Burning characteristics of candle flames in sub-atmospheric pressures: An experimental study and scaling analysis

Burning characteristics (mass burning rate, natural convection boundary layer thickness, flame height and dark zone height) of laminar diffusion flames produced by a candle at sub-atmospheric pressures in the range of P_∞?=?50–100?kPa were experimentally studied in a reduced-pressure chamber; such data are not reported to date. Scaling analysis was performed to interpret the pressure dependence. The new experimental findings for candle flames in the sub-atmospheric pressures were well interpreted by the proposed scaling laws: (1) the mass burning rate was higher for a candle with larger wick length, and it increased with increasing ambient pressure, a stagnant layer B-number model based on natural convection boundary (flame boundary layer thickness) was developed to scale the mass burning rate of candle flames at various pressures; (2) the flame boundary layer thickness was wider in lower pressure and can be well represented by a natural convection boundary layer solution; (3) flame height was higher for a candle with larger wick length, meanwhile the ratio of flame height to burning rate was independent of pressure; (4) the flame dark zone height representing a soot formation length scale changes little with pressure, meanwhile its ratio to the total flame height is scaled with pressure by $P_{\infty }^{?1/2}/L_{w,e}^{3/4}$ (L_w,e is effective wick length inside flame). This work provided new experimental data and scaling laws of candle flame behaviors in sub-atmospheric pressures, which provided information for future characterization and soot modeling for diffusion flames associated with melting and evaporation processes of solid fuels.     

Functional micropatterned surfaces prepared by simultaneous UV‐lithography and surface segregation of fluorinated copolymers

A new procedure focused on the design and preparation of structured and functional polymer surfaces by combination of two approaches acting simultaneously is developed. The elaboration of micrometer size patterned surfaces by UV‐light lithography is reported where, in addition, the surface chemical composition can be controlled by surface segregation of a fluorinated copolymer incorporated in the photopolymerizable mixture. As evidenced by contact angle and XPS measurements, the surface composition can be modified depending on such factors as with the environmental conditions or the concentration of copolymer in the blend. Moreover, the functionality of the copolymer is enhanced by the surface pattern created. As a consequence, the wettability of the films can be modified depending on the pattern and composition of the blend. By using this methodology, functional adaptive sensitive surfaces with a well‐defined topography will be obtained in one single step and without the use of tedious and time‐consuming multistep procedures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

An alternating direction scheme on a nonuniform mesh for reaction-diffusion parabolic problems

In this paper we develop a numerical method for two-dimensionaltime-dependent reaction-diffusion problems. This method, whichcan immediately be generalized to higher dimensions, is shownto be uniformly convergent with respect to the diffusion problems.This method, which can immediately be generalized to higherdimensions, is shown to be uniformly convergent with respectto the diffusion parameter.     

Antiferromagnetic versus ferromagnetic exchange interactions in bis(μ-O(oximate))dinickel(II) units for a series of closely related cube shaped carboxamideoximate-bridged Ni(4) complexes. A combined experimental and theoretical magneto-structural study

