Axial Flow Compressor Surge Signal Detection, Analysis And Application In NF-6 Transonic Wind Tunnel

The NF-6 wind tunnel is the first internationally advanced large-scale continuously-running high Reynolds number transonic airfoil wind tunnel in China, because the axial-flow compressor installed in the wind tunnel loop works under a condition of high pressure and low temperature, the flow at its entrance fluctuates greatly and would easily cause the compressor to go into surging zone, severely endangering the safety of the people working on site and make the equipment unable to work normally. When surge occurs in the compressor, the signals such as flow rate, pressure, temperature, vibration, etc., which symbolize and characterize the surge, will exhibit phenomena that are incredible and impossible under normal conventional conditions. The research on these phenomena in this dissertation is the first time in China and the references are seldom found internationally. Therefore detecting, analyzing, and predicting the signals are very important to preventing surge in scientific sense and in social value. However, the research on axial-flow surge phenomena as a cross discipline, involving aerodynamics, detecting and processing of signals, rotating machinery, control, etc., has to take many factors into account. This dissertation took the wind tunnel and the compressor as an integrated unity, exploiting the knowledge of many disciplines especially from the view of signal detecting and processing, mainly through the combination of theoretical analysis and experiments and based on the consideration of the coupling and interacting between the wind tunnel and the compressor, and produced following main creative contributions and results:1. Through analysis the conditions under which the NF-6 wind tunnel and its axial-flow compressor can jointly run stably were obtained, laying a theoretical basis for the detection of the surge signal.It was raised and put forward in the dissertation that such a complicated network of fluid flow system as the continuously-running high-speed wind tunnel be studied based on the concept of signal and system. Since the wind tunnel along with the compressor forms a closed loop, the working process of the whole system is a evolutionary process of mutually coupling and mutually interacting, which is the most significant difference between a compressor in the wind tunnel loop and that in used conventional industries. Therefore, the mutually coupling and interacting process must be taken into account when discussing the stable operating conditions and unstable working situations of the compressor in a wind tunnel. It was found from theoretical analysis that even if the compressor itself works in stable zones, the operating point of the system of the wind tunnel and the compressor might be in an unstable state and have a trend to shift to an unstable operating point of the compressor because of the coupling effect from the wind tunnel and thus, as a whole, the surge-preventing system of the compressor is a necessary precondition for the safe running of NF-6 wind tunnel.2. By analyzing the manner to detect flow rate signal a constant-annular-area method was proposed to accurately measure the flow rate through the compressor while at the same time the effect of the wind tunnel total pressure and total temperature on the measurement precision must be counted in.In view of that the flow rate signal is one of the key signals for the anti-surging control system of the axial-flow compressor to work normally in the NF-6 wind tunnel, the dissertation calculated the envelope of the range of flow rate values which the axial-flow compressor should obey when working in the NF-6 wind tunnel. At the same time, since the entrance flow entering the axial-flow compressor of the NF-6 wind tunnel is typical non-fully-developed pipe flow, the matured methods widely used in common industries for measuring flow rate are unable to be directly applied to the NF-6 wind tunnel, thus the dissertation proposed to obtain the accurate mass flow rate entering the axial-flow compressor by both measuring the average flow velocity at the cross section immediately behind the wind tunnel's first comer with a calibrated anemometer and, at the same time, measuring the total pressure and total temperature of the settling chamber of the wind tunnel. Additionally, in this dissertation, the effects of location, form, range and precision of transducers on the measured results were calculated and discussed in detail. These results gave guidance to the construction of the compressor flow rate measuring equipment in the NF-6 wind tunnel and were used to modify and improve some part of the predecessor design and contributed to the reduction of the risk of debugging, testing, and running of the equipment.3. The dissertation validated the empirical formulae for estimating the cutoff frequency of the filter through the simulation test of pressure signal detection and the analysis of the test, analyzed the signal of abrupt pressure variation when surge occurs in the compressor using wavelet transform, and found some rules in detecting stable and dynamic pressure signals.The pressure ratio signal defined as the ratio of