Douglas J. Goering

Non-conductive heat transfer associated with frozen soils

Abstract The assertion that pure conductive heat transfer always dominates in cold climates is at odds with decades of research in soil physics which clearly demonstrate that non-conductive heat transfer by water and water vapor are significant, and frequently are for specific periods the dominant modes of heat transfer near the ground surface. The thermal regime at the surface represents the effective boundary condition for deeper thermal regimes. Also, surface soils are going to respond more quickly to any climatic fluctuations; this is important to us because most facets of our lives are tied to earths surface. To accurately determine the surface thermal regime (for example, the detection of climate change), it is important to consider all potential forms of heat transfer. Gradients that have the potential to alter the thermal regime besides temperature include pore water pressure, gravitational, density, vapor pressure and chemical. The importance of several non-conductive heat transport mechanisms near the ground surface is examined. Infiltration into seasonally frozen soils and freezing (release of latent heat) of water is one mechanism for the acceleration of warming in surficial soils in the spring. Free convection due to buoyancy-induced motion of fluids does not appear to be an important heat-transfer mechanism; estimates of the Rayleigh number (the ratio of buoyancy to viscous forces) are generally around 2, which is too low for effective heat transfer. The Peclet number (ratio of convective to conductive heat transfer) is on the order of 0.25 for snowmelt infiltration and up to 2.5 for rainfall infiltration for porous organic soils. In mineral soils, both vertical and horizontal advection of heat can be neglected (Peclet number is approximately 0.001) except for snowmelt infiltration into open thermal contraction cracks. The migration of water in response to temperature or chemical gradients from unfrozen soil depths to the freezing front, and the redistribution of moisture within the frozen soil from warmer depths to colder depths, can also result in heat transfer although this effect has not been quantified here. Many of these processes are seasonal and effective only during periods of phase change when the driving gradient near the ground surface is relatively large.

A distributed thermal model for calculating soil temperature profiles and depth of thaw in permafrost regions

A spatially distributed thermal model has been developed that simulates thermal processes at the surface of the tundra, within the active layer, and in the underlying permafrost. This model was developed and applied to simulate processes on the Kuparuk River watershed on the North Slope of Alaska. Gridded meteorological data came from seven stations. Meteorologic data to calculate the surface energy balance at each of the nodes were distributed across the watershed using kriging. The kriged air temperature was also adjusted to account for elevation differences using the dry or wet adiabatic lapse rate as appropriate, and incident shortwave radiation was adjusted to consider slope effects. The equations describing the thermal processes of the surface energy balance were solved simultaneously for the surface temperature. This calculated surface temperature was then used in the subsurface finite element formulation to calculate the temperature profile and depth of thaw in the soil. Thermal properties of the soil were estimated spatially on the basis of measurements collected in typical landform vegetation units and then distributed on the basis of a vegetation map. Performance of the model was judged on the basis of comparison to measurements of soil temperatures and thaw depths. The model performs quite well in areas where subsurface thermal properties are well known. The model explains greater than 80% of variance at the surface when comparing predicted subsurface temperatures versus measured soil temperatures, and it increases in performance at greater depths. The model explains 82% of variance when comparing predicted thaw depths versus thaw depths measured over 1 km 2 grids.

Winter-time convection in open-graded embankments

Abstract Winter-time natural convection in open-graded gravel embankments is studied using a numerical representation of the unsteady two-dimensional momentum and energy equations. The analysis examines the ability of these embankments to maintain the structural integrity of thaw-unstable permafrost that often underlies roadway or airport embankments in northern climates. As a result of low ambient temperatures acting on the embankment surface during winter months, an unstable density stratification develops in the embankment. Buoyancy-driven convection of the pore air occurs in reaction to the density gradient. The convection enhances the upward transport of heat out of the embankment during winter months, thus cooling the lower portions of the embankment and underlying foundation soil. During summer months the density stratification is stable and convection does not occur. Consequently, summer-time heat transfer is dominated by thermal conduction which transports heat less effectively. The results of the present study show that the winter-time convection can lower foundation soil temperatures beneath open-graded embankments by as much as 5°C on an annual average basis compared to stan0ard sand and gravel embankments. Time varying temperatures and pore air velocites are calculated for different embankmenkt permeabilities and the results are visualized in the form of isotherm and velocity vector plots for different times of the year.

