Experimental optimization of capillary structures for loop heat pipes and heat switches

Loop Heat Pipes (LHP) and Heat Pipes (HP) are used in the thermal management of electronic devices with high-density heat dissipation. In these two-phase thermal devices, the key component is the capillary structure (CS) that pumps the working fluid using the capillary forces generated by the meniscus, which is formed due to evaporation. The performance of the LHP and the HP depends greatly on the internal structure and external configurations of the CS. However, there is not enough experimental and theoretical data on the optimization of the capillary structures of evaporators. The “inverted meniscus” scheme is the usual configuration for Loop Heat Pipes (LHP) but not for Heat Pipes (HP), which use the “classical” scheme. This paper presents the results of extensive experimental investigations on the optimization of the physical and geometrical parameters of the flat CS including thickness, configuration and size of vapor grooves, for different CS materials and working fluids. Additionally, the paper presents a comparative study between the “classical” and the “inverted meniscus” schemes of vaporization. Based on the obtained results, two examples of evaporators have been designed and evaluated.     

FIB assisted study of plasma sprayed splat-substrate interfaces: NiAl-stainless steel and alumina-NiAl combinations

This paper deals with the splat-substrate and intersplat interfaces of splats produced from Ni-5 wt.% Al and alumina based ceramic powders. The cross section of the splat-substrate interface was studied using a focused ion beam (FIB) assisted Field Emission Scanning Electron Microscope (FE-SEM). The metallic splat showed good interfacial integrity apart from a few interfacial pores resulting from entrapped gases and edge curl resulting from quenching stresses. The ceramics showed generally well adherent interfaces for both single and multilayer splats. Good adherence of both alumina splats was attributed to the improved wettability provided by the bond coat. One kind of alumina splats, namely, those obtained from agglomerated nanostructured powder was found to be free from crack and edge curl. This was attributed to the presence of a glassy phase in this splat, which can absorb quenching stresses.     

Depth-dependent magnetic characterization of Fe films on NiO(0 0 1)

We used nuclear hyperfine spectroscopies and a ⁵⁷Fe probe layer approach to study the depth-dependent magnetic properties of ultrathin Fe films on NiO(0 0 1), a system exhibiting exchange bias. Conversion electron Mössbauer spectroscopy and nuclear resonance scattering of synchrotron radiation were employed. The samples were two Fe films with a thickness (8–10 ML) slightly above the critical thickness for the onset of ferromagnetism at room temperature, in which a 2 ML-thick probe layer, enriched in the ⁵⁷Fe Mössbauer isotope, was embedded at different depths from the Fe/NiO interface. Both techniques indicate that inside the film Fe has a metallic character, while at the interface with NiO different Fe phases are present. The main conclusion is that already a few monolayers from the interface with NiO the magnetic properties of Fe are bulk-like.     

A stream function implicit finite difference scheme for 2D incompressible flows of Newtonian fluids

The development of a new algorithm to solve the Navier–Stokes equations by an implicit formulation for the finite difference method is presented, that can be used to solve two‐dimensional incompressible flows by formulating the problem in terms of only one variable, the stream function. Two algebraic equations with 11 unknowns are obtained from the discretized mathematical model through the ADI method. An original algorithm is developed which allows a reduction from the original 11 unknowns to five and the use of the Pentadiagonal Matrix Algorithm (PDMA) in each one of the equations. An iterative cycle of calculations is implemented to assess the accuracy and speed of convergence of the algorithm. The relaxation parameter required is analytically obtained in terms of the size of the grid and the value of the Reynolds number by imposing the diagonal dominancy condition in the resulting pentadiagonal matrixes. The algorithm developed is tested by solving two classical steady fluid mechanics problems: cavity‐driven flow with Re=100, 400 and 1000 and flow in a sudden expansion with expansion ratio H/h=2 and Re=50, 100 and 200. The results obtained for the stream function are compared with values obtained by different available numerical methods, to evaluate the accuracy and the CPU time required by the proposed algorithm. Copyright © 2002 John Wiley & Sons, Ltd.     

Inhibition of premixed methane flames by manganese and tin compounds

The first experimental measurements of the influence of manganese- and tin-containing compounds (MMT, TMT) on the burning velocity of methane/air flames are presented. Comparisons with Fe(CO)₅ and CF₃Br demonstrate that manganese and tin-containing compounds are effective inhibitors. The inhibition efficiency of MMT is about a factor of two less than that of iron pentacarbonyl, and that of TMT is about 26 times less effective, although TMT is still about twice as effective as CF₃Br. There exist conditions for which both MMT and TMT show a loss of effectiveness beyond that expected because of radical depletion, and the cause is believed to be particle formation. Kinetic models describing the inhibition mechanisms of manganese- and tin-containing compounds are suggested. Simulations of MMT- and TMT-inhibited flames show reasonable agreement with experimental data. The decomposition of the parent molecule for the tin and manganese species is found to have a small effect on the inhibition properties for the concentrations in this work. The inhibition effect of TMT is determined mostly by the rate of the association reaction H + SnO + M ↔ SnOH + M, and the catalytic recombination cycle is completed by the reactions SnOH + H ↔ SnO + H₂ and SnOH + OH ↔ SnO + H₂O. The inhibition mechanism by manganese-containing compounds includes the reactions: MnO + H₂O ↔ Mn(OH)₂; Mn(OH)₂ + H ↔ MnOH + H₂O, and MnOH + OH (or H) ↔ MnO + H₂O (or H₂), and the burning velocity is most sensitive to the rate of the reaction Mn(OH)₂ + H ↔ MnOH + H₂O.     

