Limited Heat Flow Research Articles

Mathematical determination of the maximum heat release for fly ash cement paste: Effect of heat flow monitoring time, calculation approach and fly ash content

The maximum hydration heat release Qmax is one of the most concerned kinetic parameters for hydration of cementitious materials. In this study, we specially investigated the effect of the mathematical calculation approach, heat flow monitoring time, and fly ash (FA) content on the obtained Qmax values of FA cement paste. The isothermal hydration heat flows of cement paste with 0%, 20%, 40% and 60% FA contents have been monitored for 250 hours at 20°C. Two conventional Qmax calculation approaches, including Knudsen's equation and the three-parameter exponential function, have been utilized to obtain Qmax values for different cementitious systems. The results indicate that irrespective of the calculation approach or monitoring time, mathematically derived Qmax linearly decreases along with the increase of FA content, which is consistent with the determined enthalpy of the binary cement system. Furthermore, the calculated Qmax increases with the extension of heat flow monitoring time, and different calculation approaches can yield varied Qmax values. However, the discrepancies between the two approaches gradually diminish as the monitoring time extends. It is recommended that for cement paste without FA addition, a minimum heat flow monitoring time of 7 days is necessary to obtain a reliable Qmax value using Knudsen's equation, while for the exponential function, a monitoring time exceeding 7 days is advised. For cement paste with FA addition, neither Knudsen's equation nor three-parameter exponential function can derive a relatively accurate Qmax value for FA within the limited heat flow monitoring duration in this study.

Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC)

Abstract. Climate change in the Arctic is leading to shifts in vegetation communities, permafrost degradation and alteration of tundra surface–atmosphere energy and carbon (C) fluxes, among other changes. However, year-round C and energy flux measurements at high-latitude sites remain rare. This poses a challenge for evaluating the impacts of climate change on Arctic tundra ecosystems and for developing and evaluating process-based models, which may be used to predict regional and global energy and C feedbacks to the climate system. Our study used 14 years of seasonal eddy covariance (EC) measurements of carbon dioxide (CO2), water and energy fluxes, and winter soil chamber CO2 flux measurements at a dwarf-shrub tundra site underlain by continuous permafrost in Canada’s Southern Arctic ecozone to evaluate the incorporation of shrub plant functional types (PFTs) in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC), the land surface component of the Canadian Earth System Model. In addition to new PFTs, a modification of the efficiency with which water evaporates from the ground surface was applied. This modification addressed a high ground evaporation bias that reduced model performance when soils became very dry, limited heat flow into the ground, and reduced plant productivity through water stress effects. Compared to the grass and tree PFTs previously used by CLASSIC to represent the vegetation in Arctic permafrost-affected regions, simulations with the new shrub PFTs better capture the physical and biogeochemical impact of shrubs on the magnitude and seasonality of energy and CO2 fluxes at the dwarf-shrub tundra evaluation site. The revised model, however, tends to overestimate gross primary productivity, particularly in spring, and overestimated late-winter CO2 emissions. On average, annual net ecosystem CO2 exchange was positive for all simulations, suggesting this site was a net CO2 source of 18 ± 4 g C m−2 yr−1 using shrub PFTs, 15 ± 6 g C m−2 yr−1 using grass PFTs, and 25 ± 5 g C m−2 yr−1 using tree PFTs. These results highlight the importance of using appropriate PFTs in process-based models to simulate current and future Arctic surface–atmosphere interactions.

The major tectonic boundaries of the Northern Red Sea rift, Egypt derived from geophysical data analysis

AbstractIn the present study, we have attempted to map the plate boundary between Arabia and Africa at the Northern Red Sea rift region including the Suez rift, Gulf of Aqaba-Dead Sea transform and southeastern Mediterranean region by using gravity data analysis. In the boundary analysis method which was used; low-pass filtered gravity anomalies of the Northern Red Sea rift region were computed. Different crustal types and thicknesses, sediment thicknesses and different heat flow anomalies were evaluated. According to the results, there are six subzones (crustal blocks) separated from each other by tectonic plate boundaries and/or lineaments. It seems that these tectonic boundaries reveal complex structural lineaments, which are mostly influenced by a predominant set of NNW–SSE to NW–SE trending lineaments bordering the Red Sea and Suez rift regions. On the other side, the E–W and N–S to NNE–SSW trended lineaments bordering the South-eastern Mediterranean, Northern Sinai and Aqaba-Dead Sea transform regions, respectively. The analysis of the low pass filtered Bouguer anomaly maps reveals that the positive regional anomaly over both the Red Sea rift and South-eastern Mediterranean basin subzones are considered to be caused by the high density of the oceanic crust and/or the anomalous upper mantle structures beneath these regions whereas, the broad medium anomalies along the western half of Central Sinai with the Suez rift and the Eastern Desert subzones are attributed to low-density sediments of the Suez rift and/or the thick upper continental crustal thickness below these zones. There are observable negative anomalies over the Northern Arabia subzone, particularly in the areas covered by Cenozoic volcanics. These negative anomalies may be attributed to both the low densities of the surface volcanics and/or to a very thick upper continental crust. On the contrary, the negative anomaly which belongs to the Gulf of Aqaba-Dead Sea transform zone is due to crustal thickening (with limited heat flow values) below this region. Additionally in this study, the crustal thinning was investigated with heat flow, magnetic and free air gravity anomalies in the Northern Red Sea rift region. In fact, the crustal thinning of the study area was also proportional to the regions of observable high heat flow values. Finally, our results were found to be well correlated with the topography, free air, aeromagnetic and heat flow dataset profiles crossing most of the study area.