the outlet total pressure to the inlet total pressure of the axial-flow compressor is another key signal on which the anti-surging control system of the axial-flow compressor relies to work normally in NF-6 wind tunnel. Since the most leading and fundamental inherent cause of surging in compressor is flow separation, thus the flow non-uniformity at the compressor entrance section would accelerate or aggravate the occurring of surging. Because of the structural limitations of the wind tunnel there exist flow parameter fluctuations and flow deflections in the entrance section of the axial-flow compressor of the NF-6 wind tunnel. Therefore adopting the pressure ratio signal obtained by only measuring total pressure as the surging-preventing control parameter of the compressor will, on the one hand, lead to narrower compressor operating zone and lower compressor efficiency, while on the other hand, increase the energy consumption and the cost of the wind tunnel tests under the same operating conditions, hence cause unnecessary waste. In this dissertation a new concept was put forward for the first time nationwide that dynamic pressure sensors be installed both in the entrance and exit of the compressor to obtain the dynamic total pressure signals at the two positions. To acquire the corresponding rules, the issues associated with measuring the dynamic pressure were simulated by an inlet test. Furthermore the singularity characteristics of the pressure signals when were analyzed using wavelet transformation when surge occurs in the compressor, which was proposed to be applied in the design of the surging-preventing system of the axial-flow compressor in the NF-6 wind tunnel to improve the rapidity and accuracy of the control system and to widen the operating envelope of the wind tunnel.4. This dissertation discussed the vibration of the axial-flow compressor's shafting of the NF-6 wind tunnel and the influence of the entering flow temperature of NF-6 wind tunnel on the surging-preventing curve, the results from the discussion gave a good guidance to the construction of the detecting and measuring system of vibration signals and temperature signals.The analysis indicated that the first-order natural vibration frequency of the structure of the axial-flow compressor's shafting in NF-6 wind tunnel fell within the excitation region corresponding to the normal working rotating speed of the compressor. Further analysis showed that the general excitation originating from motors, compressors and gears would not cause large stress of twisting vibration. Therefore the whole system could be considered to be able to run safely in the designed range of rotating speed. But if the high-speed gears have flaws the coefficients of the vibration mode of both orders are large. Meanwhile, if the frequency corresponding to the rotating speed of the compressor is approaching the first-order natural vibration frequency the whole compressor's shafting system may vibrate and hence cause the destruction of the whole structure. Therefore, in order to ensure that the whole NF-6 wind tunnel run safely a monitoring system of compressor vibration was set up guided by the above computed results. Additionally, considering the influence the entering temperature of the compressor on the surging-preventing curve, the effect of the measuring precision of temperature on the whole system was discussed and a temperature detecting, checking and protecting system was designed based on the discussions. All these measures have been utilized in the construction of NF-6 wind tunnel and laid a solid foundation for the successful running of the whole system.5. The dissertation set up an elementary mathematic model for the system, decoupled the multi-argument coupling system, and gave a generally applicable rule for the control of continuously-running wind tunnel.Analysis shows that the control system of NF-6 wind tunnel was a typical multi-argument coupling system and a typical model of multi-input and multi-output (MIMO) as viewed from the input and output. Therefore the adopting purely PID control did not meet the requirements of the NF-6 wind tunnel system control. However, from the point of view of engineering in the early period of debugging and operating of the wind tunnel, the MIMO system can be decoupled into four single separate loops and then the wind tunnel can be controlled with traditional PID technique. With the safe debugging and running of the wind tunnel guaranteed, the parameters associated with the wind tunnel running process were acquired through a massive number of wind tunnel tests. After accumulating, analyzing and processing these parameters, more advanced control mode such as artificial intelligence control could be put into use gradually to improve the running efficiency and broaden the running envelop of the wind tunnel.The above-mentioned results theoretical analysis and practical computations have been successfully used to guide the construction of the anti-surge system of the axial-flow compressor in the NF-6 wind tunnel and led to very satisfactory effects. The author hopes that the results in this dissertation will be helpful valuable to the construction of the similar wind tunnels in our country in the future.