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This study reports on combined thermal radiation, chemical reaction, and magnetic field effects on entropy generation in an unsteady nanofluid flow past an inclined cylinder using the Buongiorno model. We consider the impact of viscous dissipation, velocity slip conditions, thermal slip conditions, and the Brownian motion. The transport equations governing the flow are solved using an overlapping grid spectral collocation method. The results indicate that entropy generation is suppressed significantly by thermal radiation and chemical reaction parameters but enhanced with the magnetic field, viscous dissipation, the Brinkman number, and the Reynolds number. Also, fluid flow variables are affected by the thermophoresis parameter, the angle of cylinder inclination, and the Richardson number. We present the findings of the skin friction coefficient, the Nusselt number, and the Sherwood number. The model is applicable in fields such as the petroleum industry, building industries, and medicine. [ABSTRACT FROM AUTHOR]

Accurate information on thermal conductivity and thermal diffusivity of materials is of central importance in relation to geoscience and engineering problems involving the transfer of heat. Several methods, including the classical divided bar technique, are available for laboratory measurements of thermal conductivity, but much fewer for thermal diffusivity. We have generalized the divided bar technique to the transient case in which thermal conductivity, volumetric heat capacity and thereby also thermal diffusivity are measured simultaneously. As the density of samples is easily determined independently, specific heat capacity can also be determined. The finite element formulation provides a flexible forward solution for heat transfer across the bar, and thermal properties are estimated by inverse Monte Carlo modelling. This methodology enables a proper quantification of experimental uncertainties on measured thermal properties and information on their origin. The developed methodology was applied to various materials, including a standard ceramic material and different rock samples, and measuring results were compared with results applying traditional steady-state divided bar and an independent line-source method. All measurements show highly consistent results and with excellent reproducibility and high accuracy. For conductivity the obtained uncertainty is typically 1-3 per cent, and for diffusivity uncertainty may be reduced to about 3-5 per cent. The main uncertainty originates from the presence of thermal contact resistance associated with the internal interfaces in the bar. These are not resolved during inversion and it is imperative that they are minimized. The proposed procedure is simple and may quite easily be implemented to the many steady-state divided bar systems in operation. A thermally controlled bath, as applied here, may not be needed. Simpler systems, such as applying temperature-controlled water directly from a tap, may also be applied. [ABSTRACT FROM AUTHOR]

A recent spurt in the universal interest to study alternatives to the conventional DX refrigeration systems and use of natural refrigerants as secondary fluids has been visible. This investigation presents a comparative analysis between carbon dioxide and other working fluids for use in forced circulation-type secondary loops suitable for various refrigeration and air conditioning applications. The comparison is made on the basis of equal heat transfer rate, frictional pressure drop per unit length, temperature drop/ rise of secondary fluid and mean temperature difference between the secondary fluid and heat exchanger tube wall. Employing friction factor and heat transfer correlations, equations are obtained for diameter ratio, area ratio, mass ratio and pumping power ratio. Results show that employing carbon dioxide in place of other conventional secondary fluids except water leads to compact and lightweight heat exchangers, despite the high-pressure operation of carbon dioxide-based loops. The pumping power required is much smaller as well in case of carbon dioxide compared with all other fluids for the wide range of operating temperatures. Though results are presented here for the case of cooling only, calculations indicate similar trends for the heating configuration as well. Carbon dioxide appears to be an excellent secondary fluid for a wide range of refrigeration and air conditioning applications and it could turn out to be a disruptive technology for such applications in view of its benign environmental footprint. [ABSTRACT FROM AUTHOR]

A variety of thermal approaches are used to estimate sap flux density in stems of woody plants. Models have proved valuable tools for interpreting the behavior of heat pulse, heat balance and heat field deformation techniques, but have seldom been used to describe heat transfer dynamics for the heat dissipation method. Therefore, to better understand the behavior of heat dissipation probes, a model was developed that takes into account the thermal properties of wood, the physical dimensions and thermal characteristics of the probes, and the conductive and convective heat transfer that occurs due to water flow in the sapwood. Probes were simulated as aluminum tubes 20 mm in length and 2 mm in diameter, whereas sapwood, heartwood and bark each had a density and water fraction that determined their thermal properties. Base simulations assumed a constant sap flux density with sapwood depth and no wounding or physical disruption of xylem beyond the 2 mm diameter hole drilled for probe installation. Simulations across a range of sap flux densities showed that the dimensionless quantity k [defined as (ΔTm −ΔT)/ΔT, where ΔTm is the temperature differential (ΔT) between the heated and unheated probe under zero-flow conditions] was dependent on the thermal conductivity of the sapwood. The relationship between sap flux density and k was also sensitive to radial gradients in sap flux density and to xylem disruption near the probe. Monte Carlo analysis in which 1000 simulations were conducted while simultaneously varying thermal conductivity and wound diameter revealed that sap flux density and k showed considerable departure from the original calibration equation used with this technique. The departure was greatest for variation in sap flux density typical of ring-porous species. Depending on the specific combination of thermal conductivity and wound diameter, use of the original calibration equation resulted in an 81% under- to 48% overestimation of sap flux density at modest flux rates. Future studies should verify these simulations and assess their utility in estimating sap flux density for this widely used technique. [ABSTRACT FROM AUTHOR]