Controlling heat transfer in micro electron beam welding using volumetric heating

The small spot sizes and short residence times involved in microwelding demands the use of extremely high power density heat sources, which is beyond the feasible range in welding. The present work explores the use of volumetric heating as a key to employ such high heat intensities for microwelding. Electron beam is represented as a volumetric heat source and a numerical model is developed using commercial finite element softwares to predict the temperature distribution in the solid as a result of beam scanning. The effect of weld speed and beam interaction volume are evaluated to propose the optimum conditions of microwelding. Optimality is based on limited heat flow into the material, high controllability, and tolerable maximum temperature of the process. For the choice of AISI 304 material as an example, the optimum welding conditions required to produce a 2.5 μm weld depth are determined in the non-dimensional form of Peclet number 100 and relative beam penetration in the range of 0.8–1.2.

Sheath-Plasma Criterion of Fluid Theory with Finite Ion Temperature

with a polytropic index . The value of is important in relation to probe measurements, and one may set 1⁄4 3 for an adiabatic case (one-dimension), or 1⁄4 1 for an isothermal case. In fact, varies from three to unity owing to limited heat flow, since there is no heat flow in the adiabatic case and unhindered heat flow in the isothermal case. The value of for the sheath-plasma criterion has remained a problem. To derive the sheath-plasma criterion, equations applicable in a sheath region with space charge are used in general. However, there is another method of deriving the sheath-plasma criterion, in which equations applicable in a plasma or presheath region are used. Since quasi-neutrality is assumed in the equations in the second method, the equations become invalid in the sheath region owing to space charge, and a singularity gives a sheath-plasma boundary, where the ion drift speed is given by eq. (1) for Ti 1⁄4 0. The extension of the second method for Ti 61⁄4 0 is achieved by considering ion pressure, and shows the sheathplasma criterion indicating that the ion drift speed at the sheath-plasma boundary is given by eq. (2). As for the first method for Ti 61⁄4 0, the generalized sheath-plasma criterion of the ion velocity distribution function is obtained, although the details of the ion velocity distribution function are not determined. The purpose of this report is to find a solution of the equations in the second method for a finite ion temperature, which may vary in accordance with the polytropic approximation, because only the solution under the condition of Ti 1⁄4 0 or Ti 1⁄4 const: is known. Basic equations of the fluid theory treating a steady-state one-dimensional plasma with a uniform ion generation G (1⁄4 const:) are written as

Predicting the thermal conductivity of composite materials with imperfect interfaces

This paper compares the predicted values of the thermal conductivity of a composite made using the equivalent inclusion method (EIM) and the finite element method (FEM) using representative volume elements. The effects of inclusion anisotropy, inclusion orientation distribution, thermal interface conductance, h, and inclusion dimensions have been considered. Both methods predict similar overall behaviour, whereby at high h values, the effective thermal conductivity of the composite is limited by the inclusion anisotropy, while at lower h values, the effect of anisotropy is greatly diminished due to the more dominant effect of limited heat flow across the inclusion/matrix interface. The simulation results are then used to understand why in those cases where it has been possible to produce CNF reinforced Cu matrix composites with a large volume fraction of well dispersed CNFs, the measured thermal properties of the composite have failed to meet the expectations in terms of thermal conductivity, with measured conductivities in the range 200–300 W/m K. The simulation results show that, although degradation of the thermal properties of the CNFs and a poor interfacial thermal conductance are very likely the reasons behind the low conductivities reported, great care should be taken when measuring the thermal conductivity of this new class of materials, to avoid misleading results due to anisotropic effects.

