Prediction of Compressor Surge Using Nonlinear Mathematical Model and Realizing by MEMS Technology
by
Károly Beneda
Budapest University of Technology and Economics, Department of Aircraft and Ships

The recently appeared Micro-Electro-Mechanical Systems (MEMS) is one of the most promising innovative devices nowadays, which, spreading in many application areas, can result in a revolution of classic mechanical engineering. Besides of their small size their most important property is their power consumption, which can be magnitudes less compared to conventional actuators implying huge advantage in fields where size and mass are critical, e.g. in aerospace applications. Their relative reduced spread can be explained that one can find increased sizes in industry, where MEMS technology can’t prove. If MEMS technology is applied to machines in which the sizes are relatively small, all their advantages can be utilized.

Nowadays the utilization of turbomachines is becoming conspicuous ever than before to make their operation more efficient. The goal of this paper is to examinate the effect of applying MEMS technology on active surge control systems in the compressors of gas turbine engines. One way to increase utilization is to decrease the surge margin applied to the compressor, which was not imaginable in the past because it requires real-time data processing and actuating. The problem is highlighted because surge of compressors occur at or near the highest pressure ratio/power.

With the (passive) control systems designed based on classical mechanical engineering approach the worst case scenario was used to prevent surge. Thus, in the most cases with the high safety factor they operate too early decreasing the efficiency of the machine. A dynamic predictive control system based on real-time gathered and processed data can allow approaching the surge margin as near as possible with the safety of operating time of the actuators. The hydro-electro-mechanical actuators and huge surge bleed valves are not able to operate as quickly as needed by this task. The solution can be reducing the size of actuating elements, therefore the MEMS technology gets in focus.

In this paper we introduce the nonlinear mathematical model for simulating surge in smaller gas turbine engine compressors, such as auxiliary power units of civil aircraft, turboprop engines of smaller passenger airplanes or turbochargers of reciprocating engines. The nonlinearity of the model should be highlighted because surge is a complicated phenomenon that can’t be evaluated using linear models. Given the small size of the inspected compressors there is a possibility to apply the high-speed actuating technology of MEMS. With such a control system there will be a smaller area in the characteristic chart which can’t be utilized because of safety. It has to be emphasized that with MEMS surge control systems safety will not decrease, its advantage in high-speed actuators will sustain the prescribed safety level. To do this, it needs a real-time data acquisition system with signal processing to exactly know how far is the machine from its surge margin, or how quickly does it approach to operate in the right time causing neither early bleed nor surge itself.

The actuators of the control system are based on MEMS technology allowing a low power need solution. Using MEMS technology it is possible to design different surge control valves and actuators. The substance of the new design is a set of equally spaced small size valves in the cross-section where surge control has to be realized. This allows controlling rotating stall phenomenon with a periodic opening of the valves in the applicable sequence, bleeding the excess air only in the area where the surge begins to evolve, in the other areas, where the flow is relatively undisturbed, the unneeded bleed can be prevented, thus allowing the highest utilization of the compressor.

A nonlinear mathematical model has been developed for centrifugal compressors, which allows predicting stall and surge with a due certainty, thus the data given by a running of this model can be the input of a surge control system for the related problem. The baselines of the MEMS technology surge control system has been set, which enables an up-to-date, economical operation of the gas turbines.

References:

1. Ho, C-M., Tai, Y-C.: Review: MEMS and its application for flow control. Journal of Fluids Engineering, 1996, Vol. 118. pp 437-447.

2. The MEMS Handbook (2nd Edition, ed. By M. Gad-El-Hak), CRC Press. 2005-12-11

3. Veress, Á., Gausz, T., Rohács, J.: Near to Micro-Size Fluid Machine Design, 4th International Conference on Advanced Engineering Design, Glasgow, Scotland, UK, 2004 szeptember 5-8,

4. Bálint, G., Rohács, J.: Application of the Microelectromechanical Systems for flow Control Purposes, Proceedings of the 6th Mini Conference on Vehicle System dynamics, Identification and Anomalies, Budapest, 1998, pp. 415 – 425

5. Nobuyuki Tahara, Masahiro Kurosaki, Yutaka Ohta, Eisuke Outa, Takurou Nakajima, and Tomofumi Nakakita: Early Stall Warning Technique for Axial-Flow Compressors Journal of Turbomachinery 2007, Vol. 129, pp. 448-502.

6. M. Morini, M. Pinelli, and M. Venturini: Development of a One-Dimensional Modular Dynamic Model for the Simulation of Surge in Compression Systems, Journal of Turbomachinery 2007, Vol. 129, pp. 437-447.

Date received: March 14, 2008