Specific Temperature Profiles Research Articles

New similarity solutions for boundary-layer flow on a horizontal surface in a porous medium

New similarity solutions for free, forced, and mixed convection boundary-layer flow on a permeable horizontal surface embedded in a fluid-saturated porous medium are presented in this paper. Analytical solutions have been obtained for some specific velocity and temperature profiles. The generic case corresponding to a 1/x-type distribution of both the wall temperature and the outer velocity is discussed in detail. In the plane (U, γ) defined by the mixed convection parameter U and the suction/injection parameter γ, the existence domain of physical solutions is identified and the main characteristics (wall temperature gradients, decay rates, uniqueness, multiplicity, etc.) of these solutions are investigated systematically.

The correlation of the magnetic properties and the magnetocaloric effect in (Gd1−xErx)NiAl alloys

A study of the magnetic properties of several (Gd1−xErx)NiAl alloys (where x=0, 0.30, 0.40, 0.46, 0.50, 0.55, 0.60, 0.80, and 1.00) was undertaken using both ac and dc magnetic and heat capacity measurements in an attempt to understand the table-like magnetocaloric effect previously observed in (Gd0.54Er0.46)NiAl. Results indicate the presence of both antiferromagnetic and ferromagnetic ordering processes in all alloys containing Gd. For ErNiAl, a metamagnetic transition from an antiferromagnetic ground state was observed. Within each alloy, several magnetic transitions occur over a temperature range from 10 K [in (Gd0.20Er0.80)NiAl] up to 35 K (in GdNiAl), with all but the lowest temperature transition shifting to higher temperatures with increasing Gd content. The change in magnetic entropy (ΔSmag) induced by a change in field is observed to peak around the Néel temperature for ErNiAl while gradually broadening and shifting toward the Curie temperature as the Gd content is increased. For Gd-rich alloys, a significant contribution to ΔSmag is observed at both the low and high temperature transitions, resulting in a rounded, skewed caret-like temperature profile of the magnetocaloric effect. Factors, which are believed to contribute to this effect, include the presence and temperature spacing of multiple zero-field transitions, which most likely result from competing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. This leads to broad peaks in the magnetic heat capacity that span several transition temperatures, providing for a substantial ΔSmag over an extended temperature range. This characteristic is desired for application to magnetic refrigeration, where certain thermodynamic cycles (e.g., Ericsson cycle) require specific temperature profiles of the magnetocaloric effect in refrigerant materials (e.g., a constant change in magnetic entropy as a function of temperature within the region of cooling). In general, the best materials are those which supply the maximum amount of cooling over the widest temperature range.