The results of thermal property measurements on cores from the scientific well Yaxcopoil-1 (1511 m in depth) drilled in the Chicxulub impact structure (Mexico) are described. The thermal conductivity, thermal diffusivity, volumetric heat capacity, thermal anisotropy coefficient, thermal heterogeneity factor, and, in addition, porosity and density were measured on 451 dry and water-saturated cores from the depth interval of 404-1511 m. The acoustic velocities were determined on a subgroup of representative samples. Significant vertical short- and long-scale variations of physical properties related to the grade of shock-thermal metamorphism and correlations between thermal and other physical properties are established. Rocks of the post-impact and impact complexes differ significantly in heterogeneity demonstrating that the impact complex has larger micro- heterogeneity on sample scale. The pre-impact rocks differ essentially from the impact and post-impact rocks in the thermal conductivity, thermal diffusivity, density and porosity. The thermal anisotropy of rocks of all structural-lithological complexes is very low ( K = 1.02 ... 1.08), which is similar to the situation in the Puchezh-Katunk and Ries impact structures. Correlations are established between the thermal conductivity and elastic wave velocities measured in laboratory. For limestone-calcarenites, the thermal conductivity (λ) can be calculated from the compressional wave velocity ( V) using the formula λ= 0.346 V + 0.844, and for dolomite-anhydrites this relation has the form λ= 0.998 V + 1.163 [for λ in W (m K) and V in km s]. These correlations are used for downscaling of the sonic velocities to the decimetre scale. The effective medium theory is applied to invert the matrix thermal conductivity and pore/crack geometry from the thermal conductivity measured on the studied samples. Representative experimental data on the thermal properties for all lithological groups encountered by the Yaxcopoil-1 well essentially extend an existing database on the thermal properties of rocks of impact structures and can be used for determination of the heat flow density, interpretation of temperature logging data, theoretical modelling of heat and mass transfer processes and constructing thermal models of the Chicxulub impact structure as well as for the lithological interpretation. The research results confirm the necessity of dense sampling for the thermal property measurements to obtain reliable results in petrophysical and geothermal investigations of impact structure formations. [ABSTRACT FROM AUTHOR]

This paper integrates the research presented in the papers in this special issue of Holmér et al. and Havenith et al. [Holmér, I., Nilsson, H., Havenith, G., Parsons, K. C. (1999) Clothing convective heat exchange: proposal for improved prediction in standards and models. Annals of Occupational Hygiene, in press; Havenith, G., Holmér, I., den Hartog, E. and Parsons, K. C. (1999) Clothing evaporative heat resistance: proposal for improved representation in standards and models. Annals of Occupational Hygiene, in press] to provide a practical suggestion for improving existing clothing models so that they can account for the effects of wind and human movement. The proposed method is presented and described in the form of a basic computer program. Analytical methods (for example ISO 7933) for the assessment of the thermal strain caused by human exposure to hot environments require a mathematical quantification of the thermal properties of clothing. These effects are usually considered in terms of ‘dry’ thermal insulation and vapour resistance. This simple ‘model’ of clothing can account for the insulation properties of clothing which reduce heat loss (or gain) between the body and the environment and, for example, the resistance to the transfer of evaporated sweat from the skin, which is important for cooling the body in a hot environment. When a clothed person is exposed to wind, however, and when the person is active, there is a potentially significant limitation in the simple model of clothing presented above. Heat and mass transfer can take place between the microclimate (within clothing and next to the skin surface) and the external environment. The method described in this paper ‘corrects’ static values of clothing properties to provide dynamic values that take account of wind and human movement. It therefore allows a more complete representation of the effects of clothing on the heat strain of workers. [ABSTRACT FROM PUBLISHER]