Enhancing compressor stall and surge has a great importance for the development of turbo compressors. The application of casing treatment is an effective measure to expand the stall margin and stable operation range. Numerical investigations were conducted to predict the performance of a low flow rate centrifugal compressor with circumferential groove casing treatment in diffuser. Numerical cases with different radial location, radial width and axial depth of a circumferential single groove in different types of diffusers (vaned diffuser, half-vaned diffuser, vaneless diffuser) were carried out to compare the results. The computational fluid dynamics analyses results show that the centrifugal compressor with circumferential groove in vaned diffuser can extend stable range by about 9.1% while the efficiency over the whole operating range decreases by 0.2 to 1.7%; the results with half-vaned diffuser and vaneless diffuser can improve stable range less and the efficiency decreases more. Efforts were made to study blade level flow mechanisms to determine how the circumferential groove impacts the compressor’s stall margin and performance. The flow structures in the passage, the tip gap, and the grooves as well as their mutual interactions were plotted and analyzed. The flow transport across the tip gap in the smooth wall and the circumferential grooves were compared.
To produce a good machine tool, the thermally induced error in the machine during machining plays a crucial role and is an important issue needing to be resolved. The thermal error may account for 70% of the total error. There are three main approaches to solving the thermal error problem: preventing heat flows from hot components, designing a thermally stable structure for the machine, and compensating the thermal error using thermal error models. The first two approaches can be carried out in the primary design stage of machine tools, and they have been used in the manufacture of commercial products. The third approach, the strategy of thermal error compensation, is the most effective and popular approach. However, there are still many unsolved problems. Among these problems, the cutting conditions have a significant influence on the modeling precision of the thermal error. In this study, we develop an integral model based on the integrated grey system theory (IGST) in conjunction with a genetic-algorithm (GA)-optimized back-propagation neural network (BPNN) to investigate the influence of cutting conditions on a machine tool's thermal error. The model is chosen on account of its high ability in dealing with a small amount of training data. Results show that a single thermal error modeling formula cannot make accurate predictions for different cutting conditions. Suitable adjustment of the modeling parameters or the use of a multiple modeling scheme is needed.
A method for the speed matching of the second rotor (R2) with equal power for two rotors was proposed to avoid the overload of the second motor under low flow rate and the rapid decrease in pressure-rise and efficiency of R2 under high flow rate. The speed matching of two-stage rotors is proposed and analyzed to improve the stability margin of the counter-rotating fan (CRF). The fan performances during constant speed operating and during the speed matching operating are presented and discussed using experimental research. The results show that, the speed matching of R2 operating decreases the load of R2 under low flow rate and increases the pressure-rise and efficiency of R2 under high flow rate. Thus, the efficient working range and the blocking condition margin are increased. Reducing n1 and increasing n2 under low flow rate can regulate the position of unstable working line leftward without reducing the pressure-rise of the fan. Thus, the stability margin of the CRF is expanded.
Existing methods of improving the performance of mine counter‐rotating fans (MCRFs) through speed matching of the rotors have certain drawbacks. This study proposes a variable‐speed method for the two rotors of an MCRF to improve its performance, including the blocking margin, operational safety, high‐efficiency working range (HER), and stability margin. Experimental and numerical methods were used to study the pneumatic performance of the MCRF. The results show that the second rotor (R2) is the critical stage affecting the safety of the motor, HER, and blocking margin. A condition‐adaptive speed matching method was proposed for R2 to avoid overloading the second motor at low‐flow rates and to improve the blocking margin and HER. The first rotor (R1) is the critical stage determining the stability margin of the MCRF. Reducing n1 and increasing n2 at low‐flow rates helped extend the stability margin.
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