Thermodynamic Study of Turbofan Engine in Off-Design Conditions

Field: Engineering (Physical Sciences)

In this paper Turbofan engine performance graphs including thrust, thrust specific fuel consumption and thermal, propulsive and the overall efficiencies, also the graphs of turbofan components such as the high pressure and low pressure compressor pressure ratio, exit temperature pressure from high pressure compressor, combustor inlet temperature, corrected inlet mass flow rate of compressor and fan and bypass ratio, that is controlled by the engine control system were drawn based on inlet Mach number and various flight heights. Graphs have been analyzed and the effect of each of the aforementioned parameters was observed on performance graphs. Also, in order to yield more accurate solutions, the method of generating performance graphs has been modified. KEYWORDS: Turbofan, Off-Design ,Mach number. 1. INTRODUCTION Motor thermodynamic analysis includes assessing changes in thermodynamics of operating fluid passing into engine. This study can be divided into two entirely distinct groups: On-design analysis (Parametric Cycle Analysis) and Off-Design Analysis (engine Performance Analysis). In On-Design Analysis, the geometry of engine is not considered and in studying the performance diagram related to this analysis, each point represents a different engine. It is said that On-Design Analysis examines a Rubber Engine (1). To estimate engine performance in different air conditions, a modeling approach is needed that is capable of describing the behavior of engine components in the absence of On-Design conditions. In the late sixties, it was found sufficient to optimize the engine in a specific working point. But today, mainly for economic reasons, existence of an off-design model is necessary in early stages of design. (2). When a gas turbine engine is designed and built, its degree of freedom depends on available technology, the demands of designer and the engine's main applications (3). When the engine is installed on an aircraft, its efficiency changes with throttle setting and flying conditions and is limited by engine control system (4). The goal of all Off-Design models is to calculate fluid state in different locations of main stream in engine. Using these results, thrust, fuel consumption and all major parameters of engine components can be derived. Cohen and et al in (5) explain that performance properties of individual components of the engine can be obtained from actual test. When engine components are assembled, the performance range of each component is reduced in engine. When the engine is operating in steady speed or equilibrium, depending on the type of gas turbine, equilibrium running zone can be drawn for a range of different speeds on the compressor characteristic curve until equilibrium running line or zone is obtained. When the running condition is determined, running curves of thrust or specific fuel consumption can be achieved. Oates in (4) Explains that Off-Design Analysis can be done in two ways. The first considered case is that efficiencies of components are unknown; therefore they must be estimated as a function of Performance conditions. This method is used to calculate the primary estimate of the engine's performance. In the second case, components of engine have been built and tested; therefore performance characteristics of the engine components are available. Thus, combination performance of compressor, combustion chamber and turbine are predictable. These combined characteristics, called pumping characteristics, can be used to predict the overall performance of engine. Thus, methods of combining each of the components characteristics in order to obtain pumping characteristics are provided.Once this phase has been completed, operating characteristics will be obtained. For simplicity, the example of a turbojet engine is presented. First, simple gas generator equation is obtained and after having been connected to nuzzle, final equations are generated. Also Walsh acquires performance graphs by using β Lines and Referred Performance Charts (3). Mattingly in (6) and (1) analyzes engine performance by replacing constant values obtained in function of engine pressure ratio and temperature ratio, in a state of Off-Design, with values of same function at design point. As will be explained further in this study, recent analysis will be used for analysis of turbofan engines in a state of Off-Design. Suggestions for improving the results are also presented. Zero-dimensional model is used in this analysis. These models, with regard to their simplicity and self-explanatory nature which is independent of the exact geometry of the engine, are among the most commonly used models in the world of turbo-machinery (2).