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An analytical study of a free convection boundary layer (BL) for a laminar and steady flow over a vertical flat plate (VFP) having nonuniform thermal conditions has been investigated. The plate is subjected to a variable wall heat flux of q w ″ (x) = q r x n , where x is the distance along the VFP from the leading edge and q r is a reference heat flux. The boundary layer equations for nonuniform wall heat flux conditions have been formulated. These equations are then solved numerically for the range of exponents (n) and Prandtl numbers ( P r) of −0.25 to 1 and 0.01 to 1000 respectively. The effects of two physical parameters n and P r on the characteristic of thermal boundary layer (TBL) and hydrodynamic boundary layer (VBL) are described. Moreover, correlations of TBL and VBL thicknesses have been presented as a function of Pr and n. The TBL and VBL thickness are more influenced by Pr as compared to n. These correlations are reasonably accurate for uniform heat flux (n = 0) proposed by A. Bejan using a scale analysis and no other such correlations are available for nonuniform heat flux for a wide range of Prandtl numbers (Pr). It has been found that when P r < 1 , both the BL thicknesses decrease with the Pr till a value of P r ∼ 1 is reached. For P r > 1 , the growth of VBL and TBL thickness over the VFP increases and decreases respectively. Consequently, the local skin friction coefficient decreases and the local Nusselt number increases with the increase in P r and n at a fixed value of modified Rayleigh number. [ABSTRACT FROM AUTHOR]

A degradative dimerisation of Morita–Baylis–Hillman ketones was observed in the presence of a primary diamine. The reaction proceeded swiftly to produce methylene-bridged 1,3-dicarbonyl compounds. A brief mechanistic investigation alluded to a retro-Mannich reaction as the key step of the transformation. [ABSTRACT FROM AUTHOR]

Abstract: An aldolisation protocol for methyl vinyl ketone has been developed using an arginine mediated reaction in water. The reaction brought out an interesting contrast to the reactivity of methyl vinyl ketone with two different amino acids proline and arginine under nearly identical conditions. The aldolisation was only moderately effective with aromatic aldehydes as acceptors, but proved quite efficient when isatins were used. The latter underwent a rapid aldol addition with methyl vinyl ketone at room temperature under aqueous conditions to provide an expedient access to the corresponding 3‐substituted‐3‐hydroxyindolin‐2‐ones in good yields. These adducts were further transformed in ways characteristic to the β‐unsubstituted activated olefin moiety. The method also helped access a spirooxindole pyranone in reasonable yield. [ABSTRACT FROM AUTHOR]

A serendipitous nucleophilic activation of carboxylic acids by proline helped to achieve a direct hydrocarboxylation of methyl vinyl ketone at 60 °C under solvent-free conditions. A variety of carboxylic acids were used successfully in this oxa-Michael addition, affording useful 4-acyloxy-2-butanones in moderate yields. The reactions are carried out under solvent-free conditions, and the products are isolated in high purity by a simple work-up procedure without any need for column chromatographic purification, imparting a green quotient to the protocol. A heterodimeric non-covalent interaction between the amino acid and the carboxylic acid appears to be the most plausible mechanistic interpretation for the nucleophilic activation; additionally, the possible activation of the Michael acceptor by iminium ion formation presents an interesting case of proline acting as a bifunctional catalyst. [ABSTRACT FROM AUTHOR]

A proline-mediated Baylis–Hillman reaction of methyl vinyl ketone with aromatic aldehydes has been carried out without using any co-catalyst, under solvent-free conditions. The reaction works efficiently at 60 °C in the presence of a small amount of water to afford the Baylis–Hillman adducts in reasonable to very good yields over 8–48 h. The absence of a co-catalyst suggests that proline plays a role in the proton-transfer step of the reaction mechanism, in addition to its proposed involvement in the iminium ion formation and conjugate addition. This would, in principle, imply that proline acts as a trifunctional catalyst in the reaction, and mechanistic studies to gain a deeper understanding of this aspect should provide further insights in the future. [ABSTRACT FROM AUTHOR]

Owing to its exceptional properties at high temperature, graphite is used in several applications such as structural material and fuel block in high temperature nuclear reactors. Air ingress is one of the serious safety concerns in these reactors. Oxidation of graphite leading to increased porosity affects its mechanical strength and may lead to core collapse resulting in a severe accident. During such a scenario, generation of graphite particles could be the main hazard. Once generated, these particles often in fine and ultrafine sizes, may carry radioactivity to large distances and/or for long times. These particles owing to their higher surface to volume ratio possess an additional inhalation hazard. Ultrafine particles have the potential to enter into respiratory tract and cause damage to body organs. Coating of graphite components is preferred to reduce the oxidation induced damages at high temperatures. In the present work, effect of alumina (Al2O3) coating on the emission characteristics of particles from graphite under high temperature conditions has been investigated. Bare and Al2O3 coated graphite specimens were heated within a closed chamber at varying temperatures during these experiments. Temporal evolution of concentrations of gases (CO and CO2) and particles were measured. The results reveal that Al2O3 coating on the graphite delayed the oxidation behavior and the structure of graphite remained largely intact at high temperatures. A significant reduction in aerosol formation and CO emission was also noticed for the coated specimens. [ABSTRACT FROM AUTHOR]