A method for checking the possibility of joint work of the auxiliary power unit and the starter

This paper describes the algorithm developed by the authors for matching the workflow of the auxiliary power unit to the air turbine used when starting the engine. This technique is applied to determine the possibility of starting a gas turbine engine, calculate its time and basic parameters under all operating conditions (including in flight), as well as to select a new auxiliary power unit (APU) or an air turbine for an existing system. The developed technique considers structural, strength, operational and other limitations. The results were implemented as a computer program.


Introduction
Starting an aviation gas turbine engine (GTE) is an important step that largely determines the safety, operational efficiency and reliability of the engine and the entire aircraft. The gas turbine engine startup system (figure1) includes a whole set of various devices and blocks: a starter (usually an air turbine starter (ATS) mechanically connected to the gas turbine rotor), air and fuel supply line, automatic control systems (ACS), transmissions, power supply systems, ignition systems, etc. For a reliable engine startup, the operation of all these devices must be coordinated with each other.

Figure1.
Schematic diagram of the start-up system with an air turbine starter. The following requirements are imposed on the start-up system [1]: • the operation of the ATS must be matched with the APU operation at all operating modes (under various atmospheric conditions, speeds, and altitudes), • gas turbine engine start-up time must be minimized, • the torque on the output shaft must not exceed the maximum value according to the strength conditions of the gearbox and wheel case of the engine.
The authors of the paper solved the problem of choosing the ATS for one of the newly created gas turbine engines. The most important key to the successful operation of the start-up system is to match the work of the APU and ATS. Indeed, if the maximum efficiency of the ATS or the required design power can be achieved at compressed air flow rates or pressure levels inaccessible to the APU, the required characteristics of the entire start-up system will never be achieved.
In available scientific and technical publications [2…6], methods for matching elements of the gas turbine engine start-up system and calculating its start-up time were not found. Therefore, the authors were forced to create their own methodology for determining the possibility of joint operation of a GTE air turbo starter with an APU, determining the engine start-up time and other critical system parameters under given flight conditions.

Defining the possibility of joint work of the APU and ATS
The following assumptions are the basis of the proposed methodology for matching the ATS and APU operational processes: • the characteristics of the APU and ATS are determined separately and are presented as the dependences of the APU and ATS parameters on the pressure ratio of the working fluid π; • matching the characteristics of the APU and ATS is carried out according to the given parameter of the flow K G . It is defined as follows: where G ATSinput is the value of air mass flow rate through the ATS; * ATSinput Т is the value of the total temperature at the ATS inlet; р is the value of the total pressure at the ATS inlet.
The operational process of the APU is usually described by the dependences of the total pressure * APUoutlet р and temperature T of the air taken from the APU, on its mass flow rate G APUoutlet The APU characteristics can be obtained for several conditions of its operation, characterized, for example, by flight altitude H, flight Mach number and ambient (atmospheric) air temperature t h and by the position of the regulatory elements (for example, stagger angles α igv of the inlet guide vane (IGV) (Figures 2 and 3).
The condition for the joint work of the APU and ATS can be represented using the following equalities The developed methodology for matching the operation of ATS and APU that takes into account operational limitations can be presented as the following sequence of actions ( Figure 4).    the APU and ATS in one diagram respectively and to determine the intersection points that will be the points that meet the joint operation condition. Stage 4. If for some operational modes no joint points are found (no intersections of ATS and APU characteristics), then it is necessary to adjust the shape of ATS blades and repeat stages 1-3 determining the modified turbine characteristics using CFD.
Step 5. The parameters of the ATS operational process are determined during its joint work with the APU at each APU mode in the following sequence.
-at the intersection points of characteristics are determined at the output of the APU; -for the points of joint work of the APU and ATS, the air parameters at the ATS inlet are determined by the found values of the air parameters at the APU exit.
Thus, based on the intersection points of the above characteristics, the physical characteristics of the ATS are found when operating together with the APU in all its modes.
Stage 6. Based on calculated parameters of the ATS operational process during its joint work with the APU, the parameters of the start-up system (torque on the turbine shaft and start-up time) are calculated at each operation mode of the APU.
On the basis of the data on the torque of the output shaft, linear dependencies М torque.out.sh. =f(n out.sh ) [7] are determined for each operating mode: Based on the discovered dependence, the start time of the aircraft gas turbine engine is determined later. The calculation is carried out using the program that will be described in section 4. The coefficient B must be controlled as it provides the maximum torque at startup. Stage 7. If at least one of the found parameters of the start system does not meet the technical specifications or operational restrictions, it is necessary to adjust the shape of ATS blades and repeat stages 1-6 until the requirements are met.
Stage 8. If at all operating modes the limiting quantities (first of all, the torque on the turbine shaft) satisfy the constraints and the conditions of joint work are fulfilled, a conclusion is made about the possibility of coordinated operation of the APU and ATS for the considered modes of operation of the APU.
The developed method allows to use both experimental and calculated (design) characteristics of the APU and ATS.
The methodology was tested in assessing the possibility of joint operation of a two-stage air turbine and APU as part of a turbofan engine for a civil aviation aircraft ( Figure 5). In this figure, the shaded part of the characteristic corresponds to the operating rotational speeds of the ATS. The intersection points of the characteristics are the points where the conditions for the joint operation of the APU and the ATS (equations 5 and 6) are satisfied. An analysis of the figure shows that when using the investigated ATS, the coordinated work of the ATS and the APU was not provided for all the modes of APU operation and it is necessary to change the APU, ATS or to select new components. In addition,