Sliding mode based powertrain control for efficiency improvement in series hybrid-electric vehicles

This study involves the improvement of overall efficiency in series hybrid-electric vehicles (SHEVs) by restricting the operation of the engine to the optimal efficiency region, using a control strategy based on two chattering-free sliding mode controllers (SMCs). One of the designed SMCs performs engine speed control, while the other controls the engine/generator torque, together achieving the engine operation in the optimal efficiency region of the torque-speed curve. The control strategy is designed for application on a SHEV converted from a standard high mobility multipurpose wheeled vehicle (HMMWV) and simulated by using the Matlab-based PNGV Systems Analysis Toolkit (PSAT). The performance of the control strategy is compared with that of the original PSAT model, which utilizes PI controllers, a feedforward term for the engine torque, and comprehensive maps for the engine, generator and power converter (static only), which constitute the auxiliary power unit (APU). In this study, in spite of the simple modeling approach taken to model the APU and the optimal efficiency region, an improved performance has been achieved with the new SMC based strategy in terms of overall efficiency, engine efficiency, fuel economy, and emissions. The control strategy developed in this work is the first known application of SMC to SHEVs, and offers a simple, effective and modular approach to problems related to SHEVs

Remote power systems with advanced storage technologies for Alaskan villages

This paper presents an analytical optimization of a remote power system for a hypothetical Alaskan village. The analysis considers the potential of generating renewable energy (e.g., wind and solar), along with the possibility of using energy storage to take full advantage of the intermittent renewable sources available to these villages. Storage in the form of either compressed hydrogen or zinc pellets can then provide electricity from hydrogen or zinc–air fuel cells whenever wind or sunlight are low. The renewable system is added on to the existing generation system, which is based on diesel engines. Results indicate that significant reductions in fossil fuel consumption in these remote communities are cost effective using renewable energy combined with advanced energy storage devices. A hybrid energy system for the hypothetical village can reduce consumption of diesel fuel by about 50% with annual cost savings of about 30% by adding wind turbines to the existing diesel generators. Adding energy storage devices can further reduce fuel use, and depending on the economic conditions potentially reduce life-cycle costs. With optimized energy storage, use of the diesel gensets can be reduced to almost zero, with the existing equipment only maintained for added reliability. However, about one quarter of the original fuel is still used for heating purposes.

Removal of terrain effects from SAR satellite imagery of Arctic tundra

Synthetic aperture radar (SAR) images of the Earths terrestrial surface contain geometric and radiometric image effects which are caused by varying terrain elevation and slope. The radiometric effects tend to mask signal variations caused by other physical variables such as soil moisture and surface vegetation type, which are known to influence SAR backscatter signals. As a result, raw SAR images are of limited use in classifying surface vegetation type or quantifying the spatial distribution of soil moisture in regions of terrain relief, The authors present a technique for removing radiometric terrain effects from SAR images. Image correction was carried out in two steps. First, an existing modeling package was used in combination with digital elevation data in order to map the raw image pixels onto a geodetic coordinate system, thereby removing the geometric portion of the image distortion. Radiometric effects were then removed with the aid of a backscatter model which treats the reflected radiation as a combination of diffuse-Lambertian and specular components. Parameters in the backscatter model were determined by comparing two C-band SAR images of a test area in a region of Arctic tundra which were taken from ascending and descending orbit tracks of the ERS-1 satellite. The ascending and descending images displayed reductions in pixel value variance of 30% and 13%, respectively, after processing. Direct comparison of the two test area images reveals a dramatic improvement in image similarity after processing. >

A Novel Model Validation and Estimation Approach for Hybrid Serial Electric Vehicles

