Combustion Chambers

Thermal barrier coatings (TBCs) have been used for almost three decades to extend the life of combustors and augmentors and, more recently, stationary turbine components. Plasma-sprayed yttria-stabilized zirconia TBC currently is bill-of-material on many commercial jet engine parts. A more durable electron beam-physical vapor deposited (EB-PVD) ceramic coating recently has been developed for more demanding rotating as well as stationary turbine components. This ceramic EB-PVD is bill-of-material on turbine blades and vanes in current high thrust engine models and is being considered for newer developmental engines as well. To take maximum advantage of potential TBC benefits, the thermal effect of the TBC ceramic layer must become an integral element of the hot section component design system. To do this with acceptable reliability requires a suitable analytical life prediction model calibrated to engine experience. The latest efforts in thermal barrier coatings are directed toward c...

ABSTRACT As is well known, the increasing energy demand requires an efficient use of conventional energy sources, as well as the development of renewable technologies. The distributed generation systems entail significant benefits in terms of efficiency, emission reduction, availability and economy consequences. Renewable energy technologies are fed by intermittent resources. This feature makes the energy storage an important issue in order to improve the management or to enlarge annual operation of the facility. The use of hydrogen as an energy vector may satisfy this requirement and; at the same time, it introduces additional advantages in terms of energy efficiency and emissions reduction. This work presents an analysis based on the first and second thermodynamics law to investigate the efficiency of a hydrogen/oxygen-fueled gas turbine, which produces both electrical and thermal energy (cogeneration). A 20 kWe, microgas turbine is proposed to supply the base load demand of a residential area. The results show that the proposed facility is appropriate when the thermal energy demand is significant. We obtain an exergy efficiency of 45.7% and an energy efficiency of 89.4% regarding the lower heating value (LHV) of hydrogen. This high energy efficiency remains on the use of the liquid water effluent and the condensation heat. The main sources of irreversibility are analyzed and the effect of the design parameters on the energy and exergy efficiencies is discussed.

Modeling radiative heat transfer in combustion applications involving complex geometries and detailed spectral properties of radiative gaseous species remains a difficult challenge, especially when full coupling with detailed chemistry and fluid dynamics is required. The Monte Carlo method (MCM) usually considered as a reference “exact” method for the computation of radiative transfer is however very demanding in CPU-time. An alternative is the discrete ordinates method (DOM), based on a finite volume approach, that is more suitable for a direct coupling with computational fluid dynamics but may lack accuracy. The aim of the present paper is to propose and demonstrate the efficiency of a methodology for radiative transfer calculation, combining the advantages of both MCM and DOM. In this approach, the fast DOM is used to compute the radiative solution, and its accuracy is controlled by comparison with the exact MCM solution at a selected controlling points. A first application of th...

Abstract: This report contains the 2001 Annual Progress Reports of the postdoctoral fellows and visiting scholars of the Center for Turbulence Research. In 2001 CTR sponsored 15 resident Postdoctoral Fellows, 7 Research Associates and 3 Senior Research Fellows, hosted 7 visiting scholars and many shorter-term visitors, and supported 6 doctoral students. Most of the doctoral students engaged in turbulence research at CTR are supported by the US Office of Naval Research or the Air Force Office of Scientific Research. CTR is closely ...

Recent experience gained at AFRL with the injection of cryogenic fluids into high back-pressures is summarized. In the experimental investigations described, a jet of a cryogenic fluid, typically liquid N2, is injected into a chamber whose ambient pressure is varied to values exceeding the critical pressure of the injectant. The structure of the jet and the shear layer between the jet and the ambient has been examined. Results from visualization, jet growth rate, fractal analysis, and Raman scattering measurements indicate that the behavior of the injected fluid changes from spray-like behavior to gas jet-like behavior as pressure increased.

This work describes the methodology developed to optimize the design of the combustion chamber, the thrust generating nozzle and the cooling system of a micro-rocket using numerical simulations. The coupling between combustion, subsonic/sonic/supersonic flow transition and heat transfer inside the micro-rocket is analysed using a problem decomposition strategy and state-of-the-art numerical techniques. First, the design of the thrust-generating nozzle is optimized; then, the mixing performances and the combustion efficiency are evaluated; finally, the design of the cooling system is verified calculating the heat transfer from the hot gases to the solid shell and to the cooling fluid. Results show that sub-optimal micro-rocket design alternatives can be easily identified through self-validated numerical analyses. In this way, the number of time consuming and costly experiments required for prototypes qualification in the lab can be reduced, focusing the tests on the limited set of sub-optimal alternatives identified by numerical simulations, thus speeding up the development of new devices.

Homogeneous charge compression ignition (HCCI) is a new combustion technology that may develop as an alternative to diesel engines with high efficiency and low NOx and particulate matter emissions. This paper describes the HCCI research activities being currently pursued at Lawrence Livermore National Laboratory and at the University of California Berkeley. Current activities include analysis as well as experimental work. On analysis, we have developed two powerful tools: a single zone model and a multi-zone model. The single zone model has proven very successful in predicting start of combustion and providing reasonable estimates for peak cylinder pressure, indicated efficiency and NOX emissions. This model is being applied to develop detailed engine performance maps and control strategies, and to analyze the problem of engine startability. The multi-zone model is capable of very accurate predictions of the combustion process, including HC and CO emissions. The multi-zone model h a...

