Numerical Study on the Route of Flame-Induced Thermoacoustic Instability in a Rijke Burner

The self-excited thermoacoustic instability in a two-dimensional Rijke-type burner with a center-stabilized premixed methane–air flame is numerically studied. The simulation considers the reacting flow, flame dynamics, and radiation model to investigate the important physical processes. A finite volume-based approach is used to simulate reacting flows under both laminar and turbulent flow conditions. Chemical reaction modeling is conducted via the finite-rate/eddy dissipation model with one-step reaction mechanisms, and the radiation heat flux and turbulent flow characteristics are determined by using the P-1 model and the standard k-ε model, respectively. The steady-state reacting flow is first simulated for model verification. Then, the dynamic pressure, velocity, and reaction heat evolutions are determined to show the onset and growth rate of self-excited instability in the burner. Using the fast Fourier transform (FFT) method, the frequency of the limit cycle oscillation is obtained, which agrees well with the theoretical prediction. The dynamic pressure and velocity along the tube axis provide the acoustic oscillation mode and amplitude, also agreeing well with the prediction. Finally, the unsteady flow field at different times in a limit cycle shows that flame-induced vortices occur inside the combustor, and the temperature distribution indicates that the back-and-forth velocity changes in the tube vary the distance between the flame and honeycomb in turn, forming a forward feedback loop in the tube. The results reveal the route of flame-induced thermoacoustic instability in the Rijke-type burner and indicate periodical vortex formation and breakdown in the Rijke burner, which should be considered turbulent flow under thermoacoustic instability

Preliminary design of a 100 kW turbine generator

The National Science Foundation and the Lewis Research Center have engaged jointly in a Wind Energy Program which includes the design and erection of a 100 kW wind turbine generator. The machine consists primarily of a rotor turbine, transmission, shaft, alternator, and tower. The rotor, measuring 125 feet in diameter and consisting of two variable pitch blades operates at 40 rpm and generates 100 kW of electrical power at 18 mph wind velocity. The entire assembly is placed on top of a tower 100 feet above ground level

Tethered gravity laboratories study

The scope of the study is to investigate ways of controlling the microgravity environment of the International Space Station by means of a tethered system. Four main study tasks were performed. First, researchers analyzed the utilization of the tether systems to improve the lowest possible steady gravity level on the Space Station and the tether capability to actively control the center of gravity position in order to compensate for activities that would upset the mass distribution of the Station. The purpose of the second task was to evaluate the whole of the experiments performable in a variable gravity environment and the related beneficial residual accelerations, both for pure and applied research in the fields of fluid, materials, and life science, so as to assess the relevance of a variable g-level laboratory. The third task involves the Tethered Variable Gravity Laboratory. The use of the facility that would crawl along a deployed tether and expose experiments to varying intensities of reduced gravity is discussed. Last, a study performed on the Attitude Tether Stabilizer concept is discussed. The stabilization effect of ballast masses tethered to the Space Station was investigated as a means of assisting the attitude control system of the Station

自励燃焼不安定性に関する数値解析に関する研究

Nowadays, combustion provides more than 80% of the worldwide energy sources. However, practical combustion systems are often susceptible to combustion instabilities characterized by large-amplitude and low-frequency pressure oscillations. This dissertation presents a nonlinear analysis and a Computational Fluid Dynamics (CFD) simulation of those self-excited combustion instabilities with a Rijke tube combustor. It is necessary to learn the mechanisms of combustion instabilities in-depth to prevent combustion instabilities or control them at acceptable levels. The nonlinear analysis of the combustion is implemented first. A model of a one-dimensional (1D) Rijke tube burner with both ends opened is built. The nondimensional momentum equation and energy equation of the acoustic perturbation are derived and solved in the time domain by using the Galerkin technique. A saturated n-τ model is proposed to descript the nonlinear flame heat release rate. The time evolution of the combustion instability is calculated. The stabilities of the systems under given conditions are determined by calculating the eigenvalues. Next, the bifurcation analysis of the dynamics behavior for the Rijke burner is performed for the variation of flame location, flame heat release intensity, and the time lag between heat release and flow velocity perturbation. Nonlinear phenomena, including hysterical, critical bifurcation, and stability switching, were observed, verifying the nonlinear characteristic of the Rijke tube burner. Further, the phase diagram and Poincaré map of the limit cycle oscillations are given, showing the oscillations' periodic character. The growth rates of the onset for the exciting case and decay for the stable case are calculated too. This nonlinear analytical method helps to understand the nature of combustion instability. Second, the self-excited combustion instability in a two-dimensional (2D) Rijke tube burner with a center-stabilized premixed methane-air flame is numerically studied. The simulation considers the reacting flow, flame dynamics, and radiation model to investigate the essential physical processes. A finite volume-based approach is used to simulate reacting flows. Chemical reaction modeling is conducted via the species transport model with one-step reaction mechanisms, and the radiation heat flux is determined by using the P-1 model. The steady-state reacting flow is first simulated for model verification. Then, the dynamic pressure, velocity, and reaction heat evolutions are determined to show the onset and growth rate of self-excited instability in the burner. The growth rates of the acoustic disturbances at the onset stages are calculated by curve fitting. Using the fast Fourier transform (FFT) method, the limit cycle oscillation frequency is obtained, which agrees well with the theoretical prediction. The dynamic pressure and velocity along the tube axis provide the acoustic oscillation mode and amplitude, agreeing well with the prediction. Finally, the unsteady flow field at different times in a limit cycle shows that flame-induced vortices occur inside the combustor. The temperature distribution indicates that the back-and-forth velocity changes in the tube vary the distance between the flame and honeycomb in turn, forming a forward feedback loop in the tube. The results reveal the route of flame-induced thermoacoustic instability and indicate periodical vortex formation and breakdown in the Rijke burner. In summary, the combustion instability in a Rijke type burner is numerically investigated from the nonlinear dynamics analysis and CFD simulation. The nonlinear analysis results show that combustion instability is a nonlinear system. There are bifurcation, stability switching phenomenons that exist, and the systems may have stable and unstable results for a given state. The CFD results verify the predictions of 1D analytical results and reveal details of the dynamics flow field, showing that even in very low Reynold numbers, the coupling of flow perturbation and heat release may induce vortices. These results provide people new perspective in studying the mechanisms of combustion

Nomenclature for Aeronautics

The nomenclature for aeronautics presented in this Report No. 474 is a revision of the last previous report on this subject (i.e., Report no. 240.) This report is published for the purpose of encouraging greater uniformity and precision in the use of terms relating to aeronautics, both in official documents of the Government and in commercial publications. Terms in general use in other branches of engineering have been included only where they have some special significance in aeronautics, or form an integral part of its terminology

Redundant actuator development program

Two concepts of redundant secondary actuator mechanization, applicable to future advanced flight control systems, were studied to quantitatively assess their design applicability to an AST. The two actuator concepts, a four-channel, force summed system and a three-channel, active/standby system have been developed and evaluated through analysis, analog computer simulation, and piloted motion simulation. The quantitative comparison of the two concepts indicates that the force summed concept better meet performance requirements, although the active/standby is superior in other respects. Both concepts are viable candidates for advanced control application dependent on the specific performance requirements