regions. The simulation results show the transient cutting process including the physical phenomena for melting of the work piece as well as the plasma flow. The simulated kerf shape is compared to measured kerf under same operations. Additionally, the temperature, velocity, and current density distributions are discussed to understand the plasma characteristics during the cutting process. In an attempt to make a more reasonable kerf shape, the swirl component of the jet, the surface tension and the phase change model are investigated for improvement and discussed. Effects of metal vapor and oxidation reaction are also discussed. This work is a first attempt simulation of the plasma flow, melting, and molten metal flow in the plasma arc cutting process. As the model approaches physical reality, it gives increasingly useful insight into the relationships among operating conditions, providing very helpful directions to improve performance, and providing useful data for designing the plasma arc cutting process.