Thermo economic and risk analysis for advanced long-range aero engines

Abstract

To conceive and assess engines with minimum global warming impact and lowest cost of ownership in a variety of emission legislation scenarios, emissions taxation policies, fiscal and Air Traffic Management environments a Techno economic and Environmental Risk Assessment (TERA) model is needed. In the first part of this thesis an approach is presented to estimate the cost of maintenance and the direct operating costs of turbofan engines of equivalent thrust rating, both for long and short range applications. The three advanced types of turbofan engines analysed here are a direct drive three spool with ultra high bypass ratio, a geared turbofan with the same fan as the direct drive engine and a turbofan with counter rotating fans. The baseline engines are a three spool for long range (Trent 772b) and a two spool (CFM56-7b) for short range applications. The comparison with baseline engines shows the gains and losses of these novel cycle engines. The economic model is composed of three modules: a lifing module, an economic module and a risk module. The lifing module estimates the life of the high pressure turbine disk and blades through the analysis of creep and fatigue over a full working cycle of the engine. These two phenomena are usually the most limiting factors to the life of the engine. The output of this module is the amount of hours that the engine can sustain before its first overhaul (called time between overhauls). The value of life calculated by the lifing is then taken as the baseline distribution to calculate the life of other important modules of the engine using the Weibull approach. The Weibull formulation is applied to the life analysis of different parts of the engine in order to estimate the cost of maintenance, the direct operating costs (DOC) and net present cost (NPC) of turbofan engines. The Weibull distribution is often used in the field of life data analysis due to its flexibility—it can mimic the behavior of other statistical distributions such as the normal and the exponential. In the present work five Weibull distributions are used for five important sources of interruption of the working life of the engine: Combustor, Life Limited Parts (LLP), High Pressure Compressor (HPC), General breakdowns and High Pressure Turbine (HPT). The Weibull analysis done in this work shows the impact of the breakdown of different parts of the engine on the NPC and DOC, the importance that each module of the engine has in its life, and how the application of the Weibull theory can help us in the risk assessment of future aero engines. Then the lower of the values of life of all the distributions is taken as time between overhaul (TBO), and used into the economic module calculations. The economic module uses the time between overhaul together with the cost of labour and the cost of the engine (needed to determine the cost of spare parts) to estimate the cost of maintenance of the engine. The direct operating costs (DOC) of the engine are derived as a function of maintenance cost with the cost of taxes on emissions and noise, the cost of fuel, the cost of insurance and the cost of interests paid on the total investment. The DOC of the aircraft include also the cost of cabin and flight crew and the cost of landing, navigational and ground handling fees. With knowledge of the DOC the net present cost (NPC) for both the engine and the aircraft can be estimated over an operational period of about 30 years. The risk model uses the Monte Carlo method with a Gaussian distribution to study the impact of the variations in some parameters on the NPC. Some of the parameters considered in the risk scenarios are fuel price, interest percentage on total investment, inflation, downtime, maintenance labour cost and factors used in the emission and noise taxes. The risk analyses the influence of these variables for ten thousands scenarios and then a cumulative frequency curve is built by the model to understand the frequency of the most probable scenarios. After the conclusion of the analysis of the VITAL engines as they were specified by the Original Engine Manufacturer (OEM) (Roll – Royce, Snecma and MTU), an optimisation work was done in order to try to improve the engines. The optimisation was done using two numerical gradient based techniques Firstly the Sequential Quadratic Programming – NLPQL and secondly the Mixed Integer Optimization – MOST; the objectives of the optimisation were two: minimum fuel burn and minimum direct operating costs. Because the engines were already optimized for minimum fuel burn, the optimization for minimum fuel burn didn’t show any meaningful results; instead the results for minimum DOC showed that the engines can have some improvements. The ability of the three VITAL configurations to meet the future goals of the European Union to reduce noise and gaseous emission has been assessed and has showed that the three engines cannot fully comply with future legislation beyond 2020. In the second part of this thesis three further advanced configurations have been studied to determine whether these are potential solutions to meet the ACARE goals of 2020. For these more advanced aero engines only a performance and gaseous emissions analysis has been done, because it was no possible to do an economic analysis for the new components of these engines. These advanced configurations feature components that have been studied only in laboratories, like the heat exchangers for the ICR, the wave rotor and the constant volume combustor, and for these it has not been done a lifing analysis that is fundamental in order to understand the costs of maintenance, besides in order to do a proper direct operating costs analysis many operational flight hours are needed and none of these engine have reached TRL of 7 and more which is the stage where flight hour tests are conducted. In this thesis a parametric study on three different novel cycles which could be applied to aircraft propulsion is presented: 1. Intercooled recuperative, 2. wave rotor and 3. Constant volume combustion cycle. These three cycles have been applied to a characteristic next generation long range aero engine (geared turbofan) looking for a possible future evolution and searching for benefits on specific thrust fuel consumption and emissions. The parametric study has been applied to Top of Climb conditions, the design point, at Mach number 0.82, ISA deviation of 10 degrees and an altitude of 10686 m and at cruise condition, considering two possible designs: a) Design for constant specific thrust and b) Design for constant TET or the current technology level Both values correspond to the baseline engine. For the intercooled engine also a weight and drag impact on fuel consumption has been done, in order to understand the impact of weight increase on the benefits of the configuration, considering different values of the effectiveness of the heat exchangers, the higher the values the greater is the technical challenge of the engine. After studying the CVC and Wave rotor separately it has been decided to do a parametric study of an aero engine that comprises both configurations: the internal combustion wave rotor (ICWR). The ICWR is a highly unsteady device, but offers significant advantages when combined with gas turbines. Since it is a constant volume combustion device there is a pressure raise during combustion, this will result in having lower SFC and higher thermal efficiency. It is an advanced and quite futuristic, with a technology readiness level (TRL) of 6 or higher only by 2025, so only a preliminary performance study is done, leaving to future studies the task of a more improved analysis.