The syntheses, crystal structures, and the experimental and theoretical magnetochemical characterization for three tetrametallic Ni(II) clusters, namely, [Ni(4)(L)(4)(Cl)(2)(MeOH)(2)](ClO(4))(2)·4MeOH (1), [Ni(4)(L)(4)(N(3))(2)(MeOH)(2)](ClO(4))(2)·2MeOH (2), and [Ni(4)(L1)(4)(pyz)(2)(PhCOO)(2)(MeOH)(2)](ClO(4))(2)·7MeOH (3) (where HL and HL1 represent bipyridine-2-carboxamideoxime and pyrimidine-2-carboxamideoxime, respectively) are reported. Within the Ni(4)(2+) units of these compounds, distorted octahedral Ni(II) ions are bridged by carboxamideoximato ligands to adopt a distorted tetrahedral disposition. The Ni(4)(2+) unit, of C(2) symmetry, can also be viewed as a cube with single [O-atom] and double [NO oxime] bridging groups as atom edges, which define two almost square-planar Ni(O)(2)Ni rings and four irregular hexagonal Ni(NO)(2)Ni rings. To analyze the magnetic properties of 1-3, we have considered the simplest two-J model, where J(1) = J(2) (exchange interactions between the Ni(II) ions belonging to the Ni(O)(2)Ni square rings) and J(a) = J(b) = J(c) = J(d) (exchange interactions between the Ni(II) ions belonging to the Ni-(NO)(2)Ni hexagonal rings) with the Hamiltonian H = -J(1)(S(1)S(2) + S(3)S(4)) - J(a)(S(1)S(3) + S(1)S(4) + S(2)S(3) + S(2)S(4)). The J(1) and J(a) values derived from the fitting of the experimental susceptibility data are -5.8 cm(-1) and -22.1 cm(-1) for 1; -2.4 cm(-1) and -22.8 cm(-1) for 2, and +15.6 cm(-1) and -10.8 cm(-1) for 3. The magneto-structural results and density-functional theory (DFT) calculations demonstrate that the exchange interactions inside the Ni(μ-O)(2)Ni square rings depend on the Ni-O-Ni bridging angle (θ) and the out-of-plane angle of the NO oximate bridging group with respect to the Ni(O)(2)Ni plane (τ), whereas the interactions propagated through the Ni-N-O(Ni)-Ni exchange pathways defining the side of the hexagonal rings depend on the Ni-N-O-Ni torsion angle (α). In both cases, theoretical magneto-structural correlations were obtained, which allow the prediction of the angle for which ferromagnetic interactions are expected. For compound 3, the existence of the axial magnetic exchange pathway through the syn-syn benzoate bridge may also contribute (in addition to the θ and τ angles) to the observed F interaction in this compound through orbital countercomplementarity, which has been supported by DFT calculations. Finally, DFT calculations clearly show that the antiferromagnetic exchange increases when the dihedral angle between the O-Ni-O planes of the Ni(μ-O)(2)Ni square ring, β, increases.     

Two‐Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin‐Crossover Compounds Fe(X‐py)2[Ag(CN)2]2 (X=H, 3‐methyl, 4‐methyl, 3,4‐dimethyl, 3‐Cl)

In the series of polymeric spin‐crossover compounds Fe(X‐py)₂[Ag(CN)₂)]₂ (py=pyridine, X=H, 3‐Cl, 3‐methyl, 4‐methyl, 3,4‐dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high‐spin (HS) to the low‐spin (LS) state occurs by two‐step transitions for three out of five members of the family (X=H, 4‐methyl, and X=3,4‐dimethyl). The two other compounds (X=3‐Cl and 3‐methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin‐crossover behaviour of the compound undergoing two‐step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3‐methyl undergo light‐induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS‐to‐LS relaxation curves at different temperatures support the low‐cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4‐dimethyl the behaviour is well described by a double exponential function in accordance with the two‐step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.     

Evaluation of rejected cases in an acceptance system with data envelopment analysis and goal programming

Some DEA models have been proposed for acceptance or rejection based on a set of cases that have been previously classified. Also, a modified DEA-type linear programming model has been proposed to determine whether a new case must be accepted or rejected, depending on its location on, above, or below the sample frontier. However, these models assume that all attributes that characterize a case are discretionary. This paper extends these results by proposing a model that includes discretionary and non-discretionary attributes (inputs), and more important, a goal program which resembles a modified additive model to determine which characteristics must be changed (and by how much) in order to accept an initially rejected case. A real application is provided to illustrate the potential of the proposed ideas.     

Analysis of stochastic problem decomposition algorithms in computational grids

Stochastic programming usually represents uncertainty discretely by means of a scenario tree. This representation leads to an exponential growth of the size of stochastic mathematical problems when better accuracy is needed. Trying to solve the problem as a whole, considering all scenarios together, yields to huge memory requirements that surpass the capabilities of current computers. Thus, decomposition algorithms are employed to divide the problem into several smaller subproblems and to coordinate their solution in order to obtain the global optimum. This paper analyzes several decomposition strategies based on the classical Benders decomposition algorithm, and applies them in the emerging computational grid environments. Most decomposition algorithms are not able to take full advantage of all the computing power available in a grid system because of unavoidable dependencies inherent to the algorithms. However, a special decomposition method presented in this paper aims at reducing dependency among subproblems, to the point where all the subproblems can be sent simultaneously to the grid. All algorithms have been tested in a grid system, measuring execution times required to solve standard optimization problems and a real-size hydrothermal coordination problem. Numerical results are shown to confirm that this new method outperforms the classical ones when used in grid computing environments.