Whole-genome assembly of A02 bacteria involved in nitrogen fixation within cassava leaves.

The endophytic nitrogen (N)-fixing bacterium A02 belongs to the genus Curtobacterium (Curtobacterium sp.) and is crucial for the N metabolism of cassava ( Manihot esculenta Crantz). We isolated the A02 strain from cassava cultivar SC205 and used the 15N isotope dilution method to study the impacts of A02 on growth and accumulation of N in cassava seedlings. Furthermore, the whole genome was sequenced to determine the N-fixation mechanism of A02. Compared with low N control (T1), inoculation with the A02 strain (T2) showed the highest increase in leaf and root dry weight of cassava seedlings, and 120.3 nmol/(mL·h) was the highest nitrogenase activity recorded in leaves, which were considered the main site for colonization and N-fixation. The genome of A02 was 3,555,568 bp in size and contained a circular chromosome and a plasmid. Comparison with the genomes of other short bacilli revealed that strain A02 showed evolutionary proximity to the endophytic bacterium NS330 (Curtobacterium citreum) isolated from rice (Oryza sativa) in India. The genome of A02 contained 13 nitrogen fixation (nif) genes, including 4 nifB, 1 nifR3, 2 nifH, 1 nifU, 1 nifD, 1 nifK, 1 nifE, 1 nifN, and 1 nifC, and formed a relatively complete N fixation gene cluster 8-kb long that accounted for 0.22% of the whole genome length. The nifHDK of strain A02 (Curtobacterium sp.) is identical to the Frankia alignment. Function prediction showed high copy number of the nifB gene was related to the oxygen protection mechanism. Our findings provide exciting information about the bacterial genome in relation to N support for transcriptomic and functional studies for increasing N use efficiency in cassava.

Numerical investigation of shear layer effect on sound generation in jet diffusion flame.

Shear layer effect was pointed out to be associated with the vortex-flame interaction and leads to the difference in combustion noise. In this work, numerical simulation combining with an acoustic analogy equation was performed to investigate the shear layer effect on the sound generation by jet diffusion flames. To easily identify the cause and effect, the approach that varies the thickness of shear layer by changing the Prandtl number is adopted, which provides deeper physical insight into the characteristics of the acoustic near field. The formation and evolution mechanisms of sound sources in jet diffusion flames are interpreted. It is observed that the sound sources are concentrated in vortex centers and appear as either the sources or the drains for the sound waves. The spectral analysis shows that the low frequency sound is emitted from the flame base, while the high frequency sound is radiated from the downstream region. Moreover, the theoretical analysis based on vortical dynamics shows that the low frequency sound is primarily affected by the combustion-induced buoyancy effect, whereas the high frequency sound is determined by the baroclinic torque.

Fluctuation and extinction of laminar diffusion flame induced by external acoustic wave and source.

Acoustic wave can destabilize the flame and has a potential in firefighting, but the influences of the sound source and its frequency are still poorly understood. This work applies a loudspeaker to extinguish a laminar diffusion propane flame of 5-25 mm high, where the local sound frequency is 50-70 Hz and sound pressure is 0.8-3.2 Pa (92.0-104.1 dB). Results reveal a constant flame pulsating displacement at the extinction limit, independent of the sound environment used. Such a flame pulsating displacement is found to be caused by the motion of the speaker membrane (or diaphragm) and its induced wind, which could be two orders of magnitude larger than the displacement of the air that transmits acoustic wave. Thus, under the influence of sound source, a critical flame strain rate, stretched by the pulsating airflow, can be formulated to characterize the blow-off limit better than the local sound pressure. The sound source with a lower frequency can produce larger pulsating displacements of both membrane and flame, and thus promoting extinction. This work improves the understanding of flame dynamics under the external sound field and source, and it helps establish a scientific framework for acoustic-based fire suppression technologies.

The numerical and experimental analysis of upward flame spread over the flat surface and the wavy surface.

The corrugated cardboards have been widely used in warehouse where flame spread over its surface consisting of the flat layer and the wavy layer contributes more hazards once fire disaster happens. This paper therefore performs a numerical investigation to study upward flame spread over the cardboards with wavy structures. Comparisons between upward flame spread over the flat surface and the wavy surface of corrugated cardboards were made on the flame characteristics, flame spread rate, standoff distance, local burning rate and flame splitting. It is found that the flame gets closer to the wavy surface and a larger averaged temperature gradient is exerted on the surface, which causes the fuel to burn with a higher rate. The wavy structure is the main reason that causes a distinct difference between the flat surface and the corrugated surface, and it promotes the flame to split into more small flames, thus leading the flame to move faster. To validate the model results, experiments on the flame spreading over the fuel with wavy structure were conducted, which were in good agreement with the simulation. The results can provide a solid basis on which to guide the stacking of goods in warehouse in the future.