Measurements and numerical modeling are applied to observe and predict the interactive influence of sub-micron gas borne particles in an erosive turbulent combustion wall boundary layer. Some of the experiments and computations discussed are directed to study of turbulent free shear layers. Others ephasize wall boundary layers. The common purpose of all of these studies is to generate information on the particle interaction mechanisms active in alteration and reduction of the turbulent transport and the consequent reduction of the erosive heat and mass transfer. Primary emphasis here is given to analysis of erosive two phase wall boundary layers encountered in solid propellant combustion situations. Examples are gun barrel, rocket nozzle, and coal-fired gas turbine combustion flow fields.

As part of an ongoing effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, modeling, and experimental assessment of a catalytic combustion system. Previous work has indicated that homogenous gas-phase microcombustors are severely limited by chemical reaction timescales. Storable hydrocarbon fuels, such as propane, have been shown to blow out well below the desired mass flow rate per unit volume. Heterogeneous catalytic combustion has been identified as a possible improvement. Surface catalysis can increase hydrocarbon-air reaction rates, improve ignition characteristics, and broaden stability limits. Several radial inflow combustors were micromachined from silicon wafers using deep reactive ion etching and aligned fusion wafer bonding. The 191 mm 3 combustion chambers were filled with platinum-coated foam materials of various porosity and surface area. For near stoichiometric propane-air mixtures, ex...

A large eddy simulation (LES) methodology for turbulent flows in complex rigid geometries is developed using the immersed boundary method (IBM). In the IBM body force terms are added to the momentum equations to represent a complex rigid geometry on a fixed Cartesian mesh. IBM combines the efficiency inherent in using a fixed Cartesian grid and the ease of tracking the immersed boundary at a set of moving Lagrangian points. Specific implementation strategies for the IBM are described in this paper. A two-sided forcing scheme is presented and shown to work well for moving rigid boundary problems. Turbulence and flow unsteadiness are addressed by LES using higher order numerical schemes with an accurate and robust subgrid scale (SGS) stress model. The combined LES–IBM methodology is computationally cost-effective for turbulent flows in moving geometries with prescribed surface trajectories.Several example problems are solved to illustrate the capability of the IBM and LES methodologies. The IBM is validated for the laminar flow past a heated cylinder in a channel and the combined LES–IBM methodology is validated for turbulent film-cooling flows involving heat transfer. In both cases predictions are in good agreement with measurements. LES–IBM is then used to study turbulent fluid mixing inside the complex geometry of a trapped vortex combustor. Finally, to demonstrate the full potential of LES–IBM, a complex moving geometry problem of stator–rotor interaction is solved. Copyright © 2005 John Wiley & Sons, Ltd.

In order to gain a better understanding of some of the underlying physics associated with the interaction of high-amplitude acoustic waves and a coaxial-jet type injector similar to those used in cryogenic liquid rockets, a non-reacting-flow experimental investigation was conducted under sub-, near-, and supercritical chamber pressures, with and without acoustical excitation. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injector's exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatially-resolved mean temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed digital cameras provided information on the dynamic behavior of this jet. Mean and root mean square (RMS) values of the coaxial-jet dark-core length fluctuations were measured from the acquired images via a computer-automated m...

This paper aims to evaluate the cylinder head carbon deposit from diesel engine fuelled by four samples of diesel fuel emulsions containing 0%; 5%; 10% and 15% vol. water and 20% Palm Oil Methyl Ester (POME) were subjected to thermogravimetric analysis (TGA/DSC) in air medium. The deposit build up processes were performed on a single-cylinder direct-injection diesel engine for period of 25 h for each set of test fuel under constant speed 2500 rpm. The TGA system was used and then correlated with elemental analysis as well as infrared spectra for microscopic observations. It has been found that, as the water increases in fuel, less aromatic and less reactive of deposits would be formed. Therefore, such method of analyses can be used as an indicator to verify the stability of carbon deposit inside the combustion chamber that could substantiate the applicability of a particular fuel to be accepted. © 2008 Elsevier Ltd. All rights reserved.

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Gas turbines offer a high operational flexibility and a good turn down ratio to meet future requirements of power production. In this context, stable operation over a wide range and for different blends of fuel is requested. Thermoacoustic stability assessment is crucial for accelerating the development and implementation of new combustion systems. The results of nonlinear and linear thermoacoustic stability assessments are compared on the basis of recent measurements of flame describing functions and thermoacoustic stability of a model swirl combustor operating in the fully turbulent regime. The different assessment methods are outlined. The linear thermoacoustic stability assessment yields growth rates of the thermoacoustic instability whereas the limit cycle amplitude is predicted by the nonlinear stability method. It could be shown that the predicted limit cycle amplitudes correlate well with the growth rates of excitation obtained from linear modeling. Hence, for screening the ...

When a component, as heat shields, degrade, two stages can be distinguished. First, a potential failure appears, which could evolve to the second stage and become a functional failure. The time between these two stages is called delay-time, which has been widely studied in literature to determine when to inspect to avoid breakdown. These studies have shown only single analysis criteria to find failure finding interval (FFI). In order to overcome this limitation, we developed a novel strategy to apply multicriteria methodology to optimize FFI. Our approach considers availability, system breakdown risk and reliability analysis from a systemic perspective, studying heat shields as groups, according to their location in the combustion chamber, regardless of age defect. Hence, it is not necessary to check every one of the shields. Our results show an optimal FFI policy subject to a determined breakdown risk level. This analysis may be adaptable to other components that can be grouped tog...

This paper describes the application of high performance computing to accelerate the development of hypergolic propulsion systems for tactical missiles. Computational fluid dynamics is employed to model the chemically reacting flow within a system's combustion chamber, and computational chemistry is employed to characterize propellant physical and reactive properties. Accomplishments from the past year are presented and discussed.