Centrifugal compressors with low and moderate pressure ratio usually have characteristic with a wider stable operating region. Flat speed lines allow substantial reduction of the air mass flow rate with almost constant pressure ratio and without sudden surge occurrence. The surge is typically preceded by a stall or mild surge causing significant and thus quite easily detectable pressure oscillations. The preceding unstable operation is oscillatory yet still quite safe compared to deep surge. Research described in this paper deals with changes in both the engine slow pressures and fast parameters (pressures, noise, vibrations). Parameters were measured along constant speed lines from the operating line point down near to the compressor surge line. The goal was to describe typical changes in engine parameters and to define a stipulated limit of engine safe operation. Newly defined criteria of centrifugal compressor stable run will allow evaluation of engine operability, without the need to drive the engine into the compressor surge repeatedly.

This paper describes the design considerations and calculations of a mobile low-cost test cell for small gas turbine engines (GTE). The test bed was designed for the TS-20 jet engine, originated in a gas turbine starter and should be used mostly for educational purposes and for experimental validation of analytical computations. Limitations of the project were the engine air mass flow rate, low noise emissions for possibility of performance close to urban area, low cost and good mobility of the test cell. The design of the test cell structure, inlet and exhaust silencers from the aero-thermodynamic and sound suppression point of view is presented. The calculation results and respective design solutions are offered.

Abstract. Total pressure distortion is one of the three basic flow distortions (total pressure, total temperature and swirl distortion) that might appear at the inlet of a gas turbine engine (GTE) during operation. Different numerical parameters are used for assessing the total pressure distortion intensity and extent. These summary descriptors are based on the distribution of total pressure in the aerodynamic interface plane. There are two descriptors largely spread around the world, however, three or four others are still in use and can be found in current references.The staff at the University of Defence decided to compare the most common descriptors using basic flow distortion patterns in order to select the most appropriate descriptor for future department research. The most common descriptors were identified based on their prevalence in widely accessible publications. The construction and use of these descriptors are reviewed in the paper. Subsequently, they are applied to radial, angular, and combined distortion patterns of different intensities and with varied mass flow rates.The tests were performed on a specially designed test bench using an electrically driven standalone industrial centrifugal compressor, sucking air through the inlet of a TJ100 small turbojet engine. Distortion screens were placed into the inlet channel to create the desired total pressure distortions. Of the three basic distortions, only the total pressure distortion descriptors were evaluated. However, both total and static pressures were collected using a multi probe rotational measurement system.

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