This paper introduces a novel modeling and validation approach for hybrid electric vehicles (HEVs). The proposed dynamic modeling approach offers a more realistic simulation performance over most map-based studies of previous literature, while the novel validation approach requires no a priori information on the control algorithms running in the system but uses only measurement data collected from the actual system. A significant benefit of the proposed validation method is that it could further be used for the estimation of variables, which are unavailable for measurement, for variables such as engine torque, battery state of charge, generator torque, motor torque, fuel consumption, etc., as demonstrated in this paper. For the validation and estimation process, the simulation model must be driven with control signals obtained from the actual system, which, most of the time, are not available. To overcome this problem, in this paper, sliding-mode-control-based robust controllers are designed to emulate the engine, motor, and generator control signals to achieve minimum deviation between the variables that are calculated through the simulation model and measured from the actual system, in spite of the nonlinearities and uncertainties that are not considered in the developed model. This paper is based on the model and measurement data obtained from a series HEV, namely, the U.S. militarys high mobility multipurpose wheeled vehicle XM1124. The evaluation of the simulated and actual measurement data indicates the good performance of the developed modeling and validation technique, which is also motivating the use of the approach for the estimation of variables unavailable for measurement in a variety of systems.

The dual influence of curvature and buoyancy in fully developed tube flows

Abstract Fully developed laminar flow through a heated horizontal curved tube is studied using a two-dimensional numerical representation of the fully-elliptic Navier-Stokes and energy equations. Buoyancy and curvature effects are included and two types of thermal boundary conditions are examined. Heat transfer and pressure drop data are provided in addition to flow velocity and temperature contours. Regime maps are presented which delineate the range of combined curvature/buoyancy influence for both types of boundary conditions. The results for uniform peripheral heat flux reveal a large region of parameter space where fully developed laminar solutions could not be found.

Modeling and verification of Hybrid Electric HMMWV performance

The U.S. Military, through the Defense Advanced Research Projects Agency (DARPA) and the U.S. Army Tank-automotive and Armaments Command (TACOM) have sponsored the development of three different versions of the Hybrid Electric High Mobility Multipurpose Wheeled Vehicle (HE-HMMWV). The most recent version of this vehicle was designed by PEI Electronics, Inc. (PEI) and a limited number of prototypes have been constructed. This paper describes the modeling of this vehicle using a set of customized model components that have been formulated using the PNGV Systems Analysis Toolkit (PSAT). Model output has been obtained for the standard Federal Urban Drive Cycle (FUDS). In an effort to provide model verification, the model output data is compared to field test results obtained from a prototype vehicle driven to mimic the FUDS profile. Comparison between model output and field data shows generally good agreement demonstrating the value of model performance prediction. Additional improvements are expected to result from continuing analysis of field data and more accurate incorporation of vehicle control strategies and component behavior.

Heat Transfer in a Three Dimensional Stacked Chip Scale Package (CSP) Module

Stacking of chip scale packages (CSP) is a cost effective, manufacturable, and highly reliable approach to realizing a dense, high performance electronic system (Dewan-Sandur et al., 2006). However, system specific heat transfer and thermo-mechanical considerations severely limit the design and performance domain (Dewan-Sandur et al., 2006). Moreover, very limited generalized guidance on thermal design, except for the need to fully simulate the system, exists. This paper discusses thermal design of a forced-air cooled, eight level mu-BGA stacked CSP module and provides general guidelines as well as a non-dimensional formalism for analyzing cooling of such a 3D package. We have used a commercial CFD code (FLUENT) to analyze the steady-state thermal performance of an 8-level CSP module. The geometry is approximated by a 2D representation as the first step and is assumed to be made up of three solid layers, the polyimide tape, the elastomer adhesive and the silicon IC chip. The conjugate problem involving conduction in the solid layers and the forced convection due to air is solved by FLUENT. Temperature contours are produced for (a) power dissipation levels of 0.5, 1 and 1.5 W in the silicon chip, (b) silicon IC chip thicknesses of 50, 75 and 150 mum, (c) air gaps of 240, 320 and 400 mum and (d) air velocities of 3, 5 and 10 m/s. The effect of each of these parameters on the steady state temperature of the CSP is investigated. The results show that (1) a thicker silicon IC chip spreads heat laterally more effectively leading to more uniform temperatures along the chip, (2) the maximum temperature of the package in the center of the stack is directly proportional to the power level for a given silicon IC chip thickness and air velocity, (3) two Biot numbers (one based on the thickness of the silicon chip and the other based on the length) are identified to be crucial to the conduction along the chip and from the chip, and (4) the Nusselt number shows a dependence on the Graetz number akin to a hydrodynamically and thermally developing profile which motivates future work in this direction. Winged structures that increase the surface area for enhanced heat transfer surface are currently under active investigation.