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The end clearance of the impeller is one of the most important structural parameters in the hydraulic design of a high-speed electrical submersible pump (ESP). In this paper, an ESP with a rotating speed of 6000 r/min was taken as the research object. Numerical calculations were carried out for five different end clearance conditions of 0.1 mm, 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm, respectively, to obtain the performance and internal flow field under different situation. The simulation results were verified by the pump performance experiment. It showed that the increase of the end clearance led to a decrease of the head and efficiency of the electrical submersible pump. Through the analysis of the internal flow field, it was found that the existence of the end clearance reduced the flow rate and caused free pre-whirl. With the increase of the end clearance, the phenomenon of de-flow in the diffuser passage was aggravated, which further reduced the performance of the electrical submersible pump. Finally, the reasonable recommended value of the end clearance was given, which facilitated the optimization design and engineering application of the high-speed ESP.

Melatonin (MLT) was involved in regulating various stages of plant growth and development. However, due to the low concentration and complex matrixes of plant, the analysis of MLT is a challenging task. In this study, we developed a rapid and efficient method with simplified sample preparation by employing UPLC coupled with a high resolution Orbitrap mass spectrometry, and stable isotope-labeled MLT (MLT-d4) was first used as internal standard in the developed analytical method. In the developed method, we used one-step liquid–liquid extraction to purify the crude extracts both from shoot and root of rice for the analysis, which remarkably simplify the sample preparation process. The method exhibits high specificity and recovery yield (>96.4%). Good linearities were obtained for MLT ranging from 0.01 to 20 ng/ mL with determination coefficient (R2) of 0.9991. The limit of detection for MLT was 0.03 pg. Reproducibility of the method was evaluated by intra-day and inter-day measurements and the results showed that relative standard deviations were less than 7.2%. Moreover, MLT quantification was accomplished by using only 100 mg fresh plant tissues. Additionally, the established method was successfully applied to investigate the spatiotemporal distributions of MLT in rice under cadmium (Cd) stress condition. We found that the content of MLT in shoot and root of rice increased under Cd stress, suggesting that MLT would play a crucial role in modulating the responses to Cd stress in different plant tissues.