mW M (mw + m)

Distribution by Scientific Domains

Selected Abstracts

Heat-Transport Mechanisms in Superlattices

Yee Kan Koh
Abstract The heat transport mechanisms in superlattices are identified from the cross-plane thermal conductivity , of (AlN)x,(GaN)y superlattices measured by time-domain thermoreflectance. For (AlN)4.1 nm,(GaN)55,nm superlattices grown under different conditions, , varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest ,, , of (AlN)4 nm,(GaN)y superlattices decreases monotonically as y decreases, ,,=,6.35,W m,1 K,1 at y,=,2.2,nm, 35 times smaller than , of bulk GaN. For long-period superlattices (y,>,40,nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G,,,620 MW m,2 K,1. For y,<,40,nm, the apparent value of G increases with decreasing y, reaching G,,,2 GW m,2 K,1 at y,<,3,nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)4.1 nm,(GaN)4.9,nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2,,1014 ions cm,2 is reduced by <35%, suggesting that heat transport in these short-period superlattices is dominated by long-wavelength acoustic phonons. Calculations using a Debye-Callaway model and the assumption of a boundary scattering rate that varies with phonon-wavelength successfully capture the temperature, period, and ion-dose dependence of ,. [source]

Spring temperatures in the Sagehen Basin, Sierra Nevada, CA: implications for heat flow and groundwater circulation

Abstract Heat flow in the Sierra Nevada, CA, is low despite its young geologic age. We investigate the possibility that advective heat transport by groundwater flow leads to an underestimate of heat flow in the Sierras based purely on borehole measurements. Using temperature and discharge measurements at springs in Sagehen Basin, we find that groundwater removes the equivalent of approximately 20,40 mW m,2 of geothermal heat from the basin. This is comparable with other heat flow measurements in the region and indicates that, in this basin, at least, groundwater does transport a significant amount of geothermal heat within the basin. Additionally, we use estimates of the mean residence time of water discharged at the springs along with hourly temperature records in springs to provide constraints on groundwater flow depths within the basin. An analytical model based on these constraints indicates that the heat removed by groundwater may represent 20% to >90% of the total heat flow in the basin. Without better constraints on the regional hydrogeology and the depth of circulation, we cannot determine whether the heat discharged at the springs represents a change in the mode of heat transfer, i.e. from conduction to advection at shallow depths (<100 m) or whether this is a component of heat transfer that should be added to measured conductive values. If the latter is true, and Sagehen Basin is representative of the Sierras, basal heat flow in the Sierra Nevada may be higher than previously thought. [source]

Is advective heat transport significant at the Dead Sea basin?

Abstract An understanding of heat flux is a necessary component in reconstructing tectonic, seismic, and hydrologic models of the Dead Sea basin. Heat may be transferred by both conduction and advection by groundwater. Although the conductive heat flux in Israel has been extensively measured to be approximately 40 mW m,2, there is still a debate about the total heat flux. Recently, the discharge of hot springs along the western Dead Sea shore has been determined to be 107 m3 year,1. Simple calculations show that the heat discharged by groundwater at these hot springs is of the same order of magnitude as the measured conductive heat flux in deep boreholes. Therefore the total heat flux could be significantly higher than 40 mW m,2. However, results of numerical modeling show that the current hot-spring heat discharge is two orders of magnitude greater than that predicted for steady-state conditions and can be explained by the rapid recession of the Dead Sea. [source]

The thermal field in a basin after a sudden passive pure shear lithospheric extension and sublithospheric mechanical erosion:the case of the Tuscan Basin (Italy)

F. Mongelli
A simple new model for sudden lithospheric thinning that considers the crust to be stretched and the lower layer of the lithosphere to be partially stretched and partially mechanically eroded is proposed. This model allows calculation of the thermal field of the lithosphere during the initial warming phase and the surface uplift. Application of this model to the Tuscan Basin explains the high regional heat flux density values (>100 mW m,2,), the tectonic subsidence (about 1 km) and the average uplift (>400 m) observed in this region well. [source]

