Optimization of an Air Turbo Starter Considering its Joint Work With an Auxiliary Power Unit
Authors: Popov Grigorii , Zubanov Vasilii , Baturin Oleg , Kolmakova Daria , Novikova Yulia , Volkov Andrei 
Published: January 2021

Abstract
The authors of the paper have developed and successfully tested a method for optimizing the air starter of a gas turbine engine, considering its joint operation with the auxiliary power unit. As a result, a way to increase the efficiency of the existing launch system during the modernization of the gas turbine engine was found. Hereinafter, start efficiency is a reduction in engine start-up time and possibility of the engine start under all operating conditions.
When designing and modernizing a gas turbine engine, the greatest attention is usually paid to its main components: compressor, combustion chamber, turbine, etc. Huge efforts are spent to improve the parameters of these components, as evidenced by the huge number of publications. However, there are several “secondary” elements in the gas turbine engine. One of them is the launch system with the turbo starter, which is a small turbine driven by compressed air from the auxiliary power unit (APU). It is used to spin the engine rotor at the startup. Even though this element is small compared to the engine and it works only for a short time, the operation of a gas turbine engine is impossible without it. This system must start the engine in a short time (for military aircraft in a very short time) at any operating conditions.
The presented work appeared while verifying the possibility of using existing turbo starter for a modernized engine using modern APU fulfilling all existing operational limitations. To solve this problem, a methodology was developed for determining the possibility of joint operation of the starter turbine and the APU, and for the calculation of the parameters of the air system there. The essence of the methodology is that a characteristic of the form “flow parameter is the function of the pressure drop across the turbine” is determined for an air turbine of a turbo starter based on CFD modeling in the NUMECA program. The calculated characteristic of the turbine was obtained considering the correction factors found during verification. The calculated characteristics is in a good agreement with the experimental data.
The obtained characteristic was combined with the characteristic of the APU using the same coordinates for different flight conditions. The intersection points of the characteristics of the turbine and the APU corresponded to the operating points of the launch system. Non-intersection of the characteristics of the APU and the turbine signals the impossibility of the launch system operation at this mode. At the found operating points, the main parameters of the launch system were determined using CFD modeling. In particular, the torque values on the output shaft were checked. If it exceeded the limit value under the conditions of structural strength, work in this mode was considered as impossible. The torque value was also used to calculate the engine start time.
Based on the developed methodology for determining the possibility of joint operation of the launch system, an optimization algorithm for the turbo starter turbine was developed and implemented.
Based on the developed tools, the possibility of using existing turbo starters to launch the modernized engine was analyzed. It was found that the considered variants for air turbo starters do not meet the requirements: the first variant has a long start time, and the second one provides torque above the permissible. Using the developed algorithms, the shape of the second air turbo starter blades was optimized, which provides the modernized variant for that the permissible value of the torque on the shaft is provided with minimal changes in the design and with an acceptable start time at all operating modes.