Ahuna Mons is a 4 km tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10-500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 10(8)-10(9) years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.

The spatiotemporal evolution of temperature of leaves-on-branchlet carbon based electric double layer capacitors (EDLCs) under imposed constant current was studied using a continuum thermal model. The hot spot aggregated at the tips of graphene petals (GPs), particularly at the high concave surface, at the beginning of the charging step. As the charging proceeded, the overall temperature rose continuously, and the temperature distribution was likely uniform throughout the graphene petals due to an increasingly uniform distribution of ions on GPs surfaces. To elucidate the effects of electrode geometry on the change of temperature, several simple two-dimensional structures were also simulated in the charging step. Concave and planar structures contributed to high temperature change, while a convex structure tended to alleviate the hot spot. An insight into geometric effects on the thermal behavior may lead engineers to develop a new class of nanomaterials for supercapacitors.

This dissertation examines several questions in Martian surface processes relating to water or ice using a combination of geomorphology and modeling. I first examine sublimation of ice from new small mid-latitude craters with freshly exposed ice imaged by the High Resolution Imaging Science Experiment (HiRISE) camera. I discuss the theory of sublimation by free convection and describe a model that improves on the standard version used in the Mars literature. This model shows some differences from experimental data, but this appears to be because experimental conditions do not accurately capture the sublimation regime appropriate to the Martian surface. I use this sublimation model in concert with a thermal model and calculate sublimation rates at the sites of freshly exposed ice. Calculated sublimated thicknesses of one or more millimeters during the period when HiRISE images show ice imply that this ice is relatively pure, not pore-filling. The ice table thus revealed appears consistent with a model of the Martian subsurface in which relatively clean ice overlies pore-filling ice.Pingos are hills with cores of ice formed by freezing of liquid water under pressure. Possible pingos on Mars have been much discussed because they would have significant implications for Martian hydrological processes. I surveyed HiRISE images across a broad portion of the Martian surface searching for fractured mounds. Such features are candidate pingos, since pingos often develop surface fractures as they grow. A small number of Martian landforms, not previously identified, are morphologically consistent with pingos; however, landforms that appear related to these do show morphological differences from pingos. Other origins are possible, particularly since it is difficult to produce the requisite hydrologic conditions for pingo formation. Previously proposed pingos on Mars lack surface fracturing and are unlikely to be pingos.

This thesis work aims at developing a model in Visual Basic for Applications and Microsoft Excel that can be used to predict temperatures in semiconductor devices for two commercial products made by Voith Hydro AB, and via simulation of the model determine the maximum current that can be conducted through the two products. The two products are called field exciters. A field exciter controls the rotor current of a generator with the help of semiconductor devices. When used in a power converter, such devices give rise to losses. A certain amount of the electrical energy passing through the converter is lost in form of heat. If the thermal energy is not dissipated, the temperature in the semiconductor device will rise. This will eventually lead to device failure when the temperature exceeds a certain temperature threshold which depends on the semiconductor material. The proposed model allows to predict these losses and the corresponding temperatures for a specified field current and ambient temperature. The model was validated experimentally. A simplified brushless excitation system was designed and constructed, temperature measurements were carried out for different field currents and later used to validate the model. This thesis concludes that the model developed in Visual Basic predicts temperatures with good results for the PWM-30A but not as good for the PWM-150A. The model simulations show that the PWM-30A can operate with a continuous current of 30 A, for a short duration of 10 seconds it can step up the current to 60 A at an ambient temperature of 50 °C. When the PWM-30A is cooled by forced convection, it can conduct a continuous current of 50 A at an ambient temperature of 50 °C. During field forcing, the PWM-30A can step up the current to 100 A for a duration of 10 seconds. It has been concluded that the PWM-150A cannot, without further testing, conduct a larger current than it was originally designed for, which is 150 A continuously at an ambient temperature of 40 °C. During field forcing it can step up the current to 240 A for 10 seconds.

