Search

Industrial gas turbines are complex systems, and their proper analysis requires specific knowledge of the different components interacting with each other. One of the challenges is to accurately predict the overall performance of the system. To do so, a performance analysis tool can be used but it needs to rely on representative characteristics from the different elements. One of the key items required for the performance derivation is the understanding of the total cooling and leakage air (TCLA) and its distribution within the machine. A secondary air flow (SAF) tool is used to evaluate TCLA, but it needs to be “connected” with the overall performance model. The source locations of the SAF system are relatively straightforward to handle, but the sink (or dump) locations occurring in the turbine section require a detailed understanding of the pressure distribution within the flowpath. As a matter of fact, a change in SAF distribution to the turbine yields a change in turbine work output and load distribution that needs to be captured in the overall engine performance assessment. Furthermore, a change in turbine inlet boundary conditions also yields a change in pressure values for the SAF system. For these reasons, an accurate performance modeling of a gas turbine requires a so-called Synergy Loop to converge the overall boundary conditions of all its interacting modules and sub-models. This paper will describe the method used to integrate different commercial and in-house software to complete the Synergy Loop. The authors will also describe the different iteration steps possible between the specific components and the recommended iteration loop sequence for fast convergence.

During different airplane flight modes, various effects may appear that need to be analyzed for both the oil and the fuel system at steady-state and transient operating modes. The effects, which relate to the cold temperature, associated with fuel freeze or wax point, cause a malfunction in the fuel pumps, nozzles, and other areas of the fuel system. On the other hand, high fuel temperature also leads to negative effects — the most common failure of high-flow fuel systems is cavitation, or “vapor-lock.” The combination of too much heat or too much inlet restriction can create this operating condition, where the liquid fuel literally boils inside the fuel pump. These effects are eliminated by the fuel/oil heat exchange system. In case of low fuel temperature, the fuel is used as a refrigerant to cool down hot oil coming from bearings. And in case of high fuel temperature, the oil serves as a coolant. This paper considers the method of evaluating normal and critical aircraft engine operation modes of the oil supply system with a fuel-oil heat exchanger utilizing an unsteady-state thermal-fluid network approach. The analyses are done based on the aircraft engine example to evaluate fuel and oil systems parameters variation in time under different flight conditions — the amount of fuel in the tank, inertial thermal effects, and the response time of the system to the regulation of the heat exchanger. The article is focused on sudden switching from a high to low gas engine operating mode. Fuel consumption to the engine is reduced abruptly, but the heat transfer from the bearings to the oil is still high due to thermal inertia. In this situation, a large amount of heated fuel must be returned to the fuel tank. At a certain point in time, the temperature of the fuel can reach a critical value. At the same time bearing cooling becomes ineffective, which leads to overheating. The calculation of thermal management system was performed at nominal conditions to obtain the initial data for low power settings analysis. As results of analysis at the low power settings mode the oil temperature before fuel cooled oil cooler is reached above 138 °C, which is high value. The failure of flow return valve is considered. The variations of oil temperature after the tank and increasing of fuel temperature at the tank in case of emergency situation are obtained. The influence of cooled fuel amount on the system thermal management is analyzed.

Turbine secondary flow system calculations are usually performed by utilizing the thermal-fluid network approach. The network consists of branches and nodes. Fluid branches usually describe the flow resistance of the correspondent fluid path section, while fluid nodes are used to connect fluid branches between each other. Fluid flow in branches and nodes is described by the set of conservation equations such as mass and momentum equations. For fluid nodes, usually total or static pressure is used as a variable in these equations. However, such an approach may yield a system of equations that cannot be solved (for theoretical cases where zero resistance branches are present) or may produce significant differences in results compared to the more precise CFD solution (for real cases with resistances). This paper provides an approach on how to create a model of a fluid lossless junction in order to solve the mentioned problems. Such a junction enables the connection of any number of inflow and outflow fluid branches without bringing any flow resistance, which allows fluid branches to handle flow resistance influence by themselves. The proposed model is based on the idea, that each node should contain not just one pressure (static or total) as a variable, but two variables — both static and total pressures. Such a node can be used to model junctions of different types with flow mixing, as well as separation and sudden cross-sectional area changes. With this approach it is also possible to model chambers by modifying just one equation. Also, this new method can be applied for both compressible and incompressible calculation types and can handle chocked flows as well.