Entropy sources in a dynamical core atmosphere model

Tim Woollings
Abstract Numerical atmosphere models are not generally constructed to ensure accurate treatment of entropy, but little is known about the significance of the resulting errors. This paper examines the entropy changes during a baroclinic wave simulation in a typical dynamical core model, specifically a ,-coordinate spectral model, which includes scale-selective dissipation terms in the form of a numerical hyperdiffusion. Lagrangian entropy conservation is found to be badly represented, with numerical transport errors resulting in cross-isentrope mass fluxes which are of the same size as those associated with some real diabatic processes. In a global average, the total entropy increases at a rate of just 0.5 mW m,2K,1. This, however, is seen to be the residual of two opposing numerical effects which are several times larger, namely the destruction of entropy by dispersion and Gibbs errors, and its creation by diffusion. The entropy generated by diffusion is shown to be remarkably insensitive to the details of the diffusion scheme. This leads us to hypothesize that the entropy source from diffusion is determined by the rate at which small scales are generated by the deformation field of the large-scale flow so that, while the diffusion mechanism is clearly unrealistic, the magnitude of the entropy source is, we argue, representative of that generated by physical dissipative processes in the real atmosphere. Even in this simple model it is not possible to quantify precisely the different entropy sources and sinks which combine to give the overall entropy change. However, we can say that if there is a systematic spurious entropy source in this model, then it is small, i.e. of size 0.5 mW m,2K,1 or smaller. Copyright 2006 Royal Meteorological Society [source]

Insulating effect of coals and organic rich shales: implications for topography-driven fluid flow, heat transport, and genesis of ore deposits in the Arkoma Basin and Ozark Plateau

BASIN RESEARCH, Issue 2 2002
J.A. Nunn
ABSTRACT Sedimentary rocks rich in organic matter, such as coal and carbonaceous shales, are characterized by remarkably low thermal conductivities in the range of 0.2,1.0 W m,1 C,1, lower by a factor of 2 or more than other common rock types. As a result of this natural insulating effect, temperature gradients in organic rich, fine-grained sediments may become elevated even with a typical continental basal heat flow of 60 mW m,2. Underlying rocks will attain higher temperatures and higher thermal maturities than would otherwise occur. A two-dimensional finite element model of fluid flow and heat transport has been used to study the insulating effect of low thermal conductivity carbonaceous sediments in an uplifted foreland basin. Topography-driven recharge is assumed to be the major driving force for regional groundwater flow. Our model section cuts through the Arkoma Basin to Ozark Plateau and terminates near the Missouri River, west of St. Louis. Fluid inclusions, organic maturation, and fission track evidence show that large areas of upper Cambrian rocks in southern Missouri have experienced high temperatures (100,140 C) at shallow depths (< 1.5 km). Low thermal conductivity sediments, such as coal and organic rich mudstone were deposited over the Arkoma Basin and Ozark Plateau, as well as most of the mid-continent of North America, during the Late Palaeozoic. Much of these Late Palaeozoic sediments were subsequently removed by erosion. Our model results are consistent with high temperatures (100,130 C) in the groundwater discharge region at shallow depths (< 1.5 km) even with a typical continental basal heat flow of 60 mW m,2. Higher heat energy retention in basin sediments and underlying basement rocks prior to basin-scale fluid flow and higher rates of advective heat transport along basal aquifers owing to lower fluid viscosity (more efficient heat transport) contribute to higher temperatures in the discharge region. Thermal insulation by organic rich sediments which traps heat transported by upward fluid advection is the dominant mechanism for elevated temperatures in the discharge region. This suggests localized formation of ore deposits within a basin-scale fluid flow system may be caused by the juxtaposition of upward fluid discharge with overlying areas of insulating organic rich sediments. The additional temperature increment contributed to underlying rocks by this insulating effect may help to explain anomalous thermal maturity of the Arkoma Basin and Ozark Plateau, reducing the need to call upon excessive burial or high basal heat flow (80,100 mW m,2) in the past. After subsequent uplift and erosion remove the insulating carbonaceous layer, the model slowly returns to a normal geothermal gradient of about 30 C km,1. [source]