A Novel Gas Generator Concept for Jet Engines Using a Rotating Combustion Chamber

A gas generator — consisting of a single-stage shrouded mixed-flow compressor without a diffusor, a rotating combustion chamber, and a vaneless single-stage shrouded centripetal turbine — is presented and analyzed here. All components comprise a coherent rotating device, which avoids most of the problems usually associated with small gas generators. In other words, the concept avoids all radial clearances, it is vaneless, shortens the combustion chamber, minimizes the wetted area and enables ceramic materials to be used, due to compressive blade stresses. However, the concept faces severe structural, thermal and chemical reaction challenges. All these features and their implications are discussed and their benefits for several jet engines are quantified, mainly by means of thermodynamic cycle calculations. An upfront CFD analysis identifies a polytropic compressor efficiency of around 95%. It is then demonstrated that the concept offers a thrust-to-weight ratio which is several times higher than the standard when incorporated into small UAV-type jet engines. It also enables an attractive multistage and dual-flow, but fully vaneless design option. Lastly, a thermal efficiency increase of several percentage points would be achieved, if the concept were to be realized in the (small) core of turbofans with highest overall pressure ratios and high bypass ratios. In summary, the paper presents a gas generator approach which may be considered by designers of small jet engines like those used in UAV applications and it might even be a (challenging) long-term option for the small core engines encountered in future turbofans and turboprops.

Advancement in Heated Spin Testing Technologies

An ever more rapidly accelerating trend toward pursuing more efficient gas turbines pushes the engines to hotter and more arduous operating conditions. This trend drives the need for new materials, coatings and associated modeling and testing techniques required to evaluate new component design in high temperature environments and complex stress conditions. This paper will present the recent advances in spin testing techniques that are capable of creating complex stress and thermal conditions, which more closely represent “engine like” conditions. The data from the tests will also become essential references that support the effort in Integrated Computational Materials Engineering (ICME) and in the advances in rotor design and lifing analysis models. Future innovation in aerospace products is critically depended on simultaneous engineering of material properties, product design, and manufacturing processes. ICME is an emerging discipline with an approach to design products, the materials that comprise them, and their associated materials processing methods by linking materials models at multiple scales (Structural, Macro, Meso, Micro, Nano, etc). The focus of the ICME is on the materials; understanding how processes produce material structures, how those structures give rise to material properties, and how to select and/or engineer materials for a given application [34]. The use of advanced high temperature spin testing technologies, including thermal gradient and thermo-mechanical cycling capabilities, combined with the innovative use of modern sensors and instrumentation methods, enables the examination of gas turbine discs and blades under the thermal and the mechanical loads that are more relevant to the conditions of the problematic damages occurring in modern gas turbine engines.

Modeling Static Instabilities of Biogas Flames in a Stirred-Reactor Using Detailed Chemical Kinetics Simulations

Kinetic modeling of lean static stability limits of the combustion of biogas type fuels in a model of an ideal primary zone of a gas turbine combustor is presented here. In this study, CH4 is diluted with CO2 to simulate a range of gases representative of the products of anaerobic digestion of organic materials from different sources (e.g., landfill and animal waste digester). Fuels of this type are of interest for use in small gas turbines used in distributed generation applications. Predictions made by two detailed mechanisms (GRI-Mech 3.0 and San Diego) and one reduced mechanism (GRI-Mech 1.2, reduced) are employed to investigate the underlying kinetics near lean extinction. Approximate correlations to lean extinction are extracted from these results and compared to those of other fuels.

Engine Design Strategy for Boundary Layer Ingesting Propulsion Systems With Multiple Non-Symmetric Engine Inlet Conditions

Boundary layer ingesting inlets for hybrid wing body aircraft have been investigated at some depth in recent years due to the theoretical potential for fuel burn savings. Such savings derive from the ingestion of a portion of the low momentum wake into the propulsor to reenergize the flow, thus yielding total power savings and reducing required block fuel burn. A potential concern for BLI is that traditional concepts such as “thrust” and “drag” become less clearly defined due to the interaction between the vehicle aerodynamics and the propulsive thrust achieved. One such interaction for the HWB concept is the lateral location of the inlet on the upper surface which determines the effective Reynolds number at the point of ingestion. This is an important factor in determining the amount of power savings achieved by the system, since the boundary layer, displacement, and momentum thicknesses are functions of the local chord length and airfoil shape which are all functions of the lateral location of the engine. This poses a design challenge for engine layouts with more than two engines as at least one or more of the total engines will be operating at a different set of changing inlet conditions throughout the flight envelope. As a result, the engine operating point and propulsive performance will be different between outboard and inboard engines at flight conditions with appreciable boundary layer influence including key flight conditions for engine design: takeoff, top of climb, and cruise. The optimal engine design strategy in terms of performance to address this issue is to design separate engines with similar thrust performance. This strategy has significant challenges such as requiring the manufacturing and certification of two different engines for one vehicle. A more practical strategy is to design a single engine that performs adequately at the different inlet conditions but may not achieve the full benefits of BLI. This paper presents a technique for cycle analysis which can account for the disparity between inlet conditions. This technique was used for two principal purposes: first to determine the effect of the inlet disparity on the performance of the system; second, to analyze the various design strategies that might mitigate the impact of this effect. It is shown that a single engine can be sized when considering both inboard and outboard engines simultaneously. Additionally, it is shown that there is a benefit to ingesting larger mass flows in the inboard engine for the case with large disparity between the engine inlets.