Ortho-para conversion in solid hydrogen,catalyzed by molecular oxygen impurities

Ortho-para conversion in solid hydrogen with small amounts of molecular oxygen as impurity (0.01–0.5%) was studied by NMR in the temperature range 4.2–7 K. The observed conversion rate was substantially greater than that of the natural conversion process in pure solid H₂ and revealed a strong dependence on temperature and the oxygen content in the sample. Obviously, the conversion is catalyzed by the large paramagnetic moments of O₂ impurities. Since the influence of O₂ is of very short range, the overall rate of the catalyzed conversion in the sample is determined by the relative diffusion of ortho-H₂ and the O₂ impurities. The diffusion coefficient was found to obey an Arrhenius behavior D(T)=3(2). 10^–11.exp[96(8)/T] cm²s^–1. This temperature dependence is interpreted in terms of vacancy-particle exchange by quantum tunneling through the intervening barrier.     

High-Temperature Limit for the O2-Catalyzed Ortho-Para Conversion Rate in Solid Hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of molecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the temperature range 4.2–10.9 K. Conversion rate was mainly determined by the paramagnetic oxygen impurities. The catalyzed conversion rate constant increases with temperature in the range 4.2–8 K and becomes independent of temperature above 8 K. The high-temperature limit is 1.15(8) s ^–1 . The catalyzed conversion rate was shown to become temperature independent when the ortho-H ₂ diffusion time _d falls below the conversion time _cc in the first coordination sphere of O ₂ impurity. The obtained value of _cc =10.5(8) s was used to identify the position of O ₂ in H ₂ lattice as substitutional. The temperature dependence of the ortho-H ₂ diffusion rate near the high temperature limit was determined.     

Cerium-doped bioactive 45S5 glasses: spectroscopic,redox, bioactivity and biocatalytic properties

The ability of Ce-containing bioactive glasses, based on 45S5 Bioglass^®, to inhibit oxidative stress in terms of reduction in hydrogen peroxide and superoxide (O₂ ^?), by mimicking the catalase and superoxide dismutase activity is reported in this work. The characterization is performed on the powders of pristine glasses and after the soaking in H₂O₂ solutions and simulated body fluid. The glass samples are analysed by XPS, XRD, UV–Vis and FT-IR. The best catalyst activities are obtained for the glass with the highest content of cerium (H_5.3 = 5.3 mol% of CeO₂ in the nominal glass composition), and the best Ce³⁺/Ce⁴⁺ ratio in terms of catalase mimetic activity is found to be a function of H₂O₂ concentration. Moreover, the detailed study of the surface during the mimic enzymatic activity tests shows the formation of a Ca-P-rich layer on the glass surface, where the presence of Ce ions favours the formation of CePO₄. The phosphate in turn inhibits the formation of hydroxyapatite, decreasing the bioactivity of the glass with the highest of CeO₂ in the glass composition. This work shows the effect of Ce³⁺/Ce⁴⁺ ratio towards the catalase mimetic activity and for the first time the superoxide dismutase mimetic activity of Ce-containing 45S5-derived glasses.     

COMPUTATION OF TRANSIENT NATURAL CONVECTION IN A SQUARE CAVITY BY AN IMPLICIT FINITE-DIFFERENCE SCHEME IN TERMS OF THE STREAM FUNCTION AND TEMPERATURE

In this article we discuss the application of an implicit scheme to the solution by finite differences of transient natural convection in terms of the stream function and temperature. The second-order energy differential and fourth-order momentum equations are discretized according to the well-known alternate direction implicit (ADI) method. Then the temperature field solution is built based on the classic tridiagonal matrix algorithm (TDMA), and the stream function solution is built based on the two original hypotheses proposed here together with the penta diagonal matrix algorithm (PDMA). The time step in the algorithm is obtained analytically as a function of the Prandtl number and the size of the grid imposing the diagonal dominance condition in the pentadiagonal matrix generated by the hypothesis. We verify the hypothesis and the algorithm using the transient natural convection solution in the following parameter ranges: 10 3 r Ra r 10 6 , 10 -2 r Pr r 10 2 , 21 2 21 r grid r 61 2 61. The transient solution to the problem is presented using the average Nusselt number and local Nusselt numbers on the hot wall, CPU time, the temperature and velocity profiles in the y = 0.5 section, and the temperature in the central point of the cavity. The results of the permanent solution are presented using the maximum value of the stream function within the domain and the average Nusselt number on the cold wall as a function of the Rayleigh number, the relaxation parameter, Prandtl number, and the size of the grid. Finally, both the transient and permanent solution results are compared with the results of five other published studies.     

Optimization of the active component distribution through the catalyst bed for the case of adiabatic reactor

This article is the next one in the previous series of publications concerning the optimal active component distribution. The aim of the present work is to extend the optimization problem taking into account the energy balance. The variational problem on searching for the optimal active component distribution profile along the catalyst bed was formulated for the case of an adiabatic reactor at an arbitrary form of reaction rate expression. The Euler differential equation was derived and the existence conditions for the solution of variational problem were obtained. A numerical algorithm was suggested and the optimal profiles of the active component distribution were calculated for the first-order reaction. Under isothermal conditions with linear dependence of the reaction rate on reactant concentration at a constant mass transfer coefficient, the uniform distribution is optimal. As opposed to the case of an isothermal reactor with a first-order catalytic reaction, in an adiabatic reactor considerable economy in the active component loading might be achieved due to optimization of its distribution. It was shown that the optimal active component distribution profiles were axially decreasing for the first-order exothermic reactions and increasing for endothermic reactions. The optimal active component distribution profile was calculated for the case of methane combustion on catalytic monoliths.