Uncooled microbolometers have attracted significant interest due to their small size, low cost and low power consumption. As the application range of microbolometers broadens, increasing the dynamic range becomes one of the main objectives of microbolometer research. Targeting this objective, tunable thermal conductance microbolometers have been proposed recently, in which the thermal conductance is tuned by electrostatic actuation. Being a new concept in the field, the current tunable thermal conductance microbolometers have significant potential for improvement in design and performance. In this thesis, an extensive analysis of tunable thermal conductance microbolometers is made, an analytical model is constructed for this purpose, and solutions are proposed to some potential problems such as in-use stiction and variation in spectral response. The current thermal conductance tuning mechanisms use the substrate for electrostatic actuation, which does not support pixel-by-pixel actuation. In this thesis, a new thermal conductance tuning mechanism is demonstrated, that enables pixel-by-pixel actuation by using the micromirror as an actuation terminal instead of the substrate. In addition, a stopper mechanism is used to decrease the risk of in-use stiction. With this new mechanism, the thermal conductance can be tuned by a factor of three at relatively low voltages, making it a promising thermal conductance tuning mechanism for adaptive infrared detectors. Effective estimation of the performance parameters of a tunable thermal conductance microbolometer in the design state requires an analytical model that combines the physics of infrared radiation detection and the thermal conductance tuning mechanisms. As a part of this research, an extensive analytical model is presented, which includes the electrostatic-structural modeling of the thermal conductance tuning mechanism, and electromagnetic and thermal modeling of the microbolometer. The accuracy of the thermal model is of significant importance as the operation of the tuning mechanism within the desired range should be verified in the design stage. A thermal model based on the solution of the microbolometer heat conduction equation is established, which is easily applicable to conventional and tunable thermal conductance microbolometers of various shapes. The constructed microbolometer model is validated by experiments and finite element model simulations. Furthermore, the effect of thermal conductance tuning on spectral response is analyzed. The present thermal conductance tuning mechanisms result in variations in spectral response, which is an undesired effect in many applications. As a solution, a new microbolometer architecture is proposed, in which the spectral response is not affected by thermal conductance. The microbolometer is designed using an analytical model and its performance is characterized by finite element model simulations. To realize the proposed design, a fabrication process flow is offered. It is shown that the proposed microbolometer exhibits high performance, tunable thermal conductance and constant spectral response.

Uncooled microbolometers have attracted significant interest due to their small size, low cost and low power consumption. As the application range of microbolometers broadens, increasing the dynamic range becomes one of the main objectives of microbolometer research. Targeting this objective, tunable thermal conductance microbolometers have been proposed recently, in which the thermal conductance is tuned by electrostatic actuation. Being a new concept in the field, the current tunable thermal conductance microbolometers have significant potential for improvement in design and performance. In this thesis, an extensive analysis of tunable thermal conductance microbolometers is made, an analytical model is constructed for this purpose, and solutions are proposed to some potential problems such as in-use stiction and variation in spectral response. The current thermal conductance tuning mechanisms use the substrate for electrostatic actuation, which does not support pixel-by-pixel actuation. In this thesis, a new thermal conductance tuning mechanism is demonstrated, that enables pixel-by-pixel actuation by using the micromirror as an actuation terminal instead of the substrate. In addition, a stopper mechanism is used to decrease the risk of in-use stiction. With this new mechanism, the thermal conductance can be tuned by a factor of three at relatively low voltages, making it a promising thermal conductance tuning mechanism for adaptive infrared detectors. Effective estimation of the performance parameters of a tunable thermal conductance microbolometer in the design state requires an analytical model that combines the physics of infrared radiation detection and the thermal conductance tuning mechanisms. As a part of this research, an extensive analytical model is presented, which includes the electrostatic-structural modeling of the thermal conductance tuning mechanism, and electromagnetic and thermal modeling of the microbolometer. The accuracy of the thermal model is of significant importance as the operation of the tuning mechanism within the desired range should be verified in the design stage. A thermal model based on the solution of the microbolometer heat conduction equation is established, which is easily applicable to conventional and tunable thermal conductance microbolometers of various shapes. The constructed microbolometer model is validated by experiments and finite element model simulations. Furthermore, the effect of thermal conductance tuning on spectral response is analyzed. The present thermal conductance tuning mechanisms result in variations in spectral response, which is an undesired effect in many applications. As a solution, a new microbolometer architecture is proposed, in which the spectral response is not affected by thermal conductance. The microbolometer is designed using an analytical model and its performance is characterized by finite element model simulations. To realize the proposed design, a fabrication process flow is offered. It is shown that the proposed microbolometer exhibits high performance, tunable thermal conductance and constant spectral response.

This research focuses on the creation of a thermal estimator to be used in an integrated electromagnetic, thermo-mechanical design tool for the rapid optimal initial sizing of switched reluctance and induction machines. The switched reluctance model includes heat generation in the rotor due to core losses, heat transfer across the air gap through convection, and a heat transfer path through the shaft to ambient. Empirical Nusselt correlations for laminar shear flow, laminar flow with vortices and turbulent flow are used to estimate the convective heat transfer coefficient in the air gap. The induction model adds ohmic heat generation within the rotor bars of the machine as an additional rotor heat source. A parametric, self-segmenting mesh generation tool was created to capture the complex rotor geometries found within switched reluctance or induction machines. Modeling the rotor slot geometries in the R-θ polar coordinate system proved to be a key challenge in the work. Segmentation algorithms were established to model standard slot geometries including radial, rectangular (parallel-sided), circular and kite-shaped features in the polar coordinate system used in the R-θ solution plane. The center-node mesh generation tool was able optimize the size and number of nodes to accurately capture the cross sectional area of the feature, in the solution plane. The algorithms pursue a tradeoff between computational accuracy and computational speed by adopting a hybrid approach to estimate three dimensional effects. A thermal circuits approach links the R-θ finite difference solution to the three dimensional boundary conditions. The thermal estimator was able to accurately capture the temperature distribution in switched reluctance and induction machines as verified with experimental results.