A Novel Chemical-Looping Hydrogen Generation System With Multi-Input Fossil Fuels

This paper proposes a multi-input chemical looping hydrogen generation system (MCLH), which generates hydrogen, through the use of natural gas and coal. In this system, a new type of oven, burning coal instead of natural gas as heating resource for hydrogen production reaction, is adopted. Coal can be converted to hydrogen indirectly without gasification. Benefits from the chemical looping process, the CO2 can be captured without energy penalty. With the same inputs of fuel, the new system can product about 16% more hydrogen than that of individual systems. As a result, the energy consumption of the hydrogen production is about 165J/mol-H2. Based on the exergy analyses, it is disclosed that the integration of synthetic utilization of natural gas and coal plays a significant role in reducing the exergy destruction of the MCLH system. The promising results obtained may lead to a clean coal technology that will utilize natural gas and coal more efficiently and economically.

Staged Lean Catalytic Combustion of Gasified Biomass for Gas Turbine Applications: An Experimental Approach to Investigate Performance of Catalysts

Emission demands for gas turbine utilization will become more stringent in the coming years. Currently different techniques are used to reach low levels of NOx emissions. One possible solution is the Staged Lean Catalytic Combustion. In this concept a catalysts arrangement is used to generate high temperature combustion gases. The high temperature gases could be used to feed a second combustion stage in which more fuel is injected. In this work a series of experiments were performed at the Catalytic Combustion High Pressure Test Facility at the Royal Institute of Technology (KTH) in Sweden. The fuel used was a simulated gasified biomass and the catalytic combustor consisted of an arrangement of different catalysts, e.g. bimetallic, hexaaluminates, and perovskites catalysts. These were used as, ignition catalyst, medium temperature catalyst and high temperature catalyst respectively. The tests were performed between 5 and 13.5 bar, and the overall conversion varied between 60% and 70% and the temperature of flue gases could reach 750°C and contains high level of oxygen. The determining factor to control the exit gas temperature was the richness of the mixture (λ value). On the other hand, the increased pressure had a moderate negative effect in the overall fuel conversion. This effect is stronger at leaner mixtures compared to richer ones. Moreover, λ value and also pressure affected the temperature distribution along the reactor. The utilization of a lean catalytic combustion approach makes possible the use of a post catalytic combustion. In this region additional fuel is injected to fully burn the exiting gases and increase the exit temperature to the desired levels. This staged lean catalytic combustion approach could resemble moderate levels exhaust gas recirculation techniques and/or high air temperature combustion and it is also briefly examined in the present work.

Probabilistic and Numerical Modelling of a Lobed Mixer at Windmilling Conditions

Aero-gas turbine engines with a mixed exhaust configuration offer significant benefits to the cycle efficiency relative to separate exhaust systems, such as increase in gross thrust and a reduction in fan pressure ratio required. A number of military and civil engines have a single mixed exhaust system designed to mix out the bypass and core streams. To reduce mixing losses, the two streams are designed to have similar total pressures. In design point whole engine performance solvers, a mixed exhaust is modelled using simple assumptions; momentum balance and a percentage total pressure loss. However at far off-design conditions such as windmilling and altitude relights, the bypass and core streams have very dissimilar total pressures and momentum, with the flow preferring to pass through the bypass duct, increasing drastically the bypass ratio. Mixing of highly dissimilar coaxial streams leads to complex turbulent flow fields for which the simple assumptions and models used in current performance solvers cease to be valid. The effect on simulation results is significant since the nozzle pressure affects critical aspects such as the fan operating point, and therefore the windmilling shaft speeds and air mass flow rates. This paper presents a numerical study on the performance of a lobed mixer under windmilling conditions. An analysis of the flow field is carried out at various total mixer pressure ratios, identifying the onset and nature of recirculation, the flow field characteristics, and the total pressure loss along the mixer as a function of the operating conditions. The data generated from the numerical simulations is used together with a probabilistic approach to generate a response surface in terms of the mass averaged percentage total pressure loss across the mixer, as a function of the engine operating point. This study offers an improved understanding on the complex flows that arise from mixing of highly dissimilar coaxial flows within an aero-gas turbine mixer environment. The total pressure response surface generated using this approach can be used as look-up data for the engine performance solver to include the effects of such turbulent mixing losses.

