Observation of red emission in wurtzite ZnS nanoparticles and the investigation of phonon modes by Raman spectroscopy

Wurtzite ZnS nanoparticles are synthesized by a low temperature chemical method using ethylene glycol. Photoluminescence studies reveal a new red emission along with strong UV and blue emission known to occur from nanocrystalline ZnS. An energy level diagram involving oxygen trap levels and interstitial sulphur ions is proposed to explain the origin of the observed emission peaks. Micro-Raman spectroscopic studies indicate that the optic modes get softened whereas the second order longitudinal acoustic (LA) phonon mode get hardened in the nanocrystals. The analysis is done by taking into account the effect of the small particle size of nanoparticles as well as the existence of defects resulting in strain in the lattice.     

Study on the flow-pattern-transition instability in a natural circulation heavy water moderated boiling light water cooled reactor

A mathematical model has been developed to study the flow pattern transition instability which may occur in a boiling two-phase system. The model considers flow pattern transition criteria for vertical upward and horizontal flow in pipes to identify the flow pattern transition and flow pattern specific pressure drop models. It also considers the drift flux model to estimate the void fraction in the two-phase region. The model has been applied to predict the flow pattern transition instability in a natural circulation heavy water moderated boiling light water cooled reactor. It is found that the instability characteristics is similar to that of the Ledinegg-type instability. However, the number of multiple steady states for a given operating power can be much larger in the flow pattern transition instability as compared to that of the Ledinegg-type instability. Stability maps were plotted and compared for both the flow pattern transition instability and that of the Ledinegg-type instability. The influence of various geometric and operating parameters on this instability were investigated.     

Investigation on the characteristic of CHF in various flow pattern regimes based on look-up table data

The critical heat flux (CHF) is one of the important phenomena limiting the maximum rate of heat transfer and hence power rating of nuclear reactors. The thermal hydraulic phenomena like pressure drop, heat transfer, stability, etc. depends upon the flow pattern in the system. The CHF phenomenon is also closely related to the two-phase flow patterns. It is important to investigate the dependence of CHF on the flow pattern regimes to understand the underlying mechanisms. The present investigation reveals that CHF generally increases with mass flux in the churn/slug region. However, in the annular region the CHF decreases with increase in mass flux. Considering the dependency of the CHF trend on the flow pattern regime, it will be useful to develop CHF models, which are specific to the flow pattern regime. The data of CHF look-up table has been considered in this investigation since this approach is one of the most reliable methods for the prediction of CHF and is being used in several best-estimate thermal-hydraulic system codes, such as RELAP5, CATHARE and CATHENA. The pressure, mass flux and quality have been considered as important thermal hydraulic parameters to characterize the flow pattern during CHF under various operating condition.     

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.     

Antibacterial labdane diterpenoids of Ulva fasciata Delile from southwestern coast of the Indian Peninsula

Chromatographic purification of the dichloromethane-soluble fraction of alga, on neutral alumina, using increasing concentrations of ethylacetate/n-hexane as eluents, yielded seven labdane diterpenoids (1–7) as major constituents of green alga Ulva fasciata. Structures of these diterpenoids were established using extensive spectroscopic techniques. Antimicrobial assay showed that the compounds labda-14-ene-3α,8α-diol (2) and labda-14-ene-8α-hydroxy-3-one (4) were inhibitory to the growth of Vibrio parahaemolyticus and Vibrio alginolyticus with minimum inhibitory concentrations of 30 μg/ml by 2, and 40 μg/ml by 4, respectively against the former and 30 μg/ml by 2, and 80 μg/ml by 4, respectively, against the latter. Structure–activity relationship analyses revealed that the compounds with electronegative hydroxyl or carbonyl group(s) exhibit greater activities, apparently by proton exchange reaction with the basic aminoacyl residue at the macromolecular receptor site of virulent enzymes of pathogenic bacteria. These might provide promising therapeutic agents against infections with multi-resistant Gram-negative fish pathogenic bacteria.     

Some studies on polyblends of poly(p-phenyleneterephthalamide) and nylon-6

Polyblends of poly(p-phenyleneterephthalamide) (PPTA) and nylon-6 have been prepared by coprecipitation from solutions of the polymers in concentrated sulphuric acid. The behaviour of solutions of the polyblends in concentrated sulphuric acid was investigated. The polyblends were characterized by differential scanning calorimetry, X-ray diffraction and density measurements. The polymers were found to form a miscible blend with the composition 62.7 wt% nylon-6 and 37.3 wt% PPTA. The blends could be compacted to hard dense solids by high pressure compaction.