abstract: Thermal modeling and investigation into heat extraction methods for building-applied photovoltaic (BAPV) systems have become important for the industry in order to predict energy production and lower the cost per kilowatt-hour (kWh) of generating electricity from these types of systems. High operating temperatures have a direct impact on the performance of BAPV systems and can reduce power output by as much as 10 to 20%. The traditional method of minimizing the operating temperature of BAPV modules has been to include a suitable air gap for ventilation between the rooftop and the modules. There has been research done at Arizona State University (ASU) which investigates the optimum air gap spacing on sufficiently spaced (2-6 inch vertical; 2-inch lateral) modules of four columns. However, the thermal modeling of a large continuous array (with multiple modules of the same type and size and at the same air gap) had yet to be done at ASU prior to this project. In addition to the air gap effect analysis, the industry is exploring different ways of extracting the heat from PV modules including hybrid photovoltaic-thermal systems (PV/T). The goal of this project was to develop a thermal model for a small residential BAPV array consisting of 12 identical polycrystalline silicon modules at an air gap of 2.5 inches from the rooftop. The thermal model coefficients are empirically derived from a simulated field test setup at ASU and are presented in this thesis. Additionally, this project investigates the effects of cooling the array with a 40-Watt exhaust fan. The fan had negligible effect on power output or efficiency for this 2.5-inch air gap array, but provided slightly lower temperatures and better temperature uniformity across the array. / Dissertation/Thesis / M.S. Technology 2010

abstract: The operating temperature of photovoltaic (PV) modules has a strong impact on the expected performance of said modules in photovoltaic arrays. As the install capacity of PV arrays grows throughout the world, improved accuracy in modeling of the expected module temperature, particularly at finer time scales, requires improvements in the existing photovoltaic temperature models. This thesis work details the investigation, motivation, development, validation, and implementation of a transient photovoltaic module temperature model based on a weighted moving-average of steady-state temperature predictions. This thesis work first details the literature review of steady-state and transient models that are commonly used by PV investigators in performance modeling. Attempts to develop models capable of accounting for the inherent transient thermal behavior of PV modules are shown to improve on the accuracy of the steady-state models while also significantly increasing the computational complexity and the number of input parameters needed to perform the model calculations. The transient thermal model development presented in this thesis begins with an investigation of module thermal behavior performed through finite-element analysis (FEA) in a computer-aided design (CAD) software package. This FEA was used to discover trends in transient thermal behavior for a representative PV module in a timely manner. The FEA simulations were based on heat transfer principles and were validated against steady-state temperature model predictions. The dynamic thermal behavior of PV modules was determined to be exponential, with the shape of the exponential being dependent on the wind speed and mass per unit area of the module. The results and subsequent discussion provided in this thesis link the thermal behavior observed in the FEA simulations to existing steady-state temperature models in order to create an exponential weighting function. This function can perform a weighted average of steady-state temperature predictions within 20 minutes of the time in question to generate a module temperature prediction that accounts for the inherent thermal mass of the module while requiring only simple input parameters. Validation of the modeling method presented here shows performance modeling accuracy improvement of 0.58%, or 1.45°C, over performance models relying on steady-state models at narrow data intervals. / Dissertation/Thesis / Masters Thesis Engineering 2020

The thermal performance of the power MOSFET module is subject to change over its lifetime. This is caused by the growth of voids and other defects in the die-attach layer. The goal of this dissertation is to develop measurement techniques and finite element simulations that can measure the changes in thermal performance caused by changes in die-attach voided area. These experimental results and simulations can then be used to create predictions of the thermal performance of a particular power semiconductor module at various stages of die-attach fatigue. In the results and simulations presented, a relationship is developed between thermal impedance and void area coverage. This dissertation starts by presenting an analysis of the thermal and mechanical stresses needed for crack and void growth in the power semiconductor die-attach region. Accelerated life testing is then performed for both commercial and prototype power semiconductor devices to generate the stresses needed to precipitate void growth. Representative groups of lead and lead-free solders are then tested to compare levels of die-attach degradation under accelerated life conditions. Hardware is developed to experimentally measure thermal impedance using temperaturesensitive characteristics of the power MOSFET. The power semiconductor devices that were subjected to accelerated life testing are then measured with this hardware. The results show that die-attach voided area coverage increases thermal impedance. Representative lumped parameter thermal models that use R-C circuits are derived to demonstrate the ability of the thermal impedance analyzer to determine the differences in the die-attach layer. Finite element modeling (FEM) is then used on representative voided devices to support these results, with additional emphasis on peak temperatures caused by hotspots located over the voided areas. Experimental techniques are further applied to measurement of cooling trends that occur due to the existence of voids in the die-attach layer. These measurements are correlated with finite element thermal simulations to develop a relationship between thermal impedance, hotspot temperature, die-attach void size, and total voided area coverage. / Ph. D.