Background: Elephant grass (Pennisetum purpureum Schumach) and Hybrid Pennisetum (Pennisetum americanum × P. purpureum Schumach) are tall, fast-growing perennial C4 bunchgrasses that have been in recent developed as the most appropriate biomass feedstock in many countries for exploring various biofuel products. However, the challenges of increasing plant biomass yield and enhancing their stress tolerance, especially on marginal lands, have been existed for a long while. In the past several years, bacterial endophytes used as bio-fertilizers for improving crop production have offered an opportunity to facilitate high biomass yield of energy crops in a more sustainable manner.Results: A total of 16 endophytic bacteria strains were isolated and purified from the roots of elephant grass, which were classified into four bacterial genera: Sphingomonas, Pantoea, Bacillus, and Enterobacter. Four strains, pp01, pp02, pp04, and pp06, represented four different genera, were then selected and tested in vitro for their plant growth promoting properties, effects on plant growth and salt stress tolerance of Hybrid Pennisetum. The inoculation with these four bacterial mixture demonstrated a significant plant growth promotion for Hybrid Pennisetum from the normal to salt stress conditions at 0, 50, 100, and 200 mM NaCl, respectively. The highest promotion rate for biomass yield was 116.01 and 81.72 % for shoot fresh weight and dry weight, respectively. The bacterial strains tested were shown to solubilize insoluble phosphate, fix nitrogen, produce indole acetic acid and ammonia, but only strains from Sphingomonas, Bacillus, and Enterobacter can produce siderophore. In addition, the endophyte strains tested were all able to successfully colonize the roots of Hybrid Pennisetum, reaching upto 12.12 ± 0.98 CFU g −1 fresh roots at the 3rd day of inoculation. Conclusion:The four endophytic bacteria from elephant grass significantly promoted plant growth and biomass yield, alleviated the harmful effects of salt stress on Hybrid Pennisetum. These bacteria have indicated some unique properties that are very valuable for exploiting bio-inoculants aiding in the efforts to establish a sustainable and largescale feedstock production system for Hybrid Pennisetum, particularly, on the saline marginal lands.

For multi-stage pumps, the change of the impeller stage spacing will have a greater impact on the dynamic characteristics of the shafting rotor. The paper took XBC18-178-240LC3 vertical long shaft fire pump as the research object, using ANSYS Workbench software to study the relationship between the critical speed of the shafting rotor and the different positions of the impellers. First, 9 shafting rotors (based Z4,Z5 and Z6 models) under the different positions of the impellers were established; and then the change law of the critical speed on 9 shafting rotors in the first 12 modes was studied. The results showed that different positions of the impeller on the shaft have a great impact on critical speed of the shafting rotor. With the increase of the distance between the impeller stages, the low-order critical speed (6th order and within) of the Z4 model shafting rotor decreases. However, the low-order critical speed (5th order and within) of the Z5 and Z6 model shafting rotors decreases with the increase of the distance between the impeller stages, and the high-order critical speed doesn’t satisfy this law. Considering static characteristics of the shafting rotor and actual engineering applications, the paper selected the Z5-stage spacing 800mm shafting rotor model as the best case, providing a theoretical support for the subsequent vertical long shaft fire pump design optimization.

In this paper, a vertical long-shaft fire pump XBC18-178-240LC3 was taken as the research object to study the static characteristics of the pump rotor-shaft system under different shaft length connection. To analyse the influence of the length and number of the intermediate shaft section of the pump shaft on the deformation and equivalent stress of the shafting rotor, three shafting rotor models Z4(modified model), Z5(original model) and Z6(modified model) were designed, and then the static characteristics of them were analysed and compared under four operating conditions of 0.65Q d(0.65 times design flow),1.0Q d, 1.2Q d and 1.4Q d by numerical simulation. The results suggested that the changed rules of the three models' deformation and equivalent stress were basically the same. After the rated operating condition, the maximum deformation of the impeller of Z5 model was smaller than that of the Z4 and Z6 models. Under the four operating conditions, the maximum equivalent stress of the impeller of Z5 model was the smallest. This showed that the length and number of the intermediate shaft section affected the deformation and equivalent stress of the shaft rotor. Therefore, Z5 model was selected as the best rotor structure design scheme.