Effect of titanium on copper–titanium/carbon nanofibre composite materials

Copper/carbon nanofibre composites containing titanium varying from 0.3 wt.% to 5 wt.% were made, and their thermal conductivities measured using the laser flash technique. The measured thermal conductivities were much lower than predicted. The difference between measured and predicted values has often been attributed to limited heat flow across the interface. A study has been made of the composite microstructure using X-ray diffraction, transmission electron microscopy and Raman spectroscopy. It is shown in these materials, that the low composite thermal conductivity arises primarily because the highly graphitic carbon nanofibre structure transforms into amorphous carbon during the fabrication process.

Effect of Fibre Anisotropy on Composite Thermal Conductivity

This paper examines the effect of anisotropy on the effective composite thermal conductivity, Kc, of metal matrix composites containing carbon-based inclusions. Added effects of thermal interface conductance, hbd, and size have also been considered. It has been found that at high hbd values, the effective thermal conductivity of the composite is limited by inclusion anisotropy. At lower hbd values and smaller inclusion sizes, this effect is greatly diminished due to the more dominant effect of limited heat flow across the interface.

Geothermal Potential in Queensland

Hot rocks occur at depth over significant areas of Queensland offering a number of targets for geothermal energy extraction. On the published maps showing temperatures at 5 kilometre depths, the area beneath the Cooper and Eromanga Basins in southwest Queensland shows as the largest area of hot rocks. Much of this area is underlain by low grade meta-sedimentary rocks. The meta-sedimentary rocks are intruded by granites of different ages. In the central Eromanga Basin, the granites are of Middle Ordovician age. They are s-type granites and are not high heat producing. In the southwest, the granites are of mid Silurian age. They are also s-types and not high heat producing. None the less, they have high residual heat. The Late Carboniferous, high heat producing granite at Habernero in South Australia does cross the border into Queensland, but has no known equivalents in Queensland. High heat producing granites are common in the Mount Isa area and extend under the Eromanga Basin. High heat producing granites are present beneath the Carpentaria Basin. Several heat anomalies of unknown cause occur in western central Queensland. There are limited heat flow measurements in north Queensland. Although there are many high heat producing granites, there are no apparent insulating blanket rocks. The youngest basalt flows in north Queensland are 10,000 years old and new flows could yet occur-there may be heat associated with magma chambers. There are hot springs and warm springs in the area, but they are poorly understood. Exploration for geothermal energy will begin in Queensland in 2006 with the grant of exploration permits.

A three-dimensional model of wire array instability, ablation, and jetting

A simplified algebraic model is set up to describe the principal features of the physics of wire array behavior in the early phases. Each wire is assumed to have evolved into a liquid-vapor core surrounded by a coronal plasma which carries most of the current. This plasma is unstable to m=0 instabilities, this instability being MHD or possibly electrothermal driven by inward radial heat flow on each wire. The joule heating occurs mainly in the necks of the sausage mode; flux limited heat flow transfers the energy to the core which erodes and ablates as a plasma at the sound speed. Free expansion at this velocity occurs into the lobes of the m=0 structure, and by rocket action the J_/spl times/B_ inward global force deflects the flow into inward precursor-jets. It is shown that the magnetic Reynolds' number is less than one, and for a rapidly rising current results in negligible current or magnetic field in the precursor, as found usually in experiments. In addition, it is found that in the necks the Hall parameter is much less than one, consistent with the field-free model of flux limited heat flow.

Correlation between DSC Curves and Isobaric State Diagrams. 3. Simulation of Smeared DSC Curves

Using n-hexane (n-C6H14) and n-dodecane (n-C12H26) as a binary model system, the smearing of isobaric DSC curves, which is a result of a limited heat flow in the calorimeter, is the subject of the present paper. The underlying heat conduction process notwithstanding, the method introduced allows one to simulate the smeared traces of real isobaric DSC curves of binary systems by superimposing a large number of Gaussian curves. The resulting curves have proved to be in good agreement with the experimental data.

An Ultrastructural Study of Corneal Incisions Induced by an Excimer Laser at 193 nm

Far ultraviolet light (193 nm) produced by an excimer laser has been used to produce a variety of incisions in the corneas of anaesthetised rabbits. Tissue was lost from the site of irradiation but the nature of the underlying molecular interactions responsible for such tissue erosion remain obscure. Ultrastructural analysis of the walls of the ablated areas show damage to the adjacent structures to be confined to a zone 60 to 200 nm in width. These dimensions could either be attributed to photochemical processes in which high energy photons directly break organic molecular bond, or to thermal reactions which result in limited heat flow and damage confined to the absorption depth at 193 nm of less than 1 μm. In non-penetrating incisions that reached within 40 μm of Descemet's membrane, endothelial cells were lost beneath the line of the irradiation. This spalling of cells seemed to be generated by shock or acoustic waves.