Mission Performance Simulation of Integrated Helicopter–Engine Systems Using an Aeroelastic Rotor Model

This paper presents an integrated approach, targeting the comprehensive assessment of combined helicopter–engine designs, within designated operations. The developed methodology comprises a series of individual modeling theories, each applicable to a different aspect of helicopter flight dynamics and performance. These relate to rotor blade modal analysis, three-dimensional flight path definition, flight dynamics trim solution, aeroelasticity and engine performance. The individual mathematical models are elaborately integrated within a numerical procedure, solving for the total mission fuel consumption. The overall simulation framework is applied to the performance analysis of the Aérospatiale SA330 helicopter within two generic, twin-engine medium helicopter missions. An extensive comparison with flight test data on main rotor trim controls, power requirements and unsteady blade structural loads is presented. It is shown that, for the typical range of operating conditions encountered by modern twin-engine medium civil helicopters, the effect of operational altitude on fuel consumption is predominantly influenced by the corresponding effects induced on the engine, rather than on airframe–rotor performance. The implications associated with the implicit coupling between aircraft and engine performance, are discussed in the context of mission analysis. The potential to comprehensively evaluate integrated helicopter–engine systems within complete three-dimensional operations, using modeling fidelity designated for main rotor design applications, is demonstrated. The proposed method essentially constitutes an enabler in terms of focusing the rotorcraft design process on designated operation types, rather than on specific sets of flight conditions.

Aerodynamic Design Optimization of Nacelle and Intake

Design of nacelle and intake for an aero engine is highly complex and always a compromise between contradicting design goals. The intake has to provide air for the engine with minimal pressure loss at all flight conditions like take-off, cruise, approach, or crosswind. Further the airflow has to be evenly distributed to allow satisfying operation of the fan, and the nacelle should produce as less drag as possible. Taking this into account, the ideal lip shape alternates between thin to minimize drag during cruise and thick contour to cover the requirements at off-design conditions. To efficiently deal with these contradicting geometric design demands, a multi-objective optimization problem in 2D is defined. The major aim is to simultaneously minimize inner peak Mach number at off-design and outer peak Mach number at cruise conditions, i.e., two different flight conditions are taken into account which are evaluated in parallel. Three different parametric models are introduced to describe design changes, a first model, using superellipses and polynomials, a second one based on Class-Shape-Transformation, and a B-spline approach. To check the robustness of the design process, design of experiments for all three parameterization approaches have been performed. Based on these results proper design bounds are derived to ensure reliable CAD model generation, meshing and CFD analysis. Optimization results from a multi-objective genetic algorithm demonstrate feasibility of the proposed approach. Compared to an existing design based on human experience, the designs found are better with respect to both objectives.

Impact of Fuel Composition on Blow Off and Flashback in Swirl Stabilized Lean Premixed Combustion

Co firing of natural gas with renewable fuels such as hydrogen can reduce greenhouse gas emissions, and meet other sustainability considerations. At the same time, adding hydrogen to natural gas alters combustion properties, such as burning speeds, heating values, flammability limits, and chemical characteristics. It is important to identify how combustion stability relates to fuel mixture composition in industrial gas turbines and burners and correlate such behavior to fuel properties or operating conditions. Ultimately, it is desired to predict and prevent operability issues when designing a fuel flexible gas turbine combustor. Fuel interchangeability is used to describe the ability of a substitute fuel composition to replace a baseline fuel without significantly altering performance and operation. Any substitute fuel, while maintaining the same heating load as the baseline fuel, must also provide stable combustion with low pollutant emissions. Interchangeability indices try to predict the impact of fuel composition on lean blowoff and flashback. Correlations for operability limits have been reported, though results are more consistent for blowoff compared to flashback. Yet, even for blowoff, some disagreement regarding fuel composition effects are evident. In the present work, promising correlations and parameters for lean blow off and flashback in a swirl stabilized lean premixed combustor are evaluated. Measurements are conducted for fuel compositions ranging from pure natural gas to pure hydrogen under different levels of preheat and air flow rates. The results are used to evaluate the ability of existing approaches to predict blowoff and flashback. The results show that, while a Damköhler number approach for blowoff is promising, important considerations are required in applying the method. For flashback, the quench constant parameter suggested for combustion induced vortex breakdown was applied and found to have limited success for predicting flashback in the present configuration.